U.S. patent application number 12/360083 was filed with the patent office on 2009-07-30 for performance analyses of micromirror devices.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Jim Dunphy, Leonid Frenkel, Regis Grasser, Andrew Huibers, Satyadev Patel, Peter Richards, Greg Schaadt, Igor Volfman.
Application Number | 20090190825 12/360083 |
Document ID | / |
Family ID | 35505307 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090190825 |
Kind Code |
A1 |
Volfman; Igor ; et
al. |
July 30, 2009 |
Performance Analyses of Micromirror Devices
Abstract
The invention provides a method and apparatus for evaluating the
product quality and performances of micromirror array devices
through measurements of the electromechanical responses of the
individual micromirrors to the driving forces of electric fields.
The electromechanical responses of the micromirrors according to
the present invention are described in terms of the rotational
angles associated with the operational states, such as the ON and
OFF state angles of the ON and OFF state when the micromirror array
device is operated in the binary-state mode, and the response speed
(i.e. the time interval required for a micromirror device to
transit form one state to another) of the individual micromirrors
to the driving fields.
Inventors: |
Volfman; Igor; (Sunnyvale,
CA) ; Huibers; Andrew; (Sunnyvale, CA) ;
Patel; Satyadev; (Sunnyvale, CA) ; Richards;
Peter; (San Francisco, CA) ; Frenkel; Leonid;
(Palo Alto, CA) ; Dunphy; Jim; (San Jose, CA)
; Grasser; Regis; (Orleans, FR) ; Schaadt;
Greg; (Santa Clara, CA) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
Texas Instruments
Incorporated
Dallas
TX
|
Family ID: |
35505307 |
Appl. No.: |
12/360083 |
Filed: |
January 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10875760 |
Jun 23, 2004 |
7483126 |
|
|
12360083 |
|
|
|
|
Current U.S.
Class: |
382/141 ;
356/218 |
Current CPC
Class: |
G01N 21/55 20130101;
G01N 21/95 20130101 |
Class at
Publication: |
382/141 ;
356/218 |
International
Class: |
G06K 9/00 20060101
G06K009/00; G01J 1/42 20060101 G01J001/42 |
Claims
1. A method of evaluating a quality of a micromirror array device
having an array of micromirrors, each micromirror having a
deflectable reflective mirror plate, the method comprising: (a)
dynamically measuring a mechanical response of a mirror plate of a
micromirror in the micromirror array in response to an altering
driving force; (b) determining an instant driving force of the
altering driving force under which the mirror plate is at a desired
state of the measured mechanical response; (c) repeating steps (a)
and (b) for a number of other micromirrors in the micromirror
array; and (d) evaluating the quality of the micromirror array
device based on the determined instant driving forces.
2. The method of claim 1, wherein the step of dynamically measuring
the mechanical response further comprises: applying an altering
voltage to the mirror plate; and dynamically measuring a sequence
of rotation positions of the mirror plate in response to the
altering voltage.
3. The method of claim 2, wherein step of determining the instant
driving force further comprises: determining an instant voltage of
the altering voltage under which the mirror plate is at an ON
state.
4. The method of claim 3, wherein the ON state is a state wherein
the mirror plate is rotated 10.degree. degrees or more relative to
a substrate on which the mirror plate is formed.
5. The method of claim 4, wherein the ON state is a state wherein
the mirror plate is rotated 12.degree. degrees or more relative to
the substrate.
6. The method of claim 5, wherein the ON state is a state wherein
the mirror plate is rotated 14.degree. degrees or more relative to
the substrate.
7. The method of claim 2, wherein the altering voltage comprises an
upward sweeping edge and a downward sweeping edge, wherein the
mirror plate rotates towards an ON state during the upwards
sweeping edge, and rotates towards an OFF state during downwards
sweeping edge.
8. The method of claim 7, wherein the OFF state is a state wherein
the mirror plate is parallel to a substrate on which the mirror
plate is formed.
9. The method of claim 2, wherein the altering voltage comprises a
sequence of voltage cycles, each of which comprises a first and
second voltage peak with the first voltage peak having an amplitude
higher than a voltage required for rotating the mirror plate to an
ON state, and the second voltage peak having an amplitude that
varies over time.
10. The method of claim 9, wherein the first voltage peak of a
voltage cycle is applied to the mirror plate immediately prior to
the application of the second voltage peak of the same cycle.
11. The method of claim 9, wherein the second voltage peak of a
voltage cycle is applied to the mirror plate immediately prior to
the application of the first voltage peak of the same cycle.
12. The method of claim 2, wherein the altering voltage comprises a
sequence of voltages, the amplitude of each of which varies over
time.
13. The method of claim 2, wherein the altering voltage comprises a
set of voltage sequences, each voltage sequence having a sequence
of voltage pulses, wherein a time interval between two consecutive
voltage pulses varies over time.
14. The method of claim 2, further comprising: directing a light
beam to the mirror plates of the micromirrors in the micromirror
array; taking an image of the illuminated micromirrors; processing
the image so as to obtain an edge of each illuminated micromirror;
and selecting a micromirror to be measured.
15. The method of claim 1, further comprising: loading the
micromirror array device into a vacuum chamber of a measurement
system; and pumping out the vacuum chamber to a particular
pressure.
16. The method of claim 15, wherein the particular pressure is 1
atmosphere or less.
17. The method of claim 15, wherein the particular pressure is 20
Torr or less.
18. The method of claim 15, wherein the particular pressure is 50
mTorr or less.
19. The method of claim 14, wherein step of dynamically measuring
the mechanical response further comprises: dynamically measuring an
intensity of a light beam reflected from the mirror plate being
inspected; and from the detected illumination intensity,
determining whether the mirror plate reaches to the desired
state.
20. The method of claim 14, wherein the step of processing the
image further comprises: transforming the image into the Fourier
space; locating a plurality of peaks in the image; approximating a
horizontal and vertical pitches of the mirror plate according to
the located peaks; performing the Fourier transformation to the
images; and calculating a geometric center of each mirror plate in
the micromirror array.
21. The method of claim 20, wherein the step of approximating the
horizontal and vertical pitches further comprises: approximating
the horizontal and vertical pitches using the Siebel operator.
22. The method of claim 21, further comprising: filtering a noise
of the image before transforming the image into the Fourier
space.
23. The method of claim 21, further comprising: filtering a noise
of the image immediately after the transformation of the image into
the Fourier space.
24. The method of claim 13, further comprising: measuring a
response time for each micromirror in the micromirror array during
which the illumination intensity changes between a minimum value to
a maximum value in response to the altering voltage; and evaluating
the quality of the micromirrors based upon the measured response
time for all micromirrors of the micromirror array.
25. The method of claim 24, wherein the step of evaluating the
quality further comprises: calculating a distribution of the
response time of the micromirrors; and evaluating the quality based
upon the calculated distribution.
26. The method of claim 3, further comprising: calculating a
distribution of the measured instant voltages for the micromirrors,
wherein the instant voltage of a micromirror is the ON state
voltage; and evaluating the quality of the micromirrors based upon
the calculated distribution.
27. The method of claim 1, wherein the step of evaluating the
quality further comprises: passing the micromirror array device if
the determined instant driving force is within a predetermined
range; and failing the micromirror array device if the determined
instant driving force is beyond the predetermined range.
28. The method of claim 2, wherein the altering voltage has a
maximum value of from 10 to 70 volts.
29. The method of claim 2, wherein the altering voltage has a
maximum value of from 25 to 45 volts.
30. The method of claim 2, wherein the altering voltage has a
maximum value of from 70 to 100 volts.
31. The method of claim 14, wherein the step of taking an image of
the illuminated micromirrors further comprises: capturing a
reflected light from the illuminated micromirrors in a propagation
path of the reflected light, wherein said propagation path of the
reflected light has an angle with a propagation path of the
illumination light incident onto the micromirrors; and wherein the
angle is 10.degree. degrees or more.
32. The method of claim 31, wherein the angle is 14.degree. degrees
or more.
33. The method of claim 31, wherein the angle is 16.degree. degrees
or more.
34. The method of claim 31, wherein the angle is 18.degree. degrees
or more.
35. The method of claim 31, wherein the angle is 20.degree. degrees
or more.
36. The method of claim 31, wherein the angle is 22.degree. degrees
or more.
37-70. (canceled)
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This application is a divisional of application Ser. No.
10/875,760, filed Jun. 23, 2004.
[0002] The present invention relates to the field of
microelectromechanical devices, and more particularly to methods
and apparatus of performance evaluations through measurements of
electromechanical responses of the micromirror devices to driving
forces.
BACKGROUND OF THE INVENTION
[0003] Microelectromechanical (MEMS) devices have found many
applications in basic signal transductions. For example, MEMS-based
spatial light modulators are transducers that modulate incident
light in a spatial pattern in response to optical or electrical
inputs. The incident light may be modulated in phase, intensity,
polarization, or direction. This modulation may be accomplished
through the use of a variety of materials exhibiting magneto-optic,
electro-optic, or elastic properties. Such spatial light modulators
have many applications, including optical information processing,
display systems, and electrostatic printing.
[0004] A micromirror-based spatial light modulator is a spatial
light modulator consists of an array of micromirrors. The mirror
plates are individually addressable and deflectable with
electrostatic fields so as to modulate incident light. A typical
micromirror device comprises a deformable reflective mirror plate
held by a deformable hinge such that the mirror plate can rotate to
different positions in response to driving forces, such as
electrostatic field. According to the different rotation positions,
operation states, such as ON and OFF states in a binary operation
mode are defined. In the ON state, incident light is reflected so
as to produce a "bright" pixel on a display target, and in the OFF
state, incident light is reflected to produce a "dark" pixel on the
display target. In an application of displaying an image
represented by image pixels having "bright" and "dark" values, the
micromirrors are associated with the image pixels, and the
micromirrors are individually set to the ON or OFF states according
to the "bright" or "dark" values of the image pixels associated
with micromirrors. The collective effect of the reflection from the
micromirrors at the ON and OFF states for a given incident light is
reproduction of the image on the display target. The same operation
mechanism is applied to display applications for color images and
videos. The color image display is often performed with a color
wheel that generates the primary colors or the like. Video display
applications are often performed with a sequential color field
technique which requires the micromirrors be rotated rapidly and
frequently between the ON and OFF state so as to reflect the
appropriate "brightness" variation of the image pixels. In either
application of image and video display applications, robust
electromechanical responses to the driving forces and uniform ON
and OFF states of the micromirrors are determinative factors for
the evaluations of the product performance and quality.
[0005] Therefore, what is desired is a method and apparatus for
measuring electromechanical responses of micromirror devices.
SUMMARY OF THE INVENTION
[0006] The objects and advantages of the present invention will be
obvious, and in part appear hereafter and are accomplished by the
present invention that provides a method and apparatus for
operating pixels of spatial light modulators in display systems.
Such objects of the invention are achieved in the features of the
independent claims attached hereto. Preferred embodiments are
characterized in the dependent claims. In the claims, only elements
denoted by the words "means for" are intended to be interpreted as
means plus function claims under 35 U.S.C. .sctn. 112, the sixth
paragraph.
BRIEF DESCRIPTION OF DRAWINGS
[0007] While the appended claims set forth the features of the
present invention with particularity, the invention, together with
its objects and advantages, may be best understood from the
following detailed description taken in conjunction with the
accompanying drawings of which:
[0008] FIG. 1 is a perspective view of a portion of a micromirror
array device in which embodiments of the invention can be
implemented;
[0009] FIG. 2 is a cross-sectional view of a portion of the spatial
light modulator in FIG. 1 with the mirror plates thereof at
different rotation positions;
[0010] FIG. 3 demonstratively illustrates an exemplary image of a
portion of the micromirrors in FIG. 1;
[0011] FIG. 4 schematically illustrates a typical electromechanical
response curve of a micromirror device in FIG. 1;
[0012] FIG. 5 schematically illustrates an experimental setup for
measuring the electromechanical responses of the micromirror array
device in FIG. 1 according to an embodiment of the invention;
[0013] FIG. 6 is a flow chart showing the steps executed for
measuring the electromechanical responses of the micromirrors in
FIG. 1 according to the embodiment of the invention;
[0014] FIG. 7a is a flow chart showing the steps executed for
loading the micromirror array device into the experimental
setup;
[0015] FIG. 7b is a flow chart showing the steps executed for
setting the measurement parameters;
[0016] FIG. 7C is a flow chart showing the steps executed for
detecting the geometric centers of the individual micromirrors of
the micromirror array device;
[0017] FIG. 8 demonstratively illustrates a voltage profile of a
voltage scanning scheme for use in measuring the electromechanical
responses of the micromirrors according to an embodiment of the
invention;
[0018] FIG. 9 demonstratively illustrates a voltage scanning scheme
having a set of voltage sequences for use in measuring the
electromechanical responses of the micromirrors according to
another embodiment of the invention;
[0019] FIG. 10 demonstratively illustrates another voltage scanning
scheme having a set of voltage sequences for use in measuring the
electromechanical responses of the micromirrors according to yet
another embodiment of the invention;
[0020] FIG. 11 demonstratively illustrates yet another voltage
scanning scheme having a set of voltage sequences for use in
measuring the electromechanical responses of the micromirrors
according to yet another embodiment of the invention;
[0021] FIG. 12 schematically illustrates a simplified computing
system for use in performing the methods of the invention.
[0022] FIG. 13 demonstratively illustrates an user-interface used
for controlling the vacuum of the experimental setup;
[0023] FIG. 14 demonstratively illustrates an user-interface used
for aligning the micromirrors of the micromirror array device to
the experimental setup;
[0024] FIG. 15 demonstratively illustrates a user-interface used
for measuring the electromechanical responses of the micromirrors
with selected voltage scanning schemes according to the invention;
and
[0025] FIG. 16A is a top view of another micromirror array device
in which the embodiment of the invention can be implemented;
and
[0026] FIG. 16B is a perspective view of yet another micromirror
array device in which the embodiment of the invention can be
implemented.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0027] The invention provides a method and apparatus for evaluating
the product quality and performances of micromirror array devices
through measurements of the electromechanical responses of the
individual micromirrors to the driving forces of electric fields.
The electromechanical responses of the micromirrors according to
the present invention are described in terms of the rotational
angles associated with the operational states, such as the ON and
OFF state angles of the ON and OFF state when the micromirror array
device is operated in the binary-state mode, and the response speed
(i.e. the time interval required for a micromirror device to
transit form one state to another) of the individual micromirrors
to the driving fields.
[0028] Specifically, a driving force is applied to the mirror plate
of a micromirror being tested. In response, the mirror plate is
deflected to different rotational angles determined by the
amplitude and polarity of driving forces and the intrinsic
mechanical and electrical properties of the micromirror being
tested. The deflection of the mirror plate is monitored in a
real-time fashion through the measurement of the intensities of the
reflected light from the deflected mirror plate because the
intensities of the reflected light are determined by the deflected
positions of the individual mirror plates. And the dynamic
variations of the intensities over time carry the information on
the response speed of the mirror plate to the applied driving
force. Therefore, from the intensities and the variation of the
intensities of the reflected light, the electromechanical response
of the micromirror to the driving force can be extracted. The same
measurement can be conducted for all micromirrors of the
micromirror array device, from which the electromechanical
responses, such as the ON and OFF state angles and the response
speed of all micromirrors of the micromirror array device can be
obtained. Based on the extracted parameters, as well as
predetermined criteria, the quality and performance of the
microstructure device can be evaluated. For example, if all
micromirrors of the micromirror array device have substantially the
same ON and OFF state angle and substantially the same response
speed, or the ON and OFF state angles and the response speed
thereof have variations within respective predefined ranges, the
micromirror array device may be acceptable as a "good" product.
Otherwise, the micromirror array device is not acceptable and is
marked as a "bad" product.
[0029] The measured electromechanical responses, in turn can be
used as bases for optimizing the driving forces in practical
operations of the microstructure devices. For example, the
measurement results can be used to determine the optimum amplitudes
and/or the profiles of the driving voltages for the micromirror
array device in practical operations.
[0030] The corresponding experimental setup for measuring the
electromechanical responses of the micromirror array device
comprises an illumination system providing collimated light for
illuminating the mirror plates of the micromirrors, an image
capture device (e.g. a CCD device) for detecting intensities of the
reflected light from the mirror plates of the micromirrors, and a
set of optical elements for directing the light. A computing device
having capacities of data process and control of other functional
components of the experimental setup can also be provided for
facilitating automated measurements in accordance with the methods
of the invention. In particular, a plurality of program modules are
constructed to perform the operations of, image analyses for
determining the centers of the mirror plates, accepting parameters
from the user for instructing controlling the applications of the
driving forces to the mirror plates, analyzing the intensities of
the reflected light from the mirror plates so as to extracting
electromechanical response information of the individual
micromirrors, and generating plots as appropriate. These program
modules can be stored in and executed by the computer.
[0031] The measurement of the electromechanical responses of the
micromirrors is preferably performed under a pressure lower than 1
atmosphere, such as around 20 Torr or less, or around 50 mTorr or
less, or 15 mTorr or less.
[0032] In addition to micromirror devices, the present invention is
applicable to other type of microelectromechanical devices having
deflectable reflective planar members. For simplicity and
demonstration purposes only, the present invention will be
discussed with reference to a micromirror array device, such as a
spatial light modulator having an array of micromirrors, each of
which has a deflectable reflective mirror plate. Those skilled in
the art will certainly appreciate that the following examples are
not be interpreted as a limitation. Rather, other variations within
the spirit of the invention are also applicable.
[0033] Turning to the drawings, FIG. 1 illustrates a perspective
view of a portion of a micromirror array device 100. The
micromirror array device comprises micromirror array 106 and
electrode and circuitry array 108. Each micromirror has a mirror
plate that is held by a hinge (e.g. a torsion hinge) such that the
mirror plate can rotate along a rotation axis. According to an
embodiment of the invention, the mirror plate of each micromirror
can rotate from a natural resting state (e.g. parallel to the
substrate) to 8.degree. degrees or more, or 10.degree. degrees or
more, or 12.degree. or more, or 14.degree. degrees or more.
[0034] In this particular example, the micromirrors and electrodes
and circuitry are formed on separate substrates. Specifically, the
micromirrors are formed on substrate 102 that is light
transmissive, such as glass, while the electrodes and circuitry are
formed on substrate 104 that is a standard semiconductor wafer. The
semiconductor wafer having the electrodes and circuitry is places
proximate to the glass substrate having the micromirrors such that
the mirror plate can be rotated by an electrostatic force
established between the mirror plate and the electrode. Instead of
on separate substrates, the micromirrors and the electrodes and
circuitry can be formed on the same substrate, such as a
semiconductor wafer. In another embodiment of the invention, the
micromirror substrate can be formed on a transfer substrate that is
light transmissive. Specifically, the micromirror plate can be
formed on the transfer substrate and then the micromirror substrate
along with the transfer substrate is attached to another substrate
such as a light transmissive substrate followed by removal of the
transfer substrate and patterning of the micromirror substrate to
form the micromirror.
[0035] The micromirrors operate in binary-mode, that is, the mirror
plates of the micromirrors switch between an ON and OFF state in
performing the light modulation. In the ON state, the mirror plate
of the micromirror reflects incident light so as to generate a
"bright" pixel on a display target; and in the OFF state, the
mirror plate reflects the incident light so as to generate a "dark"
pixel on the display target. In a number of embodiments of the
invention, the micromirror array is constructed having a pitch (the
center-to-center distance between adjacent micromirrors) of 25
micrometers or less, or 10.16 micrometers or less, or from 4.38 to
10.16 micrometers. The gap between adjacent micromirrors is
approximately of 0.5 micrometers or less, or from 0.1 to 0.5
micrometer. And the mirror plate of the micromirror has a dimension
of from 20 micrometers to 10 micrometers.
[0036] For simplicity purposes, only 4.times.4 micromirrors are
illustrated in the figure. Oftentimes, the micromirror array device
has more micromirrors. For example, when the micromirror array
device is a portion of a spatial light modulator of a display
system, it may have millions of micromirrors, the number of which
determines the resolution of the display system. For example, the
spatial light modulator may have a resolution of 1024.times.768 or
higher, or 1280.times.1024 or higher, or 1640.times.1280 or higher.
Of course, the micromirror array device may have a fewer number of
micromirrors than in display, or other applications.
[0037] The operations of the individual micromirrors are determined
by the rotations of the individual mirror plates in response to the
applied electrostatic forces. Such responses can be described in
terms of the rotation angles of the mirror plates and the speeds of
the responses to the electrostatic forces. The mirror plate rotates
under an electrostatic force. For a given micromirror device, the
angle that the mirror plate can be rotated is determined by the
amplitude of the electrostatic field. When the micromirror is
operate in a binary-state including the ON and OFF state,
particular rotational angles are desired for the ON and OFF state.
Accordingly, the electrostatic forces for driving the mirror plate
to the ON and OFF state angles need to be determined. Moreover, the
time characteristic of the mirror plate in transition from one
state to another is also a critical factor, which determines the
quality of the displayed images, especially the video images.
[0038] In order to evaluate the product quality and performance of
the micromirror array device in terms of the electromechanical
responses to the electrostatic forces, the dynamic rotational
behaviors of the individual mirror plates in the presence of the
driving forces are measured through the measurements of the
intensities of the reflected light from the individual mirror
plates, and the variations of the intensities over time, which will
be discussed in detail in the following with reference to FIG.
2.
[0039] Referring to FIG. 2, the spatial light modulator in FIG. 1
is illustrated in a cross-sectional view. The deflectable
reflective mirror plates are held by deformable hinges that are
formed on glass substrate 102. Electrodes and circuitry (not shown)
are formed on the semiconductor substrate 104 for deflecting the
mirror plates. For example, the mirror plate of micromirror 110 can
be rotated in a spatial direction by an electrostatic force
established between the mirror plate and electrode 112 that is
associated with the mirror plate. The collimated incident light 118
is redirected into reflected light 120 after reflection. Depending
upon the incident angle of the light and the rotation angle of the
mirror plate, reflected light can be along the perpendicular
direction as shown in the figure. For a micromirror without
application of the driving force, such as micromirror 114 having no
electrostatic force established between the mirror plate thereof
and electrode 116, the mirror plate is at the natural resting
state, such as parallel to the substrate, as shown in the figure.
The collimated incident light 118 is thus reflected to reflected
light 122 that travels along a different direction from reflected
light 120. On image capture device 124, such as a CCD device that
is placed on top of the micromirrors, reflected light 120 generates
a "bright" pixel, while reflected light generates a "dark" pixel.
That is, the illumination intensity of the image generated by the
reflected light 120 is higher than that of reflected light 122. In
turn, the rotational positions of the mirror plates can be deducted
from the intensities of the corresponding image on the image
capture device.
[0040] As a simplified example, FIG. 3 schematically illustrates a
captured image of a set of micromirrors of the micromirror array
device at a time. Each image cell (e.g. cells 130, 128, 132, and
134) is an image of a mirror plate and is generated by the
reflected light from the mirror plate. When the micromirror is at
the ON state, the corresponding image cell is "bright"; while the
image cell is "dark" when the micromirror is at the OFF state. That
is, the darkness of the image cell reflects the rotational
positions of the mirror plate of the micromirror. For the
particular example in the figure, the cells with the shaded solid
circles have higher illumination intensities ("bright" image cells)
than those otherwise. Accordingly, the mirror plates corresponding
to the cells having the shaded solid circles are rotated to an
angle of the ON state, whereas the mirror plates corresponding to
the cells having no shaded circles stay at the OFF state.
Specifically, the mirror plates corresponding to the cells within
the matrix with the corners of 130, 128, 134, and 132 are in the ON
state, and the remaining mirror plates are in the OFF state. In
generating such an image, electrostatic fields are applied to the
mirror plates individually for rotating the mirror plate. Because
the ON and OFF rotational angles are determined by the amplitudes
of the electrostatic forces; and the ON and OFF state angles can be
monitored from the intensities of the image cells of the mirror
plates, the ON and OFF state voltages can thus be associated with
and deducted from the intensities of the image cells of the
captured image. The voltages associated with the ON and OFF states
are often referred to as the ON and OFF state voltages,
respectively. Therefore, the ON and OFF state voltages can be
measured from the measurements of the light intensities of the
corresponding image cells.
[0041] In addition to the "brightness" of the image cells of the
micromirrors, variation of the "brightness" of the image cells over
time carries the information of the response speed of the
micromirror to the electrostatic forces. For example, when a
driving force is applied to the mirror plate of a micromirror at
the OFF state, the mirror plate is rotated from the OFF state to
the ON state. Accordingly, the image cell of the mirror plate
changes from "dark" to "bright." Clearly, the speed of the mirror
plate in transiting from the OFF to the ON state is associated with
the time interval of the image cell changing from "dark" to
"bright."
[0042] With a given electrostatic force, the mirror plate has
certain response capability. FIG. 4 plots the rotation angles vs.
the voltage applied to the mirror plate of a typical micromirror
device. Both axes in the plot are in arbitrary units. During the
course of upwards voltage sweeping from zero (0) voltage, the
rotation angle increases with the increase of the voltage along
branch {circle around (1)}. At voltage V.sub.snap, the mirror plate
rotates to an angle of .sub.snap. Above V.sub.snap, the rotation
angle starts to saturate. In general, the ON state voltage is
defined as a small amount higher than the V.sub.snap, and the
corresponding rotation angle is defined as the ON state angle
.sub.ON. During the course of downwards voltage sweeping from
V.sub.ON, response hysteresis occurs. Specifically, the rotation
angle decreases with the decrease of the voltage but along branch
{circle around (2)}. Around V.sub.st, the rotation angle drops to a
value after which the rotation angle starts to decrease slowly. In
general, the OFF state voltage can be defined as a small amount
lower than voltage V.sub.st.
[0043] In order to determining the responses of the individual
micromirrors to driving electrostatic forces, an experiment setup
according to an embodiment of the invention is provided. Referring
to FIG. 5, an exemplary experimental setup for performing the
method of the present invention is schematically illustrated.
System 136 comprises light source 138 with attached fiber optic
cable 142 connected to microscope objective 144, neutral density
filter 146, diffuser 152, condenser lens 148, vacuum chamber 150 in
which micromirror device 100 is placed, sample holder 151, image
capture device 124, and computing device 154. The light source can
be DC bulbs (e.g. halogen light bulbs) or LED or other type of
light sources. The vacuum chunk may be connected to appropriate
vacuum instruments, such as pumps and valves. Sample holder 151 can
be constructed in any suitable forms. For example, the sample
holder can be a flat surface on which the sample device can be
attached and held. The sample holder can also have a supporting
surface on which the sample device to be measured can be attached
and hold. The supporting surface is preferably movable
3-dimentionally. For example, in addition to the ability of moving
in the X-Y plane, the supporting surface can be tilted at any
desired angles. In this way, the sample device attached to the
supporting surface can be rotated. In particular, the reflective
surface of the sample device can be tilted as desired--facilitating
adjustment of the propagation direction of the reflected light.
Moreover, the sample holder can be equipped with an automation
system, such as a motor, along with a position detector. In this
way, the movement of the supporting surface of the sample holder
can be automated, and the position of the supporting surface can be
determined precisely.
[0044] In accordance with an embodiment of the invention, the
components of the system in FIG. 5 are arranged according to the
configuration of the device to be measured. For example, when a
micromirror array device having an array of deflectable reflective
mirror plates is to be measured, the image capture device is
desired to be positioned in the propagation path of the reflected
light when the mirror plates are at a particular state. For
example, mirror plate can rotate to an ON state angle and an OFF
state angle, or not rotated (e.g. parallel to the substrate on
which the micromirror is formed). The image capture device can thus
be positioned in the propagation path of the reflected light from
the mirror plate at the ON state angle, the OFF state angle or
parallel to the substrate. When the image capture device is
positioned in the propagation path of the reflected light from the
mirror plate at the ON state, the detected illumination intensity
of the reflected light will increase as the mirror plate rotates
towards the ON state angle. When the photodetector is positioned in
the propagation path of the reflected light from the mirror plate
parallel to the substrate, the detected illumination intensity
decreases as the mirror plate rotates towards the ON state angle.
As a way of example, the image capture device can be positioned at
a location wherein an imaginary line connecting the device to be
measure has an angle to the incident illumination light, wherein
the angle can be 0.degree. degree, 10.degree. degrees or more,
12.degree. degrees or more, 14.degree. degrees or more, 16.degree.
degrees or more, 18.degree. degrees or more, 20.degree. degrees or
more, and 22.degree. degrees or more.
[0045] The light source emits a beam of light for the measurement
system. The light has a wavelength substantially less than the
minimum dimensions of the mirror plate. The light from the light
source is conducted to the microscope objective through the fiber
optic cable. The microscope objective forms a point light source
and emitting light passing through the diffuser. The condensing
lens preferably having a 6'' or 8''diameter collimates the diffused
light and illuminates the micromirror device within the vacuum
chamber. The incident light onto the micromirror device is
reflected by the mirror plates of the micromirrors. The reflected
light passes through the neutral density filter and is collected by
the condensing lens and focused into the image capture device. The
image capture device can be a display target, a CCD, or any other
type of devices having the function of capturing images.
[0046] Operations of the functional members of the system, such as
the vacuum chamber and the associated vacuum instruments, the image
capture device and the optical elements, can be controlled by
computing device 154 that has appropriate computer-executable
instructions for performing the controlling, which will be
discussed afterwards. Specifically, the computing device can
generate instructions for adjusting the relative positions of the
illumination system (e.g. light objective 144), the micromirror
device in the vacuum chamber, and the image capture device such
that each and every regions of the micromirror device can be
illuminated, and the reflected light such each and every regions
can be captured by the image capture device. In this embodiment, a
motorized stage can be attached to the micromirror device so as to
smoothly and accurately move the micromirror device. Moreover, the
computing device has a connection to the image capture device for
retrieving the image data from the image capture device and then
analyzing the image data.
[0047] The method of the present invention can be implemented in
many ways. In the following, an exemplary procedure according to
the method of the invention for measuring the dynamic
electromechanical response of the micromirrors will be discussed
with reference to FIGS. 6 through 12. The procedure can also be
implemented in the computing device for being automatically
executed. For this purposes, a set of programmable functional
modules are provided for interfacing the user, which will be
discussed in detail afterwards with reference to FIGS. 13 through
15.
[0048] FIG. 6 is a flow chart that presents the steps executed for
performing the measurement procedure according to the method of the
invention. The procedure starts from loading the micromirror device
into the measurement system (step 156). In fact, more than one
spatial light modulators, such as 16 or more spatial light
modulators can be loaded into the system at a measurement with each
spatial light modulator has an array of micromirrors. Before
loading, the micromirror device may have passed one or more
inspections, such as inspection with naked eyes, or other
uniformity inspections, as set forth in U.S. patent application
Ser. No. 10/875,602 filed Jun. 23, 2004, now U.S. Patent
Application Publication No. 2005-0286044 and U.S. patent
application Ser. No. 10/875,555 filed Jun. 23, 2004, now U.S. Pat.
No. 7,345,806. Both were filed on the on the same day as the
current patent application, and the subject matter of each being
incorporated herein by reference.
[0049] In order to individually drive the micromirrors with
electrostatic forces during measurements, the micromirrors are
connected to appropriate driving circuits that may be embedded
within the measurement system or installed outside the measurement
system. The loading step may have further steps, as shown in FIG.
7a.
[0050] Referring to FIG. 7, the micromirrors are loaded into the
vacuum chamber (e.g. vacuum chamber 150 in FIG. 5) (step 166).
According to an embodiment of the invention, the measurement is
preferably performed within an environment having a pressure under
1 atmosphere, more preferably around 20 Torr or less, or 50 mTorr
or less, or 15 mTorr or less. This is performed at step 168, in
which step the vacuum chamber is pumped out to the desired
pressure.
[0051] Referring back to FIG. 6, after the micromirrors to be
measured are securely installed in the measurement system with the
desired pressure, an image of the micromirrors is taken by the
image capture device (step 158). In taking the image, all
micromirrors in the inspection area are turned to the ON state such
that the cells corresponding to the micromirrors under inspection
have certain level of illumination intensities. As in the example
in FIG. 3, the micromirrors corresponding to the cells within the
matrix having the corners 128, 130, 132, and 134 are under
inspection at a time. Though as illustrated therein for simplicity
and demonstration purposes only, the number of micromirrors under
inspection (i.e. inspected simultaneously) at a time can be
35.times.35 or more, or 128.times.92 or more. Given the inspection
region, the locations of the micromirrors in the inspection region
are precisely determined and recorded (step 160 in FIG. 6). For
this purposes, the centers of the mirror plates of the micromirrors
in the inspection region are determined.
[0052] The centers of the micromirrors can be determined in many
ways. As a way of example, FIG. 7c shows the steps execute in
finding the centers of the image cells of the micromirrors in the
inspection region. Starting from step 192, the edges of the image
cells of the individual micromirrors are calculated. The edge
detection of an image can be accomplished in many ways. According
to an embodiment of the invention, a Siebel operator is applied to
the captured image for the purposes of edge detection. After the
edge detection, both horizontal and vertical edges of the image
cells corresponding to the micromirrors are obtained. Following the
edge detection, noise filtering step 194 can be optionally
performed so as to remove or reduce the noise of the image.
According to the embodiment of the invention, the image is further
processed in the Fourier space. Therefore, a Fourier transformation
is applied to the image at step 196. In the Fourier space, the
peaks of the image are located (step 198). Given the peak
positions, the pitches and rotation angles of the mirror plates are
calculated in the Fourier space (step 200) with the pitch sizes
include both pitch sizes along vertical and horizontal directions.
Such calculated pitch sizes may be different at different locations
in the image. As an optional feature, a plurality of pitch sizes at
different locations of the image can be calculated and then
averaged. Such averaged pitch size can be used as the pitch sizes
of the entire micromirror array. Specifically, the image is divided
into sub-images (step 202). Then the pitch sizes in the sub-images
are averaged at step 204. With the calculated pitch sizes and the
detected edges of the image cells of the individual micromirrors,
the geometric centers are determined at steps 208 and 210.
According to the embodiment, the geometric center detection is
performed in the real-space. Therefore, the Siebel transformation
following the Fourier transformation is applied to the image so as
to transform the image from the Fourier space back into the
real-space (step 206). At step 207, the geometric centers are
detected. Given the geometric centers, the edges, and the pitch
sizes of the image cells of the individual micromirrors, the
positions of the centers of all micromirrors of the micromirror
array device in the measurement system can be calculated (step
210).
[0053] Given the coordinates of the centers of the mirror plates
(micromirrors), each mirror plates in the inspection region can be
precisely located. In particular, the rotation positions of the
mirror plate can be derived from the illumination intensities at
the centers of the corresponding images, as shown by the shaded
solid circles in FIG. 3. Moreover, the micromirrors being inspected
can be individually identified.
[0054] Returning back to FIG. 6, following the image analyses for
determining the centers of the micromirrors, a set of measurement
parameters are determined (step 162), which is more detailed in
FIG. 7b. Referring to FIG. 7b, information on the micromirrors,
such as the identification number(s) is made of record (step 164).
The coordinates of the micromirrors at the corners of the
inspection region are saved such that each micromirror in the
inspection region can be located. In the example of the image as
shown in FIG. 3, the coordinates of the micromirrors 128, 130, 132,
and 134 are recorded. When combined with other parameters, such as
the pitch sizes along the column and row, the coordinates of each
micromirror in the inspection region can then be determined. This
is of particular importance when the measurement is to be performed
automatically with the computing device, in which case, the
computing device is capable of measuring the micromirrors
sequentially based on the coordinates of the micromirrors.
[0055] In order to measure the dynamic responses of the individual
micromirrors, a suitable voltage scan scheme is selected (step
168). The scheme defines the voltage scan profile and related
parameters. For demonstration purposes, four (4) different
voltage-scan schemes targeting at detecting different aspects of
the dynamic responses of the micromirrors to the driving
electrostatic forces will be described in the following. It will be
appreciate that other suitable voltage scan schemes without
departing from the spirit of the invention are also applicable. For
example, a voltage scan scheme combining the two or more of the
following discussed voltage scan schemes or the like are
applicable.
[0056] FIG. 8 illustrates a voltage scan profile of a static scan
scheme according to an embodiment of the invention. This scan
profile is particular useful for measuring the ON and OFF state
angle of the micromirror. The profile comprises a positive voltage
increase edge from time 0 to t.sub.Max followed by a positive
voltage decrease edge from time t.sub.Max to T.sub.1/2. The two
edges may or may not be symmetric along the t.sub.max axis. As an
optional feature, the profile may also comprise a negative voltage
increase edge from time T.sub.1/2 to t.sub.-Max followed by a
negative voltage decrease edge from time t-.sub.max to T.sub.0.
Similarly, the two edges may or may not be symmetric along the
t-.sub.max axis. However, it is preferred that the triangular
voltage profiles 0-V.sub.max-T.sub.1/2 and
T.sub.1/2-(-V.sub.Max)-T.sub.0 are symmetric around the point
(T=T.sub.1/2, V=0), even though not required. As a way of example,
t.sub.max can be from 3 to 10 sec, or from 10 sec to 50 sec, or
from 50 to 100 sec, or from 100 to 200 sec or more. V.sub.max can
be from 10 to 25 volts, or from 25 to 45 volts, or from 45 to 100
volts, or more.
[0057] During the voltage sweeping from T=0 to T=T.sub.0, the
illumination intensity of the image cell corresponding to the
mirror plate to which the voltage is applied is monitored in the
real-time fashion. For the variation of the illumination intensity,
the rotation position of the mirror plate is thus dynamically
detected. Specifically, when the mirror plate is rotated to the ON
state angle, the illumination intensity is maximized. To obtain the
voltage for the ON state angle of the micromirror, the voltage is
swept upwards from V=0 at T=0. When then voltage reaches to
V=V.sub.on at time t.sub.ON, it is observed that the illumination
intensity is maximized. Therefore, such voltage V.sub.on is defined
as the ON state voltage. For security reasons, the voltage can be
further increased a small amount to V.sub.Max at time t.sub.Max.
The voltage is then swept downwards from V=V.sub.max at
T=t.sub.max. At V=V.sub.off at time T=t.sub.off, it is observed
that the illumination intensity of the image cell corresponding to
the mirror plate is minimized. It is indicated that the mirror
plate is turned to the OFF state, such as a state when the mirror
plate is in the natural resting state or parallel to the substrate.
Such a voltage V.sub.OFF is defined as the voltage corresponding to
the OFF state. For further ensuring that the mirror plate is
returned to the OFF state, the voltage is decreased a small amount
further, such as to V=0 at T=T.sub.1/2. In the above voltage
scanning scheme, a positive voltage is applied to the mirror plate
to rotate the mirror plate. In contrast, a negative voltage can
also be applied to the mirror plate to achieve the same effect.
Specifically, the voltage sweeping can be continued at time
T=T.sub.1/2 towards the negative direction. When the negative
sweeping voltage reaches V=-V.sub.ON at time T=t.sub.-on, it is
observed that the illumination intensity of the image cell
corresponding to the mirror plate is maximized. Accordingly,
voltage V=V.sub.-ON is defined as the voltage for the OFF state.
The voltage is swept a small amount further to V.sub.max at time
T=T.sub.-max to ensure the definition of the OFF state voltage. The
sweeping voltage is then swept downwards at time T=t.sub.-max. When
the sweeping voltage is at V=-V.sub.off, it is observed that the
illumination intensity of the image cell corresponding to the
mirror plate is minimized. Such voltage is then defined as the
voltage for the OFF state. For ensuring the defined voltage for the
OFF state, the sweeping voltage is swept a small amount further
such as V=0 at T=T.sub.0.
[0058] The observed ON and OFF state voltages in different sweeping
directions can be compared so as to obtain the electromechanical
property of the micromirror. An ideal micromirror is expected to
have symmetric ON and OFF state voltages. Specifically, the ON
state voltage V.sub.on obtained from the positive voltage sweeping
has the same absolute value as the ON state voltage obtained from
the negative voltage sweeping. That is |V.sub.on|=|-V.sub.on|. The
same for the OFF state voltages, |V.sub.off|=|-V.sub.off|.
[0059] The measurement is then repeated for the remaining
micromirrors in the inspection region. After the measurements of
the micromirrors in one inspection region, the inspection region is
shifted to cover another group of micromirrors followed by the
measurements. The measurement process is repeated until the desired
electromechanical responses of all micromirrors in the spatial
light modulator are obtained. After the completion of the
measurement of one spatial light modulator, the measurement can be
continued on another spatial light modulator. Specifically,
multiple spatial light modulators can be placed in the measurement
system as shown in FIG. 5 according to an embodiment of the
invention. This is of particular importance when the measurement is
performed in a vacuum chamber (e.g. vacuum chamber 150 in FIG. 5),
because in this way, the measurement can be continuously performed
for all the spatial light modulators without extra efforts in
reloading spatial light modulators after each measurement.
[0060] With the measured electromechanical responses (e.g. the ON
and OFF state voltages) of the micromirrors in a spatial light
modulator, the quality and performance can be evaluated.
Specifically, if all micromirrors have the same ON and OFF state
voltage, or the variation of the ON and/or OFF state voltages is
within a predefined range, it can be said that the micromirror
array device is acceptable. Otherwise, the micromirror array device
is an inferior product, which can be discarded.
[0061] The measured ON and OFF state voltages, in turn, can be used
for calibrating and optimizing the driving voltages for a quality
product of the micromirror array device in operation. According to
the invention, the ON state voltage (V.sub.on) for driving the
micromirrors in operation is calculated from the measured ON state
voltages such that the mirror plate has the "fastest" response to
the driving force V.sub.on. To accomplish this, the response speed
of the mirror plate needs to be measured, which will be discussed
afterwards.
[0062] In addition to the driving voltage profile in FIG. 8, the
dynamic electromechanical response of the micromirrors can also be
measured with other driving voltage scan schemes, such as that
shown in FIG. 9. Referring to FIG. 9, the voltage scan scheme
comprises a set of driving voltage sequences, 10A, 10B, 10C, and
10D and many other similar sequences (not shown in the figure).
Each voltage sequence comprises a set of pulse structures (duty
cycles) each of which further comprises a peak having a width of
T.sub.1 followed by another peak with a width of T.sub.2. Both
T.sub.1 and T.sub.2 are longer than the required time for the
mirror plate to rotate to the ON state. The peak T.sub.1 has a
height of V.sub.max equivalent to the V.sub.max in FIG. 8. This
peak is designated for pulling the mirror plate of the micromirror
being tested to the maximum angle of the mirror plate. The peak of
T.sub.2 immediately following the peak T.sub.1 has a height varying
over time. The voltage increment of the peak at T.sub.2 can be of
any suitable value depending upon the desired precision, such as 1%
or less of V.sub.max.
[0063] During each duty cycle of a voltage sequence, the mirror
plate is pulled to its maximum rotation angle by the voltage pulse
at T.sub.1. At time t.sub.2, the voltage on the mirror plate drops
to V.sub.i and remains at the mirror plate for a time period of
T.sub.2. If the voltage V.sub.i is less than the voltage required
to rotate the mirror plate to the ON state, the mirror plate
departs from the ON state to its natural resting state. In the
corresponding image cell, the illumination intensity decreases.
However, when the voltage V.sub.i during T.sub.2 reaches or is
larger than the ON state voltage, the mirror plate stays at the ON
state. Accordingly, the illumination intensity of the image cell
remains the same. From such measurement, the ON state voltage can
be obtained.
[0064] As a way of example, voltage sequences 10A to 10D are
applied to the mirror plate of the micromirror being tested. The
voltage pulse at T.sub.2 of each duty cycle in the sequences before
10C is less than the ON state voltage. The voltage pulse at T.sub.2
of sequence 10C is equal to the ON state voltage, and the voltage
pulse at T.sub.2 of each duty cycle in the sequences after sequence
10C is larger than the ON stage voltage. The voltage sequences can
be applied sequentially to the micromirror starting from sequence
10A. Because the voltage pulses at T.sub.2 are less than the ON
state voltage, the illumination intensity of the image cell
corresponding to the micromirror being tested decreases during time
periods T.sub.2 when the voltage sequences 10A to 10B are applied.
When the voltage sequence 10C is applied, the illumination
intensity of the image cell changes during T.sub.2 time intervals,
indicating that the voltage at T.sub.2 is equal to or larger than
the ON state voltage. If the increment of the voltage at T.sub.2 of
sequence 10C from that in the applied voltage immediately prior to
sequence 10C is small enough, the voltage at T.sub.2 of sequence
10C is substantially the ON state voltage. For ensuring that the
voltage at T.sub.2 of sequence 10C is the ON state voltage,
additional voltage sequences 10D can be applied.
[0065] In accordance with another embodiment of the invention, the
voltage sweeping scheme can be inversed. Specifically, instead of
applying the above voltage sequences in an increased order of the
peak at T.sub.2, the sequences can be applied in a decreased order.
For example, sequence 10D can be applied before sequence 10C that
can be followed by sequences 10B and 10A and other sequences
consecutively.
[0066] Referring to FIG. 10, another voltage scanning scheme for
measuring the electromechanical responses of the micromirrors is
illustrated therein. According to the scheme, voltage pulses of
different voltages values are sequentially applied to the mirror
plate of the micromirror being tested. Specifically, the scheme
consists of a set of voltage sequences, and different sequences
have voltage pulses of different values. The voltage sequences can
be applied to the mirror plate in an increased or a decreased order
of the voltages. For example, the scheme consists of voltage
sequences 11A, 11B, 11C, and 11D. The voltage pulses V.sub.m in 11A
is less than those in other sequences. The voltage pulses in 11B
are higher than that in 11A, but lower than those in 11C and 11D.
For simplicity and demonstration purposes, only four (4) voltage
sequences are presented in FIG. 11. In practical, more voltage
sequences can be provided and applied in an appropriate order. In
fact, the total number of the voltage sequence depends upon the
desired precision.
[0067] As a way of example, the voltage sequences can be applied to
the mirror plate in an order of 11A, 11B, 11C, and 11D with the
voltage values increased. Assuming that before the application of
sequence 11C (i.e. during the applications of sequences 11A and
11B), the illumination intensities of the image cell corresponding
to micromirror being tested yield a "dark" image, whereas the
application of sequence 11C results in a "bright" image cell. It
can then be determined that the ON state voltage of the micromirror
is the voltage V.sub.ON in sequence 11C. Because the voltage
sequences are applied to the micromirrors with the voltages thereof
increased, the micromirror responses to the sequences of voltages
along branch {circle around (1)} in FIG. 4. The voltage sequences
can also be applied in an inversed order, such as from 11D to 11C,
then 11B followed by 11A.
[0068] The ON and OFF state voltages describe one aspect of the
electromechanical response of a micromirror. Another aspect of the
electromechanical response of a micromirror can be described in
terms of the response speed to the ON and OFF state voltages.
Specifically, the response speed measures the time interval of the
micromirror in transition from one state to another under a given
driving voltage (e.g. the ON state voltage). According to an
embodiment of the invention, the speed response can be measured
with a voltage scanning scheme having a voltage scanning profile as
shown in FIG. 11.
[0069] Referring to FIG. 11, the voltage scanning scheme consists
of a set of voltage sequences, such as 12A, 12B, 12C, and 12D. The
voltage pluses may have the same height, such as V.sub.on or
V.sub.max. The period T, thus the frequency of the voltage pulses
is the same for all voltage sequences. In one voltage sequence, the
widths of the voltage pulses W are the same. But the widths are
different in different voltage sequences. The increment of the W
for the voltage sequences consecutively applied to the micromirror
is predetermined, such as 1% or less of the applied maximum voltage
V.sub.max.
[0070] In a typical measurement with the driving voltages shown in
FIG. 11, the voltage sequences are applied to the mirror plate
being tested, while the illumination intensity of the image cell
corresponding to the micromirror is monitored at the same time. As
a way of example, assuming the speed response of the micromirror
corresponds to the voltage pulse width W of the voltage sequence
12C, voltage sequences (e.g. 12A and 12B) having the pulse widths
less than that of sequence 12C will result in a "dark" image cell
of the micromirror. When the voltage sequence 12C is applied to the
micromirror, it is observed that the illumination intensity of the
image cell of the micromirror is maximized, resulting in a "bright"
image cell. For ensuring that the natural speed response of the
micromirror corresponds to the pulse width W of voltage sequence
12C, additional voltage sequences, such as 12D can be applied to
the micromirror in test, wherein the pulse width of sequence 12D is
larger than that of sequence 12C. It should be point that, the four
(4) voltage sequences in the figure are presented therein for
demonstration and simplicity purposes only. In practice, more
voltage sequences can be provided and applied to the micromirror,
for example, between sequences 12A and 12C, and between sequences
12C and 12D.
[0071] After obtaining the speed responses of one micromirror, the
same measurement is performed for another in the inspection region
and the remaining micromirrors sequentially. Moreover, the same
measurement procedure is carried out for another group of
micromirrors in the spatial light modulator after the completion of
the measurement in one inspection area until all the micromirrors
of the spatial light modulator are tested. Because the measurement
system as shown in FIG. 5 allows for loading multiple spatial light
modulators (e.g. six) at one time, the measurement can be continued
for the rest spatial light modulators.
[0072] Returning to FIG. 8, the voltage scan scheme is selected
(e.g. from the schemes as discussed above with references to FIGS.
8 through 11) at step 168 of step 162 in FIG. 6. Returning back to
FIG. 6, the dynamic electromechanical responses of the micromirrors
are then measured at step 164 according to the parameters set at
step 162. Following the measurements, the measurement results are
then analyzed (step 165) so as to obtain the quantitative
descriptions of the electromechanical responses of the
micromirrors, such as the ON and OFF state angles, the ON and OFF
state voltages (V.sub.on and V.sub.off) and the response speed of
the micromirrors under given driving voltages. In particular, an ON
state voltage V.sub.on is obtained from the quantitative
descriptions such that the micromirrors of the micromirror array
device have the "fastest" response under V.sub.on in average. The
analyze step (step 165) can be conducted after the completion of
the measurements for the micromirrors of the entire micromirror
array device. Alternatively, the measurements can be conducted
during the measurement. For example, the analysis can be performed
after each measurement of a micromirror, or after the measurements
of a group of micromirrors (e.g. the micromirrors in the inspection
region), or after the measurements of the micromirrors of the
entire micromirror array device, or at a later time after the
measurements.
[0073] The measurement procedure may loop back to step 162 for
performing the measurements for another group of micromirrors of
the micromirror array device, or for the micromirrors on another
micromirror array device in the measurement system until all the
micromirrors of all desired micromirror array devices are
measured.
[0074] After the completion of the measurements, the micromirror
array devices are unloaded from the measurement system (step 167).
In performing the unloading, the vacuum chamber of the measurement
system is vented before unloading.
[0075] In the above discussion, a homogeneous illumination light
beam incident onto the micromirrors is preferred. However, such
homogeneous light beam may not always be ready. When an
inhomogeneous light beam is used for illuminating the system,
reflected light from the mirror plates of the micromirrors will not
be homogeneous either, and the accuracy of the measured light
intensities from the individual micromirrors can be degraded.
Moreover, the intensities of the reflected light from the
micromirrors may be out of the acceptable range of the
photodetector, in which way, the detected illumination intensity of
the reflected light will not be accurate. Even with a homogeneous
illumination light beam, the detected intensity of the illumination
system may not be accurate due to noise of the captured images of
the micromirrors in the photodetector.
[0076] An approach to solve this problem is to calibrate the
illumination intensity of the light source as disclosed in the
invention. Moreover, the solution may include a solution to depress
the noise in the captured images generated by the photodetector.
According to an embodiment of the invention, an image of the
micromirrors is taken by the photodetector when the light source is
turned off. The illumination intensity of the background noise in
captured image is analyzed and recorded. The micromirror array
device under inspection is then replaced by a reference wafer, such
as a glass plate preferably having a reflective index higher than
that of the micromirror array device. A reference illumination
intensity is then measured for the reference wafer with the light
source turned on. The noise intensity and the reference intensity
are respectively defined as the minimum and maximum illumination
intensities allowed by the photodetector. The measured illumination
intensity of the reflected light in a practical measurement is then
scaled within the dynamic intensity range between the minimum and
maximum illumination intensities.
[0077] According to an embodiment of the invention, measurement
procedures as discussed above can be implemented in a computing
device, such as computing device 154 in FIG. 5. Specifically, the
computing device controls the components of the measurement system
based on the interaction with users, or based on the control
information stored therein so as to perform the measurement
procedure. The control can be accomplished through executions of a
plurality of computer readable instructions generated from a
plurality of functional modules. FIG. 12 schematically illustrates
one exemplary computing device for implementing embodiments of the
invention. Although such devices are well known to those of skill
in the art, a brief explanation will be provided herein for the
convenience of other readers.
[0078] Referring to FIG. 12, in its most basic configuration,
computing device 180 typically includes at least one processing
unit 182 and memory 184. Depending on the exact configuration and
type of computing device, memory 184 can be volatile (such as RAM),
non-volatile (such as ROM, flash memory, etc.) or some combination
of the two.
[0079] Additionally, device 180 may also have other features and/or
functionality. For example, device 180 could also include
additional removable and/or non-removable storage including, but
not limited to, magnetic or optical disks or tape, as well as
writable electrical storage media. Such additional storage is
illustrated in FIG. 12 by removable storage 186 and non-removable
storage 188. Computer storage media includes volatile and
nonvolatile, removable and non-removable media implemented in any
method or technology for storage of information such as computer
readable instructions, data structures, program modules or other
data. The memory, the removable storage and the non-removable
storage are all examples of computer storage media. Computer
storage media includes, but is not limited to, RAM, ROM, EEPROM,
flash memory or other memory technology, CDROM, digital versatile
disks (DVD) or other optical storage, magnetic cassettes, magnetic
tape, magnetic disk storage or other magnetic storage devices, or
any other medium which can be used to store the desired information
and which can accessed by the device. Any such computer storage
media may be part of, or used in conjunction with, the device.
[0080] The device may also contain one or more communications
connections 190 that allow the device to communicate with other
devices (such as the other functional modules in FIG. 5). The
communications connections carry information in a communication
media. Communication media typically embodies computer readable
instructions, data structures, program modules or other data in a
modulated data signal such as a carrier wave or other transport
mechanism and includes any information delivery media. The term
"modulated data signal" means a signal that has one or more of its
characteristics set or changed in such a manner as to encode
information in the signal. By way of example, and not limitation,
communication media includes wired media such as a wired network or
direct-wired connection, and wireless media such as acoustic, RF,
infrared and other wireless media. As discussed above, the term
computer readable media as used herein includes both storage media
and communication media.
[0081] For facilitating the automatic control of the measurements
system for executing the desired measurement procedures with the
computing device, a set of User-Interfaces (UI) are provided
according to the invention. FIG. 13 illustrates UI 170 through
which parameters associated with the sample loading (e.g. step 156
shown in FIG. 6) can be defined. Specifically, UI 170 provides
vacuum control panel 172 for enabling the user to control the
vacuum of the system. For example, the user may activate "Vent" for
ventilating vacuum chamber 150 in FIG. 5. This is often executed
after each measurement and/or before loading one or more new sample
into the vacuum chamber. The user may activate "Idle" for
maintaining the vacuum system at its current state. When the sample
(e.g. spatial light modulators) are securely loaded into the vacuum
chamber of the system, the user may instruct the system to pumping
out the vacuum chamber to a desired pressure level, such 1
atmosphere or lower, or 20 Torr or less, or 50 mTorr or less, or 15
mTorr or less. This can be accomplished by activating "Pump." When
the system reaches the desired measurement environment (e.g. the
desire pressure), the user may instruct the system to maintain its
current state by activating "Idle." The desired measurement can
then be performed. After the completion of the measurements, the
user may instruct the measurement system to ventilating the vacuum
chamber so as to unload the sample. The ventilation of the vacuum
system can be done with the activation of "Vent."
[0082] Before (or after) performing the measurement, information
for uniquely identifying the sample to be measured can be recorded
in panel 174 as shown in the figure. This information will be
associated with the measurements results of the sample and can be
stored in the computing device or other type of storages.
[0083] In performing measurement, the rotational positions of the
mirror plates of the micromirrors are detected through the
measurements of the illumination intensities of the corresponding
image cells. To accomplish this, the image cells, especially the
centers of the image cells are required to be aligned to the
physical centers of the mirror plates when the mirror plates are at
their natural resting states, such as parallel to the substrate.
Accordingly, an alignment control mechanism is necessary. For this
reason, align control UI 176 is provided and an exemplary of which
is presented in FIG. 14.
[0084] Referring to FIG. 14, align control UI 176 consists of a
navigate control panel and an align control panels. The navigate
control panel further comprises motional control panel, in which
motion direction keys are provided. Through the motion direction
keys, the sample (e.g. the spatial light modulator in the vacuum
chamber) and/or the illumination system (e.g. 144 in FIG. 5) can be
relatively moved in "left", "right", "up" and "down" directions so
as to accomplish the alignment. The movement can control through
other control fields provided by the navigate panel. For example,
the "Amount" field, which consists of a text-input field allows for
the user to indicate the movement increment with a unit defined in
the "Unit" field that also comprises a text-input field. In
addition to step-movement, the navigate panel further provides a
"Move to" function for enabling the alignment of the illumination
system with any micromirror in the inspection field. Specifically,
the "Move to" field consists of a field of "X number" in which the
X coordinate (in terms of the number of micromirrors) of the mirror
plate to be aligned is indicted, and a field of "Y number" in which
the Y coordinate (in terms of the number of micromirrors) of the
mirror plate to be aligned is indicted. In addition to the position
control, the navigate control panel also provides the function for
controlling the optical elements of the measurements system so as
to obtain the best quality image of the micromirrors. Specifically,
a "Focus Control" is provided for controlling the optical elements,
such as projection lens 144, 146, 148, and 152.
[0085] When the micromirrors of spatial light modulator being
tested are aligned with the illumination system, the inspection
region, as well as thither position information needs to be
defined. Accordingly, the align control panel provides an "Align"
panel as shown in the figure. The "Align" panel consists of a
"Corner" panel in which the coordinates of the micromirrors at the
four (4) corners of the spatial light modulator are defined. As an
example of a spatial light modulator having 1024.times.768
micromirrors, the coordinates of the micromirror at the top-left
corner of the spatial light modulator can be set as (-1,-1). The
coordinates of the micromirror at the top-right corner can be set
as (1024,-1). The coordinates of the micromirror at the
bottom-right corner can be set as (1024, 768), and the coordinates
of the micromirror at the bottom-left corner can be set as (-1,
768). These coordinates of the corner micromirrors can be stored
through activation "Save corners" in the panel. Alternatively, the
coordinates of the corner micromirrors can be loaded from storage
through the activation of "Load Corners." In addition to indicating
the coordinates of the corner micromirrors with numbers, the "Align
Control" panel also provides "Mouse Control" function enabling the
user to control the alignment with a mouse of the computing
device.
[0086] In performing the measurement, a voltage scanning scheme is
defined, such as in step 162 of the flow chart in FIG. 6. The
voltage scanning scheme can be defined through UI 178 as shown in
FIG. 15. Referring to FIG. 15, UI 178 comprises "Polarity" panel,
"Voltage scan test settings" panel, "Data analysis settings" panel,
"Alternative scan settings" panel, and other related functional and
operational buttons, such as "Start voltage scan", "save voltage
scan", and "Analyze scan results" and an "Intensity threshold"
text-input field, and "Alternative scan settings" panel.
[0087] The "Polarity" panel provides users with a plurality of
options, such as "Positive" and "Negative" for enabling the users
to indicate the polarity of the driving voltages. The driving
voltages used for the measurements, such as those illustrated in
FIG. 8 through 11, are further defined in the "voltage scan test
settings" panel. Specifically, the voltage scan scheme can be
selected from a plurality of provided options. As an example, the
user may use "Sequence 1" and "Sequence 2" panels to define the
voltage scan sequence as shown in FIG. 8. Specifically, the upward
scanning portion can be defined in the "Sequence 1" panel. The
"Voltage Min A," "Voltage Max B" and "Voltage Min C" fields
respectively define the starting voltage, the maximum voltage
V.sub.max and the ending voltage. The downwards voltage scanning
portion of voltage profile can be defined with "Sequence 2" panel.
The "Voltage Min D," "Voltage Max E" and "Voltage Min F"
respectively define the starting voltage the downwards scanning
voltage, the maximum voltage in the negative voltage direction, and
the returning voltage. The slop of scanning voltage profile can be
defined in the "Voltage step" field, which can be a text-input
field.
[0088] The "data analysis settings" panel is provided for defining
the number of micromirrors in the inspection area. For example, the
measurement setup and the method according to the invention enable
20 or more, or 35.times.35 or more, or 128.times.92 or more
micromirrors being included in the inspection area.
[0089] In addition to the voltage profile in FIG. 8, other voltage
profiles, such that in FIG. 9 can also be defined, for example,
through the "Alternative scan settings" panel. In particular, the
starting voltage can be defined in the text-input filed "Min
Voltage;" the voltage peak of T.sub.2 can be defined in the
text-input filed "Mid Voltage;" and the voltage peak of T.sub.1 can
be defined in the text-input filed "Max Voltage." The "Voltage step
1" defines the voltage increment step of the voltage peak of
T.sub.2. In fact, the "Alternative scan settings" panel can also be
used to defining the voltage profile in FIG. 11 by, fro example,
setting the "Min Voltage" and "Mid Voltage" fields to zero, and the
"Voltage Step 1" to a desired voltage increment step. The scanning
voltage having the profile defined above can be activated and
applied to the micromirrors during measurements by checking the
"Use Alt Voltage Scan."
[0090] Given the selected and defined voltage scan scheme and
scanning voltage profile, the measurement can be initiated by
activating the functional button of "Start voltage scan." After
each scan for either a micromirror or the micromirrors within the
defined inspection area or the micromirrors of the spatial light
modulator, the scanned results can be saved through activation of
the functional button of "Save voltage scan." The results can then
be analyzed by activating the functional button of "Analyze scan
results." In the measurement, the rotational positions of the
mirror plates are detected through the measurements of the
intensities of the reflected light from the mirror plates, which is
measured through the captured images of the mirror plates. For
better presenting such intensities in the image, an intensity
threshold is determined for filtering out the intensities beyond
the threshold. As a result, the centers of the mirror plates are
more discernable; and the intensities around the center if the
mirror plate image can be more accurately compared with each
other.
[0091] In addition to selection of the scanning voltages from the
predefined (or provided) options, the method and experimental setup
including UI 178 according to the invention also enable the user to
perform the measurements with any desired voltage scanning schemes
or scanning voltage profiles. This can be accomplished through the
definition of the voltage profile with UI 178, which will not be
discussed in detail herein.
[0092] In addition to the implementation in the micromirror devices
as shown in FIG. 1, the present invention can also be implemented
in measuring other type of micromirror array devices, such as those
illustrated in FIGS. 16A and 16B.
[0093] Referring to FIG. 16A, the micromirrors are arranged in the
micromirror array such that the micromirrors in the array are
titled--that is the edges of the mirror plate of each micromirror
in the array are neither parallel to the edges of the micromirror
array nor parallel to the edges of the micromirror array device, as
set forth in U.S. patent application Ser. No. 10/698,563 to Patel,
filed on Oct. 30, 2003, the subject matter being incorporated
herein by reference.
[0094] Referring to FIG. 16B, another micromirror array device in
which embodiments of the invention can be implemented is
illustrated therein. The mirror plates of the micromirrors in the
micromirror array each have zigzagged edges. An advantage of such a
mirror plate is that the unexpected light scattering can be
reduced, thus, the contrast ratio of the displayed images can be
improved. Similar to that in FIG. 1, the micromirrors can be formed
on a light transmissive substrate 184, which can be glass. The
electrode and circuitry array can be formed on semiconductor
substrate 186 for addressing and actuating the micromirrors.
Alternatively, the micromirrors and the electrodes can be formed on
the same substrate, such as a semiconductor substrate.
[0095] It will be appreciated by those of skill in the art that a
new and useful method and a system for qualitatively evaluating
product quality of microelectromechanical devices have been
described herein. In view of the many possible embodiments to which
the principles of this invention may be applied, however, it should
be recognized that the embodiments described herein with respect to
the drawing figures are meant to be illustrative only and should
not be taken as limiting the scope of invention. For example, the
micromirror array device can be a part of a packaged device. The
device package may have the micromirror array device being
hermetically or non-hermetically sealed within the package. Those
of skill in the art will recognize that the illustrated embodiments
can be modified in arrangement and detail without departing from
the spirit of the invention. Therefore, the invention as described
herein contemplates all such embodiments as may come within the
scope of the following claims and equivalents thereof.
APPENDIX A
A Brief Description of the Sobel Detector
[0096] The Sobel operator performs a 2-D spatial gradient
measurement on an image and so emphasizes regions of high spatial
gradient that correspond to edges. Typically it is used to find the
approximate absolute gradient magnitude at each point in an input
grey-scale image.
[0097] In theory at least, the operator consists of a pair of
3.times.3 convolution masks as shown in the following. A brief
description of the convolution operator is attached in Appendix B.
One mask is simply the other rotated by 90.degree..
- 1 0 + 1 - 2 0 + 2 - 1 0 + 1 Gx ##EQU00001## + 1 + 2 + 1 0 0 0 - 1
- 2 - 1 Gy ##EQU00001.2##
Sobel Convolution Masks
[0098] These masks are designed to respond maximally to edges
running vertically and horizontally relative to the pixel grid, one
mask for each of the two perpendicular orientations. The masks can
be applied separately to the input image, to produce separate
measurements of the gradient component in each orientation (call
these Gx and Gy). These can then be combined together to find the
absolute magnitude of the gradient at each point and the
orientation of that gradient. The gradient magnitude is given
by:
|G|= {square root over (Gx.sup.2+Gy.sup.2)}
Although typically, an approximate magnitude is computed using:
|G|=|Gx|+|Gy|
which is much faster to compute.
[0099] The angle of orientation of the edge (relative to the pixel
grid) giving rise to the spatial gradient is given by:
.theta.=arctan(Gy/Gx)-3.pi./4
In this case, orientation 0 is taken to mean that the direction of
maximum contrast from black to white runs from left to right on the
image, and other angles are measured anti-clockwise from this.
Often, this absolute magnitude is the only output the user
sees--the two components of the gradient are conveniently computed
and added in a single pass over the input image using the
pseudo-convolution operator shown in the following figure.
P 1 P 2 P 3 P 4 P 5 P 6 P 7 P 8 P 9 ##EQU00002##
Pseudo-Convolution Masks Used to Quickly Compute Approximate
Gradient Magnitude
[0100] Using this mask the approximate magnitude is given by:
.parallel.G|=(P.sub.1+2.times.P.sub.2+P.sub.3)-(P.sub.7+2.times.P.sub.8+-
P.sub.9)|+|(P.sub.3+2.times.P.sub.5+P.sub.9)-(P.sub.1+2.times.P.sub.4+P.su-
b.7)|
APPENDIX B
A Brief Description of Convolution
[0101] Convolution is a simple mathematical operation which is
fundamental to many common image processing operators. Convolution
provides a way of `multiplying together` two arrays of numbers,
generally of different sizes, but of the same dimensionality, to
produce a third array of numbers of the same dimensionality. This
can be used in image processing to implement operators whose output
pixel values are simple linear combinations of certain input pixel
values.
[0102] In an image processing context, one of the input arrays is
normally just a greylevel image. The second array is usually much
smaller, and is also two dimensional (although it may be just a
single pixel thick). The following shows an example image and
kernel that we will use to illustrate convolution.
I 11 I 12 I 13 I 14 I 15 I 16 I 17 I 18 I 19 I 21 I 22 I 23 I 24 I
25 I 26 I 27 I 28 I 29 I 31 I 32 I 33 I 34 I 35 I 36 I 37 I 38 I 39
I 41 I 42 I 43 I 44 I 45 I 46 I 47 I 48 I 49 I 51 I 52 I 53 I 54 I
55 I 56 I 57 I 58 I 59 I 61 I 62 I 63 I 64 I 65 I 66 I 67 I 68 I 69
##EQU00003## K 11 K 12 K 13 K 21 K 22 K 23 ##EQU00003.2##
[0103] An example small image (left) and kernel (right) for
illustrating convolution. The labels within each grid square are
used to identify each square.
[0104] The convolution is performed by sliding the kernel over the
image, generally starting at the top left corner, so as to move the
kernel through all the positions where the kernel fits entirely
within the boundaries of the image. (Note that implementations
differ in what they do at the edges of images as explained below.)
Each kernel position corresponds to a single output pixel, the
value of which is calculated by multiplying together the kernel
value and the underlying image pixel value for each of the cells in
the kernel, and then adding all these numbers together.
[0105] So in this example, the value of the bottom right pixel in
the output image will be given by:
O.sub.57=I.sub.57K.sub.11+I.sub.58K.sub.12+I.sub.59K.sub.13+I.sub.67K.su-
b.21+I.sub.68K.sub.22+I.sub.69K.sub.23
[0106] If the image has M rows and N columns, and the kernel has m
rows and n columns, then the size of the output image will have
M-m+1 rows, and N-n+1 columns. Mathematically we can write the
convolution as:
O ( i , j ) = k = 1 m l = 1 n I ( i + k - 1 , j + l - 1 ) K ( k , l
) ##EQU00004##
wherein i runs from 1 to M-m+1 and j runs from 1 to N-n+1.
[0107] Note that many implementations of convolution produce a
larger output image than this because they relax the constraint
that the kernel can only be moved to positions where it fits
entirely within the image. Instead, these implementations typically
slide the kernel to all positions where just the top left corner of
the kernel is within the image. Therefore the kernel overlaps' the
image on the bottom and right edges. One advantage of this approach
is that the output image is the same size as the input image.
Unfortunately, in order to calculate the output pixel values for
the bottom and right edges of the image, it is necessary to invent
input pixel values for places where the kernel extends off the end
of the image. Typically pixel values of zero are chosen for regions
outside the true image, but this can often distort the output image
at these places. Therefore in general if you are using a
convolution implementation that does this, it is better to clip the
image to remove these spurious regions. Removing n-1 pixels from
the right hand side and m-1 pixels from the bottom will fix
things
* * * * *