Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-12-23
2001-02-20
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C359S295000, C359S223100, C359S248000, C345S084000, C345S085000
Reexamination Certificate
active
06191883
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of micromirror devices, more particularly to memory cell configurations suitable for use with micromirror devices in high-illumination environments.
BACKGROUND OF THE INVENTION
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 which have been developed for the fabrication of integrated circuits.
A digital micromirror device (DMD™), sometimes referred to as deformable micromirror device, is a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In 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 the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics were used to 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. 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: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
Consumer display applications have also been evolving as consumers have come to expect increasing image resolution and quality. For example, business projectors will soon be expected to have &mgr;XGA image resolution, that is be able to produce images resolutions of 1024 ×768 pixels. Micromirror-based display systems are difficult to scale to higher resolutions since more elements must be added to the micromirror array. For example, increasing the number of elements in the array increases the size of the device—thereby lowering the number of devices fabricated on a single semiconductor wafer and increasing the size of the projection optics.
While enlarging the size of the array entails several disadvantages, it is also difficult to reduce the size of the modulator elements in order to add elements to the array. The micromirror elements are micromechanical machines that cannot easily be scaled. Furthermore, the semiconductor substrate underneath the micromirrors is filled by a memory array that holds data for each micromirror. The existing memory cells cannot be reduced enough to fit under significantly reduced micromirrors without violating the minimum design rules governing the silicon processing of the SRAM cell. What is needed is a new memory design to support the fabrication of smaller micromirrors that provide less space to fabricate the memory cell.
SUMMARY OF THE INVENTION
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a very small static random access memory, and a display system utilizing the improved SRAM cell. According to one embodiment of the disclosed invention, a micromirror element is provided. The micromirror element comprises a semiconductor substrate, at least one memory cell form in the substrate, at least one address electrode connected to the memory cell, and at least one deflectable mirror supported by the substrate. The memory cell comprises: a first and second input/output node; and a first and second inverter. Each input/output node is electrically connected to the input of one inverter and the output of the other inverter. The address electrode is electrically connected to one of the input/output nodes. The deflectable member deflects when electrostatically attracted to the address electrode by a voltage differential between the address electrode and the deflectable member.
According to another embodiment of the disclosed invention, a memory cell is provided. The memory is comprised of: a first and second input/output node; and a first and second inverter. Each input/output node is electrically connected to the input of one comparator and the output of the other comparator. The address electrode is electrically connected to one of the input/output nodes. The deflectable member deflects when electrostatically attracted to the address electrode by a voltage differential between the address electrode and the deflectable member.
According to yet another embodiment of the disclosed invention, an image projection system is provided. The image projection system comprises: a light source for providing a beam of light along a light path, a micromirror device on the light path for selectively reflecting portions of the beam of light along a second light path in response to image data signals, a controller for providing image data signals to the micromirror device; and a projection lens on the second light path for focusing the selectively reflected light onto an image plane. The micromirror device comprises: a substrate, at least one memory cell fabricated on the substrate, an address electrode, and a deflectable member. The memory cell is comprised of: a first and second input/output node; and a first and second inverter. Each input/output node is electrically connected to the input of one inverter and the output of the other inverter. The address electrode is electrically connected to one of the input/output nodes. The deflectable member deflects when electrostatically attracted to the address electrode by a voltage differential between the address electrode and the deflectable member.
The primary advantage of the disclosed SRAM cell is its extremely compact layout. Because the entire SRAM cell, including the write transistor, only requires five transistors and two inputs—a word line and a bitline—the new SRAM cell is able to fit underneath a 13.8 &mgr;m micromirror cell. Prior DMD SRAM cells used six transistors and three inputs—a word line and a complementary pair of bitlines. The disadvantages of the five-transistor design—the difficulty writing to and reading from the memory cell—are overcome by a series of design and operating mode changes. The changes include mismatched gates on the inverter transistors to lower the latch point of the inverter, lowering the inverter supplies during the write operation, increasing the write enable signal applied
Huffman James D.
Miller Rodney D.
Ben Loha
Brady III Wade James
Brill Charles A.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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