June 15,1991 / Vol. 16, No. 12 / OPTICS LETTERS 967

Optically addressed thresholding very-large-scale-integration/liquid-crystal spatial light modulators

David A. Jared and Kristina M. Johnson

Optoelectronic Computing Systems Center and Department of Electrical and Computer Engineering, University of Colorado,Campus Box 425, Boulder, Colorado 80309-0425

Received February 19,1991

Two 32 X 32 array optically addressed thresholding spatial light modulators consisting of a very-large-scale-integration chip and a ferroelectric liquid-crystal modulator are presented. The intensity contrast ratios rangefrom 3:1 and 11:1, and the response times range from 500 ps to 6 ms.

Recently a new class of spatial light modulator (SLM)consisting of a liquid-crystal modulator on top of avery-large-scale-integration (VLSI) chip has been de-veloped.1-7 The VLSI chip can contain combinationsof photodetectors, analog/digital electronics, and met-al pads to modulate a liquid crystal. The liquid-crys-tal material is sandwiched between the VLSI chip anda sheet of glass coated with a transparent conductor.Optically or electronically addressed devices can befabricated depending on the design of the chip. Inthis manner, it is possible to design special-purpose,well-integrated SLM's that can perform specific func-tions in optoelectronic computing architectures.These devices have the following advantages: ease ofdesign and fabrication, ease of integration with stan-dard electronics, low power requirements, relativelyinexpensive fabrication, high frame rates, and the pos-sibility of fabricating large two-dimensional arrays.

This Letter focuses on two optically addressedSLM's (OASLM's). The OASLM's consist of a 32 X32 array of phototransistors, amplifiers, and modulat-ing pads. Three OASLM's are fabricated on the samechip: two thresholding OASLM's and a logarithmicOASLM. Only the two thresholding OASLM's arediscussed here. The chip is fabricated by using the 2-,gm, n-well, low-noise analog complementary metal-oxide semiconductor (CMOS) process provided byMOSIS (the Defense Advanced Research ProjectsAgency/National Science Foundation silicon broker-age service). The three OASLM's can be seen in thephotograph of the chip in Fig. 1. A smectic C* and adistorted helix ferroelectric liquid crystal (FLC) areused as the modulating electro-optic material.8

The circuit at each pixel consists of a phototransis-tor, an amplifier, and an inverter driver as shown inFig. 2. The parasitic bipolar phototransistor P isformed by placing a p-diffusion region in an n well.The collector is the substrate, the base is the isolated nwell, and the emitter is the p diffusion. A substratecontact surrounds the phototransistor to isolate thedetector from bulk effects. Light incident upon thephototransistor generates a base current 1B and anemitter current IE = 1IB, where a is the combinedquantum efficiency and gain of the phototransistor.

On a separate test chip, fi is typically measured to beapproximately 130.

The first thresholding OASLM [Fig. 2(a)] uses twodiode-connected transistors to convert the emittercurrent into a voltage, Vp Vdd - (2kT/q)ln(IE/Io), forinput to the transconductance amplifier, where Io isthe transistor drain to source current at a zero gate tosource voltage. The output current of the transcon-ductance amplifier is IA = Ib tanh[(2q/T)(Vp - Vth)],where Ib is the current in the current source transistorQ7 controlled by the external voltage Vb and Vth is anexternal voltage. Essentially, the hyperbolic-tangentfunction produces the threshold: when Vp > Vth, IA -Ib, and when Vp < Vth, IA #-Ib. The conductance oftransistors Q4 and Qe is used to convert the current-type output to a voltage-type output. A CMOS in-verter is then used to threshold the output signal V0 ut

Fig. 1. Photograph of the chip containing three OASLM's.

0146-9592/91/120967-03$5.00/0 © 1991 Optical Society of America

968 OPTICS LETTERS / Vol. 16, No. 12 / June 15,1991

VV to the glass cover with conductive epoxy. An align-(a) La01 103 04 08 ment layer of rubbed polyvinyl alcohol is applied only

on the glass cover. The glass is positioned on the chipE -Ab~vout using a specially designed mechanical jig. The jig

consists of an X-Y translation stage for positioning08 1 3the chip and three micrometers for lowering the glass

| Cn l + z r 06- cover onto the chip. The thickness of the air gapEl Vth between the chip and the glass surface is measured by

p ̂ n , r using a Pohl interferometer. White-light fringes areV v07lrl'b also used to position the glass cover over the chip.

Once the desired thickness is achieved, the glass coveris epoxied to the chip and removed from the jig. Thechips are filled with the FLC in a vacuum. Two types

(a) 4.0

_Vout

M

.>

>0

3.0

2.0

1.0

0.0

Fig. 2. Schematics of the OASLM's: (a) the first thresh-olding OASLM, (b) the second thresholding OASLM.

1.0 2.0 3.0Vth , Volts

4.0

Intensity25 nW/cm

2

-------- 100 nW/cm2.............. 1.0 piW/cm2~~ 5.0 pN/cnr2

........ 10.0 pW/crn2

.e*---*- -20.0,uW/crr21 00.0 pW/cm2

- - - - 900.0 pW/crrfurther and to drive the liquid-crystal modulating pad.The response of the first OASLM to various inputintensities is shown in Fig. 3(a).

The second thresholding OASLM [Fig. 2(b)] usestwo current mirrors to mirror the current IE into acurrent comparator. The output current of the com-parator is IA = Ib - 2IE. Again, a CMOS inverter isused to drive the modulating pad. The only differ-ence between the circuit for the first and secondOASLM is that the photoemitter current is not con-verted into a voltage in the second OASLM. Rather,the photoemitter current is mirrored directly into acurrent comparator. The response of the secondOASLM to various input intensities is shown in Fig.3(b).

The pixels for both OASLM's are located on 100 rmX 50 ,Am centers. The phototransistor and the modu-lating pad are side by side. The n-well region of thephototransistor is 40 Arm X 36 Am. The modulatingpad is made in second-level metal. The modulatingpad is 46 /Am X 50 ,um, with a 38 rm X 42 Am cut in theoverglass on top of it. All the circuitry is locatedunderneath the modulating pad.

After the chips are fabricated through MOSIS, theFLC and the glass cover are placed on the chip. Theglass cover is 6 mm X 7 mm X 4 mm cut from a BK-7optical flat (X/10). Indium tin oxide is evaporated onone side of the glass to form the transparent electrode,and an antireflection coating is placed on the otherside. A layer of chromium is evaporated along oneedge of the glass marginally overlapping the indiumtin oxide. This allows for a wire to be easily attached

(b)

.u

0

Intensity100 nW/cm

2

------.-- 1.0 pWicm2..--*a>>- .- -a 10.0 pW/cnr

100.0 pW/crn2.. -* - 900.0 pW/crrn

Vth. Volts

Fig. 3. Voltage on the modulating pad, Vout, as a function ofthe threshold voltage Vth for various input optical intensitiesfor (a) the first OASLM and (b) the second OASLM.

View devicethrough microscope

microscopeblective

Fiber bundlelight source

Illumination frommovesbie | I ;';.. ........ microscopeopaquescreen | VLSIOASLM

Distance

Fig. 4. Experimental arrangement.

Increasingintensity

II XtI AA_

June 15,1991 / Vol. 16, No. 12 / OPTICS LETTERS 969

(a)

(b)

Fig. 5. Photograph of the response of (a) the first thresh-olding OASLM and (b) the second OASLM using a smecticC* FLC when an input intensity difference is incident uponthe device.

of FLC material are used, a smectic C* FLC (BritishDrug House SCE13) and a distorted-helix FLC (Hoff-man-LaRoche 6304). The smectic C* FLC essentiallyprovides binary modulation, while the distorted-helixFLC allows for analog modulation.

The OASLM's are tested by using a reflection,polarizating microscope. The experimental arrange-ment is shown in Fig. 4. The white-light illuminationsource from the microscope is used as both a write anda read beam. A second white-light source from anincoherent fiber bundle is also used to illuminate theOASLM's. The second source is incident upon theOASLM's at a glancing angle. Because of the glancingangle, the second source cannot be seen through themicroscope. An opaque screen is placed between thesecond source and the OASLM's to shadow an edgeacross the OASLM's. This produces a step in thewrite-beam intensity. By adjusting the thresholdvoltage, Vth, the output of the OASLM's can bethresholded on either side of the shadow. Figure 5shows the output of the OASLM's when the devicesare shadowed. In these experiments, Vdd = 5 V, andthe voltage on the glass electrode is Velectrode = 2.5 V.

The intensity contrast ratio Ion/Ioff is measured byimaging a single modulating pad on a photodiode andchanging the threshold voltage. A helium-neon laser

with X = 633 nm is used. The contrast ratios of thefirst and second OASLM's, respectively, are 5:1 and11:1 for the smectic C* FLC and 3:1 and 10:1 for thedistorted-helix FLC.

The electrical response time of the OASLM's ismeasured by focusing all the light from the array ontoa photodiode and driving the threshold voltage with asquare wave with an amplitude of 4.9 V. The 10% to90% response time is measured. The devices exhibitan asymmetry in the on and off response times. Theon response times of the first and second OASLM's,respectively, are 3 and 6 ms for the smectic C* FLCand 1.25 and 1.4 ms for the distorted-helix FLC. Theoff response times of the first and second OASLM's,respectively, are 1.7 and 4 ms for the smectic C* FLCand 500 and 500 ,m for the distorted-helix FLC.

The optical response time of the OASLM's is mea-sured similarly to the electrical response time, but theread and write beams are modulated by a chopper.Because of this optical arrangement, only the on or offresponse time can be measured. The optical responsetimes measured are identical to the electrical responsetime.

In conclusion, two thresholding optically addressedspatial light modulators consisting of a FLC modula-tor on top of a VLSI chip are presented. The devicesconsist of a 32 X 32 array of pixels on 50 ,m X 100 ,mcenters. A smectic C* and distorted-helix FLC areused. The intensity contrast ratios range from 3:1 to11:1, and the response times range from 500 Ms to 6 ins.

Discussions with Tim Slagle, Rich Turner, and Kel-vin Wagner were enjoyed while developing this pro-ject. David Doroski and Chongchang Mao helpedsubstantially in filling the chips with the FLC. Thisresearch was supported by the National Science Foun-dation Engineering Research Centers for Optoelec-tronic Computing Systems (grant CDR862228) andthe Martin Marietta Corporation. Graduate Fellow-ship support from NASA-Johnson Space Flight Cen-ter for David A. Jared is gratefully acknowledged.

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