Abstract:
An electro-optical device may be defined using metallic standoffs between a top plate and a substrate, such as a silicon substrate in a liquid crystal on silicon (LCOS) technology. In one embodiment, the metallic standoffs may be formed from a metal layer, such as metal four layer, above the metal layer used to form the metal pixel mirrors. In this way, relatively constant and uniform cell thicknesses may be achieved without significantly increasing the processing overhead.

Description:
BACKGROUND 
     This invention relates generally to electro-optical devices such as liquid crystal devices. 
     Liquid crystal displays use a spatial light modulator (SLM) made up of a top plate and a substrate which surround a liquid crystal material. Conventionally, the region for the liquid crystal material is defined by spacer balls which may be distributed over the substrate. In addition, it is To known to fabricate insulating spacers directly on a silicon substrate. The function of the spacers is to maintain the distance between the top plate and the substrate and to define the region for the liquid crystal. 
     Liquid crystal devices using liquid crystal over a silicon substrate (LCOS) technology may form large screen projection displays or smaller displays (using direct viewing rather then projection technology). Typically, the liquid crystal material is suspended over a thin passivation layer. A glass plate with an indium tin oxide (ITO) layer covers the liquid crystal, creating the liquid crystal unit sometimes called a cell. The glass layer is typically suspended over the liquid crystal by a gasket that surrounds the cell array. 
     A silicon substrate may define a large number of pixels. Each pixel may include semiconductor transistor circuitry in one embodiment. The pixel may have a top reflective layer. An electrical potential may be applied to an electro-optical material using the reflective layer. A transparent top plate may have an inner transparent conductive layer that acts as an electrode that works with the reflective layer. An electrical field generated at each pixel may alter optical characteristics of an electro-optical material between the silicon substrate and the top plate. For example, the polarization of light passing through the electro-optical material may be altered. As another example, the electro-optical material may change its light transmission characteristics. 
     In some cases a standoff may be used to support the top plate. However, it is desirable to maintain a constant cell gap, thereby creating a constant liquid crystal thickness over the entire area of a cell array. To varying degrees, the use of discrete spacers is a simple way of providing this spacing, but in many cases they may not provide a sufficiently constant cell thickness. 
     Another technique uses an oxide spacer which may be formed on top of the pixel mirrors. However, this technique has the disadvantage that it requires extra processing steps. 
     Thus, there is a continuing need for a way to improve the uniformity of the spacing between the top plate and substrate in electro-optical devices without unduly increasing processing overhead. 
     SUMMARY 
     In one embodiment of the present invention, an electro-optical device includes a top plate and a silicon substrate. A metallic standoff is situated between the substrate and the top plate. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of one embodiment of the present invention; 
     FIG. 2 is a cross-sectional view taken generally along the line,  2 — 2  in FIG. 1; and 
     FIG. 3 is an enlarged cross-sectional view taken vertically through one of the standoffs shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an electro-optical device  10 , such as a spatial light modulator (SLM), includes a substantially light transmissive top plate  26  and a substrate  11 . A plurality of metallic standoffs  28 , including the standoffs  28   a  and  28   b , provide spacing between the substrate  11  and the top plate  26 . The metallic standoffs  28  may be formed using conventional semiconductor manufacturing processing. For example, in one embodiment of the invention, they may be formed (by etching, for example) from the metal four (or higher) metal layer used in conventional semiconductor processing. Thus, little or no additional processing overhead may result from forming spacers in this way. In addition, the spacers provide a very well defined cell thickness across even large arrays of cells. 
     The metallic standoffs  28  may be formed entirely or partially of a metallic material. Suitable metallic materials include metals, such as aluminum, copper, and tungsten metal alloys, and polysilicon including metal silicides. In some embodiments the standoffs  28  may be formed of an electrically conductive material such as metal or doped polysilicon. 
     Referring to FIG. 2, each cell may include a reflective mirror  18 . In the illustrated embodiment, the cell  18  is rectangular or square and a standoff  28   a-d  is formed at each of the cell&#39;s corners. In one embodiment, each of the standoffs partly overlaps each cell to assist in spacing a total of four adjacent cells. Of course, in other embodiments a higher or lower number of standoffs may be used. In addition, while the standoffs  28  are illustrated as having a truncated frustoconical shape, other standoff shapes may be used as well, including cylindrical, pyramidal, and rectangular solid shapes. 
     Referring now to FIG. 3, the detailed configuration of one embodiment of the present invention is illustrated. In this embodiment, an LCOS structure is defined using the silicon substrate  11  having doped regions  36  formed therein. In one illustrated embodiment, four or more metal layers are provided including a metal one layer  12  which is spaced by an interlayer dielectric (ILD)  34  from a metal two layer  14  and a metal three layer  18 , which may form a pixel mirror. In one embodiment the metal two layer may provide light blocking and the metal one layer may provide the necessary interconnections for the semiconductor devices. This leaves the metal four (or higher) layer to form the standoffs  28 . The pixel mirror  18  may electrically connect to the metal one layer  12  using a via  16 , which in one embodiment of the invention may be formed of tungsten. 
     A gap  32  may exist between adjacent cells as illustrated in FIG.  2 . Thus, the layer  18  may be part of one cell while the layer  18   a  is the beginning of an adjacent cell. A standoff  28  may straddle at least two adjacent cells, and in some cases, such as the one illustrated in FIG. 2, each standoff  28  may straddle four adjacent cells. 
     In one embodiment of the invention, the standoffs  28  may be covered by a dielectric material  30  formed during passivation. Typically, the material  30  is deposited using conventional techniques. Suitable materials for the layer  30  include oxides and nitrides. 
     The metallic standoffs  28  may be effectively free floating or they may be coupled to a biasing potential. While the illustrated standoff is conical, other shapes including those having more vertical sides (using less isotropic etches for example) may be defined as well. 
     In this way, existing four (or more) layer metal semiconductor fabrication processes may be used to form spacers in the form of metallic standoffs without adding significant processing overhead. One masking step may define the standoffs, which may be etched in the conventional fashion to form the standoffs. The metallic layer that forms the standoffs for the electro-optical device, may be used for conventional connections in other areas of the die. 
     The top plate  26  may be coated with an indium tin oxide (ITO) layer  24 . In embodiments where the insulating layer  30  is omitted, a polyimide layer may be used between the indium tin oxide layer  24  and the upper end of the metallic standoff  28 . The polyimide layer may provide electrical isolation. Other insulating materials may be coated on the ITO layer  24  in place of or in addition to the polyimide layer. 
     After fabrication, in one embodiment, a liquid crystal material is added to the region  22  using conventional techniques. For example, a gasket material (not shown) may encircle the cell array between the top plate  26  and the substrate  11 . A notch (not shown) formed in the gasket allows liquid crystal material to be wicked through the notch into the region  22 . The notch may thereafter be sealed using conventional techniques to retain the liquid crystal material in the region  22 . 
     Suitable electro-optical material may include liquid crystals, such as a ferroelectric liquid crystal, a polymer dispersed liquid crystal, a twisted nematic liquid crystal, and a polymer stabilized cholesteric texture liquid crystal. 
     While the present invention has been described with respect to limited member of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the present invention.