Abstract:
A method of forming an improved liquid-crystal-on-silicon display and resultant display is described, in which the device structure is enhanced by the photolithography building of alignment posts among the mirror pixels of the microdisplay.

Description:
This is a division of patent application Ser. No. 09/262,910, filing date Mar. 5, 1999, Liquid-Crystal-On-Silicon Display With Photolithographic Alignment Posts And Optical Interference Layers, assigned to the same assignee as the present invention now U.S. Pat. No. 6,498,635. 
     This is related to patent application Ser. No. 09/262,000 filed Mar. 3, 1999 (CS98-076). 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to the method of fabrication of alignment posts and optical interference layers directly on silicon wafers with liquid crystal material and to the unique resulting device structure. 
     (2) Description of the Prior Art 
     The picture quality of liquid crystal displays from a simple seven segments to millions of pixels is determined by the structure used to control the variation of thickness and position leads after wafer processing. There are known processes for creating insulated alignment posts based on preformed glass microspheres and rods; relatively low series resistance posts can be obtained by means of selective deposition of polysilicon and metallic silicide. (Making of metallic vias and contacts are a comparatively well known processing art.) 
     U.S. Pat. No. 5,498,925 to Bell et al describes the formation of posts in flat panel displays using processes based on a heat-treated slurry or paste upon a glass plate. U.S. Pat. No. 5,597,736 teaches the function of a light-blocking layer deposited upon a semiconductor substrate material that can emit light. Until now, it has been difficult to construct alignment posts using photolithography and also add optical interference layers simultaneously onto a semiconductor substrate material. 
     BRIEF SUMMARY OF THE INVENTION 
     A principal object of the present invention is to describe a new structure for building a flat-panel liquid-crystal display upon an integrated circuit (IC) die with inter-related alignment between the posts supporting the overlaying glass cover plate and optical interference layers (OIL) employed to improve image quality. The alignment posts are made of silicon nitride, and the OIL of silicon nitride/oxide multilayers. 
     Another object of this invention is to describe effective and very manufacturable methods of photolithographic formation of insulating alignment posts (also called studs or pillars). These methods can be used in processing many different device types, and are described in this application for liquid crystal display devices as a way of illustrating their embodiment at a pixel density beyond that achievable with preformed micro-glass spheres and rods. 
     A further object of the present invention is to describe methods of deposition for both the posts and the optical interference layers that are independent of each other and retain their desired feature during deposition of subsequent features. 
     In accordance with the objects of this invention, a new method of forming insulating material alignment posts associated with active device structures is achieved. A silicon wafer having patterned active device therein and thereon is formed and the insulating material alignment posts are deposited in a pattern over the pattern of the active device structures. Furthermore a device structure that combines insulating materials for alignment posts and optical interference layers associated with an active device structure in a silicon body is achieved. A silicon semiconductor wafer having patterned active device structures and alignment posts is formed and the whole structure is covered with an optical interference layer; 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, there is shown: 
     FIGS. 1 through 3 schematically illustrate in cross-sectional representation a preferred embodiment of the device structure of the present invention. The process flow for making the alignment posts and the optical interference layers is shown in FIGS. 4 to  10 . 
     FIG. 11 schematically illustrates in cross-sectional representation the final embodiment of this invention for this liquid-crystal-on-silicon display device. The bonding pad is shown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now more particularly to FIG. 1, here is shown a portion of a partially completed integrated circuit liquid-crystal display. The glass cover plate  10  provides the transparent enclosure for the external incident light to be reflected back to an observer. The strength of reflected light is dependent on the light polarization, absorption and light scattering properties of each liquid-crystal display pixel, which is controlled by the electrical field established within the liquid crystal material  11 . The IC die  12 , separated from the glass plate by the alignment posts  14  generates these E-fields. The resultant space between the glass plate and the silicon wafer is filled with the chosen liquid crystal material. Light, either provided or ambient, enters the open face of the liquid-crystal-on silicon and is reflected from the underlying pixels to form a viewable pattern of polarized light, i.e. the image is viewed directly or projected through an optical polarized system. Such microdisplays may contain more than one-thousand columns by more than one-thousand rows (totally over one-million pixels) in a square surface area less the one inch diagonal; the pixel pitch can be made less than 10 micrometers (microns), and has been achieved as low as 5 microns with an interpixel gap of one micron or less. When alignment posts are placed between adjacent pixels, the interpixel gap has been fabricated at a typical distance of 0.6 microns with a range from 0.55 to 0.85 microns. 
     The glass cover has a transparent conductive coating on its inner surface, which is the common electrode for the entire pixel array. 
     FIG. 2 illustrates the bonding pads  20  on the IC die to which the external wires  22  are attached. The silicon wafer contains the embedded control circuits that activate the pixel patterns in the viewable area  24 . The refractive index anisotropy of the liquid crystal is influenced by the electric fields above the IC surface. A small change in voltage makes a large change in the optical transmission. Because this invention teaches the photolithographic making of patterns of alignment posts, the ratio of pixels  30  to posts  14  is often fabricated in the range 20:1 to 50:1. As shown in FIG. 3, the photolithographic method permits these alignment posts to be constructed in the space between adjacent pixels. 
     The process steps for making the alignment posts and optical interference layers is shown in FIGS. 4 to  10 . Starting with FIG. 4, the conductive metallic layer  30  is formed over the silicon oxide  40 , which is formed on top of the metal layer  41  on the IC. A silicon semiconductor substrate  15  has active devices therein and an insulating layer  16  upon which metal layer  41  is formed. Then a photoresist layer is laid down over the metal  30  to construct the pixels. The photoresist is exposed and a portion removed to provide that each pixel retains a metallic layer, which shall act as a mirror reflector for the light incident upon said pixel. If aluminum were chosen as the metallic mirror, its 90% reflectivity would improve to near 100% with the addition of the overlaid optical interference layer. The remaining photoresist is stripped and the entire structure is covered with silicon nitride  50  by PECVD and to a thickness of between about five-thousand to ten-thousand Angstroms as in FIG.  5 . Then the standard photoresist and etching methods are used to create the posts mask  60  shown in FIG.  6 . The alignment posts  14  are created by selection of a chemical removal process that stops at the metal  30  and the silicon oxide  40 . 
     After the resist is stripped, as shown in FIG. 7, the device is ready to be covered with the optical interference layers  80 . Optical interference layers are used to improve light reflections. Careful attention is required in constructing these optical interference layers so as not to disturb any underlying devices and posts. The OIL covering is considered to be an optional addition. The added expense of the OIL covering may be avoided in some cases of lower image quality. The LCD-on-silicon device could be completed and used functionally without adding the OIL after the formation of the alignment posts. 
     The optional optical interference layer coating is composed of multiple layers of insulating material with properties of varying optical indices of refraction, as shown in FIG.  8 . Oxides and nitrides have been applied in the fabrication of these multilayer stacks. It is critical to have SiOx as the bottom layer to match the index of refraction of the substrate. 
     Durable, uniform and reproducible amorphous silicon nitride multilayer coatings deposited by PECVD are known optical interference filters in the near infrared. Optical interference coatings for the visible spectrum made from four or more layers of oxide/nitride/oxide/nitride formed for this application are considered to be novel and worthy for this patent application. 
     To attach the wires to the IC via bonding pads, a photoresist mask is formed with openings over the location of the bond pads and the material is removed to metallic layer  41 . Then the photomask is removed, leaving the finished device structure shown in FIG.  10 . 
     The existence of numerous alignment posts permits the usage of ultra-thin glass plate or cover over the liquid crystal. This implementation results in lightweight displays for portable applications. Glass covers supported by alignment posts are typically 0.5 millimeters in thickness, and can range from 0.2 to 0.8 millimeters. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.