U.S. patent application number 12/946583 was filed with the patent office on 2011-03-10 for methods for forming layers within a mems device using liftoff processes.
This patent application is currently assigned to QUALCOMM MEMS Technologies, Inc.. Invention is credited to Chun-Ming Wang.
Application Number | 20110058243 12/946583 |
Document ID | / |
Family ID | 37451248 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110058243 |
Kind Code |
A1 |
Wang; Chun-Ming |
March 10, 2011 |
METHODS FOR FORMING LAYERS WITHIN A MEMS DEVICE USING LIFTOFF
PROCESSES
Abstract
Certain MEMS devices include layers patterned to have tapered
edges. One method for forming layers having tapered edges includes
the use of an etch leading layer. Another method for forming layers
having tapered edges includes the deposition of a layer in which
the upper portion is etchable at a faster rate than the lower
portion. Another method for forming layers having tapered edges
includes the use of multiple iterative etches. Another method for
forming layers having tapered edges includes the use of a liftoff
mask layer having an aperture including a negative angle, such that
a layer can be deposited over the liftoff mask layer and the mask
layer removed, leaving a structure having tapered edges.
Inventors: |
Wang; Chun-Ming; (Fremont,
CA) |
Assignee: |
QUALCOMM MEMS Technologies,
Inc.
San Diego
CA
|
Family ID: |
37451248 |
Appl. No.: |
12/946583 |
Filed: |
November 15, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12702132 |
Feb 8, 2010 |
7835093 |
|
|
12946583 |
|
|
|
|
11506770 |
Aug 18, 2006 |
7660058 |
|
|
12702132 |
|
|
|
|
60710019 |
Aug 19, 2005 |
|
|
|
Current U.S.
Class: |
359/239 ;
438/669 |
Current CPC
Class: |
G02B 26/001 20130101;
B81B 2203/053 20130101; B81C 2201/0167 20130101; B81B 2203/0163
20130101; B81B 2203/0307 20130101; Y10S 359/90 20130101; B81B
2201/047 20130101; B81B 3/0072 20130101; B81C 1/00142 20130101;
B81B 2203/019 20130101; B81C 1/00666 20130101; G02B 26/0841
20130101 |
Class at
Publication: |
359/239 ;
438/669 |
International
Class: |
G02B 26/02 20060101
G02B026/02; H01L 21/302 20060101 H01L021/302 |
Claims
1. An electromechanical device, comprising: an electrode layer
located over a substrate; a conductive movable layer located over
the conductive layer, wherein at least a portion of the conductive
movable layer is spaced apart from the electrode layer by an air
gap; at least one support structure located adjacent the conductive
movable layer, wherein an outer periphery of the support structure
comprises an outwardly tapered edge.
2. The electromechanical device of claim 1, additionally comprising
a dielectric layer located over the electrode layer.
3. The electromechanical device of claim 1, wherein the at least
one support structure underlies at least a portion of the
conductive movable layer.
4. The electromechanical device of claim 1, wherein the at least
one support structure overlies at least a portion of the conductive
movable layer.
5. The electromechanical device of claim 1, wherein the at least
one support structure is formed via a liftoff process.
6. The electromechanical device of claim 1, wherein the
electromechanical device comprises an interferometric
modulator.
7. The electromechanical device of claim 1, additionally
comprising: a processor that is configured to communicate with the
conductive movable layer, the processor being configured to process
image data; and a memory device that is configured to communicate
with the processor.
8. The electromechanical device of claim 7, additionally comprising
a driver circuit configured to send at least one signal to the
conductive movable layer.
9. The electromechanical device of claim 8, additionally comprising
a controller configured to send at least a portion of the image
data to the driver circuit.
10. The electromechanical device of claim 7, additionally
comprising an image source module configured to send the image data
to the processor.
11. The electromechanical device of claim 10, wherein the image
source module comprises at least one of a receiver, transceiver,
and transmitter.
12. The electromechanical device of claim 7, additionally
comprising an input device configured to receive input data and to
communicate the input data to the processor.
13. A method of fabricating an electromechanical device, the method
comprising: forming an electrode layer over a substrate; forming a
sacrificial layer over the electrode layer; forming a conductive
movable layer over the sacrificial layer; forming a mask having at
least one aperture extending therethrough, the mask having a
negative angle at the edge of the at least one aperture; depositing
a layer of support structure material over the mask and the at
least one aperture; and removing the mask to form a support
structure.
14. The method of claim 13, wherein forming the support structure
is done prior to forming the conductive movable layer.
15. The method of claim 13, wherein forming the conductive movable
layer is done prior to forming the support structure.
16. The method of claim 13, additionally comprising performing a
release etch to remove the sacrificial layer, forming an air gap
between portions of the conductive movable layer and the electrode
layer.
17. An electromechanical device, comprising: an electrode layer
located over a substrate; a conductive movable layer located over
the conductive layer, wherein at least a portion of the conductive
movable layer is spaced apart from the electrode layer by an air
gap; at least one support structure located adjacent the conductive
movable layer, wherein the at least one support structure comprises
means for facilitating conformal deposition of an overlying
layer.
18. The electromechanical device of claim 17, wherein the means for
facilitating conformal deposition of an overlying layer comprise an
outwardly tapered edge at an outer periphery of the support
structure.
19. The electromechanical device of claim 17, wherein the at least
one support structure underlies at least a portion of the
conductive movable layer.
20. The electromechanical device of claim 17, wherein the at least
one support structure overlies at least a portion of the conductive
movable layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/702,132, filed on Feb. 8, 2010, now issued
as U.S. Pat. No. 7,835,093, which is a continuation of U.S. patent
application Ser. No. 11/506,770, filed on Aug. 18, 2006, now issued
as U.S. Pat. No. 7,660,058, which claims the benefit of U.S.
Provisional Application No. 60/710,019, filed Aug. 19, 2005, each
one of which is hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] Microelectromechanical systems (MEMS) include micro
mechanical elements, actuators, and electronics. Micromechanical
elements may be created using deposition, etching, and/or other
micromachining processes that etch away parts of substrates and/or
deposited material layers or that add layers to form electrical and
electromechanical devices. One type of MEMS device is called an
interferometric modulator. As used herein, the term interferometric
modulator or interferometric light modulator refers to a device
that selectively absorbs and/or reflects light using the principles
of optical interference. In certain embodiments, an interferometric
modulator may comprise a pair of conductive plates, one or both of
which may be transparent and/or reflective in whole or part and
capable of relative motion upon application of an appropriate
electrical signal. In a particular embodiment, one plate may
comprise a stationary layer deposited on a substrate and the other
plate may comprise a metallic membrane separated from the
stationary layer by an air gap. As described herein in more detail,
the position of one plate in relation to another can change the
optical interference of light incident on the interferometric
modulator. Such devices have a wide range of applications, and it
would be beneficial in the art to utilize and/or modify the
characteristics of these types of devices so that their features
can be exploited in improving existing products and creating new
products that have not yet been developed.
SUMMARY OF THE INVENTION
[0003] In one embodiment, a method of fabricating a MEMS device is
provided, the method including forming an electrode layer, forming
a mask over the electrode layer, patterning the electrode layer
using the mask to form an isolated electrode member, and tapering
the electrode member to form an outwardly tapering edge portion,
where at least a portion of the mask remains in place during the
tapering.
[0004] In another embodiment, a method of fabricating a MEMS device
is provided, the method including forming a sacrificial layer over
a substrate, patterning the sacrificial layer to form apertures,
forming a support layer over the patterned sacrificial layer,
forming a mask over the support layer, patterning the support layer
using the mask to form at least one support structure located at
least partially within the aperture in the sacrificial layer, and
tapering the support structure to form a tapered edge, where at
least a portion of the mask remains in place during the
tapering.
[0005] In another embodiment, a method of fabricating a MEMS device
is provided, the method including forming an electrode layer over a
substrate, forming a sacrificial layer over the electrode layer,
patterning the sacrificial layer to form at least one tapered
aperture extending through the sacrificial layer, forming a mask
layer including an aperture overlying the tapered aperture
extending through the sacrificial layer, the mask layer including
an overhanging portion extending around the aperture in the mask
layer, depositing a support layer over the mask layer, removing the
mask layer via a liftoff process, thereby forming a support
structure located at least partially within the tapered aperture
extending through the sacrificial layer, and forming a movable
layer adjacent the support structure.
[0006] In another embodiment, a method of fabricating a MEMS device
is provided, the method including forming an electrode layer over a
substrate, forming a mask over the electrode layer, where the mask
including a negative angle extending about the periphery of the
mask, depositing a layer of sacrificial material over the mask,
removing the mask via a liftoff process, forming a sacrificial
layer including a tapered aperture, forming a support structure
located at least partially within the tapered aperture, and forming
a movable layer adjacent the support structure.
[0007] In another embodiment, a method of fabricating a MEMS device
is provided, the method including forming a lower electrode layer
over a substrate, forming a sacrificial layer over the electrode
layer, forming a mask layer overlying the sacrificial layer, the
mask layer including an aperture having an overhanging portion
extending around an edge of the aperture, depositing a layer of
electrode material over the mask layer, removing the mask layer via
a liftoff process, thereby forming at least an isolated electrode
member having an outwardly tapering edge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an isometric view depicting a portion of one
embodiment of an interferometric modulator display in which a
movable reflective layer of a first interferometric modulator is in
a relaxed position and a movable reflective layer of a second
interferometric modulator is in an actuated position.
[0009] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device incorporating a 3.times.3 interferometric
modulator display.
[0010] FIG. 3 is a diagram of movable mirror position versus
applied voltage for one exemplary embodiment of an interferometric
modulator of FIG. 1.
[0011] FIG. 4 is an illustration of a set of row and column
voltages that may be used to drive an interferometric modulator
display.
[0012] FIG. 5A illustrates one exemplary frame of display data in
the 3.times.3 interferometric modulator display of FIG. 2.
[0013] FIG. 5B illustrates one exemplary timing diagram for row and
column signals that may be used to write the frame of FIG. 5A.
[0014] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a visual display device comprising a plurality of
interferometric modulators.
[0015] FIG. 7A is a cross section of the device of FIG. 1.
[0016] FIG. 7B is a cross section of an alternative embodiment of
an interferometric modulator.
[0017] FIG. 7C is a cross section of another alternative embodiment
of an interferometric modulator.
[0018] FIG. 7D is a cross section of yet another alternative
embodiment of an interferometric modulator.
[0019] FIG. 7E is a cross section of an additional alternative
embodiment of an interferometric modulator.
[0020] FIG. 8 is a top plan view of an array of interferometric
modulator elements in which the individual elements comprise
support structures.
[0021] FIGS. 9A-9J are schematic cross-sections illustrating a
method for fabricating an interferometric modulator element
comprising support structures located over a movable layer.
[0022] FIGS. 10A-10F are schematic cross-sections illustrating a
method for fabricating an interferometric modulator element
comprising a reflective layer which is partially separated from a
mechanical layer.
[0023] FIGS. 11A-11B are schematic cross-sections illustrating a
method for forming a MEMS structure having a tapered edge through
the use of an etch leading layer.
[0024] FIGS. 12A-12B are schematic cross-sections illustrating an
alternate method for forming a MEMS structure having a tapered edge
through the use of an etch leading layer.
[0025] FIG. 13 is a schematic cross-section illustrating a step in
a method for forming a MEMS structure having a tapered edge by
altering the properties of the support structure layer during
formation of the MEMS structure layer.
[0026] FIG. 14 is a schematic cross-section illustrating a step in
a method for forming a MEMS structure having a tapered edge by
forming a series of sublayers having differing properties to form
the MEMS structure layer.
[0027] FIG. 15 is a schematic cross-section illustrating a step in
a method for forming a MEMS structure having a tapered edge by
forming an overlying mask layer having poor adhesion to the MEMS
structure layer.
[0028] FIGS. 16A-D are schematic cross-sections illustrating a
method for forming a MEMS structure having a tapered edge through
the use of an iterative ashing and etching process.
[0029] FIGS. 17A-17E are schematic cross-sections illustrating a
method for forming a MEMS structure having a tapered edge though
the use of multiple etches.
[0030] FIGS. 18A-18C are schematic cross-sections illustrating a
method for forming a support structure having tapered edges through
the use of a liftoff process.
[0031] FIGS. 19A-19C are schematic cross-sections illustrating a
method for forming a mask layer having a negative taper for use in
a liftoff process.
[0032] FIG. 20 is a schematic cross-section illustrating a step in
a method for forming a sacrificial layer having a tapered
aperture.
[0033] FIG. 21 is a schematic cross-section illustrating a step in
a method for forming an isolated electrode member having a tapered
edge.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0034] The following detailed description is directed to certain
specific embodiments of the invention. However, the invention can
be embodied in a multitude of different ways. In this description,
reference is made to the drawings wherein like parts are designated
with like numerals throughout. As will be apparent from the
following description, the embodiments may be implemented in any
device that is configured to display an image, whether in motion
(e.g., video) or stationary (e.g., still image), and whether
textual or pictorial. More particularly, it is contemplated that
the embodiments may be implemented in or associated with a variety
of electronic devices such as, but not limited to, mobile
telephones, wireless devices, personal data assistants (PDAs),
hand-held or portable computers, GPS receivers/navigators, cameras,
MP3 players, camcorders, game consoles, wrist watches, clocks,
calculators, television monitors, flat panel displays, computer
monitors, auto displays (e.g., odometer display, etc.), cockpit
controls and/or displays, display of camera views (e.g., display of
a rear view camera in a vehicle), electronic photographs,
electronic billboards or signs, projectors, architectural
structures, packaging, and aesthetic structures (e.g., display of
images on a piece of jewelry). MEMS devices of similar structure to
those described herein can also be used in non-display applications
such as in electronic switching devices.
[0035] In the fabrication of MEMS devices which comprise a series
of sequentially deposited layers, it is often advantageous to
provide tapered or angled structures, such as an underlying layer
or structure having a tapered edge in order to facilitate the
conformal deposition of overlying layers without breaks or weak
thinner portions. In certain embodiments, etching methods can be
used to provide such tapered edges to, e.g., mirror/electrodes or
support structures. In particular embodiments, these etching
methods may include the use of etch leading layers overlying the
layers to be etched. In other particular embodiments, these etching
methods may include an iterative etching and/or ashing process. In
other embodiments, liftoff processes can be used to form structures
having tapered edges.
[0036] One interferometric modulator display embodiment comprising
an interferometric MEMS display element is illustrated in FIG. 1.
In these devices, the pixels are in either a bright or dark state.
In the bright ("on" or "open") state, the display element reflects
a large portion of incident visible light to a user. When in the
dark ("off" or "closed") state, the display element reflects little
incident visible light to the user. Depending on the embodiment,
the light reflectance properties of the "on" and "off" states may
be reversed. MEMS pixels can be configured to reflect predominantly
at selected colors, allowing for a color display in addition to
black and white.
[0037] FIG. 1 is an isometric view depicting two adjacent pixels in
a series of pixels of a visual display, wherein each pixel
comprises a MEMS interferometric modulator. In some embodiments, an
interferometric modulator display comprises a row/column array of
these interferometric modulators. Each interferometric modulator
includes a pair of reflective layers positioned at a variable and
controllable distance from each other to form a resonant optical
cavity with at least one variable dimension. In one embodiment, one
of the reflective layers may be moved between two positions. In the
first position, referred to herein as the relaxed position, the
movable reflective layer is positioned at a relatively large
distance from a fixed partially reflective layer. In the second
position, referred to herein as the actuated position, the movable
reflective layer is positioned more closely adjacent to the
partially reflective layer. Incident light that reflects from the
two layers interferes constructively or destructively depending on
the position of the movable reflective layer, producing either an
overall reflective or non-reflective state for each pixel.
[0038] The depicted portion of the pixel array in FIG. 1 includes
two adjacent interferometric modulators 12a and 12b. In the
interferometric modulator 12a on the left, a movable reflective
layer 14a is illustrated in a relaxed position at a predetermined
distance from an optical stack 16a, which includes a partially
reflective layer. In the interferometric modulator 12b on the
right, the movable reflective layer 14b is illustrated in an
actuated position adjacent to the optical stack 16b.
[0039] The optical stacks 16a and 16b (collectively referred to as
optical stack 16), as referenced herein, typically comprise several
fused layers, which can include an electrode layer, such as indium
tin oxide (ITO), a partially reflective layer, such as chromium,
and a transparent dielectric. The optical stack 16 is thus
electrically conductive, partially transparent, and partially
reflective, and may be fabricated, for example, by depositing one
or more of the above layers onto a transparent substrate 20. The
partially reflective layer can be formed from a variety of
materials that are partially reflective such as various metals,
semiconductors, and dielectrics. The partially reflective layer can
be formed of one or more layers of materials, and each of the
layers can be formed of a single material or a combination of
materials.
[0040] In some embodiments, the layers of the optical stack 16 are
patterned into parallel strips, and may form row electrodes in a
display device as described further below. The movable reflective
layers 14a, 14b may be formed as a series of parallel strips of a
deposited metal layer or layers (orthogonal to the row electrodes
of 16a, 16b) deposited on top of posts 18 and an intervening
sacrificial material deposited between the posts 18. When the
sacrificial material is etched away, the movable reflective layers
14a, 14b are separated from the optical stacks 16a, 16b by a
defined gap 19. A highly conductive and reflective material such as
aluminum may be used for the reflective layers 14, and these strips
may form column electrodes in a display device.
[0041] With no applied voltage, the cavity 19 remains between the
movable reflective layer 14a and optical stack 16a, with the
movable reflective layer 14a in a mechanically relaxed state, as
illustrated by the pixel 12a in FIG. 1. However, when a potential
difference is applied to a selected row and column, the capacitor
formed at the intersection of the row and column electrodes at the
corresponding pixel becomes charged, and electrostatic forces pull
the electrodes together. If the voltage is high enough, the movable
reflective layer 14 is deformed and is forced against the optical
stack 16. A dielectric layer (not illustrated in this Figure)
within the optical stack 16 may prevent shorting and control the
separation distance between layers 14 and 16, as illustrated by
pixel 12b on the right in FIG. 1. The behavior is the same
regardless of the polarity of the applied potential difference. In
this way, row/column actuation that can control the reflective vs.
non-reflective pixel states is analogous in many ways to that used
in conventional LCD and other display technologies.
[0042] FIGS. 2 through 5B illustrate one exemplary process and
system for using an array of interferometric modulators in a
display application.
[0043] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device that may incorporate aspects of the
invention. In the exemplary embodiment, the electronic device
includes a processor 21 which may be any general purpose single- or
multi-chip microprocessor such as an ARM, Pentium.RTM., Pentium
II.RTM., Pentium III.RTM., Pentium IV.RTM., Pentium.RTM. Pro, an
8051, a MIPS.RTM., a Power PC.RTM., an ALPHA.RTM., or any special
purpose microprocessor such as a digital signal processor,
microcontroller, or a programmable gate array. As is conventional
in the art, the processor 21 may be configured to execute one or
more software modules. In addition to executing an operating
system, the processor may be configured to execute one or more
software applications, including a web browser, a telephone
application, an email program, or any other software
application.
[0044] In one embodiment, the processor 21 is also configured to
communicate with an array driver 22. In one embodiment, the array
driver 22 includes a row driver circuit 24 and a column driver
circuit 26 that provide signals to a display array or panel 30. The
cross section of the array illustrated in FIG. 1 is shown by the
lines 1-1 in FIG. 2. For MEMS interferometric modulators, the
row/column actuation protocol may take advantage of a hysteresis
property of these devices illustrated in FIG. 3. It may require,
for example, a 10 volt potential difference to cause a movable
layer to deform from the relaxed state to the actuated state.
However, when the voltage is reduced from that value, the movable
layer maintains its state as the voltage drops back below 10 volts.
In the exemplary embodiment of FIG. 3, the movable layer does not
relax completely until the voltage drops below 2 volts. Thus, there
exists a window of applied voltage, about 3 to 7 V in the example
illustrated in FIG. 3, within which the device is stable in either
the relaxed or actuated state. This is referred to herein as the
"hysteresis window" or "stability window." For a display array
having the hysteresis characteristics of FIG. 3, the row/column
actuation protocol can be designed such that during row strobing,
pixels in the strobed row that are to be actuated are exposed to a
voltage difference of about 10 volts, and pixels that are to be
relaxed are exposed to a voltage difference of close to zero volts.
After the strobe, the pixels are exposed to a steady state voltage
difference of about 5 volts such that they remain in whatever state
the row strobe put them in. After being written, each pixel sees a
potential difference within the "stability window" of 3-7 volts in
this example. This feature makes the pixel design illustrated in
FIG. 1 stable under the same applied voltage conditions in either
an actuated or relaxed pre-existing state. Since each pixel of the
interferometric modulator, whether in the actuated or relaxed
state, is essentially a capacitor formed by the fixed and moving
reflective layers, this stable state can be held at a voltage
within the hysteresis window with almost no power dissipation.
Essentially no current flows into the pixel if the applied
potential is fixed.
[0045] In typical applications, a display frame may be created by
asserting the set of column electrodes in accordance with the
desired set of actuated pixels in the first row. A row pulse is
then applied to the row 1 electrode, actuating the pixels
corresponding to the asserted column lines. The asserted set of
column electrodes is then changed to correspond to the desired set
of actuated pixels in the second row. A pulse is then applied to
the row 2 electrode, actuating the appropriate pixels in row 2 in
accordance with the asserted column electrodes. The row 1 pixels
are unaffected by the row 2 pulse, and remain in the state they
were set to during the row 1 pulse. This may be repeated for the
entire series of rows in a sequential fashion to produce the frame.
Generally, the frames are refreshed and/or updated with new display
data by continually repeating this process at some desired number
of frames per second. A wide variety of protocols for driving row
and column electrodes of pixel arrays to produce display frames are
also well known and may be used in conjunction with the present
invention.
[0046] FIGS. 4, 5A, and 5B illustrate one possible actuation
protocol for creating a display frame on the 3.times.3 array of
FIG. 2. FIG. 4 illustrates a possible set of column and row voltage
levels that may be used for pixels exhibiting the hysteresis curves
of FIG. 3. In the FIG. 4 embodiment, actuating a pixel involves
setting the appropriate column to -V.sub.bias, and the appropriate
row to +.DELTA.V, which may correspond to -5 volts and +5 volts,
respectively Relaxing the pixel is accomplished by setting the
appropriate column to +V.sub.bias, and the appropriate row to the
same +.DELTA.V, producing a zero volt potential difference across
the pixel. In those rows where the row voltage is held at zero
volts, the pixels are stable in whatever state they were originally
in, regardless of whether the column is at +V.sub.bias, or
-V.sub.bias. As is also illustrated in FIG. 4, it will be
appreciated that voltages of opposite polarity than those described
above can be used, e.g., actuating a pixel can involve setting the
appropriate column to +V.sub.bias, and the appropriate row to
-.DELTA.V. In this embodiment, releasing the pixel is accomplished
by setting the appropriate column to -V.sub.bias, and the
appropriate row to the same -.DELTA.V, producing a zero volt
potential difference across the pixel.
[0047] FIG. 5B is a timing diagram showing a series of row and
column signals applied to the 3.times.3 array of FIG. 2 which will
result in the display arrangement illustrated in FIG. 5A, where
actuated pixels are non-reflective. Prior to writing the frame
illustrated in FIG. 5A, the pixels can be in any state, and in this
example, all the rows are at 0 volts, and all the columns are at +5
volts. With these applied voltages, all pixels are stable in their
existing actuated or relaxed states.
[0048] In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and
(3,3) are actuated. To accomplish this, during a "line time" for
row 1, columns 1 and 2 are set to -5 volts, and column 3 is set to
+5 volts. This does not change the state of any pixels, because all
the pixels remain in the 3-7 volt stability window. Row 1 is then
strobed with a pulse that goes from 0, up to 5 volts, and back to
zero. This actuates the (1,1) and (1,2) pixels and relaxes the
(1,3) pixel. No other pixels in the array are affected. To set row
2 as desired, column 2 is set to -5 volts, and columns 1 and 3 are
set to +5 volts. The same strobe applied to row 2 will then actuate
pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other
pixels of the array are affected. Row 3 is similarly set by setting
columns 2 and 3 to -5 volts, and column 1 to +5 volts. The row 3
strobe sets the row 3 pixels as shown in FIG. 5A. After writing the
frame, the row potentials are zero, and the column potentials can
remain at either +5 or -5 volts, and the display is then stable in
the arrangement of FIG. 5A. It will be appreciated that the same
procedure can be employed for arrays of dozens or hundreds of rows
and columns. It will also be appreciated that the timing, sequence,
and levels of voltages used to perform row and column actuation can
be varied widely within the general principles outlined above, and
the above example is exemplary only, and any actuation voltage
method can be used with the systems and methods described
herein.
[0049] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a display device 40. The display device 40 can be,
for example, a cellular or mobile telephone. However, the same
components of display device 40 or slight variations thereof are
also illustrative of various types of display devices such as
televisions and portable media players.
[0050] The display device 40 includes a housing 41, a display 30,
an antenna 43, a speaker 45, an input device 48, and a microphone
46. The housing 41 is generally formed from any of a variety of
manufacturing processes as are well known to those of skill in the
art, including injection molding and vacuum forming. In addition,
the housing 41 may be made from any of a variety of materials,
including, but not limited to, plastic, metal, glass, rubber, and
ceramic, or a combination thereof. In one embodiment, the housing
41 includes removable portions (not shown) that may be interchanged
with other removable portions of different color, or containing
different logos, pictures, or symbols.
[0051] The display 30 of the exemplary display device 40 may be any
of a variety of displays, including a bi-stable display, as
described herein. In other embodiments, the display 30 includes a
flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD
as described above, or a non-flat-panel display, such as a CRT or
other tube device, as is well known to those of skill in the art.
However, for purposes of describing the present embodiment, the
display 30 includes an interferometric modulator display, as
described herein.
[0052] The components of one embodiment of the exemplary display
device 40 are schematically illustrated in FIG. 6B. The illustrated
exemplary display device 40 includes a housing 41 and can include
additional components at least partially enclosed therein. For
example, in one embodiment, the exemplary display device 40
includes a network interface 27 that includes an antenna 43, which
is coupled to a transceiver 47. The transceiver 47 is connected to
a processor 21, which is connected to conditioning hardware 52. The
conditioning hardware 52 may be configured to condition a signal
(e.g., filter a signal). The conditioning hardware 52 is connected
to a speaker 45 and a microphone 46. The processor 21 is also
connected to an input device 48 and a driver controller 29. The
driver controller 29 is coupled to a frame buffer 28 and to an
array driver 22, which in turn is coupled to a display array 30. A
power supply 50 provides power to all components as required by the
particular exemplary display device 40 design.
[0053] The network interface 27 includes the antenna 43 and the
transceiver 47 so that the exemplary display device 40 can
communicate with one or more devices over a network. In one
embodiment, the network interface 27 may also have some processing
capabilities to relieve requirements of the processor 21. The
antenna 43 is any antenna known to those of skill in the art for
transmitting and receiving signals. In one embodiment, the antenna
transmits and receives RF signals according to the IEEE 802.11
standard, including IEEE 802.11(a), (b), or (g). In another
embodiment, the antenna transmits and receives RF signals according
to the BLUETOOTH standard. In the case of a cellular telephone, the
antenna is designed to receive CDMA, GSM, AMPS, or other known
signals that are used to communicate within a wireless cell phone
network. The transceiver 47 pre-processes the signals received from
the antenna 43 so that they may be received by and further
manipulated by the processor 21. The transceiver 47 also processes
signals received from the processor 21 so that they may be
transmitted from the exemplary display device 40 via the antenna
43.
[0054] In an alternative embodiment, the transceiver 47 can be
replaced by a receiver. In yet another alternative embodiment, the
network interface 27 can be replaced by an image source, which can
store or generate image data to be sent to the processor 21. For
example, the image source can be memory device such as a digital
video disc (DVD) or a hard-disc drive that contains image data, or
a software module that generates image data.
[0055] The processor 21 generally controls the overall operation of
the exemplary display device 40. The processor 21 receives data,
such as compressed image data from the network interface 27 or an
image source, and processes the data into raw image data or into a
format that is readily processed into raw image data. The processor
21 then sends the processed data to the driver controller 29 or to
the frame buffer 28 for storage. Raw data typically refers to the
information that identifies the image characteristics at each
location within an image. For example, such image characteristics
can include color, saturation, and gray-scale level.
[0056] In one embodiment, the processor 21 includes a
microcontroller, CPU, or logic unit to control operation of the
exemplary display device 40. The conditioning hardware 52 generally
includes amplifiers and filters for transmitting signals to the
speaker 45, and for receiving signals from the microphone 46. The
conditioning hardware 52 may be discrete components within the
exemplary display device 40, or may be incorporated within the
processor 21 or other components.
[0057] The driver controller 29 takes the raw image data generated
by the processor 21 either directly from the processor 21 or from
the frame buffer 28 and reformats the raw image data appropriately
for high speed transmission to the array driver 22. Specifically,
the driver controller 29 reformats the raw image data into a data
flow having a raster-like format, such that it has a time order
suitable for scanning across the display array 30. Then the driver
controller 29 sends the formatted information to the array driver
22. Although a driver controller 29, such as a LCD controller, is
often associated with the system processor 21 as a stand-alone
Integrated Circuit (IC), such controllers may be implemented in
many ways. They may be embedded in the processor 21 as hardware,
embedded in the processor 21 as software, or fully integrated in
hardware with the array driver 22.
[0058] Typically, the array driver 22 receives the formatted
information from the driver controller 29 and reformats the video
data into a parallel set of waveforms that are applied many times
per second to the hundreds and sometimes thousands of leads coming
from the display's x-y matrix of pixels.
[0059] In one embodiment, the driver controller 29, the array
driver 22, and the display array 30 are appropriate for any of the
types of displays described herein. For example, in one embodiment,
the driver controller 29 is a conventional display controller or a
bi-stable display controller (e.g., an interferometric modulator
controller). In another embodiment, the array driver 22 is a
conventional driver or a bi-stable display driver (e.g., an
interferometric modulator display). In one embodiment, a driver
controller 29 is integrated with the array driver 22. Such an
embodiment is common in highly integrated systems such as cellular
phones, watches, and other small area displays. In yet another
embodiment, the display array 30 is a typical display array or a
bi-stable display array (e.g., a display including an array of
interferometric modulators).
[0060] The input device 48 allows a user to control the operation
of the exemplary display device 40. In one embodiment, the input
device 48 includes a keypad, such as a QWERTY keyboard or a
telephone keypad, a button, a switch, a touch-sensitive screen, or
a pressure- or heat-sensitive membrane. In one embodiment, the
microphone 46 is an input device for the exemplary display device
40. When the microphone 46 is used to input data to the device,
voice commands may be provided by a user for controlling operations
of the exemplary display device 40.
[0061] The power supply 50 can include a variety of energy storage
devices as are well known in the art. For example, in one
embodiment, the power supply 50 is a rechargeable battery, such as
a nickel-cadmium battery or a lithium ion battery. In another
embodiment, the power supply 50 is a renewable energy source, a
capacitor, or a solar cell including a plastic solar cell, and
solar-cell paint. In another embodiment, the power supply 50 is
configured to receive power from a wall outlet.
[0062] In some embodiments, control programmability resides, as
described above, in a driver controller which can be located in
several places in the electronic display system. In some
embodiments, control programmability resides in the array driver
22. Those of skill in the art will recognize that the
above-described optimizations may be implemented in any number of
hardware and/or software components and in various
configurations.
[0063] The details of the structure of interferometric modulators
that operate in accordance with the principles set forth above may
vary widely. For example, FIGS. 7A-7E illustrate five different
embodiments of the movable reflective layer 14 and its supporting
structures. FIG. 7A is a cross section of the embodiment of FIG. 1,
where a strip of metal material 14 is deposited on orthogonally
extending supports 18. In FIG. 7B, the moveable reflective layer 14
is attached to supports 18 at the corners only, on tethers 32. In
FIG. 7C, the moveable reflective layer 14 is suspended from a
deformable layer 34, which may comprise a flexible metal. The
deformable layer 34 connects, directly or indirectly, to the
substrate 20 around the perimeter of the deformable layer 34. These
connections are herein referred to as support structures, which can
take the form of isolated pillars or posts and/or continuous walls
or rails. The embodiment illustrated in FIG. 7D has support
structures 18 that include support plugs 42 upon which the
deformable layer 34 rests. The movable reflective layer 14 remains
suspended over the cavity, as in FIGS. 7A-7C, but the deformable
layer 34 does not form the support posts by filling holes between
the deformable layer 34 and the optical stack 16. Rather, the
support posts 18 are formed of a planarization material, which is
used to form the support post plugs 42. The embodiment illustrated
in FIG. 7E is based on the embodiment shown in FIG. 7D, but may
also be adapted to work with any of the embodiments illustrated in
FIGS. 7A-7C, as well as additional embodiments not shown. In the
embodiment shown in FIG. 7E, an extra layer of metal or other
conductive material has been used to form a bus structure 44. This
allows signal routing along the back of the interferometric
modulators, eliminating a number of electrodes that may otherwise
have had to be formed on the substrate 20.
[0064] In embodiments such as those shown in FIG. 7, the
interferometric modulators function as direct-view devices, in
which images are viewed from the front side of the transparent
substrate 20, the side opposite to that upon which the modulator is
arranged. In these embodiments, the reflective layer 14 optically
shields the portions of the interferometric modulator on the side
of the reflective layer opposite the substrate 20, including the
deformable layer 34. This allows the shielded areas to be
configured and operated upon without negatively affecting the image
quality. Such shielding allows the bus structure 44 in FIG. 7E,
which provides the ability to separate the optical properties of
the modulator from the electromechanical properties of the
modulator, such as addressing and the movements that result from
that addressing. This separable modulator architecture allows the
structural design and materials used for the electromechanical
aspects and the optical aspects of the modulator to be selected and
to function independently of each other. Moreover, the embodiments
shown in FIGS. 7C-7E have additional benefits deriving from the
decoupling of the optical properties of the reflective layer 14
from its mechanical properties, which are carried out by the
deformable layer 34. This allows the structural design and
materials used for the reflective layer 14 to be optimized with
respect to the optical properties, and the structural design and
materials used for the deformable layer 34 to be optimized with
respect to desired mechanical properties.
[0065] In certain embodiments, it may be desirable to provide
additional support to a movable layer such as the movable
reflective layer 14 illustrated in FIG. 7A, or the combination of
mechanical layer 34 and movable reflective layer 14 of FIGS. 7C-7E.
In optical MEMS devices, such as interferometric modulators, the
movable layer may comprise a reflective sublayer and a mechanical
sublayer, as will be discussed in greater detail below. Such
support may be provided by a series of support structures which may
be located along the edges of an individual modulator element
and/or in the interior of such an element. In various embodiments,
these support structures may be located either over or underneath a
movable layer. In alternate embodiments, support structures may
extend through an aperture formed in the mechanical layer, such
that support is provided from both above and below the mechanical
layer. As used herein, the term "rivet" generally refers to a
patterned layer overlying a mechanical layer in a MEMS device,
usually in a recess or depression in the post or support region, to
lend mechanical support for the mechanical layer. Preferably,
though not always, the rivet includes wings overlying an upper
surface of the mechanical layer to add stability and predictability
to the mechanical layer's movement. Similarly, support structures
underlying a mechanical layer in a MEMS device to lend mechanical
support for the mechanical layer are generally referred to herein
as support "posts." In many of the embodiments herein, the
preferred materials are inorganic for stability relative to organic
resist materials.
[0066] An exemplary layout of such support structures is shown in
FIG. 8, which depicts an array of MEMS elements. In certain
embodiments, the array may comprise an array of interferometric
modulators, but in alternate embodiments, the MEMS elements may
comprise any MEMS device having a movable layer. It can be seen
that support structures 62, which in the illustrated embodiment are
underlying post structures but may in other embodiments include
overlying rivet structures, are located both along the edges of a
movable layer 66 and in the interior of a MEMS element, in this
example an interferometric modulator element 60. Certain support
structures may comprise rail structures 64, which extend across the
gap 65 between two adjacent movable layers 66. It can be seen that
movable layer 66 comprises a strip of deformable material extending
through multiple adjacent elements 60 within the same column. The
rail structures 64 run parallel with lower electrodes which define
rows crossing the upper electrodes defined by the strips of the
movable layer 66. The support structures 62 serve to stiffen the
movable layer 66 within the elements or pixels 60, and together
with the rail structures 64, separate the upper and lower
electrodes to define cavities in which the upper electrode can move
vertically.
[0067] Advantageously, these support structures 62 are made small
relative to the surrounding area of the modulator element 60. As
the support posts constrain deflection of the movable layer 66 and
may generally be opaque, the area underneath and immediately
surrounding the support structures 62 is not usable as active area
in a display, as the movable layer in those areas is not movable to
a fully actuated position (e.g., one in which a portion of the
lower surface of the movable layer 14 of FIG. 7A is in contact with
the upper surface of the optical stack 16). Because this may result
in undesirable optical effects in the areas surrounding the post, a
dark or "black" mask layer may advantageously be provided between
the support structures and the viewer to avoid excessive reflection
in these regions that may wash out the intended color.
[0068] In certain embodiments, these support structures may
comprise a depression in the movable layer, along with a
substantially rigid structure(s) above and/or below the movable
layer which helps to maintain the shape. While such support
structures may be formed of a polymer material, an inorganic
material having greater rigidity is preferably used, and provides
advantages over similar structures comprising polymeric
materials.
[0069] For instance, a polymeric support structure may not maintain
a desired level of rigidity over a wide range of operating
temperatures, and may be subject to gradual deformation or
mechanical failure over the lifetime of a device. As such failures
may affect the distance between the movable layer and the optical
stack, and this distance at least partially determines the
wavelengths reflected by the interferometric modulator element,
such failures may lead to a shift in the reflected color due to
wear over time or variance in operating temperatures. Other MEMS
devices experience analogous degradation over time when supports
are formed of polymeric material.
[0070] One process for fanning an interferometric modulator element
comprising underlying post support structures is described with
respect to FIGS. 9A-9J. In FIG. 9A, it can be seen that a
transparent or light-transmissive substrate 70 is provided, which
may comprise, for example, glass or a transparent polymeric
material. A conductive layer 72, which may comprise
indium-tin-oxide (ITO), is then deposited over the transparent
substrate and a partially reflective layer 74, which may comprise
chromium, is deposited over the conductive layer 72. Although in
one embodiment conductive layer 72 may comprise ITO, and may be
referred to as such at various points in the below specification,
it will be understood that the conductive layer 72 may comprise any
suitable conductive material, and need not be transparent for
non-optical MEMS structures. Similarly, although sometimes referred
to as a chromium layer, partially reflective layer 74 may comprise
any suitable partially reflective layer, and may be omitted for
non-optical MEMS structures.
[0071] The conductive layer 72 and partially reflective layer 74
are then patterned and etched to form bottom electrodes, also
referred to as row electrodes, which run cross-wise (e.g.,
perpendicular) to the movable layer 66 of FIG. 8 and which will be
used to address a row of MEMS elements. In certain embodiments, the
conductive and partially reflective layers 72 and 74 may
advantageously also be patterned and etched to remove the ITO and
chromium underlying the areas where the support post structures
will be located, forming apertures 76 as depicted in FIG. 9B. This
patterning and etching is preferably done by the same process which
forms the row electrodes. The removal of ITO and chromium (or other
conductive materials) underlying the support structures helps to
minimize risk of shorting between an overlying conductive layer,
such as the movable layer, and the bottom electrode. Thus, FIG. 9B
and the subsequent figures depict a cross-section of a continuous
row electrode formed by layers 72 and 74, in which isolated
apertures 76 have been etched, taken along a line extending through
those apertures. In other embodiments in which the conductive layer
72 and partially reflective layer 74 are not etched to form
apertures 76, a dielectric layer, discussed below, may provide
sufficient protection against shorting between the bottom electrode
and the movable layer.
[0072] The conductive layer 72 and partially reflective layer 74
may be patterned via photolithography and etched via, for example,
commercially available wet etches. Chromium wet etches include
solutions of acetic acid (C.sub.2H.sub.4O.sub.2) and cerium
ammonium nitrate [Ce(NH.sub.4).sub.2(NO.sub.3).sub.6]. ITO wet
etches include HCl, a mixture of HCl and HNO.sub.3, or a mixture of
FeCl.sub.3/HCl/DI in a 75%/3%/22% ratio and H.sub.2O. Once the
apertures 76 have been formed, a dielectric layer 78 is deposited
over the conductive and partially reflective layers 72 and 74, as
seen in FIG. 9C, forming the optical stack 16. In certain
embodiments, the dielectric layer may comprise SiO.sub.2 or
SiN.sub.x, although a wide variety of suitable materials may be
used.
[0073] The thickness and positioning of the layers forming the
optical stack 16 determines the color reflected by the
interferometric modulator element when the element is actuated
(collapsed), bringing the movable layer 66 into contact with the
optical stack. In certain embodiments, the optical stack is
configured such that the interferometric modulator element reflects
substantially no visible light (appears black) when the movable
layer is in an actuated position. Typically, the thickness of the
dielectric layer 78 is about 450 .ANG., although it will be
understood that the desired thickness will vary based on both the
refractive index of the material and the desired color reflected by
the interferometric modulator in a collapsed state. While
illustrated for simplicity as planar (which can be achieved if the
dielectric layer 78 is a spin-on glass), the dielectric layer 78 is
typically conformal over the patterned lower electrode formed from
layers 72 and 74.
[0074] As seen in FIG. 9D, a layer 82 of sacrificial material is
then deposited over the dielectric layer 78. In certain
embodiments, this sacrificial layer 82 is formed from a material
which is etchable by fluorine-based etchants, particularly
XeF.sub.2. For example, the sacrificial layer 82 may be formed from
molybdenum or amorphous silicon (a-Si). In other embodiments, the
sacrificial layer may comprise tantalum or tungsten. Other
materials which are usable as sacrificial materials include silicon
nitride, certain oxides, and organic materials. The thickness of
the deposited sacrificial layer 82 will determine the distance
between the optical stack 16 and the movable layer 66, thus
defining the dimensions of the interferometric gap 19 (see FIG.
7A). As the height of the gap 19 determines the color reflected by
the interferometric modulator element when in an unactuated
position, the thickness of the sacrificial layer 82 will vary
depending on the desired characteristics of the interferometric
modulator. For instance, in an embodiment in which a modulator
element that reflects green in the unactuated position is formed,
the thickness of the sacrificial layer 82 may be roughly 2000
.ANG.. In further embodiments, the sacrificial layer may have
multiple thicknesses across an array of MEMS devices, such as in a
multicolor display system where different interferometric gap sizes
are used to produce different colors.
[0075] In FIG. 9E, it can be seen that the sacrificial layer 82 has
been patterned and etched to form tapered apertures 86. The
apertures 86 overlie the apertures 76 cut into the layers 72 and 74
of ITO and chromium. These apertures 86 may be formed by masking
the sacrificial layer, using photolithography, and then performing
either a wet or dry etch to remove portions of the sacrificial
material. Suitable dry etches include, but are not limited to,
SF.sub.6, CF.sub.4, Cl.sub.2, or any mixture of these gases with
O.sub.2 or a noble gas such as He or Ar. Wet etches suitable for
etching Mo include a PAN etch, which may be a mix of phosphoric
acid, acetic acid, nitric acid and deionized water in a 16:1:1:2
ratio. Amorphous silicon can be etched by wet etches including KOH
and HF Nitrate. Preferably, however a dry etch is used to etch the
sacrificial layer 82, as dry etches permit more control over the
shape of tapered apertures 86.
[0076] In FIG. 9F a layer 84 of inorganic post material is
deposited over the patterned sacrificial layer 82, such that the
inorganic post layer 84 also coats the side walls and the base of
the tapered apertures 86. In certain embodiments, the inorganic
post layer 84 is thinner than the sacrificial layer 82, and is
conformal over the sacrificial layer 82. In certain embodiments,
the inorganic post layer 84 may comprise silicon nitride
(SiN.sub.x) or SiO.sub.2, although a wide variety of other
materials may be used, some of which are discussed in greater
detail below.
[0077] In FIG. 9G, the inorganic post layer 84 is patterned and
etched to form inorganic posts 88. Thus, the inorganic post layer
84 is preferably selectively etchable with respect to the
underlying sacrificial layer 82, so as to permit etching of
inorganic post layer 84 while leaving the sacrificial layer 82
unaffected. However, if the inorganic post layer 84 is not
selectively etchable relative to the sacrificial layer 82, an etch
stop layer (not shown) may be provided between the inorganic post
layer 84 and the sacrificial layer 82.
[0078] It can be seen in FIG. 9G that the edges of the inorganic
posts 88 preferably taper which, like the tapered or sloped
sidewalls of the apertures 86, facilitates continuous and conformal
deposition of overlying layers. It can be seen that the post
structure 88 in the illustrated embodiment has a thickness which is
thinner than that of the sacrificial layer 82, and comprises a
substantially flat base portion 89, a sloped sidewall portion 87,
and a substantially horizontal wing portion 85 which extends over a
portion of the sacrificial material. Thus, the post 88
advantageously provides a substantially flat surface at the edge of
the post for supporting an overlying movable layer 66 (see FIG.
9H), minimizing stress and the resultant undesired deflection which
might occur if the movable layer 66 were deposited over a less flat
edge. Details of how to taper structures like the post 88 of FIG.
9H are discussed with respect to FIGS. 11A-21, below.
[0079] In one embodiment, the inorganic post layer 84 and resultant
post 88 comprise diamond-like carbon (DLC). In addition to being
extremely hard and stiff (roughly 10.times. harder than SiO.sub.2),
the DLC inorganic post layer 84 can be etched with an O.sub.2 dry
etch. Advantageously, an O.sub.2 dry etch is highly selective
relative to a wide variety of sacrificial materials, including but
not limited to Mo and a-Si sacrificial material, as well as other
sacrificial materials discussed above. An inorganic post comprising
DLC thus provides a very stiff post, lessening the likelihood and
amount of downward flexure of the edges of the support post 88 when
overlying moving or mechanical layers are pulled downward during
MEMS operation, while permitting the use of an etch which is
relatively benign to a wide variety of materials.
[0080] In FIG. 9H, it can be seen that the components which will
form the movable layer 66 (see, e.g., moveable reflective layer 14
in FIG. 7A) are then deposited over the etched sacrificial layer
82, lining the tapered apertures 86. In the embodiment of FIG. 9H,
a highly reflective layer 90, also referred to as a mirror or minor
layer, is deposited first, followed by a mechanical layer 92. The
highly reflective layer 90 may be formed from a specular metal,
such as aluminum or an aluminum alloy, due to their high
reflectance over a wide spectrum of wavelengths. The mechanical
layer 92 may comprise a metal such as Ni and Cr, and is preferably
formed such that the mechanical layer 92 contains residual tensile
stress. The residual tensile stress provides the mechanical force
which tend to pull the movable layer 66 away from the optical stack
16 when the modulator is unactuated, or "relaxed." For convenience,
the combination of the highly reflective layer 90 and mechanical
layer 92 is collectively referred to as the movable layer 66,
although it will be understood that the term movable layer, as used
herein, also encompasses a partially separated mechanical and
reflective layer, such as the mechanical layer 34 and the movable
reflective layer 14 of FIG. 7C.
[0081] In an embodiment in which the sacrificial layer is to be
"release" etched by a XeF.sub.2 etch, both the reflective layer 90
and the mechanical layer 92 are preferably resistant to XeF.sub.2
etching. If either of these layers is not resistant, an etch stop
layer may be used to protect the non-resistant layer surface
exposed to the release etch. Similarly, the post 88 is preferably
resistant to the release etch, or is alternately protected by an
etch stop layer. It can also be seen that the taper of the edges of
the posts 88 facilitates the conformal deposition of the reflective
layer 90 and mechanical layer 92. Absent this taper, it may be
difficult to deposit these layers such that the layers have
substantially uniform thicknesses over surfaces outside and within
the apertures 86.
[0082] In an alternate embodiment, the movable layer 66 may be a
single layer which is both highly reflective and has the desired
mechanical characteristics. However, the deposition of two distinct
layers permits the selection of a highly reflective material, which
might otherwise be unsuitable if used as the sole material in a
movable layer 66, and similarly allows selection of a suitable
mechanical layer (with some flexibility and inherent tension)
without regard to its reflective properties. In yet further
embodiments, the movable layer may be a reflective sublayer which
is largely detached from the electrical and mechanical layer, such
that the reflective layer may be translated vertically without
bending (see, e.g., FIGS. 10A-10F and attendant description).
[0083] In other embodiments in which the MEMS devices being formed
comprise non-optical MEMS devices (e.g., a MEMS switch), it will be
understood that the movable layer 66 need not comprise a reflective
material. For instance, in embodiments in which MEMS devices such
as MEMS switches are being formed comprising the support structures
discussed herein, the underside of the movable layer 66 need not be
reflective, and may advantageously be a single layer, selected
solely on the basis of its mechanical properties or other desirable
properties.
[0084] Next, in FIG. 9I, it can be seen that photolithography is
used to pattern the mechanical layer 92, and etch the movable layer
66 (i.e., the mechanical layer 92 and the reflective layer 90) to
form etch holes 100, which expose portions of the sacrificial layer
82, in order to facilitate "release" etching of the sacrificial
layer. In certain embodiments, multiple etches are employed to
expose the sacrificial layer. For example, if the mechanical layer
92 comprises nickel and the reflective layer 90 comprises aluminum,
HNO.sub.3 may be used to etch the mechanical layer 92, and
phosphoric acid or a base such as NH.sub.4OH, KOH, THAM, or NaOH
may be used to etch the reflective layer 90. This patterning and
etching may also be used to define the strip electrodes seen in
FIG. 8, by etching gaps 65 between strips of the movable layer 66
(see FIG. 8), separating columns of MEMS devices from one
another.
[0085] Finally, in FIG. 9J, it can be seen that a release etch is
performed to remove the sacrificial layer, creating the
interferometric gap 19 through which the movable layer 66 can move.
In certain embodiments, a XeF.sub.2 etch is used to remove the
sacrificial layer 82. Because XeF.sub.2 etches the preferred
sacrificial materials well, and is extremely selective relative to
other materials used in the processes discussed above, the use of a
XeF.sub.2 etch advantageously permits the removal of the
sacrificial material with very little effect on the surrounding
structures.
[0086] As discussed above, certain embodiments of MEMS devices, and
in particular interferometric modulators, comprise a movable layer
comprising a reflective layer which is partially detached from a
mechanical layer. FIGS. 10A-10F illustrate an exemplary process for
forming separate mirror structures underlying the mechanical layer
in such a MEMS device, which in the illustrated embodiment is an
interferometric modulator. This process may include, for example,
the steps described with respect to FIGS. 9A-9D, in which an
optical stack is deposited, and a sacrificial layer is deposited
over the optical stack.
[0087] In FIG. 10A, it can be seen that a reflective layer 90 is
deposited over the sacrificial layer 82. In certain embodiments,
the reflective layer 90 may comprise a single layer of reflective
material. In other embodiments, the reflective layer 90 may
comprise a thin layer of reflective material with a layer of more
rigid material (not shown) overlying the thin layer of sacrificial
material. As the reflective layer of this embodiment will be
partially detached from an overlying mechanical layer, the
reflective layer 90 preferably has sufficient rigidity to remain in
a substantially flat position relative to the optical stack 16 even
when partially detached, and the inclusion of a stiffening layer on
the side of the reflective layer located away from the optical
stack can be used to provide the desired rigidity.
[0088] In FIG. 10B, the reflective layer 90 of FIG. 10A is
patterned to form a patterned minor layer 200. In one embodiment,
the patterned minor layer 200 comprises a contiguous layer in which
apertures corresponding to the locations of (but wider or narrower
than) support structures have been formed. In another embodiment,
the patterned minor layer 200 may comprise multiple reflective
sections detached from one another.
[0089] In FIG. 10C, a second sacrificial layer 196 is deposited
over the patterned mirror layer 200. Preferably, the second
sacrificial layer 196 is formed from the same material as the first
sacrificial layer 82, or is etchable selectively with respect to
surrounding materials by the same etch as the first sacrificial
layer 82.
[0090] In FIG. 10D, tapered apertures 86 are formed which extend
through both the second sacrificial layer 196 and the first
sacrificial layer 82. It can also be seen in FIG. 35D that an
aperture 208 is formed in a portion of the second sacrificial layer
196 overlying the patterned mirror layer 200, exposing at least a
portion of the patterned minor layer 200.
[0091] In FIG. 10E, a mechanical layer 92 is deposited over the
patterned sacrificial layers 196 and 82 and exposed portions of the
patterned minor layer 200. In particular, it can be seen that the
mechanical layer 92 at least partially fills the aperture 208 (see
FIG. 10D), such that a connector portion 202 connecting the
mechanical layer 92 and the patterned mirror layer 200 is
formed.
[0092] In FIG. 10F, a release etch is performed which removes both
the first sacrificial layer 82 and the second sacrificial layer
196, thereby forming an interferometric gap 19 between the
patterned minor layer 200 and the optical stack. Thus, an optical
MEMS device is formed, which includes a movable layer 66 comprising
a mechanical layer 92 from which a patterned mirror layer 200 is
suspended, where the patterned mirror layer 200 is partially
detached from the mechanical layer 92. This optical MEMS device,
may be, for example, an interferometric modulator such as that
described with respect to FIG. 7C and elsewhere throughout the
application. In non-optical MEMS, the suspended upper electrode
need not be reflective.
[0093] In the embodiment of FIG. 10F, it is undesirable for the
patterned mirror layer 200 to have a re-entrant edge surface, as
undercutting the mirror layer may lead to undesirable optical
effects if incident light is reflected off of the re-entrant edge
surface and reflected towards the viewer. Desirably, the patterned
mirror layer 200 comprises a flat lower surface, such that any
light reflected by the patterned mirror layer 200 is reflected in a
uniform direction. One method for ensuring that the patterned
mirror layer 200 comprises a substantially flat surface without
significant undercut, is to etch the mirror layer such that the
patterned mirror layer comprises a tapered edge. In addition, if
the patterned mirror layer 200 comprises a tapered edge, the second
sacrificial layer and other overlying layer s9e.g., the mechanical
layer) can be more reliably deposited conformally over the
patterned mirror layer 200, with less risk of thickness
nonuniformity and stresses created at sharper 90 degree
corners.
[0094] Thus, it is often desirable to form tapered edges in a
variety of locations throughout a MEMS device, including when
overlying layers are to be deposited conformally over the patterned
layer, or when it is desirable to avoid a re-entrant profile.
Several methods are disclosed below for the formation of such
tapered edges. While they are described primarily with respect to
the post embodiment discussed above with respect to FIGS. 9A-9J,
other uses of the structures and methods are contemplated (such as
in the patterning of the mirror layer 200 of FIGS. 10A-10F) and can
be achieved through modification of the below structures and
methods. In certain embodiments, these processes include patterning
a layer to form a desired structure and tapering the edge of that
structure. Although sometimes described as two distinct steps, it
will be understood that the patterning and tapering will often
comprise a single step, and that in further embodiments, partial
tapering will occur while the structure is being patterned, such
that the patterning and tapering need may be done
simultaneously.
[0095] In one embodiment, described with respect to FIGS. 11A-11B,
an etch leading layer is used to achieve an etch having the desired
taper. In FIG. 11A, it can be seen that a layer of inorganic post
material 84 is deposited over a sacrificial layer 82. An etch
leading layer 270 is deposited over the post layer 84, and a mask
272 is formed over the area where the inorganic post will be. In
this embodiment, the etch leading layer is selected to be a
material which, for particular etches, will have an etch rate which
is faster than the etch rate of the underlying material, or the
post material 84 in the illustrated embodiment. In certain
embodiments, the post layer 84 may comprise SiO.sub.2 or SiN.sub.x.
The etch leading layer may comprise a wide variety of materials,
including an SiO.sub.2 or SiN.sub.x layer which are deposited at a
low temperature; are significantly more porous than the underlying
layer, are deposited with a different composition of nitrogen in
the SiN.sub.x layer; or are formed to be particularly oxygen or
nitrogen rich. Similarly, a hydrogen rich SiN.sub.x layer may be
used, or a layer of SiN.sub.x deposited at low temperatures. Si is
etched faster than SiO.sub.2 when a HF/HNO.sub.3 etch is used, and
Al will etch faster than SiO.sub.2 when an HF+H.sub.3PO.sub.4 etch
is used. Also, Mo will etch faster than SiO.sub.2 when either a
HF/HNO.sub.3 or an HF/HNO.sub.3/H.sub.3PO.sub.4 etch is used. BHF
is commonly used to etch SiN.sub.x, although other suitable
etchants may be used. A wide variety of other etches and etch
leading layers may be used depending on the composition of the
inorganic post layer 84. Depending on the etchant to be used, the
mask 272 may be formed of, for example, a hard mask such as Ni, or
a photoresist mask. In particular, when a wet etch is used, the
mask 272 may comprise a hard mask.
[0096] FIG. 11B depicts the embodiment of FIG. 11A after being
exposed to an etchant, preferably an isotropic etchant. As can be
seen, the etch leading layer 270 has been etched faster than the
post layer 84, exposing both the top and the side of the post layer
84 to the etchant. Because the post layer 84 is etched from the top
and from the side, a tapered side has been formed by this etch.
Subsequently, one or both of the mask 272 and the remaining etch
leading layer 270 may be removed (not shown).
[0097] The structure and method of FIGS. 11A-11B may be adapted for
use in forming a patterned mirror layer having a tapered edge. In
one embodiment, the inorganic post layer 84 of FIGS. 11A-11B may be
replaced by a layer of aluminum, and the etch leading layer 270 may
be, for example, a layer of aluminum deposited such that it will
etch faster. This can be achieved, for instance, though deposition
of the etch leading layer at a higher pressure than the deposition
of the lower aluminum layer. In an alternate embodiment, the etch
leading layer may comprise an aluminum alloy having a higher etch
rate than aluminum. In still other embodiments, any suitable
material having the desired mechanical properties and a higher etch
rate than aluminum may be used.
[0098] A variation of the above embodiment is discussed with
respect to FIGS. 12A-12B, and includes the step of FIG. 11A. In
this embodiment, the etch leading layer 270 and the underlying
layer (in the illustrated embodiment, inorganic post material 84)
are selectively etchable relative to one another. In FIG. 12A, it
can be seen that the etch leading layer 270 has been selectively
etched relative to the post layer 84 via a first (preferably
isotropic) etch, exposing part of the top of the post layer 84. In
FIG. 12B, a second (preferably isotropic) etch has been used to
etch the post layer 84 both from above and from the side, resulting
in the tapered edge seen in the figure. The angle of the taper can
be controlled both by the extent to which the first etch is
permitted to remove the etch leading layer 270, and by the rate at
which the second etch diffuses into the space between the mask 272
and the post layer 84. If the second etch etches the post layer 84
such that it undercuts the etch leading layer 270, a third etch,
which may use the same etch as the first etch, may be performed in
order to remove the overhanging portion of the etch leading layer
270, such that a tapered edge is formed without undesirable
overhanging portions.
[0099] The structure and method of FIGS. 12A-12B may also be
adapted for use in forming a patterned mirror layer having a
tapered edge. In one embodiment, the inorganic post layer 84 of
FIGS. 12A-12B may be replaced by a layer of aluminum, and the etch
leading layer 270 may comprise a layer of nickel, which can be
selectively etched relative to aluminum. The nickel etch leading
layer may first be etched, such as via a HNO.sub.3 wet etch, which
is selective with respect to the aluminum layer. The aluminum layer
may then be etched via an H.sub.3PO.sub.4 wet etch, which is
selective with respect to the nickel layer. If necessary, a
subsequent HNO.sub.3 etch is performed to remove any portions of
the nickel layer overhanging the tapered aluminum layer.
[0100] As discussed above, one or both of the etch leading layer
and the mask may be removed after etching the inorganic post layer.
In a further embodiment, the etch leading layer of any of the
embodiments discussed herein may be selected so that it can be
etched away or otherwise removed in a single process. In a
particular embodiment, the etch leading layer is a polymeric
material, such as PMGI or PMMA, and the mask is a photoresist mask.
The undercut can be achieved during the photolithography process
and it does not require an extra etch step. In this embodiment, a
single etch (different from the first etch) an remove both the PR
mask and the polymeric etch leading layer, simplifying the
fabrication process.
[0101] In another embodiment, the desired taper can be achieved
through the deposition of a post layer 84 which has differing
properties at different heights in the layer, such that the upper
portion of the layer 84 will be etched at a faster rate than the
lower portion of the layer 84, rather than employing a separate
etch leading layer. FIG. 13 depicts such an embodiment, in which
the properties of the layer 84 are varied during the formation
(e.g., deposition) of the layer, such that the layer 84 has been
etched into a tapered shape. Such varying properties can be
achieved in multiple ways. For example, during plasma chemical
vapor deposition, process conditions such as the gas, power and/or
pressure may be varied (graded through the thickness) in order to
make the composition of later-deposited material more quickly
etchable by a particular etch. This variation can also be achieved
while using a sputter etch process
[0102] Similarly, as can be seen in FIG. 14, a single layer 84 of
inorganic post material with varying properties can be approximated
through the deposition of multiple layers having slightly different
properties. In this embodiment, three separate layers 84a, 84b, and
84c are fabricated, such that layers 84b and 84c have an etch rate
faster than 84a, and 84c has an etch rate faster than 84b, upon
exposure to a particular etchant. In various embodiments, more or
less layers of varying or uniform thickness can be deposited to
form such a stratified post layer. As discussed above, various
properties of a CVD or sputter etch process can be modified between
the deposition of the various post layers in order to achieved the
desired relative etch rates for each layer. This process can also
be applied to the deposition of a mirror layer or any other
layer.
[0103] In another embodiment, as depicted in FIG. 15, a mask layer
272 can be deposited over the post layer 84 such that the adhesion
between the mask 272 and the post layer 84 is intentionally poor.
During etching of the post layer 84, the mask 272 will pull away
from the post layer 84, permitting the post layer 84 to be etched
from above, as well as from the side, creating the desired tapered
shape. Preferably, the etch used to etch the post layer 84 is a wet
etch or an isotropic dry etch. The resist adhesion can be modified
during the fabrication process by, for example, lowering the bake
temperature of the mask 272, or chemically treating the upper
surface of the post layer 84 to reduce adhesion with the resist 272
or other mask material. This process can also be applied to the
etching of a mirror layer or any other layer.
[0104] In yet another embodiment, a tapered edge can be
approximated through the deposition of a photoresist mask over the
inorganic post layer and the use of successive etching and ashing
processes to gradually remove portions of that photoresist mask,
forming a staircase-like structure. FIGS. 16A-16D depict a method
of forming such a staircase pattern at the edge of an inorganic
structure such as the post of FIG. 9J. In FIG. 16A, it can be seen
that an etch barrier layer 280 has been formed between a layer 84
of post material and the sacrificial layer 82. A mask layer 282 is
then deposited over the post layer 84.
[0105] In FIG. 16B, an etch is performed to remove a portion of the
exposed post layer 84. As shown in the illustrated embodiment, this
etch may only remove a portion of the exposed post layer 84, but in
other embodiments, the first etch may etch the entire exposed
portion of the post layer, stopping on the etch stop layer 280.
[0106] In FIG. 16C, a portion of the mask 282 has been removed,
such as by an ashing process, and another etch is performed to
remove another newly exposed portion of the post layer 84. This
process is then repeated until the post layer 84 has been etched
all the way down to the etch barrier layer 282 in certain places,
as can be seen in FIG. 36D. Thus, inorganic posts 290 may be
fabricated, having edges with a staircase-like profile that
approximates a tapered edge. While any appropriate number of
successive etching and ashing (or other mask reduction or
shrinkage) steps can be used, in one embodiment, three such
iterations provide an acceptable approximation of a tapered
edge.
[0107] This iterative etching and ashing (or other mask reduction
or shrinkage) process can also be applied to the etching of a
mirror layer. On one embodiment, the mirror layer is masked, and a
first etch, which may be a H.sub.3PO.sub.4 etch, is used to etch a
portion of the exposed mirror layer. The mask is then partially
ached, exposing a previously unetched portion of the mirror layer,
and the mirror layer is then etched via a second etch, which in
certain embodiments may be the same as the first etch, and in other
embodiments may be a different etch, such as a TMAH etch, which may
be more selective with respect to the underlying sacrificial or
etch stop layer.
[0108] In another embodiment, an alternate iterative etching
process may be used to form a patterned mirror layer having a
tapered edge. In FIG. 17A, it can be seen that a reflective layer
90 has been deposited over the first sacrificial layer 82, and that
an etch leading layer 290 has been deposited over the reflective
layer 90, followed by a photoresist mask 292. In the illustrated
embodiment, the etch leading layer 290 is etchable by an etch
(e.g., fluorine-containing) which will also etch the sacrificial
layer 82, and in particular may comprise the same material as the
sacrificial layer, e.g., molybdenum.
[0109] In FIG. 17B, the reflective layer 90 has been partially
etched, such as through the use of a PAN etch, which will etch both
aluminum and molybdenum, although it can be seen that the
underlying sacrificial layer 82 has not been exposed by this etch.
In FIG. 17C, the remainder of the exposed reflective layer 90 has
been etched using an etch which is selective with respect to the
sacrificial material, such as an H.sub.3PO.sub.4 etch. By not
exposing the underlying sacrificial material to an etch which will
significantly etch the sacrificial material, undercut of the
reflective layer 90 can be avoided, and the desired tapered shape
obtained.
[0110] In FIG. 17D, the mask 292 is been stripped. In FIG. 17E, the
etch leading layer 290 has been removed by an etch which is
selective with respect to the reflective layer 90, such as an
SF.sub.6/O.sub.2 etch. Although a portion of the underlying
sacrificial material 92 may be removed by this etch, it will not
have an effect on the tapered shape of the patterned reflective
layer 90, as the etch is selective with respect to the reflective
layer 90, and will not etch the reflective layer 90 from
beneath.
[0111] It will be understood that although the above methods for
forming a tapered edge have been discussed primarily with respect
to shaping the edges of posts, these methods may be modified and
applied to forming tapered edges of other layers in the disclosed
MEMS devices, such as an electrode layer or a sacrificial layer,
particularly when opening tapered apertures in support regions. In
addition, the processes disclosed and described herein may be
modified to incorporate these steps whenever the processes could
benefit from the etching of a layer such that it comprises tapered
edges. These processes may also be utilized in conjunction with
other methods of forming MEMS devices, such as the disclosed
interferometric modulators.
[0112] In conjunction with, or in place of, the forming of tapered
edges on layers, the deposition of overlying layers can also be
facilitated by several alternate methods of improving step coverage
at edges of layers. In certain embodiments, the sputter deposition
may be biased, so that some or all of the material being deposited
is deposited at an angle, so as to provide better coverage at
corners, reducing the need for tapering the steps. In another
embodiment, the substrate on which these layers are being deposited
may be tilted at an angle to the target structure, and in further
embodiments may be rotated during deposition. In another
embodiment, the profile of the target structure used in sputter
deposition may be optimized so that certain portions are at desired
angles to the substrate onto which the material sputtered. In yet
another embodiment, substrate on which the layer is being deposited
is rocked back and forth.
[0113] Sputter etching may also be advantageously be utilized in
the processes described herein. In addition to roughening and
cleaning a surface on which another layer is to be deposited, as
discussed above, sputter etching may be used in conjunction with
the sputter deposition. In one embodiment, a thicker layer than
necessary is deposited, and then sputter etched to the desired
thickness, providing a smoother layer (e.g., by rounding corners).
In a further embodiment, sputter etching and deposition can be
alternated. A biased sputter may also be used to improve step
coverage on conductive substrates, conductive layers, or substrates
on conductive supports (e.g., chucks). Other deposition processes
may also be used, such as atomic layer deposition (ALD) atmospheric
pressure chemical vapor deposition (AP-CVD), low pressure chemical
vapor deposition (LP-CVP), high-density plasma chemical vapor
deposition (HDP-CVD), and in further embodiments these deposition
processes may be modified to include biasing. Any of these methods
may be used in conjunction with any of the processes described
herein to improve step coverage. Furthermore, in certain
embodiments, the use of these methods may reduce or eliminate the
need to form tapered edges on layers, or augment the advantages of
tapered edges.
[0114] In other embodiments, lift-off processes can be utilized to
form the desired layers during fabrication of the interferometric
modulator. In one embodiment, described with respect to FIGS.
18A-18C, a lift-off process can be utilized to form a desired
support structure adjacent a movable layer, and in particular a
support post structure underlying the movable layer.
Advantageously, this lift-off process permits the deposition of
layers, such as inorganic posts, having the desired taper,
facilitating the deposition of overlying layers. In addition, this
process may eliminate the need to selectively etch the post
material relative to the underlying material. In FIG. 18A, it can
be seen that a layer 82 of sacrificial material has been patterned
and etched to form tapered apertures 86, and that a mask 222 having
a negative angle at the edge is deposited over the sacrificial
layer 82, such that the mask 222 does not extend to the edge of the
aperture 86. In certain embodiments, this mask 222 comprises
photoresist material (and in a particular embodiment may be a
bilayer photoresist, to facilitate formation of the negative
angle), but may also be a hard mask. In a particular embodiment,
the mask 222 may comprise the photoresist used to pattern the
sacrificial layer 82.
[0115] In FIG. 18B, it can be seen that a layer of post material 84
is deposited over the mask 222. The negative angle of the mask 222
results in the deposition of post material 84 in the shape of the
desired inorganic post 188, having the tapered edges due to shadow
effects on the deposition. Depending on the thickness of the mask
222 and the degree of the negative angle, the shape of the mask 222
also advantageously results in a gap between the post material 84
above the mask 222, and the post material 84 which will form the
inorganic post. In this embodiment, the mask 222 is advantageously
thicker than the materials to be deposited, permitting access to
the mask 222 by etchant or ashing chemicals. Preferably, the post
material is deposited via physical vapor deposition (e.g., an
evaporation or sputtering process), so as to avoid deposition of
material on the underside of the mask 222 which would inhibit the
liftoff of the undesired portion of material overlying the mask
222. However, if such connections are fanned, for example if a CVD
process is used to deposit the post layer 84, ultrasonic energy can
be used to break any connecting portions, as they will likely be
thin in the shadowed region under the mask overhang relative to the
thickness of the post layer 84 generally.
[0116] In FIG. 18C, it can be seen that a liftoff process is used
to remove the mask 222, removing the post material 84 overlying the
mask 222 at the same time. This liftoff process may comprise an
etch to remove the mask 222, such as a wet etch, or a gaseous or
vaporous etch. The liftoff process may also comprise a wrinkle bake
and rinsing process to remove those materials which are no longer
attached. The fabrication process further comprises the subsequent
deposition of a movable layer, patterning, and releasing the MEMS
device, as discussed above.
[0117] A similar liftoff process may also be utilized to form rivet
structures overlying a movable layer. In particular, the process of
FIGS. 18A-18C may be modified by depositing a movable layer (such
as the movable layer 66) over the patterned sacrificial layer prior
to depositing the mask. A layer of the desired rivet material is
then deposited over the mask, and the mask is then removed, lifting
off the excess rivet layer, forming a rivet. Advantageously, as
discussed above, this may eliminate the need to selectively etch
the rivet layer relative to an underlying mechanical layer. Thus, a
rivet may be formed from the same material as the mechanical layer
without the need for an etch barrier layer between the two
layers.
[0118] In one embodiment, the mask 222 having a negative angle can
be formed through the use of a photoresist mask in conjunction with
an underlying layer of liftoff material. FIGS. 19A-19C depict a
process for forming such a mask. In FIG. 19A, it can be seen that a
layer of liftoff material 224 has been deposited over an underlying
layer 226 (which may be, for example a sacrificial layer), and a
layer of photoresist material 228 has been deposited over the
liftoff layer 224. In FIG. 19B, the photoresist material 228 has
been patterned and selectively removed to form an aperture 229,
which will permit etching of the underlying liftoff layer 224.
[0119] In FIG. 19C, it can be seen that the liftoff layer 224 has
been selectively etched with some degree of isotropy to laterally
recess under the photoresist 228 and to form a cavity 230, with
edges having a negative angle or overhang, as discussed above. This
negative angle can be achieved, for example, by selectively
overetching the liftoff layer 224. In one embodiment, the liftoff
layer 224 comprises a polyimide release layer. In one embodiment,
the liftoff layer 224 may itself be photopatternable. In one
embodiment, the liftoff layer 224 may be bilayer photoresist with
undercut formed by development. In one embodiment, as previously
depicted in FIG. 18) the mask 222 may be formed from a single
liftoff layer, rather than from a liftoff layer in conjunction with
a photoresist mask.
[0120] A liftoff process may be utilized to form other desired
shapes during the fabrication of supports for a MEMS device, such
as the illustrated interferometric modulator. In another
embodiment, described with respect to FIG. 20, a liftoff process is
utilized to form tapered apertures in the sacrificial layer 82. In
this embodiment, a mask 222 is first formed over the optical stack
16, prior to deposition of the sacrificial layer 82. As can be
seen, this mask 222 comprises negative angles which control the
shape of the deposited sacrificial layer 82. Still with respect to
FIG. 20, it can be seen that a layer 82 of sacrificial material has
been deposited, such that the sacrificial material located on the
substrate will have a shape with the desired tapered apertures upon
liftoff of the mask 222 and the overlying sacrificial material.
[0121] In another embodiment, described with respect to FIG. 21, a
liftoff process is used to form isolated electrode members, which
in an embodiment comprising an optical MEMS may be mirrors. This
embodiment includes the steps of FIG. 9A-9D. In FIG. 21, it can be
seen that a mask 222 is deposited over the sacrificial layer 82,
the mask layer having 222 a re-entrant profile along the side of
the mask layer 222, and that a layer of reflective material 90 is
then deposited over the mask layer 222. In a later step, the mask
222 is removed by a liftoff process, along with the overlying
portions of the reflective layer 90, forming isolated electrode
members, which in this embodiment comprise mirrors. The fabrication
process may continue as discussed with respect to FIGS.
10C-10F.
[0122] It will be understood that various combinations of the above
embodiments are possible. Various other combinations of the methods
discussed above are contemplated and are within the scope of the
invention. In addition, it will be understood that structures
formed by any of the methods above may be utilized in combination
with other methods of forming structures within MEMS devices.
[0123] It will also be recognized that the order of layers and the
materials forming those layers in the above embodiments are merely
exemplary. Moreover, in some embodiments, other layers, not shown,
may be deposited and processed to form portions of an MEMS device
or to form other structures on the substrate. In other embodiments,
these layers may be formed using alternative deposition,
patterning, and etching materials and processes, may be deposited
in a different order, or composed of different materials, as would
be known to one of skill in the art.
[0124] It is also to be recognized that, depending on the
embodiment, the acts or events of any methods described herein can
be performed in other sequences, may be added, merged, or left out
altogether (e.g., not all acts or events are necessary for the
practice of the methods), unless the text specifically and clearly
states otherwise.
[0125] While the above detailed description has shown, described,
and pointed out novel features of the invention as applied to
various embodiments, it will be understood that various omissions,
substitutions, and changes in the form and details of the device of
process illustrated may be made by those skilled in the art without
departing from the spirit of the invention. As will be recognized,
the present invention may be embodied within a form that does not
provide all of the features and benefits set forth herein, as some
features may be used or practiced separately from others.
* * * * *