U.S. patent application number 10/922565 was filed with the patent office on 2005-03-03 for method for making a micromechanical device by removing a sacrificial layer with multiple sequential etchants.
Invention is credited to Heureux, Peter J., Huibers, Andrew G., Patel, Satyadev R., Schaadt, Gregory P..
Application Number | 20050045276 10/922565 |
Document ID | / |
Family ID | 23127633 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050045276 |
Kind Code |
A1 |
Patel, Satyadev R. ; et
al. |
March 3, 2005 |
Method for making a micromechanical device by removing a
sacrificial layer with multiple sequential etchants
Abstract
An etching method, such as for forming a micromechanical device,
is disclosed. One embodiment of the method is for releasing a
micromechanical structure, comprising, providing a substrate;
providing a sacrificial layer directly or indirectly on the
substrate; providing one or more micromechanical structural layers
on the sacrificial layer; performing a first etch to remove a
portion of the sacrificial layer, the first etch comprising
providing an etchant gas and energizing the etchant gas so as to
allow the etchant gas to physically, or chemically and physically,
remove the portion of the sacrificial layer; performing a second
etch to remove additional sacrificial material in the sacrificial
layer, the second etch comprising providing a gas that chemically
but not physically etches the additional sacrificial material.
Another embodiment of the method is for etching a silicon material
on or within a substrate, comprising: performing a first etch to
remove a portion of the silicon, the first etch comprising
providing an etchant gas and energizing the etchant gas so as to
allow the etchant gas to physically, or chemically and physically,
remove the portion of silicon; performing a second etch to remove
additional silicon, the second etch comprising providing an etchant
gas that chemically but not physically etches the additional
silicon.
Inventors: |
Patel, Satyadev R.; (Elk
Grove, CA) ; Huibers, Andrew G.; (Mountain View,
CA) ; Schaadt, Gregory P.; (Santa Clara, CA) ;
Heureux, Peter J.; (Felton, CA) |
Correspondence
Address: |
REFLECTIVITY, INC.
350 POTRERO AVENUE
SUNNYVALE
CA
94085
US
|
Family ID: |
23127633 |
Appl. No.: |
10/922565 |
Filed: |
August 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10922565 |
Aug 19, 2004 |
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10154150 |
May 22, 2002 |
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6800210 |
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60293092 |
May 22, 2001 |
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Current U.S.
Class: |
156/345.43 |
Current CPC
Class: |
B81C 2201/0109 20130101;
B81C 1/00547 20130101; B81C 1/00476 20130101; B81C 2201/112
20130101; B81C 2201/0132 20130101; B81C 2201/0133 20130101; B82Y
30/00 20130101 |
Class at
Publication: |
156/345.43 |
International
Class: |
C23F 001/00 |
Claims
We claim:
1. An apparatus comprising: an etching chamber; connected to the
etching chamber, a first source of etchant capable of etching in a
plasma state; and connected to the etching chamber, a second source
of etchant different from the first source of etchant and capable
of etching in a non-plasma state.
2. The apparatus of claim 1, further comprising an RF source.
3. The apparatus of claim 1, further comprising a source of
stiction treatment connected to the etching chamber.
4. The apparatus of claim 3, wherein the stiction treatment source
is a source of a chlorosilane.
5. The apparatus of claim 1, further comprising a load lock
selectively in fluid communication with the etching chamber.
6. The apparatus of claim 1, wherein the chamber comprises two
chambers, one chamber having the RF source with the first source of
etchant connected thereto, a second chamber having the second
source of etchant connected thereto.
7. The apparatus of claim 1, wherein the first source of etchant is
a source of a hydrocarbon, fluorocarbon or SF6 and the second
source of etchant is a source of noble gas halide or
interhalogen.
8. The apparatus of claim 7, wherein the first source of etchant is
a source of fluorocarbon and the second source of etchant is
bromine trifluoride or xenon difluoride.
9. The apparatus of claim 7, further comprising a source of at
least one of Ar, O2, He and N2.
10. The apparatus of claim 1, further comprising a recirculation
line for selectively recirculating etchant gas.
11. The apparatus of claim 1, wherein the etchants of the first and
second sources are gases capable of etching silicon when released
into the etching chamber, the gas from the second source capable of
etching silicon in a non-plasma state.
12. An apparatus comprising: a first etching chamber; connected to
the first etching chamber, a source of a first etchant capable of
etching in a plasma state; and a second etching chamber; connected
to the second etching chamber, a source of a second etchant
different from the first source of etchant and capable of etching
in a non-plasma state.
13. The apparatus of claim 12, wherein said first etching chamber
comprises an RF source.
14. The apparatus of claim 12, further comprising a source of
stiction treatment.
15. The apparatus of claim 14, wherein the stiction treatment
source is a source of a chlorosilane.
16. The apparatus of claim 12, further comprising a load lock.
17. The apparatus of claim 12, wherein the first etchant is a
hydrocarbon, fluorocarbon or SF6 and the second etchant is a noble
gas halide or interhalogen.
18. The apparatus of claim 17, wherein the first etchant is a
source of fluorocarbon and the second etchant is bromine
trifluoride or xenon difluoride.
19. The apparatus of claim 17, further comprising a source of at
least one of Ar, O2, He and N2.
20. The apparatus of claim 12, further comprising a recirculation
line for selectively recirculating etchant gas.
21. The apparatus of claim 12, wherein the first and second
etchants are gases capable of etching silicon.
22. The apparatus of claim 12, wherein the second etchant is xenon
difluoride.
23. The apparatus of claim 22, wherein the first etchant is a
fluorocarbon.
24. The apparatus of claim 22, wherein the first chamber is
constructed so as to form a plasma of the first etchant.
25. The apparatus of claim 24, wherein a silicon sample to be
etched is disposed within the first chamber and the first etchant
is capable of being energized within the first chamber to etch the
silicon sample
26. The apparatus of claim 12, wherein the first chamber is
constructed so as to form charged species or radicals from the
first etchant.
27. The apparatus of claim 12, wherein the first etchant has the
formula CxFy.
28. The apparatus of claim 27, wherein the first etchant is
selected from C5F12, C3F6, C2F6, C3F8, C4F8, CF4, C2F4, CF2, C2F6,
C4F10, C6F14.
29. The apparatus of claim 28, wherein the second etchant is
selected from bromine trifluoride, bromine trichloride and xenon
difluoride.
30. The apparatus of claim 12, wherein the first chamber is a
plasma etching system and the second chamber is a xenon difluoride
etching system.
31. The apparatus of claim 12, wherein the first chamber comprises
top and bottom electrodes.
32. The apparatus of claim 31, wherein the top and bottom
electrodes are separated by a grounded diffuser plate.
33. The apparatus of claim 12, wherein the first etching chamber is
a plasma etching chamber that can be operated in remote plasma
mode.
34. The apparatus of claim 12, wherein the first chamber is
constructed to perform sputter etching or accelerated ion-assisted
etching.
35. The apparatus of claim 12, wherein the first chamber is
constructed for performing a first etch of a sample and the second
chamber is constructed for performing a second etch of the
sample.
36. The apparatus of claim 35, wherein the apparatus allows for
performing first and second etches on a substrate in the first and
second chambers respectively without exposing the substrate to
ambient.
37. The apparatus of claim 12, further comprising a source of
SF6.
38. The apparatus of claim 22, further comprising a source of an
alkyl chlorosilane.
39. The apparatus of claim 12, wherein the first chamber comprises
a means to energize the first etchant.
40. The apparatus of claim 21, wherein the first chamber comprises
a pair of parallel plate electrodes disposed in the first
chamber.
41. The apparatus of claim 40, further comprising a source of
electric power to supply power to the electrodes so that gas
discharging takes place to generate gas plasma.
42. The apparatus of claim 12, wherein the first etchant has the
formula CxFyHz.
43. The apparatus of claim 42, wherein the first etchant is
selected from C3HF6, C3H2F6, C3H3F5, CH2F2, C3HF7.
Description
CROSS-REFERENCE TO RELATED CASES
[0001] This application is a divisional of Ser. No. 10/154,150 to
Patel, et al filed May 22, 2002, the subject matter being
incorporated herein by reference.
BACKGROUND
[0002] A wide variety of micro-electromechanical devices (MEMS) are
known, including accelerometers, DC relay and RF switches, optical
cross connects and optical switches, microlenses, reflectors and
beam splitters, filters, oscillators and antenna system components,
variable capacitors and inductors, switched banks of filters,
resonant comb-drives and resonant beams, and micromirror arrays for
direct view and projection displays. There are a wide variety of
methods for forming MEMS devices, including a) forming
micromechanical structures monolithically on the same substrate as
actuation or detection circuitry, b) forming the micromechanical
structures on a separate substrate and transferring the formed
structures to a circuit substrate, c) forming circuitry on one
substrate and forming micromechanical elements on another substrate
and bonding the substrates side by side or in a flip-chip type
arrangement, or d) forming micromechanical structures without any
circuitry. Regardless of the actual method used, at some point in
the manufacturing process for making MEMS devices, a sacrificial
layer is generally removed in order to release the micromechanical
structure. The released structure is then able to be actively
actuated or moved, such as pivoting or rotation of a micromirror
for a projection display or optical switch, or movement during
sensing, such as an accelerometer in an automobile airbag
system.
SUMMARY OF THE INVENTION
[0003] In its most simple form, the invention is directed to
etching a material where a first etch removes a portion of the
material and fully or partially physically removes the material,
and where a subsequent etch removes additional material and removes
the material chemically but not physically. The material can be a
semiconductor material such as silicon, and the areas removed can
be of any dimensions such as an elongated trench, a well or other
area limited in size, or even an entire area across a substrate.
The result of the first and second etches can also result in an
undercut such as for microfluidic channels or for a thermal sensor,
or for simply removing material in an IC process.
[0004] In another embodiment, the invention is directed to
releasing a micromechanical structure, comprising: providing a
substrate; providing a sacrificial layer directly or indirectly on
the substrate; providing one or more micromechanical structural
layers on the sacrificial layer; performing a first etch to remove
a portion of the sacrificial layer, the first etch comprising
providing an etchant and energizing the etchant so as to allow the
etchant to physically, or chemically and physically, remove the
portion of the sacrificial layer; and performing a second etch to
remove additional sacrificial material in the sacrificial layer,
the second etch comprising providing a second ethant that
chemically but not physically etches the additional sacrificial
material.
[0005] Another embodiment of the method is for etching a material
on or within a substrate, comprising: performing a first etch to
remove a portion of the material, the first etch comprising
providing an etchant and energizing the etchant so as to allow the
etchant to physically, or chemically and physically, remove the
portion of the material; and performing a second etch to remove
additional silicon, the second etch comprising providing an etchant
that chemically but not physically etches the additional
material.
[0006] Also disclosed is an apparatus that comprises an etching
chamber; connected to the etching chamber, a first source of
etchant capable of etching a target material at least partially
physically; and connected to the etching chamber, a second source
of etchant different from the first source of etchant and capable
of etching the target material chemically but not physically.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIGS. 1A to 1E illustrate one method for forming
micromirrors;
[0008] FIG. 2 is a top view of a micromirror showing line 1-1 for
taking the cross section for FIGS. 1A to 1E;
[0009] FIGS. 3A to 3E illustrate the same method as in FIGS. 1A to
1D but taken along a different cross section;
[0010] FIG. 4 is a top view of a mirror showing line 3-3 for taking
the cross section for FIGS. 3A to 3E;
[0011] FIG. 5 is an illustration of a second embodiment of a
micromirror in the present invention;
[0012] FIGS. 6A to 6C are cross sectional views of a method of
making the micromirrors of FIG. 5, taken along line 6-6 in FIG.
5;
[0013] FIGS. 7A to 7C are cross sectional views of the method
illustrated in FIGS. 6A to 6C, taken along line 7-7 in FIG. 5;
[0014] FIG. 8 is an illustration of the I/O pads and Si backplane
for the embodiment of the invention using separate substrates;
[0015] FIGS. 9A to 9D are cross sectional views illustrating the
dual etching steps in the method of the present invention;
[0016] FIG. 10 is an isometric view of a released
microstructure;
[0017] FIG. 11 is a cross sectional view of a second etch performed
without the first etch;
[0018] FIG. 12 is a view of one embodiment of an apparatus for
performing etching in the present invention;
[0019] FIG. 13 is a view of another embodiment of the apparatus of
the present invention; and
[0020] FIG. 14 is an illustration of further details of the etching
chamber for one embodiment of the invention.
DETAILED DESCRIPTION
[0021] Micromechanical Structure Fabrication:
[0022] Processes for microfabricating a MEMS device such as a
movable micromirror and mirror array are disclosed in U.S. Pat.
Nos. 5,835,256 and 6,046,840 both to Huibers, the subject matter of
each being incorporated herein by reference. A similar process for
forming MEMS movable elements (e.g. mirrors) on a wafer substrate
(e.g. a light transmissive substrate or a substrate comprising CMOS
or other circuitry) is illustrated in FIGS. 1 to 4. By "light
transmissive", it is meant that the material will be transmissive
to light at least in operation of the device (The material could
temporarily have a light blocking layer on it to improve the
ability to handle the substrate during manufacture, or a partial
light blocking layer for decreasing light scatter during use.
Regardless, a portion of the substrate, for visible light
applications, is preferably transmissive to visible light during
use so that light can pass into the device, be reflected by the
mirrors, and pass back out of the device. Of course, not all
embodiments will use a light transmissive substrate). By "wafer" it
is meant any substrate on which multiple microstructures or
microstructure arrays are to be formed and which allows for being
divided into dies, each die having one or more microstructures
thereon. Though not in every situation, often each die is one
device or product to be packaged and sold separately. Forming
multiple "products" or dies on a larger substrate or wafer allows
for lower and faster manufacturing costs as compared to forming
each die separately. Of course the wafers can be any size or shape,
though it is preferred that the wafers be the conventional round or
substantially round wafers (e.g. 4", 6", 8" or 12" in diameter) so
as to allow for manufacture in a standard foundry.
[0023] FIGS. 1A to 1D show a manufacturing process for a
micromechanical mirror structure. As can be seen in FIG. 1A, a
substrate such as glass (e.g. Corning 1737F), quartz, Pyrex.TM.,
sapphire, (or silicon alone or with circuitry thereon) etc. is
provided. The cross section of FIGS. 1A-D is taken along line 1-1
of FIG. 2. An optional block layer on the glass surface (not shown)
can be provided to block light (incident through the light
transmissive substrate during use) from reflecting off of the hinge
and potentially causing diffraction and lowering the contrast ratio
(if the substrate is transparent).
[0024] As can be seen in FIG. 1B, a sacrificial layer 14, such as
amorphous silicon, is deposited. The thickness of the sacrificial
layer can be wide ranging depending upon the movable element/mirror
size and desired tilt angle, though a thickness of from 500 .ANG.
to 50,000 .ANG., preferably around 5000 .ANG. is preferred.
Alternatively the sacrificial layer could be polysilicon, silicon
nitride, silicon dioxide, polyimide or other organic material, etc.
depending upon the materials selected for the structural layers. A
lithography step followed by a sacrificial layer etch forms holes
16a,b in the sacrificial silicon, which can be any suitable size,
though preferably having a diameter of from 0.1 to 1.5 um, more
preferably around 0.7.+-.0.25 um. The etching is performed down to
the glass/quartz substrate or down to the block layer if present.
Preferably if the glass/quartz layer is etched, it is in an amount
less than 2000.ANG..
[0025] At this point, as can be seen in FIG. 1C, a first layer 18
is deposited by chemical vapor deposition. Preferably the material
is silicon nitride or silicon oxide deposited by any suitable
method such as sputtering, LPCVD or PECVD, however other materials
such as polysilicon, silicon carbide or an organic compound could
be deposited at this point (of course the sacrificial layer and at
least the etchant of the second etch--to be described below--should
be adapted to the material used). The thickness of this first layer
can vary depending upon the movable element size and desired amount
of stiffness of the element, however in one embodiment the layer
has a thickness of from 100 to 3200 .ANG., more preferably around
1100 .ANG.. The first layer undergoes lithography and etching so as
to form holes (0.5 to 1 um in diameter) for posts for holding the
MEMS structure on the substrate.
[0026] A second layer 20 (the "hinge" layer) is deposited as can be
seen in FIG. 1D. By "hinge layer" it is meant the layer that
defines that portion of the device that flexes to allow movement of
the device. The hinge layer can be disposed only for defining the
hinge, or for defining the hinge and other areas such as the
mirror. In any case, it is preferred that the first layer is
removed in hinge areas prior to depositing the hinge material
(second layer). The material for the second (hinge) layer can be
the same (e.g. silicon nitride) as the first layer or different
(silicon oxide, silicon carbide, polysilicon, etc.) and can be
deposited by any suitable method such as sputtering or chemical
vapor deposition as for the first layer. The thickness of the
second/hinge layer can be greater or less than the first, depending
upon the stiffness of the movable element, the flexibility of the
hinge desired, the material used, etc. In one embodiment the second
layer has a thickness of from 50 .ANG. to 2100 .ANG., and
preferably around 500 .ANG.. In another embodiment, the first layer
is deposited by PECVD and the second layer by LPCVD.
[0027] As also seen in FIG. 1D, a reflective and conductive layer
22 is deposited. The reflective/conductive material can be gold,
aluminum or other metal, or an alloy of more than one metal though
it is preferably aluminum deposited by PVD. The thickness of the
metal layer can be from 50 to 2000 .ANG., preferably around 500
.ANG.. It is also possible to deposit separate reflective and
conductive layers. An optional metal passivation layer (not shown)
can be added, e.g. a 10 to 1100 .ANG. TiN or TiON layer deposited
by PECVD. Then, photoresist patterning on the metal layer is
followed by etching through the metal layer with a suitable metal
etchant. In the case of an aluminum layer, a chlorine (or bromine)
chemistry can be used (e.g. a plasma/RIE etch with Cl.sub.2 and/or
BCl.sub.3 (or Cl2, CCl4, Br2, CBr.sub.4, etc.) with an optional
preferably inert diluent such as Ar and/or He).
[0028] In the embodiment illustrated in FIGS. 1A to 1D, both the
first and second layers are deposited in the area defining the
movable element, whereas the second layer, in the absence of the
first layer, is deposited in the area of the hinge. It is also
possible to use more than two layers to produce a laminate movable
element, which can be desirable particularly when the size of the
movable element is increased such as for switching light beams in
an optical switch. A plurality of layers could be provided in place
of single layer 18 in FIG. 1C, and a plurality of layers could be
provided in place of layer 20 and in place of layer 22. Or, layers
20 and 22 could be a single layer, e.g. a pure metal layer or a
metal alloy layer or a layer that is a mixture of e.g. a dielectric
or semiconductor and a metal. Some materials for such layer or
layers that could comprise alloys of metals and dielectrics or
compounds of metals and nitrogen, oxygen or carbon (particularly
the transition metals) are disclosed in U.S. provisional patent
application 60/228,007, the subject matter of which is incorporated
herein by reference.
[0029] Whatever the specific combination, it is desirable that the
first/reinforcing layer(s) is provided and patterned (at least in
the hinge area) prior to depositing and patterning the hinge
material and metal. In one embodiment, the reinforcing layer is
removed in the area of the hinge, followed by depositing the hinge
layer and patterning both reinforcing and hinge layer together.
This joint patterning of the reinforcing layer and hinge layer can
be done with the same etchant (e.g. if the two layers are of the
same material) or consecutively with different etchants. The
reinforcing and hinge layers can be etched with a chlorine
chemistry or a fluorine chemistry where the etchant is a
perfluorocarbon or hydrofluorocarbon (or SF6) that is energized so
as to selectively etch the reinforcing and/or hinge layers both
chemically and physically (e.g. a plasma/RIE etch with CF.sub.4,
CHF.sub.3, C.sub.3F.sub.8, CH.sub.2F.sub.2, C.sub.2F.sub.6,
SF.sub.6, etc. or more likely combinations of the above or with
additional gases, such as CF.sub.4/H.sub.2, SF.sub.6/Cl.sub.2, or
gases using more than one etching species such as CF.sub.2Cl.sub.2,
all possibly with one or more optional inert diluents). Of course,
if different materials are used for the reinforcing layer and the
hinge layer, then a different etchant can be employed for etching
each layer. Alternatively, the reflective layer can be deposited
before the first (reinforcing) and/or second (hinge) layer. Whether
deposited prior to the hinge material or prior to both the hinge
material and the reinforcing material, it is preferable that the
metal be patterned (e.g. removed in the hinge area) prior to
depositing and patterning the hinge material.
[0030] FIGS. 3A to 3D illustrate the same process taken along a
different cross section (cross section 3-3 in FIG. 4) and show the
sacrificial layer 14 deposited on the light transmissive substrate
10, followed by layers 18, 20 and the metal layer 22. The cross
sections in FIGS. 1A to 1D and 3A to 3D are taken along
substantially square mirrors in FIGS. 2 and 4 respectively.
[0031] It should also be noted that materials and method mentioned
above are examples only, as many other method and materials could
be used. For example, the Sandia SUMMiT process (using polysilicon
for structural layers) or the Cronos MUMPS process (also
polysilicon for structural layers) could be used in the present
invention. Also, a MOSIS process (AMI ABN-1.5 um CMOS process)
could be adapted for the present invention, as could a MUSiC
process (using polycrystalline SiC for the structural layers) as
disclosed, for example, in Mehregany et al., Thin Solid Films, v.
355-356, pp. 518-524, 1999. Also, though PVD and CVD are referred
to above, other thin film deposition methods could be used for
depositing the layers, including spin-on, anodization, oxidation,
electroplating and evaporation.
[0032] After forming the microstructures as in FIGS. 1 to 4 on the
first wafer, it is preferable to remove the sacrificial layer so as
to release the microstructures (in this case micromirrors). The
release is described in more detail hereinbelow. This release can
be performed at the die level, though it is preferred to perform
the release at the wafer level. FIGS. 1E and 3E show the
microstructures in their released state. As can be seen in FIG. 1E,
posts 2 hold the released microstructure on substrate 10.
[0033] An alternate embodiment to that illustrated in FIG. 1-4, is
illustrated in FIGS. 5 to 7. As can be seen in FIG. 5, the mirror
formed is not square. The micromirrors of the present invention
need not be square but can have other shapes that may decrease
diffraction and increase the contrast ratio, depending upon the
position of the light source. Such mirrors are disclosed in U.S.
provisional patent application 60/229,246 to Ilkov et al. filed
Aug. 30, 2000, and U.S. patent application Ser. No. 09/732,445 to
Ilkov et al. filed Dec. 7, 2000, the subject matter of each being
incorporated herein by reference. Also, the mirror hinges can be
flexure hinges as illustrated in the above-mentioned applications
and as shown in FIG. 5 of the present application.
[0034] FIGS. 6A to 6C are cross sections taken along line 6-6 of
FIG. 5. As can be seen in FIG. 6A, a substrate 1 is provided. A
sacrificial layer 2 is deposited thereon and patterned so as to
form holes 6A, 6B. The sacrificial material can be any suitable
sacrificial material known in the art, such as amorphous silicon,
silicon nitride, silicon oxynitride, silicon dioxide, PSG,
amorphous silicon, etc. On layer 2 is deposited a micromechanical
structural layer 7 (FIG. 6B) of a material different than that of
the sacrificial layer (e.g. polysilicon if the sacrificial layer is
silicon dioxide, silicon nitride if the sacrificial layer is
amorphous silicon or polyimide, etc.). As can be seen in FIG. 6C,
an additional structural layer 8 is deposited (after removing part
of layer 7 in the hinge areas--not evident in this cross section),
followed by depositing a reflective layer 9 (e.g. a metal such as
Al, Ag, Au etc.). Layers 2, 7, 8 and 9 can be deposited by any
known methods depending upon the material (spin-on for an organic
material such as polyimide, chemical vapor deposition or sputtering
for silicon or silicon compounds, sputtering for metal, etc.)
and/or as disclosed herein with respect to the other figures.
Finally, as illustrated in FIG. 6C, layers 7 to 9 are patterned by
depositing and patterning photoresist followed by etching with a
suitable etchant selected for the material(s) being etched (e.g.
chlorine chemistry for a metal layer, hydrocarbon or fluorocarbon
(or SF6) plasma for silicon or silicon compound layers, etc.). Not
shown is the final removal of the sacrificial layer 2, discussed
further herein below.
[0035] FIGS. 7A to 7C illustrate the same process as in FIGS. 6A to
6C, but are cross sectional views taken along line 7-7 of FIG. 5.
As can be seen in FIG. 7A, the sacrificial layer 2 is deposited on
substrate 1, followed by, in FIG. 7B, depositing layer 7. After
deposition of layer 7, portions are removed (see gaps in layer 7 in
FIG. 7B). This can be performed with a chlorine , chlorine
compound, hydrocarbon, fluorocarbon or other known plasma etch
selected based on the composition of layer 7. Then, as can be seen
in FIG. 7C, layers 8 and 9 are deposited over all areas (not shown)
followed by patterning to form hinges in the gaps in layer 7 and a
corresponding reflective movable mirror element. The hinges,
therefore, are made of layers 8 and 9 (e.g. a silicon or silicon
compound layer and a metal layer) and the mirror area is formed of
layers 7 to 9. Of course there are many variations to the above,
such as putting the metal layer down first, forming the hinges or
the entire device from a silicon compound-metal alloy (such as in
application 60/228,007 mentioned above), or using a single silicon
or silicon compound layer and a single metal layer.
[0036] Circuitry:
[0037] In the present invention, the circuitry can be formed
together on the same substrate as the microstructures, such as in
U.S. Pat. Nos. 5,061,049, 5,527,744, and 5,872,046. If the
microstructures are not formed monolithically on the same wafer as
the circuitry, then a second substrate can be provided having
circuitry thereon (or, circuitry could be provided on both the
first wafer and the replacement substrate if desired). If the
microstructures are micromirrors, then it may be preferable to form
circuitry and electrodes on a second wafer substrate with at least
one electrode electrostatically controlling one pixel (one
micromirror on the first wafer portion of the die) of the
microdisplay. The voltage on each electrode on the surface of the
backplane determines whether its corresponding microdisplay pixel
is optically `on` or `off,` forming a visible image on the
microdisplay. Details of the backplane and methods for producing a
pulse-width-modulated grayscale or color image are disclosed in
U.S. patent application Ser. No. 09/564,069 to Richards, the
subject matter of which is incorporated herein by reference.
[0038] The display pixels themselves, in a preferred embodiment,
are binary, always either fully `on` or fully `off,` and so the
backplane design is purely digital. Though the micromirrors could
be operated in analog mode, no analog capability is necessary. For
ease of system design, the backplane's I/O and control logic
preferably run at a voltage compatible with standard logic levels,
e.g. 5V or 3.3V. To maximize the voltage available to drive the
pixels, the backplane's array circuitry may run from a separate
supply, preferably at a higher voltage.
[0039] One embodiment of the backplane can be fabricated in a
foundry 5V logic process. The mirror electrodes can run at 0-5V or
as high above 5V as reliability allows. The backplane could also be
fabricated in a higher-voltage process such as a foundry Flash
memory process using that process's high-voltage devices. The
backplane could also be constructed in a high-voltage process with
larger-geometry transistors capable of operating at 12V or more. A
higher voltage backplane can produce an electrode voltage swing
significantly higher than the 5-7V that the lower voltage backplane
provides, and thus actuate the pixels more robustly.
[0040] In digital mode, it is possible to set each electrode to
either state (on/off), and have that state persist until the state
of the electrode is written again. A RAM-like structure, with one
bit per pixel is one architecture that accomplishes this. One
example is an SRAM-based pixel cell. Alternate well-known storage
elements such as latches or DRAM (pass transistor plus capacitor)
are also possible. If a dynamic storage element (e.g. a DRAM-like
cell) is used, it is desirable that it be shielded from incident
light that might otherwise cause leakage.
[0041] The perception of a grayscale or full-color image will be
produced by modulating pixels rapidly on and off, for example
according to the method in the above-mentioned U.S. patent
application Ser. No. 09/564,069 to Richards. In order to support
this, it is preferable that the backplane allows the array to be
written in random-access fashion, though finer granularity than a
row-at-a-time is generally not necessary.
[0042] It is desirable to minimize power consumption, primarily for
thermal reasons. Decreasing electrical power dissipation will
increase the optical/thermal power budget, allowing the
microdisplay to tolerate the heat of more powerful lamps. Also,
depending upon the way the microdisplay is assembled
(wafer-to-wafer join+offset saw), it may be preferable for all I/O
pads to be on one side of the die. To minimize the cost of the
finished device it is desirable to minimize pin count. For example,
multiplexing row address or other infrequently-used control signals
onto the data bus can eliminate separate pins for these functions
with a negligible throughput penalty (a few percent, e.g. one clock
cycle for address information per row of data is acceptable). A
data bus, a clock, and a small number of control signals (5 or
less) are preferred.
[0043] In use, the die can be illuminated with a 200 W or more arc
lamp. The thermal and photo-carrier effects of this may result in
special layout efforts to make the metal layers as `opaque` as
possible over the active circuitry to reflect incident optical
energy and minimize photocarrier and thermal effects. An on-chip PN
diode could be included for measuring the temperature of the
die.
[0044] In one embodiment the resolution is XGA, 1024.times.768
pixels, though other resolutions are possible. A pixel pitch of
from 5 to 24 um is preferred (e.g. 14 um). The size of the
electrode array itself is determined by the pixel pitch and
resolution. A 14 um XGA device's pixel array will therefore be
14.336.times.10.752 mm.
[0045] As can be seen in FIG. 8, the I/O pads (88) can be placed
along the right edge of the die, as the die is viewed with pixel
(0,0) (89 in FIG. 5) at the top left corner. Putting the pads on
the `short` (left/right) edge (87) of the die is preferable due to
the slightly reduced die size. The choice of whether the I/O should
go on the left vs. right edge of the die is of little importance
since the display controller ASIC may support mirroring the
displayed image in the horizontal axis, the vertical axis, or both.
If it is desired to orient the display with the I/O on the left
edge, the image may simply be rotated 180 degrees by the external
display controller. The electrode voltage during operation is, in
the low state 0V and in the high state preferably from 5 to 7 V (or
12V or higher in the higher voltage design). Of course other
voltages are possible, though lower actuation voltages are
preferred. In one embodiment the electrodes are metal squares,
though other geometries are possible. Standard CMOS passivation
stackup over the electrodes can be provided.
[0046] Assembly:
[0047] After depositing and patterning the various micromechanical
layers, the substrate itself, or a deposited sacrificial material,
is removed in order to release the micromechanical structures.
Removal of substrate of deposited material can also be simply for
undercutting (e.g. for a thermal sensor) or for forming wells or
trenches (e.g in an integrated circuit process). In any case, the
removal/etching of material is preferably performed immediately
prior to bonding the just-etched substrate to another substrate
(e.g. a) a circuit substrate as disclosed hereinabove, b) a
permanent silicon, glass or other substrate such as when the
micromechanical structures are formed monolithically on the same
substrate as actuation, detection or other circuitry, or c) a
removable "sacrificial" substrate such as disclosed in U.S. patent
application 60/276,222 to Patel et al. filed Mar. 15, 2001.
Regardless of the type and purpose of the second substrate to be
attached, any known substrate or specifically wafer bonding process
could be used, including epoxy bonding (disclosed further below),
anodic bonding, fusion bonding, metal thermocompression bonding,
etc.). In one embodiment of the invention, the substrate with
(preferably released) micromechanical structures, or undercut
structures, is bonded to the second substrate with the application
of epoxy. Before or after such substrate bonding, an optional
anti-stiction treatment or other passivation treatment, or
treatment for improving epoxy bond strength, can be applied. If an
anti-stiction treatment is performed, in a preferred embodiment the
treatment is a self assembled monolayer or lubricant. The
anti-stiction layer is preferably formed by placing the device in a
liquid or gas silane, preferably an alkyl silane, e.g. a
chlorosilane known in the art. Of course, many different silanes
and other materials are known in the art for their ability to
reduce surface contact forces and provide anti-stiction for MEMS
structures.
[0048] The release of the micromechanical structures in the present
invention (e.g. micromirrors)--or simple etching--is a multi-step
process. A first etch is performed that has relatively low
selectivity (e.g. less than 200:1, preferably less than 100:1 and
more preferably less than 35:1 or even 10:1). A second etch follows
has higher selectivity (e.g. greater than 100:1, preferably greater
than 200:1 and more preferably greater than 500:1 or even 1000:1).
The first etch is preferably a gas etch where the etchant is
preferably a fluoride etchant, more preferably an energized
fluoride gas. The energized fluoride gas is energized with, for
example, light (e.g. UV light), an electric field, or other fields
or energy to energize the gas beyond its normal energy as a gas at
a particular temperature, such as into a plasma state. This
energizing of the gas of the invention gives it a physical
component to its etching behavior, in addition to a chemical
component. Specific examples for energizing the etchant of the
first etch include using a pair of parallel plate electrodes
disposed in a chamber with a gas, and applying electric power of
high frequency to the electrodes so that gas discharging takes
place to generate gas plasma. Besides reactive ion etching and
plasma etching, there are EDR dry etching methods, ion beam etching
methods and photo excited etching methods. The first etchant could
also be a noble gas which is energized so as to cause a purely
physical etch in the first etch (e.g. an Ar or Xe sputter etch).
These methods for the first etch in the invention preferably
accomplish the initial etch by causing an interaction physically
(Ar sputter) or chemically and physically (plasma fluoride
compound) between the energized gas and the material to be removed
in making the MEMS device. The first etch, therefore, is preferably
the result of at least energetic bombardment of the sacrificial
material (e.g. by charged species such as positive ions, electrons
or negative ions), and possibly additionally a chemical reaction
between the etchant gas or gases (e.g. by radicals) and the
sacrificial material.
[0049] The first etch utilizes a halogen containing etchant gas
that removes the silicon containing sacrificial material both
chemically and physically and is preferably a fluorocarbon compound
which has carbon and fluorine components (a perfluorocarbon), or
carbon, fluorine and hydrogen components (a hydrocarbon).
Chlorofluorocarbons and bromofluorocarbons (e.g. C2F2Cl2, C3F4Cl2,
CFCl, C3F2Cl2Br2, CF3Cl, C2F2Br2, CFBr and CF2ClBr) are also
possibilities, though they are banned in most countries of the
world. If the etchant gas is of the formula CxFy, it can be C5F12,
C3F6, C2F6, C3F8, C4F8, CF4, C2F4, CF2, C2F6, C4F10, C6F14 or other
etchant consisting of carbon and fluorine as is known in the art.
If the etchant gas is a hydrocarbon of the formula CxFyHz, it can
be C3HF6, C3H2F6, C3H3F5, CH2F2, C3HF7 or other etchant consisting
of carbon, fluorine and hydrogen as is known in the art. Or, the
etchant of the first etch could be an oxygenated perfluorocarbon,
such as CF3OCHFCF3 or CF3CF2OCF2CHF2. The first etch can
alternatively utilize sulfur hexafluoride, or an energized
interhalogen or a noble gas halide that etches the sacrificial
material both chemically and physically (e.g. RIE/plasma XeF2, IF5,
BrCl3, BrF3, IF7, ClF3, ClF5, IC1, IBr, etc.).
[0050] In one embodiment, the first etch gas is excited with
multiple or single wavelengths in the ultraviolet region,
preferably in the UV-C region, such as with synchrotron radiation,
or preferably with a xenon flash lamp (200 nm and above), a
photoionization lamp such as a Cathodeon photoionization lamp (112
nm to 250 nm), a vacuum ultra violet lamp such as a Cathodeon
(Deuterium arc source) vacuum ultra violet lamp (112 nm up to 900
nm), or a McPherson Deuterium VUV (emissions continuous between 165
nm and 370 nm). Of course the spectrum or specific wavelength can
be tailored to the etchant gas being used. For example, a single
wavelength excimer laser could also be used, that has a wavelength
that corresponds to an absorption wavelength of the etchant gas.
For example, if the first etch uses XeF2, then an excimer laser
emitting a wavelength of 157 nm could be used to photoionize the
XeF2 gas. For example, a fluorine laser (e.g. a VUV 157 nm GAM
laser, Lambda-Physik Novaline F1030--1000 Hz 157 nm fluorine laser,
or a Cymer ELX-6500 1000 Hz 157 nm fluorine laser) that is scanned
over the substrate surface or exposes the entire substrate or
portions thereof (e.g. die portions) at the same time due to
magnification with CaF2 optics (e.g. a catadioptric lens
system)--with or without a mask to protect micromechanical
structures--can be utilized. In the alternative, a plasma etching
system, e.g. from MRC, Drytek or Applied Materials, could be used
to energize the first etch gas.
[0051] One or more additional gases can be mixed with the
aforementioned etchants for the first etch, including one or more
of O2, an inert gas such as Xe or Ar, N2, F2, H2, CO, NxFy (e.g.
NF3), SixFy (e.g. SiF4) or an additional fluorocarbon (with or
without a hydrogen component) as above. The exact mixture of gases
for the first etch can be optimized for the sacrificial material as
known in the art, though it is not necessary that the selectivity
be optimized (rather that the primary fluoride containing gas and
any additional gases be capable of etching silicon and/or silicon
compounds when energized). Regardless of which gas or gases are
used in the first energized etch, it is preferred that the first
etch not proceed all the way through the thickness of the
sacrificial layer. In most cases, the first etch should proceed
through 1/4 or less, or preferably {fraction (1/10)} or less of the
total thickness of the sacrificial layer. Also, it is preferred
that the etch proceed for less than 20 minutes, and more preferably
less than 10 minutes. The preferred etching depth is 500 angstroms
or less and preferably less than 250 angstroms. Such limits on the
first etch should result in substantially no undercutting (of etch
material from under the micromechanical structural material).
[0052] The second etch utilizes an etchant gas capable of
spontaneous chemical etching of the sacrificial material,
preferably isotropic etching that chemically (and not physically)
removes the sacrificial material. Such chemical etching and
apparatus for performing such chemical etching are disclosed in
U.S. patent application Ser. No. 09/427,841 to Pate et al. filed
Oct. 26, 1999, and in U.S. patent application Ser. No. 09/649,569
to Patel at al. filed Aug. 28, 2000, the subject matter of each
being incorporated herein by reference. Preferred etchants for the
second etch are gas phase fluoride etchants that, except for the
optional application of temperature, are not energized. Examples
include HF gas, noble gas halides such as xenon difluoride, and
interhalogens such as IF5, BrCl3, BrF3, IF7 and ClF3. The second
etch may comprise additional gas components such as N2 or an inert
gas (Ar, Xe, He, etc.). Though such gases can be used in the first
etch, the difference is that in the first etch they are energized
(e.g to a plasma state) to physically and chemically etch the
sacrificial material, whereas in the second etch, except for
optional heating, the gas is not energized and chemically etches
the sacrificial material isotropically. In this way, the remaining
sacrificial material is removed and the micromechanical structure
is released. In one aspect of such an embodiment, BrF3 or XeF2 are
provided in a plasma-etching chamber with diluent (e.g. N2 and He).
A plasma etch for 1 to 90 minutes, depending upon the concentration
of etchant used, is followed by a non-plasma chemical etch using
the same interhalogen or noble gas halide.
[0053] In one embodiment, the first etch removes sacrificial
material exposed between micromechanical elements to be released
that are from 1/4 to 5 um, preferably from 1/2 to 1 um spaced apart
from each other, thus removing "strips" of sacrificial material
having an effective width of e.g. from 1/2 to 1 um. The depth of
sacrificial material removed in the first etch is from 10 to 100
angstroms and is less than {fraction (1/10)}th, preferably less
than {fraction (1/20)}th of the total depth of sacrificial material
to be removed by both the first and second etchants. The material
removed between each microstructure and substrate has a length and
width of from 10 to 1000 um (preferably from 25 to 100 um) and a
depth of from 0.25 to 50 um (preferably from 1 to 10 um)--with in
most cases the etching undercuts and releases structural layers
having a surface area of from 100 to 2500 um.sup.2.
[0054] Referring again to FIGS. 1-4, it can be seen that a metal
layer (e.g. aluminum) in this embodiment is provided prior to
performing the first and second etches. As such, in a preferred
embodiment of the invention the first and second etches would
minimally harm any metal (e.g. Al) provided as part of the
microstructures or metallic interconnects, while at the same time
being preferably relatively non-selective so as to remove residues
such as photoresist, photoresist developer or remover/cleaner, as
well as oxides of silicon, silicon, etc. An industry standard HF
(gas or liquid) wash would not meet these preferred characteristics
(higher Al damage, low Si etching, etc.).
[0055] The methods discussed generally above, can be implemented in
a number of ways. For example, a glass wafer (such as a Coming
1737F, Eagle 2000, quartz or sapphie wafer) can be provided and
coated with an opaque coating, such as a Cr, Ti, Al, TaN,
polysilicon or TiN or other opaque coating at a thickness of 2000
angstroms (or more depending upon the material) on the backside of
the wafer, in order to make the transparent substrate temporarily
opaque for handling. Then, in accordance with FIGS. 1-4, after an
optional adhesion layer is deposited (e.g. a material with dangling
silicon bond such as SiNx--or SiOx, or a conductive material such
as vitreous carbon or indium tin oxide) then a sacrificial material
of hydrogenated amorphous silicon is deposited (gas=SiH4 (200
sccm), 1500 sccm of Ar, power=100 W, pressure=3.5 T, temp=380 C,
electrode spacing=350 mil; or gas=150 sccm of SiHy, 100 sccm of Ar,
power=55 W, pressure=3 Torr, temp=380 C, electrode spacing=350 mil;
or gas=200 sccm SiH4, 1500 sccm Ar, power=100 W, temp=300 C,
pressure=3.5 T; or other process points in between these settings)
on the transparent wafer at a thickness of 5000 Angstroms in a
plasma enhanced chemical vapor deposition system such as an Applied
Materials P5000. Or, the sacrificial material could be deposited by
LPCVD at 560 C, along the lines set forth in U.S. Pat. No.
5,835,256 to Huibers et al., incorporated herein by reference. Or,
the sacrificial material could be deposited by sputtering, or could
be a non-silicon containing material such as an organic material
(to be later removed by, e.g. plasma oxygen ash). The a-Si is
patterned (photoresist and etched by a chlorine chemistry, e.g.
Cl2, BCl3 and N2), so as to form holes for attachment of the mirror
to the glass substrate. A first layer of silicon nitride, for
creating stiffness in the mirror and for connecting the mirror to
the glass, is deposited by PECVD (RF power=150 W, pressure=3 Torr,
temp=360 C, electrode spacing=570 mils, gas=N2/SiH4/NH3
(1500/25/10); or RF power=127 W, pressure=2.5 T, temp=380 C,
gas=N2/SiH4/NH3 (1500/25/10 sccm), electrode spacing=550 mil, or
other process parameters could be used, such as power at 175 W and
pressure at 3.5 Torr) at a thickness of 900 Angstroms and is
patterned (pressure=800 mT, RF power=100 to 200 W, electrode
spacing=0.8 to 1.1 mm, gas=CF4/CHF3/Ar (60 or 70/40 to 70/600 to
800 sccm, He=0 to 200 sccm), so as to remove the silicon nitride in
areas in which the mirror hinges will be formed. Next, a second
layer of silicon nitride is deposited by PECVD (RF power=127 W,
pressure=2.5 T, temp=380 C, gas=N2/SiH4/NH3 (1500/25/10 sccm),
electrode spacing=550 mil) at a thickness of 900 Angstroms. Then,
Al is sputtered onto the second silicon nitride layer at a
thickness of 500 Angstroms at a temp of from 140 to 180 C,
power=2000 W, Ar=135 sccm. Or, instead of Al, the material could be
an aluminum alloy (Al--Si (1%), Al--Cu (0.5%) or AlSiCu or AlTi, as
well as an implanted or target doped aluminum. The aluminum is
patterned in the P5000 with a chlorine chemistry (pressure=40 mT,
power=550 W, gas=BCl3/Cl2/N2=50/15/30 sccm). Then, the SiN layers
are etched (pressure=100 mT, power=460 W, gas=CF4/N2 ({fraction
(9/20)} sccm), followed by ashing in a H2O+O2+N2 chemistry in
plasma. Next, the remaining structures are ACT cleaned (acetone+DI
wafer solution) and spun dry. (this clean can also be done with EKC
Technology's EKS265 photoresist residue remover or other solvent
based cleaner) After resist coating the frontside of the wafer
having the microstructures thereon, the backside TiN is etched in a
BCl3/Cl2/CF4 chemistry in plasma (or other metal etchant from CRC
Handbook of Metal Etchants)--or polished or ground off using CMP,
or removed with acid vapor such as HF--followed by a second ACT
clean (acetone+DI wafer solution) and a second spin dry. The wafer
is singulated into individual die and each die is exposed to 300 W
CF4 plasma (pressure=150 Torr, 85 sccm for 60 seconds, followed by
300 sec etch in a mixture of He, XeF2 and N2 (etch pressure 158
mTorr). The etch is performed by providing the die in a chamber of
N2 at around 400 mTorr. A second area/chamber has therein 3.5 mTorr
XeF2 and 38.5 mTorr He. A barrier between the two areas/chambers is
removed, resulting in the combined XeF2, He and N2 etching
mixture.
[0056] Or, the transparent wafer (e.g. Corning 1737F) is coated
with TiN at a thickness of 2000 angstroms on the backside of the
glass wafer. Then, in accordance with FIGS. 1-4, without an
adhesion layer, a sacrificial material of hydrogenated amorphous
silicon is deposited (power=100 W, pressure=3.5 T, temp=300 C,
SiH4=200 sccm, Ar=1500 sccm, or pressure=2.5 Torr, power=50 W,
temp=360 C, electrode spacing=350 mils, SiH4 flow=200 sccm, Ar
flow=2000 sccm) on a glass wafer at a thickness of 5300 Angstroms
in an Applied Materials P5000. The a-Si is patterned (photoresist
and etched by a chlorine chemistry, e.g. C12, BCl3 and N2 -50 W),
so as to form holes for attachment of the mirror to the glass
substrate. A first layer of silicon nitride, for creating
stifffiess in the mirror and for connecting the mirror to the
glass, is deposited by PECVD (pressure=3 Torr, 125 W, 360 C,
gap=570, SiH 4=25 sccm, NH3=10 sccm, N2=1500 sccm) at a thickness
of 900 Angstroms and is patterned (CF4/CHF3), so as to remove the
silicon nitride in areas in which the mirror hinges will be formed.
Next, a second layer of silicon nitride is deposited by PECVD (same
conditions as first layer) at a thickness of 900 Angstroms. Then,
Al is sputtered (150 C) onto the second silicon nitride layer at a
thickness of 500 Angstroms. The aluminum is patterned in the P5000
with a chlorine chemistry (BCl3, Cl2, Ar). Then, the SiN layers are
etched (CHF3, CF4), followed by ashing in a Hitachi barrel asher
(O2, CH3OH at 250 C). Next, the remaining structures are cleaned
with EKC Technology's EKS265 photoresist residue remover. After
resist coating the frontside of the wafer having the
microstructures thereon, the backside TiN is etched in a SF6/Ar
plasma, followed by a second clean and a second spin dry.
[0057] After depositing the sacrificial and structural layers on a
wafer substrate, the wafer is singulated and each die then is
placed in a Drytek parallel plate RF plasma reactor. 100 sccm of
CF4 and 30 sccm of O2 flow to the plasma chamber, which is operated
at about 200 mtorr for 80 seconds. Then, the die is etched for 300
seconds at 143 mTorr etch pressure (combined XeF2, He and N2). The
etch is performed by providing the die in a chamber of N2 at around
400 mTorr. A second area/chamber has therein 5.5 mTorr XeF2 and 20
mTorr He. A barrier between the two areas/chambers is removed,
resulting in the combined XeF2, He and N2 etching mixture. The
above could also be accomplished in a parallel plate plasma etcher
with power at 300 W CF4 (150 Torr, 85 sccm) for 120 seconds.
Additional features of the second (chemical, non-plasma) etch are
disclosed in U.S. patent application Ser. No. 09/427,841 to Patel
et al. filed Oct. 26, 1999, and U.S. patent application Ser. No.
09/649,569 to Patel et al. filed Aug. 28, 2000, the subject matter
of each being incorporated herein by reference.
[0058] As can further be seen in FIGS. 9A to D, a substrate 10
(silicon or glass) has a sacrificial silicon or silicon compound
layer 20 disposed thereon. One or more structural layers 30 are
provided (and patterned) on the sacrificial layer 20. Residue 22a
and 22b from prior processing steps for forming the micromechanical
structures prior to release are disposed on sacrificial layer 20.
As can be seen in FIG. 9B, after a first fully physical or
physical/chemical etch, a first portion of the sacrificial layer 20
(along with residue 22a and 22b) is removed. Then, as can be seen
in FIG. 9C, the remainder of sacrificial layer 20 is in the process
of being removed by a purely chemical etch, which ultimately
results in releasing the micromechanical structure 30 as can be
seen in FIG. 9D and FIG. 10. As can be seen in FIG. 11, if the
first etch is not performed prior to the second etch, then uneven
etching results as illustrated by lines 32a and 32b.
[0059] The apparatus for performing the etching of the present
invention can be seen in FIG. 12 to 14. As can be seen in FIG. 12,
an apparatus is provided that includes a source chamber 51
(containing, for example, xenon difluoride crystals for the second
etch--the crystals maintained at a temperature of 28.5.degree. C.
at which temperature the sublimation pressure of the crystals is
5-11 mbar (4-8 torr)), an expansion chamber 52 having a volumetric
capacity of 29 cubic inches (0.46 liter) to receive xenon
difluoride gas from the source chamber 51, with a shutoff valve 53
joining these two chambers, an etch chamber 54 having a volumetric
capacity of 12 cubic inches (0.18 liter) to contain the sample
microstructure to be etched, the etch chamber 54 fed by the
expansion chamber 52 through a further shutoff valve 55. Also
included in the apparatus is a first gas source 56 communicating
with the expansion chamber 52 through a further shutoff valve 57, a
second gas source 58 communicating with the expansion chamber
through a separate shutoff valve 59, a vacuum pump 61 and
associated shutoff valves 62, 63 to control the evacuation of the
chambers, a third gas source 64 serving as a pump ballast with an
associated shutoff valve 65 to prevent backstreaming from the pump
61, and manually operated needle valves 66, 67, 68 to set the gas
flow rates through the various lines and to permit fine adjustments
to the pressures in the chambers. The expansion chamber 52 and the
etch chamber 54 were both maintained at a temperature of
35.0.degree. C., while different gases were placed in the first and
second gas sources for the various etches.
[0060] Although not shown in the drawing, the apparatus may be
varied to improve the sample uniformity and reduce the total etch
time (by actively moving reaction products away from etch sites and
replenishing the etch site with reactant) by placing an agitator in
the etch chamber 54, by including a circulation line between the
etch and expansion chambers with a pump in the line to circulate
the gas mixture through the etch chamber 54 and the expansion
chamber 52, or by using both of these methods.
[0061] The general procedure followed in these experiments began
with the evacuation of both the expansion chamber 52 and the etch
chamber 54, followed by venting both chambers to atmospheric
pressure with gas from the first gas source 56 by opening the two
shutoff valves 57, 55, between this gas source and the two
chambers. The sample was then placed in the etch chamber 54 (with
the shutoff valves 57, 55 open during the sample insertion) which
was then sealed, and both the expansion chamber 52 and the etch
chamber 54 were evacuated. All valves were then closed.
[0062] The connecting valve 55 between the expansion chamber 52 and
the etch chamber 54 was opened, and the shutoff valve 57 at the
outlet of the first gas source 56 was opened briefly to allow the
gas from the first gas source to enter the expansion and etch
chambers to a pressure of about 630 mbar (470 torr). The shutoff
valve 57 was then closed. The connecting valve 55 was then closed,
and the expansion chamber 52 was evacuated and isolated. The supply
valve 53 from the xenon difluoride source chamber 51 was then
opened to allow xenon difluoride gas to enter the expansion chamber
to a pressure above 8 mbar (6 torr) (due to the higher temperature
of the expansion chamber). The supply valve 53 was then closed,
outlet valve 63 was opened, and the needle valve 67 was opened
slightly to lower the xenon difluoride pressure in the expansion
chamber to 6.7 mbar (5 torr). Both the outlet valve 63 and the
needle valve 67 were then closed. The shutoff valve 59 at the
second gas source 58 was then opened and with the assistance of the
needle valve 66, gas from the second gas source was bled into the
expansion chamber to a pressure of about 27 mbar (20 torr). At this
point the expansion chamber 52 contained xenon difluoride at 7 mbar
(5 torr) plus gas from the second gas source 18 at 20 mbar (15
torr), while the etch chamber 54 contained gas from the first gas
source at 630 mbar (470 torr).
[0063] The connecting valve 55 between the expansion chamber 52 and
the etch chamber 54 was then opened to allow the gas mixture from
the expansion chamber to enter the etch chamber as the gases from
the two chambers became mixed and distributed between the chambers,
thereby beginning the etch process. The etch chamber thus contained
xenon difluoride at a partial pressure of 4.7 mbar (3.5 torr) gas
from the first gas source at a partial pressure of 180 mbar (140
torr) and gas from the second gas source at a partial pressure of
14 mbar (11 torr), thereby resulting in a (second gas):(first
gas):(xenon difluoride) volume ratio of 3:39:1. The etch process
was continued for as long as needed to remove all of the
sacrificial layer, as determined visually, then discontinued.
[0064] FIG. 13 represents a different apparatus than that
illustrated in FIG. 12. In FIG. 13, the etchant gas for the second
etch originates in a source chamber 71. If xenon difluoride is
used, effective results can be achieved by maintaining the crystals
under 40 degrees C. (e.g. at a temperature of 28.5.degree. C.).
(Xenon difluoride is only one of several etchant gases that can be
used. Examples of other gases are mentioned elsewhere herein.) The
sublimation pressure of xenon difluoride crystals at 28.5.degree.
C. is 5-11 mbar (4-8 torr). An expansion chamber 72 receives xenon
difluoride gas from the crystals in the source chamber(s) 71, and a
shutoff valve 73 is positioned between the source and expansion
chambers. The sample to be etched 74 is placed in an etch chamber
75, which can contain a baffle 76 and a perforated plate 77. A
reciprocating pump is positioned between the expansion chamber 72
and the etch chamber 75.
[0065] Also shown are four individual gas sources 79, 90, 114 and
117 supplying the expansion chamber 72 through shutoff valves 91,
92, 116 and 119, a vacuum pump 123 and associated shutoff valves
94, 95, 96, 97, and 98 to control the evacuation of the chambers, a
third gas source 99 serving as a pump ballast with an associated
shutoff valve 100 to prevent backstreaming from the pump 123, and
manually operated needle valves 101, 102, 103, 104, 105, 111, 115
and 118 to set the gas flow rates through the various lines and to
permit fine adjustments to the pressures in the chambers. When
xenon difluoride is used, the expansion chamber 72 and the etch
chamber 75 are typically maintained at around room temperature
(e.g. 25.0.degree. C.). However, the expansion chamber and etch
chamber could also be heated (e.g. to between 25 and 40 degrees
C.), though this would likely be performed in conjunction with
directly cooling the sample being processed. A recirculation line
106 permits gas to flow continuously through the etch chamber 75 in
a circulation loop that communicates (via valves 96, 97, and 104,
105) with the expansion chamber 72 and reenters the etch chamber 75
by way of the reciprocating pump 78. Valve 112 permits gas transfer
between expansion chamber 72 and etch chamber 75 via a portion of
the recirculation line 106 without traversing recirculation pump
78. Valve 113 in path 110 permits introduction of etchant gas into
the expansion chamber 72 to replenish the etchant mixture during
the etching process.
[0066] The valves are preferably corrosive gas resistant
bellows-sealed valves, preferably of aluminum or stainless steel
with corrosive resistant O-rings for all seals (e.g. Kalrez.TM. or
Chemraz.TM.). The needle valves are also preferably corrosion
resistant, and preferably all stainless steel. As an optional
feature, a filter 109 can be placed in the recirculation line 106
to remove etch byproducts from the recirculation flow, thereby
reducing the degree of dilution of the etchant gas in the flow. The
filter can also serve to reduce the volume of effluents from the
process. The etch chamber 75 can be of any shape or dimensions, but
the most favorable results will be achieved when the internal
dimensions and shape of the chamber are those that will promote
even and steady flow with no vortices or dead volumes in the
chamber interior. A preferred configuration for the etch chamber is
a circular or shallow cylindrical chamber, with a process gas inlet
port at the center of the top of the chamber, plus a support in the
center of the chamber near the bottom for the sample, and an exit
port in the bottom wall or in a side wall below the sample support.
The baffle 76 is placed directly below the entry port. The
perforated plate 77 is wider than the baffle 76 and preferably
transmits all gas flow towards the sample.
[0067] The etching chamber of both FIGS. 12 and 13 can be provided
so as to be capable of energizing one or more gases for the first
etch. For example, the etching chamber can be provided with a
system for creating a plasma in the etching chamber. As can be seen
in FIG. 14, top and bottom electrodes are separated by a grounded
diffuser plate 42 that allows gas to be transported between the
upper and lower areas. When the lower electrode is powered, the
system can operate like a conventional RIE, whereas when the upper
electrode is powered, the plasma is confined to region 41 between
the upper electrode and ground grid 42. In this mode ("remote
plasma mode") the substrate or wafer 43 is shielded from ion
bombardment but free radicals and neutral species can be readily
transported to the substrate surface.
[0068] The first etch in the present invention can involve one or
more of sputter etching, chemical etching, and accelerated
ion-assisted etching (each capable of being caused by the plasma
system of FIG. 14, though other ways of causing these types of
etching are known). In accelerated ion-assisted etching, like the
sputtering process, ions are accelerated by the sheath potential.
But, unlike sputter etching, the purpose of the accelerated ions
are not to sputter away the surface, but rather to damage the
surface only, leaving dangling bonds and dislocations in the
surface. This is to modify the surface into a more reactive form so
that the damaged surface will react with the neutral etchants more
easily. Sputter etching is a purely physical process whereby
surface materials are being ejected by impinging ions. The ions are
propelled by the sheath potentials. Thus, they acquire energy and
momentum to knock off the surface materials when they hit on the
surface. The pressure has to be low in order for the surface
materials to move across the reactor onto opposing surfaces. This
is also to prevent ejected materials from colliding with the gas
molecules and thus back-scattering onto the surface. Chemical
etching (during a plasma etch), on the other hand, is a spontaneous
reaction between plasma-generated neutral species and substrate
material to form volatile gaseous reaction products.
[0069] The apparatus for providing the physical or
physical/chemical etching can be within the same chamber as for the
second etch, as noted above, as part of a second apparatus separate
from the apparatus for the second etch, or within a separate
chamber but as part of the same apparatus as that used for the
second etch. Being provided as part of the same apparatus, whether
in the same chamber or not, allows for the first and second etches
to take place without exposing the substrate being etched to
ambient. In a preferred embodiment, the substrate being etched is
not exposed to gases other than gases used in the first or second
etch process. A load lock (not shown) can also be provided with the
appropriate valves for evacuating the load lock chamber.
[0070] In addition to the etchant for the second etch, illustrated
as chamber 71 in FIG. 13, one or more sources of additional gases,
such as O2, SF6, a source of the first etchant (e.g. a hydrocarbon
or fluorocarbon), N2, Ar, He, or other diluent gas sources or other
sources for providing chemical or physical etching, as well as a
source of stiction-reducing agent (e.g. an alkyl chlorosilane)
could be connected to the etching chamber(s). These additional gas
sources (potentially in liquid or solid form under pressure) are
illustrated as sources 79, 90, 114 and 117 in FIG. 13. Of course
additional sources of gases for introduction to the etching
apparatus could be provided, and could be provided to separate
chambers, depending upon whether a single or multiple chamber
apparatus is used.
[0071] After releasing the micromechanical structure(s), the first
wafer with such structures thereon can be packaged (e.g. if
circuitry is provided on the first wafer), or the first wafer can
be bonded to another wafer having circuitry thereon, in a
"flip-chip" type of assembly. The bonding of the circuitry wafer to
the first wafer holding the microstructures can be by anodic
bonding, metal eutectic bonding, fusion bonding, epoxy bonding, or
other wafer bonding processes known in the art. A preferred bonding
method is bonding with an IR or UV epoxy such as disclosed in U.S.
Pat. No. 5,963,289 to Stefanov et al, "Asymmetrical Scribe and
Separation Method of Manufacturing Liquid Crystal Devices on
Silicon Wafers", which is hereby incorporated by reference. In
order to maintain separation between the bonded wafers, spacers can
be mixed into the epoxy. The spacers can be in the form of spheres
or rods and can be dispensed and dispersed between the first wafer
and sealing wafer in order to keep the sealing wafer spaced away
from the first wafer (so as to avoid damage to the microstructures
on the first wafer). Spacers can be dispensed in the gasket area of
the display and therefore mixed into the gasket seal material prior
to seal dispensing. This is achieved through normal agitated mixing
processes. The final target for the gap between the first wafer and
sealing wafer can be from 1 to 100 um. This of course depends upon
the type of MEMS structure being encapsulated and whether it was
surface or bulk micromachined (bulk micromachined structures may
not need any spacers between the two wafers). The spheres or rods
can be made of glass or plastic, preferably an elastically
deforming material. Alternatively, spacer pillars can be
microfabricated on at least one of the wafer substrates. In one
embodiment, pillars/spacers are provided only at the edge of the
array. In another embodiment, pillars/spacers can be fabricated in
the array itself. If the spacers are micro-fabricated spacers, they
can be formed on the lower wafer, followed by the dispensing of an
epoxy, polymer, or other adhesive (e.g. a multi-part epoxy, or a
heat or UV-cured adhesive) adjacent to the micro-fabricated
spacers. The adhesive and spacers need not be co-located, but could
be deposited in different areas on the lower substrate wafer.
Alternative to glue, a compression bond material could be used that
would allow for adhesion of the upper and lower wafers. Spacers
micro-fabricated on the lower wafer (or the upper wafer) and could
be made of polyimide, SU-8 photo-resist.
[0072] Then, the two wafers are aligned. If precision alignment is
desired, alignment of the opposing electrodes or active viewing
areas may involve registration of substrate fiducials on opposite
substrates. This task accomplished with the aid of video cameras
with lens magnification. The machines range in complexity from
manual to fully automated with pattern recognition capability.
Whatever the level of sophistication, they accomplish the following
process: 1. Dispense a very small amount of a UV curable adhesive
at locations near the perimeter and off of all functional devices
in the array; 2. Align the fiducials of the opposing substrates
within the equipment capability; and 3. Press substrates and UV
tack for fixing the wafer to wafer alignment through the remaining
bonding process (e.g., curing of the internal epoxy).
[0073] The final cell gap can be set by pressing the previously
tacked laminates in a UV or thermal press. In a UV press, a common
procedure would have the substrates loaded into a press where at
least one or both of the press platens are quartz, in order to
allow UV radiation from a UV lamp to pass unabated to the gasket
seal epoxy. Exposure time and flux rates are process parameters
determined by the equipment and adhesive materials. Thermally cured
epoxies may require that the top and bottom platens of a thermal
press be heated. The force that can be generated between the press
platens is typically many pounds. With thermally cured epoxies,
after the initial press the arrays are typically transferred to a
stacked press fixture where they can continue to be pressed and
post-cured. In one embodiment, the epoxy between the first wafer
and sealing wafer is only partially cured so as to allow easier
removal of the sealing wafer. After the sealing wafer is removed,
this epoxy can be optionally cured. An epoxy can be selected that
adheres less well (depending upon the wafer materials) than other
epoxies, so as to allow for easier removal of the sealing wafer
after singulation. Also, UV epoxy and IR epoxy can be used at the
same time, with the UV epoxy being cured prior to IR cure.
[0074] Once the wafers have been bonded together to form a wafer
assembly, the assembly can be separated into individual dies.
Scribes are placed on the respective substrates in an offset
relationship at least along one direction. The units are then
separated, resulting in each unit having a ledge on each end of the
die. Such a ledge can also allow for electrical testing of each
die, as electrical contacts can be exposed on the ledge (e.g., if
circuitry has been formed together with the microstructures on the
first wafer). The parts can then be separated from the array by
venting the scribes on both substrates. Automatic breaking can be
done by commercially available guillotine or fulcrum breaking
machines. The parts can also be separated by hand.
[0075] Separation may also by done by glass scribing and partial
sawing of one or both substrates. Sawing is preferably done in the
presence of a high-pressure jet of water. Moisture must not be
allowed to contact the microstructures. Therefore, at gasket
dispense, an additional gasket bead must be dispensed around the
perimeter of the wafer, or each gasket bead around each die must
fully enclose the die area so that water can not enter and touch
the microstructures. Preferably, however, the end of each
scribe/saw lane must be initially left open, to let air vent during
the align and press processes. After the array has been pressed and
the gasket material fully or partially cured, the vents are then
closed using either the gasket or end-seal material. The glass is
then scribed and sawed.
[0076] Alternatively, both the first wafer and sealing wafer
substrates may be partially sawed prior to part separation. With
the same gasket seal configuration, vent and seal processes as
described above, saw lanes are aligned to fiducials on the sealing
substrate. The glass is sawed to a depth between 25% and 95% of its
thickness. The first wafer substrate is sawed and the parts
separated as described above.
[0077] The first wafer, upon which the micromechanical structures
are formed and released, can be any suitable substrate for the
particular MEMS microstructure (and optionally circuitry) formed
thereon, such as a light transmissive substrate such as glass,
borosilicate, tempered glass, quartz or sapphire, or any other
suitable light transmissive material. Or, the first wafer could be
a metal, ceramic or preferably a semiconductor wafer (e.g. silicon
or GaAs). An anti-reflective coating can be applied to the glass
either before processing begins on the glass, or preferably at the
time of packaging.
[0078] There are many variations possible to the preferred
embodiments disclosed above. For example, the second etch, instead
of using the previously-mentioned gas phase fluoride non-plasma
etchants, could instead use a gas phase acid, such as (non-plasma)
HF, HBr, HI, Cl2, combinations thereof (and any such acid(s) with
or without H2), non-energized except for being at a high
temperature (e.g. 900 C or above). Or, either the first or second
etch could include BI3, BBr3, BCl3 or AICl3 (plasma etch for the
first etch or non-plasma chemical etch for the second). As with any
of the etchants, the etch can be performed in pulse or continuous
mode.
[0079] It should be noted that the invention is applicable to
forming micromirrors such as for a projection display or optical
switch, or any other MEMS device that would benefit from protection
of the microstructures during wafer singulation. If an optical
switch is the microstructure being protected, mirrors with multiple
hinges can be provided on the first wafer so as to allow for
multi-axis movement of the mirror. Such multi-axis movement,
mirrors for achieving such movement, and methods for making such
mirrors are disclosed in U.S. patent application Ser. No.
09/617,149 to Huibers et al. filed Jul. 17, 2000, the subject
matter of which is incorporated herein by reference.
[0080] Of course, the microstructure need not be a movable mirror
(for a projection display, for optical switching, or even for data
storage), but could be one or more accelerometers, DC relay or RF
switches, microlenses, beam splitters, filters, oscillators and
antenna system components, variable capacitors and inductors,
switched banks of filters, resonant comb-drives and resonant beams,
etc. Any MEMS structure, particularly a released or movable
structure, could benefit from the release method described
herein.
[0081] The invention has been described in terms of specific
embodiments. Nevertheless, persons familiar with the field will
appreciate that many variations exist in light of the embodiments
described herein.
* * * * *