U.S. patent application number 11/959813 was filed with the patent office on 2008-04-24 for structure and method for releasing stressy metal films.
This patent application is currently assigned to PALO ALTO RESEARCH CENTER INCORPORATED. Invention is credited to Christopher L. Chua, David K. Fork, Koenraad F. Van Schuylenbergh.
Application Number | 20080095996 11/959813 |
Document ID | / |
Family ID | 38174151 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080095996 |
Kind Code |
A1 |
Chua; Christopher L. ; et
al. |
April 24, 2008 |
STRUCTURE AND METHOD FOR RELEASING STRESSY METAL FILMS
Abstract
A method and structure for forming a spring structure that
avoids undesirable kinks in the spring is described. The method
converts a portion of a release layer such that the converted
portion resists etching. The converted portion then serves as an
anchor region for a spring structure deposited over the release
layer. When the non-converted portions of the release layer are
etched, the spring curls out of the plane of a plane.
Inventors: |
Chua; Christopher L.; (San
Jose, CA) ; Fork; David K.; (Los Altos, CA) ;
Van Schuylenbergh; Koenraad F.; (Fremont, CA) |
Correspondence
Address: |
PATENT DOCUMENTATION CENTER
XEROX CORPORATION
100 CLINTON AVENUE SOUTH, XEROX SQ. 20 TH FLOOR
ROCHESTER
NY
14644
US
|
Assignee: |
PALO ALTO RESEARCH CENTER
INCORPORATED
3333 Coyote Hill Road
Palo Alto
CA
94304
|
Family ID: |
38174151 |
Appl. No.: |
11/959813 |
Filed: |
December 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11300872 |
Dec 15, 2005 |
|
|
|
11959813 |
Dec 19, 2007 |
|
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|
Current U.S.
Class: |
428/212 |
Current CPC
Class: |
Y10T 428/24942 20150115;
B81C 1/0015 20130101 |
Class at
Publication: |
428/212 |
International
Class: |
B32B 7/02 20060101
B32B007/02 |
Claims
1. An intermediate structure for forming a spring comprising: a
substrate; a release material deposited as a release layer, the
release region including two regions with different etch
characteristics, the two regions including an anchor region that
resists a selective etch and a release region that is susceptible
to a selective etch, the different etch characteristics caused by a
treating at least one of the two different regions to change the
chemical structure of at least one of the two different region;
and, an overlying material deposited over the release layer, an
anchor portion of the overlying material deposited over the anchor
region and a release portion of the overlying material deposited
over the release region.
2. The intermediate structure of claim 1 wherein the treating of
the at least one of the two different regions occurs after the
deposition of the overlying material.
3. The intermediate structure of claim 1 wherein the anchor region
is an oxidized release material.
4. The intermediate structure of claim 1 wherein the anchor region
is a cross linked polymer.
5. The intermediate structure of claim 1 wherein the anchor region
includes boron and silicon.
6. The intermediate structure of claim 1 wherein the anchor region
and the release region are the same thickness.
7. The intermediate structure of claim 1 wherein the anchor region
is thicker than the release region.
8. The intermediate structure of claim 1 wherein the spring
material is a metal that includes a plurality of sublayers, the
atomic spacing of the lower sublayers is smaller such that an
internal stress gradient exists in the spring material.
9. A spring structure comprising: a substrate; a release layer
deposited over the substrate, at least one region of the release
layer has been chemically altered such that an anchor region
resists a selective etch and a release region is susceptible to a
selective etch; and, a spring material including an anchor portion
coupled to the anchor region of the release layer, the spring
material further including a release portion that curls out of a
plane parallel to the substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of U.S. Application. No. 11/300,872
filed Dec. 15, 2005 by the same inventors and claims priority
therefrom. This divisional application is being filed in response
to a restriction requirement in that prior application.
BACKGROUND
[0002] Spring structures in MicroElectroMechanical systems (MEMS)
have become increasingly important in a wide variety of
applications. These applications include electronic packaging, test
and measurement probing, integrated high quality factor inductors,
electrical interconnects, fluid distribution and printing
applications.
[0003] Traditional methods of forming small spring structures have
disadvantages. Often the fabrication process used to produce these
springs produce sharp angled bends or "kinks" in the spring at or
near the point where the spring lifts up from the substrate such as
is shown in FIG. 6. When the angled bend is oriented in a direction
that opposes the spring flex the angled bend serves as a weak point
that is prone to failure with repeated use.
[0004] Alternate fabrication techniques exist to avoid such bends.
However, these alternative techniques use precisely controlled
timing of etch rates. Etch rates depend on many parameters such as
etchant concentrations, the exact composition and thickness of the
layer being etched, and the spring geometry. Consistently
reproducing the multiple parameters is difficult in a commercial
production environment.
[0005] Thus a method for forming spring structures that does not
rely on timing to control etching and results in spring structures
that do not form angled bends that are susceptible to breakage is
needed.
SUMMARY
[0006] A method of forming a suspended structure is described. The
method converts a region of a release layer to form an anchor of
the suspended structure. A release layer is first deposited over a
substrate. At least one region of the release layer is treated to
alter the chemical structure such that an anchor region of the
release layer is resistant to a selective etchant and a release
region of the release layer is etchable by the selective etchant.
The selective etchant is then used to perform an etch of the
release layer such that the release region releases a release
portion of an overlying layer, the release portion of the overlying
layer to form a suspended structure, an anchor portion of the
overlying layer remains attached to the anchor region. The method
described is useful for forming various structures, but has
particular application in the fabrication of spring structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIGS. 1-3 provide a schematic side view of an example
lithography operation for forming a stressed metal spring.
[0008] FIG. 4 shows a top view and FIG. 5 a side view of a spring
fabrication method where the release layer is patterned prior to
deposition of the spring material thereby allowing a portion of the
spring material to adhere directly to an underlying substrate.
[0009] FIG. 6 shows a side view of a spring fabricated using a
release layer patterned prior to deposition of the spring
material.
[0010] FIG. 7 shows a top view and FIG. 8 a side cross sectional
view of a spring material deposited over a continuous release layer
where an anchor region of the continuous release layer has been
altered to resist etching.
[0011] FIG. 9 is a flow chart that describes an example procedure
of forming a suspended structure such as a spring by oxidizing a
portion of a continuous release layer.
[0012] FIG. 10 shows an example side cross sectional view of a
spring fabricated by converting an anchor region of a release
layer.
[0013] FIG. 11 shows a side cross sectional view of a spring
material deposited over a continuous release layer where the anchor
region of the continuous release layer expands when it is altered
to resist etching.
[0014] FIG. 12 shows a side cross sectional view of a spring
fabricated using the method of FIG. 9 when an expansion of the
release layer anchor region occurs.
[0015] FIG. 13 shows a top view of an example membrane
structure.
[0016] FIG. 14 shows a side cross sectional view of the membrane
structure of FIG. 13.
DETAILED DESCRIPTION
[0017] A planar self terminating release process for fabricating
small suspended structures is described. The technique converts
selected areas of a release layer to an etch resistant material. An
overlying material is deposited over the release layer, the
overlying material includes an anchor portion deposited over the
converted etch resistant material. After etching the release layer,
the etch resistant material serves as an anchor region that fixes
an anchor portion of the overlying material to an underlying
substrate. The remaining portion of the overlying material has been
undercut and thus remains suspended. The described technique is
particularly useful for forming spring structures.
[0018] Micro-spring structures are typically formed by depositing a
spring material over a release layer. As used herein, "spring
material" is broadly defined as any material upon which a stress
may be induced to cause the material to curl out of a plane.
Typically, after curling out of the plane, the spring material is
flexible and can apply a force approximately proportional to
displacement over a small distance. Example spring materials
include MoCr alloys sputtered at different ambient pressures to
induce a stress gradient within the thickness of the material, Ni
electroplated using different chemistries to produce internal
stress differences between the different layers, layers of bi-morph
materials that have different internal stresses, and layers of
bi-morph materials that have different thermal expansion
coefficients. One particularly useful spring material is a
"stressed metal". As used herein, "stressed metal" is defined as a
spring structure with an internal stress gradient. Stressed metals
are typically formed by depositing multiple sublayers, each
sublayer deposited at a different temperature or pressure such that
differing atomic packing densities in each sublayer result in an
the internal stress gradient. A detailed description of forming a
stressed metal spring is provided in U.S. Pat. No. 6,528,350
entitled "Method for Fabricating a Metal Plated Spring Structure"
by David Fork and U.S. Pat. No. 5,613,861 entitled
"Photolithographically Patterned Spring Contact" by Smith et al.
which is hereby incorporated by reference.
[0019] FIGS. 1-3 provide a schematic side view of an example
lithography operation for forming a stressed metal spring. In FIG.
1, a release layer 104 and a seed layer 108 are deposited over a
substrate 100. Release layer 104 is selected to be a material that
can be easily etched to "release" a spring material that is
subsequently deposited over the release layer. In one embodiment,
release layer 104 is a sputtered titanium (Ti) layer.
[0020] Seed layer 108 is deposited over the release layer. Seed
layer 108 facilitates growth or deposition of spring materials
deposited over seed layer 108. An example seed layer is a gold (Au)
layer deposited by sputtering techniques.
[0021] It is sometimes advantageous to combine release layer 104
and seed layer 108 into a single layer or use a single material for
both layers. Combining the two layers reduces the number of
deposition operations during fabrication. Examples of combined
seed/release layer materials include, but are not limited to,
silicon-nitride (SiN), titanium (Ti), copper (Cu) and/or nickel
(Ni) deposited in a single layer over substrate 100.
[0022] In FIG. 2, a spring material 204 is deposited over release
layer 104. In one example, spring material 204 is a metal such as
nickel (Ni) deposited in a series of sublayers, 208, 212, 216 to
create an internal stress gradient. Electroless or electroplating
techniques may be used to deposit the spring material. In one
example fabrication technique, the built in stress gradient is
obtained by plating from two baths with different characteristics
or by varying the current density during plating. A detailed
description of forming such stress gradients during plating is
provided in U.S. Pat. No. 6,528,350 entitled "Method for
Fabricating a Metal Plated Spring Structure" by Fork et al. issued
Mar. 4, 2003 which is hereby incorporated by reference.
[0023] In an alternate stressed metal fabrication technique,
metallic layers are sputtered under gradually decreasing pressure
such that the atomic spacing is larger in the upper sublayers of
the spring material. The lower densities in the spring upper
sublayers produce a stress gradient. A detailed description of
using sputtering to form a stress gradient is provided in U.S. Pat.
No. 5,613,861 entitled "Photolithographically Patterned Spring
Contact" by Donald Smith et al. and hereby incorporated by
reference in its entirety.
[0024] Although FIG. 2 shows a "stressed metal" spring material, it
should be understood that the spring material is not limited to
such metals. For example, a bimorph or bimetallic material may be
used as a spring material. Temperature or other parameter changes
induce stresses in the bimorph or bimetallic material causing the
spring release portion to curl out of a spring plane. As used
herein, the "spring plane" is the plane in which the release
portion of the spring is originally deposited and is a plane
typically parallel to the top surface of the release layer.
[0025] In order to release the spring material, part of the release
layer is etched away. In one embodiment, the etching is an undercut
wet etch. For example, the TFT Ti etch solution manufactured by the
Transcene Company of Danvers, Mass. 01923 USA is an appropriate
etchant for a Ti release layer. For SiN release layers,
hydrofluoric and buffered hydrofluoric acid solutions may be used.
In another embodiment, a XeF2 gas would be used to undercut vapor
etch a silicon release layer. FIG. 3 shows etching of the release
layer 304 to release a release portion 308 of spring material 312.
The internal stress gradient in spring material 312 causes the
spring material to uplift or curl out of the spring plane.
[0026] In the structure of FIG. 3, an unetched section of release
layer 304 attaches an anchor portion 316 of spring material 312 to
substrate 300. One method of fabricating the structure of FIG. 3
uses the spring itself as a mask for the etchant. When the spring
is used as the mask, the spring geometry defines the unetched
portion of release layer 304. The etchant gradually undercuts mask
from the spring material 312 perimeter. To maintain the unetched
section under anchor portion 316, the center of anchor portion 316
is further from the perimeter edge than any point in a release
portion of the spring. Typically, this is done by making the anchor
portion 316 of the spring material wider than the release portion
308 of spring material 312.
[0027] When etching, release portion 308 is released first and the
etching process terminated before the entire spring anchor is
undercut. The described technique is outlined in the previously
referenced U.S. Pat. No. 5,613,861. However, such a technique
limits the amount of control over the etching process as well as
the spring shape design. In particular, precise timing of the
etching process and larger anchor areas are needed. Furthermore,
even when done properly, a substantial undercut 324 of the anchor
portion 316 occurs. Release masks 328 patterned to cover wider
areas than the anchor portion 316 can reduce but not eliminate the
undercut 324.
[0028] In an alternate spring fabrication method, the release layer
is patterned prior to deposition of the spring material thereby
allowing the spring material to adhere directly to the underlying
substrate. FIG. 4 shows a top view and FIG. 5 a side view of such a
structure. FIG. 4 shows release layer 404 patterned to expose
substrate 400 prior to deposition of spring material 408.
[0029] FIG. 5 shows a side view of the patterned release layer
including the spring structure prior to spring release. As can be
seen from FIG. 5, an anchor portion 504 of spring material 408
adheres directly to substrate 400 and a release portion 508 of
spring material 408 rests over release layer 404. Thus when etching
release layer 404, the process is self terminating in that all of
release layer 404 may be removed with well defined spring take-off
points. As used herein, "self terminating" means that further
exposure to a selective etchant will not remove additional release
layer material. As used herein, "take-off point" is defined as the
point on the spring at which the bottom surface of the spring no
longer contacts one of either the release layer or the
substrate.
[0030] However, the embodiment of FIG. 5 presents a problem in that
after removal of release layer 404, and uplift of release portion
508, a discontinuity or "kink" occurs in the spring material at the
former edge of release layer 404. FIG. 6 shows the discontinuity
604 in the first derivative of the spring material 408 after
uplift. An upward directed interconnect segment 616 connects the
spring anchor portion 620 to the spring release portion 624. A
first angle 608 ranging between 0 and 180 degrees forms between the
spring anchor plane 612 and the interconnect segment 616. As used
herein, "spring anchor plane" 612 is defined as the plane that
includes the spring anchor portion 620. A second angled edge 628
connects the termination of interconnect segment 616 and the
beginning of release portion 624. The angled edges at the
discontinuity represent weak points in the spring structure that
lead to higher spring failure rates.
[0031] In order to either change the orientation of the
discontinuity or to avoid the discontinuity altogether, an
alternative fabrication method using a continuous release layer is
described in FIGS. 7-12 and the associated text. In the system
described in the following description, an anchor portion of the
release layer is converted to a non-etchable or etch resistant
material prior to or after an overlying material, usually a spring
material deposition. As used herein, "non-etchable" or "etch
resistant" means that the material resists etching by the selective
etchant selected to remove non-anchor portions of the release
layer, thus enabling selective etching. It should be understood
that "etch resistant" does not mean that the material will resist
etching by all etchants that may be available. The spring anchor
portion is deposited over the converted etch resistant material and
a spring release portion is deposited over the non-converted
etchable portion of the release material.
[0032] FIG. 9 is a flow chart that describes an example procedure
for forming a suspended structure from an overlying material. As
used herein, a "suspended structure" is defined as a structure that
includes an anchor portion that is supported by an underlying layer
and a suspended or overhang portion, (called herein, a release
portion) that is prevented from dropping by its lateral attachment
to the anchor portion. The resulting suspended structures formed
may include, but are not limited to, cantilever structures,
membrane structures and stressed metal springs. A brief description
of example structures will follow, however, the fabrication
description will primarily focus on stressed metal springs as one
example.
[0033] Cantilever structures are suspended free hanging beams that
remain in substantially the same plane after the release process as
before the release layer is removed. These structures are used in
many micro-electro mechanical system applications such as
mechanical resonators for diagnosing the vibration characteristics
of machine blocks. They can also form part of an electronic filter
for tuning electronic signals.
[0034] Spring structures are similar to cantilever structures
except the released portion of springs is designed to bend up or
down out of the plane after release. Spring structures usually have
a curved profile in the release portion, but the release portion
can have different radius of curvatures at different positions. A
part of the released portion may have a straight profile. Stressed
metal springs are a class of springs made by engineering the stress
properties of the springs to make the release portion bend in or
out of the plane after an underlying release layer is removed.
[0035] Membrane structures are suspended sheets supported by two or
more anchor portions. The sheet can either pop in or out of the
plane after release or can remain on the same plane as before
release. FIG. 13 shows a top view of an example membrane structure
1300 including a suspended portion 1304 supported by four anchors
1308. FIG. 14 shows a side cross sectional view of membrane
structure 1300 along line 1404. Membrane structures are used in
many applications including micro-mirrors in optical cross connect
switches as taught in U.S. Pat. No. 6,411,427 entitled "Structure
for an Optical Switch on a Glass Substrate" by Peeters et al. and
as a Fabry-Perot element in wavelength tunable vertical-cavity
surface-emitting lasers and optical filters.
[0036] In the flow chart of FIG. 9, the final structure of a
stressed metal spring is formed by converting a portion of a
continuous release layer. As used herein, "continuous release
layer" is defined as a release layer that exists and is continuous
under every point of a deposited spring material such that no part
of the spring material directly contacts the underlying substrate.
FIG. 7 shows a top view and FIG. 8 shows a side cross sectional
view of the formed structure at various points during
fabrication.
[0037] In block 904 of FIG. 9, continuous release layer 702 of FIG.
7 is deposited over a substrate 800. In one embodiment, the
continuous release layer is an etchable material such as a-Si
(amorphous silicon), titanium, polymers such as Benzocyclobutene,
or aluminum. Regions of the release layer are then identified as
anchor regions 708 and release regions 704. In order to prevent
anchor regions 708 from being etched, the chemical structure of
anchor region 708 will be altered to make the anchor region 708 of
release layer 702 resistant to etching. One method of altering the
chemical structure to resist etching is through oxidation.
[0038] In block 908 of FIG. 9, a mask is deposited to protect
release regions 704 of release layer 702. The unmasked portions of
release layer 702, the anchor regions, are then exposed to a
process or "treated" to alter the chemical structure of the anchor
regions. Altering the chemical structure is broadly defined to
include any alteration to the molecular structure that changes the
properties of the material. Examples of chemical alteration
include, but are not limited to crosslinking polymers when exposed
to radiation, oxidation mechanisms, and implantation of impurities.
In FIG. 9, the altering process described is an oxidation
mechanism. Thus block 912 describes placing the spring sample in a
standard wet oxidation furnace. The mask protects release regions
704, thus only anchor regions of release layer 708 are oxidized.
Although oxidation of the entire thickness of anchor portion 708
provides the best resistance to etching, partial conversion where
only a bottom portion of anchor region 708 may suffice in limited
etching situations.
[0039] Oxidation converts anchor region 708 into an oxidized
material. In the case of a-Si release layers, anchor region 708
converts into SiO2. If aluminum or a high aluminum containing
material such as AlAs is used as the release layers, the oxidized
regions are typically converted to Al3O2. Although a-Si and
Aluminum materials have been described, almost any material that is
more resistant to etching upon oxidation may be used.
[0040] In block 916, an overlying layer of material such as spring
material 712 is deposited over continuous release layer 902. In
general "overlying material" is broadly defined to include any
material that lies directly over and is supported by the release
layer. The overlying material may be made from a variety of
materials, although in the described spring fabrication, the
overlying material is usually a metal or metal alloy. A spring
anchor portion 804 is deposited over oxidized anchor region 708 and
a spring release portion 808 is deposited over unoxidized release
region 704. In one embodiment, spring material is a stressed metal
as described in the description associated with FIGS. 1-3. As
previously discussed, electroless or electroplating techniques may
be used to deposit the spring material. In one embodiment, the
built in stress gradient is obtained by plating from two baths with
different stress characteristics or by varying the current density
during plating. In an alternate technique, metallic layers are
sputtered under gradually decreasing pressure such that the atomic
spacing is larger in the upper spring sublayers resulting in a
stress gradient.
[0041] After spring material 712 deposition, the sample is
selectively etched in block 920. The selective etch etches the
unoxidized release regions 704 but the treated oxidized anchor
regions 708 resist etching. Assuming an a-Si release layer, an
appropriate selective etchant is XeF2 gas which removes the Si but
does not remove SiO2. Typically the selective etch etches up to the
border of the release region and the anchor region of the release
layer. Thus usually the selective etch is maintained long enough to
remove all the release layer material in the release region.
[0042] FIG. 10 shows the resultant spring structure after etching.
In FIG. 10, the spring 1004 is smooth and continuous between an
anchor portion 804 and suspended portion or a release portion 808.
As used herein, "smooth" means the first derivative of the spring
is continuous across the entire spring surface and particularly in
the region between anchor portion 804 and release portion 808. Thus
a "smooth" surface is characterized by the absence of "kinks".
[0043] Although FIG. 9 shows one embodiment for depositing a
suspended structure such as a spring structure over a treated
release layer, other embodiments are possible. For example, in an
alternative embodiment, the treatment of the release layer to make
the anchor region resistant to etching or the release region
susceptible to etching may occur after the overlying material such
as the spring material is deposited. In an oxidation
implementation, this may be accomplished by, performing block 916
of FIG. 9 ahead of block 908. Perforations or apertures in the
spring anchor portion may be used to facilitate oxidation of the
release layer under the spring anchor portion. Treating or
oxidizing the release layer after spring deposition facilitates
alignment by allowing the anchor portion 804 of FIG. 10 to be
self-aligned to the etch-resistant layer 708.
[0044] An alternate method of treating a release layer after
overlying material or spring material deposition uses ion
implantation. In one example method of ion implantation, boron
atoms are implanted in an a-Si (amorphous silicon) release layer.
Initially, a spring material is deposited over the release layer.
After deposition, the spring material can be patterned into the
desired spring shape. Alternately, patterning the spring material
can also be postponed to a later processing step. A mask such as
photoresist is patterned over the spring material layer. The mask
defines which regions of the release layer are to be treated, in
this case, converted into an etch resistant material. A standard
ion implantation machine implants Boron ions into the release
layer. The masking material blocks the Boron ions. However, the
boron ions easily penetrate unmasked areas of the thin spring
material. The Boron-doped regions become selectively resistant to a
selective etchants such as KOH. The result is similar to the result
obtained by oxidizing the underlying release layers through
perforations in the spring material.
[0045] Although processes have been described to treat the anchor
region and thereby convert an anchor region of a generally etchable
release layer into a etch resistant material; the reverse process
of converting a release region of the release material to make the
release region more etchable relative to the unconverted material
is also possible. For example, release region 704 can be oxidized
while anchor region 708 remains the original unmodified release
material. A selective etchant that only etches the treated release
region is then selected. One example of such a selective etchant is
hydrofluoric acid which selectively etches the oxidized material in
release region 704 but not untreated or unoxidized release layer
material in anchor region 708. The selective etching thus releases
spring section 708.
[0046] FIGS. 7, 8 and 10 show an ideal structure. In actual
formation, some materials, such as silicon, expand when oxidized.
Silicon oxidation results in expansion of the oxidized layer by 127
percent. Thus, a 20 nm thick layer of Si can become a 45 nm thick
layer when oxidized. FIG. 11 shows the effects of such an expansion
prior to spring release. In FIG. 11, a silicon release layer is
deposited over a substrate 1100. An anchor region 1104 of the
release layer is oxidized. Resulting anchor region 1104 of release
layer 1108 is thicker than the release region 1112. Thus after
spring material 1116 is deposited over release layer 1108, the
anchor portion 1120 of spring material 1116 is in an anchor plane
1118 that is higher than the release portion 1124 of the spring
material.
[0047] FIG. 12 shows the spring structure of FIG. 11 after release.
In the structure of FIG. 12, a discontinuity or "kink" occurs due
to the fact that the release layer 1108 surface was not planar. In
particular, anchor region 1104 top surface was above the release
region 1108 top surface. Thus, the take off point 1204 where
release portion 1208 begins to curl out of a plane parallel to the
substrate is below the anchor plane that includes spring anchor
portion 1212.
[0048] Interconnect segment 1220 connects spring anchor portion
1212 to spring release portion 1208. In particular, interconnect
segment 1220 forms a first angle 1224 with spring anchor 1212.
Unlike the kink of FIG. 6, the take off point is below the anchor
plane. After release, the downward oriented bend of interconnect
segment 1220 results in a spring oriented such that spring
deflection along direction 1224 does not place excessive stress on
the spring structure. Thus, unlike the bend of FIG. 6, spring
breakage at the discontinuity is less of a problem. In practice,
the downward kink in FIG. 12 is very small compared to the bend in
FIG. 6. Particularly when the spring material is deposited before
oxidation, the spring material 1120 and 1124 in FIG. 11 does not
stretch much when release section 1104 is oxidized. In contrast,
the upward bend 628 in FIG. 6 is formed by depositing spring
material over a patterned step on the release layer. Depositing the
spring over the step makes the upward bend a part of the spring
shape by design.
[0049] To form the smooth spring structure of FIGS. 7, 8, and 10, a
method of chemically altering an anchor region 708 of a release
layer without a volumetric change is needed. One example method of
inducing such a chemical change is to implant or otherwise diffuse
Boron into a Si release layer in the anchor region. A boron
concentration of 1020 per cubic centimeter is sufficient to render
the anchor portion 708 non-etchable by KOH/water/alcohol wherein
the activating alcohols may include isopropanol, secondary butanol,
or tertiary butanol. However, one disadvantage of the described
process is that KOH reacts with some common spring materials such
as MoCr, thus the use of KOH limits the choice of spring materials
to materials that withstand KOH such as Ni.
[0050] Other methods and materials may also be used to treat a
region of release layer 702 of FIG. 7 to resist etching. For
example, instead of oxidation, photodefinable dielectrics and
photodefinable metal oxide films may be used for the release layer.
Examples of photodefinable dielectrics include, but are not limited
to HD-40001-line photodefinable polyiminide, SU-8, photo-Ormocers,
and photo-Benzocyclobutene (BCB). Examples of photodefinable metal
oxide films include those made by Brewer Science, Inc. of 2401
Brewer Drive, Rolla, Mo. 65401-9926 USA.
[0051] When using photodefinable material, only the anchor region
708 of release layer 702 is exposed to radiation. A mask deposited
over release regions 704 may be used to control exposure to
radiation. The mask protects release region 704 from the radiation.
Alternatively, lasers may be used instead of masks. The lasers
carefully direct radiation to irradiate only the anchor region 708
of release layer 702. Depending on the photocurable material used,
the radiation may be visible light, UV light, or X-ray radiation.
When photodefinable polymer is used for the release layer, the
radiation is typically ultraviolet radiation (UV radiation) having
a wavelength between 230 nm and 400 nm. A commonly used exposure
wavelength is the 365 nm i-line of mercury lamps. The UV induces a
chemical change by cross linking polymers in the anchor region of
the release layer making the anchor region more resistant to
etching.
[0052] After exposure to radiation, a selective etchant or
developer removes the unexposed portions of the release layer 702.
The actual selective etchant used depends on the material used for
the release layer, but an example selective etchant is _BCB
developers developed by Dow Chemical Co. (Midland, Mich.) when
Benzocyclobutene is used for the release layer. Thermal curing
after spring release may also further fix the anchor portion 708 of
release layer 702.
[0053] Most of the prior description has been focused on converting
an anchor region of a continuous release layer to make the anchor
region resistant to etching. The described technique may be
modified such that instead of converting an anchor region, two
different materials are deposited. The two materials include a
first etch resistant material deposited in the anchor region and a
second etchable material deposited in the release region. Although
using two entirely different materials in the anchor region and the
release region provides flexibility in material selection,
depositing two different materials involves additional masking and
deposition steps thereby increasing fabrication costs. Furthermore,
depositing two different materials extremely close together, with
proper alignment, and approximately equivalent thicknesses is a
complicated process that can result in lower yields.
[0054] The preceding specification has included numerous details
that are intended to provide examples and to facilitate
understanding of the invention and various uses of the invention.
These details should not be used to limit the invention. For
example, although the overlying material has been described as a
spring material, usually a stressed metal material, other overlying
materials may also be used. The formed structure is not necessarily
even a spring. Thus, the invention should only be limited by the
claims, as originally presented and as they may be amended, to
encompass variations, alternatives, modifications, improvements,
equivalents, and substantial equivalents of the embodiments and
teachings disclosed herein, including those that are presently
unforeseen or unappreciated, and that, for example, may arise from
applicants/patentees and others.
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