U.S. patent application number 10/790945 was filed with the patent office on 2004-09-02 for electrically isolated support for overlying mem structure.
Invention is credited to Rodgers, Murray Steven.
Application Number | 20040171206 10/790945 |
Document ID | / |
Family ID | 31715223 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040171206 |
Kind Code |
A1 |
Rodgers, Murray Steven |
September 2, 2004 |
Electrically isolated support for overlying MEM structure
Abstract
MEM devices are fabricated with integral dust covers, cover
support posts and particle filters for reduced problems relating to
particle contamination. In one embodiment, a MEM device (10)
includes an electrostatic actuator (12) that drives a movable frame
(14), a displacement multiplier (16) for multiplying or amplifying
the displacement of the movable frame (14), and a displacement
output element (18) for outputting the amplified displacement. The
actuator (12) is substantially encased within a housing formed by a
cover (36) and related support components disposed between the
cover (36) and the substrate (38). Electrically isolated support
posts may be provided in connection with actuator electrodes to
prevent contact between the cover and the underlying electrodes.
Such a support post may also incorporate an electric filter element
for filtering undesired components from a drive signal. Particle
filters may be provided in connection with etch release holes or
other openings in order to further protect against particle
contamination.
Inventors: |
Rodgers, Murray Steven;
(Albuquerque, NM) |
Correspondence
Address: |
MARSH FISCHMANN & BREYFOGLE LLP
Suite 411
3151 South Vaughn Way
Aurora
CO
80014
US
|
Family ID: |
31715223 |
Appl. No.: |
10/790945 |
Filed: |
March 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10790945 |
Mar 2, 2004 |
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10224207 |
Aug 20, 2002 |
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6700173 |
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Current U.S.
Class: |
438/231 |
Current CPC
Class: |
B81B 7/0012
20130101 |
Class at
Publication: |
438/231 |
International
Class: |
H01L 021/8238 |
Claims
What is claimed:
1. A MEM apparatus comprising: a substrate; a first structure
supported on said substrate; an electrostatic component disposed
between said substrate and said first structure and extending
across an area of said substrate; and at least one support
structure disposed within said area of said substrate for
supporting said first structure.
2. A MEM apparatus as set forth in claim 1, wherein said
electrostatic component comprises a movable electrode.
3. A MEM apparatus as set forth in claim 1, wherein said
electrostatic component comprises a static electrode.
4. A MEM apparatus as set forth in claim 1, wherein said
electrostatic component includes a movable electrode and a static
electrode.
5. A MEM apparatus as set forth in claim 4, wherein said support
structure is positioned proximate to said static electrode.
6. A MEM apparatus as set forth in claim 1, wherein said support
structure has a height, relative to an axis extending between said
first structure and said substrate, greater than a height of said
electrostatic component, wherein said support structure maintains a
separation between said first structure and said electrostatic
component.
7. A MEM apparatus as set forth in claim 1, wherein said at least
one support structure comprises multiple support structures
distributed across an area of said first structure.
8. A MEM apparatus as set forth in claim 1, wherein said support
structure is substantially electrically isolated from said
electrostatic component.
9. An MEM apparatus as set forth in claim 1, wherein said
electrostatic element comprises a movable element that is movable
across a range of positions and said support structure is
positioned to avoid mechanical interference with said movable
element as said movable element moves across said range of
positions.
10. A MEM apparatus comprising: a substrate; a first structure
supported on said substrate; a movable component disposed between
said substrate and said first structure and extending across an
area of said substrate; and at least one support structure disposed
within said area of said substrate for supporting said first
structure.
11. A MEM apparatus as set forth in claim 10, wherein said movable
component includes a movable electrode and a static electrode and
said support structure is positioned proximate to said static
electrode.
12. A MEM apparatus as set forth in claim 10, wherein said support
structure has a height, relative to an axis extending between said
first structure and said substrate, greater than a height of said
movable component, wherein said support structure maintains a
separation between said first structure and said movable
component.
13. A MEM apparatus as set forth in claim 10, wherein said at least
one support structure comprises multiple support structures
distributed across an area of said first structure.
14. A MEM apparatus as set forth in claim 10, wherein said support
structure is substantially electrically isolated from said movable
component.
15. An MEM apparatus as set forth in claim 10, wherein said movable
component is movable across a range of positions and said support
structure is positioned to avoid mechanical interference with said
movable component as said movable component moves across said range
of positions.
16. A MEM apparatus, comprising: a movable optical component; an
actuator mechanism for effecting movement of said optical
component; a cover extending over at least a portion of said
actuator mechanism, said cover further extending across an area;
and at least one support structure, disposed within said area, for
supporting said cover.
17. A MEM apparatus as set forth in claim 16 wherein said actuator
mechanism comprises a movable electrode and a static electrode and
said support structure is positioned proximate to said static
electrode.
18. A MEM apparatus as set forth in claim 16, wherein said support
structure has a height, relative to an axis extending between said
cover and a bottom surface of said actuator mechanism, greater than
a height of said actuator mechanism, wherein said support structure
maintains a separation between said cover and said actuator
mechanism.
19. A MEM apparatus as set forth in claim 16, wherein said at least
one support structure comprises multiple support structures
distributed across an area of said cover.
20. A MEM apparatus as set forth in claim 16, wherein said support
structure is substantially electrically isolated from said actuator
mechanism.
21. An MEM apparatus as set forth in claim 16, wherein said
actuator mechanism comprises a movable component that is movable
across a range of positions and said support structure is
positioned to avoid mechanical interference with said movable
component as said movable component moves across said range of
positions.
22. A MEM apparatus, comprising: a substrate; and a micromachined
structure formed on said substrate including: an electrical lead
supported on said substrate; and filter structure, supported on
said substrate, for filtering an undesired electrical component
from said lead.
23. An apparatus as set forth in claim 22, wherein said filter
comprises structure for establishing a capacitance between said
structure and said electrical lead.
24. A method for use in constructing a MEM device, comprising the
steps of: first establishing an active component on a substrate,
said active component comprising one of movable component and an
electrostatic component, said active component extending across an
area of said substrate; second establishing an overlying structure
extending over at least a portion of said active component; and
third establishing at least one support structure within said area
for supporting said overlying structure.
25. A method as set forth in claim 24, wherein said active
component comprises a movable electrode and a static electrode and
said step of third establishing comprises positioning said support
structure proximate to said static electrode.
26. A MEM apparatus as set forth in claim 24, wherein said step of
third establishing comprises forming said support structure such
that said support structure has a height, relative to an axis
extending between said overlying structure and said substrate,
greater than a height of said active component, wherein said
support structure maintains a separation between said overlying
structure and said active component.
27. A method as set forth in claim 24, wherein said step of third
establishing comprises forming multiple support structures
distributed across an area of said overlying structure.
28. A method as set forth in claim 24, wherein said step of third
establishing comprises substantially electrically isolating said
support structure from said active component.
29. A method as set forth in claim 24, wherein said active
component comprises a movable component that is movable across a
range of positions and said step of third establishing comprises
positioning said support structure to avoid mechanical interference
with said movable component as said movable component moves across
said range of positions.
Description
RELATED APPLICATIONS
[0001] This patent application claims priority from U.S. patent
application Ser. No. 10/224,207 that was filed on Aug. 20, 2002,
entitled "ELECTRICALLY ISOLATED SUPPORT FOR OVERLYING MEM
STRUCTURE". The entire disclosure of U.S. patent application Ser.
No. 10/224,207 is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to micromechanical
or microelectromechanical (collectively "MEM") systems and, in
particular, to the provision and use of covers in connection with
components or substrate areas of such systems. Such covers extend
over and may substantially encase the protected areas or components
to protect against particle contamination.
BACKGROUND OF THE INVENTION
[0003] MEM systems include highly miniaturized devices that employ
electrical and/or mechanical components formed on a substrate.
There are a number of fabrication technologies, collectively known
as micromachining, for producing MEM systems. One type of
micromachining process is surface micromachining. Surface
micromachining generally involves deposition and photolithographic
patterning of alternate layers of structural material (typically
polycrystalline silicon, termed polysilicon) and sacrificial layers
(typically silicon dioxide, termed oxide) on a silicon wafer
substrate material. Using a series of deposition and patterning
steps, functional devices are constructed layer by layer. After a
device is completed, it is released by removing all or some of the
remaining sacrificial material by exposure to a selective etchant
such as hydrofluoric acid, which does not substantially attack the
polysilicon layers.
[0004] A potential problem in connection with MEM systems relates
to particle contamination. Particle contamination can potentially
impair or disable a system by interfering with the electrical
signals and/or mechanical movements of some or all of the
electrical and/or mechanical devices. Electrostatic components,
such as actuators, are particularly susceptible to particle
contamination as particles may be electrically attracted to such
components and may cause electrical shorts. Various movable
elements may be susceptible to mechanical interference due to
particle contamination. Such contamination can occur during
construction/assembly or during operation. Completed systems are
typically packaged so as to reduce exposure to potential
contaminants from the ambient environment, but significant levels
of contaminants may still occur within such packaging, thereby
reducing yield and potentially allowing for malfunctions after
system deployment. In many environments, including MEM-based
optical switches, such malfunctions could entail substantial
expense and inconvenience, e.g., associated with switch down time,
network reconfiguration and repair or replacement.
SUMMARY OF THE INVENTION
[0005] The present invention is directed to shielding components of
a MEM system or substrate areas (together with any overlying
structure) from particle contamination. In this manner the yield
and reliability in operation of MEM systems can be improved.
Additionally, reduced susceptibility of MEM systems to particle
contamination allows for construction and assembly of MEM systems
under more practical conditions relating to cleanliness, thereby
reducing costs. The invention thereby facilitates more practical
and cost effective MEM system construction and assembly, including
for high criticality applications such as MEM-based optical
switches.
[0006] In accordance with one aspect of the present invention, a
cover is provided to protect an active component of a MEM apparatus
from particle contamination. The cover extends over and,
preferably, substantially encases the active component. The
associated MEM apparatus includes a substrate, an active component
formed on the substrate, and a cover formed on the substrate and
extending over the active component. An associated process involves
establishing an active component on a substrate and establishing a
cover on the substrate extending over the active component. The
active component and cover are preferably formed on the substrate
by a surface micromachining process.
[0007] The active component may include an electrostatic element
and/or a movable element. In this regard, an electrostatic actuator
is an example of a component that includes both electrostatic and
movable elements. As noted above, electrostatic elements are a
particular concern with respect to particle contamination because
such elements may attract charged particles and such particles may
cause short circuits or other malfunctions. In this regard,
electrostatic components include components that receive a voltage
in operation such that an electrical potential is established
relative to other components or structure of the device. Similarly,
movable elements are a concern with respect to particle
contamination because particles may mechanically interfere with
movement.
[0008] The cover may extend over the entirety of the active
component or over an area of the component, e.g., a critical area
with respect to movement or likelihood of particle attraction. It
will be appreciated that in some cases, such as typical actuator
implementations, the cover will include openings or otherwise
terminate so as to allow the covered component to mechanically
and/or electrically interface with cooperating elements. Moreover,
the cover may be an uninterrupted web of material or may be
intermittent (e.g., formed as a grid or screen) or otherwise
include openings. In this regard, openings may be provided to
facilitate penetration of an etchant during a release process. In
cases where the cover includes openings, such openings are
preferably dimensioned to minimize penetration of potentially
harmful particles, e.g., having a maximum dimension of less than
about 5 microns and, more preferably, less than about 2 microns.
Filters may be provided in connection with such openings to further
reduce the potential for particle contamination.
[0009] In one embodiment, the MEM apparatus is an optical control
apparatus such as for moving a micromirror, microlens, shutter or
other movable optical component. The apparatus includes: a movable
optical component; an actuator mechanism, formed on a substrate,
for effecting movement of the optical component; and a cover
supported on the substrate and extending over the actuator
mechanism. The actuator is preferably movable in response to
electric control signals and may include at least one electrostatic
element and at least one movable link for use in transmitting
motion to the optical component. The cover may extend over at least
a portion of the electrostatic element and/or link. Such an
apparatus may be implemented in connection with micromirror-based
optical systems such as 1.times.N or N.times.N optical
cross-connect switches, multiplexers, demultiplexers,
spectrometers, etc.
[0010] It has been recognized that structural issues have the
potential to interfere with successful implementation of covers, or
other large area structures, for certain applications. In
particular, in order to provide the desired particle protection in
connection with certain components such as certain electrostatic
actuators, the cover may be required to extend over a substantial
area, e.g., the cover may have a maximum dimension of greater than
hundreds of microns or even greater than several millimeters. In
such cases, the cover may be drawn along an axis transverse to the
substrate surface (e.g., down towards underlying structure) so as
to potentially cause short circuits or otherwise interfere with
operation of adjacent components or prevent proper release. This
may be a particular concern where the cover extends over very large
areas or where the cover extends over electrostatic elements that
may attract the cover. Other forces that may act on the cover
include meniscus forces, stiction and loads from interconnected
structure.
[0011] In this regard, in accordance with another aspect of the
present invention, at least one support structure such as a post is
used to support an overlying structure of a MEM apparatus. The
corresponding apparatus includes: a substrate; an active component
supported on the substrate and extending across a first area of the
substrate; an overlying structure supported on the substrate and
extending over the first area; and a support structure disposed in
the first area for supporting the overlying structure. The active
component may include an electrostatic and/or a movable element.
The overlying structure may be a cover or other element. The
support structure preferably extends across space occupied by
active component between the overlying structure and the substrate.
For example, the support structure may extend from the substrate to
the overlying structure.
[0012] The support structure can be implemented so as to minimize
the potential for electrical or mechanical interference with the
active component. In this regard, where the active component
includes movable elements, the position of the support structure
can be selected with due regard for the expected range of motion of
the movable elements so as to avoid mechanical interference between
the support structure and movable elements. Where the active
component includes electrostatic elements, the support structure
may be configured to avoid disruption or contact with elements
and/or may be otherwise electrically isolated therefrom.
[0013] According to another aspect of the present invention, an
electronic filter may be integrally formed as part of a MEM
apparatus. Various types of MEM devices include conductors for
transmitting signals such as control signals for controlling
movement or other operation of active components. In some cases,
very accurate control of these components may be required.
Unfortunately, high performance microelectromechanical actuation
systems may be susceptible to very low levels of electrical noise
or other artifacts of the control signals. The potential for such
problems increases with progressive miniaturization.
[0014] An apparatus according to this aspect of the present
invention includes: a substrate; an electrical conductor supported
on the substrate; and a filter formed on the substrate for
filtering artifacts from an electrical signal transmitted by the
conductor. For example, the filter may function to apply a
capacitance in the pathway of the conductor or in parallel with an
electrical feature of the conductor pathway. The filter may thereby
provide a frequency dependent filtering function. In one
embodiment, filter material is formed in proximity to the conductor
but separated from the conductor by air or insulating material. The
filter material may be grounded or otherwise controlled to have
desired characteristics. A capacitance is thereby established
between the conductor and adjacent structure. The capacitance may
be selected to impart desired filtering characteristics, e.g.,
through appropriate selection of materials, dimensions,
configurations and electrical properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] For a more complete understanding of the present invention
and further advantages thereof, reference is now made to the
following Detailed Description taken in conjunction with the
drawings, in which:
[0016] FIG. 1 is a perspective view of a MEM device including a
dust cover in accordance with the present invention;
[0017] FIG. 2 is a perspective view of a base structural layer of
the MEM device of FIG. 1;
[0018] FIG. 3 is a perspective view showing a first sacrificial
layer of the MEM device of FIG. 1;
[0019] FIG. 4 is a perspective view showing a second structural
layer of the MEM device of FIG. 1;
[0020] FIG. 5 is a perspective view showing a second sacrificial
layer of the MEM device of FIG. 1;
[0021] FIG. 6 is a perspective view showing a third structural
layer of the MEM device of FIG. 1;
[0022] FIG. 7 is a perspective view showing a third sacrificial
layer of the MEM device of FIG. 1;
[0023] FIG. 8 is a perspective view showing a fourth structural
layer of the MEM device of FIG. 1;
[0024] FIG. 9A is a perspective, partial cross-sectional view
showing an electrical contact of the MEM device of FIG. 1;
[0025] FIG. 9B is a perspective partial cross-sectional view
showing shielded electrodes in combination with a cover in
accordance with the present invention;
[0026] FIGS. 10A and 10B show a close up of the interface between
the actuator and the displacement multiplier of the MEM device of
FIG. 1;
[0027] FIG. 11 is a close up perspective view showing the relative
geometry of the outer support posts and electrodes of the MEM
device of FIG. 1;
[0028] FIG. 12 is a perspective, close up view showing the
interface between the central support posts and the movable frame
of the MEM device of FIG. 1;
[0029] FIG. 13A is a perspective view showing the relative geometry
between a portion of the cover and underlying electrodes of the MEM
device of FIG. 1;
[0030] FIG. 13B is a partial perspective view of a MEM device in
accordance with the present invention showing the interface between
electrodes and electrically isolated support posts;
[0031] FIG. 14 is a bottom perspective view of the structure of
FIG. 13B;
[0032] FIG. 15 is a top perspective view, partially cut away
showing details of the structure of FIG. 13B;
[0033] FIG. 16 illustrates an example of a microelectromechanical
system configured with a filter system according to the present
invention;
[0034] FIG. 17 illustrates an example of a filter system according
to the present invention;
[0035] FIG. 18 illustrates an example of the fabrication of the
filter system of FIG. 2;
[0036] FIG. 19 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0037] FIG. 20 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0038] FIG. 21 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0039] FIG. 22 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0040] FIG. 23 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0041] FIG. 24 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0042] FIG. 25 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0043] FIG. 26 illustrates additional details of the fabrication of
the filter system of FIG. 2;
[0044] FIG. 27 illustrates another example of a filter system
according to the present invention;
[0045] FIG. 28 illustrates another example of a filter system
according to the present invention;
[0046] FIG. 29 illustrates another example of a filter system
according to the present invention; and
[0047] FIG. 30 illustrates another example of a filter system
according to the present invention.
DETAILED DESCRIPTION
[0048] Reference will now be made to the accompanying drawings,
which at least assist in illustrating the various pertinent
features of the present invention. For purposes of illustration,
the following description is related to the formation of covers and
support posts for covers or other overlying layers for
microelectromechanical (MEM) systems, although it will be
appreciated that the such structure is useful for both
micromechanical and microelectromechanical systems. In addition,
one or more micro-devices or microstructures may define any given
micromechanical or microelectromechanical system.
[0049] Surface micromachining is a preferred type of technique for
fabricating the structures described herein, although other
techniques may be utilized as well. Moreover, in certain instances
it may be desirable to use a combination of two or more fabrication
techniques to define a given MEM system. Since surface
micromachining is a preferred fabrication technique for the MEM
systems described herein, the basic principles of surface
micromachining will first be described. Initially, 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.
[0050] Surface micromachining generally entails depositing
typically alternate layers of structural material and sacrificial
material using an appropriate substrate which functions as the
foundation for the resulting microstructures. A dielectric
isolation layer will typically be formed directly on an upper
surface of the substrate on which a MEM 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 MEM system, which is thereafter
fabricated thereon. Other layers of sacrificial and structural
materials are then sequentially deposited to define the various
microstructures and devices of the MEM system. Various patterning
operations may be executed on one or more of these layers before
the next layer is deposited to define the desired microstructure.
After the various microstructures are defined in this general
manner, the desired portions of the various sacrificial layers are
removed by exposing the "stack" to one or more etchants. This is
commonly called "releasing." During releasing, at least certain of
the microstructures are released from the substrate to allow some
degree of relative movement between the microstructure(s) and the
substrate. In certain situations, not all of the sacrificial
material used in the fabrication is removed during the release. For
instance, sacrificial material may be encased within a structural
material to define a microstructure with desired characteristics
(e.g., a prestressed elevator microstructure). Also, portions of
the sacrificial layers may be retained for support.
[0051] 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 ready availability and material
compatibility. The substrate again is essentially a foundation on
which the microstructures are fabricated. This foundation material
is generally stable to the processes that are being used to define
the microstructure(s) and does not 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 ideal, however, the etch
selectivity in some cases may be 5:1 or even lower. 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).
[0052] As discussed above, one aspect of the present invention
relates to providing a dust cover to protect particular components
or areas of a MEM system from particle contamination. In the
following discussion, the invention is set forth in the context of
a dust cover for covering and substantially encasing an
electrostatic actuator of a MEM system. The dust cover has
particular advantages for such an application because, as noted
above, components with electrostatic and/or moving elements, such
as electrostatic actuators, are particularly susceptible to short
circuits, mechanical obstruction, or other malfunctions due to
particle contamination. It will be appreciated, however, that the
invention is not limited to such a context.
[0053] Referring first to FIGS. 1 and 8, perspective views of a MEM
device 10 are shown. The illustrated device 10 is an electrostatic
actuator such as may be used for effecting movement of a movable
component. The nature of the movable component depends on the
purpose of the MEM system. One example would be a movable mirror of
an optical switch. An example of such a device is disclosed in U.S.
patent application Ser. No. 09/966,963, entitled "Large Tilt Angle
MEM Platform", filed on Sep. 27, 2001, which is incorporated herein
by reference in its entirety. The device 10 generally includes an
electrostatic actuator 12 (FIG. 8), that drives a movable frame 14,
a displacement multiplier 16 for multiplying or amplifying the
displacement of the movable frame 14, and a displacement output
element 18 for outputting the amplified displacement. The structure
and operation of such a displacement multiplier 16 is generally set
forth in U.S. patent application Ser. No. 6,174,179, by Kota et
al., issued on Jan. 16, 2001, which is incorporated herein by
reference in its entirety. Generally, the displacement multiplier
is driven at input port 20 by the movable frame 14. The
displacement multiplier 16 functions to amplify this input motion
so that displacement output element 18 moves in concert with the
movable frame 14 but across a range of movement that is
substantially greater than that of the movable frame 14. The output
element 18, in turn, is mechanically linked to the movable mirror
or other element that is driven, at least in part, by the device
10.
[0054] As generally shown in FIGS. 8 and 11, and described in more
detail below, the actuator 12 includes a number of electrodes 22
that are used to drive the frame 14. These include fixed electrodes
24 and movable electrodes 26. Electrical signals can be applied to
the electrodes 22 via leads 28 and 30 that terminate in bonding
pads 32 and 34. Accordingly, a signal such as a voltage potential
applied across the bonding pads 32 and 34 is, in turn, applied at
the electrodes 22. By applying such a signal at the electrodes 22,
an electrostatic force is selectively applied as between the fixed
and movable electrodes 24 and 26 so as to move the movable
electrodes 26 relative to the fixed electrodes 24. The movable
electrodes 26 are associated with the movable frame 14 such that
the control signals are used to controllably drive the frame 14
and, in turn, the displacement output element 18.
[0055] As discussed above, MEM components that include an
electrostatic and/or a movable element are particularly susceptible
to problems associated with particle contamination. The illustrated
actuator 12 is an example of a component that includes both
electrostatic and movable elements. In particular, as discussed
above, a voltage potential is applied across the fixed and movable
electrodes 24 and 26 in operation in order to create a drive force
for effecting movement of the frame 14. Such potentials may attract
particles. Moreover, very close spacing between the movable and
fixed electrodes 24 and 26 may be achieved during operation. Thus,
very small particles, e.g., on the order of one micron, may create
short circuits. Furthermore, it is apparent that even small
particles could mechanically interfere with movement of the movable
electrodes 26, the frame 14 or other movable elements.
[0056] Thus, in accordance with the present invention, the actuator
12 is substantially encased within a housing formed by a cover top,
cover walls and related support components disposed between the
cover top 36 and the electrical interconnect layer 38 (FIG. 9A).
The cover top 36 is shown in FIG. 1. In FIG. 8, the cover top 36 is
illustrated as being raised so that the underlying components
including the actuator 12, peripheral support structure 40, and
support posts 42 can be seen. It will be appreciated that the cover
top 36 and cover support structure 40 do not necessarily sealingly
enclose the actuator. In this regard, as shown in FIG. 1, the cover
top 36 includes a number of etch release holes 44. These etch
release holes 44 allow for penetration of an etchant to facilitate
the release process discussed above. It will be appreciated that,
in the absence of such release holes 44, complete and timely
penetration of the etchant across the area of the actuator 12 would
be difficult. These etch release holes 44 are preferably
distributed substantially uniformly across the area of the cover 36
and may be dimensioned to reduce penetration of potentially harmful
particles. For example, in the illustrated embodiment, etch release
holes 44 may have a diameter of approximately 1.25 microns.
[0057] The effectiveness of the cover top 36 in preventing particle
contamination may further be enhanced through the use of filters in
connection with the etch release holes 44, as discussed below. The
illustrated cover top 36 and related support assembly also provide
an opening 46 (See, FIGS. 10A and 10B, where the cover top 36 is
shown as being transparent in FIG. 10A for purposes of
illustration) to permit the frame 14 to interface with the
displacement multiplier 16 and associated structure. This opening
46 can be dimensioned so as to allow the desired mechanical
interface between the frame 14 and displacement multiplier 16 while
minimizing the opportunity for particle penetration. In the
illustrated embodiment, the opening 46 provides a clearance 48 of
no more than about 2 microns and more preferably no more than about
1 micron between the moving structure of the frame 14 on the one
hand and the peripheral cover support structure 40 and cover 36 on
the other hand.
[0058] FIGS. 2-8 illustrate the MEM device 10 in layer by layer
detail. It will be appreciated that FIGS. 2-8 do not fully
illustrate the production sequence. For example, in FIGS. 2-8, the
various sacrificial layers are shown as they would be formed after
the release step using the etchant. Thus, FIGS. 2-8 illustrate the
form of the finished product layer by layer for purposes of
clarity.
[0059] As previously discussed, a dielectric isolation layer is
generally first provided on the substrate. A first structural layer
is then usually formed on the dielectric isolation layer. This
initial structural layer is patterned with conductors and utilized
for establishing various electrical interconnections for the MEM
device. This structural layer 50 and the associated conductors 52
are shown in FIG. 2. In particular, the leads 28 and 30 to the
bonding pads 32 and 34 and conductors 52 for forming connections to
the electrodes 22 (not shown in FIG. 2) can be seen. These
conductors are used to provide voltage signals to drive the
electrodes 22.
[0060] FIG. 9A shows the connection of the voltage electrical input
54 to the electrical interconnect layer 50 of FIG. 2. As shown in
FIG. 9A, the connection is formed from beneath. That is, the
electrical input 54 is connected to the electric structural layer
50 via penetration through layer 38 and the dielectric isolation
layer 41.
[0061] The illustrated electrical interface accommodates shielded
conductors as described in copending U.S. patent application Ser.
No. 10/099,720 entitled "Multi-Level Shielded Multi-Conductor
Interconnect Bus for MEMS", which is incorporated herein by
reference. In particular, that application discloses conductors
that are electrically isolated from adjacent conductors by way of
certain isolation structure. Such isolation structure may be
incorporate a cover structure as shown in simplified form in FIG.
9B. In particular, FIG. 9B shows two electrode lines 900 and 902
substantially encased within shield structure 904. Although not
shown, it will be appreciated that additional electrical and/or
mechanical structure such as an actuator assembly may be included
in the device 906 with appropriate connections to the lines 900 and
902. Although two lines 900 and 902 are shown, it will be
appreciated that certain actuator designs including those described
above, can be implemented with a single drive line and a ground. In
such cases, one of the conductors 900 or 902 could be omitted or
branched off to provide separate drive circuitry.
[0062] In the illustrated embodiment, the shield structure includes
shield walls 908, extending longitudinally along the length of the
lines 900 and 902, supporting a shield cover 910, such that the
walls 908 and cover 910 substantially encase the lines 900 and 902
for particle protection. The walls 908 are supported on bases 912
that extend through the dielectric layer 914 to the substrate 916.
In this manner, the entire structure 904, together with any desired
additional components or device 906 can be maintained at a ground
or reference potential, thereby improving isolation between the
lines 900 and 902 and reducing cross-talk or interference. The
illustrated device 906 includes support walls 918 to support
further structures as desired.
[0063] FIG. 3 shows the first sacrificial layer 56 which forms the
first layer of the peripheral cover support structure 40, and
various support posts for supporting the cover top 36, actuator
electrodes 24 and displacement multiplier 16. These ports include
outer support posts 60 and central support posts 58 for supporting
the cover top 36 as discussed in more detail below.
[0064] FIG. 4 illustrates the next structural layer 61 which forms
a first layer of the electrodes 22, frame 14, and displacement
multiplier 16. This structural layer also forms another layer of
the peripheral cover support structure 40, outer support posts 58,
and central support posts 60 for supporting the cover top 36.
[0065] As shown, the frame portion of the structural layer is
formed with elongate slots 62 around the central support posts 60.
These elongate slots 62 accommodate reciprocating motion of the
frame 14 without mechanical interference due to the central support
posts 60.
[0066] FIG. 5 illustrates the next sacrificial layer 64. This
sacrificial layer 64 is used to provide a number of support posts
66 for interconnecting upper and lower levels of the actuator 12
and the displacement multiplier 16. This layer 64 also provides a
further layer of the peripheral cover support structure 40, outer
support posts 58 and center support posts 60 for supporting the
cover top 36.
[0067] FIG. 6 illustrates the next structural layer 68. This
structural layer 68 forms an upper layer of the movable frame 14,
as well as an upper layer of the displacement multiplier 16. This
layer 68 also provides the next layer of the peripheral cover
support structure 40, outer support posts 58 and center support
posts 60 for supporting the cover top 36.
[0068] Again, the frame portion of this structural layer 68 is
formed with elongate slots 62 around the central support posts 60.
These elongate slots 62 accommodate reciprocating motion of the
frame 14 without mechanical interference due to the central support
posts 60. This geometry is best seen in FIG. 12.
[0069] FIG. 7 shows the next sacrificial layer 72. This layer 72
provides the next layer of the peripheral cover support structure
40, outer posts 58 and central posts 60 for supporting the cover
top 36. In particular, this sacrificial layer 72 provides a
vertical separation between the cover top 36 and the actuator
assembly 12. This sacrificial layer also is used to provide support
posts 74 for an upper layer of the displacement multiplier 16.
[0070] Finally, FIG. 8 shows the uppermost structural layer 76 of
the illustrated MEM device 10. This layer 76 is used to form the
cover top 36 (shown as being raised for purposes of illustration)
and the uppermost layer of the displacement multiplier 16.
[0071] FIG. 11 shows a close-up of the outer posts 58 fabricated
around the electrode region. These posts 58 are preferably
positioned close to the electrode region to reduce the likelihood
of contact between the cover top 36 and the electrodes 24 and 26.
The various sacrificial and structural layers of the posts 58 can
be readily seen in this perspective view.
[0072] As noted above, the cover top 36 is generally maintained at
ground potential. The underlying electrodes 24 and 26 are
electrically biased. An attractive force is therefore exerted on
the cover top 36 to pull the cover top 36 down towards the
electrodes 24 and 26. Contact between the cover 36 and electrodes
24 and 26 would cause an electrical short and device failure.
Further protection against such an occurrence may be provided by
establishing support posts in the area of the electrodes 24 and 26.
This may be understood by reference to FIGS. 13A and 13B. FIG. 13A
illustrates a cover top 36 constructed as described above in
connection with FIGS. 1-12. As shown, there are substantial areas
where the cover top 36 extends over the electrodes 24 without
support.
[0073] FIG. 13B illustrates a modification where electrically
isolated supports 60 are provided in the area of the electrodes
24'. Such supports 60' may be provided in connection with the fixed
electrodes or in connection with the movable electrodes provided
that the movable electrodes are formed to accommodate movement
without electrical and/or mechanical interference due to the
support posts 60'. In particular, FIG. 13B illustrates electrically
isolated support posts 60' extending through an electrical
conductor 80 of a base structural layer and through the vertical
layer stack forming a stationary electrode 24'. Although the
electrically isolated supports are illustrated as supporting a
cover top, it will be appreciated that such electrically isolated
posts, e.g., used in connection with a stationary or movable
electrode, may be used to support various types of layers overlying
a MEM component, especially an active component including
electrostatic and/or movable elements.
[0074] FIG. 14 is a bottom view, i.e., up through a transparent
substrate, showing details of the anchoring of the electrically
isolated support posts 60'. As shown, the voltage conductor 80
loops around each central support post 60'. Typically the support
post will be held at ground potential. Optional nitride cuts under
each post 60' allow the post 60' to be anchored to the substrate
thereby adding mechanical rigidity and providing an electrical path
to the underlying substrate on which the posts terminate.
[0075] FIG. 15 is a cut away view further showing how the isolated
posts 60' extend through the layer stack and how the posts 60'
interface with the voltage conductor 80. Such posts 60' may be used
to serve other functions in addition to support for a cover or
other overlying structure. In particular, the base structural layer
of the posts 60' may be used to provide an electrical filter. As
discussed above, the voltage conductor 80 is used to provide
control signals to operate the actuator. In many applications, such
as use of the actuators to move a micromirror of an optical
cross-connect switch, very precise movement of the actuator may be
required. Such precise control may be difficult due to electrical
noise. Such noise may become particularly problematic in connection
with increased miniaturization of the electrostatic elements. In
the illustrated embodiment, a space 82 is provided between the base
layer of the support post and the conductor loop. This base layer
of the support posts 60', like the remainder of the support posts,
is maintained at ground potential. As a result, a capacitance is
provided between the support posts 60' and surrounding structure.
This capacitance can serve to filter the signal transmitted through
the conductor 80 on a wavelength-dependent basis, e.g., to help
diminish high frequency noise, including quantization noise from
D/A converters. The nature of this capacitance and the resulting
filtering function can be altered as desired for particular
applications through appropriate control of the post/conductor
spacing, the potential difference between the post and conductor,
material properties including any dopants and the like. In this
manner, a cleaner drive signal can be provided to the conductor 80
for improved control.
[0076] FIG. 16 illustrates an exemplary MEM system 100
incorporating such electrically isolated posts with integral
filters and further configured with multiple particle filters,
e.g., 102, 104, and 106 according to the present invention.
Although these filters are illustrated and described below as
depending from an overlying layer such as a cover surface, it will
be appreciated that such filters could be integrated into a support
wall or other structure. MEM systems constructed by MEMX, Inc. of
Albuquerque, N.Mex., such as MEM system 100 may include a first
layer 108 that provides electrical interconnections and as many as
five or more additional layers of mechanical polysilicon layers
that form functional elements ranging from simple cantilevered
beams to complex microengines connected to a gear train. MEM system
100 also includes a cover 110 to protect the electrical and
mechanical layers 108 and 112-116 from particle contamination. Etch
release apertures 118A-F in the cover 110 provide a means to
introduce etchant during the release step to remove the remaining
sacrificial material and release the mechanical and electrical
devices in the layers 108 and 112-116. Such etch release apertures
are required to allow penetration of the etchant for releasing the
structure during the final fabrication steps. The etch release
apertures 118A-F are typically on the order of about 1.25 microns
in size. Particle filters, e.g., 102, 104 and 106, are preferably
formed around the etch release apertures 118A-F and operate to trap
particles that may enter the MEM system 100 through the apertures
118A-F, thereby assuring that virtually no contamination may occur
in the MEM system 100. The filters, e.g., 102-106, which are
described in detail below, thus allow penetration of the etchant
but impede ingress of particles of a size that may obstruct
movement or cause short circuits.
[0077] FIG. 17 illustrates a cut away perspective view of the
particle filter 102. For purpose of illustration, the following
description will now be directed toward the operation and
fabrication of the illustrated particle filter 102, having an
exemplary configuration and associated fabrication process. It will
be appreciated however, that the following discussion applies
equally to the particle filters 104 and 106, as well as other
particle filters described herein, as well as other configurations
and processes according to the invention.
[0078] The particle filter 102 includes a filter bottom 200 and
filter wall 202. The filter wall 202 is interconnected to the
filter bottom 200 by support feature 206, referred to herein as
anchor post 206. The filter wall 202 may also be formed from at
least one depending portion of the cover 110 over MEM system 100.
In other words, a filter top may be provided by forming the filter
wall 202, anchor 206 and cover 110 from the same deposition layer
or integrally or otherwise interconnected layer portions in the MEM
system 100.
[0079] In that regard, the filter wall 202 and filter bottom 200
define a particle trap 208 formed at the mating but non-sealably
interconnected intersection of the filter wall 202 and filter
bottom 200. That is, the filter wall 202 and bottom 200 interface
so as to provide one or more openings dimensioned to allow
penetration of etchant but capture certain particles that may have
passed through an etchant aperture, e.g., 118A. As illustrated on
FIG. 17, the filter wall 202 and filter bottom 200 are not actually
connected, but rather, define a gap or space along the intersection
that forms the particle trap 208. In this case, the anchor post 206
provides the interconnection between the filter wall 202 and filter
bottom 200, via the filter top/cover 110. As may be appreciated,
the dimension of the gap 208 is defined by the size of particle to
be trapped within the filter 102. In this regard, the dimension of
the gap 208 is preferably, in the range of 0.1 micron to 0.5 micron
and more preferably is 0.2 micron. Operationally, the particle trap
208 effectively traps particles entering the particle filter 102
within the gap 208, thereby preventing the particles from
contaminating the mechanical and electrical micro-devices in the
layers 108 and 112-116.
[0080] FIGS. 18-26 illustrate one example of the fabrication of the
particle filter 102. Only those portions of the MEM system 100 that
are relevant to the present invention will be described herein.
Those skilled in the art will appreciate, however, that since the
particle filter 102 is preferably fabricated using micromachining,
various other combinations of depositions and surface machining
that are within the scope of the present invention exist to produce
particle filters according to the principles disclosed herein.
[0081] Referring to FIG. 18, there is shown a cross sectional view
of the fabrication process for the particle filter 102 completed to
the structural layer 310 forming the filter bottom 200.
Specifically, the structure of FIG. 3 includes a substrate 300,
dielectric isolation layers, 302 and 304, a pair of sacrificial
layers, 306 and 308, and a structural layer 310. It should be noted
that the sacrificial layers 306 and 308 may alternatively be
structural layers such as structural layers 114 and 116. However,
for purposes of clarity, the fabrication of the particle filter 102
is illustrated in FIGS. 18-26 utilizing sacrificial layers 306 and
308. In other words, to provide a clearer understanding of the
present invention, sacrificial layers, 306 and 308, are shown on
FIGS. 18-26 rather than structural layers 114 and 116.
[0082] The dielectric isolation layers, 302 and 304, may be a
thermal oxide layer and silicon nitride layer respectively, formed
by a conventional thermal diffusion process as is well known in the
integrated circuit art. The term "substrate" as used herein means
those types of structures that can be handled by the types of
equipment and processes that are used to fabricate microdevices
and/or microstructures on, within, and/or from a substrate using
one or more micro-photolithographic patterns.
[0083] Exemplary materials for the sacrificial layers, 306 and 308,
as well as other sacrificial layers utilized to form the particle
filter 102 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). Exemplary materials for the structural layer 310
as well as other structural layers that form the particle filter
102 include doped or undoped polysilicon and doped or undoped
silicon. Exemplary materials for the substrate 300 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, and chemical-mechanical polishing (CMP) as
examples.
[0084] After formation of the structure of FIG. 18, the structural
layer 310 may be patterned using photolithographic masking and
etching into the shape of the filter bottom 200, as illustrated in
FIG. 19. In this regard, a thin layer of light sensitive
photoresist may be spun onto the layer 310. The layer 310 may then
be exposed to light using a mask. After etching, the remaining
photoresist may then be stripped away resulting in the structure of
FIG. 19. As will become apparent from the following description,
the filter bottom 200 may be patterned into a variety of shapes as
a matter of design choice to accommodate different spatial
configurations and limitations within a MEM system, such as MEM
system 100.
[0085] Referring to FIG. 20, after patterning of the filter bottom
200, another layer 500 of sacrificial material is deposited onto
the patterned layer 310. It should be noted, however, that while
the sacrificial layer 500 is shown in a planarized state, such as
could be achieved through chemical-mechanical polishing,
planarization is not necessary to the fabrication of the particle
filter 102. Referring to FIG. 21, the sacrificial layer 500 is
patterned using a cut etch to form a circumferential annular void
600 within the sacrificial layer 500. The circumferential annular
void 600 will eventually become the filter wall 202 for the
particle filter 102. It should also be noted that the void 600 is
etched all the way down to the structural layer 310/filter bottom
200 and slightly overlaps the side of the structural layer 310 or
in other words the top portion of the filter bottom 200. The
overlap is not necessary to the formation of the particle filter
102, but rather, increases the efficiency of the particle filter
102 as it forms the lip (shown on FIG. 17) of the particle trap
208, which further restricts particles passing through the particle
trap 208.
[0086] Referring to FIG. 22, after etching of the void 600, a thin
layer of sacrificial material 700 is applied to backfill void 600.
The thickness of the backfill layer 700 determines the gap spacing
of the particle trap 208 and therefore is precisely controlled
during the backfill process. In that regard, the thickness of the
backfill layer 700 is preferably in the range of 0.1 micron to 0.5
micron and more preferably is 0.2 micron. It should also be noted
since the layer 700 is the same material as the sacrificial layer
500 it essentially becomes part of the layer 500 as shown in FIG.
23. Alternatively, a timed etch to the desired depth may be
utilized to form the void 600, thus eliminating the need for the
backfill layer 700. As will be appreciated by those skilled in the
art, however, the backfill method eliminates many of the
difficulties associated with timed etching, e.g. knowledge of the
precise thickness of the sacrificial layer 500. Still referring to
FIG. 23, the sacrificial layer 500 including the added material of
layer 700 is again patterned using a cut etch to form a
substantially central annular void 800. The central annular void
800 will eventually become the anchor post 206 for the particle
filter 102.
[0087] Referring to FIG. 24, after the sacrificial backfill layer
700 is deposited and void 800 etched, another structural layer 900
is deposited and planarized. Again as will be appreciated the
planarization is not necessary to the formation and/or operation of
the particle filter 102. The structural layer 900 forms the filter
wall 202 and the top cover 110. Referring to FIG. 25, after
deposition of the layer 900, etch release apertures 118A are cut
into the layer 900 to provide the means for introducing the
chemical etchant used to release the particle filter 102 and or
other microdevices and/or microstructures in a MEM system, such as
MEM system 100.
[0088] Referring to FIG. 26, the etch release step utilizes a
selective etchant that etches away exposed portions of the
sacrificial layers 306, 308, and 500 over time, while leaving the
polysilicon structural layers 302, 304, and 310 intact to
form/release the particle filter 102. 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)).
[0089] The completed particle filter 102 is supported in the MEM
system 100 by the filter top/cover 110, which in turn supports the
filter bottom 200 via the anchor post 206. Advantageously, this
permits the formation of the particle trap 208 around the etch
release apertures 118A. Also advantageously, in this regard, the
particle filter 102 virtually eliminates the possibility of
particle contamination as particles entering through the etch
release apertures 118A are trapped by the particle trap 208. As
stated above, the etch release apertures are on the order 1.25
microns in size while the particle trap is on the order of 0.2
micron in size.
[0090] Referring to FIGS. 27-30, a further advantage of the present
invention is provided through various alternative embodiments of
the present particle filter. The present particle filter can be
constructed in a variety of geometrical shapes as a matter of
design choice. Those skilled in the art will appreciate the slight
variations in etching to achieve the various designs illustrated in
FIGS. 27-30, and thus, a description is omitted for the purpose of
brevity. Additionally, those skilled in the art will appreciate
that the particle filters 1200-1500 are for purpose of illustration
and not limitation and that numerous other designs can be formed
according to the principles of the present invention.
[0091] The particle filters 1200-1500 operate substantially
similarly to the particle filter 102 in that they include a
particle trap defined by mating, but non-interconnected surfaces,
of a filter wall and a filter bottom connected to the filter wall
through a support feature. The particle filters 1200-1500, however,
provide the advantage of accommodating various different spatial
limitations created by the different microstructures that can be
included in a MEM system such as MEM system 100. For example,
particle filter 1300 includes a slightly smaller filter bottom and
is externally supported by an anchor post 1304. Particle filters
1200, 1400 and 1500 all include variations of the principles of the
present invention and may be incorporated into one or more MEM
systems as a matter of design choice. In addition, it will be
appreciated that a MEM system, such as system 100, could include
one or more of the different filter designs, e.g. 102, and
1200-1500, in a single system as a matter of design choice.
[0092] Those skilled in the art will appreciate variations of the
above-described embodiments that fall within the scope of the
invention. As a result, the invention is not limited to the
specific examples and illustrations discussed above, but only by
the following claims and their equivalents.
* * * * *