Abstract:
Packaging of MEMS and other devices, and in some cases, devices that have vertically extending structures. Robust packaging solutions for such devices are provided, which may result in superior vacuum performance and/or increased protection in some environments such as high-G environments, while also providing high volume throughput and low cost during the fabrication process.

Description:
FIELD  
       [0001]     The present invention relates generally to the field of semiconductor manufacturing and Micro Electro Mechanical Systems (MEMS). More specifically, the present invention relates to methods for packaging of MEMS and other devices, and in some cases, devices that have vertically extending structures thereon.  
       BACKGROUND  
       [0002]     Microelectromechanical system (MEMS) devices often employ semiconductor fabrication techniques to create small mechanical structures on the surface of a substrate such as a wafer. In the production of MEMS gyroscopes and accelerometers, for example, such fabrication techniques are often used to create a number of moving structures that can be used to sense displacement and/or acceleration in response to movement of the device about an input or rate axis. In navigational and communications systems, for example, such moving structures can be used to measure and/or detect variations in linear and/or rotational motion of an object traveling through space. In other applications, such as automotive systems, for example, such moving structures can be used in vehicle dynamic control (VDC) systems and antilock braking system (ABS) to sense changes in vehicle and/or tire motion.  
         [0003]     The packaging of such MEMS devices remains a significant hurdle in the overall fabrication process. In many cases, MEMS die include a MEMS side and a back side. The back side of the MEMS die is often bonded to the floor of a cavity in a MEMS package. Wire bond pads on the MEMS side of the MEMS die are typically wire bonded to bond pads in or along the MEMS package cavity. Finally, a package lid is typically secured to the top of the MEMS package to provide a hermitic seal for the MEMS package cavity. In some cases, the lid is secured in a vacuum or partial vacuum to provide a desired environment for the enclosed MEMS device. When a partial vacuum is used, and in some embodiments, an inert gas may be introduced when the lid is secured to the top of the MEMS package so that an inert gas is back filled into the enclosure housing the MEMS device, but this is not required.  
         [0004]     Due to their size and composition, the mechanical structures of many MEMS devices are susceptible to damage in high-G applications, and from particles, moisture or other such contaminants that can become entrained within the MEMS package cavity. In addition, and in some cases, the difficulty in accurately regulating the pressure within the MEMS package cavity during the fabrication process can affect the performance characteristics of the MEMS device, often reducing its efficacy in detecting subtle changes in motion. Furthermore, some MEMS devices have vertically extending structures that extend up from the MEMS die, which in some cases, may present a challenge for flip-chip die bonding. As such, there is a need for robust packaging solutions for MEMS devices that offer superior vacuum performance and/or increased protection in some environments such as high-G environments, while also providing high volume throughput and low cost during the fabrication process.  
       SUMMARY  
       [0005]     The present invention relates to the packaging of MEMS and other devices, and in some cases, devices that have vertically extending structures thereon. The present invention may provide robust packaging solutions for such devices, which may result in superior vacuum performance and/or increased protection in some environments such as high-G environments, while also providing high volume throughput and low cost during the fabrication process.  
         [0006]     In one illustrative embodiment, a MEMS die is provided that includes a MEMS device secured to a substrate. The MEMS device may include one or more suspended structures positioned vertically above the substrate. The suspended structures may be located in a first region of the substrate, and a second region may extend around the periphery of the first region of the substrate. In some cases, a seal ring is disposed on the second portion of the substrate, wherein the seal ring extends around the first portion of the substrate. A plurality of bond pads may be positioned along the second portion of the substrate. The plurality of bond pads may be positioned inside of the seal ring (e.g. between the seal ring and the one or more suspended structures) and/or outside of the seal ring, as desired.  
         [0007]     Such a MEMS device may be packaged in a corresponding MEMS package. The MEMS package may have a package body that has a recess in a surface thereof to form a cavity. The cavity may be adapted to receive the one or more suspended structures of the MEMS device. A seal ring may be situated on the package body, which encircles the recess. The seal ring may be adapted to be in registration with the seal ring of the MEMS die when the MEMS die and MEMS package are brought together. A plurality of bond pads may be disposed on the surface of the package body, wherein one or more of the bond pads are in registration with one or more of the bond pads of the MEMS die.  
         [0008]     The MEMS die may be flipped over and so that the one or more suspended structures of the MEMS device extend at least partially into the cavity of the MEMS package. The seal ring of the MEMS die may then be aligned with the seal ring of the MEMS package. In some cases, a solder pre-form may be placed between the seal ring of the MEMS die and the seal ring of the MEMS package, which when heated, may form a hermitic seal between the substrate of the MEMS die and the MEMS package. In other cases, a sufficient quantity of bonding material such as gold or aluminum may be provided along the seal ring, and the MEMS die and the MEMS package may be sealed together along the seal ring using thermo-compression bonding. In yet other cases, the MEMS die and the MEMS package may be sealed together along the seal ring using resistance welding, eutectic bonding, or using any other suitable bonding approach.  
         [0009]     In some cases, the MEMS die and MEMS package may be sealed together in a low pressure environment, such as a vacuum environment. This may result in a sealed low pressure environment in the cavity, which for some MEMS devices, may be desirable.  
         [0010]     One or more of the bond pads of the MEMS die may be bonded to one or more bond pads of the MEMS package. In some cases, the one or more bond pads of the MEMS die are bonded to one or more bond pads of the MEMS package at the same time as the MEMS die and the MEMS package are sealed together along their seal rings. The one or more bond pads of the MEMS die may be bonded to the one or more bond pads of the MEMS package using any suitable bonding approach. For example, the one or more bond pads of the MEMS die may be bonded to the one or more bond pads of the MEMS package by, for example, soldering, eutectic bonding, thermo-compression bonding, resistance welding, adhesives, or by any other suitable attachment process.  
         [0011]     Also, it is contemplated that the seal rings of the MEMS die and the MEMS package may be secured by one attachment process, and the bond pads may be secured with the same or different attachment process, as desired. For example, the seal rings may be secured by soldering, and the bond pads may be secured by thermo-compression bonding, or visa versa. When the seal rings are secured by soldering, the seal rings may be made from a material or material system that allows solder to wet to the MEMS die and MEMS package. As noted above, and in some cases, a solder pre-form is provided and placed between the seal rings of the MEMS die and the MEMS package to help form the seal between the seal rings.  
         [0012]     In some cases, the MEMS package may be picked and placed into a bonding chamber. The MEMS package may be photo-registered (e.g. using pattern recognition) for placement accuracy. The MEMS die may in some cases be placed into a flipper station that flips the MEMS die so that the MEMS side of the MEMS die faces down toward the MEMS package. The MEMS die may then be picked by a tool head and photo-registered (e.g. using pattern recognition) for placement accuracy. The tool head may move the MEMS die into position over the MEMS package. The bonding chamber may be evacuated to approximately 10×10 −5  torr, as desired. Heat may be applied to the MEMS die and/or MEMS package, sometimes via the tool head, which may melt the solder pre-form and/or prepare the MEMS package for thermo-compression bonding with the MEMS die. The tool head may also apply force to the MEMS die to help form the bond, creating a hermetically sealed cavity with the MEMS device therein and electrically connected the bond pads to the MEMS package bond pads. The cavity may then be cooled and vented. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]      FIG. 1  is a schematic cross-sectional side view of an illustrative MEMS die, solder pre-form and MEMS package;  
         [0014]      FIG. 2  is schematic top view of the MEMS die, solder pre-form and MEMS package of  FIG. 1 ;  
         [0015]      FIG. 3  is a schematic cross-sectional side view of the illustrative MEMS die, solder pre-form and MEMS package of  FIGS. 1-2  after assembly;  
         [0016]      FIG. 4  is a schematic cross-sectional side view of another illustrative MEMS die, solder pre-form and MEMS package;  
         [0017]      FIG. 5  is a schematic cross-sectional side view of the illustrative MEMS die, solder pre-form and MEMS package of  FIG. 4  after assembly; and  
         [0018]      FIG. 6  is a schematic cross-sectional side view showing an illustrative method for making a MEMS die having an upper sense plate.  
     
    
     DESCRIPTION  
       [0019]     The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized. While the fabrication of MEMS inertial sensors such as MEMS gyroscopes and MEMS accelerometers is specifically discussed, it should be understood that the fabrication steps and structures described herein can be utilized in the packaging of other types of MEMS devices such as electrostatic actuators, optical lenses, RF switches, relay switches, microbolometers, devices with actuatable micro-mirrors, pressure sensors and/or any other suitable device (MEMS or not), as desired.  
         [0020]     Referring now to  FIG. 1 , an illustrative method of packaging a MEMS device will now be described. The illustrative method begins with the steps of providing a MEMS die, generally shown at  10 , having a MEMS gyroscope device  11  secured to a substrate  12 . The MEMS gyroscope device  11  may include one or more suspended structures positioned vertically above the substrate as shown. The suspended structures may be located in a first region  16  (see  FIG. 2 ) of the substrate  12 , and a second region may extend around the periphery of the first region  16  of the substrate  12 . MEMS gyroscopes are often used to sense angular displacement or movement. In many cases, MEMS gyroscopes includes two proof masses that are suspended above a substrate, and are driven electrostatically 180° out of phase along a drive plane. A lower sense plate is often provided under each of the proof masses, often directly on the substrate, to detect deflections in the positions of the proof masses caused by rotation or angular displacement of the gyroscope sensor. An upper sense plate  24  may also be provided above the proof masses to increase the sensitivity of the gyroscope, but this is not required.  
         [0021]     In the illustrative embodiment, MEMS gyroscope  11  includes moving components  18  and  20  (e.g. proof masses), and corresponding sense plates  22  and  24 . Sense plate  24  may be supported by a support structure  16 . In some cases, the MEMS gyroscope  11  may be made by micromachining a silicon substrate, the result of which is bonded to a glass (e.g. Pyrex™) substrate  12 . In some cases, the glass substrate  12  may include one or more patterned metal layers that form, for example, lower sense plates  22  and well as I/O traces. This, however, is only illustrative, and it is contemplated that the MEMS die  10  may be made from any number of materials or material systems including, for example, quartz, silicon, gallium arsenide, germanium, glass, and/or any other suitable material. It should also be understood that other types of MEMS or other devices (e.g. accelerometers, electrostatic actuators, optical lenses, RF switches, relay switches, microbolometers, devices with actuatable micro-mirrors, pressure sensors, etc.) can be packaged in accordance with the present invention, as desired.  
         [0022]     The illustrative MEMS die  10  also includes a number of bond pads  26 . The bond pads  26  may be electrically connected (not illustrated) to the MEMS device  11 , and in particular to one or more sense plates  22 ,  24 , one or more proof masses  20 , and/or other components or parts of the MEMS device  11 , as desired. The bond pads  26  may be positioned between a patterned seal ring  32  and the MEMS device  11 , but this is not required. For example, one or more bond pads  26   a  may be positioned outside of patterned seal ring  32  (see  FIG. 2 ).  
         [0023]     The bond pads  26  may be connected by leads or traces running on, for example, a surface of the substrate  12 , as desired. Each bond pad  26  may include a protrusion of material such as gold or lead or any material or combination of materials suitable to promote the formation of an electrical connection between the bond pad on the MEMS die  10  and the corresponding bond pad on the MEMS package  14 , as further described below. The protrusion may be a solid layer or may be a plurality of bumps or concentric rings, as desired.  
         [0024]     In some embodiments, the MEMS die  10  may also include a patterned seal ring  32 . The patterned seal ring  32  may be formed by a deposition of material or other suitable technique. When a soldering process is used to bond the MEMS die  10  to the MEMS package  14  along the seal ring  32 , the seal ring  32  may be made from gold, lead, tin, aluminum, platinum or other suitable materials or combination of materials suitable for providing a good wetting surface for the solder. Of course, if the sealing mechanism does not rely on solder, the patterned seal ring  32  may be made from a different material or may not be provided at all. For example, a glass frit seal may be used along the seal ring  32  to bond and seal the MEMS die  10  to the MEMS package  14 , particularly if the MEMS die  10  and/or MEMS package  14  include ceramic or the like. In another example, when a thermo-compression bonding process is used to bond the MEMS die  10  to the MEMS package  14  along the seal ring  32 , the seal ring  32  may include a bonding material such as gold, silver, lead, tin, aluminum, or the like, which after sufficient heat and pressure are applied, will form the desired thermo-compression bond.  
         [0025]     The seal ring  32  may completely encircle the MEMS device  11 , and in some cases, the bond pads  26 . Patterned seal ring  32  may be electrically isolated from the MEMS device  11  and from the bond pads  26 . The electrical isolation may be made particularly robust when, for example, resistance welding is used to bond the MEMS die  10  to the MEMS package  14  along the seal ring  32 .  
         [0026]     The illustrative MEMS package  14  shown in  FIG. 1  includes bond pads  28  and/or  28   a  that are configured to be in registration with or otherwise mate with bond pads  26  and/or  26   a  of MEMS die  10 . In the illustrative embodiment, bond pads  28  and  28   a  are electrically connected with leads  30 , which permit the MEMS package  14  to be connected to a larger circuit, such as to bond pads on a printed circuit board (not shown). As can be seen in  FIG. 4 , the leads  30  may extend above the MEMS die  10  after final assembly so that the leads  30  can be used to mount the resulting package directly onto a printed wiring board, multi-chip package or other structure, as desired. Alternatively, and as shown in  FIG. 3 , the leads  30  need not extend above the MEMS die  10 , in which case a hole, recess or other suitable structure may be provided in the printed wiring board, multi-chip package or other structure to accommodate the MEMS die  10 , or a raised ring or other structure may extend up from the printed wiring board, multi-chip package or other structure to accommodate the MEMS die  10 .  
         [0027]     MEMS package  14  may be made from any number of materials or material systems including, for example, ceramic, quartz, silicon, glass or other suitable materials. In some cases, the materials used for MEMS package  14  may be selected to help reduce or relieve mechanical stress and/or strain that may occur between the MEMS die  10  and the MEMS package  14  as the components go through various temperatures during operation and/or fabrication.  
         [0028]     As noted above, the MEMS package  14  may include a patterned seal ring  34  that is configured to be in registration or mate with the seal ring  32  of the MEMS die  10 . The seal ring  34  may be formed like seal ring  32  or may be formed using techniques suitable to the material of the MEMS package  14 . The seal ring  34  may be electrically isolated from bond pads  28  and leads  30 .  
         [0029]     In some cases, the MEMS package  14  may include a cavity  33  with a cavity perimeter  33   a , which is adapted to receive part of the MEMS die  10 , such as the one or more suspended structures of MEMS device  11  (see  FIG. 3 ). A getter  38  may be provided on a surface of the cavity  33  or on another suitable surface such as structure  16  of the MEMS device  11 . The getter may be deposited using sputtering, resistive evaporation, e-beam evaporation or other suitable deposition technique and may be made from zirconium, titanium, boron, cobalt, calcium, strontium, thorium, combinations thereof or other suitable getter material. The getter may be selected to chemically absorb some or all of the gases anticipated to outgas into the cavity  33 , such as water vapor, oxygen, carbon monoxide, carbon dioxide, nitrogen, hydrogen, and/or other gases, as desired.  
         [0030]     In some cases, a solder pre-form  36  may be provided. The solder pre-form  36  may be sized to correspond to patterned seal rings  32  and  34 . Solder pre-form  36  may be formed of indium, lead, tin, gold, other suitable metals or suitable alloys thereof. The solder pre-form  36  may be a separate component placed on the MEMS package  14  during the assembly process. In one illustrative embodiment, solder pre-form  36  is a solder layer deposited onto MEMS package  14  or MEMS die  10  using deposition or other suitable technique.  
         [0031]      FIG. 2  is schematic view depicting the top of the MEMS package  14 , the bottom of the MEMS die  10 , and the solder pre-form  36  of  FIG. 1 . The solder pre-form  36  and seal rings  32  and  34  are shown as having generally the same shape so they mate with each other to form a seal during processing. They are shown as generally rectangular but may be any desired shape. Bond pads  26  and  28  are configured so that they too will mate during the sealing process, and form electrical connections between the MEMS die  10  and the MEMS package  14 .  
         [0032]     When a solder is used to form the seal between the MEMS die  10  and the MEMS package  14 , the MEMS package  14  may be picked and placed into a bonding chamber, and a solder pre-form  36  may be placed on seal ring  34 . The position of the MEMS package  14  may be sensed or verified using photo-registration (e.g. using pattern recognition) or any other suitable technique, as desired. A MEMS die  10  may then be picked and placed, sometimes using a flipper station to first flip the MEMS die  10  so that the MEMS side of the MEMS die  10  faces the MEMS package  14 . A tool may be provided to pick up the MEMS die  10  from the back side. The MEMS die  10  may be photo-registered (e.g. using pattern recognition) for placement accuracy, if desired. One illustrative tool may include a pressure plate for applying pressure to the MEMS die  10  opposite the seal ring and/or the bond pads. The pressure plate may surround a vacuum cup by which the MEMS die  10  is picked.  
         [0033]     When so provided, heat may be applied, sometimes via the tool, to melt the solder perform and/or prepare the MEMS die  10  and/or MEMS package  14  for bonding. The MEMS die  10  may in some cases be kept at a lower temperature, if desired, or brought to the same temperature as the MEMS package  14 .  
         [0034]     In some cases, a controlled environment may be created in the bonding chamber. For example, gases may be extracted from the bonding chamber to form a controlled vacuum pressure therein. The controlled vacuum pressure may be, for example, 1 atmosphere, 0.5 atmosphere, less than 100×10 −5  torr, less than 50×10 −5  torr, less than 15×10 −5  torr, or less than 10×10 −5  torr. In some cases, once the gases are extracted from the bonding chamber, one or more inert gasses may be introduced or otherwise backfilled into the chamber. The backfilled inert gas(es) may be at any pressure, but in some cases, may be less than 10×10 −2  torr, less than 50×10 −3  torr, less than 20×10 −3  torr, or less than 50×10 −4  torr. For some applications, the backfilled inert gas(es) may be about 18×10 −3  torr.  
         [0035]     The tool may then bring the MEMS die  10  into engagement with the MEMS package  14 , and may apply heat and/or pressure to help form the seal between the seal rings and to simultaneously form electrical connections between corresponding bond pads of the MEMS die  10  and the MEMS package  14 . The now formed MEMS package may include the MEMS device  11  in the chamber  33 , as best seen in  FIG. 3 . The getter  38  may be activated by heat or other means, if desired.  
         [0036]     In some cases, the bond pads of the MEMS die  10  and the bond pads of the MEMS package  14  may be secured by thermo-compression bonding. When so provided, the bond pads of the MEMS die  10  and/or the MEMS package  14  may includes bumps formed from a sufficient quantity of bonding material such as gold, silver, lead, tin, aluminum, or the like. In some embodiments, the bonding material is formed of a single material such as either gold or aluminum. In other embodiments, the bonding material is formed of different materials.  
         [0037]     A bonding force can then be applied between the MEMS die  10  and the MEMS package  14  which is sufficient to secure the MEMS die  10  to the MEMS package  14 . This bonding force can be any useful force such as, for example, at least 25,000 kg force, or 50,000 kg force, or 100,000 kg force per cumulative gram of bonding material used for all bond pads. While the bonding force is applied, the bonding material may be heated sufficient to aid in securing the MEMS die  10  to the MEMS package  14 . The heat can be any useful amount sufficient to raise the temperature of the bonding material to a temperature greater than 300, 350, 450, or 500 degrees C., as desired. In some cases, the bond pads may be thermo-compression bonded in accordance with co-pending U.S. patent application Ser. No. 10/878,845, filed Jun. 28, 2004, and entitled “Methods and Apparatus For Attaching A Die To A Substrate”, which is incorporated herein by reference.  
         [0038]     Of course, other suitable equipment and techniques may be used to package the MEMS device  10 . For example, a hinged chamber may be provided that flips the MEMS die  10  over to the MEMS package  14 . Alternatively, or in addition, the entire process may take place in a larger chamber so that multiple MEMS die  10  may be simultaneously bonded to multiple corresponding MEMS packages  14 , as desired. It is also contemplated that the operability of the MEMS device may be verified prior to or after the MEMS die and the MEMS package are secured together.  
         [0039]      FIG. 4  is a schematic cross-sectional side view of another illustrative MEMS die, solder pre-form and MEMS package. This illustrative embodiment is similar to that shown and described above with respect to  FIGS. 1-3 , but in this case, the MEMS package  14  includes on its perimeter a riser  42  that extends beyond MEMS die  10  so that leads  30  may more easily be connected to a circuit board or other component. That is, in the illustrative embodiment of  FIG. 4 , the MEMS package  14  may be configured and used as a leadless chip carrier (LCC) package. In the illustrative embodiment, riser  42  extends beyond the back side of substrate  12  of MEMS die  12 , but it is contemplated that rise  42  may merely extend far enough to permit leads  30  to bond to an adjacent circuit board. For example, in some cases, riser  42  may extend only far enough for the leads  30  to be flush or nearly flush with the back side of substrate  12  of MEMS die  10 . In any event, and as can be seen, the substrate  12  of MEMS die  10  may be somewhat protected when the packaged MEMS device is mounted to a circuit board or the like, because it is situated between the MEMS package  14  and the circuit board or the like (not shown).  FIG. 5  is a schematic cross-sectional side view of the illustrative MEMS die  10 , solder pre-form  36  and MEMS package  14  of  FIG. 4  after assembly.  
         [0040]      FIG. 6  is a schematic cross-sectional side view showing an illustrative method for making a MEMS die having an upper sense plate, such as MEMS die  10 . In the illustrative method, a first wafer  50  is provided. The first wafer  50  may be a glass (e.g. Pyrex™) wafer, or may be any other suitable wafer as desired. A MEMS device  52   a , such as a MEMS gyroscope, accelerometer or other structure, may be bonded to the first wafer  50 . In the illustrative embodiment, a number of MEMS devices  52   a - 52   e  are bonded to the first wafer  50 ; one for each MEMS die  60   a - 60   e.    
         [0041]     In some cases, the MEMS devices  52   a - 52   e  may include one or more suspended structures positioned vertically above the substrate such as shown in  FIG. 1 . In many cases, MEMS gyroscopes includes two proof masses that are suspended above a substrate, and are driven electrostatically 180° out of phase along a drive plane. A lower sense plate may be provided under each of the proof masses, often directly on the first wafer  50 , to detect deflections in the positions of the proof masses caused by rotation or angular displacement of the gyroscope sensor. In some cases, the MEMS devices  52   a - 52   e  may be made by micromachining a silicon substrate, the result of which is bonded to the first wafer  50 . This, however, is only illustrative, and it is contemplated that the MEMS devices  52   a - 52   e  may be made from any number of materials or material systems including, for example, quartz, silicon, gallium arsenide, germanium, glass, and/or any other suitable material.  
         [0042]     The illustrative MEMS devices  52   a - 52   e  may also include a number of bond pads (see  FIG. 1 ). The bond pads may be electrically connected (not illustrated) to the MEMS devices  52   a - 52   e , and in particular to one or more of the sense plates, one or more proof masses, and/or other components or parts of the MEMS devices  52   a - 52   e , as desired. The bond pads may be positioned between a patterned seal ring (see  FIG. 1 ) and the MEMS devices  52   a - 52   e , but this is not required. The bond pads may be connected by leads or traces running on, for example, a surface of the first wafer  50 , as desired. A patterned seal ring may completely encircle the MEMS devices  52   a - 52   e  for each MEMS die  60   a - 60   e , and in some cases, encircle the corresponding bond pads, but this is not required (see  FIG. 1 ).  
         [0043]     A second wafer  54  may also be provided. The second wafer  54  may be a glass (e.g. Pyrex™) wafer, or may be any other suitable wafer as desired. In the illustrative embodiment, a number of recesses  56   a - 56   e  may be etched or otherwise formed in the surface of the second wafer  54 . The depth of recesses  56   a - 56   e  may be adapted to result in a desired spacing between the back wall of the recesses  56   a - 56   e  and the proof masses or other structures of the MEMS devices  52   a - 52   e . A metal or other conductive layer may patterned on or in the recesses  56   a - 56   e  to form one or more upper sense plates, if desired. The patterned or other conductive layer may extend up the edge of the recesses  56   a - 56   e  and make electrical contact with corresponding pads on the first wafer  50 , once the second wafer  54  is bonded to the first wafer, as further described below.  
         [0044]     Additional recesses  58   a - 58   d  may also be etched or otherwise provided in the second wafer  54 . In the illustrative embodiment, the depth of recesses  58   a - 58   d  may be set so that a saw blade or the like can cut or otherwise remove regions  62   a - 62   d  of the second wafer  54  without engaging or otherwise damaging the first wafer  50 , as further described below. In the illustrative embodiment, the depth of recesses  58   a - 58   d  may be greater than the depth of recesses  56   a - 56   e.    
         [0045]     The second wafer  54  may be flipped and bonded to the first wafer  50  such that the metal or other conductive layer on or in the recesses  56   a - 56   e  form one or more upper sense plates for the MEMS devices  52   a - 52   e . The second wafer  54  may be bonded to the first wafer  52  using any suitable method. In one illustrative embodiment, the second wafer  54  is bonded to the first wafer  52  using anodic bonding.  
         [0046]     Once the second wafer  54  has been bonded to the first wafer  52 , the resulting wafer pair may be diced to provide individual MEMS die  60   a - 60   e . In the illustrative embodiment, a saw blade or the like can cut or otherwise remove regions  62   a - 62   d  of the second wafer  54 , preferably without engaging or otherwise damaging the first wafer  50 . Next, a saw blade or the like can be used to cut or otherwise remove regions  66   a - 66   d  of the first wafer  50  to separate the individual MEMS die  60   a - 60   e  from each other.  
         [0047]     Having thus described the several embodiments of the present invention, those of skill in the art will readily appreciate that other embodiments may be made and used which fall within the scope of the claims attached hereto. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention.