Optics: measuring and testing – Of light reflection
Reexamination Certificate
2002-05-22
2004-09-07
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06788416
ABSTRACT:
BACKGROUND
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching similar to those developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors.
Digital micromirror devices have been utilized in optical display systems. In these display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of many digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane that, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Thin hinge structures, which restrict the deformation to a relatively small region of the device, limit the amount of light scattered and improve image quality.
Torsion beam devices enabled the use of dark field optical systems. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface. The rigid member or mirror is suspended by, and typically centered on, the torsion hinge-allowing the mirror to rotate by twisting the torsion hinge. Address electrodes are formed on the substrate beneath the mirror on either side of the torsion hinge axis. Electrostatic attraction between an address electrode and the mirror, which in effect form the two plates of an air gap capacitor, is used to rotate the mirror.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by using an elevated mirror to block most of the light from reaching the torsion beam hinges. The elevated mirror is connected to an underlying torsion beam or yoke by a support post. The yoke is attached to the torsion hinges, which in turn are connected to rigid support posts. Because the structures that support the mirror and allow it to rotate are underneath the mirror instead of around the perimeter of the mirror, virtually the entire surface of the device is used to fabricate the mirror. Since virtually all of the light incident to a hidden-hinge micromirror device reaches an active mirror surface—and is thus either used to form an image pixel or reflected away from the image to a light trap—the hidden-hinge device's contrast ratio is much higher than the contrast ratio of previous devices.
Images are created by positioning the DMD so that a light beam strikes the DMD at an angle equal to twice the angle of rotation. In this position, the mirrors fully rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lens-transmitting light to a pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lens-leaving the corresponding pixel dark.
Full-color images are generated either by using three micromirror devices to produce three single-color images, or by sequentially forming three single-color images using a single micromirror device illuminated by a beam of light passing through three color filters mounted on a rotating color wheel.
An example of a small portion of a digital micromirror array is depicted in FIG.
1
. In
FIG. 1
, a small portion of a digital micromirror array
100
with several mirrors
102
is depicted. Some of the mirrors
102
have been removed to show the underlying structure of the DMD array.
FIG. 2
is an exploded close-up of one individual mirror
102
of a DMD array. The electrical interconnections and operations of the individual micromirrors
102
are described in further detail in U.S. Pat. No. 6,323,982 entitled Yield Superstructure for Digital Micromirror Device to Larry J. Hornbeck, which is hereby incorporated by reference.
A representative embodiment of an existing DMD testing apparatus is depicted in FIG.
3
. In
FIG. 3
, an incandescent light
305
provides light that is captured by a set of focusing optics
310
. The focusing optics
310
focus the captured light so as to create a substantially uniform and collimated set of light beams
315
. The light beams
315
are directed to the DMD device under test (DUT) so as to create a reflected set of light beams
325
. The reflected light beams
325
may be directed to a reflector screen
330
so that the reflected patterns of light generated by the DMD device under test
320
may be viewed by a person. According to this embodiment, a pattern generator
335
is connected to the DMD device under test
320
so that a variety of patterns may be applied to the micro mirrors on the DMD device under test
320
. By using these patterns, different characteristics of the DMD device under test
320
may be analyzed.
One problem associated with existing DMD testing apparatuses
300
is that they can only identify problems on a macroscopic scale. In other words, the system depicted in
FIG. 3
can only be used to detect problems that are visible to the naked eye, such as twinkle or stuck pixels. Another problem associated with existing DMD testing apparatuses
300
is that the incandescent light source
105
cannot be switched on and off at high speed, thus limiting its effectiveness in measuring the transient response of a DMD device under test
320
. Yet another problem associated with existing DMD testing apparatuses
300
is that the light generated by the incandescent light source
305
is not coherent and is polychromatic. This can create focusing and interference problems for the focusing optics
310
and for the DMD device under test
320
.
There is therefore a need in the art for an improved DMD testing apparatus that can characterize the transient behavior of individual mirrors in a DMD device under test. There is also a need for a testing apparatus that utilizes a high-speed monochromatic light source so that stroboscopic bursts of monochromatic light can be applied to the DMD device under test. There is also a further need for a device and method that can produce quantifiable data corresponding to the transient behavioral characteristics of individual micromirrors in a DMD array in a very short period of time.
BRIEF SUMMARY
Each individual mirror in an DMD micromirror array can be modeled as behaving in a deterministic manner. In other words, each time a mirror is stimulated by a particular set of signals, the mirror will behave in the same manner. This being the case, if the DMD array is driven with a periodic set of signals, each mirror in the DMD array will react with a periodic response. Each individual mirror in a DMD array, however, may have a unique response.
Because each mirror can be made to behave in a periodic manner, the measurements of the transient effects exhibited by individual micromirr
Brady III Wade James
Brill Charles A.
Stafira Michael P.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
LandOfFree
Method and apparatus for dynamic DMD testing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for dynamic DMD testing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for dynamic DMD testing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3262939