Guardring DRAM cell

Details Guardring DRAM cell Guardring DRAM cell

2001-05-30

2002-02-05

Epps, Georgia (Department: 2873)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S297000, C257S313000

Reexamination Certificate

active

06344672

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.
Due to the extreme environments in which micromirror devices are operated, photogenerated carriers in the semiconductor substrate are a concern to the stability of a charge storage device such as a DRAM. Attempts to eliminate the photogenerated carriers have focused on metal light shields to prevent photons from reaching the silicon substrate and creating photocarriers. While largely successful, metal shields require an additional metal layer on the surface of the micromirror devices. This metal layer that not only drives up the processing and cost associated with the fabrication of micromirrors, but also lowers the yield of the micromirror fabrication process. What is needed is a better system and method for eliminating or minimizing the effects of photocarriers in micromirror substrates.
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 an improved DRAM cell for use in a high-intensity ambient light environment, a method of forming the improved DRAM cell, and a display system utilizing the improved DRAM cell. According to one embodiment of the disclosed invention, a micromirror device is provided. The micromirror device comprises a substrate, at least one memory cell, at least one electrode, and at least one deflectable member. An active collection region forms the bottom plate of the capacitor and acts as a guardring that recombines photocarriers before they reach the address node between a transistor and capacitor in the memory cell.
According to one embodiment of the disclosed invention, the active collector region is an n-doped semiconductor region encircling said address node. Alternatively, the active collector region is a p-doped semiconductor region on an n-type substrate. The address node typically is located directly beneath the deflectable member on the substrate to take maximum advantage of the shielding provided by the deflectable member.
According to yet another embodiment of the disclosed invention, a memory cell is provided. The memory cell comprises a substrate, at least one transistor, at least one capacitor, an address node connecting the at least one transistor, and an active collector region. The active collector region is fabricated in the substrate in a position to recombine photocarriers traveling through the substrate from reaching the address node. According to one embodiment of the disclosed invention, the active collector region encircles the address node and forms the bottom plate of the capacitor. The active collector region typically is an n-doped semiconductor region on a p-type substrate, but alternatively is formed by a p-doped semiconductor region on an n-type substrate.
According to yet another embodiment of the disclosed invention, an image projection system is provided. The image projection system comprises a light source, a micromirror device, a controller, and a projection lens. The micromirror device is positioned on a light path and selectively reflects portions of a beam of light along a second light path in response to image data signals. The controller provides the image data signals to the micromirror device, and the projection lens focuses the selectively reflected light onto an image plane. The micromirror device comprises a substrate, at least one memory cell, an active collector region, at least one address electrode, and at least one deflectable member. The active collector region positioned to block photocarriers traveling through the substrate from reaching an address node between a transistor and capacitor in the memory cell.
The memory cell described above provides or enables several improvements to the conventional micromirror device. First, the use of a DRAM cell reduces the number of transistors required to from a memory cell from five to only one. Eliminating four transistors from each memory cell greatly reduces the number of interconnections that must be formed to allow the memory cell to operate. Because there are less interconnections, one metal interconnect layer can be eliminated. Eliminating an entire metal layer reduces the cost of the device, but also reduces the light shielding provided by the interconnections. Nevertheless, the increase light immunity provided by the DRAM cell described herein enables the use of a DRAM cell in a micromirror device in spite of the reduced shielding provided by the remaining interconnections.
The elimination of four transistors also reduces the physical size of the memory cell. The small memory cell enables the use of smaller mirror elements. The smaller mirror elements provide increased image resolution for a given micromirror array size.
The new DRAM design provides a 3-4× improvement in light immunity. Light immunity is measured by the time the capacitor can hold an effective potential across the plates of the capacitor. Since the retention o

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