Abstract:
A method for assembling a micro-electromechanical system (MEMS) device that includes a micro-machine is described. The method comprises forming the micro-machine on a die, the die having a top surface and a bottom surface, providing a plurality of die bonding pedestals on a surface of a housing, and mounting at least one of the top surface of the die and components of the micro-machine to the die bonding pedestals such that a bottom surface of the die at least partially shields components of the micro-machine from loose gettering material.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to manufacturing of Micro Electromechanical System (MEMS) devices, and more specifically to, getter devices and problems caused by gettering materials within MEMS devices.  
           [0002]    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.  
           [0003]    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. One example of an inertial sensor that utilizes a MEMS device is a gyroscope.  
           [0004]    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. Getters are sometimes attached to the housing to facilitate removal of water vapor and hydrogen, for example.  
           [0005]    Getters can however, release particles that can interfere with operation of the MEMs device. In one example, a MEMS gyroscope and other MEMS based inertial devices can be exposed to high-G forces that may cause an amount of particles to be released from the getter, and come into contact with moving components of the MEMS device.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    In one aspect, a method for assembling a micro-electromechanical system (MEMS) device that includes a micro-machine is provided. The method comprises forming the micro-machine on a die, the die having a top surface and a bottom surface, providing a plurality of die bonding pedestals on a surface of a housing, and mounting at least one of the top surface of the die and components of the micro-machine to the die bonding pedestals such that a bottom surface of the die at least partially shields components of the micro-machine from loose gettering material.  
           [0007]    In another aspect, a micro-electromechanical system (MEMS) device is provided that comprises a micro-machine comprising a die and at least one each of a proof mass, a motor drive comb, a motor pick-off comb, and a sense plate. The MEMS device also comprises a housing configured to hold the micro-machine, a cover to be attached to the housing, to form a substantially sealed cavity, and a getter within the substantially sealed cavity. The micro machine is attached to the housing such that the die shields the proof mass, the motor drive comb, the motor pick-off comb, and the sense plate from particles that become dislodged from the getter.  
           [0008]    In still another aspect, a micro-electromechanical system (MEMS) gyroscope is provided that comprises a housing and a micro-machine comprising a die, at least one sense plate, at least one proof mass suspended a distance from said at least one sense plate, at least one motor drive comb and at least one motor pick-off comb. The gyroscope also comprises a getter comprising gettering material and a cover attached to the housing forming a substantially sealed cavity for the micro-machine and getter. The die is mounted within the cavity such that any gettering material that becomes dislodged from the getter is at least partially prevented from contacting the sense plates, the proof masses, the motor drive combs, and the motor pick-off combs.  
           [0009]    In yet another aspectt, a method for mounting a micro-machine portion of a micro-electromechanical system (MEMS) within a housing portion of the MEMS that also contains a getter is provided. The method comprises forming a micro-machine on a die and orienting the micro-machine within the housing such that the die is between the getter and components of the micro-machine.  
           [0010]    In yet still another aspect, a micro-electromechanical system (MEMS) accelerometer is provided that comprises a housing, a micro-machine, a getter, and a cover attached to the housing. The micro-machine comprises a die, at least one sense plate, at least one proof mass suspended a distance from the at least one sense plate, at least one motor drive comb and at least one motor pick-off comb. The getter includes a gettering material, and the cover and housing are configured to form a substantially sealed cavity for the micro-machine and getter. The die is mounted within the cavity such that gettering material that becomes dislodged from the getter is substantially blocked from contacting the sense plates, proof masses, motor drive combs, and motor pick-off combs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a side view of a known MEMS device utilizing a getter.  
         [0012]    [0012]FIG. 2 is a side view of a MEMS device where the micro-machine is mounted in a flipped configuration.  
         [0013]    [0013]FIG. 3 is a schematic view of a MEMS gyroscope which can be produced utilizing the micro-machine described with respect to FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]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 . Die  110  is attached to a bottom surface  122  of housing  102  utilizing contacts  124 . Contacts  124  are sometimes referred to as die bonding pedestals. In one embodiment, the attachment of die  110 , contacts  124 , and housing  102  is accomplished utilizing a thermocompression bonding process or another known bonding process.  
         [0015]    Upon attachment of micro-machine  108  to housing  102 , cover  104  is attached to housing  102  to form 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, and a vacuum is formed 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 die chip  110  and therefore may oscillate within the vacuum of cavity  126 .  
         [0016]    However, the seal between housing  102  and cover  104  is typically not absolute. In one embodiment, a getter  130  which includes a gettering material (not shown) is attached to a getter substrate  132 . Getter substrate  132  is then attached to cover  104 . Getter  130  removes water vapor or other gases (e.g. hydrogen) within cavity  126 , as is known in the art. These gases are known to permeate the seal between housing  102  and cover  104  over time and are also known to be emitted over time (into cavity  126 ) by the materials which make up housing  102  and cover  104 . Removal of the water vapor and gases facilitates maintaining the environment within cavity  126 . The gettering material of getter  130  is typically particle based, and as described above, some gettering material may break free from getter  130 .  
         [0017]    [0017]FIG. 2 illustrates a side view of a MEMS device  200  that includes a housing  200  onto which a cover  204  is attached to provide a substantially sealed cavity  206 . MEMS device  200  includes a micro-machine  208  that is attached to housing  202  in a flipped configuration. The term flipped, as used herein, refers to a mounting orientation of a micro-machine within a housing which is upside down as compared to known mounting orientations. Micro-machine  208  includes a die  210 , proof masses  214 , motor drive combs  216 , and motor pick-off combs  218 . Micro-machine  208  further includes sense plates  220  which form parallel plate capacitors with proof masses  214 . In one embodiment, sense plates  220  are metal films that have been deposited and patterned onto die  210 . Proof masses  214 , motor drive combs  216 , motor pick-off combs  218 , and sense plates  220  are mounted onto die  210  utilizing known processes. However, rather than mounting a bottom surface  222  of die  210  directly to die bonding pedestals, as is done in known MEMS devices, micro-machine  208  is flipped over before being attached to the die bonding pedestals, and therefore other portions of micro-machine  208  are attached to the die bonding pedestals, as further described below.  
         [0018]    As shown in FIG. 2, motor drive combs  216  and a top surface  224  of die  210  is attached to die bonding pedestals  226 , which are located on a bottom surface  228  of housing  202 , typically through a thermocompression bonding process. In one embodiment, die bonding pedestals  226  are gold contacts. By flipping micro-machine  208 , die  210  is also flipped, and bottom surface  222  of die  210  provides protection for operational and moveable portions (e.g. proof masses  214  and sense plates  220 , and portions of motor drive combs  216  and motor pick-off combs  218 ) of micro machine  208 . Protection is provided such that particles of gettering material which become dislodged from getter  230 , for example, due to vibration, are blocked from components of micro-machine  208 , due to the orientation of micro-machine  208  with respect to getter  230 .  
         [0019]    Orientation of die  210  and arrangement of die bonding pedestals  226  also allows such pedestals to be utilized as electrical contacts for components of micro-machine  208 . Referring again to FIG. 2, pedestals  240  are in contact with electrical nodes  242  on die  210 , and pedestals  244  are in electrical contact with motor drive combs  216 . Pedestals  240  and  244  also provide electrical contact with circuits outside of housing  202 , for example, through one of a plurality of electrical conductors  250 . One electrical conductor  250 , is illustrated as providing an electrical path from bottom surface  228  of housing  202  to an exterior surface  254  of housing  202 . A number of such electrical connections utilizing electrical conductors similar to conductor  250  are further described with respect to FIG. 3 below. Additional connections to such conductors can be made to components of micro-machine  208  with additional pedestals  226 .  
         [0020]    MEMS device  200  may comprise more or fewer components than described. For instance, while four electrical connections are illustrated (e.g. four pedestals  226 ), 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 (proof masses, drive combs, pick-off combs, etc.) may be present in MEMS device  200  other than those components above described. Further, components of MEMS device  200  may comprise multiple functions. Micro-machine  208  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 device  200 . The illustrations in the Figures are intended to show embodiments for mounting a micro-machine that provides protection from dislodged gettering material rather than provide a description of a specific MEMS device.  
         [0021]    [0021]FIG. 3 is a schematic illustration of a MEMS gyroscope  400  which incorporates a micro-machine oriented similarly to micro-machine  208 , described with respect to FIG. 2. Such an orientation has been referred to herein as a flipped or upside down orientation. MEMS gyroscope  400  includes a housing  402  (similar to housing  202  (shown in FIG. 2)) that includes therein a micro-machine which is a tuning fork gyroscope (TFG)  404 . Housing  402  is sealed with a cover (not shown). Housing  402  may be 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  402  may provide a structure to co-locate elements of TFG  404  and/or locate other elements within a close proximity of one another within the housing  402 . TFG  404 , in one embodiment, is located within a substantially sealed cavity  406  which is formed by bonding the cover to housing  402 .  
         [0022]    In one embodiment, TFG  404  includes proof masses  414 , motor drive combs  416 , motor pick-off combs  418 , and sense plates  420  constructed on a wafer. A pre-amplifier  422  may be included within housing  402  and is electrically connected or coupled to each proof mass  414  and sense plate  420  combination, for example, through die bonding pedestals  226  (shown in FIG. 2). Pre-amplifier  422  and TFG  404  may both be formed on a common substrate and, in one embodiment, are electrically connected. In other embodiments, pre-amplifier  422  is electrically connected to proof masses  414 . An output of pre-amplifier  422  is sent to sense electronics  424 , or alternatively, pre-amplifier  422  may be incorporated within sense electronics  424 .  
         [0023]    In addition, an output  426  of motor pick-off combs  418  is transferred to feedback monitors  428 . Feedback monitors  428  provide output signals  430  to drive electronics  432 , which power motor drive combs  416 . Alternatively, feedback monitors  428  may be incorporated within drive electronics  432 . MEMS gyroscope  400  may also include a system power source and other operational electronics, which are not shown in FIG. 3 for ease of illustration.  
         [0024]    In other embodiments (not shown) one or more of pre-amplifier  422 , sense electronics  424 , feedback monitors  428 , and drive electronics  432  may be mounted on bottom surface  222  (shown in FIG. 2) of die  210  (shown in FIG. 2). To make electrical connections between these components and components external to housing  202  (shown in FIG. 2), housing  202  can be configured with electrical leads  106  and electrical connections  109 , similar to those shown in FIG. 1.  
         [0025]    Motor drive combs  416  excite the proof masses  414  using electrostatic forces by applying a voltage to electrodes of proof masses  414 . Motor pick-off combs  418  monitor the excitation or oscillation of proof masses  414  by monitoring voltage signals on electrodes on proof masses  414 . Motor pick-off combs  418  output a feedback signal to feedback monitors  428 . Feedback monitor  428  provides an output  430  which is input to drive electronics  432 . If proof masses  414  begin to oscillate too fast or too slow, drive electronics  432  may adjust an oscillation frequency such that proof masses  414  vibrate at a resonant frequency. Excitation at such a frequency may enable a higher amplitude output signal to be generated. Many or all of the above described electrical interconnections may be accomplished utilizing die bonding pedestals when the micro-machine is in a flipped configuration.  
         [0026]    While operation of gyroscope  400  is described, such operation is not likely if particles of gettering materials, for example, as described above, are released within cavity  406 . By orienting the micro-machine in an upside down or flipped configuration, a secondary cavity is essentially obtained which substantially reduces probabilities of gettering particles coming into contact with components, of the micro-machine, including those components which need to be able to move freely for proper operation.  
         [0027]    Such a flipped micro-machine configuration is further usable in other sensor based-devices. It is contemplated to utilize the flipped micro-machine orientation and method 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.  
         [0028]    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.