Abstract:
One method of the present invention includes preparing a die with traces and pads as desired for the intended use of the die. A MEMS device is mounted to the die. The die is then mounted to a substrate of the same material as the die. The substrate is then mounted to a package. The die and/or the substrate may be flip-chip mounted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Microelectromechanical systems (MEMS) devices or computer chips are often mounted on dies. Stresses due to die mounting can affect the performance of MEMS devices. The stresses change with temperature because of the differing coefficients of thermal expansion (CTE) of the package and the die; dies made of silicon or borosilicate glass typically have a lower CTE than that of the package, so as temperature changes, the package stresses the die. Thus, as shown in  FIGS. 1A and 1B , a die  10  with a lower CTE than a package  12  are heated when attaching the die  10  to the package  12 . When the assembly  14  cools, the package experiences more shrinkage than the die, and creates stress at attachment points  16 . The stress affects the performance of the MEMS device  9 . 
         [0002]    The stresses also change due to shock when wire bonds change shape, which causes problems for capacitive sensors that require stable geometry for stable output. Additionally, making wire bonds at the package level makes the parts more expensive because each part must be handled separately, as well as increasing the chance of damage to the assembly. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention provides a system and method of reducing the stress associated with a die mount, as well as eliminating the wire bonding operation. 
         [0004]    In an example embodiment, a die is prepared with traces and pads as desired for the intended use of the die. A MEMS device is mounted to the die. The die is then mounted to a substrate of the same material as the die. [MEMS TO DIE ATTACH STEP.]The substrate is then mounted to a package. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0005]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0006]      FIG. 1A  is a cross-section of a package-mounted die according to the prior art at a temperature sufficient to attach the die to the package; 
           [0007]      FIG. 1B  is a cross-section of the package-mounted die of  FIG. 1A  after it has cooled; 
           [0008]      FIG. 2A  is a perspective view of a package-mounted die according to the present invention; 
           [0009]      FIG. 2B  is an exploded perspective view of the package-mounted die of  FIG. 2A ; 
           [0010]      FIG. 2C  is a cross-sectional view of the package-mounted die of  FIG. 2A  through the plane AA′; 
           [0011]      FIG. 3  is an alternate embodiment of a cover plate according to the present invention; and 
           [0012]      FIG. 4  is a flow diagram of a method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    With reference to  FIGS. 2A-2C , a device  20  according to an embodiment of the present invention is shown. A MEMS device  22 , which may be an accelerometer, a gyroscope, or any other suitable MEMS device, is mounted to a rectangular block die  24  during the MEMS build. The die  24  is mounted to a rectangular block cover plate  26  via gold bumps  30  (which can be approximately 0.001″ to 0.005″ in diameter [0.001 TO 0.005 INCHES IN DIAMETER?], and the die  24  can be flip-chip mounted to the cover plate  26 . The gold bumps  30  provide all of the structural strength necessary to prevent dislodging the die  24  during use. The MEMS device  22  can be protected from contact during flip-chip mounting using etched features or process controls. The cover plate  26  is mounted to a package  28 , a rectangular block including a recess  23  (or wirebond shelf) with a step  25 , sized and shaped to receive the MEMS device  22 , the die  24 , and the cover plate  26 . The cover plate  26  is mounted to the step  25  of the package  28  via gold bumps  32 , and the cover plate  26  can be flip-chip mounted to the package  28 . The package  28  can be a Leadless Ceramic Chip Carrier (LCCC). Electrically conductive traces  34  from the MEMS device  22  to the gold bumps  30 , and conductive traces  36  from the gold bumps  30  to the gold bumps  32  provide a connection between the MEMS device  22  and the package  28  so that the MEMS device  22  can be in electrical contact with devices or systems (not shown) outside the package as necessary depending on the intended use of the device  22 . 
         [0014]    The die  24  has a preselected CTE, and the package  28  has a preselected CTE that is higher than the CTE of the die  24 . The cover plate  26  is preferably made of the same material as the die  24  with the same CTE as the die  24 . Alternatively, the cover plate  26  is made of a different material than the die  24 , as long as the CTE of the cover plate  26  has an intermediate CTE higher than the CTE of the die  24  and lower than the CTE of the package  28 , and preferably the CTE of the cover plate  26  is closer to the CTE of the die  24  than the package  28 . 
         [0015]      FIG. 3  shows a portion of an alternate embodiment of the present invention. The cover plate  26  includes a composite of first and second materials  27 , 29 . The die has a first CTE. The second material  29  has a CTE greater than the CTE of the die  24  and less than the CTE of the first material  29 . The first material  27  has a CTE less than the CTE of the package  24 . The package  24  is attached to the first material  27 , and the die is attached to the second material  29 . 
         [0016]      FIG. 4  shows a process flow diagram of a method  40  for creating an embodiment of the present invention. At a block  42 , a MEMS device is mounted to a die having a CTE equal or greater than the CTE of the MEMS device. At a block  44 , the die is mounted to a cover plate having a CTE greater than the CTE of the die and less than the CTE of a package. Finally, at a block  46 , the die is mounted to the package. 
         [0017]    Note that the electrically conductive traces  34 ,  36  electrically connect the MEMS device to the package. The traces are mechanically more stable under shock than wirebonds, because they are printed on the package and cover plate. 
         [0018]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, the embodiments disclosed use gold bumps, but solder balls, conductive polymers, or directional conductors may be used instead. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.