U.S. patent application number 10/076222 was filed with the patent office on 2003-06-26 for whole wafer mems release process.
Invention is credited to Dewa, Andrew S..
Application Number | 20030116815 10/076222 |
Document ID | / |
Family ID | 26757815 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030116815 |
Kind Code |
A1 |
Dewa, Andrew S. |
June 26, 2003 |
WHOLE WAFER MEMS RELEASE PROCESS
Abstract
A process for manufacturing a wafer having a multiplicity of
MEMS devices such as mirrors with gimbals formed thereon is
disclosed. The devices on the wafer include features defined by a
wide line between features which extend completely through the
wafer, and have a ratio of greater than about 4:1 with respect to
the narrow lines which separate individual devices. Each individual
device is separated by narrow gaps or line widths which are, for
example, about 10 .mu.m. Thus, the etching process is controlled
such that the features defined by the wide lines are etched
completely through, whereas the individual devices are separated by
narrow lines which are not etched completely through the wafer.
Therefore, the multiplicity of devices remain attached together
even after the wafer is released from a backing wafer. Thus, the
wafer with the many devices still attached together allows further
processing such as packaging, testing, transport, etc. without the
required handling of individual devices.
Inventors: |
Dewa, Andrew S.; (Plano,
TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
26757815 |
Appl. No.: |
10/076222 |
Filed: |
February 14, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60342248 |
Dec 21, 2001 |
|
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|
Current U.S.
Class: |
257/432 ; 438/51;
438/65 |
Current CPC
Class: |
B81C 1/00896 20130101;
B81C 1/00619 20130101 |
Class at
Publication: |
257/432 ; 438/51;
438/65 |
International
Class: |
H01L 021/00 |
Claims
I claim:
1. A process for manufacturing a plurality of MEMS devices on a
first layer of material of a selected thickness comprising:
attaching said first layer of material to a backing layer of
material; defining features on each individual ones of said
plurality of MEMS devices with first lines having at least a first
selected width; defining boundary lines between individual ones of
said plurality of MEMS devices with second lines having a width
that is less than said first selected width; simultaneously etching
said first lines and said second lines until said first lines
defining device features are etched through said selected
thickness; stopping said etching before said second lines defining
boundaries are etched through said first selected thickness; and
separating said first layer with said plurality of devices attached
together from said backing layer.
2. The process of claim 1 and comprising further processing of said
separated first layer.
3. The process of claim 2 when said further processing comprising
testing said devices while still attached together on said first
layer.
4. The process of claim 2 wherein said further processing comprises
separating each individual device of said first layer from each
other.
5. The process of claim 1 wherein said further processing comprises
cleaning said devices while still attached together subsequent to
said separation step.
6. The process of claim 1 and further comprising packing said
separated wafer with said devices still attached together for
storage or shipping.
7. The process of claim 1 wherein said first width of said first
lines have a ratio greater than 4:1 with respect to said width of
said second lines.
8. The process of claim 1 wherein said first selected width is at
least about 50 .mu.m and said second width is about 10 .mu.m.
9. The process of claim 1 wherein said first layer of material is
selected from the group consisting of silicon, gallium arsenide,
quartz and silicon carbide.
10. The process of claim 9 wherein said first layer of material is
silicon.
11. A process for manufacturing a plurality of gimbal mirror
devices on a first layer of material of a selected thickness
comprising: attaching said first layer of material to a backing
layer of material; defining features on each individual ones of
said plurality of gimbal mirror devices with first lines having at
least a first selected width; defining boundary lines between
individual ones of said plurality of gimbal mirror devices with
second lines having a width that is less than said first selected
width; simultaneously etching said first lines and said second
lines until said first lines defining gimbal mirror features are
etched through said first selected thickness; stopping said etching
before said second lines defining boundaries are etched through
said first selected thickness; and separating said first layer with
said plurality of gimbal mirror devices attached together from said
backing layer.
12. The process of claim 11 further comprising testing individual
devices defined on said first layer.
13. The process of claim 11 further comprising separating each
individual gimbal mirror device from said first layer.
14. The process of claim 11 wherein said further processing
comprises cleaning said gimbal mirror while still attached together
subsequent to said step of separating said first layer from said
backing layer.
15. The process of claim 11 and further comprising packing said
separated wafer with said devices still attached together for
storage or shipping.
16. The process of claim 11 wherein said first lines have a width
at least equal to about 50 .mu.m and said second lines have a width
of about 10 .mu.m.
17. The process of claim 11 wherein said first layer is a silicon
wafer.
18. A wafer defining a plurality of MEMS devices attached together
comprising: at least two features of said MEMS devices separated by
first lines etched completely through said wafer, said first line
having at least a first selected width; second lines etched part
way through said wafer defining individual ones of said plurality
of MEMS devices, said second lines having a second width which is
less than said first selected width.
19. The wafer of claim 18 wherein said width of said first lines
have a ratio greater than 4:1 with respect to said width of said
second lines.
20. The wafer of claim 18 wherein said first selected width is at
least about 50 .mu.m and said second width is about 10 .mu.m.
21. A wafer defining a plurality of gimbal mirror devices attached
together comprising: at least two features of said gimbal mirror
devices separated by first lines etched completely through said
wafer, said first lines having at least a first selected width;
second lines etched part way through said wafer defining individual
ones of said plurality of gimbal mirror devices, said second lines
having a second width which is less than said first selected width.
Description
[0001] This patent claims the benefit of U.S. Provisional Patent
Application No. 60/342,248, filed Dec. 21, 2001, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This unit relates generally to apparatus and methods for
manufacturing MEMS (micro-electromechanical systems) by forming a
multiplicity of such devices on a silicon wafer. More specifically,
the invention relates to such a manufacturing process which allows
further processing and/or testing before each individual device is
separated from the silicon wafer.
BACKGROUND OF THE INVENTION
[0003] Texas Instruments presently manufactures a two-axis analog
micromirror MEMS device fabricated out of a single piece of
material (such as silicon, for example) typically having a
thickness of about 115 .mu.m. The die layout consists of an oval
micromirror, normally 3.8 mm.times.3.2 mm supported on a gimbal
frame by two silicon torsional hinges. The gimbal frame is attached
to the die frame by another orthogonal set of torsional hinges. The
micromirror die (i.e. each individual device) is fabricated by
etching the 115 .mu.m thick silicon wafer in a specialized ICP
(Inductively Coupled Plasma) plasma reactor.
[0004] MEMS devices are becoming more and more available and
common. However, these devices are extremely small compared to
regular machines, but still very large when compared to the
individual circuits or components and features found on IC's and
other electronic chips. Some MEMS devices such as the digital
micromirror device arrays produced by Texas Instruments are made
significantly smaller than most other types of MEMS devices, but
are also very large compared to components on an IC or other chips
and use existing geometry and patterning techniques common for the
productions of semiconductor circuits. For example, small MEMS
devices such as gimbal supported mirror 32 shown in FIG. 2D used
for optical switching of transmitted data streams are presently on
the order of 3.2.times.3.8 mm, whereas the mirrors on micromirror
arrays used for display devices are typically between about 15-20
microns on a side. Thus, it is seen that MEMS devices are not
comfortably compared with either full-size machines or devices
(they are much smaller) or a true array of micro devices such as
IC's, memory chips, and the like (they are much larger).
[0005] The present invention relates to individual mirror devices
formed on a wafer using processing steps some of which have
similarity to steps used in manufacturing IC's and other
semiconductor devices.
SUMMARY OF THE INVENTION
[0006] The present invention provides a process for manufacturing a
plurality of MEMS devices on a first layer of material, such as for
example, a thin wafer of silicon typically having a thickness of
about of 115 .mu.m. The process comprises attaching the thin
silicon wafer to a carrier or backing wafer and then defining
features for each individual device of said plurality of devices
with a first line width. The boundary or separation lines between
the individual ones of the plurality of devices are defined with a
second line width that has a thickness less than the thickness of
the first line width used to define the device features.
[0007] After placing both the lines which define the features of
the individual devices and the boundary or separation lines between
individual devices, the wafer while attached to the backing wafer
is etched such that the lines which define the features of the
device are etched through the selected thickness. However, the
etching is stopped before the thinner lines which define boundaries
of the individual devices are etched through the thickness of the
wafer. This is possible because of the phenomenon called
microloading. Microloading is the differential etch rate between
wide lines and narrow lines (wide lines etch faster) in a plasma
reactor. Thus, it is seen that the individual devices are formed
because of the fast etch rate of the wide lines, while at the same
time all of the devices on the wafer remain attached together
because of the slower etch rate of the thin separation line. The
wafer with the devices still attached together is then separated
from the backing layer. It should also be noted that the wafer with
the devices could be silicon or another suitable material. Further,
the wafer may also undergo other processes before the device is
etched. For example, electronics, sensors or other mechanical
features can be created by standard IC or MEMS fabrication before
the process step of etching through the wafer is accomplished.
[0008] Therefore, according to embodiments of the present
invention, the silicon wafer with all of the attached devices
etched therein can then be further processed. For example, further
processing may comprise testing of the torsional gimbals of the
individual mirrors by moving the mirrors by either soft directed
currents of air or spring pins. This is a much faster process than
having to handle and test the gimbals on each separated mirror. In
addition, it is also possible to better clean the attached mirror
on the wafer after it has been released from its backing layer than
it is to handle each individual device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The above-mentioned features as well as other features of
the present invention will be more clearly understood from
consideration of the following description in connection with the
accompanying drawings in which:
[0010] FIGS. 1A through 1H illustrate the various steps of a
manufacturing process;
[0011] FIG. 2A illustrates a mirror wafer;
[0012] FIG. 2B shows a top view and a side view of the fixture for
catching the individual mirror devices upon release;
[0013] FIG. 2C is a cross-section of the apparatus used to release
the silicon device from the backing wafer used by a manufacturing
process;
[0014] FIG. 2D is an enlarged view of a pocket on the fixture of
FIG. 2B and also shows an individual mirror device caught by the
fixture of FIG. 2B;
[0015] FIGS. 3A and 3B illustrate two methods of removing wafer
waste areas greater than about 50 .mu.m;
[0016] FIG. 3C illustrates how device features having a separation
less than a narrow line width may be formed by a first method;
[0017] FIG. 4 illustrates the use of wide separation lines to
define device features and narrow separation lines to define
devices according to the present invention;
[0018] FIG. 5 illustrates how device features having a separation
less than a selected line width may be formed according to the
present invention;
[0019] FIGS. 6A through 6C illustrates a method of manually
separating the device wafer from the backing wafer according to the
present invention; and
[0020] FIG. 7 illustrates a method of separating the individual
devices on a wafer.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] The process flow of one method of manufacturing two-axis
analog micromirror MEMS devices wherein the individual dies,
elements or devices are diced or separated by the same through the
wafer etch that forms the features of the mirror is disclosed in
FIGS. 1A-1H. As shown in FIG. 1A, a 115 .mu.m thick wafer 10 is
bonded to a carrier or backing wafer 12 (see FIG. 1B). Optional
alignment marks 14 may then be etched into the thin wafer material
or other suitable material using a resist layer 16 along with
photolithography and plasma etching as shown in FIG. 1C. After the
plasma etch, the resist 16 used to form the optional alignment
marks 14 is then stripped as shown in FIG. 1D. The features of the
micromirror or MEMS devices are then patterned with
photolithography as indicated by line gaps 18 and 20 patterned in a
second resist layer 22 as is well known by those skilled in the
art. This arrangement is shown in FIG. 1E. As shown in FIG. 1F, the
mirror features formed by gap or line pattern 18 and 20 are then
etched completely through the wafer 10 as indicated by reference
numbers 24 and 26 using a special ICP plasma reactor and the Bosch
process U.S. Pat. No. 5,498,312. It is important to note at this
point that according to this method of manufacturing, at the same
time the mirror features such as were etched completely through the
wafer 10, the line patterns or etches such as indicated at etch 26
used to separate the individual dies or mirrors as indicated at
etch line 24 are also etched completely through the wafer 10. After
the etching process, the second photo resist layer 22 is stripped
away, and the wafer still bonded to the backing wafer is given a
gold coat 28 such as shown in FIG. 1G. Finally, the mirror die or
individual mirrors are released from the carrier wafer 12 as shown
in FIG. 1H. This is accomplished by placing the combination carrier
or backing wafer 12 and the wafer 10 in a solvent bath to dissolve
the agent bonding the carrier wafer 12 and wafer 10 together. The
bonding agent is typically a photo resist. Therefore, according to
one embodiment, the solvent for separating the backing wafer 12
from the wafer 10 is acetone.
[0022] Referring now to FIGS. 2A, 2B, 2C and 2D, there is shown the
wafer 10 with the individual mirrors etched therein, a fixture for
catching the individual dies or mirrors after they are released
from the backing or carrier wafer 12 (top and side view shown in
FIG. 2B), and a cross-sectional view of the solvent bath with the
wafer 10 and fixture of 2B in place as used during the release
process (FIG. 2C). As shown in FIG. 2A, the embodiment illustrates
178 individual mirrors or dies etched into the wafer 10. Likewise,
the fixture of 2B shows an equal number or 178 pockets such as
pocket 30 more clearly seen in the broken out blown up illustration
of FIG. 2D, which catch the individual mirrors or dies, such as
mirror and gimbal structure 32, after they are released. The mirror
wafer 10 is aligned on the fixture of FIG. 2B so that each
individual mirror is over a pocket 30 that catches the mirror after
release. As shown in FIG. 2C, the bonded wafer 10 is loaded upside
down in the fixture so that gravity will pull the individual
mirrors down into an aligned pocket as they are released from the
carrier wafer 12.
[0023] This process requires non-standard semiconductor practices
and consequently experiences some problems that may reduce yield.
For example, each individual die or mirror can have residue on the
die resulting from the release process; (2) each of the die can get
drying spots where they land on the released fixture; (3) some
breakout pieces of the original wafer 10 (to be discussed
hereinafter) can get stuck to the mirror die; and (4) some of the
die or individual mirrors 32 simply never get released from the
carrier wafer 12 or they get re-stuck to the carrier wafer 12 when
the acetone or alcohol used in a subsequent rinse dries (due to
capillary forces). Furthermore, as mentioned, this process is also
different from standard semiconductor assembly practices because it
is very difficult to ship the individual dies that have been
released from the carrier wafer since they break rather easily
during routine handling. Also, there is no way other than an
optical inspection of each individual die or mirror to identify the
known good mirrors. However, optical inspection of such small items
is extremely difficult and expensive. There are also no mechanical
or electrical tests that can be performed on the individual mirrors
or dies while they are still bonded to the backing layer to verify
whether the mirrors are good or faulty.
[0024] Consequently, since it is very difficult to ship, (if
shipping is to occur) the individual dies because they are fragile
and cannot be shipped using the accepted methods for shipping
electronic die, such as gel-track trays or chip trays. Therefore
according to this process, the bonded combination wafer 10 and
backing wafer 12 must be shipped. Thus, the release process must
also be transferred to the assembly vendor. This means that there
may be no yield data available on the mirror dies until final
testing of the assembled micromirrors and may result in an
inability to determine the cause of defaults or the particular
process steps or areas where the defaults occur.
[0025] The present invention relates to individual mirror devices
formed on a wafer using processing steps some of which have
similarity to steps used in manufacturing IC's and other
semiconductor devices. Referring again to the process discussed
with respect to FIGS. 1A through 1H, it is noted that the described
process follows "mask" guidelines which required all features on
the wafer or each individual device to be created by etching
trenches, for example, having a 10 .mu.m width. This rule or
guideline was typically included or followed because of
"microloading" which occurs with plasma etching. As discussed
above, microloading results because lines of different widths etch
at different rates, and more specifically, "wide" lines etch at a
faster rate than "thin" lines. Thus, to provide consistency in
etching of features, a standard rule is that all lines including
features and separating lines are to be etched by lines 10 .mu.m in
width. Consequently, as shown in FIGS. 3A and 3B, if an area 34
(FIG. 3B), that is larger than 10 .mu.m is to be removed, the
process discussed above with respect to FIGS. 1A through 1h
required etching 10 .mu.m lines 37 around the area to be removed so
as to leave a break-away area or piece 36 as shown in FIG. 3A. For
example in FIGS. 3A and 3B, the area to be removed is 50 .mu.m.
This break-away piece or area 36 is then removed after the etching
release process. The break-away area or piece 36 will typically
simply fall away after the individual dies or mirrors are removed
from the backing wafer 12. A potential problem with this process is
that sometimes the break-away areas or pieces 36 are not removed,
but instead, stick to one of the mirror devices and cause a
failure. FIG. 3C illustrates the gimbal support structure 38 and a
mirror 40 attached to the gimbal support structure 38 by a torsion
hinge 42, as well as a blow up view of an alignment stop 44 (there
may be more than one) between structure 38 and mirror 40 as formed
by this process. This illustration shows how all etch lines may be
limited to a minimum of 10 .mu.m, yet some parts of the structure
may be divided by a spacing less than 10 .mu.m.
[0026] Other difficulties or problems with the above discussed
method are when the layer 10 with the individual devices was
released from the backing wafer 12.
[0027] The process of this invention uses the differences in
"microloading" or in etch rates of wide lines and narrow lines
advantageously. For example, the process of the present invention
may follow the method discussed above with respect to FIG. 1A
through 1H from FIG. 1A through FIG. 1D. However, as shown in FIG.
4, according to the present invention, narrow lines 46 are used as
the dividing or separation lines between individual dies (devices
such as the mirror device 48 and 50), whereas "wide" etching lines
such as lines 52 in the illustrated embodiment have a ratio of
greater than 4:1 with respect to the narrow lines 46 and are used
to define features of an individual device formed on the 115 .mu.m
wafer 10. As examples only, the individual devices are separated by
lines having a width of 10 .mu.m, and the features in FIGS. 4, 5
and 7 are shown as being defined by lines equal to or greater than
50 .mu.m. For example, in the embodiment shown in FIG. 4, line 52
separates mirror 54 from gimbal structure 56. Therefore, as also
illustrated, the features on individual devices formed by the fast
etching 50 .mu.m lines (such as line 52) are completely etched
through the 115 .mu.m wafer 10 before the slower etching 10 .mu.m
separation lines 46 are able to etch through the wafer. FIG. 5
shows formation of the torsional hinge or support 58 and an
alignment stop 60 using 50 .mu.m lines for separating features as
formed by the process of the present invention. It will be
appreciated by those skilled in the art that although silicon is
often preferred for such processes, other suitable materials such
as, but not limited to, gallium arsenide, quartz and silicon
carbide may also be used.
[0028] Therefore, by stopping the etching process after the
complete etching of the wide lines (e.g. line 52), but before the
narrow lines (e.g. line 46) can etch through the wafer, all of the
individual devices (or according to the embodiment discussed above
the individual 178 mirrors and their gimbal structure) are all
still attached to each other. This allows the multiplicity of
devices etched into the 115 .mu.m wafer 10 to be removed from the
backing wafer 12 still in the shape of a wafer or as a single unit.
Since all of the individual devices or "dies" are still attached to
each other, they are all more likely to separate from the backing
wafer 12 than was the case using the method discussed above with
respect to FIGS. 1A-1H and 2A-2C. Thus, the yield will
increase.
[0029] A process for releasing wafer 10 from its backing wafer 12
and then cleaning the released "etched" wafer 10 is illustrated and
discussed with respect to FIGS. 6A through 6C. For example, as
shown in FIG. 6C, the wafer combination 62 consisting of etched
wafer 10 and backing wafer 12 is soaked in acetone 64 for a
selected period of time to substantially dissolve the adhesive (for
example, resist) which bonds the wafer 10 to backing wafer 12. Then
as shown in FIG. 6B, an edge of the wafer 10 with the individual
devices etched therein is then gripped such as by tweezers 66 and
slid or pulled off of the carrier wafer 12 as shown in FIG. 6C.
This process can also be done by automated tooling. The removed
wafer 10 is then preferably soaked in a fresh bath of clean acetone
for about five minutes to remove any residue so as to avoid spots
on the devices. The micromirror wafer should quickly be placed in
the fresh bath to assure that the wafer stays wet with acetone.
After the wafer has been soaked in the fresh acetone bath, the
wafer is preferably rinsed in a hot IPA bath for about five
minutes. The wafer is then removed from the hot IPA bath. The
removal of the wafer 10 from the hot IPA bath may be a slow process
so that the IPA sheets off of the wafer or alternately, the wafer
may be dried using an IPA vapor dryer.
[0030] As was disclosed above, it is extremely difficult to test
the individual mirrors after they have been separated from each
other according to the process discussed with respect to FIGS.
1A-1H. However, it is now possible to carefully clamp the etched
wafer 10 with all 178 mirrors and gimbals in a fixture and then
test the individual devices or mirrors to determine defects by
applying a slight force. For example, a spring pin or air pressure
may be used to verify proper movement of the mirrors. Subsequent to
testing, the individual devices or gimbal mirror a structure on the
wafers such as structures 48 and 50 can then be separated from each
other by using a punch 68 and anvil 70 to crack the connecting
material 72 which remains in the area of the 10 .mu.m lines 46
after etching. This is shown in FIG. 7.
[0031] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. It is therefore
intended that the appended claims encompass any such modifications
or embodiments.
* * * * *