Abstract:
Manufacture a spatial light modulator by fabricating a first set of micro-mirrors, then subsequently fabricating a second set of micro-mirrors interspersed with the first set, to reduce space between adjacent micro-mirrors versus what could be done by simultaneously fabricating all the micro-mirrors as one set.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to spatial light modulators, and more particularly to a spatial light modulator having its array of micro-mirrors manufactured in two sets. 
     2. Background Art 
     FIG. 1 illustrates a top view of a simplified spatial light modulator (SLM)  10  such as is known in the art. The SLM includes an array of reflective micro-mirrors  12  each of which constitutes an electrode of a respective display pixel. The other electrode of the display pixels is formed as a transparent electrode layer (not shown, on top of an area of liquid crystal (not shown). 
     Due to limitations in existing fabrication techniques, the array of micro-mirrors is arranged such that there is a space or gap  14  between adjacent micro-mirrors. Currently, this space accounts for upward of 10% of the surface area of the pixel array. This means that upward of 10% of the light striking the array is lost, and is not reflected back through the liquid crystal. This lost light is actually of absorbed by the SLM, causing heating and other problems. Furthermore, the spacing places limitations on the scalability of the micro-mirror array, making it increasingly difficult to effectively decrease the micro-mirror size and increase the resolution of the device. 
     FIG. 2 illustrates a cross-sectional view of a portion of the SLM  10 . The device is built upon a substrate  18 , such as silicon. Conductive vias  16  or other interconnecting structures are built into the substrate. The micro-mirrors  12  are fabricated in electrical contact with the vias, with spacing  14  between adjacent neighbors. The micro-mirrors are encased in one or more insulative, antireflective layers. These layers can include a first layer  20 , such as SiO 2 , and a second layer  22 , such as Si 3 N 4 , which is fabricated over the first layer. A layer of liquid crystal material  24  is sandwiched between these layers and a glass layer  26  which holds the transparent electrode layer  28 . When an electrical potential is applied between a micro-mirror and the transparent electrode layer, the region of liquid crystal between them becomes transparent. The degree of transparency can be adjusted, such as by altering the electrical potential or by phase width modulating its signal, to give plural levels of transparency, and thus plural levels of reflected light. A typical SLM offers 256 levels of color resolution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
     FIG. 1 shows a top view of a spatial light modulator having an array of micro-mirrors, spaced apart as dictated by manufacturing technology limitations of the prior art. 
     FIG. 2 shows a cross-section view of the spatial light modulator of FIG.  1 . 
     FIGS. 3-10 show a manufacturing sequence of the invention. 
     FIG. 11 shows a top view of a spatial light modulator built according to the invention. 
     FIG. 12 shows a detailed, close-up, top view of micro-mirrors in the array built according to the invention. 
     FIG. 13 shows a top view of an alternative embodiment of the array of micro-mirrors. 
     FIG. 14 shows one embodiment of a method of fabrication of the spatial light modulator. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 3-10 illustrate one exemplary embodiment of a method of fabricating a spatial light modulator (SLM)  30  so as to overcome spatial limitations of the prior art. The reader will appreciate that FIGS. 3-10 have been greatly simplified, such as by showing a greatly reduced number of components. 
     FIG. 3 shows the substrate  18  upon which the SLM is based. The substrate may be silicon, gallium arsenide, or other suitable material. The substrate is equipped with vias  32 ,  34  or other suitable electrical connections to the memory (not shown) built into the semiconductor substrate. These may be logically divided into a first set of connections  32 , and a second set of connections  34 , which are interspersed. A first set of micro-mirrors  36  are fabricated in electrical contact with respective ones of the first set of connections  32 . The second set of connections  34  is left unconnected at this point. In one embodiment, the first micro-mirrors may be constructed of aluminum having a thickness of roughly 1,500 Å. In other embodiments, the aluminum may be 1,000 Å thick, or up to 20,000 Å thick. 
     FIG. 4 shows an insulative, antireflective layer  38  such as SiO 2  which is formed over the first set of contacts and the exposed substrate. In one embodiment, the insulative, antireflective layer  38  can be 250 Å thick. 
     FIG. 5 shows a hole  39  which has been etched or otherwise formed through the insulative layer to expose one of the second set of connections  34 . In one embodiment, the hole comprises approximately 1% of the area of the micro-mirror to be connected through it. In various other embodiments, the hole may be the same size as the underlying via, or somewhat smaller, or somewhat larger, depending upon the particulars of the fabrication process, the application, and so forth. 
     FIG. 6 shows the formation of a micro-mirror  42  of the second set, in electrical contact with its corresponding member of the second set of connections  34 . In one embodiment, the second micro-mirrors can be fabricated from aluminum having a thickness of roughly 1,250 Å as measured from the top of layer  38 . In some embodiments, the thickness of the second micro-mirrors differs from that of the first micro-mirrors by the thickness of the insulative layer  38 , so the surfaces of the first and second micro-mirrors are substantially coplanar. 
     The first and second micro-mirrors may be fabricated of any suitably reflective and conductive material. Typically, this will be a metal, such as aluminum, silver, or the like. 
     FIG. 7 shows the formation of an additional insulative, antireflective layer  40 , which insulates the second set of micro-mirrors. In one embodiment, this additional insulative layer can be roughly 2,000 Å thick, then, by chemical mechanical polishing, decreased down to a thickness of about 750 Å, which will also planarize the surface, as shown. The reader will appreciate that, in some embodiments where the first insulative layer  38  and the second insulative layer  40  are fabricated of sufficiently similar materials, they may effectively blend into a single insulative layer; thus, in FIGS. 8-10, the first insulative layer  38  is no longer shown. 
     FIG. 8 shows the formation of a second insulative, antireflective layer  22  such as Si 3 N 4  or other suitable material. In some embodiments, this layer, or perhaps other layers, may be omitted. In one embodiment, both antireflective layers can be roughly 750 Å thick. 
     FIG. 9 shows the addition of a layer of liquid crystal material  24 . 
     FIG. 10 shows the addition of glass  24 , with a film or layer of conductive electrode  44 . 
     FIG. 11 is a top view of the SLM  30  built according to this invention, in which the spacing  46  between adjacent micro-mirrors is substantially reduced from that ( 14 ) of the prior art. In one embodiment, the micro-mirrors are interspersed in a checkerboard pattern, as shown. Other patterns may, of course, be practicable given the teachings of this disclosure. 
     The reader will appreciate that the insulative layer  38  can be very thin, such as 250 Å, and that this is a much smaller spacing than the smallest spacing ( 14  in FIGS. 1 and 2) which lithography techniques permit between the simultaneously-fabricated single set of micro-mirrors ( 12  in FIGS. 1 and 2) in the prior art. Thus, by fabricating adjacent micro-mirrors separately, the invention permits tighter spacing of the micro-mirrors. 
     FIG. 12 illustrates this principle in further detail. For ease of visualization, the first set of micro-mirrors  36  are shown in solid lines, while the second set of micro-mirrors  42  are shown in dashed lines. In one embodiment, the micro-mirrors are substantially square or rectangular, having a width W and a height T, with a horizontal distance H or a vertical distance V between adjacent neighbors. The horizontal and vertical directions may collectively be termed lateral directions. By fabricating the micro-mirrors as two checkerboarded sets, the distance S between adjacent neighbors within in the same set is dramatically larger than the distance H or V which would apply if the micro-mirrors were fabricated as a single set. (The vertical distance between adjacent neighbors in one of the two sets, corresponding generally to S, is not shown, in the interest of simplicity.) 
     In some embodiments, it may be found that the diagonal distance D between diagonal neighbors in the same set of two sets can become the limiting factor in determining how closely the micro-mirrors can be spaced within the same set. 
     FIG. 13 illustrates one embodiment of a solution to this problem. By altering the shape of the micro-mirrors, the diagonal distance B can be made substantially greater than the distance D (of FIG.  12 ). One suitable alternative shape is an eight-sided, nearly square shape, with the corners slightly relieved, as shown. 
     The reader will appreciate that the grid need not be regular nor rectangular, and that the micro-mirrors can be of any suitable shape, spacing, and gridding. For example, hexagonal pixels in a honeycomb grid could be a desirable solution in some applications. 
     FIG. 14 illustrates one exemplary embodiment of a method  60  of manufacturing the SLM of this invention. After the substrate is properly prepared ( 61 ), the first and second sets of vias or interconnects are fabricated ( 62 ). Then the first set of micro-mirrors is fabricated ( 63 ), and an insulative layer of SiO 2  may be formed ( 64 ) overlying the first set of micro-mirrors. The second set of vias or interconnects is exposed ( 65 ) such as by etching holes through the insulative layer. The second set of micro-mirrors is fabricated ( 66 ), and another insulative SiO 2  layer may be formed ( 67 ) overlying the second set of micro-mirrors. The insulative layers may blend into a single insulative layer, as previously mentioned. The insulative layer is CMP polished ( 68 ) down to a predetermined thickness suitable to make the layer antireflective. Another antireflective layer Si 3 N 4  may be formed ( 69 ) overlying this SiO 2  layer. A layer of liquid crystal material is deposited ( 70 ) over the whole array, and the entire structure is overlayed ( 71 ) with a layer of glass or other suitable material, in which the transparent electrode layer has been formed. 
     The reader should appreciate that drawings showing methods, and the written descriptions thereof, should also be understood to illustrate machine-accessible media having recorded, encoded, or otherwise embodied therein instructions, functions, routines, control codes, firmware, software, or the like, which, when accessed, read, executed, loaded into, or otherwise utilized by a machine, will cause the machine to perform the illustrated methods. Such media may include, by way of illustration only and not limitation: magnetic, optical, magneto-optical, or other storage mechanisms, fixed or removable discs, drives, tapes, semiconductor memories, organic memories, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW, Zip, floppy, cassette, reel-to-reel, or the like. They may alternatively include down-the-wire, broadcast, or other delivery mechanisms such as Internet, local area network, wide area network, wireless, cellular, cable, laser, satellite, microwave, or other suitable carrier means, over which the instructions etc. may be delivered in the form of packets, serial data, parallel data, or other suitable format. The machine may include, by way of illustration only and not limitation: microprocessor, embedded controller, PLA, PAL, FPGA, ASIC, computer, smart card, networking equipment, or any other machine, apparatus, system, or the like which is adapted to perform functionality defined by such instructions or the like. Such drawings, written descriptions, and corresponding claims may variously be understood as representing the instructions etc. taken alone, the instructions etc. as organized in their particular packet/serial/parallel/etc. form, and/or the instructions etc. together with their storage or carrier media. The reader will further appreciate that such instructions etc. may be recorded or carried in compressed, encrypted, or otherwise encoded format without departing from the scope of this patent, even if the instructions etc. must be decrypted, decompressed, compiled, interpreted, or otherwise manipulated prior to their execution or other utilization by the machine. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
     If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.