Patent Publication Number: US-RE39642-E

Title: Spatial light modulator and method

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to spatial light modulators, and more particularly to improvements therein which result in low voltage, high speed devices with high contrast ratios. 
     PLZT (lanthanum modified lead zirconium titanate) is well known for use in spatial light modulators (SLMs), or “light valves”. U.S. Pat. No. 5,198,920 by the present inventor gives general background on spatial light modulators. Also see the article “Photoconductive Activated Light Valve for High Definition Projection System”, by Garth Gobeli and Thomas Toor, 172/SPIE vol. 1664 High-Resolution Displays and Projection Systems (1992). 
       FIG. 1  shows a prior art spatial light modulator formed in a relatively thick (e.g., 200 microns) substrate  10  of PLZT. On the top surface  15 A there are formed various positive electrodes, for example, positive electrodes  11  and  13 , and various negative electrodes, such as negative electrodes  12  and  14 , interspaced between the positive electrodes. The opposite voltages applied to the positive and negative electrodes produce “fringing” electric fields  15  between the positive and negative electrodes underneath upper surface  15  of PLZT substrate  10 . The presence or absence of the fringing fields  15  modulate or control the amount of light that can pass through the spatial light modulator or light valve  1 . 
     A substantial problem of the above prior art of  FIG. 1  is that the fringing electric fields  15  are relatively nonuniform. This non-uniformity results in the necessity of applying large voltage differences, usually more than 60 volts, between the positive and negative electrodes in order to achieve the desired level of light modulation. It would be very desirable to be able to use lower electrode voltage differences of less than 60 volts. The non-uniformity of the fringing electric field also means that memory mode material cannot be employed as a substrate. 
     Very thin layers of PLZT material of thickness in the range from 0.01 to 0.08 microns have been fabricated by depositing PLZT material onto a suitable substrate, using sputtering or liquid phase deposition techniques. Such very thin PLZT films require high activation voltages which lie well to the left of the minimum “A” shown in the curve of “PLZT operating voltage versus PLZT layer thickness” shown in subsequently described FIG.  8 . Also, PLZT substrates with thicknesses in the 100 to 200 micron range have been fabricated by conventional grinding and polishing techniques, but the substrates of this thickness range require very high operating voltages that lie far to the right of the minimum “PLZT operating voltage versus PLZT layer thickness” shown in the curve of FIG.  8 . 
     There would be a great many applications in the fields of optical computing, optical projectors, and large dynamic range cameras, for a two-dimensional spatial light modulator array in which each spatial light modulator cell operates with voltages less than approximately 60 volts, at operating speeds of more than approximately one million operations per second, and with a contrast ratio of greater than approximately 512 to 1. 
     Until now, no available or proposed spatial light modulator has been capable of meeting all three of these operating objectives. For example, typical liquid crystal devices (LCDs) operate at low voltages (less than 5 volts) but are quite slow, typically switching at about 30 frames per second, and have a low contrast ratio, typically about 12 to 1. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a spatial light modulator capable of operating at low voltages of less than approximately 60 volts. 
     It is another object of the invention to provide a spatial light modulator which can be directly driven by silicon LSI or VLSI integrated circuits. 
     It is another object of the invention to provide a spatial light modulator having high operating speed capability which allows more than approximately one million switching or modulation operations per second. 
     It is another object of the invention to provide a spatial light modulator having a device contrast ratio which is higher than approximately 512 to 1. 
     It is another object of the invention to provide a spatial light modulator which has a “modulation depth” that is continuously or nearly continuously attainable, to at least 1 point in 512 so as to have at least 512 distinguishable gray scale contrasts. 
     It is another object of the invention to provide a spatial light modulator with a layer of solid state electro-optical material having an optimum thickness that results in an optimally low activation voltage. 
     It is another object of the invention to provide a practical technique for making a layer of PLZT of optimum thickness as to achieve optimally low activation voltage in a spatial light modulator. 
     Briefly described, and in accordance with one embodiment thereof, the invention provides a spatial light modulator including a thin layer ( 10 A) of solid-state electro-optical material having parallel, opposite first ( 15 A) and second ( 15 B) surfaces, an elongated first electrode ( 22 ) disposed on the first surface and generally oriented in a first direction, and a plurality of spaced, elongated second electrodes ( 23 ) disposed on the second surface and generally oriented in a second direction. Regions in the electro-optical material between portions of the first electrode and portions of the second electrodes define pixel regions ( 17 ) through which light selectively passes in response to differences between control voltages applied to the first electrode and the plurality of second electrodes, respectively. In the described embodiment, the first electrode is of serpentine shape, with “pixel-defining” first portions (B) of the first electrode being oriented in the second direction and second portions (A) of the first electrode located between the first portions being oriented in the first direction, which in one described embodiment is perpendicular to the second direction. The layer of solid-state electro-optical material has a thickness in the range of approximately 5 to 15 microns ((micrometers), and is composed of PLZT. In one embodiment, a plurality of thin regions ( 25 ) of insulator material are disposed on the first surface between the second portions (A) of the first electrode and the electro-optical material. In one embodiment, the spatial light modulator includes a plurality of the first electrodes ( 22 ), the first electrodes and the second electrodes defining a rectangular array of pixel regions ( 17 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view diagram of a typical prior art PLZT spatial light modulator. 
         FIG. 2  is a partial sectional view of a “penetrating field” spatial light modulator of the present invention. 
         FIG. 3  is a plan view illustrating a one-dimensional spatial light modulator of the present invention. 
         FIG. 4  is a plan view of a two-dimensional spatial light modulator according to the present invention. 
         FIG. 4A  is a partial section view of the spatial light modulator shown in FIG.  4 . 
         FIG. 5  is a partial section view of a particular type of PLZT spatial light modulator of the present invention using fringing fields in conjunction with transparent electrodes used to modulate the fringing fields by retarding or enhancing them. 
         FIG. 6  is a diagram of a device that can be used in conjunction with grinding and polishing techniques used to provide thin PLZT layers in accordance with the present invention. 
         FIG. 6A  is a diagram of an interferometric system for measuring the thickness of the PLZT layer being lapped or polished using the device of FIG.  6 . 
         FIG. 6B  is a diagram useful in explaining the operation of system of FIG.  6 A. 
         FIG. 7  is a partial section view illustrating a “stacked” or “complex” spatial light modulator having transmission properties which are the combined transmission properties of the stacked individual spatial light modulators. 
         FIG. 8  is a diagram illustrating a plot of the spatial light modulator operating voltage versus PLZT layer thickness when it is configured as a Kerr cell. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  illustrates the basic improvement of the present invention, which includes a PLZT layer that is 5-15 microns thick, which is much more of an optimum thickness than in the most relevant prior art of which the applicant is aware. Obtaining PLZT layers of thickness in this range is very problematic because such a thin wafer can not be handled without considerable risk of breakage. However, grinding and polishing techniques can be used in accordance with the present invention to obtain such thin wafers, which then are attached onto a thicker, more rigid, transparent substrate, such as sapphire, indicated by dashed lines  20  in  FIG. 2 , to provide structural rigidity. 
     In accordance with the present invention, the thinness of PLZT layer lOA is what allows electrodes to be placed on both the top surface  15 A and the bottom surface  15 B of PLZT wafer lOA so as to produce effective “penetrating” electrical fields while maintaining inter-electrode spacings sufficiently small that operating voltages do not exceed 60 volts. (Operating voltage is defined as the voltage that when applied to the device will produce a polarization rotation of 90 degrees and consequently will provide the maximum attainable transmission through a Kerr cell.). Specifically, electrodes  22  are located on the top surface of PLZT layer  10 A, and negative electrodes  23  are located on the bottom surface. The electrode patterns can be formed by conventional photolithographic processes similar to those used in the semiconductor industry to pattern the various layers on integrated circuits. 
     As a practical matter, the lower electrodes  23  may be initially patterned on thick substrate  20  before PLZT layer lOA is attached to substrate  20 , and the upper electrodes  22  can be patterned on the upper surface  15 A after PLZT layer lOA is affixed to thick substrate  20  and then lapped and polished using the device of  FIG. 6  to obtain the desired 5-15 micron thickness of PLZT layer  10 A. 
     In any case, the provision of both upper electrodes  22  and lower electrodes  23  on a thin PLZT substrate of 5-15 micron thickness results in “penetrating” electric fields  15 A that extend all the way through the PLZT wafer  10 A, instead of “fringing” fields  15  as shown in prior art FIG.  1 . The “penetrating” fields  15 A are much more uniform than the fringing fields  15  in FIG.  1 . Furthermore, the thinness of PLZT layer lOA results in the advantage that the top electrodes  22  and the bottom electrodes  23  can be placed more closely together on the top surface  15 A and the bottom surface  15 B, respectively. The electrode spacing on a surface of a penetrating field type of spatial light modulator must be fairly close to the thickness of the layer of light medium, i.e., the PLZT layer  10 A. Consequently, lower ioltages are required to produce a polarization rotation of 90 degrees and hence the maximum attainable Kerr cell transmission. 
     For the structure shown in  FIG. 2 , my computations indicate that with a PLZT layer thickness of 8 microns, upper electrodes  22  are 2 microns wide and spaced 18 microns apart, and bottom electrodes  23  are 2 microns wide and spaced midway “between” pairs of top electrons  22  as illustrated, for applied “operating voltage” between the (+) electrodes  22  and the (−) electrodes  23  of less than 60 volts to be sufficient to modulate the penetrating fields  15 A enough to produce 90 degrees of “rotation” of light passing through PLZT layer  15 A. Although I have not yet obtained measurements of the activation or operating voltages of the structure of  FIG. 2  with the above-indicated dimensions, I believe, on the basis of both my computations and experiences to date, that whenever I do obtain such measurements, the activation voltage for the optimum PLZT layer thickness may be as low as 30 volts, and perhaps even as low as 15 volts. 
       FIG. 3  illustrates a “plan view” of a “one-dimensional” arrangement of electrodes  22  and  23  of FIG.  2 . The thin PLZT layer  10 A is omitted for convenience of illustration, and narrow lines are drawn to indicate the electrodes, which actually have a significant width, as seen in the section view of FIG.  2 . In  FIG. 3 , top conductor  22  extends in a snakelike pattern on the top surface of PLZT wafer  10 A, and the various electrodes  23  are formed on the bottom surface  15 B of PLZT wafer  10 A, and are arranged to “fan out” in the directions on opposite sides of conductor  22 . This “fan out” has the advantage of allowing use of conveniently sized and conveniently located contact or bonding pads. 
     The array shown in  FIG. 3  can be made “two-dimensional” as shown in  FIGS. 4 and 4A  by providing a number of parallel top electrode traces  22 - 1 . 2  . . . M that run parallel to but are spaced from each other on the top surface  15 A, and by extending the bottom electrodes  23 - 1 . 2  . . . N so they are “shared” among the parallel top electrode traces  22 . 
     More specifically, in  FIGS. 4 and 4A , a number of generally horizontal serpentine electrodes  22 - 1 . 2  . . . M are provided in parallel relationship as shown in  FIG. 4  on the top surface  15 A of the thin (5-15 microns) PLZT layer  10 . On the top surface  15 B of thin PLZT layer  10 A, a plurality of straight horizontal strips  25 - 1 . 2  . . . M of oxide composed of spin-on glass, sputtered SiO 2 , or other appropriate insulating layer are provided, so as to partially span the “U” or “inverted U” shaped portions of electrodes  22 - 1 . 2  . . . M, respectively. A plurality of vertical metal strips  23 - 1 . 2  . . . N are formed on the bottom surface  15 B of the thin PLZT layer  10 A after the oxide strips  23 - 1 . 2  . . . N. The oxide strips  25 - 1 . 2  . . . M prevent the “horizontal” portions “A” of electrodes  23 - 1 . 2  . . . N from contacting the PLZT material. 
     The “vertical” portions “B” of electrodes  23 - 1 . 2  . . . N define pixel areas or regions  17  of PLZT layer  10 A through which light may pass under control of voltages applied to electrodes  22  and  23 , respectively. 
     The purpose of the oxide strips  25 - 1 ,  25 - 2 , . . .  25 -M is to provide an insulated region under-lying all of the horizontal portions A of the “snake-shaped” upper electrodes  22 - 1 . 2  . . . M This serves the purpose of preventing significant electric fields and associated “cross talk” from occurring between these horizontal portions of the electrode structure and the lower, y-oriented, electrodes. The upper structure thus provides contact to PLZT layer  10 A only along the y-oriented portion of its total length. Thus, the oxide strips  25 -M and the “snake-shaped” electrodes  22 - 1 . 2  . . . M are on the same (top) surface of PLZT layer  10 A, with the oxide being deposited first, and with portions of the snake-shaped electrodes  22 - 1 . 2  . . . M on the oxide strips  25 - 1 . 2  . . . M, respectively. The oxide strip layers,  25 - 1 . 2  . . . M in  FIG. 4 , are deposited onto the top surface of the PLZT. The snake-shaped electrodes,  22 - 1 . 2  . . . M are then deposited onto the top surface of PLZT layer  1 OA such that the horizontal (x-oriented) segments thereof lay on top of the oxide layers  25 - 1 . 2  . . . M, respectively. The only portions of the electrodes  22 - 1 . 2  . . . M that contact the PLZT substrate  10 A are the vertical (y-oriented) segments. The bottom side electrodes  23 - 1 . 2  . . . N are then deposited so that they are positioned equidistant from adjacent vertical top surface electrode segments. 
     The only electric fields that penetrate the PLZT layer  10 A are between the back surface electrodes and the vertical (y) segments of the top surface electrodes. The electric fields between the horizontal (x-oriented) segments and the back surface electrodes are scre(end out by the intervening oxide layers. 
     This set of masks can also be employed to fabricate a one-surface-only spatial light modulator device as follows: 
     (a) First, the snake-shaped electrode structures  22 - 1 . 2  . . . M are deposited. (b) Next, the oxide strips  25 - 1 . 2  . . . M are deposited in the positions illustrated so that they overlay the horizontal x-oriented segments of the top surface electrodes  22 - 1 . 2  . . . M, respectively. (c) Finally, the y-oriented electrodes  23 - 1 . 2  . . . N are deposited onto the same surface of the PLZT layer  10 A in the positions illustrated. 
     This configuration depends on utilizing the fringing electric fields between the electrodes  23 - 1 . 2  . . . N and the vertical segments of the initially deposited snake-shaped electrodes  22 - 1 . 2  . . . M to activate the spatial light modulator. 
     In the spatial light modulator shown in  FIGS. 3 and 4 , the widths of the serpentine top conductors  22 - 1 . 2  . . . M might be 2 to 4 microns, and the center-to-center spacing of each SLM cell might be 20 to 25 microns. The width of the bottom electrodes  23  might be 2 to 4 microns. 
       FIG. 8  shows a graph of PLZT operating voltage applied between the (+) and (−) electrodes as a function of the thickness of the PLZT layer  10 A. The PLZT operating voltage is the voltage require to produce a phase change of 90 degrees in the light propagating through the spatial light modulator. There is a minimum at the PLZT layer thickness indicated by “A”, because as the thickness of the PLZT layer  10 A decreases, the electric field intensity of the “penetrating” electric field produced between the (+) and (−) electrodes increases, increasing the amount of phase shifting. However, there is an opposing effect in that the decreasing thickness of PLZT layer  1 OA results in less distance through which the light can propagate, and this tends to decrease the amount of phase shift produced. These opposing phenomena result in an “optimum” thickness. 
       FIG. 6  indicates how the 5-15 micron thick layer  10 A of PLZT material can be produced. For example, a thick piece of PLZT material can be bonded onto a pyrex microscope slide  46  or the like using a suitable epoxy compound. The pyrex slide  46  is then mounted on a movable piston  34  disposed within a jig or cylinder  36 . The position of piston  34  within the jig  36  is established by a number of adjustable positioning assemblies each including a stiff compression spring  35  and an adjustment screw  39  (or equivalent mechanical adjustment device) that adjusts the position of piston  34  within jig  36 . One end of compression spring  35  exerts a downward force on piston  34 . The opposite end of compression spring  35  exerts an upward force against top member  36 A of jig  36 . Each adjusting screw  39  extends through a corresponding clearance hole  40  in the top member  36 A, through a corresponding compression spring  35 , into a corresponding threaded hole  41  in piston  34 . The enlarged heads of adjusting screws  39  abut the top surface of top member  36 A, and act as limit stops to limit the extended lapping or polishing of PLZT layer  10 A by lapping/polishing wheel  37 . The uniformity of the thickness of PLZT layer  10 A can be adjusted by means of adjusting screw  39 . 
     The assembly is used to press the exposed surface of the PLZT layer  10  against the grinding surface of a metallographic lapping wheel  37 . A suitable grinding or polishing compound or grit or substance is provided to obtain the desired amounts of grinding or polishing during various phases of the lapping procedure. The adjusting screws  39  are individually adjusted to control the tilt and pressure of the surface of PLZT layer  10 A being lapped or polished. When a suitable surface finish is obtained, that surface can be bonded to a suitable substrate, such as a sapphire substrate having “bottom” electrodes  23  already formed thereoni. The opposite face of the sapphire substrate then could be bonded to a suitable pyrex slide, positioned on piston  34 , and the lapping/polishing procedure could be repeated until the desired 5-15 micron thickness of the PLZT layer  10  is achieved. Further processing to provide the top electrodes  22  then could be performed. 
       FIG. 6A  schematically illustrates how the amount of adjustment by means of screws  39  in  FIG. 6  can be determined to provide the distribution of pressure on the surface of PLZT layer  10 A needed to achieve uniform thickness of PLZT layer  10 A as lapping or polishing progresses. The entire assembly  36  shown in  FIG. 6  can be removed from lapping wheel  37  and positioned in an Michelson interfero;meter set-up  61  including a light source  53 , a micrometer-adjustable reference mirror  52 , a beam splitter  55  and a compensator  58 . Rays such as ray  54  from light source  53  are split by beam splitter  55 . Part of ray  54  is reflected as ray  54 A to reference mirror  52 . Part of ray  54  passes through beam splitter  55  and emerjes as ray  54 B. Ray  54 B passes through compensating element  58  of thickness and composition identical to beam splitter  55  and emerges as ray  54 C. Ray  54 C then impinges on the surface of the PLZT layer  10 A being polished. Ray  54 C is reflected as ray  57 A, which passes back through compensator  58  and emerges as ray  57 B. Ray  57 B is reflected by beam splitter  55  as ray  57 C. Ray  54 A is reflected by reference mirror  52  as ray  56 A, which passes through beam splitter  55  and emerges as ray  56 B. 
     Rays  56 B and  57 C form an interference pattern which can be observed by a detector or human eye  60 . The observed fringe pattern can indicate how uniform the surface being polished of PLZT layer  10 A is, how thick it is, and how parallel it is to the upper surface of sapphire substrate  20 . Light source  53  is initially a laser source. The adjusting screws  39  of the tilt adjust mechanism (shown as block  36 B in  FIG. 6A ) are adjusted to produce a suitable fringe pattern, as observed by human eye  60 , indicating the orientation of reference mirror  52  and PLZT layer  10 A is precisely 90 degrees. 
     Then the laser light source is replaced by a white light source. A single dark fringe then will be observed if the above distances of reference mirror  52  and PLZT layer  10 A from beam splitter  55  are identical to within 0.1 micron. After such a fringe is found, reference mirror  52  is adjusted to obtain a single dark fringe on PLZT layer  10 A as indicated in “A” of  FIG. 6B , and the reading of the micrometer adjustment of mirror  52  is noted. Then, mirror  52  is again adjusted to produce the single dark fringe on the surface of sapphire substrate  20  as shown in “B” of FIG.  6 B. The difference between the two micrometer adjustment readings of the positions of reference mirror  52  is equal to the present thickness of PLZT layer  10 A. 
     If the thickness of PLZT layer  10 A is not precisely uniform, the fringe orientation will be different (for example, as in “C” of  FIG. 6B ) on sapphire substrate  20  than on PLZT layer  10 A. If that is the case, the adjustment screws  39  can be adjusted so that as the lapping/polishing continues the pressure of PLZT layer  10 A adjust lapping wheel  37  is greatest where the most PLZT material needs to removed so the thickness will be precisely uniform at the end of the lapping and polishing process. One skilled in the art can interpret the observed fringe pattern to determine when the thickness of PLZT layer  10 A is uniform, and when the thickness is not uniform, one skilled in the art can readily determine how much to adjust the tilt of piston  34  within jig  36  by adjusting screws  39 . The process could be accomplished under computer control in a manufacturing environment to interpret the fringe pattern and, if necessary, adjust the piston tilt for further polishing. 
     The above technique, by providing PLZT layers in the 5-15 micron thickness range, is a practical way of fabricating PLZT substrates of precisely the thickness needed to achieve the minimum operating voltage at the low point “A” of the PLZT operating voltage versus PLZT thickness curve of FIG.  8 . 
     More complex spatial light modulator structures then can be formed by vertically “stacking” such two-dimensional spatial light modulators one on top of the other, as shown in FIG.  7 . For example, in  FIG. 7  two two-dimensional spatial light modulators  47  and  48  are vertically stacked, with a deposited layer of transparent insulating material  45  separating the electrodes  22  of spatial light modulator  47  from those of spatial light modulator  48 . Numeral  50  indicates light rays passing through a particular pixel, the width of which is indicated by arrow  17 . Each of the spatial light modulators  47  and  48  can be similar or identical to the one described with reference to  FIG. 2 , and the same reference numerals are used in  FIG. 7  as in  FIG. 2  to designate various parts. Different voltages can be applied to the various electrodes to “address” or “select” various spatial light modulator cells to influence the individual control of the various “pixels” or “voxels” of the complex spatial light modulator. 
     There are numerous ways to address a two dimensional array of cells. The most prevalent, which is employed in LCD displays is known as active matrix display (AMD). In such a system, the individual cells are designated by their x and y (column and row addresses. Part of the activation voltage is placed on each of the x and y lines aind a transistor which is unique to each cell is activated by the sum of the x and y voltages, but not by any individual x or y voltage. Such transistor then provides the desired activation vfoltage and holds that voltage until that cell is again addressed in a subsequent writing cycle. 
       FIG. 5  illustrates another spatial light modulator structure of the present invention, in which both positive and negative electrodes  11 - 14  are formed on the top surface  15 A, just as in the prior art structure shown in FIG.  1 . However, in  FIG. 1 , transparent electrodes  31  and  32  are formed on the bottom surface of thin PLZT layer  10 A. (For convenience, the thick, transparent substrate on which PLZT layer  10 A is attached is not shown.) 
     The voltages applied to bottom electrode  31  and  32  in  FIG. 5  can “modulate” the fringing fields  15  in FIG.  5 . For example, a sufficiently positive voltage applied to transparent electrode  31  “enhances” or strengthens the fringing field  15  as shown, producing a strong modulating effect by increasing the intensity of the fringing fields  15 . On the other hand, a negative voltage applied to one of the transparent bottom electrodes, for example electrode  32 , has the effect of “retarding” or suppressing the fringing field above it. In the example of  FIG. 5 , fringing field  15 X is retarded by the (−) voltage applied to transparent electrode  32 . 
     This feature gives added flexibility in providing x and y select capability to allow convenient x,y addressing of individual pixels iii an array of spatial light modulator cells. 
     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make the various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. It is intended that all combinations of elements and steps which perform substantially the same function in substantially the same way to achieve the same result are within the scope of the invention.