Abstract:
Method and apparatus for forming an array of reflective elements for spatial light modulation. The array includes a substrate supporting electronically addressable actuators, each associated with a corresponding reflective element, a coupling attaching each actuator to the corresponding reflective element to place each reflective element in a substantially planar surface. Each electronically addressable actuator responds to predetermined addressing from a processing circuit to reposition the corresponding reflective element out of the planar surface a predetermined distance identified in the predetermined electronic addressing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/161,939, filed Oct. 28, 1999, which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with U.S. Government Support under Contract Number F08630-00-C-006, awarded by the Air Force Office of Scientific Research. The Federal Government therefore has certain rights in the invention. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     Spatial light modulation is used in the fields of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. There optical beams are deflected by mirror arrays where it is desired to be able to individually phase adjust the reflected light from each mirror. Because of the large number of mirrors in such arrays it is important to be able to achieve high speeds in the addressing of each mirror as well as accurate control of each mirror&#39;s displacement. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes a mirror array for use in spatial light modulation. A CMOS circuit is provided on a substrate in an array corresponding to the placement desired for each mirror. A low temperature procedure making use of wet soluble polymer photoresists, sputter deposition and ion etching is then utilized to create a structure above the CMOS circuit comprising a metalized or metal diaphragm supported by flexures from the substrate. The diaphragm and circuit each include opposite plates of a capacitor. The application of a voltage between the circuit and the diaphragm causes the diaphragm to be attracted or repelled by electrostatic forces. In the micron and submicron size of the mirror assemblies and arrays, voltages of a few volts compatible with CMOS circuitry is able to create a half micron displacement. 
     The fabrication process then uses the same low temperature procedures to create a mirror on top of the diaphragm and supported from it by a single support post. Once released from any polymer used in the fabrication process, the mirror has bending stresses released by a sputter removal of surface layers until a planar surface is achieved. Prior to release, mirror surface roughness can be removed by polishing. 
    
    
     DESCRIPTION OF THE DRAWING 
     These and other features of the present invention are more fully set forth in the detailed description below and in the accompanying drawing of which: 
     FIG. 1 is a view of an interferometric microscopic photograph of an array according to the invention; 
     FIG. 2 is an optical microscope view of another array according to the invention; 
     FIG. 3 is a sectional view of three elements of an array according to the invention; 
     FIG. 4 is a perspective view of a diaphragm according to the invention; 
     FIGS. 5A and 5B are diagrammatic top and side views of a diaphragm according to the invention; 
     FIG.  6 -FIGS. 14A, B are top and side sectional views illustrating processing steps in the fabrication of an array according to the invention; 
     FIGS. 15-16 illustrate the presence of stress in a mirror element according to the invention; 
     FIGS. 17-19 illustrate steps in relieving stress in mirror elements according to the invention; 
     FIGS. 20-21 illustrate first and second embodiments for electronically driving mirrors according to the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a driven array of mircomechanically produced mirrors useful in spatial light modulation (SLMs) to form an optical image. SLMs have application in optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. 
     The invention provides a new type of SLM useful in phase-only optical correlators. An array of mirrors is provided that can be moved over a full wavelength allowing 360 degrees of phase control. Interferometric and optical microscopic images of such an array  12  are illustrated in FIG. 1 and 2. A single 300 micron square mirror  14  is shown raised 2 microns. An expanded view of three mirror assemblies of an array according to the invention is shown in FIG.  3 . There three mirrors  16  are provided, typically as metalizations deposited from a sputter or other low temperature deposition, along with struts  18  supporting the mirror from a diaphragm or platform  20 . The diaphragms  20  are themselves the product of a metalization deposition similar to that for the mirrors  16 . They are produced leaving a cavity  22  under and between them and a substrate  24 . Beneath the diaphragms  20  CMOS circuits  26  are provided to generate an electrostatic force that attracts the diaphragms  20  and in turn the mirrors  16  an amount corresponding to input signals to the CMOS circuits as described below. The processing of the diaphragms  20  and mirrors  16  using low temperature processing such as polymer photoresists and sputter depositions are more fully described below. 
     The structure of the diaphragms  20  is more fully shown in the perspective view of FIG.  4  and diagrammatic views of FIGS. 5A and B. As shown there, the diaphragms  20  have peripheral flexures  30  leading from diametrically opposite corners to support posts  32 , all produced in a sputter deposition as described below. 
     The process of formation of the mirror array begins, as shown in FIG. 6, with the formation by micromechanical, LSI type processing, on a typically silicon substrate  42 , of a set of typically CMOS circuits  40 , one for each mirror element to be generated. The circuits  40 , as more fully described below, are fed by a data bus  44  from on chip circuitry  46  responsive in turn to signals from a CPU  48  or other off chip processor. 
     Because the CMOS circuits  40  are fabricated first, the remaining structural processing is a low temperature procedure that prevents thermal damage to the circuits  40 . That processing is initiated as shown in FIGS. 7A and B by depositing, such as by spin drying, a polymer photoresist layer  50  over the substrate  42 . The polymer is chosen to be releasable after processing at low temperatures such as by wet etching in a solvent, or possible reactive ion etching in an oxygen plasma. The polymer photoresist layer  50  is exposed and developed to leave apertures  52  in FIGS. 8A and B for the deposition of a metalization or metal layer  54  for the anchors and for the diaphragm and its flexures described above. The metal deposition  54  as shown in FIGS. 9A and B can be sputtered material such as a chromium-aluminum composite, aluminum, gold, or nickel. 
     The metal layer is patterned and etched in FIGS. 10A and B to leave the flexure  30  supported diaphragm  20 . A polymer photoresist and reactive ion etch, such as in a chlorine atmosphere, may be used to create and separate the flexures  30  and diaphragm  20 . 
     At this point, the photoresist may be released by wet solvent procedures as shown in FIGS. 11A and B or the mirror structure may be begun as shown in FIGS. 12A and B using a second layer of polymer resist  56 . The resist is patterned to leave upon being developed an aperture  58  for the formation in the steps of FIGS. 13A and B of a metalized or metal layer  60  for the mirror. The procedures are similar to those in forming the metal layer  54  for the diaphragms  20 . The metal layer  60  is patterned and etched as before in FIGS. 14A and B to separate the mirror structures  62  and their struts or posts  64 . Finally the whole structure is subjected to a wet solvent procedure to remove all polymer. At some point in the procedure, such as at FIGS. 13A and B, before polymer release, the device may be given a surface polishing to remove surface roughness and improve the quality of reflection. 
     In some cases and in reference to FIGS. 16-19, the mirror  62  will exhibit a stress induced curvature resulting from the stresses built into the metal layer  60  during formation and release of supporting polymer at the conclusion of fabrication. These stresses  64 , as shown in FIG. 16, vary over the depth of the mirror  62 , and in fact change polarity. Thus, the stresses can be balanced giving a planar mirror surface by removal of portions of the mirror element  62  until a point is reached where the stresses combine to keep the mirror surface flat. This point can be reached in the process of removal of surface layers as shown in FIG.  19 . The procedure for removal may utilize an ion beam  70  in an argon atmosphere to cut back the mirror surface. The process can be monitored by an interferometer  72  to detect the point of maximum flatness. 
     The circuitry  26  shown in FIG. 3 can be of several forms as illustrated in FIGS. 20 and 21. In FIG. 20 a CPU  80 , off chip, applies instructions including addressing information designating, in a repeating sequence over the whole array  12 , each of mirrors  16  to be moved and data typically in the form of a voltage indicating the amount and polarity of displacement of that mirror. This information is fed to the circuit  26  at each mirror assembly to an address decoder  82  and voltage decoder  84  where the voltage is stored in a capacitive memory  86 . A driver gate  88  is activated when the corresponding mirror is addressed to apply that voltage through it to a capacitive plate  90  which in turn applies an electrostatic force to the actuator diaphragm  20 . Sufficient motion can be achieved with a low voltage of, for example a few volts compatible with CMOS circuitry, to achieve the 360 degree change in light phase on the mirrors. 
     An alternative CMOS circuit is illustrated in FIG. 21 where an on or off chip processor  100  applies via a data bus  102  to respective decoders  104  addressing and displacement information. In a typical application of eight bit data, a 256×256 mirror array can be addressed and a data byte of eight bits used to achieve a resolution of 256 displacement positions. In this example, the decoder determines from the addressing when its corresponding mirror is being addressed and then uses each of the eight bits to apply a low voltage to corresponding capacitive plates in an array  108 . The plates are sequentially sized, typically each plate being twice the size of its neighbor. Each data bit applies or does not apply a fixed voltage to the corresponding plate based on the bit being of one state or the other, achieving a combined force proportional to the area of activated plates and a resolution of 256 positions. 
     The invention can be broadly scaled to different size arrays and mirror areas. A total mirror displacement of half a micron can be provided to achieve the desired phase change in the optical spectrum. The spacing of the diaphragms  20  and CMOS circuits  26  is a function of the voltage available and the total desired displacement, response time and other factors within the grasp of those skilled in the art. 
     The invention is not intended to be limited by any of the above description and is to be interpreted on the scope of the following claims.