March 1, 1990 / Vol. 15, No. 5 / OPTICS LETTERS
`
`291
`
`Ferroelectric-liquid-crystal/silicon-integrated-circuit spatial
`light modulator
`
`L. K. Cotter
`
`Displaytech, Inc., 2200 Central Avenue, Boulder, Colorado 80301
`
`T. J. Drabik
`School of Electrical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
`
`R. J. Dillon and M. A. Handschy
`Displaytech, Inc., 2200 Central Avenue, Boulder, Colorado 80301
`
`Received October 2, 1989; accepted December 26, 1989
`We present the design and characterization of a spatial light modulator (SLM) comprising a ferroelectric-liquid-
`crystal light-modulating layer on top of a silicon integrated circuit. Our SLM consists of two electrically addressed
`arrays on the same integrated-circuit die. The first, a 1 x 128 linear array with a 20-jim center-to-center element
`spacing, used shift register addressing, while the second, a 64 X 64 square array with 60-jim pitch, used static random
`access memory addressing. The resulting SLM could be addressed at frame rates of up to 4.5 kHz and gave single-
`element intensity contrast ratios of 12:1.
`
`A high-performance spatial light modulator (SLM) is
`the key component for many proposed optical com-
`puters. The SLM, a typically two-dimensional array
`of light modulators, spatially encodes data bits onto a
`light beam as an image. It is by processing these
`entire image frames in rapid succession that the com-
`puters achieve high speed. Consequently, the frame
`rate of the SLM limits overall computing speed. We
`present here a SLM that exploits the low-voltage, low-
`power, fast electro-optic properties of ferroelectric liq-
`uid crystals (FLC's) to attain a high frame rate. We
`put a FLC light-modulating layer directly atop a sili-
`con integrated circuit (IC), using functional elements
`of the IC to apply the required driving voltages to the
`FLC layer.
`Lee et al.1 use a similar approach with
`(Pbl-xLax)(ZryTil- ) 1-./4O 3 (PLZT) light modulators,
`but the problems of integrating the light modulator
`with the silicon circuitry are much more difficult for
`the ceramic PLZT than for the organic liquid FLC.
`FLC's are directly compatible with standard IC pro-
`cessing, as has been demonstrated by Drabik and Gay-
`lord, who fabricated FLC light modulators on a silicon
`die processed according to standard complementary
`metal oxide semiconductor (CMOS) steps but con-
`taining no functional circuitry.2 Picart et al. 3 have
`applied both nematics and FLC's to circuits with very-
`large-scale integration (VLSI) to obtain average and
`real-time visualization of circuit voltages. McKnight
`et al. 4 have demonstrated small SLM's using the slow-
`er nematic liquid crystals on a static random access
`memory (SRAM) IC. Nematics on silicon IC's were
`first used to make miniature displays.5' 6 Our FLC/IC
`hybrid approach, which was independently proposed
`by Collings et al., 7 who called it the "active-back-
`plane" SLM, permits frame times equal to the FLC
`
`switching time independent of the number of elements
`in the SLM, in contrast to commonly used passive
`matrix addressing techniques7 whose line-at-a-time
`character will likely limit frame rates to the order of
`100 Hz. 8
`To exploit the electro-optic properties of FLCs most
`advantageously, we prefer the surface-stabilized FLC
`geometry,9' 10 where the FLC is confined between elec-
`trode plates spaced closely enough to unwind the
`FLC's intrinsic helical structure permanently. Figure
`1(a) shows our hybrid SLM with the FLC layer be-
`tween an IC (the bottom plate) and a glass coated on
`its inner surface with an indium tin oxide transparent
`electrode (the top plate). Metal electrode pads on the
`IC upper surface define reflective SLM elements.
`The FLC film is optically uniaxial with birefringence
`An - 0.15; voltages applied between an IC pad and the
`
`(a)
`
`(b)
`
`integrated-circuit die
`
`WORD
`
`Fig. 1. (a) Construction of the FLC/silicon IC SLM. (b)
`Schematic diagram of the SLM square-array SRAM ele-
`ment. The element's state is changeable only when the
`WORD line is high; otherwise the static latch formed by the
`cross-coupled inverters holds the state last determined by
`the BIT and iiT lines. The latch output is buffered by the
`third inverter onto the FLC driving pad.
`
`0146-9592/90/050291-03$2.00/0
`
`© 1990 Optical Society of America
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`FNC 1024
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`OPTICS LETTERS / Vol. 15, No. 5 / March 1, 1990
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`glass common electrode select according to their sign
`between two FLC optic-axis orientations, both lying
`nearly parallel to the plates but differing in direction
`by a characteristic angle 0 that can be -45' by appro-
`priate choice of FLC material and driving voltage.
`We performed intensity modulation with this switch-
`able wave plate by illuminating it with polarized light
`whose polarization direction was parallel to the optic
`In this state the
`axis of one of the FLC's states.
`reflected light has the incident polarization and is
`rejected by a crossed analyzer following a beam split-
`In the other FLC state the inci-
`ter (the dark state).
`dent light is resolved into ordinary and extraordinary
`modes, ideally of equal amplitude (0 = 45°). The FLC
`film introduces a phase shift between these modes; by
`a choice of the FLC film thickness d such that the
`phase shift is 7r, i.e., And/X = 1/4, the reflected light
`will be linearly polarized at an angle 20 to the incident
`polarization and hence passed by the analyzer (the
`bright state).
`We put two electrically addressed arrays on the
`same IC die. The first, a 1 X 128 one-dimensional
`array with a 20-Mm center-to-center element spacing,
`used shift register addressing, while the second, a 64 X
`64 two-dimensional array with a 60-Mim pitch, used
`SRAM addressing. The addressing circuitry was op-
`timized for a high frame rate. A single cell of the
`in Fig. 1(b),
`schematically
`SRAM array, illustrated
`comprises a six-transistor static RAM cell" augment-
`ed with an inverter to isolate the load of the switching
`electrode from the cell's state. This permits latching
`of a state into a cell in a few nanoseconds without
`waiting a number of microseconds for the liquid crys-
`tal driven by the cell to complete switching. Addi-
`tional provisions for speed in the two-dimensional de-
`vice design are a byte-wide write operation to reduce
`the number of write cycles required per frame and a
`row-page writing mode. In the row-page mode, the
`WORD line selecting a row is held high while all eight
`bytes in that row are written; therefore the WORD line
`need be toggled high and low only once per written
`row. Inclusion of these design features has led to the
`capability of writing a complete frame into the two-
`dimensional device in less than the switching time of
`the FLC material used; thus the cell outputs can be
`made to change nearly in unison. In the one-dimen-
`sional device the shift register cells were also buffered
`from the FLC material with extra inverters. This
`permits clocking of the shift register a high speeds
`(-10 MHz) and therefore the loading of a new 128-
`long frame in less than the switching time of the FLC
`cells.
`The IC was designed by us and fabricated at MO-
`SIS,12 the Defense Advanced Research Projects Agen-
`cy/National Science Foundation silicon brokerage ser-
`in a 3-Mtm CMOS dou-
`vice. The chip was fabricated
`ble-metal p-well process and could provide 0- or 5-V
`drive to the reflective pads. By connecting the glass
`common electrode to 2.5 V, either plus or minus 2.5 V
`could be applied to the FLC. The functional cell
`in the lower levels of the IC was made from
`circuitry
`field-effect transistors of doped silicon and oxide lay-
`ers, interconnected by conductors in added polysilicon
`and first-metal layers. Each cell drives a reflective
`
`pad in a second-level metal layer. In the final layer of
`sputtered passivation glass we specified an opening
`over each reflective pad, which provided both electri-
`cal contact between each element's electrode and the
`FLC and part of the spacer layer separating the pad
`from the top electrode plate of the completed device.
`The elements were on 60-mm centers, with square sec-
`ond-metal pads 55 Am on a side centered under open-
`ings 45 ,im on a side in the passivation glass.
`After testing the electrical function of the IC's, we
`assembled the chips into complete FLC cells. We
`applied a rubbed nylon alignment layer to a clean
`indium tin oxide-coated glass plate and soldered on a
`fine wire to permit connection to the indium tin oxide.
`After dusting off the IC with a gentle, filtered static-
`free air stream, we laid the glass plate directly on the
`die, with the glass narrow enough to fit between the
`rows of wire bonds on the two opposing edges of the die
`and long enough to permit the soldered wire to hang
`over the edge of the die. The assembly was clamped
`together and filled under vacuum with liquid crystal
`ZLI-4003.13 Our first SLM's were held together only
`by the liquid crystal's surface tension. This technique
`proved successful, although to preserve the cell paral-
`lelism for long periods we found it necessary to glue
`the glass plate to the IC at the expense of small por-
`tions of the arrays' being lost owing to glue flow be-
`tween the glass plate and the IC. A number of perma-
`nent bonding techniques have been used to join sub-
`strates in situations with requirements and conditions
`similar to ours14; we anticipate little difficulty in
`adapting a bonding technique to the production of
`silicon/FLC devices.
`Figure 2 shows a microphotograph of the square
`array. The surface of the square array is much like
`that of a waffle, with the passivation glass overlayer
`forming the ridges and the metal reflectors sitting at
`the bottom of the FLC-filled wells. The highest
`points on the ridges, over the addressing lines, contact
`and support the glass plate. Judged by comparing
`their largely yellow color between crossed polarizers
`
`Fig. 2. Reflected light microphotograph of a portion of the
`64 X 64 square array between crossed polarizers. The array.
`has been oriented so that the FLC state in half the elements,
`has its optic axis parallel to the polarizer. These elements
`appear dark, while the other elements appear bright, their
`FLC axis making an angle -25° to the polarizer. The ele-
`ments are on 60-Am centers.
`
`

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`March 1, 1990 / Vol. 15, No. 5 / OPTICS LETTERS
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`293
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`would drop to 500 Hz, while if FLC dissipation were
`the only limit, 70 kHz would be allowable. However,
`power dissipation by the silicon active backplane is
`likely to be more limiting than dissipation from the
`FLC.7 FLC/IC SLM's of a quality satisfactory for
`optical processors will also require further research to
`produce flat arrays with optically uniform elements,
`which can be based in part on IC planarization pro-
`cesses.' 6
`This research was supported by a contract from the
`Defense Advanced Research Projects Agency. T. J.
`Drabik was supported in part by a grant from the Joint
`Services Electronics Program under contract
`DAAL03-87-K-0059.
`
`References
`
`1. S. H. Lee, S. C. Esener, M. A. Title, and T. J. Drabik,
`Opt. Eng. 25, 250 (1986).
`2. T. J. Drabik and T. K. Gaylord, in Technical Digest of
`the 1988 Optical Society of America Annual Meeting
`(Optical Society of America, Washington, D.C., 1988), p.
`99.
`3. B. Picart, L. Dugoujon, 0. Petit, C. Destrade, C. L6on, H.
`T. Nguyen, and J. P. Marcerou, Proc. Soc. Photo-Opt.
`Instrum. Eng. 1080, 131 (1989).
`4. D. J. McKnight, D. G. Vass, and R. M. Sillitto, Appl.
`Opt. 28, 4757 (1989).
`5. K. Kasahara, T. Yanagisawa, K. Sakai, T. Adachi, K.
`Inoue, T. Tsutsumi, and H. Hori, in Conference Record
`of the 1980 Biennial Display Research Conference (In-
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`York, 1980), p. 96.
`6. W. A. Crossland, B. Needham, and P. W. Ross, Proc.
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`7. N. Collings, W. A. Crossland, P. J. Ayliffe, D. G. Vass,
`and I. Underwood, Appl. Opt. 28, 4740 (1989).
`8. P. W. Ross, in Conference Record of the 1988 Interna-
`tional Display Research Conference (Institute of Elec-
`trical and Electronics Engineers, New York, 1988), p.
`185.
`9. N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36,899
`(1980).
`10. K. Skarp and M. A. Handschy, Mol. Cryst. Liq. Cryst.
`165, 439 (1988).
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`Analysis of VLSI Circuits (Addison-Wesley, Reading,
`Mass., 1985), p. 390.
`12. MOS Implementation Service, University of Southern
`California, Information Sciences Institute, 4676 Admi-
`ralty Way, Marina del Ray, Calif. 90292-6695.
`13. EM Chemicals, 5 Skyline Drive, Hawthorne, N.Y. 10532.
`14. C. D. Fung, P. W. Cheung, W. H. Ko, and D. G. Fleming,
`eds., Micromachining and Micropackaging of Trans-
`ducers (Elsevier, Amsterdam, 1985).
`15. M. A. Handschy, K. M. Johnson, G. Moddel, and L. A.
`Pagano-Stauffer, Ferroelectrics 85, 279 (1988).
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`Electrical and Electronics Engineers, New York, 1984).
`
`Fig. 3. Higher-magnification microphotograph of the
`square array showing structure within the elements due to
`the underlying circuitry topography.
`
`with FLC cells of known thickness, the elements in the
`two-dimensional array have a FLC film thickness of
`approximately 1 ttm. Structure visible within the
`bright elements in the square array (Fig. 3) is due to
`the underlying circuitry, which makes the metal re-
`flector bumpy; these bumps cause variations in the
`thickness of the FLC layer that result in the observed
`variations of appearance within each element. In the
`linear array (not shown) all circuitry is off to the side
`and the reflectors themselves are flat with no underly-
`ing structure.
`We measured intensity contrast ratios of 12:1 on
`single elements within the square array using He-Ne
`laser light whose 633-nm wavelength should have been
`close to the transmittance peak indicated by the cells'
`yellow color. To measure the SLM's frame rate we
`drove the square array alternately all bright and all
`dark; the highest frequency attained without attenua-
`tion of the peak-to-peak optical response was 2.25
`kHz, for a frame rate of 4.5 kHz. The optical rise and
`fall times were FLC limited at 120 Msec. At this frame
`rate the 64 X 64 SLM has a data throughput of 18
`Mbit/sec.
`In conclusion, we have demonstrated how two rela-
`tively mature technologies, silicon VLSI and FLC's,
`can be simply combined to make a hybrid SLM. The
`FLC light modulator's low drive voltage requirements
`makes it a good match to the characteristics of the
`standard CMOS IC. Its low power dissipation, unlike
`that of most light-modulation technologies, prevents
`thermal dissipation from limiting the frame speed.
`For example, even allowing for 10-W/cm 2 dissipation,
`an efficient PLZT/IC SLM is estimated to be thermal-
`ly limited to a 50-kHz frame rate'; a FLC SLM could
`be framed 10 times more frequently.' 5 At a thermal
`dissipation level that could be achieved without spe-
`cial measures, say 100 mW/cm2, the PLZT frame rate