Two-Dimensional Silicon/Ferroelectric Liquid Crystal Spatial Light Modulators
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Georgia Institute of Technology, School of Electrical Engineering and Microelectronics Research Center, Atlanta, Georgia, 30332-0250 ABSTRACT Spatial light modulator (SLM) technology based on commodity silicon fabrication processes and employing a thin, ferroelectric liquid crystal light modulating layer at the chip's surface, has advanced to the point of producing high-performance, practically useful devices. Electrically addressed display devices with 256 x 256 resolution, 100 : 1 contrast, and 10 kHz frame rate, as well as optically addressed, "smart" SLMs that perform spatio-temporal filtering on images, have resulted in the course of the author's involvement with Displaytech, Inc. The silicon technology infrastructure is a juggernaut that produces regular and significant advances in capability. An optoelectronics technology based on foundry silicon can exploit these advances directly and realize rapid improvements. The gains in the density, speed, and optical performance of the Si/FLC devices presented here, illustrate this perspective. The favorable speed-power performance of these devices is due to an excellent match in the "terminal characteristics" of FLC materials with those of fine-line MOSFETs. The impact of these characteristics on overall device performance is discussed in some depth.
INTRODUCTION Spatial light modulator (SLM) technologies have received fresh interest with the widening of applications for optical signal processing and the rising demand for smallformat displays. Wide recognition of the versatility of silicon as an electronic material and the wealth of knowledge available on it, has motivated researchers to devise SLM 1 technologies that combine silicon with various light-modulating materials and devices. It is appealing to use standard or nearly standard commodity CMOS fabrication services provided by "silicon foundries," to produce chips that realize all but the lightmodulating function of an electrically or optically addressed SLM. Post-processing steps can then be performed to incorporate a light modulating layer onto the chips to turn them into complete SLMs. The rationale for this approach is that the investment in time and money for silicon fabrication is enormous, and makes commodity silicon into a bargain that is hard to beat. The cost of using standard processes is so low compared to the alternative, that it is worth adapting one's design approach to the nature of this resource. A challenge is to accommodate those characteristics of foundry VLSI that are at odds with requirements for good SLMs. The properties of the light modulating material incorporated with the VLSI, determine the usefulness and practicability of the resulting SLM technology. Ferroelectric liquid crystals (FLCs), with switching speeds at or below 100 js, operate 100 times faster than the twisted-nematic materials used in early liquid crystal light valves. Further, their mechanical properties are convenient. Being isotropic liquids at convenient temperatures (around 1000 C), FLCs are compatible with straight
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