Abstract:
A method for increasing the bonding strength between a die and a housing for the die is described where a micro-electromechanical system (MEMS) device is formed on the die. The method comprises depositing a plurality of clusters of contact material onto a bottom surface of the housing, placing the die onto the clusters, and subjecting the housing, the clustered contacts, and the die to a thermocompression bonding process.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to manufacturing of Micro Electromechanical System (MEMS) devices, and more specifically, to attaching dies of MEMS devices to chip carriers. 
   Micro-electromechanical systems (MEMS) include electrical and mechanical components integrated on the same substrate, for example, a silicon substrate. Substrates for MEMS devices are sometimes referred to as dies. The electrical components are fabricated using integrated circuit processes, while the mechanical components are fabricated using micromachining processes that are compatible with the integrated circuit processes. This combination makes it possible to fabricate an entire system that fits within a chip carrier using standard manufacturing processes. 
   One common application of MEMS devices is utilization within inertial sensor. The mechanical portion of the MEMS device provides the sensing capability for the inertial sensor, while the electrical portion of the MEMS device processes the information received from the mechanical portion. Examples of inertial sensors that utilize MEMS devices include gyroscopes and accelerometers. 
   The MEMS production process involves the placement of the operational portion of the MEMS device, sometimes referred to as a micro-machine, within a chip carrier or housing, which is then hermetically sealed. In one known placement process, the die or substrate on which the operational portion of the MEMS device is formed is attached to gold contacts in the carrier using a thermocompression bonding process. However, this thermocompression process involves use of forces that sometimes result in damage to the die, e.g., cracks in the die. The cracks in the die can result in reduced strength in the bond between the chip carrier and the die. Since MEMS devices are often utilized in high gravitational force (high-G) environments, the bond strength between the chip carrier and the die is important. Should the chip carrier and the die become separated, operation of the MEMS device could be compromised. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a method for bonding a die and a housing for the die is provided. A micro-electromechanical system (MEMS) device is formed on the die and the method comprises depositing a plurality of clusters of contact material onto a bottom surface of the housing, placing the die onto the clusters, and subjecting the housing, the clustered contacts, and the die to a thermocompression bonding process. 
   In another aspect, a micro-electromechanical system (MEMS) device is provided which comprises a micro-machine formed on a die, a housing having a bottom surface configured to hold the micro-machine, and a plurality of contact clusters on the bottom surface of the housing. Each cluster includes a plurality of individual contacts which is utilized to bond the die to the housing through a thermocompression process. 
   In still another aspect, a micro-electromechanical system (MEMS) gyroscope is provided. The MEMS gyroscope comprises a housing, a die, and a micro-machine formed on the die which includes at least one sense plate, at least one proof mass suspended above the at least one sense plate, at least one motor drive combs and at least one motor pick-off comb. The gyroscope further comprises a plurality of contact clusters between the die and the housing, each cluster comprising a plurality of individual contacts utilized to bond the die to the housing through a thermocompression process. 
   In yet another aspect, a method for forming a thermocompression bond between a die and a housing is provided. The die has a micro-electromechanical system (MEMS) machine formed thereon and the method comprises depositing a plurality of clusters of contact material between the die and the housing and forming the bond between the die and the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a known MEMS device. 
       FIG. 2  is a top view of a known MEMS device housing showing contacts for a die. 
       FIG. 3  is a top view of a MEMS device housing showing clustered contacts for attaching a die to the housing. 
       FIG. 4  is a side view of a MEMS device which utilizes clustered contacts. 
       FIG. 5  is a schematic view of a MEMS gyroscope which can be produced utilizing the clustered contacts and housing of FIGS.  3  and  4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a diagram of one known embodiment of a Micro-Electromechanical System (MEMS)  100 . MEMS  100  includes a housing  102  (sometimes referred to as a chip carrier) to which a cover  104  is eventually attached in order to form a sealed cavity. Electrical leads  106  provide electrical connections to a micro-machine  108  which includes a die  110  that is attached to housing  102 . As shown in  FIG. 1 , electrical connections  109  are provided through housing  102  to external devices (not shown). For example, in the case of a MEMS tuning fork gyroscope, micro-machine  108  includes proof masses  114 , motor drive combs  116 , and motor pick-off combs  118 . Micro-machine  108  further includes sense plates  120  which form parallel plate capacitors with proof masses  114 . In one embodiment, sense plates  120  are metal films that have been deposited and patterned onto die  110 . 
   Upon completion of the micro-machine portion of MEMS device  100 , cover  104  is attached to housing  102 , forming a substantial hermetic seal. In one embodiment, a cavity  126  is formed when cover  104  is attached to housing  102 . Cavity  126  is first evacuated to remove any gases (i.e. oxygen, hydrogen, water vapor) within cavity  126 . Cavity is then backfilled with a dry gas to a controlled pressure. Typically the dry gas is an inert gas, for example, nitrogen or argon. In another embodiment, cover  104  is attached to housing  102  under vacuum conditions, resulting in vacuum conditions within cavity  126 . Cavity  126  provides an environment that allows components of micro-machine  108  to move freely. For example, proof masses  114  may be movably coupled to micro-machine chip  108  and therefore may oscillate within the vacuum of cavity  126 . Before cover  104  is attached to housing  102 , die  110  is mounted to housing  102  through a thermocompression bonding process utilizing a plurality of contacts  128 . In one embodiment, contacts  128  are made from gold. Since the bonding process utilizes pressure, die  110  is sometimes prone to cracking. The cracking of die  110  could affect operation of MEMS device  100 , especially in high-G environments. 
     FIG. 2  is a top view of housing  102  with die  110  and cover  104  removed, illustrating contacts  128  placed within a bottom surface  140  of housing  102  before die  110  is attached to housing  102  (contacts  128 ) with the thermocompression bonding process. In the embodiment shown, nine contacts  128  are deposited onto bottom surface  140 . In one embodiment, before die  110  is attached with the thermocompression process, contacts have a diameter of about 5 mil. 
     FIG. 3  is a top view of a housing  202 , similar to housing  102  (shown in FIG.  2 ), except that housing  202  has clusters  220  of contacts  228  on a bottom surface  240  of housing  202 . Housing  202  includes a same number (e.g. nine) of clusters  220  as housing  102  (shown in  FIG. 2 ) has contacts  128  (shown in FIG.  2 ). Clusters  220  each include four contacts  228 , although fewer or more contacts  228  can be incorporated into each cluster  220 . In the embodiment shown, nine clusters  220  of contacts  228  are deposited onto bottom surface  140 . By incorporating cluster  220  of contacts  228 , a greater surface area of die  110  is in contact with contacts  228  than with contacts  128 , which as described below, results in a stronger bond between die  110  and housing  202 . In one embodiment, each contact  228  of cluster  220  has a diameter of about two mil. 
     FIG. 4  illustrates a side view of an embodiment of a MEMS device  200  which incorporates cluster contacts  228  as described with respect to FIG.  3 . As compared to contacts  128 , clusters of contacts  228  provide the attachment mechanism for forming thermocompression bonds with a die, while distributing the bond area over more of the die surface, which allows the die to better withstand the pressures utilized in thermocompression bonding as compared to known attachment methods. The higher pressures result in stronger thermocompression bonds with the die than the bonds that are formed in utilizing the known attachment methods (e.g. use of single contacts  128 ). The stronger bonds are sufficiently strong enough to allow operation of a MEMS device on a die that operates in a high-G environment. In addition, the larger surface area of contact also helps to prevent damage to the die. In one embodiment, individual contacts  228  within clusters  220  are about two mil in diameter before contact with a die, and before the thermocompression process. 
   MEMS devices  100  and  200  may comprise more or fewer components than described. For instance, while two electrical contacts  106  are illustrated, those skilled in the art will recognize that a MEMS device may comprise more than two contacts and/or extruding pins as well. Additionally, more or fewer members may be present in MEMS devices  100  and  200  other than those components above described. Further, components of MEMS devices  100  and  200  may comprise multiple functions. Micro-machine  110  may be any such electromechanical machine used in accordance with MEMS and MEMS based devices. In addition, alternate packages may be used as well to provide a housing for MEMS  100  and  200 . The illustrations in the Figures are intended to show embodiments for attaching a MEMS device within a housing utilizing clustered contacts  228  rather than provide a description of a specific MEMS device. 
     FIG. 5  is a schematic illustration of MEMS gyroscope  300  configured to incorporate clustered contacts  228  (shown in  FIGS. 3 and 4 ) to attach die  110  (shown in  FIG. 4 ) to a housing through thermocompression bonding. In one embodiment, MEMS gyroscope  300  includes housing  302  that includes therein, for example, a tuning fork gyroscope (TFG)  304  on die  110  (shown in FIG.  4 ). Housing  302  is configured to be sealed with cover  104  (shown in FIG.  4 ). Housing  302  is typically one of a plastic package, a small outline integrated circuit (SOIC) package, a ceramic leadless chip carrier, a plastic leaded chip carrier (PLCC) package, a quad flat package (QFP), or other housings as known in the art. Housing  302  provides a structure to co-locate elements of TFG  304  and/or locate other elements within a close proximity of one another within the housing  302 . TFG  304 , in one embodiment, is located within a substantially sealed cavity  306  which is formed by bonding cover  104  to housing  302 . 
   In one embodiment, TFG  304  includes proof masses  114 , motor drive combs  116 , motor pick-off combs  118 , and sense plates  120  constructed from a wafer. A pre-amplifier  310  is included within housing  302  and is electrically connected or coupled to each proof mass  114  and sense plate  120  combination. In one embodiment, pre-amplifier  310  and TFG  304  are formed on a common substrate (e.g. die  110 ) and, in one embodiment, are electrically connected. In other embodiments, pre-amplifier  310  is electrically connected to proof masses  114 . An output of pre-amplifier  310  is sent to sense electronics  312 , or alternatively, pre-amplifier  310  is incorporated within sense electronics  312 . 
   In addition, an output  314  of motor pick-off combs  118  is transferred to feedback monitors  316 . Feedback monitors  316  provide output signals  318  to drive electronics  320 , which power motor drive combs  116 . Alternatively, feedback monitors  316  are incorporated within drive electronics  320 . MEMS gyroscope  300  also includes a system power source and other operational electronics, which are not shown in  FIG. 5  for ease of illustration. 
   Motor drive combs  116  excite the proof masses  114  using electrostatic forces by applying a voltage to electrodes of proof masses  114 . Motor pick-off combs  118  monitor the excitation or oscillation of proof masses  114  by monitoring voltage signals on electrodes on proof masses  114 . Motor pick-off combs  118  output a feedback signal to feedback monitors  316 . Feedback monitor  316  provides an output  318  which is input to drive electronics  320 . If proof masses  114  begin to oscillate too fast or too slow, drive electronics  320  may adjust an oscillation frequency such that proof masses  114  vibrate at a resonant frequency. Excitation at such a frequency may enable a higher amplitude output signal to be generated. 
   As above described, incorporation of clustered contacts  228  (shown in  FIGS. 3 and 4 ) provide additional surface area and support when attaching a die, for example, die  110  (shown in  FIG. 4 ) to housing  102  utilizing a thermocompression process. In certain embodiments, clustered contacts  228  provide two or more times as many contact points for attaching a die as compared to known attachment methods utilizing single contacts  128  (shown in FIGS.  1  and  2 ). 
   Utilization of clustered contacts  228 , provides the advantages of thermocompression bonding techniques as known, while also providing a stronger bond between the dies and housings than is provided through utilization of single contacts. Clustered contacts are further usable in other sensor based-devices, including sensor devices where the micro-machine is oriented within a housing with an orientation that is upside down as compared as compared to known mounting orientations. It is also contemplated to utilize the clustered contact attachment methods described herein in a variety of MEMS devices, including, but not limited to, MEMS inertial measurement units, gyroscopes, pressure sensors, temperature sensors, resonators, air flow sensors, and accelerometers. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.