Abstract:
A low-cost, high-performance, reliable micromirror package ( 300 ) that replaces the ceramic substrate in conventional packages with a printed circuit board substrate ( 30 ) and a molded plastic case ( 33 ), and the cover glass with a window ( 36 ), preferably an optically clear plastic window. The printed circuit board substrate ( 30 ) allows for either external bond pads or flex cable connection of the micromirror package to the projector&#39;s motherboard. These packages support flexible snap-in, screw-in, ultrasonic plastic welding, or adhesive welding processes to overcome the high cost seam welding process of many conventional packages.

Description:
This application is a Divisional of Application Ser. No. 09/991,446, filed Nov. 9, 2001, now U.S. Pat. No. 6,704,131, which ciaims priority under 35 USC §119(e)(1) of Provisional Application No. 60/249,105, filed Nov. 16, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the packaging of semiconductor chips and particularly to that of micro-electrical mechanical systems (MEMS) such as micromirror devices. 
     2. Description of the Related Art 
     Packaging is a critical part of producing a high-performance MICROMIRROR for use in optical applications. Typically in a micromirror, the mirrors land on the substrate surface of the device. To avoid the mirrors from sticking, it is necessary that the micromirror be packaged in a controlled environment, with minimum amounts of moisture, adhesives, dust, and other contaminants. In order to provide such an environment, hermetically sealed packages are often used. However, the cost of such packages and the negative impact on the assembly process is extremely high. 
     The packaging of micromirror chips for use in projection display and other electro-optical applications has continued to present a cost barrier that contributes to higher prices for these products. A lower cost micromirror package is required to reduce the cost of these high-resolution, digital projectors. Today&#39;s micromirror packages are mostly built on custom designed ceramic substrates and have expensive glass covers (lids), which are seam welded or fixed in place with an adhesive. These packages are not only expensive, but they require a low throughput process that reduces the product cycle time. 
     FIG. 1 is a drawing of a welded package. This is a hermetically sealed package that consists of a ceramic substrate  10  with a built-in Kovar lid-attaching ring  11  on its surface and a lid assembly, which consists of a Kovar frame  12  with built-in an optical quality glass window  13 . In the assembly process the micromirror die  14  is attached to the substrate to provide both mechanical and thermal stability, as well as an electrical ground plane. The micromirror leads are bonded to pads on the substrate  10  which extend to external package pads on the edges or bottom of the package. The package is filled with an inert gas and then the lid assembly is seam welded at the two mating surfaces  15  between the lid frame  12  and lid-attaching ring  11 . 
     The adhesive sealed package shown in FIG. 2 has been used in place of the welded package to lower the cost and improve the manufacturing throughput. This package is similar in that it has a ceramic substrate  20  but the cover glass  21  (lid) is a single piece of optical quality glass. In this case, the micromirror  22  is mounted and bonded out in the substrate&#39;s  20  cavity and then the cover glass  21  is attached using an adhesive. The main advantages are that the assembly process is somewhat simplified, which improves the manufacturing throughput and the glass covers  21  are simple pieces of glass that can be sourced from various vendors. 
     The micromirror packages discussed above perform very well but are too expensive and tend to limit cost reduction efforts due to their high cost material and labor content. What is needed is a simple micromirror packaging approach that is low cost, easily assembled, and reliable. The invention disclosed herein fulfills this need. 
     SUMMARY OF THE INVENTION 
     This invention discloses a low cost, high performance, reliable micromirror package that overcomes many of the shortcomings of the conventional ceramic packages commonly used. This approach replaces the ceramic substrate with a printed circuit board substrate, the ceramic case with a molded plastic case, and the cover glass with an optical quality plastic window or lid. The printed circuit board substrate allows for either external bond pads or flex cable connection of the micromirror package to the projector&#39;s motherboard. These packages support flexible snap-in, screw-in, ultrasonic plastic welding, or adhesive welding processes to overcome the low throughput, high cost, seam welding process of many conventional packages. 
     Other advantages include the following: 
     1. Requires no seam welding, 
     2. readily supports assembly automation, 
     3. uses multiple source, commodity piece parts, 
     4. light weight, 
     5. has built-in thermal plane on the bottom of the printed circuit board substrate, 
     6. supports standard chip connection methods, such as edge pad, grid-pad, or flex cable, and 
     7. lower cost. 
    
    
     DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
     The included drawings are as follows: 
     FIG. 1 is a perspective view of a conventional welded hermetically sealed, ceramic, micromirror package. (prior art) 
     FIG. 2 is a perspective view of a conventional epoxy sealed, ceramic, micromirror package. (prior art) 
     FIG. 3 a  is a perspective view of the low-cost, snap-on window, micromirror package of a first embodiment of this invention. 
     FIG. 3 b  is a sectional view of one portion of the package of FIG. 3 a  showing the lid retention mechanism. 
     FIG. 4 is an exploded view of the micromirror package in FIG. 3 a.    
     FIG. 5 a  is a perspective view of the low-cost, adhesive attached window, micromirror package of another embodiment of this invention. 
     FIG. 5 b  is a sectional side view of a portion of the package of FIG. 5 a  showing the adhesively sealed lid. 
     FIG. 6 is an exploded view of the micromirror package in FIG. 5 a.    
     FIG. 7 is perspective view of the low-cost micromirror package of this invention configured with flexible interconnect cables. 
     FIG. 8 is a system level diagram of a one-micromirror projection display system incorporating the low-cost micromirror package of this invention. 
     FIG. 9 is a system level diagram of a 3-micromirror projection display system incorporating three of the low-cost micromirror packages of this invention. 
    
    
     DETAILED DESCRIPTION 
     This invention discloses a low-cost, easily assembled micromirror package. The packages of this invention use fiberglass printed circuit board substrates, molded plastic parts, and lightweight plastic windows instead of ceramic parts and glass windows. The details of the two embodiments are discussed below. 
     FIG. 3 a  shows a first embodiment of the micromirror package  300  of this invention. The package is comprised of a printed circuit board (PCB) base (substrate)  30 , a molded plastic case  33 , a top window-retaining ring  37 , and a plastic optical window  36 , enclosing a micromirror  38 . This invention also allows for the attachment of an optical aperture in the window area of the package, which will prevent stray light from entering around the edge of the package where it can bounce around and contaminate the light coming from the micromirror mirrors. The high cost ceramic used in many conventional packages is replaced by the lower cost PCB  30  (example—FR4) and molded plastic  33  assembly. The PCB substrate  30  contains circuit traces  31 , which are used to bond out the micromirror&#39;s  38  leads to the outside by means of either side contacts  32  or a grid-pad matrix (not shown) on the bottom of the package  300 . Bond wires  39  are shown connecting the micromirror  38  to the PCB traces  31 . A thermal ground plane is also included on the bottom surface of the PCB  30 , which is in effect the bottom of the package  300 . The plastic case  33  is molded around the PCB  30  substrate to form a seal at the bottom of the package. There is an O-ring mating flange  34 , shown in FIG. 3 b , located on the top of the plastic case  33 . The plastic case  33  also has snap-pockets where the window  36  mounting-clamp  37  attaches to the assembly. 
     FIG. 3 b  is a sectional view of one side of the package showing the top window  36  mounting and clamping mechanism. As shown, the plastic case  33  has snap-pockets  40  molded into it to contain the optical plastic window-clamping ring  37 . An O-ring  35  sits on the O-ring-seal flange  34  and then the optical plastic window  36  sits on top of the O-ring. Finally, the mounting clamp  37  is placed over and around the perimeter of the top window  36  and pressed down, compressing the O-ring  35 , locking the snap-hooks  41  into the snap-pockets  40  formed in the molded plastic case  33 , sealing the top portion of the package. 
     FIG. 4 is an exploded view of the micromirror package of the first embodiment of this invention. The PCB  30  and the molded plastic case  33  are mated together in the mold when the plastic case is manufactured, with the PCB  30  becoming the bottom of the package. The micromirror  38  is attached to the PCB  30  and bonded out to the circuit interconnect traces  31 , shown in FIG. 3 a , using standard semiconductor processes. An O-ring  35  is then placed on the O-ring-seal flange  34  surface, shown in FIG. 3 a , of the package. Finally, the window  36 , preferably an optically clear plastic window or a glass window, is placed on top of the O-ring  35 , the mounting clamp  37  is place over and around the edges of the window  36 , and the snap-hooks of retaining ring  37  are snapped into the snap-pockets  40  in the plastic case  33 , compressing the O-ring, to provide a lightweight, sealed assembly. 
     FIG. 5 a  is a drawing showing a second embodiment of the micromirror package  500  of this invention, which uses an adhesive to attach the optically clear top window  56 . The package is comprised of a printed circuit board (PCB) base  50 , a molded plastic case  53  having an adhesive-seal flange (surface)  54 , and an optical window  56 , and encloses a micromirror  57 . This package is similar to that of the first embodiment except for the way the optical window  56  is attached. The bottom of the package is a PCB  50  with circuit traces  51  bringing the micromirror leads out to edge pads  52  or to a bottom grid-pad matrix. As in the earlier case, the micromirror is attached to the PCB and bond wires  58  are attached between the micromirror input/output pads and the PCB traces  51 . A thermal ground plane is also included on the bottom surface of the PCB  50 , which is in effect the bottom of the package  500 . The primary difference in this embodiment is that the O-ring mating flange of the earlier embodiment is replaced with an adhesive-seal flange  54 , which is an integral part of the molded plastic case  53 . In this case, the snap pockets in the plastic case are no longer required. This package has all the benefits of the earlier package; low-cost, lightweight, easy assembly, and good reliability. 
     FIG. 5 b  is a sectional view of the optical plastic window  56  mounting technique for the package in the second embodiment of the invention. As shown, the plastic case  53  has an adhesive-seal flange  54  built into it. The adhesive  55  is dispensed on top of the seal flange  54  surface and the plastic window  56  sits on top of the adhesive  55 . The adhesive is then activated and cured to bond the window  56  to the molded case  53  to provide a completely sealed package. 
     FIG. 6 is an exploded view of the low-cost micromirror package of the second embodiment of this invention. The PCB  50  and the molded plastic case  53  are mated together in the mold when the plastic case is manufactured, with the PCB  50  becoming the bottom of the package. The micromirror  57  is attached to the PCB  50  and bonded out to the circuit interconnect traces  51  (FIG. 5 a ) using standard semiconductor processes. An adhesive  55  is then dispensed on to the adhesive-seal flange  54  surface of the molded plastic case  53  and the optically clear plastic window  56  is placed on top of the adhesive  55 . Finally, the adhesive is activated and cured to properly seal the package. 
     FIG. 7 shows the micromirror packages of this invention with flexible interconnect cables integrally built into the package. Although FIG. 7 shows the first embodiment of the invention, either of the two packages discussed above can be configured with flexible interconnect cables. The package shown in FIG. 7, with flex-cable interconnect capability, is comprised of a printed circuit board base  30  with PCB leads  31 , a molded case  33  with a snap connected optical plastic window  36 , a mounted micromirror  38 , bonding wires  39  connecting the micromirror  38  to the PCB traces  31 , and flexible cables  70  (two shown) with lead traces  72  and attached connectors  71 . This configuration can have up to 4 flex-cables. The connectors  71  are used to connect the micromirror(s) into a motherboard or other bus. 
     FIG. 8 shows a system level block diagram for a single micromirror projection display system. The system is comprised of a light source  80 , a first condenser lens  81 , a motor/color filter wheel assembly  82 , a second condenser lens  83 , a low-cost micromirror in the package of this invention  84 , a fixed or zoom projection lens  85 , and a viewing screen  86 . 
     Another example of a high-brightness micromirror projection display, which uses three of the low-cost micromirror packages of this invention, is shown in FIG.  9 . This system is comprised of a lamp (light source) and reflector assembly  90 , a condenser lens  91 , a turning mirror  92 , a total internal reflective (TIR) prism  93 , three micromirrors (for red, green, and blue light) in the low-cost package of this invention  94 , color splitting/color combining prisms  95 , a fixed or zoom lens  96 , and a viewing screen  97 . 
     While this invention has beer described in the context of two preferred embodiments, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.