Abstract:
An improved spatial light modulator package comprising a spatial light modulator  1006  attached to a central region of a substrate  1004 , a sealing ring  1002  on said substrate  1004  around the central region thereof, a window frame  402  attached to the sealing ring  1002 , and a window  404  glued to the window frame  402 . Gluing the window  404  to the window frame  402  avoids distortion of the glass that occurs when the window is heat bonded to the window frame, and avoids having to grind and polish the glass window after it is bonded to the window frame.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC §119(e)(1) of provisional application No. 60/070,152 filed Dec. 31, 1997. 
     The following patent and/or commonly assigned patent application is hereby incorporated herein by reference: 
     U.S. Pat. No. 5,610,625 filing date Jun. 7, 1995 issue date Mar. 11, 1997 title Monolithic Spatial Light Modulator and Memory Package. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of integrated circuit packaging, particularly to light modulator packaging, and more particularly to windows for digital micromirror device packages. 
     BACKGROUND OF THE INVENTION 
     Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, metal sputtering, and plasma etching which have been developed for the fabrication of integrated circuits. 
     Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a particular type of micromechanical devices. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable. 
     Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator which uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs are typically operated in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems. 
     Micromirror devices are used to create image displays by selectively reflecting light from the micromirrors. Micromirror devices are also very sensitive to contamination from outside particles, therefore they must be contained in a sealed environment to protect the micromirror device from the outside particles. Additionally, the micromirror package must have a precisely formed window in the package to allow light to reach the micromirror device and reflect therefrom out of the package. 
     Micromechanical devices in general, and micromirror device in particular, often require nontraditional packages in order fully to operate to their full potential. These special packages raise the finished cost of the device, and often cost more than the device they package. Perhaps the best example of a micromechanical device that requires special packaging is a micromirror device. Because of the strict optical constraints placed on the package window, prior art windows for micromirror assemblies are very expensive to manufacture. Therefore, an improved low-cost window for a micromechanical device is needed. 
     SUMMARY OF THE INVENTION 
     Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for an improved windowed package lid. One embodiment of the claimed invention provides a method of packaging a device comprising the steps of providing a ceramic package substrate defining a central region, attaching a metal sealing ring to the ceramic package substrate, bonding the device to the central region of the ceramic package substrate, gluing a window to a window frame, and welding the window frame to the sealing ring. 
     Another embodiment of the present invention provides a micromechanical package lid. The package lid comprises a glass window and a metal window frame glued to said glass window. Typical package lids also have at least one anti-reflective coating on said glass window. Typical package lids also have a metal aperture deposited on said glass window. 
     Another embodiment of the disclosed invention provides a device package comprising a substrate having a central region, a sealing ring on the substrate around the central region, a device attached to the central region of the substrate, a window frame attached to the sealing ring, and a window glued to the window frame. The device package has an inner chamber enclosing the device formed by the substrate, sealing ring, window frame, and window. 
     Yet another embodiment of the disclosed invention provides a projection display system. The projection display system comprises a light source for projecting a beam of light along a light path, a spatial light modulator device in the light path for selectively reflecting portions of the beam of light along the projection light path, and projection optics along said projection light path for focusing said selectively reflected portions of said beam of light onto an image plane. 
     The spatial light modulator device in the disclosed projection display system is comprised of a micromechanical spatial light modulator packaged within a chamber created by a ceramic substrate, a sealing ring attached to said ceramic substrate, a window frame attached to the sealing ring, and a glass window glued to the sealing ring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a top view of a window assembly for a micromechanical device package according to the prior art. 
     FIG. 2 is a front view of the prior art window assembly of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the prior art window assembly of FIG.  1 . 
     FIG. 4 is a top view of a window assembly for a micromechanical device package according to one embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of the window assembly of FIG.  4 . 
     FIG. 6 is a cross-sectional view of one portion of the window assembly of FIG. 4 showing the attachment of a glass window to a metal frame. 
     FIG. 7 is a front view of the window assembly of FIG. 4 showing the preferred location of an optional getter. 
     FIG. 8 is a bottom view of the window assembly of FIG. 4 showing the preferred location of an optional getter. 
     FIG. 9 is a side view of a micromechanical device package showing the attachment of the window lid of FIG. 4 to the remainder of the package. 
     FIG. 10 is a schematic view of a micromirror-based projection system utilizing an adhesive-sealed device package lid according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior art windowed package lids, as shown in FIGS. 1-3, require an extensive amount of individual processing which dramatically drives up the cost of producing the package lids. The package lid  100  shown in FIGS. 1-3 is fabricated from a Kovar frame  102  which has been treated to ensure a good glass to metal seal when melted glass is pressed into the frame  102 . The frame has a surface roughness of 32 on both sides of a welding flange  103 . The frame is nickel plated to a thickness of between 0.0001 and 0.0003 inches and then gold plated to a minimum thickness of 0.00005 inches. 
     Corning  7056  glass, with a refractive index of 1.487+/−0.010 for light having a wavelength of 0.545 μm is melted and pressed into the frame  102  for form a window. At a temperature of about 800° C., the glass is pressed into the frame  102  to form a window  104 . After pressing the glass window  104  into the frame, the window  104  must extend above and below the frame  102  to allow grinding equipment to surface the glass. After the glass window has cooled, it is ground and polished to a surface flatness of four fringes spherical power and two fringes irregularity. 
     Once both surfaces of the glass window have been ground to the required tolerance, a metal layer is deposited on the inner surface  106  of the glass window  104 . The metal layer typically has a reflectance of at least 90% for all wavelengths between 0.42 μm and 0.80 μm. The metal layer is patterned to form an aperture  108 . After opening the aperture in the metal layer, both surfaces of the window, including the metal layer, are coated with an antireflective coating. The antireflective coating ensures no more than 0.5% of the light between 0.42 μm and 0.80 μm is reflected by the surfaces of the window  104  and at least 98% of the light between 0.42 μm and 0.80 μm is transmitted. The outer layer of the antireflective coating on the inner surface of the finished lid  106  must be magnesium fluoride. Some versions of the lid only have an antireflective coating on the inner surface, and some versions do not include metal aperture layer. 
     The new adhesive-sealed lid  400 , shown in FIG. 4, dramatically reduces the cost of packaging a micromechanical device. Whereas the lid of the prior art relied upon a glass to metal seal formed at a temperature of 800° C., a process which required each lid window to be individually ground and polished, the lid disclosed herein is fabricated at temperatures below 200° C., allowing the use of float glass sheets. Most of the processing is done to the glass in sheet form, allowing simultaneous processing of 10-20 lids. 
     According to one embodiment of the present invention, a window frame  402  is formed from a sheet of Kovar or Alloy 42. Since the new lid window design does not rely on a high-temperature glass to metal seal, the window frame  402  may be a flat sheet instead of the thick window frame  102  of prior art package designs. After fabricating the window frame  402  the frame is plated with nickel and, according to some embodiments, gold. According to one embodiment of the disclosed invention, the frame has a surface roughness of 32 on both sides and is nickel plated to a thickness of between 0.0001 and 0.0003 inches and then gold plated to a minimum thickness of 0.00005 inches. 
     The dimensions of the window frame  402 , as well as the dimensions of the window  404 , depend on the size of the ceramic substrate to which the window will be attached, as well as other system requirements, such as a preferred glass to device distance. While all the dimensions are understood to depend on the particular application for which the package lid is intended, a typical window frame  402  is 1.090 inches by 1.400 inches, 0.0085 inches thick, and has an opening in the center region that is 0.615 inches by 0.449 inches. 
     The glass portion of the new lid is preferably fabricated from polished float glass, or an equivalent. A sheet of glass has a metal layer sputtered onto the device side  406  of the glass. The metal layer may be either a highly reflective layer or a non-reflective layer depending on how the optical system tries to control extraneous light. Aluminum, chrome, and silver are useful metals for the metal layer, and are often sputtered over an oxide deposited on the glass to improve adhesion between the glass and the metal layer. The metal layer is then patterned to form a 0.615 by 0.449 inch aperture  408  in the center. After forming the aperture layer, an antireflective coating is fabricated on both sides of the glass, typically having a magnesium fluoride layer as the outer layer on both sides of the glass. 
     After the antireflective coatings and the aperture metalization are applied to the glass in sheet form, the sheets of glass are scribed and broken into individual windows. 
     According to various alternative embodiments of the disclosed invention, the metal aperture layer may be sputtered on the glass and patterned after the formation of the antireflective coating. Some applications may only require the antireflective coating to be deposited on one side, probably the device side  406 , of the window  404 . Additionally, some applications place very restrictive tolerances on the position of the aperture. Extremely tight aperture tolerances require the glass to be scribed and broken to form individual windows prior to the formation of the metal aperture layer. 
     The window frame  402  and the window  404  are typically epoxied together to form a semi-hermetic seal which is impermeable to many common gasses. The adhesive joint is formed by coating the region of the window frame  402  where the window  404  will be applied with adhesive, preferably in either paste or film form. The completed window  404  is then brought into contact with the window frame  402  and the adhesive is allowed to cure. Since the adhesive cures at a much lower temperature (−25° to +200° C.) than the window forming process of the prior art (+800° C.), there is less stress on the bond allowing the use of material having greater differences in their coefficient of thermal expansion. 
     To reduce device failures caused by molecules trapped in the package or generated during device operation, some embodiments of the disclosed invention attach a getter to the inside of the lid. FIGS. 7 and 8 show one location for two getter strips  802  on the inside of the package lid. Getters are chosen from existing getter materials based on the quantity, size, and type of molecules to be trapped. One of the principle functions of the getters is to absorb gasses, particularly water vapor, from the device chamber. 
     After the adhesive has cured, the flange of the window frame  402  is seam welded to a metal ring  1002  (shown in FIG. 9) on the ceramic portion  1004  of the device package, completing the packaging of the micromechanical device  1006 . To ensure proper access to the flange, an interface zone  410 , shown in FIG. 4, is kept clear of the window  404  and adhesive. Prior to attaching the window  404  and frame  402 , the micromechanical device  1006 , or other device to be packaged, is electrically connected to the substrate by bond-out wires  1008 . Bond-out wires  1008  electrically connect the device  1006  to contacts on the exterior of the package through wiring in the ceramic substrate  1004 . 
     Preferably, the completed lid is not seam welded onto the remainder of the package until after the adhesive has fully cured. Welding the lid to the remainder of the package prior to completion of the curing process results in the adhesive outgassing vapors into the package chamber  1010  which are often harmful to the packaged device. 
     FIG. 10 is a schematic view of an image projection system  1100  using an improved micromirror package  1102  according to the present invention. In FIG. 10, light from light source  1104  is focused on the improved micromirror package  1102  by lens  1106 . Although shown as a single lens, lens  1106  is typically a group of lenses and mirrors which together focus and direct light from the light source  1104  onto the surface of the micromirror device  1102 . Mirrors on the micromirror device that are rotated to an off position reflect light to a light trap  1108  while mirrors rotated to an on position reflect light to projection lens  1110 , which is shown as a single lens for simplicity. Projection lens  1110  focuses the light modulated by the micromirror device  1102  onto an image plane or screen  1112 . Mirrors in the exterior border region of micromirror device  1102  direct the light impinging on the border region to the light trap  1108 , ensuring that the border region of the display  1114  is very dark and creating a sharp contrast with the interior image portion  1116  of the image plane. 
     Thus, although there has been disclosed to this point a particular embodiment for an adhesive-sealed window lid for an integrated circuit package and method therefore etc., it is not intended that such specific references be considered as limitations upon the scope of this invention except insofar as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims.