Abstract:
Methods and systems for limiting sensor motion. An embodiment of the invention uses unattached stud bumps to create a shock cage between a spring-mounted pad and a base substrate or a stop ring.

Description:
GOVERNMENT INTEREST 
       [0001]    The invention described herein was made in the performance of work under U.S. Government Contract No. DE-EE0002754 titled Orientation Module 300 and sponsored by the Department of Energy. The Government may have rights to portions of this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    When mounting sensors, performance can be affected, due to stresses transmitted to the sensor device. Typically, isolators are used to reduce transmitted stresses. The effectiveness of an isolator can be limited by the requirements that it survive large deflections from the assembly process and shock and vibration during use. Often an isolator that is “softer” (more flexible) would do the best job of isolation, but might deflect too much during use and, thus, break. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention includes a method for setting small gaps (creating a shock “cage”) utilizing commonly used stud bumps. 
         [0004]    An exemplary isolator device includes a base substrate, a plurality of first metal stud bumps being bonded to the base substrate, a plurality of second metal stud bumps, and a support layer. The support layer includes a support ring, a floating raft configured to receive a sensor device and a plurality of springs. The support ring is bonded to the plurality of first metal stud bump. The floating raft is flexibly attached to the support ring via the springs. The plurality of second metal stud bumps is bonded to only one of the base substrate or the floating raft. The first and second metal stud bumps have a compressed height dimension. The compressed height dimension of the second metal stud bumps is less than the compressed height dimension of the first metal stud bumps. 
         [0005]    In one aspect of the invention, the device further includes a stop ring having a hole with a width dimension, a plurality of third metal stud bumps being bonded to the support ring and a plurality of fourth metal stud bumps being bonded to only one of the floating raft or the stop ring. The third and fourth metal stud bumps have a compressed height dimension. The compressed height dimension of the fourth metal stud bumps is less than the compressed height dimension of the third metal stud bumps. 
         [0006]    This invention uses only methods and structures already required by the process of die mounting to very accurately and repeatedly set stop gaps. It is highly automatable. It can be used to limit the stress seen by the isolator during die attachment or to cage the part completely for operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0008]      FIG. 1  is a perspective view of a sensor package formed in accordance with an embodiment of the present invention; 
           [0009]      FIGS. 2 and 3  are cross-sectional views of the package shown in  FIG. 1 ; and 
           [0010]      FIGS. 4-1  through  4 - 8  are cross-sectional views of steps of an exemplary process for forming a sensor package in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIGS. 1-3  show a sensor package  20  formed in accordance with an embodiment of the present invention. The sensor package  20  includes an isolation layer  30 , a sensor device  40  mounted to the isolation layer  30 , and upper and lower substrate sections  32 ,  34  that are mounted to the isolation layer  30  using stud bump  36 , such as gold stud bumps (GSBs) or physically comparable stud bumps. The upper substrate section (hereinafter “stop ring”)  34  includes a hole  38  for receiving the sensor device  40  but not a raft  50  that is used to support the sensor device  40 . 
         [0012]    The isolator layer  30  includes a raft (a center isolation bond pad)  50  connected to a support structure (i.e., stop ring)  52  via springs  56  formed into the material of the isolation layer  30 . Other support structures are possible, including multiple isolation bond pads or a support “ring” divided into many discrete sections. It is possible to form the isolator layer  30  from various metals, silicon, or glasses using appropriate techniques, such as machining (e.g., ultrasonic, bulk), sintering, electronic discharge machining (EDM), etching (e.g., acid for metals, reactive ion etch for silicon) and other techniques well understood by the various industries. In one embodiment, the isolator layer  30  is formed by a deep reactive ion etching (DRIE) process. 
         [0013]    In one embodiment, the sections  32 ,  34  and the isolation layer  30  are enclosed within a ceramic, plastic, and/or alumina package that encloses and protects the sensor device  40 . 
         [0014]    The stop ring  34  is formed of any suitable material (e.g., metal, silicon, or glass). The inner hole  38  is smaller than the raft  50 , thus allowing the stop ring  34  to interfere with undesired movements of the raft  50 . In one embodiment, the stop ring includes fingers (not shown) that reach over the raft or the stop ring is formed using multiple separate pieces. 
         [0015]    The sensor device  40  is any sensor requiring a firm mounting, which does not induce stress due to coefficient of thermal expansion (CTE) mismatch and external forces on the package  20 . For example, the sensor device  40  is an accelerometer, a gyro, a pressure sensor, or other sensing device. 
         [0016]    In one embodiment, the stud bumps  36  compress in proportion to force applied. The stud bumps  36  get progressively harder to compress as they are crushed and do provide a small spring-back force. The stud bumps  36  adhere to metallized portions of the various surfaces. Metallizations are not applied where there is a desire to not have surface-to-bump bonding. 
         [0017]      FIGS. 4-1  through  4 - 8  show steps of a process for making a sensor package, such as that shown in  FIG. 1 . First, as shown in  FIG. 4-1 , an isolation layer  100  is etched to form a support ring  102 , springs  104 , and a raft  106 . Then, stud bumps  110  are bonded to the raft  106  and the support ring  102  using previously applied metallized pads. At this point, the stud bumps  110  are not compressed. 
         [0018]    Next, as shown in  FIG. 4-2 , a stop ring  114  having a hole  116  (previously etched/machined) is compressed to the stud bumps  110 , thereby causing them to compress and causing the stud bumps  110  to bond to previously applied metallizations located on the support ring  102  and raft  106 . Only the stud bumps  110  bonded to the support ring  102  bond to previously applied metallizations on the stop ring  114 . 
         [0019]    As shown in  FIG. 4-3 , the raft  106  is further compressed into the stop ring  114 , causing further compression of the raft stud bumps  110 . This can be done a number of ways, such as flipping the assembly and applying a force only to the raft  106  or applying force to the stop ring  114  while the raft  106  makes contact with a pedestal (as shown). 
         [0020]    As shown in  FIG. 4-4 , stud bumps  122  are added to a base substrate  120  in order to match those on the isolation layer  100 . The stud bumps  122  are added onto previously applied metallized pads. 
         [0021]    As shown in  FIG. 4-5 , the base substrate  120  is bonded to the isolation layer  100 . Because of the spring  104 , the stud bumps  122  that are under the support ring  102  become compressed. The raft  106  does not include any metallization on a bottom surface, thereby ensuring no bond occurs with the stud bumps  122 . 
         [0022]    Next, as shown in  FIG. 4-6 , stud bumps  130  are applied to a top of the raft  106  through the hole  116  over previously applied metallized pad(s). In another embodiment, the bumps  130  are applied when the bumps  110  are applied or are attached to a mechanism (i.e., sensor)  136  before the mechanism  136  is attached to the raft  106 . 
         [0023]    As shown in  FIG. 4-7 , a force is applied to the mechanism  136  located over the stud bumps  130  on top of the raft  106 , thereby causing the mechanism  136  to bond to the stud bumps  130  while depressing the stud bumps  122  under the raft  106 . 
         [0024]    Then, as shown in  FIG. 4-8 , after the force is released from the mechanism  136 , the springs  104  cause the raft  106  to return to a null position. In the null position shock gaps  140  are formed between the raft  106  and the stud bumps  122  and between the stud bumps  122  on the raft  106  and the stop ring  114 . The isolation raft  106  with the mechanism  136  floats in neutral position with small gaps  140  to limit movement due to shock/vibration, thereby preventing breakage of the springs  104  during excessive shock events. 
         [0025]    In the case where no travel cage is required, the same concept is used to support floating parts of isolator (“rafts”) during diebond so that delicate isolation springs do not break. In this example, steps  1 ,  2 , and  3  ( FIGS. 4-1  through  4 - 3 ) are skipped. Only the isolator is bonded to the substrate (no stop ring in this case). Force used to bond mechanism to isolation raft will compress bumps beneath. This will reduce the travel of the isolator during bonding so springs do not break, and after bond there will be a small gap between isolator and substrate. 
         [0026]    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 order of stud bumping and bonding of the bumps can be arranged differently than what is shown in the Figures. 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.