U.S. patent application number 10/879496 was filed with the patent office on 2005-03-17 for method for making a microstructure by surface micromachining.
Invention is credited to Rodgers, M. Steven, Sniegowski, Jeffry J..
Application Number | 20050059184 10/879496 |
Document ID | / |
Family ID | 25283029 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050059184 |
Kind Code |
A1 |
Sniegowski, Jeffry J. ; et
al. |
March 17, 2005 |
Method for making a microstructure by surface micromachining
Abstract
Various methods for forming surface micromachined
microstructures are disclosed. One aspect relates to executing
surface micromachining operation to structurally reinforce at least
one structural layer in a microstructure. Another aspect relates to
executing the surface micromachining operation to form a plurality
of at least generally laterally extending etch release channels
within a sacrificial material to facilitate the release of the
corresponding microstructure.
Inventors: |
Sniegowski, Jeffry J.;
(Tijeras, NM) ; Rodgers, M. Steven; (Albuquerque,
NM) |
Correspondence
Address: |
MARSH, FISCHMANN & BREYFOGLE LLP
3151 SOUTH VAUGHN WAY
SUITE 411
AURORA
CO
80014
US
|
Family ID: |
25283029 |
Appl. No.: |
10/879496 |
Filed: |
June 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10879496 |
Jun 29, 2004 |
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09840716 |
Apr 23, 2001 |
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6756317 |
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Current U.S.
Class: |
438/48 ;
438/52 |
Current CPC
Class: |
H01L 21/302 20130101;
B81C 1/00547 20130101 |
Class at
Publication: |
438/048 ;
438/052 |
International
Class: |
H01L 021/00 |
Claims
1-66. Canceled
67. A method for making a surface micromachined microstructure,
comprising the steps of: forming a first sacrificial layer over a
first substrate, wherein said first substrate comprises an upper
surface that extends in a lateral dimension; forming a plurality of
conduits that are hollow, embedded, have a closed perimeter that is
defined at least in part by said first sacrificial layer, and
extend in said lateral dimension; forming a first structural layer
over said first sacrificial layer and after said forming a first
sacrificial layer step, wherein said first structural layer extends
in said lateral dimension and comprises a first perimeter that
defines a lateral extent of said first structural layer, wherein
said plurality of conduits extend in said lateral dimension
underneath said first structural layer such that said first
structural layer overlies said plurality of conduits; and removing
said first sacrificial layer after said forming a first structural
layer step, wherein said removing step comprises flowing an etchant
within at least some of said plurality of conduits, and wherein
said forming a plurality of conduits step is completed before said
forming a first structural layer step.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to making a
microstructure by surface micromachining. One aspect relates to
making a structurally reinforced microstructure to provide a
flatter profile on one or more of its structural layers. Another
aspect relates to providing a plurality of at least generally
laterally extending etch release channels to facilitate the release
of the microstructure from the substrate.
BACKGROUND OF THE INVENTION
[0002] There are a number of microfabrication technologies that
have been utilized for making microstructures (e.g.,
micromechanical devices, microelectromechanical devices) by what
may be characterized as micromachining, including LIGA
(Lithographie, Galvonoformung, Abformung), SLIGA (sacrificial
LIGA), bulk micromachining, surface micromachining, micro
electrodischarge machining (EDM), laser micromachining, 3-D
stereolithography, and other techniques. Bulk micromachining has
been utilized for making relatively simple micromechanical
structures. Bulk micromachining generally entails cutting or
machining a bulk substrate using an appropriate etchant (e.g.,
using liquid crystal-plane selective etchants; using deep reactive
ion etching techniques). Another micromachining technique that
allows for the formation of significantly more complex
microstructures is surface micromachining. Surface micromachining
generally entails depositing alternate layers of structural
material and sacrificial material using an appropriate substrate
which functions as the foundation for the resulting microstructure.
Various patterning operations may be executed on one or more of
these layers before the next layer is deposited so as to define the
desired microstructure. After the microstructure has been defined
in this general manner, the various sacrificial layers are removed
by exposing the microstructure and the various sacrificial layers
to one or more etchants. This is commonly called "releasing" the
microstructure from the substrate, typically to allow at least some
degree of relative movement between the microstructure and the
substrate. Although the etchant may be biased to the sacrificial
material, it may have some effect on the structural material over
time as well. Therefore, it is generally desirable to reduce the
time required to release the microstructure to reduce the potential
for damage to its structure.
[0003] Microstructures are getting a significant amount of
attention in the field of optical switches. Microstructure-based
optical switches include one or more mirror microstructures. Access
to the sacrificial material that underlies the support layer that
defines a given mirror microstructure is commonly realized by
forming a plurality of small etch release holes down through the
entire thickness or vertical extent of the mirror microstructure
(e.g., vertically extending/disposed etch release holes). The
presence of these small holes on the upper surface of the mirror
microstructure has an obvious detrimental effect on its optical
performance capabilities. Another factor that may have an effect on
the optical performance capabilities of such a mirror
microstructure is its overall flatness, which may be related to the
rigidity of the mirror microstructure. "Flatness" may be defined in
relation to a radius of curvature of an upper surface of the mirror
microstructure. This upper surface may be generally convex or
generally concave. Known surface micromachined mirror
microstructures have a radius of curvature of no more than about
0.65 meters.
BRIEF SUMMARY OF THE INVENTION
[0004] The present invention is generally embodied in a method for
making a microstructure by surface micromachining. In this method,
at least one and more typically a plurality of at least generally
laterally extending etch release channels or conduits are formed
within a sacrificial material. This sacrificial material is used to
fabricate the microstructure on an appropriate substrate. At least
some of this sacrificial material is removed when the
microstructure is released from the substrate (and thereby
encompassing the situation where all of this sacrificial material
is removed).
[0005] A first aspect of the present invention is embodied in a
method for making a microstructure by surface micromachining that
includes forming a first structural layer over a sacrificial
material. "Over" includes being deposited directly on the substrate
or being deposited on an intermediate layer that is disposed
between the subject sacrificial layer and the substrate. "On" in
contrast means that there is an interfacing relation. In any case,
a plurality of hollow etch release pipes, channels, conduits, or
the like extend at least generally laterally through/within this
sacrificial material. Lateral or the like, as used herein, means
that the etch release conduits are disposed or oriented in a
direction which is at least generally parallel with the substrate.
Although the etch release conduits will typically extend laterally
at a constant elevation relative to the substrate, such need not
necessarily be the case. Ultimately, at least some of the
sacrificial material is removed at least in part by allowing an
etchant to flow through any and all of these hollow etch release
conduits (and thereby encompassing the situation where all of this
sacrificial layer is removed).
[0006] Various refinements exist of the features noted in relation
to the first aspect of the present invention. Further features may
also be incorporated in the first aspect of the present invention
as well. These refinements and additional features may exist
individually or in any combination. In one embodiment, at least one
end of at least one etch release conduit may be disposed at least
generally at the same radial position as a perimeter of the first
structural layer. Hereafter, "at least generally at the same radial
position" in relation to any end of any etch release conduit or
structure used to define the same means within 50 .mu.m of the
radial position of the perimeter of the relevant structural layer
in one embodiment, more preferably within 25 .mu.m of the radial
position of the perimeter of the relevant structural layer in
another embodiment, and even more preferably at the same radial
position as the perimeter of the relevant structural layer. As
such, the etchant does not have to etch in from the perimeter "too
far" before encountering an open end of one or more etch release
conduits associated with the first aspect. Reducing the time
required for the etchant to reach the etch release conduits should
at least to a point reduce the overall time for accomplishing the
release of the microstructure from the substrate.
[0007] Various layouts of the plurality of etch release conduits in
the noted lateral dimension may be utilized in relation to the
first aspect. In one embodiment, each of the plurality of etch
release conduits may be disposed in non-intersecting relation, in
another embodiment the plurality of etch release conduits are
disposed in at least substantially parallel relation, and in yet
another embodiment at least some of the etch release conduits
intersect. The plurality of etch release conduits may extend at
least generally toward (and thereby including to) a common point,
such as one that corresponds with a center of the first structural
layer in the lateral dimension. All etch release conduits need not
extend the same distance toward this common point, although such
may be the case. Some of the plurality of etch release conduits may
extend at least generally toward (and thereby including to) a first
common point (e.g., one that corresponds with a center of the first
structural layer in the lateral dimension), while some of the
plurality of etch release conduits may extend toward a different
common point. The plurality of etch release conduits may also
extend in the lateral dimension in a variety of configurations. In
one embodiment, the plurality of etch release conduits are at least
generally axially extending, while in another embodiment the
plurality of etch release conduits are non-linear (e.g., in a
sinusoidal configuration that is at least generally parallel with
the substrate).
[0008] In one embodiment of the first aspect, the laterally
extending etch release conduits are completely defined before the
stack that includes the microstructure being fabricated by the
methodology of the first aspect is exposed to a release etchant for
removing the first sacrificial layer. One particularly desirable
application for the first aspect is in the formation of a mirror
microstructure or multiple mirror microstructures for a surface
micromachined optical system that has at least some degree of
movement relative to the first substrate. Because of the presence
of the plurality of laterally extending etch release conduits,
there is no need to have a plurality of etch release apertures or
holes that extend entirely down through the entire vertical extent
of the first structural layer to remove the sacrificial material
that directly underlies this first structural layer. That is, there
is no need for a plurality of vertically disposed etch release
apertures, with "vertical" being at least generally opposite
"lateral." As such, the upper surface of the first structural layer
retains a very smooth surface that lacks any such vertically
disposed etch release apertures, which makes the microstructure
that is made by the methodology of the first aspect particularly
suited for use in optical applications. This is true whether the
upper surface of the first structural layer is the actual optical
surface for the mirror microstructure, or whether a film or other
layer is deposited on the first structural layer to provide more
desirable optical properties/characteristics.
[0009] The sacrificial material that directly underlies the first
structural layer used by the first aspect may actually be defined
by the sequential deposition of multiple sacrificial layers, one on
top of the other. That is, a lower portion of this sacrificial
material may be formed as one layer, and an upper portion of this
sacrificial material may be subsequently formed in overlying
relation. Consider a case where a first sacrificial layer is formed
over the first substrate, and where a first intermediate layer is
formed on this first sacrificial layer. The first intermediate
layer may be patterned to define a first subassembly (e.g., a
plurality of strips). Portions of the first sacrificial layer are
exposed by a patterning of the first intermediate layer to define
the first subassembly. An upper portion of the first sacrificial
layer (i.e., something less than the entirety of the first
sacrificial layer) may be etched an amount after this patterning
such that at least part of the first sacrificial layer that
underlies at least part of the first subassembly is removed (e.g.,
using a timed etch). The first subassembly may be disposed directly
on the first sacrificial layer, directly on a structural layer, or
directly on both the first sacrificial layer and a structural layer
(e.g., for the case where there is both a sacrificial material and
a structural material at the same level within a stack which
contains the microstructure being fabricated by the methodology of
the subject first aspect).
[0010] The "gap" that now exists between the first subassembly and
that portion of the etched first sacrificial layer that is directly
beneath the first subassembly may be characterized as an undercut.
A second sacrificial layer may be formed on at least the first
sacrificial layer. One could characterize this as "backfilling."
Notwithstanding this characterization of the backfilled sacrificial
material as a "second sacrificial layer", the first and second
sacrificial layers may in fact be indistinguishable from each other
and may in effect define a continuous structure. Nonetheless, the
sacrificial material that has been characterized as the second
sacrificial layer will not fill the entire extent of each of the
undercuts. This failure to fill the undercuts defines the plurality
of etch release conduits that are associated with the first aspect
of the present invention. One could visualize that the first
subassembly acts as an umbrella of sorts that prevents the material
that has been characterized as the second sacrificial layer from
totally filling the noted undercuts that are protected by the first
subassembly. Typically the second sacrificial layer will also cover
the first subassembly. In this case and possibly in other instances
during the fabrication of the microstructure associated with the
first aspect, it may be desirable to planarize an upper surface of
a given layer before depositing the next layer thereon. One
appropriate technique for providing this planarization function is
chemical mechanical polishing.
[0011] The first subassembly may be a reinforcing structure for the
first structural layer, in which case the first subassembly would
exist in the microstructure that is defined by the methodology of
the first aspect. Reinforcement may be provided by structurally
interconnecting the first structural layer with the first
subassembly through the above-noted second sacrificial layer which
may be deposited on the first subassembly in addition to the first
sacrificial layer as noted. Further reinforcement of the first
structural layer may be accomplished by structurally
interconnecting the first subassembly with a structural layer that
underlies the first sacrificial layer. In both cases, the actual
reinforcement structure could be in the form of a plurality of
posts or columns that are disposed in spaced relation, in the form
of a plurality of at least generally laterally extending ribs or
rails, or in the form of a grid-like reinforcement structure.
[0012] The first subassembly need not remain in the microstructure
that may be defined by the methodology of the first aspect. That
is, the first subassembly need not be part of the final
microstructure that is ultimately fabricated by the methodology of
the first aspect. In this case, the only purpose of the first
subassembly would be to at least assist in the formation of the
plurality of etch release conduits that are associated with the
first aspect of the present invention. This "temporary" first
subassembly may be in the form of a plurality of rails that are
formed on or in a sacrificial material that is used in the
methodology of the first aspect. Removal of the first subassembly
from the final microstructure being made by the subject first
aspect may be desirable in order to retain a low mass for this
microstructure (e.g., in an upper structural layer of such a
microstructure).
[0013] One way in which the first subassembly may be removed or
alleviated from the final microstructure being made by the first
aspect of the present invention is to form the first subassembly
from a material that would be etched away along with the various
sacrificial layers, although possibly at a different rate.
Appropriate materials for the first subassembly in this case
include silicon nitride, poly-silicon-germanium, or any other
material that is soluble in the release etch or other etchant that
will not have an adverse effect on any of the structural layers
that may be included in the microstructure being made by the
methodology of the subject first aspect. Having the first
subassembly be of a reduced thickness may also contribute to the
first subassembly being removed along with the sacrificial material
within a desired time when releasing the microstructure made by the
methodology of the first aspect. In one embodiment where the first
subassembly is formed from silicon nitride and in the form of a
plurality of strips, the first subassembly has a thickness or
vertical extent of typically less than about 1,500 .ANG. for this
purpose.
[0014] Another option for creating the plurality of at least
generally laterally extending etch release channels in accordance
with the first aspect entails forming a first intermediate layer
that will underlie the first sacrificial layer. This first
intermediate layer may be patterned to define a plurality of at
least generally laterally extending strips that are disposed in
non-intersecting relation over at least a portion of their length.
These strips are spaced relatively close to each other such that
when the first sacrificial layer is deposited on the first
intermediate layer, the sacrificial material is unable to entirely
fill the space between the adjacent strips. More specifically, an
upper portion of the space between adjacent strips will "close off"
during the deposition of the material that defines the first
sacrificial layer before a lower portion of the space between the
adjacent strips has had a chance to be filled with the material
that defines the first sacrificial layer. Each "unfilled" void
between adjacent pairs of strips defines one of the plurality of
etch release channels referenced in relation to the first aspect.
The manner in which the voids are formed may be characterized as
"keyholing." Keyholing in relation to the first aspect is a result
a relatively close spacing between adjacent strips in relation to
their thickness or vertical extent. In one embodiment, a ratio of
the height of these strips to the spacing between adjacent strips
is at least about 1:1.
[0015] Further options exist for creating the plurality of at least
generally laterally extending etch release channels in accordance
with the first aspect. One way is to use multiple, different
etchants. A first etchant that is not selective to the first
sacrificial layer may be used to form the plurality of at least
generally laterally extending etch release channels. A second,
different etchant that is selective to the first sacrificial layer
may thereafter be directed through the plurality of at least
generally laterally extending etch release channels or conduits
(again created/defined by the first etchant) to remove the first
sacrificial layer. The first etchant may be selective to a material
that forms a plurality of at least generally laterally extending
etch release rails that are embedded or encased within the first
sacrificial layer or at least in a sacrificial material. Any
appropriate layout may be utilized for these plurality of at least
generally laterally extending etch release rails, including a
plurality of separate and discrete etch release rails, a network or
grid of interconnected etch release rails, or some combination
thereof.
[0016] The intermediate structure in the fabrication of the
microstructure in accordance with the first aspect may be
characterized as a stack, and includes the various layers that are
sequentially deposited on the substrate, and thereafter possibly
patterned. This stack includes an exterior surface that is opposite
the first substrate. Access to at least one of the etch release
rails in the above-noted two etchant example may be provided by a
first runner that extends from this exterior surface of the stack
and at least generally toward the first substrate to a level such
that it may structurally interconnect with at least one of the
plurality of etch release rails. The same material that defines the
etch release rails may define this first runner, such that the
first etchant will first remove the first runner, and then each
etch release rail that is structurally interconnected therewith
(either directly or indirectly). Multiple first runners may be
provided for accessing the plurality of etch release rails,
multiple etch release rails may be accessed by a single first
runner, or some combination thereof.
[0017] A second aspect of the present invention is embodied in a
method for making a microstructure in which a plurality of at least
generally laterally extending etch release channels or conduits are
formed within a sacrificial material that is used to build/assemble
the microstructure on an appropriate substrate, but which is at
least in part removed when the microstructure is released from this
substrate. These etch release channels do not exist until the
release of the microstructure is initiated in the second aspect. In
this regard, the method of the second aspect includes forming a
first intermediate layer on top of a first sacrificial layer or
possibly on top of the substrate. This first intermediate layer is
patterned to define a plurality of at least generally laterally
extending first strips that sit on top of the first sacrificial
layer. A second sacrificial layer is thereafter deposited on that
portion of the first sacrificial layer which was exposed by the
patterning of the first intermediate layer so as to be disposed at
least alongside the first strips. Although not fundamentally
required by the second aspect, the second sacrificial layer may
also be disposed on top of the first strips as well. In any case, a
first structural layer is formed on top of the second sacrificial
layer. Both the first and second sacrificial layers are removed at
least in part using an appropriate etchant. Generally, those
portions of the second sacrificial layer that interface with or are
disposed adjacent to the first strips etch at a greater rate than
other portions of the second sacrificial layer which effectively
defines a plurality of at least generally laterally extending etch
release pipes, channels, conduits or the like. A plurality of
hollow and at least generally laterally extending etch release
channels or conduits (e.g., disposed at least generally parallel
with an upper surface of the first substrate) are thereby formed in
the second sacrificial layer by this differential etch rate.
Although these etch release conduits will typically be disposed at
a constant elevation relative to the substrate, such need not
necessarily be the case. Ultimately, at least part of the second
sacrificial layer, as well as the first sacrificial layer, are
removed at least in part by allowing an etchant to flow through any
and all of the noted conduits after the formation of the same in
the releasing operation (and thereby encompassing the situation
where all of the first and second sacrificial layers are removed,
as well as the first intermediate layer if the same is not a
structural material as discussed below).
[0018] Various refinements exist of the features noted in relation
to the second aspect of the present invention. Further features may
also be incorporated in the second aspect of the present invention
as well. These refinements and additional features may exist
individually or in any combination. The noted differential etch
rate is believed to be based upon the density of that portion of
the second sacrificial layer in proximity to the first strips being
less than a density of the remainder of the second sacrificial
layer, as well as a density of the first sacrificial layer as well
for that matter. The differential density is due to the sticking
characteristics of the depositing atoms or molecules, which depends
on the characteristics of the deposition technique. For example,
the density of add-on molecules in a plasma-enhanced chemical vapor
deposition (PECVD) system depends on directional orientation of the
surface to the plasma body. In this way, atoms or molecules
striking the surface have different energy available to them to aid
in their positioning in low-energy (i.e. high-density) positions on
the surface.
[0019] In one embodiment of the second aspect, at least one end of
at least one first strip may be disposed at least generally at the
same radial position as a perimeter of the first structural layer.
Since the first strips effectively define the etch release
conduits, at least one end of at least one etch release conduit may
be disposed at this same radial position as well. As such, the
etchant does not have to etch in from the perimeter "too far"
before encountering a region that will have a higher etch rate, and
that again defines a corresponding etch release conduit. The
parameters mentioned above in relation to the first aspect
regarding this feature are equally applicable to this second
aspect.
[0020] Various layouts of the plurality of first strips (and
thereby the etch release conduits) in the noted lateral dimension
may be utilized in accordance with the second aspect. In one
embodiment, each of the plurality of at least generally laterally
first strips are further disposed in non-intersecting relation, in
another embodiment each of the plurality of first strips are
disposed in at least substantially parallel relation, and in yet
another embodiment at least some of the first strips intersect. The
plurality of first strips may extend at least generally toward (and
thereby including to) a common point, such as one that corresponds
with a center of the first structural layer in the lateral
dimension. All of the first strips need not extend the same
distance toward this common point, although such may be the case.
Some of the plurality of strips may extend at least generally
toward (and thereby including to) a first common point (e.g., one
that corresponds with a center of the first structural layer in the
lateral dimension), while some of the plurality of first strips may
extend toward a different common point. The plurality of first
strips utilized by the second aspect may also extend in the lateral
dimension in a variety of configurations. In one embodiment, the
plurality of first strips are at least generally axially extending,
while in another embodiment the plurality of first strips are
non-linear (e.g., in a sinusoidal configuration within a plane that
is parallel with the substrate).
[0021] One particularly desirable application for the second aspect
is in the formation of a mirror microstructure or multiple mirror
microstructures for a surface micromachined optical system that has
at least some degree of movement relative to the first substrate.
Because of the different etch rates that result from the way in
which the microstructure is made by the methodology of the second
aspect, there is no need to have a plurality of vertically disposed
etch release apertures that extend down entirely through the second
structural layer to release the mirror microstructure from the
first substrate by the removal of the underlying first and second
sacrificial layers. As such, the upper surface of the second
structural layer retains a very smooth surface that lacks any such
vertically disposed etch release apertures, which makes the
microstructure that is made by the methodology of the second aspect
particularly suited for use in optical applications. This is true
whether the upper surface of the second structural layer is the
actual optical surface for the mirror microstructure, or whether a
film or other layer is deposited on the second structural layer to
provide more desirable optical properties/characteristics.
[0022] Typically the second sacrificial layer will also cover the
first strips (again, formed from the first intermediate layer) such
that they are effectively embedded between the first and second
sacrificial layers. In this case, it may be desirable to planarize
an upper surface of the second sacrificial layer before depositing
the first structural layer thereon. It may also be desirable to
planarize an upper surface of other layers within the
microstructure made in accordance with the methodology of the
second aspect as well before depositing another layer thereon. One
appropriate technique for executing this planarization function is
chemical mechanical polishing.
[0023] The first strips may be a reinforcing structure for the
first structural layer and would then exist in the microstructure
that is made by the methodology of the second aspect. Reinforcement
may be provided by structurally interconnecting the first
structural layer with the first strips through the above-noted
second sacrificial layer which may be deposited on the first strips
in addition to the first sacrificial layer as noted. Further
reinforcement of the first structural layer may be accomplished by
structurally interconnecting the first strips with a structural
layer that underlies the first sacrificial layer and thereby the
first structural layer. In both cases, the actual reinforcing
structure could be in the form of a plurality of posts or columns
that are disposed in spaced relation, in the form of a plurality of
at least generally laterally extending ribs or rails, or in the
form of a grid-like reinforcement structure.
[0024] A third aspect of the present invention is embodied in a
method for making a surface micromachined microstructure. A first
sacrificial layer is formed over a first substrate in a manner so
as to define a plurality of at least generally laterally extending
low density regions therein. A first structural layer is thereafter
formed over the first sacrificial layer. The release of the first
structural layer from the first substrate is affected by removing
the first sacrificial layer with an appropriate etchant. The
etching rate within the low density regions of the first
sacrificial layer is greater than in other regions of the first
sacrificial layer.
[0025] Various refinements exist of the features noted in relation
to the third aspect of the present invention. Further features may
also be incorporated in the third aspect of the present invention
as well. These refinements and additional features may exist
individually or in any combination. The higher etch rate in the low
density regions of the first sacrificial layer may define a
plurality of at least generally laterally extending etch release
pipes, channels, conduits, or the like, which in turn should reduce
the time required to completely release the microstructure from the
first substrate. Although the low density regions will typically be
disposed at a constant elevation relative to the substrate, such
need not be the case.
[0026] In one embodiment of the third aspect, at least one end of
at least one low density region may be disposed at least generally
at the same radial position as a perimeter of the first structural
layer. Since the low density regions effectively define the etch
release conduits, at least one end of at least one etch release
conduit may be disposed at this same radial position as well. As
such, the etchant does not have to etch in from the perimeter "too
far" before encountering a low density region for definition of an
etch release conduit(s). The parameters mentioned above in relation
to the first aspect regarding this feature are equally applicable
to this third aspect.
[0027] Various layouts of the plurality of low density regions, and
thereby the etch release conduits, in the noted lateral dimension
may be utilized in accordance with the third aspect. In one
embodiment, each of the plurality of at least generally laterally
extending low density regions are disposed in non-intersecting
relation, in another embodiment each of the plurality of low
density regions are disposed in at least substantially parallel
relation, and in yet another embodiment at least some of the low
density regions intersect. The plurality of low density regions may
extend at least generally toward (and thereby including to) a
common point, such as one that corresponds with a center of the
first structural layer in the lateral dimension. All of the low
density regions need not extend the same distance toward this
common point, although such may be the case. Some of the plurality
of low density regions may extend at least generally toward (and
thereby including to) a first common point (e.g., one that
corresponds with a center of the first structural layer in the
lateral dimension), while some of the plurality of low density
regions may extend toward a different common point. The plurality
of low density regions utilized by the third aspect may also extend
in the lateral dimension in a variety of configurations. In one
embodiment, the plurality of low density regions are at least
generally axially extending, while in another embodiment the
plurality of low density regions are non-linear (e.g., in a
sinusoidal configuration within a plane that is at least generally
parallel with the substrate).
[0028] One way in which the low density regions associated with the
third aspect may be formed is by forming a second sacrificial layer
over the first substrate, and then patterning the same to define a
plurality of at least generally laterally extending etch release
conduit apertures. Each of these etch release conduit apertures is
defined by first and second sidewalls that are disposed in spaced
relation to each other. The first sacrificial layer is formed such
that the material of the first sacrificial layer is deposited
within these etch release conduit apertures. The first sacrificial
layer may be deposited on the top of the second sacrificial layer
as well. In any case, the low density regions associated with the
third aspect will thereby exist along the first and second
sidewalls of each of the etch release conduit apertures.
[0029] One advantage of the above-noted method for defining the low
density regions in accordance with the third aspect, and thereby
for defining a plurality of etch release channels, is that a layout
of the low density regions may define a network or grid-like
structure or such that a plurality of these low density regions
cross and/or are interconnected in some manner. For instance, the
patterning of the second sacrificial layer could define a repeating
pattern of interconnected "diamonds," a honeycomb or honeycomb-like
structure, or the like. This ability to define a network could
further enhance the distribution of the etchant during the release
of the first structural layer from the first substrate. Another
advantage of this particular method for defining the low density
regions, and thereby for defining a plurality of etch release
channels, is that the same does not require the use of any
structural layer or material for the formation thereof. Therefore,
this particular embodiment of the third aspect could be used to
enhance the release of a simple, single structural layer in a
microstructure.
[0030] The above-noted methodology for defining the low density
regions in accordance with the third aspect may also be utilized
where structural reinforcement of the first structural layer is
desired. Structural reinforcement of the first structural layer may
be realized by having an appropriate reinforcement structure
cantilever downwardly from a lower surface of the first structural
layer. Another way to structurally reinforce the first structural
layer is to structurally interconnect the first structural layer
with an underlying structural layer. The only limitation on the use
of any such reinforcement structure is that it should not extend
downwardly through any of the noted low density regions so as to
cut off any etch release channels. This may be done in variety of
manners. Consider the case where the second sacrificial layer is
patterned in the form of a honeycomb. A plurality of columns or
posts could extend downwardly from the first structural layer
through the "closed cell" portions of the honeycomb without
intersecting with any of the low density regions which define the
profile of the honeycomb (in plan view).
[0031] Notwithstanding the advantages of the above-noted method for
forming the low density regions in accordance with the third
aspect, these low density regions may also be defined in the manner
discussed above in relation to the second aspect. Therefore, those
features discussed above in relation to the second aspect may be
used in this third aspect as well.
[0032] A fourth aspect of the present invention is embodied in a
method for making a surface micromachined microstructure. A first
sacrificial layer is formed over a first substrate. A first
structural layer is formed over the first sacrificial layer. The
release of the first structural layer from the first substrate is
affected by what may be characterized as a two step etch. In this
regard, a first etchant may be used to form a plurality of at least
generally laterally extending etch release channels or conduits
within the first sacrificial layer or so as to otherwise be
embedded within a sacrificial material. A second, different etchant
that is selective to the first sacrificial layer may thereafter be
directed through the plurality of at least generally laterally
extending etch release channels (again created/defined by the first
etchant) to remove the first sacrificial layer.
[0033] Various refinements exist of the features noted in relation
to the fourth aspect of the present invention. Further features may
also be incorporated in the fourth aspect of the present invention
as well. These refinements and additional features may exist
individually or in any combination. The first etchant may be
selective to a material that forms a plurality of at least
generally laterally extending etch release rails that are embedded
or encased within the first sacrificial layer or at least in a
sacrificial material. Any appropriate layout may be utilized for
these plurality of at least generally laterally extending etch
release rails, including a plurality of separate and discrete etch
release rails, a network or grid of interconnected etch release
rails, or some combination thereof.
[0034] The intermediate structure in the fabrication of the
microstructure in accordance with the fourth aspect may be
characterized as a stack, and includes the various layers that are
sequentially deposited on the substrate, and thereafter possibly
patterned. This stack includes an exterior surface that is opposite
the first substrate. Access to at least one of the noted etch
release rails may be provided by a first runner that extends from
this exterior surface of the stack and at least generally toward
the substrate to a level such that it may interconnect with at
least one of the plurality of etch release rails. The same material
that defines the etch release rails may define this first runner,
such that the first etchant will first remove the first runner, and
then each etch release rail interconnected therewith. Multiple
first runners may be provided for accessing the plurality of etch
release rails, multiple etch release rails may be accessed by a
single first runner, or some combination thereof.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0035] FIG. 1A is a plan view of one embodiment of surface
micromachined optical system that includes a movable mirror
microstructure.
[0036] FIG. 1B is a plan view of another embodiment of a surface
micromachined optical system that includes a movable mirror
microstructure.
[0037] FIG. 1C is a bottom view of the surface micromachined
optical system of FIG. 1B.
[0038] FIG. 2A is a cross-sectional view of one embodiment of a
mirror microstructure that may be used in a surface micromachined
optical system.
[0039] FIG. 2B is a cross-sectional view of a portion of the mirror
microstructure of FIG. 2A with an optical coating thereon.
[0040] FIG. 3 is a cross-sectional view of the mirror
microstructure of FIG. 2A along line 3-3.
[0041] FIG. 4 is a cross-sectional view of another embodiment of a
mirror microstructure that may be used in a surface micromachined
optical system.
[0042] FIG. 5 is a cross-sectional view of another embodiment of a
mirror microstructure that may be used in a surface micromachined
optical system.
[0043] FIG. 6 is a cross-sectional view of the mirror
microstructure of FIG. 5 taken along line 6-6, as well as of the
mirror microstructure of FIG. 7 taken along line 6-6.
[0044] FIG. 7 is a cross-sectional view of another embodiment of a
mirror microstructure that may be used in a surface micromachined
optical system.
[0045] FIG. 8 is a cross-sectional view of the mirror
microstructure of FIG. 7 taken along line 8-8.
[0046] FIG. 9A is a cross-sectional view of another embodiment of a
mirror microstructure that may be used in a surface micromachined
optical system.
[0047] FIG. 9B is a cross-sectional view of the mirror
microstructure of FIG. 9A taken along line 9B-9B.
[0048] FIG. 10A is a cross-sectional view of another embodiment of
a mirror microstructure that may be used in a surface micromachined
optical system.
[0049] FIG. 10B is a cross-sectional view of the mirror
microstructure of FIG. 10A taken along line 10B-10B.
[0050] FIG. 10-C is a cross-sectional view of the mirror
microstructure of FIG. 10A taken along line 10C/D-10C/D.
[0051] FIG. 10D is a cross-sectional view of a variation of the
mirror microstructure of FIG. 10A taken along line 10C/D-10C/D.
[0052] FIG. 11A is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0053] FIG. 11B is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0054] FIG. 11C is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0055] FIG. 11D is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0056] FIG. 11E is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0057] FIG. 11F is a cross-sectional view of another embodiment of
a rail layout for structural reinforcement and/or rapid etch
release.
[0058] FIGS. 12A-M are sequential views of one embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0059] FIGS. 13A-M are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0060] FIGS. 14A-F are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0061] FIGS. 15A-G are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0062] FIGS. 16A-C are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0063] FIGS. 17A-G are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
[0064] FIG. 18A is a top/plan view of one embodiment of an etch
release conduit aperture grid that may be defined using the
methodology of FIG. 17A-G, and at a point in time in the process
corresponding with FIG. 17B and along line 18A-18A in FIG. 17B.
[0065] FIG. 18B is a cutaway view of the embodiment of FIG. 18A, at
a point in time in the process corresponding with FIG. 17F and
along line 18B-18B in FIG. 17F.
[0066] FIGS. 19A-F are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system, and which uses the same technique for forming
a plurality of etch release conduits as the method of FIGS.
17A-G.
[0067] FIGS. 20A-D are sequential views of another embodiment for
making one embodiment of a microstructure for a surface
micromachined system.
DETAILED DESCRIPTION OF THE INVENTION
[0068] The present invention will now be described in relation to
the accompanying drawings which at least assist in illustrating its
various pertinent features. Surface micromachined microstructures
and methods of making the same are the general focus of the present
invention. Various surface micromachined microstructures and
surface micromachining techniques are disclosed in U.S. Pat. Nos.
5,783,340, issued Jul. 21, 1998, and entitled "METHOD FOR
PHOTOLITHOGRAPHIC DEFINITION OF RECESSED FEATURES ON A
SEMICONDUCTOR WAFER UTILIZING AUTO-FOCUSING ALIGNMENT"; U.S. Pat.
No. 5,798,283, issued Aug. 25, 1998, and entitled "METHOD FOR
INTEGRATING MICROELECTROMECHANICAL DEVICES WITH ELECTRONIC
CIRCUITRY; U.S. Pat. No. 5,804,084, issued Sep. 8, 1998, and
entitled "USE OF CHEMICAL MECHANICAL POLISHING IN MICROMACHINING";
U.S. Pat. No. 5,867,302, issued Feb. 2, 1999, and entitled
"BISTABLE MICROELECTROMECHANICAL ACTUATOR"; and U.S. Pat. No.
6,082,208, issued Jul. 4, 2000, and entitled "METHOD FOR
FABRICATING FIVE-LEVEL MICROELECTROMECHANICAL STRUCTURES AND
MICROELECTROMECHANICAL TRANSMISSION FORMED, the entire disclosures
of which are incorporated by reference in their entirety
herein.
[0069] The term "sacrificial layer" as used herein means any layer
or portion thereof of any surface micromachined microstructure that
is used to fabricate the microstructure, but which does not exist
in the final configuration. Exemplary materials for the sacrificial
layers described herein include undoped silicon dioxide or silicon
oxide, and doped silicon dioxide or silicon oxide ("doped"
indicating that additional elemental materials are added to the
film during or after deposition). The term "structural layer" as
used herein means any other layer or portion thereof of a surface
micromachined microstructure other than a sacrificial layer and a
substrate on which the microstructure is being fabricated.
Exemplary materials for the structural layers described herein
include doped or undoped polysilicon and doped or undoped silicon.
Exemplary materials for the substrates described herein include
silicon. The various layers described herein may be
formed/deposited by techniques such as chemical vapor deposition
(CVD) and including low-pressure CVD (LPCVD), atmospheric-pressure
CVD (APCVD), and plasma-enhanced CVD (PECVD), thermal oxidation
processes, and physical vapor deposition (PVD) and including
evaporative PVD and sputtering PVD, as examples.
[0070] In more general terms, surface micromachining can be done
with any suitable system of a substrate, sacrificial film(s) or
layer(s) and structural film(s) or layer(s). Many substrate
materials may be used in surface micromachining operations,
although the tendency is to use silicon wafers because of their
ubiquitous presence and availability. The substrate is essentially
a foundation on which the microstructures are fabricated. This
foundation material must be stable to the processes that are being
used to define the microstructure(s) and cannot adversely affect
the processing of the sacrificial/structural films that are being
used to define the microstructure(s). With regard to the
sacrificial and structural films, the primary differentiating
factor is a selectivity difference between the sacrificial and
structural films to the desired/required release etchant(s). This
selectivity ratio is preferably several hundred to one or much
greater, with an infinite selectivity ratio being preferred.
Examples of such a sacrificial film/structural film system include:
various silicon oxides/various forms of silicon; poly
germanium/poly germanium-silicon; various polymeric films/various
metal films (e.g., photoresist/aluminum); various metals/various
metals (e.g., aluminum/nickel); polysilicon/silicon carbide;
silicone dioxide/polysilicon (i.e., using a different release
etchant like potassium hydroxide, for example). Examples of release
etchants for silicon dioxide and silicon oxide sacrificial
materials are typically hydrofluoric (HF) acid based (e.g.,
undiluted or concentrated HF acid, which is actually 49 wt % HF
acid and 51 wt % water; concentrated HF acid with water; buffered
HF acid (HF acid and ammonium fluoride)).
[0071] Only those portions of a surface micromachined
microstructure that are relevant to the present invention will be
described herein. There may and typically will be other layers that
are included in a given surface micromachined microstructure, as
well as in any system that includes such microstructures. For
instance and in the case where the surface micromachined
microstructures described herein are utilized as a movable mirror
microstructure in a surface micromachined optical system, a
dielectric isolation layer will typically be formed directly on an
upper surface of the substrate on which such a surface
micromachined optical system is to be fabricated, and a structural
layer will be formed directly on an upper surface of the dielectric
isolation layer. This particular structural layer is typically
patterned and utilized for establishing various electrical
interconnections for the surface micromachined optical system which
is thereafter fabricated thereon.
[0072] One embodiment of at least a portion of a surface
micromachined optical system 2 is presented in FIG. 1A. The surface
micromachined optical system 2 is fabricated on a substrate (not
shown) and includes at least one microstructure in the form of a
mirror microstructure 6. The surface micromachined optical system 2
may and typically will include multiple mirror microstructures 6
that are disposed/arranged in the form of an array (not shown),
although there may be applications where only a single mirror
microstructure 6 is required. The mirror microstructure 6 is
interconnected with the substrate by a plurality of suspension
springs 8. One end of each spring 8 is interconnected with the
mirror microstructure 6, while its opposite end is interconnected
with a structural support 4 that is in turn interconnected with the
substrate (possibly through one or more underlying structural
layers, which is the configuration shown in FIG. 1A).
[0073] A lower electrode 10 is disposed below the mirror
microstructure 6 in spaced relation and is interconnected with the
substrate as well. An electrical contact 12a is interconnected with
the lower electrode 10, while an electrical contact 12b is
interconnected with the mirror microstructure 6. Appropriate
voltages may be applied to both of the electrical contacts 12a, 12b
to move the mirror microstructure 6 toward or away from the lower
electrode 10 (and thereby the substrate) into a desired position to
provide an optical function. This movement is typically at least
generally perpendicular relative to the lower electrode 10 and the
substrate. The mirror microstructure 6 is commonly referred to as a
piston mirror based upon the described motion.
[0074] Another embodiment of at least a portion of a surface
micromachined optical system 16 is presented in FIGS. 1B-C. The
surface micromachined optical system 16 is fabricated on a
substrate (not shown) and includes at least one microstructure in
the form of a mirror microstructure 20. The surface micromachined
optical system 16 may and typically will include multiple mirror
microstructures 20 that are disposed/arranged in the form of an
array (not shown), although there may be applications where only a
single mirror microstructure 20 is required. The mirror
microstructure 20 is interconnected with the substrate by a pair of
suspension springs 22. An imaginary line that extends between the
springs 22 defines an axis about which the mirror microstructure 20
may pivot. One end of each spring 22 is interconnected with the
mirror microstructure 20, while its opposite end is interconnected
with a structural support 18 that is in turn interconnected with
the substrate.
[0075] A pair of lower electrodes 24 are disposed below the mirror
microstructure 20 and are interconnected with the substrate as
well. One electrode 24a is associated with one side of the
above-noted pivot axis, while the electrode 24b is associated with
the opposite side of the above-noted pivot axis. There is one
electrical contact 26a electrically interconnected with each lower
electrode 24, while there is an electrical contact 26b electrically
interconnected with the mirror microstructure 20. Appropriate
voltages may be applied to appropriate ones of the contacts 26 to
pivot the mirror microstructure 20 about its pivot axis in the
desired direction and amount into a desired position to provide an
optical function.
[0076] Details regarding a particular configuration of a mirror
microstructure in a surface micromachined optical system, such as
for the mirror microstructures 6 and 20 in the surface
micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C,
respectively, are presented in FIGS. 2A and 3 in the form of a
two-layered mirror microstructure 30. The mirror microstructure 30
is made on an appropriate substrate 54 by surface micromachining
techniques. Components of the mirror microstructure 30 include a
first structural layer or support 34 that is spaced vertically
upward relative to the substrate 54 (e.g., disposed at a higher
elevation relative to the substrate 54), a second structural layer
or support 38 that is spaced vertically upward relative to the
first structural layer 34 (e.g., disposed at a higher elevation
relative to the substrate 54), and a plurality of separate and
discrete columns or posts 50 that are disposed in spaced relation.
The columns 50 extend between and fixedly interconnect the first
structural layer 34 and the second structural layer 38 for
providing structural reinforcement for the microstructure 30 and,
more particularly, the second structural layer 38. The columns 50
may be disposed in either equally spaced relation or the spacing
between adjacent columns 50 may vary in at least some manner.
Therefore, the second structural layer 38 and first structural
layer 34, along with the interconnecting columns 50, may be moved
simultaneously if acted upon by any interconnected actuator to
provide a desired/required optical function.
[0077] An upper surface 46 of the second structural layer 38 is or
may include an optically reflective layer or film. That is, the
materials that are used to define the second structural layer 38
may provide the desired/required optical properties/characteristics
for the mirror microstructure 30. More typically a separate layer
or film 48 (FIG. 2B) will be deposited on an upper surface 46 of
the second structural layer 38 to realize the desired/required
optical properties/characteristics. Appropriate materials that may
be deposited on the second structural layer 38 for providing the
desired/required optical properties include gold, silver, and
aluminum for metal coatings. For metals, gold and an associated
adhesion layer are preferable to obtain a suitable reflectance.
[0078] Depending upon the method of manufacture, a plurality of
small etch release holes (not shown) may be formed through the
entire vertical extent of the first and/or second structural layer
34, 38 to allow for the removal of any sacrificial layer(s) that is
disposed between the first structural layer 34 and the second
structural layer 38 of the mirror microstructure 30, and that is
disposed between the first structural layer 34 and the substrate
54, respectively, when the mirror microstructure 30 is released
from the substrate 54 (i.e., during the fabrication of the mirror
microstructure 30). For instance, using the general principles of
the manufacturing technique represented in FIGS. 12A-M to define
the microstructure 30 would require such etch release holes. It
should be appreciated that having etch release holes that extend
entirely through the second structural layer 38, and which are
thereby exposed on its upper surface 46, may have an adverse effect
on its optical performance capabilities. Certain degradations in
optical performance may be acceptable in some instances. However,
the mirror microstructure 30 also may be made without having any
etch release holes that extend down through the second structural
layer 38, including in accordance with the methodology represented
in FIG. 17A-G.
[0079] Various desirable characteristics of the types of reinforced
mirror microstructures described herein are addressed following the
discussion of various other embodiments. Certain parameters are
used in this summarization. One such parameter is the diameter of
the uppermost structural layer in the mirror microstructure, and
that is represented by the dimension "d.sub.SL" ("SL" being an
acronym for "structural layer"). "Diameter" does not of course
limit this uppermost structural layer to having a circular
configuration (in plan view), but is a simply the distance of a
straight cord or line that extends laterally from one location on a
perimeter of this uppermost structural layer, through a center of
this uppermost structural layer, and to another location on the
perimeter of this uppermost structural layer. The dimension
d.sub.SL for the case of the mirror microstructure 30 thereby
represents the diameter of the second structural layer 38 (the
distance from one location on a perimeter 44 of the second
structural layer 38, through a center 42 of the second structural
layer 38, and to another location on this perimeter 44).
[0080] Another parameter that is used in the summarization of the
desirable characteristics of the mirror microstructures disclosed
herein is the distance from the center (in the lateral dimension)
of the uppermost structural layer in the mirror microstructure to
that reinforcing structure (by engaging a lower surface of this
uppermost structural layer) which is closest to the center of this
uppermost structural layer. This is represented by the dimension
"d.sub.RS" ("RS" being an acronym for "reinforcing structure"). The
dimension d.sub.RS for the case of the mirror microstructure 30
represents the distance from the center 42 of the second structural
layer 38 to that column 50 which is closest to the center 42. A
final parameter that is used in the noted summarization is the
radius of curvature of the uppermost structural layer in the mirror
microstructure (i.e., the amount which this uppermost structural
layer is "cupped" or "bulged"). This is represented by the
dimension RC. The dimension RC for the case of the mirror
microstructure 30 thereby defines the radius of curvature of the
second structural layer 38.
[0081] Another configuration of a mirror microstructure for a
surface micromachined optical system, such as for the mirror
microstructures 6 and 20 in the surface micromachined optical
systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented
in FIG. 4 in the form of a three-layered mirror microstructure 58.
The mirror microstructure 58 is fabricated on a substrate 60 by
surface micromachining techniques. Components of the mirror
microstructure 58 include a first structural layer or support 62
that is spaced vertically upward relative to the substrate 60
(e.g., disposed at a higher elevation than the substrate 60), a
second structural layer or support 78 that is spaced vertically
upward relative to the first structural layer 62 (e.g., disposed at
a higher elevation relative to the substrate 60), a third
structural layer or support 94 that is spaced vertically upward
relative to the second structural layer 78 (e.g., disposed at a
higher elevation relative to the substrate 60), a plurality of
separate and discrete first columns or posts 74 that are disposed
in spaced relation to each other, and a plurality of separate and
discrete second columns or posts 90 that are also disposed in
spaced relation to each other. The first columns 74 extend between
and fixedly interconnect the first structural layer 62 and the
second structural layer 78, while the second columns 90 extend
between and fixedly interconnect the second structural layer 78 and
the third structural layer 94, all for structurally reinforcing the
mirror microstructure 58 and, more particularly, the third
structural layer 94. Therefore, the first structural layer 62, the
second structural layer 78, the interconnecting columns 74, the
third structural layer 94, and the interconnecting columns 90 may
be moved simultaneously if acted upon by any interconnected
actuator to provide a desired/required optical function.
[0082] The first columns 74 may be disposed in either equally
spaced relation or the spacing between adjacent columns 74 may vary
in at least some manner. The same applies to the columns 90. The
plurality of first columns 74 may be offset in relation to the
plurality of second columns 90 or such that no first column 74 is
axially aligned (in the vertical direction) with any second column
90 as shown. However, other relative positionings between the
plurality of first columns 74 and plurality of second columns 90
may be utilized as well, including where one or more of the
plurality of first columns 74 is at least partially aligned with a
second column 90.
[0083] An upper surface 102 of the third structural layer 94 is or
includes an optically reflective layer or film. That is, the
materials that are used to define the third structural layer 94 may
provide the desired/required optical properties/characteristics for
the mirror microstructure 58. More typically a separate layer or
film will be deposited on the third structural layer 94 to realize
the desired/required optical properties/characteristics. Those
materials discussed above in relation to the mirror microstructure
30 for this purpose may be utilized by the mirror microstructure 58
and in the general manner illustrated in FIG. 2B.
[0084] Depending upon the method of fabrication, a plurality of
small release holes (not shown) may be formed through the entire
vertical extent of one or more of the first structural layer 62,
the second structural layer 78, and the third structural layer 94
to allow for the removal of any underlying and adjacently disposed
sacrificial layer(s), or when the mirror microstructure 58 is
released from the substrate 60 (i.e., during the fabrication of the
mirror microstructure 58). For instance, using the general
principles of the manufacturing technique represented in FIGS.
12A-M to define the mirror microstructure 58 would require such
etch release holes. It should be appreciated that having etch
release holes that extend entirely through the third structural
layer 94, and which are thereby exposed on its upper surface 102,
may have an effect on its optical performance capabilities. Certain
degradations in optical performance may be acceptable in some
instances. However, the mirror microstructure 58 also may be made
without having any etch release holes that extend down through the
third structural layer 94, including in accordance with the
methodology represented in FIG. 17A-G.
[0085] Certain parameters are identified on FIG. 4 and that are
addressed in the above-noted summarization of certain desirable
characteristics that follows below. The dimension "d.sub.SL" for
the case of the mirror microstructure 58 represents the diameter of
the third structural layer 94 (e.g., a line that extends laterally
from one location on a perimeter 70 of the third structural layer
94, through a center 66 of the third structural layer 94, and to
another location on this perimeter 70) that is being structurally
reinforced collectively by the plurality of columns 90, the second
structural layer 78, the plurality of columns 74, and the first
structural layer 62. The dimension "d.sub.RS" for the case of the
mirror microstructure 58 represents the distance from the center 66
of the third structural layer 94 to that column 90 which is closest
to the center 66. Finally, RC for the case of the mirror
microstructure 58 represents the radius of curvature of the third
structural layer 94.
[0086] Another configuration of a mirror microstructure for a
surface micromachined optical 10 system, such as for the mirror
microstructures 6 and 20 in the surface micromachined optical
systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented
in FIGS. 5-6 in the form of a two-layered mirror microstructure
106. The mirror microstructure 106 is fabricated on a substrate 108
by surface micromachining techniques. Components of the mirror
microstructure 106 include a first structural layer or support 110
that is spaced vertically upward relative to the substrate 108
(e.g., disposed at a higher elevation relative to the substrate
108), a second structural layer or support 122 that is spaced
vertically upward relative to the first structural layer 110 (e.g.,
disposed at a higher elevation relative to the substrate 108), and
a plurality of at least generally laterally extending ribs or rails
118 (i.e., with their length dimension being measured in a lateral
dimension or at least generally parallel with the substrate 108).
The upper and lower extremes of the rails 118 extend between and
fixedly interconnect the first structural layer 110 and the second
structural layer 122 to structurally reinforce the mirror
microstructure 106 and, more particularly, the second structural
layer 122. Therefore, the second structural layer 122 and first
structural layer 110, along with the interconnecting rails 118, may
be moved simultaneously if acted upon by any interconnected
actuator to provide a desired/required optical function.
[0087] The rails 118 further extend at least generally laterally
from one location on or at least generally proximate to a perimeter
116 of the microstructure 106 (e.g., within 50 .mu.m of the
perimeter 116, more preferably within 25 .mu.m of this perimeter
116) to another location on or at least generally proximate to this
perimeter 116 (e.g., within 50 82 m of the perimeter 116, more
preferably within 25 .mu.m of this perimeter 116). In the
illustrated embodiment, the rails 118 are of an axial or linear
configuration in the lateral dimension, and further are disposed in
at least generally parallel and equally spaced relation.
[0088] An upper surface 126 of the second structural layer 122 is
or includes an optically reflective layer or film. That is, the
materials that are used to define the second structural layer 126
may provide the desired/required optical properties/characteristics
for the mirror microstructure 106. More typically a separate layer
or film will be deposited on the second structural layer 122 to
realize the desired/required optical properties/characteristics.
Those materials discussed above in relation to the mirror
microstructure 30 for this purpose may be utilized by the mirror
microstructure 106 and in the general manner illustrated in FIG.
2B.
[0089] There are a number of significant advantages in relation to
the design utilized by the mirror microstructure 106 of FIGS. 5-6.
First is in relation to its optical characteristics. The upper
surface 126 of the second structural layer 122 of the mirror
microstructure 106 need not and preferably does not include any
vertically disposed etch release holes which extend downwardly
therethrough in order to allow for the removal of any sacrificial
material from between the first structural layer 110 and the second
structural layer 122 when the microstructure 106 is released from
the substrate 108, which significantly enhances the optical
performance capabilities of the mirror microstructure 106
(vertically disposed etch release holes would extend through the
first structural layer 110 to remove any sacrificial material
between the first structural layer 110 and the substrate 108). That
is, preferably the upper surface 126 of the mirror microstructure
106 is continuous and devoid of any indentations, etch release
holes, or the like (depressions or indentations that may develop on
the upper surface 126 of the second structural layer 122 from the
manufacture of the mirror microstructure 106 may be addressed by a
planarization operation). Exactly how the mirror microstructure 106
may alleviate the need for the etch release holes in the second
structural layer 122 will be discussed in more detail below in
relation to the manufacturing methodologies represented in FIGS.
12A-M, 13A-M, 15A-G, and 17A-G. Suffice it to say for present
purposes that the rails 118 may at least assist in the definition
of a plurality of at least generally laterally extending etch
release pipes, channels, or conduits in a sacrificial layer(s) that
is disposed between the second structural layer 122 and the first
structural layer 110 during the manufacture of the mirror
microstructure 106. Any rails described herein that provide this
function are characterized as etch release rails or the like. How
the rails 118 are oriented relative to each other, or stated
another way the layout of the rails 118, may have an effect on
their ability to create this plurality of etch release channels or
conduits within any sacrificial layer that is disposed between the
first structural layer 110 and the second structural layer 122
during the manufacture of the mirror microstructure 106 by surface
micromachining techniques. The general design considerations for
etch release rails is summarized below following the description of
other embodiments of microstructures that include etch release
rails.
[0090] Another function provided by the plurality of rails 118 is
structural reinforcement of the second structural layer 122. That
is, the plurality of rails 118 structurally interconnect the second
structural layer 122 with the first structural layer 110. The
structural reinforcement function of the rails 118 would not be
adversely affected, and may in fact improve, by having at least
some of the rails 118 be disposed in intersecting relation (e.g.,
see FIGS. 9A-B).
[0091] Certain parameters are identified on one or more of FIG. 5-6
and that are addressed in the above-noted summarization of
desirable characteristics that follows below. The dimension
"d.sub.SL" for the case of the mirror microstructure 106 represents
the diameter of the second structural layer 122 (e.g., the distance
of a line that extends from one location on the perimeter 116 of
the second structural layer 122, through a center 114 of the second
structural layer 122, and to another location on this perimeter)
that is being structurally reinforced collectively by the plurality
of rails 118 and the first structural layer 110. The dimension
"d.sub.RS" for the case of the mirror microstructure 106 represents
the distance from the center 114 of the second structural layer 122
to that rail 118 that is closest to the center 114, which is "0" in
the illustrated embodiment (FIG. 6) since one of the rails 118
actually extends through the center 114. It should be appreciated
that a rail 118 need not extend through the center 114, in which
case d.sub.RS would have a value greater than "0." Finally, RC for
the case of the mirror microstructure 106 represents the radius of
curvature of the second structural layer 122.
[0092] Another configuration of a mirror microstructure for a
surface micromachined optical system, such as for the mirror
microstructures 6 and 20 in the surface micromachined optical
systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented
in FIGS. 6-8 in the form of a three-layered mirror microstructure
130. The mirror microstructure 130 is fabricated on an appropriate
substrate 132 by surface micromachining techniques. Components of
the mirror microstructure 130 include a first structural layer or
support 134 that is spaced vertically upward relative to the
substrate 132 (e.g., disposed at a higher elevation than the
substrate 132), a second structural layer or support 146 that is
spaced vertically upward relative to the first structural layer 134
(e.g., disposed at a higher elevation relative to the substrate
132), a third structural layer or support 158 that is spaced
vertically upward relative to the second structural layer 146
(e.g., disposed at a higher elevation relative to the substrate
132), a plurality of at least generally laterally extending first
rails 142, and a plurality of at least generally laterally
extending second rails 154. The first rails 142 extend between and
fixedly interconnect the first structural layer 134 and the second
structural layer 146, while the second rails 154 extend between and
fixedly interconnect the second structural layer 146 and the third
structural layer 158, all to structurally reinforce the mirror
microstructure 130 and, more particularly, the third structural
layer 158. Therefore, the first structural layer 134, the second
structural layer 146, the interconnecting rails 142, the third
structural layer 158, and the interconnecting rails 154 may be
moved simultaneously if acted upon by any interconnected actuator
to provide a desired/required optical function.
[0093] The rails 154 extend at least generally laterally from one
location on or at least generally proximate to a perimeter 138 of
the third structural layer 158 (e.g., within 50 .mu.m of the
perimeter 138, more preferably within 25 .mu.m of this perimeter
138) to another location on or least generally proximate to this
perimeter 138 (e.g., within 50 .mu.m of the perimeter 138, more
preferably within 25 .mu.m of this perimeter 138). In the
illustrated embodiment, the plurality of rails 154 are of an axial
or linear configuration in the lateral dimension, and further are
disposed in at least generally parallel and equally spaced
relation. The plurality of rails 142 are also of an axial or linear
configuration in the lateral dimension, and further are disposed in
at least generally parallel and equally spaced relation.
[0094] An upper surface 162 of the third structural layer 158 is or
includes an optically reflective layer or film. That is, the
materials that are used to define the third structural layer 158
may provide the desired/required optical properties/characteristics
for the mirror microstructure 130. More typically a separate layer
or film will be deposited on the third structural layer 158 to
realize the desired/required optical properties/characteristics.
Those materials discussed above in relation to the mirror
microstructure 30 for this purpose may be utilized by the mirror
microstructure 130 and in the general manner illustrated in FIG.
2B.
[0095] There are a number of significant advantages in relation to
the design utilized by the mirror microstructure 130. First is in
relation to its optical characteristics. The upper surface 162 of
the third support layer 158 of the mirror microstructure 130 need
not and preferably does not include any vertically disposed etch
release holes in order to allow for the removal of any sacrificial
material from between the third structural layer 158 and the second
structural layer 146 when the microstructure 130 is released from
the substrate 132, which significantly enhances the optical
capabilities of the mirror microstructure 130. That is, preferably
the upper surface 162 of the mirror microstructure 130 is
continuous and devoid of any indentations, holes, or the like
(depressions or indentations that may develop on the upper surface
162 of the third support layer 158 during the manufacture the
microstructure 130 may be addressed by a planarization operation).
Exactly how the mirror microstructure 130 may alleviate the need
for any vertically disposed etch release holes in the third
structural layer 158 will be discussed in more detail below in
relation to the manufacturing methodologies represented in FIGS.
12A-M, 13A-M, 15A-G, and 17A-G. Suffice it to say for present
purposes that the rails 154 may at least assist in the definition
of a plurality of laterally extending etch release pipes, channels,
or conduits in a sacrificial layer(s) that is disposed between the
second structural layer 146 and the third structural layer 158
during the manufacture of the mirror microstructure 130. As such,
the rails 154 may be characterized as etch release rails as noted
above. Again and in this case, how the rails 154 are oriented
relative to each other, or stated another way the layout of the
rails 154, may have an effect on their ability to create this
plurality of etch release channels or conduits within any
sacrificial layer that is disposed between the second structural
layer 146 and the third structural layer 158 during the manufacture
of the mirror microstructure 130 by surface micromachining
techniques. The general design considerations for etch release
rails again are summarized below following the description of the
various embodiments of microstructures that include such etch
release rails.
[0096] In the event that it would be desirable to avoid the use of
etch release holes in the second structural layer 146 to allow for
the removal of any sacrificial layer(s) that is disposed between
the second structural layer 146 and the first structural layer 134,
the characteristics noted above in relation to the rails 154 could
be utilized by the rails 142 as well. Etch release holes would
likely be required in the first structural layer 134 in order to
allow for the removal of any sacrificial layer(s) that is disposed
between the first structural layer 134 and the substrate 132 during
the release of the microstructure 130 from the substrate 132.
[0097] Another function provided by the plurality of first rails
142 and second rails 154 is structural reinforcement of the mirror
microstructure 130, and more particularly the third structural
layer 158. It is believed that enhanced structural reinforcement is
realized by having the plurality of first rails 142 disposed in a
different orientation within the lateral dimension than the
plurality of second rails 154. In the illustrated embodiment, the
plurality of first rails 142 are disposed at least substantially
perpendicular to the plurality of second rails 154 in the lateral
dimension. The structural reinforcement function of the rails 142
and 154 would not be adversely affected, and may fact improve, by
having at least some of the rails 142 be disposed in intersecting
in relation and/or by having at least some of the rails 154 be
disposed in intersecting relation (e.g., see FIGS. 9A-B).
[0098] Certain parameters are identified on one or more of FIG. 6-8
in relation to the mirror microstructure 130 and that are addressed
in the above-noted summarization of desirable characteristics that
follows below. The dimension "d.sub.SL" for the case of the
microstructure 130 represents the diameter of the third structural
layer 158 (e.g., the distance of a straight line that extends from
one location on the perimeter 138 of the third structural layer
158, through a center 136 of the third structural layer 158, and to
another location on this perimeter 138) that is being structurally
reinforced collectively by the plurality of rails 154, the second
structural layer 146, the plurality of rails 142, and the first
structural layer 134. The dimension "d.sub.RS" for the case of the
microstructure 130 represents the distance from the center 136 of
the third structural layer 158 to that rail 154 that is closest to
the center 136, which is "0" in the illustrated embodiment since
one of the rails 154 actually extends through the center 136. It
should be appreciated that a rail 154 need not extend through the
center 136, in which case d.sub.RS would have a value greater than
"0." Finally, RC for the case of the mirror microstructure 130
represents the radius of curvature of the third structural layer
158.
[0099] Another configuration of a mirror microstructure for a
surface micromachined optical system, such as for the mirror
microstructures 6 and 20 in the surface micromachined optical
systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented
in FIGS. 9A-B in the form of a two-layered mirror microstructure
166. The mirror microstructure 166 is fabricated on a substrate 168
by surface micromachining techniques. Components of the mirror
microstructure 166 include a first structural layer or support 170
that is spaced vertically upward relative to the substrate 168
(e.g., disposed at a higher elevation relative to the substrate
168), a second structural layer or support 172 that is spaced
vertically upward relative to the first structural layer 170 (e.g.,
disposed at a higher elevation relative to the substrate 168), a
plurality of at least generally laterally extending first rails 174
that are disposed in spaced relation, and a plurality of at least
generally laterally extending second rails 178 that are also
disposed in spaced relation. Both the first rails 174 and the
second rails 178 extend between and fixedly interconnect the first
structural layer 170 and the second structural layer 172 to
structurally reinforce the microstructure 166 and, more
particularly, the second structural layer 172. In the illustrated
embodiment, the plurality of first rails 174 are disposed in at
least substantially parallel and equally spaced relation, as are
the plurality of second rails 178. However, the plurality of first
rails 174 are not disposed in the same orientation as the plurality
of second rails 178 in the lateral dimension. In the illustrated
embodiment, the first rails 174 are disposed at least substantially
perpendicular to the second rails 178. As such, the first
structural layer 170 and second structural layer 172, as well as
the interconnecting rails 174, 178, may be moved simultaneously if
acted upon by any interconnected actuator to provide a
desired/required optical function.
[0100] The plurality of first rails 174 and the plurality of second
rails 178 will not form the type of etch release channels or
conduits in any sacrificial layer that may exist between the first
structural layer 170 and the second structural layer 172 during the
manufacture of the mirror microstructure 166 using surface
micromachining in comparison to the mirror microstructures 106 and
130 discussed above. As such, a plurality of small vertically
disposed etch release holes (not shown) will typically extend
through the entire vertical extent of the second structural layer
172 to allow for the removal of any sacrificial layer(s) that is
disposed between the first structural layer 170 and the second
structural layer 172 of the mirror microstructure 166 used in the
assembly thereof (a plurality of small vertically disposed etch
release holes (not shown) will also typically extend through the
entire vertical extent of the first structural layer 170 to allow
for the removal of any sacrificial layer(s) that is disposed
between the first structural layer 170 and the substrate 168 of the
mirror microstructure 166 to release the microstructure 166 from
the substrate 168). This again may have an adverse impact on the
optical performance capabilities of an upper surface 176 of the
second structural layer 172. However, the microstructure 166 still
has desirable structural reinforcement characteristics. In this
regard, the plurality of first rails 174 and the plurality of
second rails 178 may be viewed as defining a grid 177 having a
plurality of closed cells 179 (i.e., having a closed boundary). Any
appropriate configuration may be used to define the perimeter of
these closed cells 179 (e.g., a honeycomb, cylindrical), and each
such closed cell 179 need not be of the same configuration.
[0101] An upper surface 176 of the second structural layer 172 is
or includes an optically reflective layer or film of the type
discussed above in relation to the mirror microstructure 30. Due to
the likely existence of the plurality of vertically disposed etch
release holes in the second structural layer 172, the upper surface
176 will at least have a plurality of dimples or the like which may
have an effect on its optical performance capabilities. As such,
the principal advantage of the mirror microstructure 166 is the
structural reinforcement of the second structural layer 172 that is
provided by the plurality of rails 174, 178 that structurally
interconnect the second structural layer 172 with the first
structural layer 170.
[0102] Certain parameters are identified on one or more of FIGS.
9A-B and that are addressed in the above-noted summarization of
desirable characteristics that follows below. The dimension
"d.sub.SL" for the case of the microstructure 166 represents the
diameter of the second structural layer 172 (e.g., the distance of
a straight line that extends from one location on a perimeter 173
of the second structural layer 172, through a center 171 of the
second structural layer 172, and to another location on this
perimeter 173) that is being structurally reinforced collectively
by the plurality of rails 174, the rails, 178, and the first
structural layer 170. The dimension "d.sub.RS" for the case of the
microstructure 166 represents the distance from the center 171 of
the second structural layer 172 to that portion of the grid 177
that is closest to the center 171. Finally, RC for the case of the
microstructure 166 represents the radius of curvature of the second
structural layer 172.
[0103] Another configuration of a mirror microstructure for a
surface micromachined optical system, such as for the mirror
microstructures 6 and 20 in the surface micromachined optical
systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented
in FIGS. 10A-C in the form of a reinforced, single-layer mirror
microstructure 384. The mirror microstructure 384 is fabricated on
a substrate 386 by surface micromachining techniques and is
separated therefrom by a space 392. Other structural components
could be disposed within this space 392, although any such
structures would typically be spaced from the mirror microstructure
384 at least when providing its optical function. Components of the
mirror microstructure 384 include a structural layer or support 404
that is spaced vertically upward relative to the substrate 386
(e.g., disposed at a higher elevation relative to the substrate
386); a plurality of at least generally laterally extending rails
400 that are fixedly interconnected with the structural layer 404,
that extend toward but not to the substrate 386, and that are
disposed in a first orientation; and a plurality of at least
generally laterally extending rails 396 that are fixedly
interconnected with the lower extreme of the rails 400 at a
plurality of discrete locations, that extend toward but not to the
substrate 386, and that are disposed in a second orientation that
is different than the first orientation. In the illustrated
embodiment, the rails 400 are disposed at least generally
perpendicular to the rails 396, although other relative
orientations or relative angular positions could be utilized.
Therefore, the structural layer 404, as well as the rails 400 and
396, may be moved simultaneously if acted upon by any
interconnected actuator to provide a desired/required optical
function.
[0104] At least the rails 400, and possibly the rails 396, extend
at least generally laterally from one radial location corresponding
with or at least generally proximate to a perimeter 390 of the
microstructure 384 (e.g., within 50 .mu.m of the perimeter 390,
more preferably within 25 .infin.m of this perimeter 390) to
another radial location corresponding with or at least generally
proximate to this perimeter 390 (e.g., within 50 .mu.m of the
perimeter 390, more preferably within 25 .mu.m of this perimeter
390). In the illustrated embodiment, the rails 400 are of an axial
or linear configuration in the lateral dimension, and are further
disposed in at least generally parallel and equally spaced
relation, while the rails 396 are also of an axial or linear
configuration in the lateral dimension, and are further disposed in
at least generally parallel and equally spaced relation.
[0105] An upper surface 406 of the structural layer 404 is or
includes an optically reflective layer or film. That is, the
materials that are used to define the structural layer 404 may
provide the desired/required optical properties/characteristics for
the mirror microstructure 384. More typically a separate layer or
film will be deposited on the structural layer 404 to realize the
desired/required optical properties/characteristics. Those
materials discussed above in relation to the mirror microstructure
30 for this purpose may be utilized by the mirror microstructure
384 and in the general manner illustrated in FIG. 2B.
[0106] There are a number of significant advantages in relation to
the design utilized by the mirror microstructure 384. First is in
relation to its optical characteristics. The upper surface 406 of
the support layer 404 of the mirror microstructure 384 need not and
preferably does not include any vertically disposed etch release
holes which extend entirely through the structural layer 404 in
order to allow for the removal of any sacrificial material from
between the structural layer 404 and the substrate 386 when the
microstructure 384 is released from the substrate 386, which
significantly enhances the optical performance capabilities of the
mirror microstructure 384. That is, preferably the upper surface
406 of the mirror microstructure 384 is continuous and devoid of
any indentations, etch release holes, or the like (depressions or
indentations that may develop on the upper surface 406 of the
structural layer 404 from the manufacture of the mirror
microstructure 384 may be addressed by a planarization operation).
Exactly how the mirror microstructure 384 may alleviate the need
for the etch release holes in the structural layer 404 will be
discussed in more detail below in relation to the manufacturing
methodologies represented in FIGS. 12A-M, 13A-M, 15A-G, and 17A-G.
Suffice it to say for present purposes that the rails 396, the
rails 400, or both may at least assist in the definition of a
plurality of at least generally extending etch release pipes,
channels, or conduits in a sacrificial layer(s) to allow for a more
expedient removal of the same and without the need for any
conventional etch release holes that extend downwardly entirely
through the structural layer 404. As such, the plurality of rails
396 and 404 may both be characterized as etch release rails as
noted above. Again and in this case, how the rails 400 are oriented
relative to each other and how the rails 396 are oriented relative
to each other, or stated another way the layout of the rails 396
and 404, may have an effect on their respective abilities to these
etch release conduits within any sacrificial layer that is disposed
between the structural layer 404 and the substrate 386 during the
manufacture of the mirror microstructure 384 by surface
micromachining techniques. The general design considerations for
etch release rails again are summarized below following the
description of the various embodiments of microstructures that
include such etch release rails.
[0107] Another function provided by the plurality of rails 400 and
the plurality of rails 396 is structural reinforcement of the
microstructure 384 and, more particularly the structural layer 404.
In the illustrated embodiment, the rails 400 provide enhanced
stiffness in effectively one direction and the plurality of rails
396 provide enhanced stiffness in effectively one direction as
well, but different from that associated with the rails 400. It is
believed that enhanced structural reinforcement is realized by
having the plurality of rails 400 disposed in a different
orientation within the lateral dimension than the plurality of
rails 396. In the illustrated embodiment, the plurality of rails
400 are disposed at least substantially perpendicular to the
plurality of rails 396 in the lateral dimension. The structural
reinforcement function of the rails 400 and 396 would not be
adversely affected, and may in fact improve, by having at least
some of the rails 400 be disposed in intersecting in relation
and/or by having at least some of the rails 396 be disposed in
intersecting relation (e.g., see FIGS. 9A-B).
[0108] Certain parameters are identified on one or more of FIGS.
10A-C in relation to the mirror microstructure 384 and that are
addressed in the above-noted summarization of the desirable
characteristics that follows below. The dimension "d.sub.SL" for
the case of the microstructure 384 represents the diameter of the
structural layer 404 (e.g., the distance of a straight line that
extends from one location on the perimeter 390 of the structural
layer 404, through a center 388 of the structural layer 404, and to
another location on this perimeter 390) that is being structurally
reinforced collectively by the plurality of rails 400 and the
plurality of rails 396. The dimension "d.sub.RS" for the case of
the microstructure 384 represents the distance from the center 388
of the structural layer 404 to that rail 400 that is closest to the
center 388. Finally, RC for the case of the mirror microstructure
384 represents the radius of curvature of the structural layer
404.
[0109] FIG. 10D presents a variation of the mirror microstructure
384 of FIGS. 10A-C. The only difference between the FIG. 10A-C and
FIG. 10D configurations is that the rails 400 in the FIG. 10A-C
configuration are replaced with a plurality of columns or posts
400' in the FIG. 10D configuration. The FIG. 10D configuration
still provides at least some degree of structural reinforcement for
the structural layer 404, and also allows for formation of etch
release conduits along the length of the rails 396.
[0110] Other layouts of rails that provide/allow for the formation
of the etch release conduits in a sacrificial layer(s) (i.e., etch
release rails), are within the scope of the present invention.
Representative examples of other etch release rail layouts that may
be appropriate for forming etch release channels and that may be
used in any of the above-described embodiments of mirror
microstructures are presented in FIGS. 11A-F. FIG. 11A illustrates
an embodiment in which a plurality of rails 412 are fixed to and
extend away from a structural layer or support 408. These rails 412
may "cantilever" from this structural layer 408, or alternatively
may interconnect the structural layer 408 with another structural
layer (not shown) to provide a multi-layered mirror microstructure.
In any case, the rails 412 have what may be characterized as a
sinusoidal lateral dimension or a sinusoidal configuration in plan
view, and further extend from one location on or at least generally
proximate to a perimeter 410 (e.g., within 50 .mu.m of this
perimeter 410, more preferably within 25 .mu.m of this perimeter
410) of the structural layer 408 to another location on or at least
generally proximate to this perimeter (e.g., within 50 .mu.m of
this perimeter 410, more preferably within 25 .mu.m of this
perimeter 410). Stated another way, the plurality of rails 412
extend sinusoidally within a plane that is at least generally
parallel with the substrate 408. In the illustrated embodiment,
none of the rails 412 intersect and adjacent rails 412 are nested
to a degree such that the peaks 416 of one rail 412 extend at least
partially within the space defined by a corresponding trough 420 of
the adjacent rail 412. Stated another way, the peaks 416 of one
rail 412 preferably extend beyond a line that is tangent to the
troughs 420 of an adjacent rail 412. The "amplitude" of this
sinusoidal configuration need not remain constant along the length
of the rails 412. The rails 412 also may be characterized as
"meandering" so as to provide enhanced stiffness in more than one
dimension. Other "meandering" configurations than the sinusoidal
type illustrated in relation to FIG. 11A may be utilized as well
for structural reinforcement and to allow for the formation of etch
release channels (e.g., in zig-zag fashion).
[0111] FIG. 11B illustrates an embodiment in which a plurality of
rails 436 are fixed to and extend away from a structural layer or
support 424. These rails 436 may "cantilever" from this structural
layer 424, or alternatively may interconnect the structural layer
424 with another structural layer (not shown) to provide a
multi-layered mirror microstructure. In any case, instead of being
disposed in parallel relation, the plurality of rails 436 are at
least generally radially disposed or extending (but still within
the lateral dimension). In the illustrated embodiment, the
plurality of rails 436 extend from or at least generally proximate
to a perimeter 428 (e.g., within 50 .mu.m of this perimeter 428,
more preferably within 25 .mu.m of this perimeter 428) of the
structural layer 424 toward, but not to, a center 432 of the
structural layer 424.
[0112] FIG. 11C illustrates an embodiment in which a plurality of
rails 452, 456 are fixed to and extend away from a structural layer
or support 440. These rails 452, 456 may "cantilever" from this
structural layer 440, or alternatively may interconnect the
structural layer 440 with another structural layer (not shown) to
provide a multi-layered mirror microstructure. In any case, instead
of being disposed in parallel relation, the plurality of rails 452,
456 are at least generally radially disposed or extending (but
still within the lateral dimension). In the illustrated embodiment,
the plurality of rails 452, 456 extend from or at least generally
proximate to a perimeter 444 (e.g., within 50 .mu.m of this
perimeter 444, more preferably within 25 .mu.m of this perimeter
444) of the structural layer 440 at least toward a center 448 of
the structural layer 440. The rails 452 extend further toward the
center 448 than the rails 456. This reduces the potential for
adversely affecting the formation of the etch release channels at
radially inward locations (i.e., at locations that are closer to
the center 448).
[0113] FIG. 11D illustrates an embodiment in which a plurality of
first rails 508 and a plurality of second rails 512 are fixed to
and extend away from a structural layer or support 500. These rails
508, 512 may "cantilever" from this structural layer 500, or
alternatively may interconnect the structural layer 500 with
another structural layer (not shown) to provide a multi-layered
mirror microstructure. In any case, instead of being disposed in
parallel relation, the plurality of first rails 508 and the
plurality of second rails 512 are at least generally radially
disposed or extending (but still within the lateral dimension). In
the illustrated embodiment, the plurality of first rails 508 and
the plurality of second rails 512 extend from or at least generally
proximate to a perimeter 504 (e.g., within 50 .mu.m of this
perimeter 504, more preferably within 25 .mu.m of this perimeter
504) of the structural layer 500 toward, but not to, a center 506
of the structural layer 500. Generally, the plurality of first
rails 508 extend further toward the center 506 than do the
plurality of second rails 512, and at least one second rail 512 is
disposed between adjacent pairs of first rails 508.
[0114] FIG. 11E illustrates an embodiment in which a plurality of
first rails 528, second rails 532, third rails 540, and fourth
rails 548 are fixed to and extend away from a structural layer or
support 516. Although these rails 528, 532, 540, and 548 are
illustrated as being simply a line in FIG. 11E, it should be
appreciated that these rails 528, 532, 540, and 548 may be of any
appropriate width. These rails 528, 532, 540, and 548 may
"cantilever" from this structural layer 516, or alternatively may
interconnect the structural layer 516 with another structural layer
(not shown) to provide a multi-layered mirror microstructure. In
the illustrated embodiment, all of the rails 528, 532, 540, and 548
are not disposed in parallel relation to each other, and
furthermore all of the rails 528, 532, 540, and 548 are not
radially disposed or extending within the lateral dimension (i.e.,
all rails 528, 532, 540, and 548 do not extend toward a point
corresponding with a center 524 of the structural layer 516).
Instead, in the illustrated embodiment: 1) the plurality of first
rails 528 extend from or at least generally proximate to a
perimeter 520 (e.g., within 50 .mu.m of this perimeter 520, more
preferably within 25 .mu.m of this perimeter 520) of the structural
layer 516 to a point corresponding with the center 524 of the
structural layer 516--that is, the rails 528 intersect at a point
corresponding with the center 524, and effectively define a column
or post that is disposed at the center 524; 2) a pair of second
rails 532 are disposed between each adjacent pair of first rails
528, and extend from or at least generally proximate to the
perimeter 520 (e.g., within 50 .mu.m of this perimeter 520, more
preferably within 25 .mu.m of this perimeter 520) of the structural
layer 516 to an intersection 536--that is, a pair of second rails
532 that terminate at an intersection 536 are "nested" between each
adjacent pair of first rails 528; 3) a pair of third rails 540 are
disposed within the space that is inward of each pair of second
rails 532 that are joined at an intersection 536, and extend from
or at least generally proximate to the perimeter 520 (e.g., within
50 .mu.m of this perimeter 520, more preferably within 25 .mu.m of
this perimeter 520) of the structural layer 516 to an intersection
544--that is, a pair of third rails 540 that terminate at an
intersection 544 are "nested" between each adjacent pair of second
rails 532 that are joined at an intersection 536; and 4) a pair of
fourth rails 548 are disposed within the space that is inward of
each pair of third rails 540 that are joined at an intersection
544, and extend from or at least generally proximate to the
perimeter 520 (e.g., within 50 .mu.m of this perimeter 520, more
preferably within 25 .mu.m of this perimeter 520) of the structural
layer 516 to an intersection 552--that is, a pair of fourth rails
548 that terminate at an intersection 552 are "nested" between each
adjacent pair of third rails 540 that are joined at an intersection
544. Therefore, the layout presented in FIG. 11E has at least some
etch release rails that intersect at one common point (e.g., rails
528 that intersect at a point corresponding with the center 524),
while other rails intersect at a different common point (e.g., at
intersections 536, 544, 552).
[0115] FIG. 11F illustrates an embodiment in which a plurality of
first rails 568, second rails 572, third rails 576, and fourth
rails 580 are fixed to and extend away from a structural layer or
support 556. These rails 568, 572, 576, and 580 may "cantilever"
from this structural layer 556, or alternatively may interconnect
the structural layer 556 with another structural layer (not shown)
to provide a multi-layered mirror microstructure. In any case,
instead of being disposed in parallel relation, the plurality of
rails 568, 572, 576, and 580 are at least generally radially
disposed or extending (but still within the lateral dimension).
That is, the plurality of rails 568, 572, 576, and 580 extend from
or at least generally proximate to a perimeter 560 (e.g., within 50
.mu.m of this perimeter 560, more preferably within 25 .mu.m of
this perimeter 560) of the structural layer 556 at least toward a
point corresponding with a center 564 of the structural layer 556.
Generally, the rails 568, 572, 576, and 580 terminate at different
radial positions relative to the center 564 of the structural layer
556. In the illustrated embodiment: 1) the plurality of first rails
568 extend from or at least generally proximate to the perimeter
560 (e.g., within 50 .mu.m of this perimeter 560, more preferably
within 25 .mu.m of this perimeter 560) of the structural layer 556
to the center 564 of the structural layer 556--that is, the
plurality of first rails 568 intersect at a common point
corresponding with the center 564, and effectively define a column
or post that is disposed at the center 564; 2) one second rail 572
is disposed between each adjacent pair of first rails 568, and
extends from or at least generally proximate to the perimeter 560
(e.g., within 50 .mu.m of this perimeter 560, more preferably
within 25 .mu.m of this perimeter 560) of the structural layer 556
toward, but not to, the center 564 of the structural layer 556; 3)
one third rail 576 is disposed between each first rail 568 and an
adjacent second rail 572, and extends from or at least generally
proximate to the perimeter 560 (e.g., within 50 .mu.m of this
perimeter 560, more preferably within 25 .mu.m of this perimeter
560) of the structural layer 556 toward, but not to, the center 564
of the structural layer 556; and 4) one fourth rail 580 is disposed
between each pair of adjacent rails 568, 572, and 576, and extends
from or at least generally proximate to the perimeter 560 (e.g.,
within 50 .mu.m of this perimeter 560, more preferably within 25
.mu.m of this perimeter 560) of the structural layer 556 toward,
but not to, the center 564 of the structural layer 556. The second
rails 572 extend further toward the center 564 than the third rails
576, and the third rails 576 extend further toward the center 564
than the fourth rails 580.
[0116] Various layouts of etch release rails have been described
above. However, it should be appreciated that these layouts are
merely representative of the various ways in which etch release
rails may be patterned to define a plurality of etch release
conduits. Any layout of etch release rails may be utilized that
will allow for the removal of one or more sacrificial layers in a
desired manner by providing or allowing for the formation of a
plurality of at least generally laterally extending etch release
conduits or channels within one or more sacrificial layers through
which an appropriate etchant may flow during the release of the
corresponding microstructure from its substrate. What etch release
rail layouts are appropriate for this purpose is based upon a
number of factors. Generally, the etchant will proceed at least
generally perpendicularly away from its corresponding etch release
conduit. At least one and more typically a pair of etch release
conduits will extend at least generally along the lateral extent of
each etch release rail. A targeted total etch release time will be
established or specified. The maximum total etch release time for
the types of microstructures described herein is preferably that
which does not significantly damage any of the structural layers of
the corresponding microstructure by exposure of the same to the
etchant. The etch rate will also be known and for design purposes
may be assumed to be constant. Therefore, a determination may then
be made as to how far from a given etch release conduit the etchant
will proceed in the specified total etch release time to create
what may be characterized as a projected etch release void. So long
as the projected etch release voids between adjacent etch release
rails abut or more preferably overlap to a degree, or stated
another way such that the entirety of the space between adjacent
etch release rails is defined by at least one and more typically a
plurality of projected etch release voids, the proposed layout of
the etch release rails will be appropriate for affecting the
release.
[0117] Another way to characterize an appropriate layout of etch
release rails is in relation to a maximum desired spacing between
adjacentmost etch release rails. Each etch release rail should be
positioned such that the maximum space between a given etch release
rail and an adjacentmost etch release rail is no more than about
twice the linear distance that an etchant will proceed through a
sacrificial layer in a specified total etch release time. In one
embodiment, this maximum spacing is less than about 100 microns,
and in another embodiment is within a range of about 50-75
microns.
[0118] Another important factor in relation to the various mirror
microstructures discussed above, as well as in relation to the
methods and mirror microstructures to be discussed below, is the
surface topography of the uppermost structural layer of these
mirror microstructures. This is particularly an issue when
reinforcing structures extend or depend from the lower surface of
this uppermost structural layer. These types of structures are
generally formed by first forming a plurality of apertures within a
layer of sacrificial material, and then depositing a structural
material over this layer of sacrificial material (discussed in more
detail below). That portion of the structural material that is
deposited within the apertures formed in the layer of sacrificial
material defines the reinforcing structures. The surface topography
that is desired for the upper surface of the uppermost structural
layer of any of the mirror microstructures described herein, as
deposited, or stated another way before being planarized, is one
where the maximum distance between any peak and valley on the upper
surface of this uppermost structural layer is less than the maximum
thickness of this uppermost structural layer. This type of surface
topography allows the upper surface of the uppermost structural
layer to thereafter be planarized a reasonable amount to yield the
desired degree of optical flatness.
[0119] There are a number of ways in which the desired surface
topography can be realized for the case where reinforcing
structures extend from the lower surface of the uppermost
structural layer of the mirror microstructure. One is to form this
uppermost structural layer for the mirror microstructure to a
sufficient thickness such that appropriate planarization techniques
(e.g., chemical mechanical polishing) may be utilized to reduce the
surface roughness of this surface to a desired level without an
undesirable amount of thinning of this uppermost structural layer
at any location. A sufficient thickness for the uppermost
structural layer in this scenario is where the thickness of the
uppermost structural layer is thicker than the underlying layer of
sacrificial material by an amount such that after being planarized
(e.g., by chemical mechanical polishing) to realize the desired
optical surface, the uppermost structural lay will still have
sufficient mechanical integrity. Another option for the case when
the thickness of the uppermost structural layer of the mirror
microstructure is comparable to the thickness of the underlying
layer of sacrificial material is to control the width of the
apertures in this layer of sacrificial material that again are used
to define the reinforcing structures that extend or depend from the
lower surface of the uppermost structural layer of the mirror
microstructure.
[0120] The mirror microstructures described herein that
structurally reinforce the uppermost structural layer, that have
one or more structures extending or depending from the lower
surface of the uppermost structural layer for purposes of providing
etch release rails, or both, have a desired surface topography on
the uppermost structural layer. This may be provided or realized by
the sizing of the structures that extend or depend from the lower
surface of the uppermost structural layer. Generally, the maximum
width of any individual structure that extends or depends from the
lower surface of the uppermost structural layer (e.g., the width of
a rail, the diameter of a post or column) should be less than twice
the thickness of the uppermost structural layer to provide the
desired surface topography based solely on the selection of
reinforcement structure width. This "thickness of the uppermost
structural layer" is that thickness that is disposed above the
depending reinforcing structure, or stated another way the
thickness at a location that is between any adjacent depending
reinforcing structures. This provides a desirable surface
topography for the uppermost structural layer, namely one that may
be planarized (e.g., by chemical mechanical polishing) a reasonable
amount to yield a desired degree of optical flatness while allowing
the uppermost structural layer to retain sufficient mechanical
integrity. These same principles may be applied to any underlying
structural layer of the mirror microstructure as well to improve
the surface topography. It should be noted that the maximum width
of the reinforcing structures that extend or depend from the lower
surface of the uppermost structural layer becomes less important as
the ratio of the thickness of the uppermost structural layer to the
thickness of the underlying sacrificial layer increases. Therefore,
some combination of structural layer thickness and reinforcement
structure width may provide the noted desired surface topography as
well.
[0121] The various reinforcing structures discussed above may be
used at any level within a mirror microstructure. That is, even
though a particular reinforcing structure may have only been
described herein in relation to a two-layered structure does not
mean that the same could not be used to structurally reinforce a
single structural layer of a mirror microstructure (i.e., so as to
cantilever from the same) or to structurally interconnect adjacent
but spaced structural layers in a mirror microstructure having
three or more spaced structural layers. Moreover, any combination
of the above-noted reinforcing structures may be used in any
combination in a microstructure having three or more spaced
structural layers (e.g., to interconnect any two adjacent but
spaced structural layers), or may cantilever from a lower surface
of a structural layer to structurally reinforce the same in the
general manner, for instance, of the microstructure 302 to be
discussed below in relation to the methodology of FIGS. 15A-G.
[0122] The above-noted reinforcing structures may also utilize any
appropriate vertical and/or horizontal cross-sectional
profiles/configurations, and further may extend in the lateral
dimension in any appropriate manner (e.g., axially, sinusoidally,
meandering, "zig-zagging"). Although the above-noted reinforcing
structures have been illustrated as being at least generally
perpendicular to an interconnected structural layer(s), such need
not be the case. Moreover, all reinforcing structures need not
necessarily be disposed in the same vertical orientation (i.e., the
same may be disposed at one or more different angles relative to
vertical).
[0123] It should be appreciated that the mirror microstructures 30,
58, 106, 130, 166, 384, and 384' discussed above may be
incorporated into any surface micromachined system and at any
elevation within such a system, and that these microstructures 30,
58, 106, 130, 166, 384, and 384' may be appropriate for
applications other than as a mirror. Characterizing the various
structural layers in these microstructures 30, 58, 106, 130, 166,
384, and 384' as "first," "second" and the like also does not
necessarily mean that these are the first, second, or the like
structural layers that are deposited over the associated substrate,
although such may be the case. It should also be appreciated that
the characterization of the various structural layers of these
microstructures 30, 58, 106, 130, 166, 384, and 384' as "first,"
"second," and the like also does not mean that the same must be
"adjacent" structural layers in any surface micromachined system
that includes these microstructures 30, 58, 106, 130, 166, 384, and
384'. There may be one or more intermediate structural layers that
are deposited over the associated substrate at an elevation that is
between what has been characterized as "first" and "second"
structural layers or the like, although any such structural layers
will have been removed from the area occupied by the
microstructures 30, 58, 106, 130, 166, 384, and 384' (e.g., there
may be one or more of structural layers that are "off to the side"
or laterally disposed relative to a given microstructure for
various purposes). It should also be appreciated that the various
structural layers of the noted microstructures 30, 58, 106, 130,
166, 384, and 384' may also be defined by one or more structural
layers, such as involving multiple and spaced in time
depositions.
[0124] One key advantage of the microstructures 30, 58, 106, 130,
166, 384, and 384' discussed above is their structurally reinforced
nature. Some of these reinforcement alternatives may require etch
release holes through one or more of the various structural layers
depending upon the particular manufacturing technique that is
employed, which may degrade the performance of the microstructures
30, 58, 106, 130, 166, 384, and 384' in a given application (e.g.,
when functioning as a mirror in an optical system, and where etch
release holes are required for the uppermost structural layer
because of the reinforcement structure that was utilized). A
certain amount of degradation may be acceptable based upon the
enhanced structural rigidity realized by these microstructures, and
in certain applications the existence of the noted etch release
holes may be irrelevant or at least of reduced significance.
However, certain of the reinforcement structures utilized by the
above-noted microstructures may actually enhance the release of the
microstructure from the associated substrate and altogether
alleviate the need for vertically disposed etch release holes in
one or more structural layers. These techniques will be discussed
in more detail below in relation to FIGS. 12-15, 17, and 20.
[0125] The above-described structurally reinforced mirror
microstructures will typically have a minimum surface area of about
2,000 .mu.m.sup.2 with a minimum lateral dimension of about 50
.mu.m for the optically functional surface of the mirror
microstructure (e.g., in the case of a circular mirror
microstructure, this minimum lateral dimension would be its
diameter; in the case of a square mirror microstructure, this
minimum lateral dimension would be the length of any of its four
sides; in the case of a rectangular mirror microstructure, this
minimum lateral dimension would be the length of the shortest of
its four sides). Moreover, the above-described structurally
reinforced mirror microstructures will utilize a structural layer
with the optically functional surface that has a maximum film
thickness of about 10 .mu.m in one embodiment, and more typically
about 6 .mu.m in another embodiment. This "maximum film thickness"
does not include the thickness of any reinforcement structure that
extends or depends from a lower surface of the relevant structural
layer.
[0126] The actual amount by which the above-noted microstructures
are structurally reinforced is affected by one or more
characteristics of the reinforcing structures that are used. There
may be two extremes in relation to the number or density of
reinforcing structures that are used. The first is having a high
density for the individual reinforcing structures, which is limited
only by the design rules and minimum dimensions of the process
technology, as well as the ability to insure that the etchant for
the release can adequately access the sacrificial films for their
removal. The second extreme is to go to a very sparse or low
density for the reinforcing structures, which gets to the point of
doing limited reinforcing or stiffening of the associated
structural layer(s). The benefit of the former is to provide a
maximum effect of reinforcement or stiffening of the associated
structural layer(s) or microstructure, but at the expense of
density and therefore total mass of the microstructure (i.e., in
the case where the microstructure is a mirror, the microstructure
may be very stiff, but too massive to allow for the use or
realization of rapid switching speeds). Therefore, the optimum
spacing and size of the reinforcing structures will oftentimes be
an engineering compromise between stiffness and mass. If total mass
does not matter, but stiffness is at a premium, then using a high
density for the reinforcing structures, as allowed by the design
and processed rules, would be optimum for the particular
application. However, if the microstructure is a mirror or some
other structure which must switch or otherwise move at a relatively
fast rate (e.g., in sub-millisecond times), then a calculation of
total mass versus available actuation forces will likely determine
the appropriate density for the reinforcing structures. Mass may
also be an issue for those microstructures that are moved in
relation to the physical size of any associated actuator, the
amount of voltage required to accomplish the desired movement, or
both.
[0127] There are a number of ways in which the microstructures 30,
58, 106, 130, 166, 384, and 384' may be at least generally
characterized. One or more of the microstructures 30, 58, 106, 130,
166, 384, and 384' may be characterized as including: 1) at least
two separate and distinct (i.e., not interconnected and disposed in
spaced relation) reinforcing structures (e.g., at least two
separate and discrete columns or posts; at least two separate and
discrete rails); 2) at least two separate and distinct reinforcing
structures that are disposed at different and radially spaced
locations or positions in the lateral dimension, or at least two
different portions of what may be characterized as a single
reinforcing structure (e.g., the "grid" defined by the rails 174
and rails 178 in the microstructure 166 of FIGS. 9A-B) being
disposed at different and radially spaced locations or positions in
this lateral dimension; 3) having a ratio of d.sub.RS/d.sub.SL that
is no more than about 0.5, and thereby including a ratio of "0.",
for instance where d.sub.RS is "0" (i.e., where there is a
reinforcing structure at the center of the structural layer being
structurally reinforced); or 4) or any combination thereof.
Visualization of the second noted characterization may be enhanced
by a reference to the FIG. 2-3 embodiment where at least some of
the plurality of columns 50 of the microstructure 30 are clearly
disposed at a different distance from a center 42 of the
microstructure 30, as well as the FIG. 9A-B embodiment where at
least some of the rails 174 and at least some of the rails 178 are
disposed at different radial positions and are in spaced
relation.
[0128] Another way of characterizing the microstructures 30, 58,
106, 130, 166, 384, and 384' is in relation to a radius of
curvature RC of the uppermost structural layer (e.g., the amount by
which the uppermost structural layer is "bowed" or "dished"). The
radius of curvature RC may have its center on either side of the
uppermost structural layer in the microstructures 30, 58, 106, 130,
166, 384, and 384'. That is, the upper surface of the uppermost
structural layer in the microstructures 30, 58, 106, 130, 166, 384,
and 384' may be generally concave or generally convex. In one
embodiment, the uppermost structural layer of the microstructures
30, 58, 106, 130, 166, 384, and 384' has a radius of curvature RC
that is at least about 1 meter, and in another embodiment that is
at least about 2 meters. The reinforcement configuration used by
the microstructure 166 in a three-layered mirror microstructure has
been fabricated with a radius of curvature RC that is about 14
meters. It should be noted that increasing the stiffness of a
microstructure does not in and of itself mean that the radius of
curvature of an uppermost structural layer of this microstructure
will in turn be increased. That is, the case of a constant stress
gradient through a plate or a beam leads to the same radius of
curvature independent of thickness to first order. This then
indicates that simple stiffening by adding thickness to a plate or
a beam does not, in and of itself, necessarily lead to a flatter
structure with a greater radius of curvature. However, the complex
method of reinforcing microstructures in the manner disclosed
herein can lead to significant internal stress compensation between
and within the individual structural layers of these reinforced
microstructures, such that greater flatness (i.e., a larger radius
of curvature for the uppermost structural layer) can be realized in
addition to achieving greater stiffness.
[0129] Another characterization that can be made in relation to the
microstructures 30, 58, 106, 130, 166, 384, and 384' is the effect
of the various reinforcing structures/layouts on overall structural
stiffness of the microstructures 30, 58, 106, 130, 166 384, and
384', which can be characterized by moment of inertia. In very
general terms, the moment of inertia for a simple rectangular
cross-section is I=bh.sup.3/12, where h is the plate thickness and
b the width. This illustrates the general idea or concept that
structural stiffness is a cubic function of the corresponding
thickness. Thus, going from a 2.25 .infin.m thick single structural
layer to an approximately 11.0 .mu.m multi-layered, structurally
reinforced microstructure of the type contemplated by the
microstructures 30, 58, 106, 130, 166, 384, and 384' implies an
approximate increase of stiffness by a factor of
11.sup.3/2.25.sup.3=117, or roughly two orders of magnitude (with
the orders by 10-based). This is important when there is a need for
a structure to not deform out of plane, or in the case of a mirror
microstructure that is coated with a reflective gold layer to not
be deformed by the stress in the gold layer. In other words,
because of thermal mismatch for example, the gold can be in
relative tension to the underlying structural layer (e.g.,
polysilicon), and thus would have the tendency to cause the
underlying structural layer to curl into a cup shape. The dramatic
increase in stiffness by reinforcement will keep the mirror
microstructure much flatter. Also, during temperature cycles, the
gold-coated plate will not change its RC as much (i.e., it will be
much more mechanically stable). The overall complexity of the
geometry and the deposition and anneal precludes a simple
`estimate` of the resulting RC especially at the current levels
being obtained. Empirical determination is less time consuming and
more accurate.
[0130] Investigations are still being undertaken in relation to
evaluating the structural reinforcement of the microstructures 30,
58, 106, 130, 166, 384, and 384'. Generally, it is believed that a
structurally reinforced three-layered microstructure will be more
rigid than a structurally reinforced two-layered microstructure. In
this case, any combination of the above-noted reinforcing
structures that structurally interconnect adjacent structural
layers in the microstructures 30, 58, 106, 130, 166, 384, and 384'
may be utilized in a three-layered microstructure in accordance
with one or more principles of the present invention. However,
evaluations are still ongoing in relation to "optimizing" the
structural reinforcement of the microstructures 30, 58, 106, 130,
166, 384, and 384' in the general manner described herein. There
may be instances where different combinations of the above-noted
reinforcing structures may be more appropriate for when the
associated microstructure is used in a given application or in
certain conditions.
[0131] Various microstructure fabrication methods will now be
described. Each of the various fabrication methods to be discussed
that utilize reinforcement/etch release rails may also use the
above-noted guidelines for maximum reinforcement structure widths
to realize the desired surface topography for the uppermost
structural layer prior to planarizing the same. Each of the various
fabrication methods to be discussed which define etch release
conduits should be implemented such that at least one end of at
least one etch release conduit is disposed at or at least generally
proximate to a perimeter of the microstructure being fabricated
(e.g., within 50 microns of this perimeter in one embodiment, and
more preferably within 25 microns of this perimeter in another
embodiment). This again reduces the amount of time that the release
etchant must "etch in" from this perimeter before reaching the etch
release conduit(s), and thereby allows the release to be finished
within a desired amount of time. Multiple ends of etch release
conduits or etch release conduit accesses are preferably disposed
at this radial position. It should be noted that sacrificial
material is disposed about the perimeter of the microstructures
that are fabricated in accordance with the following. Therefore,
absent a preformed via or the like, the release etchant must first
etch down to the level(s) at which the etch release conduits are
disposed. However, the time required for the release etchant to go
down to the level(s) of the etch release conduits is not that
significant due to the rather minimal vertical distance which these
microstructures extend above their corresponding substrate.
[0132] One method for making a microstructure is illustrated in
FIGS. 12A-M. This methodology may be utilized to make the mirror
microstructure 106 of FIGS. 5-6, and the principles of this
methodology may be utilized in/adapted for the manufacture of the
mirror microstructure 130 of FIG. 7, the mirror microstructure 384
of FIGS. 10A-C, and the mirror microstructure 384' of FIG. 10D, as
well as the variations therefore that are presented in FIGS. 11A-F.
In addition to being able to form a desired reinforcing structure,
the methodology of FIGS. 12A-M further provides a desired manner
for releasing the microstructure at the end of processing by
forming a plurality of at least generally laterally extending etch
release conduits in one or more of its sacrificial layers to
facilitate the removal thereof to provide the releasing
function.
[0133] FIG. 12A illustrates a substrate 182 on which a
microstructure 180 (FIG. 12M) will be fabricated by surface
micromachining techniques. Multiple layers are first sequentially
deposited/formed over the substrate 182. A first sacrificial layer
186 is deposited over the substrate 182 as illustrated in FIG. 12B,
a first structural layer 190 is deposited on the first sacrificial
186 as illustrated in FIG. 12C, and a second sacrificial layer 194
is deposited on the first structural layer 190 as illustrated in
FIG. 12D. The second sacrificial layer 194 is then patterned to
define a plurality of interconnect apertures 198 as illustrated in
FIG. 12E. These interconnect apertures 198 at a minimum allow for
establishing a structural connection with the first structural
layer 190, and may be in the form of at least generally laterally
extending grooves or trenches (e.g., to define a plurality of rails
or at least the lower portion thereof, such as the type utilized by
the mirror microstructure 106 of FIG. 5, the mirror microstructure
130 of FIG. 7, and the mirror microstructure 384 of FIG. 10A, as
well as the variations therefore illustrated in FIGS. 11A-F). These
interconnect apertures 198 could also be in the form of a plurality
of separate and discrete holes that are disposed in spaced relation
to define a plurality of posts or columns that would structurally
interconnect with the underlying first structural layer 190 (e.g.,
similar to the manner in which the columns 50 interconnect with the
underlying first structural layer 34 in the case of the mirror
microstructure 30 of FIG. 2A). Using the methodology of FIGS. 12A-M
to make the mirror microstructure 384 of FIGS. 10A-C or the mirror
microstructure 384' of FIG. 10D would not require this type of a
patterning of the second sacrificial layer 194 to define the
plurality of interconnect apertures 198, since the rails 396 of the
microstructure 384/384' do not interconnect with any underlying
structural layer.
[0134] A second structural layer 202 is deposited on the second
sacrificial layer 194 as illustrated in FIG. 12F. The material that
defines the second structural layer 202 is also deposited within
and at least substantially fills the interconnect apertures 198
that were previously formed in the second sacrificial layer 194,
and such may be characterized as being part of the second
structural layer 202. This portion of the second structural layer
202 may be characterized as a plurality of first reinforcement
sections 214 that will be the lower extreme of a reinforcing
assembly 208 for the microstructure 180 that is being fabricated.
Although FIG. 12F shows an intersection between the lower extreme
of each of the first reinforcement sections 214 and the upper
extreme of the first structural layer 190, typically such an
intersection will not exist and instead will at least appear to be
continuous.
[0135] The second structural layer 202 is then patterned to define
a plurality of at least generally laterally extending second
reinforcement sections 210 for the reinforcing assembly 208, as
illustrated in FIG. 12G (e.g., to define a plurality of rails or at
least the middle portion thereof of the type utilized by the
microstructure 106 of FIG. 5, the microstructure 130 of FIG. 7, the
microstructure 384 of FIGS. 10A-C, and the microstructure 384' of
FIG. 10D, as well as the variations therefore presented in FIGS.
11A-F). Each second reinforcement section 210 is disposed directly
above (e.g., vertically aligned) with at least one first
reinforcement section 214 (if used), although the second
reinforcement sections 210 will typically have a slightly larger
width than any corresponding first reinforcement section(s)
214.
[0136] A third sacrificial layer 218 is then deposited on the
second reinforcement sections 210 that were formed from the second
structural layer 202 as illustrated in FIG. 12H. The upper surface
of this third sacrificial layer 218 will typically have a wavy or
uneven contour. Generally, those portions of the third sacrificial
layer 218 that are disposed over the second reinforcement sections
210 will be disposed at a higher elevation than those portions of
the third sacrificial layer 218 that are disposed between adjacent
second reinforcement sections 210. Therefore, the upper surface of
the third sacrificial layer 218 will typically be planarized in an
appropriate manner, such as by chemical mechanical polishing to
yield a sufficiently flat upper surface for the third sacrificial
layer 218, as illustrated in FIG. 12I.
[0137] The third sacrificial layer 218 is then patterned to define
a plurality of interconnect apertures 222 as illustrated in FIG.
12J. These interconnect apertures 222 at a minimum allow for
establishing a structural interconnection with the second
reinforcement sections 210, and may be in the form of at least
generally laterally extending grooves or trenches (e.g., to define
a plurality of rails or at least the upper portion thereof of the
type utilized by the microstructure 106 of FIG. 5, the
microstructure 130 of FIG. 7, and the microstructure 384 of FIG.
10A, as well as the variations therefore illustrated in FIGS.
11A-F), or may be in the form of a plurality of separate and
discrete holes that are disposed in spaced relation (e.g., to
define a plurality of posts or columns of the type utilized by the
mirror microstructure 384' of FIG. 10D). Each interconnect aperture
222 is disposed directly above (e.g., vertically aligned) a
corresponding second reinforcement section 210, although the second
reinforcement sections 210 will typically have a slightly larger
width than their corresponding interconnect aperture(s) 222.
[0138] A third structural layer 226 is deposited on the third
sacrificial layer 218 as illustrated in FIG. 12K. The material that
defines the third structural layer 226 is also deposited within and
at least substantially fills the interconnect apertures 222 that
were previously formed in the third sacrificial layer 218, and such
may be characterized as being part of the third structural layer
226. This portion of the third support layer 226 may be
characterized as a plurality of third reinforcement sections 230
that are the upper extreme of the reinforcing assembly 208 for the
microstructure 180 that is being fabricated. Although FIG. 12K
shows an intersection between the lower extreme of each of the
third reinforcement section 230 and the upper extreme of their
corresponding second reinforcement 210, typically such an
intersection will not exist and instead will at least appear to be
continuous.
[0139] The upper surface of the third structural layer 226 will
typically have a wavy or uneven contour as illustrated in FIG. 12K.
Generally, those portions of the third structural layer 226 that
are disposed between adjacent third reinforcement sections 230 will
be recessed to a degree. Therefore, the upper surface of the third
structural layer 226 will typically be planarized in an appropriate
manner, such as by chemical mechanical polishing, to yield a
sufficiently flat upper surface for the third structural layer 226
as illustrated in FIG. 12L. This completes the definition of the
microstructure 180. It should be appreciated that the system that
includes the microstructure 180 will likely include other
components than those illustrated in FIGS. 12A-M and that may
interface with the microstructure 180 in some manner (e.g., one or
more actuators).
[0140] The microstructure 180 is now ready to be released.
"Released" means to remove each of the sacrificial layers of the
surface micromachined system and thereby including the first
sacrificial layer 186, the second sacrificial layer 194, and the
third sacrificial layer 218. An etchant is used to provide the
releasing function. The manner in which the microstructure 180 was
formed in accordance with the methodology of FIGS. 12A-M reduces
the time required for the sacrificial layers 194 and 218 to be
totally removed. Ultimately, a plurality of at least generally
laterally extending etchant flow pipes, channels, or conduits are
formed in the third sacrificial layer 218. Those portions of the
third sacrificial layer 218 that are positioned against/near the
second reinforcement sections 210 that were formed from the second
structural layer 202 are believed to be less dense than the
remainder of the third sacrificial layer 218 since the etch rate is
greater in proximity to the second reinforcement sections 210 and
including along the length thereof. Recall that the third
sacrificial layer 218 was deposited after the second reinforcement
sections 210 were formed, which creates these low density regions.
Low density regions in the third sacrificial layer 218 thereby
exist along the entire length of both sides of each second
reinforcement 210. Principally these low density regions will exist
along a sidewall 212 of each of the second reinforcement section
210 (e.g., the vertically disposed/extending portion of the second
reinforcement section 210). The etch rate will be greater in the
low density regions of the third sacrificial layer 218 than
throughout the remainder of the third sacrificial layer 218. This
will effectively form at least two etch release pipes, channels or
conduits in the third sacrificial layer 218 along the side of each
second reinforcement section 210. The development of the etch
release channels during the initial portion of the release etch
provides access to interiorly disposed locations within the
sacrificial layers for the etchant to complete the release before
the etchant has any significant adverse effect on the
microstructure 180. Notwithstanding the characterization of the
structures 214, 210, and 230 as "reinforcement sections," it should
be appreciated that the entire focus of the methodology of FIGS.
12A-M could in fact be to simply provide a plurality of at least
generally laterally extending etch release conduits, to in turn
provide a "rapid etch release function" for the microstructure 180.
That is, it is not required that the structures 214, 210, and 230
actually structurally reinforce the microstructure 180, although
such is preferably the case. Therefore, the structures 210 that
provide for the definition of the low density regions in the third
sacrificial layer 218, and thereby the etch release conduits, could
also be properly characterized as etch release rails or the
like.
[0141] The microstructure 180 of FIG. 12M has a desired surface
topography on its third structural layer 226 using the above-noted
principles. For the case where the thickness of third structural
layer 226 is comparable or less than the thickness of the
underlying third sacrificial layer 218, the maximum lateral
dimension of any rail upper section 230 (designated as "w" in FIG.
12M) again should be less than twice the thickness of the uppermost
structural layer (designated as "t" in FIG. 12M, and which does not
include the depending structure). This again provides a desirably
smooth surface topography for the third structural layer 226, which
is desired for optical applications. Having the third structural
layer 218 be of a thickness which is greater than the thickness of
the third sacrificial layer 218 reduces the effects of the width of
the interconnect apertures 222 on the surface topography of the
third structural layer 218.
[0142] The basic principle for forming etch release channels or
conduits encompassed by the methodology represented in FIGS. 12A-M
is that low density regions of sacrificial material are formed when
the sacrificial material is deposited along at least generally
vertically disposed surfaces of an etch release rail, and that the
same each effectively defines a etch release channel or conduit.
These etch release rails may exist at any desired level within the
microstructure being fabricated and yet still provide this low
density region formation function. Moreover, these etch release
rails do not need to be structurally interconnected with the
uppermost structural layer of the microstructure being fabricated
to provide this low density region formation function. For
instance, these etch release rails instead could be anchored to the
underlying substrate or another underlying structural layer. In
fact, these etch release rails need not remain in the final
structure of the microstructure being fabricated at all, but
instead may be removed during the release of the microstructure
from the substrate.
[0143] Another method for making a microstructure is illustrated in
FIGS. 13A-M. This methodology may be utilized to make the mirror
microstructure 106 of FIGS. 5-6, and the principles of this
methodology may be utilized in/adapted for the manufacture of the
mirror microstructure 130 of FIG. 7, the mirror microstructure 384
FIGS. 10A-C, and the mirror microstructure 384' of FIG. 10D, as
well as the variations therefore presented in FIGS. 11A-F. In
addition to being able to form a desired reinforcing structure, the
methodology of FIGS. 13A-M further provides a desired manner for
releasing the microstructure at the end of processing by forming a
plurality of at least generally laterally extending etch release
conduits in one or more of its sacrificial layers to facilitate the
removal thereof to provide the releasing function. In contrast to
the methodology of FIGS. 12A-M, the methodology of FIGS. 13A-M
forms these etch release conduits during the fabrication of the
microstructure (i.e., there is at least one more deposition after
these etch release conduits are formed). Stated another way, the
plurality of etch release in the case of the methodology of FIGS.
13A-M exist before the microstructure and sacrificial layers are
exposed to any etchant for providing the release function.
[0144] FIG. 13A illustrates a substrate 234 on which a
microstructure 232 will be fabricated. Multiple layers are first
sequentially deposited/formed over the substrate 234. A first
sacrificial layer 238 is deposited over the substrate 234 as
illustrated in FIG. 13B, a first structural layer 242 is deposited
on the first sacrificial layer 238 as illustrated in FIG. 13C, and
a second sacrificial layer 246 is deposited on the first structural
layer 242 as illustrated in FIG. 13D. The second sacrificial layer
246 is then patterned to define a plurality of interconnect
apertures 250 as illustrated in FIG. 13E. These interconnect
apertures 250 at a minimum allow for establishing a structural
connection with the first structural layer 242, and may be in the
form of at least generally laterally extending grooves or trenches
(e.g., to define a plurality of rails or at least the lower portion
thereof of the type utilized by the microstructure 106 of FIG. 5
and the microstructure 130 of FIG. 7, as well as the variations
therefore illustrated in FIGS. 11A-F). The interconnect apertures
250 could also be in the form of a plurality of separate and
discrete holes that are disposed in spaced relation to define a
plurality of posts or columns that would structurally interconnect
with the underlying first structural layer 242 (e.g., similar to
the manner in which the columns 50 interconnect with the underlying
first structural layer 34 in the case of the mirror microstructure
30 of FIG. 2A). Using the methodology of FIGS. 13A-M to make the
microstructure 384 of FIGS. 10A-C and microstructure 384' of FIG.
10D would not require this patterning of the second sacrificial
layer 246 to define the plurality of interconnect apertures 250,
since the rails 396 of the microstructure 384/384' do not
interconnect with an underlying structural layer.
[0145] A second structural layer 254 is deposited on the second
sacrificial layer 246 as illustrated in FIG. 13F. The material that
defines the second structural layer 254 is also deposited within
and at least substantially fills the interconnect apertures 250
that were previously formed in the second sacrificial layer 246,
and such may be characterized as being part of the second
structural layer 254. This portion of the second support layer 254
may be characterized as a plurality of first reinforcement sections
258 that are the lower extreme of a reinforcing assembly 256 for
the microstructure 232 that is being fabricated (FIG. 13M).
Although FIG. 13F shows an intersection between the lower extreme
of each of the first reinforcement sections 258 and the upper
extreme of the first structural layer 242, typically such an
intersection will not exist and instead will at least appear to be
continuous.
[0146] The second structural layer 254 from FIG. 13F is then
patterned to define a plurality of at least generally laterally
extending second reinforcement sections 262 for the reinforcing
assembly 256 (FIG. 13M), as illustrated in FIG. 13G (e.g., to
define a plurality of rails or at least the middle portion thereof
of the type utilized by the microstructure 106 of FIG. 5, the
microstructure 130 of FIG. 7, the microstructure 384 of FIGS.
10A-C, and the mirror microstructure 384' of FIG. 10D, as well as
the variations therefore presented in FIGS. 11A-F). Each second
reinforcement section 262 is disposed directly above (e.g.,
vertically aligned) with at least one first reinforcement section
258 (if any), although the second reinforcement sections 262 will
typically have a slightly larger width than their corresponding
first reinforcement section(s) 258 (if any). At this time an upper
portion of the second sacrificial layer 246 is removed as
illustrated in FIGS. 13H. That is, a portion of the second
sacrificial layer 246 remains after this removal operation. The
second sacrificial layer 246 extends above the first structural
layer 242 a distance which is less than the distance which the
lower extreme of the second reinforcement sections 262 are disposed
above the first structural layer 242. As, there is now a gap or an
undercut 266 beneath the lower extreme of the second reinforcement
sections 262 and the upper surface of the second sacrificial layer
246. One way in which only an upper portion of the second
sacrificial layer 246 may be removed is by a timed or controlled
etch. It is also basically a requirement that this etch be of an
isotropic type in order to form the undercuts 266. This is most
easily accomplished using a liquid HF-based etchant. An anisotropic
dry plasma etch, for example, would only etch straight down and the
undercuts 266 (and thereby the etch release channels 270 noted
below) would not subsequently be formed.
[0147] A third sacrificial layer 274 is then deposited on the
second reinforcement sections 262 that were formed from the second
structural layer 254 and on the second sacrificial layer 246 as
illustrated in FIG. 13I. Although FIG. 131 shows an intersection
between the third sacrificial layer 274 and the second sacrificial
layer 246, typically this will not be the case such that the third
sacrificial layer 274 and the second sacrificial layer 246 will
appear to be continuous. Not all portions of the undercuts 266 will
"fill" with the material that defines the third sacrificial layer
274, for example if the formation of the third sacrificial layer
274 is done with a PECVD oxide. These resulting voids define a
plurality of at least generally laterally extending etch release
channels 270. One of these etch release channels 270 is disposed on
and extends along the entire length of each side of an upper
extreme of the first reinforcement sections 258 of the reinforcing
assembly 256 (and/or beneath the second reinforcement sections 262
along both sides thereof).
[0148] The upper surface of the third sacrificial layer 274 may
retain a wavy or uneven contour after being deposited. The upper
surface of the third sacrificial layer 274 may then be planarized
in an appropriate manner, such as by chemical mechanical polishing,
to yield a sufficiently flat upper surface for the third
sacrificial layer 274 and as illustrated in FIG. 13I. In any case,
the third sacrificial layer 274 is then patterned to define a
plurality of interconnect apertures 278 as illustrated in FIG. 13J.
These interconnect apertures 278 at a minimum allow for
establishing a structural interconnection with the second
reinforcement sections 262, and may be the form of at least
generally laterally extending grooves or trenches (e.g., to define
a plurality of rails or at least the upper portion thereof of the
type utilized by the microstructure 106 of FIG. 5, the
microstructure 130 of FIG. 7, and the microstructure 384 of FIGS.
10A-C, as well as the variations therefore illustrated in FIGS.
11A-F), or may be in the form of a plurality of separate and
discrete holes that are disposed in spaced relation (e.g., to
define a plurality of posts or columns of the type utilized by the
microstructure 384' of FIG. 10D). In any case, each interconnect
aperture 278 is disposed directly above (e.g., vertically aligned)
with a corresponding second reinforcement section 262, although the
second reinforcement sections 262 will typically have a slightly
larger width than their corresponding interconnect aperture(s)
278.
[0149] A third structural layer 282 is then deposited on the third
sacrificial layer 274 as illustrated in FIG. 13K. The material that
defines the third structural layer 282 is also deposited within and
at least substantially fills the interconnect apertures 278 that
were previously formed in the third sacrificial layer 274, and such
may be characterized as being part of the third structural layer
282. This portion of the third structural layer 282 may be
characterized as a plurality of third reinforcement sections 286
that are the upper extreme of the reinforcing assembly 256 for the
microstructure 232 that is being fabricated. Although FIG. 13K
shows an intersection between the lower extreme of each of the
third reinforcement sections 286 and the upper extreme of their
corresponding second reinforcement section 262, typically such an
intersection will not exist and instead will at least appear to be
a continuous structure.
[0150] The upper surface of the third structural layer 282 will
typically have a wavy or uneven contour, such as a plurality of
laterally disposed and axially extending depressions. Generally,
those portions of the third structural layer 282 that are disposed
between adjacent third reinforcement sections 286 will be recessed
to a degree. Therefore, the upper surface of the third structural
layer 282 will typically be planarized in an appropriate manner,
such as by chemical mechanical polishing, to yield a sufficiently
flat upper surface for the third structural layer 282 and as
illustrated in FIG. 13L. This completes the definition of the
microstructure 232. It should be appreciated that the system that
includes the microstructure 232 will likely include other
components than those illustrated in FIGS. 13A-M and that may
interface with the microstructure 232 in some manner (e.g., one or
more actuators).
[0151] The microstructure 232 is now ready to be released.
"Released" means to remove each of the sacrificial layers in the
system, and thereby including the first sacrificial layer 238, the
second sacrificial layer 246, and the third sacrificial layer 274.
An etchant is used to provide the releasing function. Predefined
flow paths for this etchant are defined in and extend through
portions of the second sacrificial layer 246 and the third
sacrificial layer 274 in the form of the above-noted etch release
channels 270. The existence of the etch release channels 270
provides access to interiorly disposed locations within the
sacrificial layers 246, 274 for the etchant to complete the release
before the etchant has any significant adverse effect on the
microstructure 232. Notwithstanding the characterization of the
structures 258, 262, and 286 as "reinforcement sections," it should
be appreciated that the entire focus of the methodology of FIGS.
13A-M could in fact be to simply provide a plurality of at least
generally laterally extending etch release conduits, to in turn
provide a "rapid etch release function" for the microstructure 232.
That is, it is not required that the structures 258, 262, and 286
actually structurally reinforce the microstructure 232, although
such is preferably the case. Therefore, the structures 258 and/or
262 that provide for the definition of the etch release conduits
270 could also be properly characterized as etch release rails or
the like.
[0152] The basic principle for forming etch release channels or
conduits encompassed by the methodology represented in FIGS. 13A-M
is that one or more undercuts may be formed under an etch release
rail in such a manner that the subsequent deposition of a
sacrificial material will not entirely fill these undercuts,
thereby leaving a void that defines an etch release channel or
conduit. These etch release rails may exist at various levels
within the microstructure being fabricated and yet still allow for
the formation of etch release channels or conduits in this same
general manner. Moreover, these etch release rails do not need to
be structurally interconnected with the uppermost structural layer
of the microstructure being fabricated to allow for the formation
of etch release channels or conduits in this same general manner.
For instance, these etch release rails instead could be anchored to
the underlying substrate or another underlying structural layer. In
fact, these etch release rails need not remain in the final
structure of the microstructure being fabricated at all, but
instead may be removed during the release of the microstructure
from the substrate as in the case of the methodology of FIGS.
14A-F.
[0153] Another method for making another embodiment of a
microstructure 358 is illustrated in FIGS. 14A-F. FIG. 14A
illustrates that a first sacrificial layer 360 has been deposited
over a substrate 356, and that an intermediate layer 364 has been
deposited directly on the first sacrificial layer 360. The first
sacrificial layer 360 is formed from a different material than the
intermediate layer 364. In one embodiment, the first sacrificial
layer 360 is formed from those types of materials identified above
as being appropriate for the types of sacrificial layers described
herein, while the intermediate layer 364 may be materials such as
silicon nitride, polygermanium, or the like. The material that is
selected for the intermediate layer 364 should be at least
partially soluble in the etchant that is used to remove the first
sacrificial layer 360, or in a secondary etchant that does not
affect the various structural layers of the microstructure 358 and
that can be applied after the release etchant. For example, if
polygermanium were used for the intermediate layer 364, it will not
dissolve in a release etch that uses HF, but could be dissolved
subsequently in a short hydrogen peroxide bath that would not
adversely affect the polysilicon that may be used for the various
structural layers of the microstructure 358. In any case, of the
intermediate layer 364 is then patterned to define a plurality of
at least generally laterally extending strips 368 as illustrated in
FIG. 14B, and which function as etch release rails. As such, any of
the layouts noted above for etch release rails may be utilized.
However, unlike other etch release rails described herein that also
provide a reinforcing function, the strips 368 will be removed
during the release of the microstructure 358 that is formed by the
methodology of FIGS. 14A-F. In order to ensure the complete removal
of the strips 368, they should be of a reduced thickness. In one
embodiment, the thickness or vertical extent of the silicon nitride
strips 368 is no more than about 1500 .ANG..
[0154] At this time, part of an upper portion of the first
sacrificial layer 360 is removed as illustrated in FIG. 14C.
Generally, a portion of the first sacrificial layer 360 is removed
from under the plurality of strips 368 along both of its edges to
define a gap or an undercut 370 along both edge portions of each
strip 368. The entirety of the first sacrificial layer 360 remains
directly under a portion of the strips 368 in the form of a
pedestal or the like to support the same. One way in which only an
upper portion of the first sacrificial layer 360 may be removed is
by a timed or controlled etch. It is also basically a requirement
that this etch be of an isotropic type in order to form the
undercuts 370. This is most easily accomplished using a liquid
HF-based etchant. An anisotropic dry plasma etch, for example,
would only etch straight down and the undercuts 370 (and thereby
the etch release channels 380 noted below) would not subsequently
be formed.
[0155] A second sacrificial layer 372 is then deposited on the
strips 368 and on the first sacrificial layer 360 as illustrated in
FIG. 14D. Although FIG. 14D shows an intersection between the
second sacrificial layer 372 and the first sacrificial layer 360,
typically this will not be the case such that the second
sacrificial layer 372 and the first sacrificial layer 360 will
appear to be continuous. Not all portions of the undercuts 370 will
"fill" with the material that defines the second sacrificial layer
372, for example if the formation of the second sacrificial layer
372 is done with a PECVD oxide. These resulting voids define a
plurality of at least generally laterally extending etch release
channels 380. One of these etch release channels 380 is disposed on
and extends along the entire length of each bottom side portion of
each of the strips 368.
[0156] The upper surface of the second sacrificial layer 372 may
retain a wavy or uneven contour after being deposited (not shown).
The upper surface of the second sacrificial layer 372 may then be
planarized in an appropriate manner, such as by chemical mechanical
polishing, to yield a sufficiently flat upper surface for the third
sacrificial layer 372 and as illustrated in FIG. 14D. In any case,
a first structural layer 376 is deposited on the second sacrificial
layer 372 as illustrated in FIG. 14E which defines the entirety of
a microstructure 358. It should be appreciated that the system that
includes the microstructure 358 will likely include other
components than those illustrated in FIGS. 14A-F and that may
interface with the microstructure 358 in some manner (e.g., one or
more actuators).
[0157] The microstructure 358 is now ready to be released.
"Released" means to remove each of the sacrificial layers in the
system, and thereby including the first sacrificial layer 360 and
the second sacrificial layer 372. The plurality of strips 368 are
also removed in this release. Etchants are used to provide the
releasing function. Predefined flow paths for this etchant are
defined in and extend through portions of the second sacrificial
layer 372 and the first sacrificial layer 360 in the form of the
above-noted etch release channels 380. The existence of the etch
release channels 380 provides access to interiorly disposed
locations within the sacrificial layers 360, 372 for the etchant to
complete the release before the etchant has any significant adverse
effect on the microstructure 358.
[0158] The manner in which the etch release channels 380 are formed
is similar to the methodology of FIGS. 13A-M. The primary
difference is that the methodology of FIGS. 14A-F does not define
any reinforcing structure for its microstructure 358, whereas the
methodology of FIGS. 13A-M does define a reinforcing assembly 256
for its microstructure 232. A related difference is that the strips
368 in the methodology of FIGS. 14A-F are removed during the
release etch or in a post-release etch, whereas the reinforcing
assembly 256 in the methodology of FIGS. 13A-M is not removed
during the release.
[0159] Another method for making another embodiment of a reinforced
microstructure 302 is illustrated in FIGS. 15A-G. In addition to
being able to form a desired reinforcing structure, the methodology
of FIGS. 15A-G further provides a desired manner for releasing the
microstructure 302 at the end of processing by forming a plurality
of at least generally laterally extending etch release channels in
one or more of its sacrificial layers to facilitate the removal
thereof when releasing the microstructure 302. Any of the layouts
noted above for etch release rails may be used to form these etch
release channels, but in the manner set forth in relation to FIGS.
15A-G.
[0160] FIG. 15A illustrates a substrate 300 on which the
microstructure 302 will be fabricated. Multiple layers are first
sequentially deposited/formed over the substrate 300. A first
sacrificial layer 304 is deposited over the substrate 300, and a
first structural layer 308 is then deposited on the first
sacrificial layer 304. The first support layer 308 is then
patterned to define a plurality of lower reinforcement sections 312
of a reinforcing assembly 310 for the microstructure 302 as
illustrated in FIG. 15B. These lower reinforcement sections 312 are
at least generally laterally extending. Adjacent lower
reinforcement sections 312 are separated by a spacing 314 that is
also thereby at least generally laterally extending as well.
Generally, a relationship between the distance between adjacent
lower reinforcement sections 312 and the height or vertical extent
of the lower reinforcement sections 312 is selected to allow a
plurality of etch release channels to be defined in the spacings
314. In one embodiment: 1) the width or lateral extent of each of
the spacings 314, or stated another way the distance between
adjacent lower reinforcement sections 312 measured parallel with an
upper surface of the substrate 300, is no more than about 1.5
.mu.m; and 2) the height or vertical extend of each of the lower
reinforcement sections 312 is at least about 1.5 .mu.m. Stated
another way, a ratio of the height of a given lower reinforcement
section 312 to a width or lateral extent between this lower
reinforcement section 312 and an adjacent lower reinforcement
section 312 (i.e., one of the rail spacings 314) is at least about
1:1.
[0161] A second sacrificial layer 316 is deposited on the first
structural layer 308 as illustrated in FIG. 15C. The second
sacrificial layer 316 extends within and occupies a portion of each
of the spacings 314 that were previously formed from the first
structural layer 308. However, the material that defines the second
sacrificial layer 316 does not fill or occupy the entirety of the
spacings 314 due to the relative close spacing between adjacent
lower reinforcement sections 312 in relation to the height or
vertical extent of the lower reinforcement sections 312. This may
be characterized as "keyholing." In any case, the remaining voids
in the lower portion of each of the spacings 314 define a plurality
of at least generally laterally extending etch release channels
320.
[0162] The upper surface of the second sacrificial layer 316 may
retain a wavy or uneven contour. The upper surface of the second
sacrificial layer 316 may then be planarized in an appropriate
manner, such as by chemical mechanical polishing, to yield a
sufficiently flat upper surface for the second sacrificial layer
316 and as illustrated in FIG. 15C. In any case, the second
sacrificial layer 316 is then patterned to define a plurality of
interconnect apertures 324 as illustrated in FIG. 15D. These
interconnect apertures 324 at a minimum allow for establishing a
structural connection with the lower reinforcement sections 312,
and may be in the form of at least generally laterally extending
grooves or trenches (e.g. to define rails), or may be in the form
of a plurality of separate and discrete holes that are disposed in
spaced relation (e.g. to define a plurality of columns or posts).
In any case, each interconnect aperture 324 is disposed directly
above (e.g., vertically aligned) a corresponding lower
reinforcement section(s) 312, although the lower reinforcement
sections 312 will typically have a slightly larger width than their
corresponding interconnect aperture(s) 324.
[0163] A second structural layer 328 is deposited on the second
sacrificial layer 316 as illustrated in FIG. 15E. The material that
defines the second structural layer 328 is also deposited within
and at least substantially fills the interconnect apertures 324
that were previously formed in the second sacrificial layer 316,
and such may be characterized as being part of the second
structural layer 328. This portion of the second structural layer
328 may be characterized as a plurality of upper reinforcement
sections 332 that are the upper extreme of the reinforcing assembly
310 (FIG. 15G) for the microstructure 302 that is being fabricated.
Although FIG. 15E shows an intersection between the lower extreme
of each of the upper reinforcement sections 332 and the upper
extreme of their corresponding lower reinforcement section 312,
typically such an intersection will not exist and instead will at
least appear to be a continuous structure.
[0164] The upper surface of the second structural layer 328 will
typically have a wavy or uneven contour or at least a plurality of
laterally disposed and preferably axially extending depressions.
Generally, those portions of the second structural layer 328 that
are disposed between adjacent upper reinforcement sections 332 will
be recessed to a degree. Therefore, the upper surface of the second
structural layer 328 will typically be planarized in an appropriate
manner, such as by chemical mechanical polishing, to yield a
sufficiently flat upper surface for the second structural layer 328
and as illustrated in FIG. 15F. This completes the definition of
the microstructure 302. It should be appreciated that the system
that includes the microstructure 302 will likely include other
components than those illustrated in FIGS. 15A-G and that may
interface with the microstructure 302 in some manner (e.g., one or
more actuators).
[0165] The microstructure 302 is now ready to be released.
"Released" means to remove each sacrificial layer and thereby
including the first sacrificial layer 304 and the second
sacrificial layer 316. An etchant is used to provide the releasing
function. Predefined flow paths for this etchant are defined in and
extend through portions of the first sacrificial layer 304 and the
second sacrificial layer 316 in the form of the above-noted etch
release channels 320. The existence of the etch release channels
320 provides access to interiorly disposed locations within the
sacrificial layers for the etchant to complete the release before
the etchant has any significant adverse effect on the
microstructure 302.
[0166] As opposed to other of the reinforced microstructures
disclosed herein, the microstructure 302 is only a single
structural layer (second structural layer 328) that is structurally
reinforced by a plurality of "cantilevered" structures extending
downwardly therefrom, namely the plurality of at least generally
laterally extending lower reinforcement sections 312.
Notwithstanding the characterization of the structures 312 and 332
as "reinforcement sections," it should be appreciated that the
entire focus of the methodology of FIGS. 15A-G could in fact be to
simply provide a plurality of at least generally laterally
extending etch release conduits, to in turn provide a "rapid etch
release function" for the microstructure 302. That is, it is not
required that the structures 312 and 332 actually structurally
reinforce the microstructure 302, although such is preferably the
case. Therefore, the structures 312 could also be characterized as
etch release rails or the like.
[0167] The basic principle for forming etch release channels or
conduits encompassed by the methodology represented in FIGS. 15A-G
is that the deposition of a sacrificial material onto a layer
having at least one and more typically a plurality slots having a
height that is at least as great as the width produces a keyholing
effect at the bottom of the slot, which in turn defines an etch
release channel or conduit. The layer with the noted types of slots
may exist at various levels within the microstructure being
fabricated and yet still allow for the formation of etch release
channels or conduits in this same general manner. Moreover, a layer
with the noted types of slots does not need to be structurally
interconnected with the uppermost structural layer of the
microstructure being fabricated to allow for the formation of etch
release channels or conduits in this same general manner. For
instance, the layer with the noted types of slots instead could be
anchored to the underlying substrate or another underlying
structural layer. In fact, this layer with the noted types of slots
need not remain in the final structure of the microstructure being
fabricated at all, but instead may be removed during the release of
the microstructure from the substrate.
[0168] Another method for making another embodiment of a reinforced
microstructure 338 is illustrated in FIGS. 16A-C. FIG. 16A
illustrates a substrate 340 on which the microstructure 338 will be
fabricated. A first sacrificial layer 340 is deposited over the
substrate 336. The first sacrificial layer 340 may actually be a
plurality of sacrificial layers that are deposited at different
times in the process. In any case, the first sacrificial layer 340
is patterned to define a plurality of at least generally laterally
extending apertures 344 as illustrated in FIG. 16B. These apertures
344 do not extend down through the entire thickness of the first
sacrificial layer 340 in the illustrated embodiment, and may be
made by a timed etch. These apertures 344 may also be made by first
etching down to the substrate 336, and then backfilling with a
sacrificial material in a subsequent deposition to provide
apertures 344 of the desired depth.
[0169] The apertures 344 effectively function as a mold cavity and
may be in any desired shape for the resulting reinforcing
structure. For instance, the apertures 344 may be arranged to
define a plurality of at least generally laterally extending ribs
or rails (e.g., similar to the rails 118 of the mirror
microstructure 106 of FIG. 5; the rails 154 or 142 of the mirror
microstructure 130 of FIG. 7). Another option would be to arrange
the apertures 344 to define a waffle pattern, honeycomb pattern,
hexagonal pattern, or the like (e.g., a grid or network of
reinforcing structures), such as defined by a plurality of
intersecting rails that utilized by the mirror microstructure 166
of FIGS. 9A-B.
[0170] A first structural layer 348 is deposited on the first
sacrificial layer 340 as illustrated in FIG. 16C. The first
structural layer 348 extends within and occupies at least
substantially the entirety of each of the apertures 344 that were
previously formed in the first sacrificial layer 340. Those
portions of the first structural layer 348 that are disposed within
the apertures 344 may be characterized as a reinforcement
structures 352 that "cantilever" or extend downwardly from the
first structural layer 348 at least generally toward the underlying
substrate 336.
[0171] The upper surface of the first structural layer 348 may
retain a wavy or uneven contour. The upper surface of the first
structural layer 348 may then be planarized in an appropriate
manner, such as by chemical mechanical polishing, to yield a
sufficiently flat upper surface for the first support layer 348.
Thereafter, the first structural layer 348 is released by removing
the first sacrificial layer 340. A plurality of small etch release
holes (not shown) will extend through the entire vertical extent of
the first structural layer 348 to allow for the removal of the
first sacrificial layer 340 that is disposed between the first
structural layer 348 and the substrate 336. Therefore, the primary
benefit of the design of the microstructure 338 is the structural
reinforcement of the first structural layer 348 and the existence
of a relatively large space between the lower extreme of the
reinforcing assembly 352 and the substrate 336.
[0172] Another method for making a microstructure is illustrated in
FIGS. 17A-G. This methodology may be utilized to make the mirror
microstructure 106 of FIGS. 5-6, and the principles of this
methodology may be utilized in/adapted for the manufacture of the
mirror microstructure 30 of FIGS. 2-3, the mirror microstructure 58
of FIG. 4, the mirror microstructure 130 of FIG. 7, the mirror
microstructure 384 of FIGS. 10A-C, and the mirror microstructure
384' of FIG. 10D, as well as the variations therefore that are
presented in FIGS. 11A-F. In addition to being able to form a
desired reinforcing structure, the methodology of FIGS. 17A-G
further provides a desired manner for releasing the microstructure
at the end of processing by forming a plurality of at least
generally laterally extending etch release conduits in one or more
of its sacrificial layers to facilitate the removal thereof to
provide the releasing function.
[0173] Multiple layers are first sequentially deposited/formed over
an appropriate substrate 448 as illustrated in FIG. 17A, including
a first sacrificial layer 449, a first structural layer 450, and a
second sacrificial layer 454. The second sacrificial layer 454 is
then patterned to define a plurality of etch release conduit
apertures 458 as illustrated in FIG. 17B. These etch release
conduit apertures 458 may be in the form of at least generally
laterally extending grooves or trenches, and nonetheless are
defined by a pair of at least generally vertically disposed and
spaced sidewalls 462.
[0174] A third sacrificial layer 466 is then deposited on the
second sacrificial layer 454 as illustrated in FIG. 17C. The
material that defines the third sacrificial layer 466 is also
deposited within and at least substantially fills the etch release
conduit apertures 458 that were previously formed in the second
sacrificial layer 454, and such may be characterized as being part
of the third sacrificial layer 466. Although FIG. 17C shows an
intersection between the third sacrificial layer 466 and the second
sacrificial layer 454, typically such an intersection will not
exist and instead will at least appear to be continuous. In any
case, that portion of the third sacrificial layer 466 that is
deposited alongside the sidewalls 462 of the etch release conduit
apertures 458 will be of a lower density than other portions of the
third sacrificial layer 466, as well as the second sacrificial
layer 454 and first sacrificial layer 449 for that matter. These
low density regions ultimately become a plurality of etch release
channels as will be discussed in more detail below. It should be
appreciated that the spacing between the sidewalls 462 of each
aperture 458 may also be subject to the types of "keyholing"
effects discussed above in relation to the methodology of FIGS.
15A-G. That is, in the event that the ratio of the height of the
sidewalls 462 of a given etch release conduit aperture 458 to the
spacing between these two sidewalls 462 is at least about 1:1, an
etch release conduit or channel will also develop in the lower
portion of this etch release conduit aperture 458 due to the
"closing" off of the upper portion of this etch release conduit
aperture 458 during the deposition of the third sacrificial layer
466.
[0175] The upper surface of the third sacrificial layer 466 may
retain a wavy or uneven contour, as illustrated in FIG. 17C.
Generally, those portions of the third sacrificial layer 466 that
are disposed over the etch release conduit apertures 458 may be
disposed at a lower elevation than those portions of the third
sacrificial layer 466 that are disposed between adjacent etch
release conduit apertures 458. In the event that this is the case
and as illustrated in FIG. 17D, the upper surface of the third
sacrificial layer 466 may be planarized in an appropriate manner,
such as by chemical mechanical polishing, to yield a sufficiently
flat upper surface for the stack as thus far defined. This
planarization may totally eliminate the third sacrificial layer 466
except from within the etch release conduit apertures 458 as shown,
or the third sacrificial layer 466 may remain as a continuous layer
on the first sacrificial layer 454 (not shown, but in the manner
depicted in FIG. 19D discussed below).
[0176] Reinforcement structures may be incorporated into the
microstructure using the method of FIGS. 17A-G. In this regard, the
second sacrificial layer 454, as well as any overlying portion of
the third sacrificial layer 466, may be patterned to define a
plurality of interconnect apertures 470 as illustrated in FIG. 17E.
Notably, these interconnect apertures 470 are disposed between the
etch release conduit apertures 458 that now have the material of
the third sacrificial layer 466 therein. That is, preferably none
of the interconnect apertures 470 extend downwardly through and/or
intersect any of the etch release conduit apertures 458 that now
include material from the third sacrificial layer 466. These
interconnect apertures 470 allow for establishing a structural
interconnection with the first structural layer 450, and may be in
various forms. For instance, these interconnect apertures 470 may
be in the form of at least generally laterally extending grooves or
trenches (e.g., to define a plurality of rails or at least the
upper portion thereof and of the type utilized by the
microstructure 106 of FIG. 5 and the microstructure 130 of FIG. 7,
as well as the variations therefore illustrated in FIGS. 11A-F), or
may be in the form of a plurality of separate and discrete holes
that are disposed in spaced relation (e.g., to define a plurality
of posts or columns of the type utilized by the mirror
microstructure 30 of FIGS. 2-3).
[0177] A second structural layer 474 is deposited on any exposed
portions of the second sacrificial layer 454 and the third
sacrificial layer 466, as illustrated in FIG. 17F. The material
that defines the second structural layer 474 is also deposited
within and at least substantially fills the interconnect apertures
470 within the second sacrificial layer 454, and such may be
characterized as being part of the second structural layer 474.
This portion of the second structural layer 474 may be
characterized as a plurality of reinforcement sections 478 for the
microstructure 476 that is being fabricated. Although FIG. 17F
shows an intersection between the lower extreme of each of the
reinforcement sections 478 and the upper extreme of first
structural layer 450, typically such an intersection will not exist
and instead will at least appear to be continuous.
[0178] The upper surface of the second structural layer 474 may
retain have a wavy or uneven contour. Generally, those portions of
the second structural layer 474 that are disposed over the
reinforcement sections 478 may be recessed to a degree. Therefore,
the upper surface of the second structural layer 474 may be
planarized in an appropriate manner, such as by chemical mechanical
polishing, to yield a sufficiently flat upper surface for the
second structural layer 474 and as illustrated in FIG. 17F. This
completes the definition of the microstructure 476. It should be
appreciated that the system that includes the microstructure 476
will likely include other components than those illustrated in
FIGS. 17A-G and that may interface with the microstructure 476 in
some manner (e.g., one or more actuators). Moreover, it should be
appreciated that the steps illustrated in FIGS. 17A-G may be
repeated in an appropriate manner in order to define a
microstructure of the type presented in FIGS. 4 and 7 (e.g., three
or more spaced, but structurally interconnected, structural
layers).
[0179] The microstructure 476 is now ready to be released.
"Released" means to remove each of the sacrificial layers of the
surface micromachined system and thereby including the first
sacrificial layer 449, the second sacrificial layer 454, and the
third sacrificial layer 466. An etchant is used to provide the
releasing function. The manner in which the microstructure 476 was
formed in accordance with the methodology of FIGS. 17A-G reduces
the time required for the sacrificial layers 454 and 466 to be
totally removed. Ultimately, a plurality of at least generally
laterally extending etchant flow pipes, channels, or conduits are
formed in the third sacrificial layer 466. Those portions of the
third sacrificial layer 466 that are positioned alongside the
sidewalls 462 of the etch release conduit apertures 458 that were
formed in the second sacrificial layer 454 are less dense than the
remainder of the third sacrificial layer 466. The etch rate will be
greater in the low density regions of the third sacrificial layer
466 than throughout the remainder of the third sacrificial layer
466. This will effectively form an etch release pipe, channel or
conduit in the third sacrificial layer 466 along each sidewall 462.
The development of the etch release channels during the initial
portion of the release etch provides access to radially inwardly
disposed locations within the sacrificial layers for the etchant to
complete the release before the etchant has any significant adverse
effect on the microstructure 476.
[0180] The methodology represented by FIGS. 17A-G provides a number
of advantages. One is that the first sacrificial layer 454 may be
patterned to define any appropriate arrangement for the etch
release conduit apertures 458 within/throughout a sacrificial
layer(s) (and thereby an arrangement for the low density regions
which will ultimately define the etch release conduits), including
an arrangement where one or more of these etch release conduit
apertures 458 intersect. That is, the formation of the etch release
conduits is not adversely affected by having the low density
regions intersect.
[0181] FIGS. 18A-B present one arrangement where the first
sacrificial layer 454 has been patterned to define a network 482 or
grid of one embodiment of intersecting/interconnected etch release
conduit apertures 458. The network 482 of
intersecting/interconnected etch release conduit apertures 458
increases the amount of the sacrificial layer that is initially
exposed to the release etchant within radially inward locations,
and thereby should reduce the total amount of time required to
release the microstructure 476 that is being formed. The various
etch release conduit apertures 458 may be routed to define a
desired network 482 for distribution of the release etchant
throughout the first sacrificial layer 454 to not only reduce this
total etch release time, but to also allow for use of a desired
reinforcing structure. In this regard, FIG. 18B illustrates one
embodiment that may be used for the reinforcement sections 478 in
combination with the network 482 of FIG. 18A. The reinforcement
sections 478 of FIG. 18B are in the form of a plurality of spaced
posts or columns that structurally interconnect the first
structural layer 450 and the second structural layer 474 of the
microstructure 459 (e.g., for defining a microstructure of the type
illustrated in FIG. 2A).
[0182] Another advantage associated with the manner in which the
etch release conduits are formed in the methodology of FIGS. 17A-G,
is that these etch release conduits may be formed without requiring
the use of any reinforcement structures. This is illustrated by the
sequential view presented in FIGS. 19A-F. Multiple layers are
sequentially deposited/formed over an appropriate substrate 488,
including a first sacrificial layer 490 as illustrated in FIG. 19A.
The first sacrificial layer 490 is then patterned to define a
plurality of etch release conduit apertures 492 as illustrated in
FIG. 19B. These etch release conduit apertures 492 may be of the
type used by the methodology of FIGS. 17A-G, and are defined by a
pair of at least generally vertically disposed and spaced sidewalls
494.
[0183] A second sacrificial layer 496 is then deposited on the
first sacrificial layer 490, as illustrated in FIG. 19C. The
material that defines the second sacrificial layer 496 is also
deposited within and at least substantially fills the etch release
conduit apertures 492 that were previously formed in the first
sacrificial layer 490, and such may be characterized as being part
of the second sacrificial layer 496. Although FIG. 19C shows an
intersection between the second sacrificial layer 496 and the third
sacrificial layer 490, typically such an intersection will not
exist and instead will at least appear to be continuous. In any
case, that portion of the second sacrificial layer 496 that is
deposited alongside the sidewalls 494 of the etch release conduit
apertures 492 will be of a lower density than other portions of the
second sacrificial layer 496, as well as the first sacrificial
layer 490 for that matter.
[0184] The upper surface of the second sacrificial layer 496 may
retain a wavy or uneven contour, as illustrated in FIG. 19C. The
upper surface of the second sacrificial layer 496 may be planarized
in an appropriate manner, such as by chemical mechanical polishing,
to yield a sufficiently flat upper surface for the stack as thus
far defined and as illustrated in FIG. 19D. This planarization may
totally eliminate the second sacrificial layer 496 except from
within the etch release conduit apertures 492 (not shown), or the
second sacrificial layer 496 may remain as a continuous layer on
the first sacrificial layer 490 as shown in FIG. 19D.
[0185] A first structural layer 498 is then deposited on any
exposed portions of the first sacrificial layer 490 and the second
sacrificial layer 496, as illustrated in FIG. 19E. This completes
the definition of the microstructure 486. It should be appreciated
that the system that includes the microstructure 486 will likely
include other components than those illustrated in FIGS. 19A-F and
that may interface with the microstructure 486 in some manner
(e.g., one or more actuators).
[0186] The microstructure 486 is now ready to be released.
"Released" means to remove each of the sacrificial layers of the
surface micromachined system and thereby including the first
sacrificial layer 490 and the second sacrificial layer 496. An
etchant is used to provide the releasing function. The manner in
which the microstructure 486 was formed in accordance with the
methodology of FIGS. 19A-F reduces the time required for the
sacrificial layers 490 and 496 to be totally removed. Ultimately, a
plurality of at least generally laterally extending etchant flow
pipes, channels, or conduits are formed in the second sacrificial
layer 496. Those portions of the second sacrificial layer 496 that
are positioned alongside the sidewalls 494 of the etch release
conduit apertures 492 that were formed in the first sacrificial
layer 490 are less dense than the remainder of the second
sacrificial layer 496. The etch rate will be greater in the low
density regions of the second sacrificial layer 496 than throughout
the remainder of the second sacrificial layer 496. This will
effectively form an etch release pipe, channel or conduit in the
second sacrificial layer 496 along each sidewall 494. The
development of the etch release channels during the initial portion
of the release etch provides access to radially inwardly disposed
locations within the sacrificial layers for the etchant to complete
the release before the etchant has any significant adverse effect
on the microstructure 486.
[0187] Etch release channels that exist prior to the release of the
microstructure 486 may also be formed by the "keyholing" concept
discussed above in relation to FIGS. 15A-G. Generally, a
relationship between the width of the etch release conduit
apertures 492 and the height or vertical extent of these etch
release conduit apertures 492 may be selected to allow a plurality
of etch release channels to be defined in the lower portion of
these apertures 492. When this relationship is selected in the same
general manner discussed above in relation to FIGS. 15A-G, the
second sacrificial layer 496 will extend within and occupy only a
portion of each of the apertures 492 that were previously formed
from the first sacrificial layer 490. However, the material that
defines the second sacrificial layer 496 will not fill or occupy
the entirety of the apertures 492 due to the relative close spacing
between the sidewalls 494 that define the etch release conduit
apertures 492 in relation to the height or vertical extent of these
apertures 492 (i.e., a void will remain in the lower portion of
each aperture 492). This again may be characterized as "keyholing."
In any case, the remaining voids in the lower portion of each of
the etch release conduit apertures 492 will define a plurality of
at least generally laterally extending etch release channels.
[0188] Another method for making a microstructure is illustrated by
reference to FIGS. 20A-D. The fundamental principles of this
methodology may be utilized to make the mirror microstructure 30 of
FIG. 2A, the mirror microstructure 58 of FIG. 4, the mirror
microstructure 106 of FIGS. 5-6, the mirror microstructure 130 of
FIG. 7, the mirror microstructure 384 of FIGS. 10A-C, the mirror
microstructure 384' of FIG. 10D, as well as the variations
therefore that are presented in FIGS. 11A-F. In addition to
accommodating the fabrication of at least certain types of
reinforcing structures for the microstructure, the methodology
embodied by FIGS. 20A-D further provides a desired manner for
releasing the microstructure at the end of fabrication by forming a
plurality of at least one at least generally laterally extending
etch release conduit within one or more of its sacrificial layers
to facilitate the removal of this sacrificial material during the
release of the microstructure from the substrate.
[0189] Multiple layers of at least two different types of materials
are sequentially deposited to define a stack 594 on a substrate 582
that is appropriate for surface micromachining. The various
deposition and patterning steps that may yield the configuration
illustrated in FIG. 20A have been sufficiently described in
relation to other embodiments, and will not be repeated. The stack
594 includes a microstructure 592, which in the illustrated
embodiment is in the form of a single structural layer 590 that is
disposed in spaced relation to the substrate 582. Any configuration
that is appropriate for the manner of defining etch release
channels in the manner contemplated by FIGS. 20A-D may be utilized
for the microstructure 592, including where the structural layer
590 is structurally reinforced in an appropriate manner. In any
case, the stack 594 also includes an etch release conduit fill
material 586 that is encased within a sacrificial material 584 both
within the area occupied by the microstructure 592 and laterally
beyond the microstructure 592 or "off to the side" of the
microstructure 592. Generally, this etch release conduit fill
material 586 is removed by a first etchant to form at least one
laterally extending etch release conduit 602 at least somewhere
underneath at least one structural layer of the microstructure 592.
That is, the first etchant is more selective to the etch release
conduit fill material 586 than the sacrificial material 584 in an
amount such that it does not remove any significant portion of the
sacrificial material 584. Thereafter, a second, different etchant
(i.e., a release etchant) enters the etch release conduit(s) 602.
This second, different etchant removes at least that portion of the
sacrificial material 584 which is accessible through the etch
release conduit(s) 602 to release the microstructure 592 from the
substrate 582. That is, the second etchant is more selective to the
sacrificial material 584 than the structural layer 590 in an amount
such that it does not remove any significant portion of the
structural layer 590.
[0190] The etch release conduit fill material 586 may be disposed
at one or more levels relative to the substrate 582 within the
microstructure 592. An at least generally vertically extending
runner 588 of etch release conduit material 586 is disposed
laterally beyond the area occupied by the microstructure 592 (i.e.,
off to the side), and extends at least generally downwardly from an
uppermost exterior surface 596 of the stack 594 toward the
substrate 582 to the etch release conduit fill material 586 at one
or more of the levels within the stack 594. Any appropriate number
of runners 588 may be utilized, and each may be of any appropriate
configuration.
[0191] The etch release conduit fill material 586 at any level
within the stack 594 may occupy the entirety of this level under at
least one structural layer of the microstructure 592. More
typically, a patterning operation will have been done at a given
level to define an appropriate layout of etch release rails from
the etch release conduit fill material 586. FIG. 20B illustrates
one embodiment of a layout of etch release rails 598 of etch
release conduit fill material 586 that are in the form of a grid or
network. Other layouts for the etch release rails 598 may be
utilized, including without limitation those discussed above. In
the case of the embodiment of FIG. 20B, sacrificial material 584 is
also disposed at the same level within the stack 594 as the etch
release rails 598 (i.e., in the space between adjacent etch release
rails 598). Reinforcement structures 600 also may be disposed at
the same level within the stack 594 as the etch release rails 598
in this same space as well if desired. These reinforcement
structures 600 are separated from the etch release conduit fill
material 586 by sacrificial material 584. That is, the etch release
conduit fill material 584 is encased within sacrificial material
584.
[0192] Regardless of the layout of the etch release rails 598 of
etch release conduit fill material 586, the microstructure 592 is
released in the same general manner. Initially, the stack 594 is
exposed to a first etchant that is selective to the etch release
conduit fill material 586. In one embodiment, the etch release
conduit fill material 586 is the same material that is used to form
the various structural layers that define the microstructure 592.
In this case, it is necessary for the various structural layers of
the microstructure 592 to be isolated from the etch release conduit
fill material 586 by sacrificial material 584. There may be
instances where a particular etchant may be sufficiently selective
to the etch release conduit fill material 586 so as to not require
this isolation of the structural material of the microstructure 592
from the etch release conduit fill material 586.
[0193] The first etchant removes the etch release conduit fill
material 586 within the runner 588 to define an access 604, as well
as any etch release rails 598 of etch release conduit fill material
586 connected therewith. This first etchant preferably does not
remove any significant portion of any sacrificial material 584 that
encases the etch release rails 598 and/or the runner(s) 588. The
resulting void by this removal of material of the etch release
rails 598 defines at least one etch release conduit 602 that is at
least generally laterally extending and disposed under at least one
structural layer of the microstructure 592 (FIG. 20C). In the
layout of etch release rails 598 illustrated in FIG. 20B, there
would be a grid or network of etch release conduits 602 within the
sacrificial material 584 disposed under the area occupied by the
structural layer 590.
[0194] There is a separate and distinct second etching operation in
accordance with the methodology of FIGS. 20A-D. After the first
etching operation has been executed to define at least one and more
typically a plurality of etch release conduits 602, the stack 594
undergoes a second etching operation. The second etching operation
uses a second etchant that is different from the first etchant, and
that is selective to the sacrificial material 584. This second
etchant flows down through the various accesses 604 that may be
associated with the stack 594 and into any etch release conduit 602
fluidly interconnected therewith. The second etchant removes any
sacrificial material 584 in contact therewith to release the
microstructure 592 from the substrate 582 (FIG. 20D). In the event
that the etch release conduit fill material 586 is polysilicon,
representative examples for the first etchant would be potassium
hydroxide, tetramethylammonium hydroxide, and xenon difluoride.
Assuming that the sacrificial material 584 is doped or undoped
silicone dioxide or silicone oxide, representative examples for the
second etchant would be HF-based, including those identified
above.
[0195] The foregoing description of the present invention has been
presented for purposes of illustration and description.
Furthermore, the description is not intended to limit the invention
to the form disclosed herein. Consequently, variations and
modifications commensurate with the above teachings, and skill and
knowledge of the relevant art, are within the scope of the present
invention. The embodiments described hereinabove are further
intended to explain best modes known of practicing the invention
and to enable others skilled in the art to utilize the invention in
such, or other embodiments and with various modifications required
by the particular application(s) or use(s) of the present
invention. It is intended that the appended claims be construed to
include alternative embodiments to the extent permitted by the
prior art.
* * * * *