Patent Publication Number: US-2016229684-A1

Title: Mems device including support structure and method of manufacturing

Description:
RELATED APPLICATIONS 
     This patent application claims priority from U.S. 61/881,643, the disclosures of which are incorporated herein, in their entirety, by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to MicroElectroMechanical Systems (MEMS) devices and more specifically to a MEMS device for reducing sensitivity to external forces or pressure. 
     BACKGROUND 
     Micro Electro Mechanical Systems (MEMS) devices, in particular accelerometers and angular rate sensors or gyroscopes (i.e. inertial sensors), are being used in a steadily growing number of applications. Due to the significant increase in consumer electronics applications for MEMS sensors such as smart phones, optical image stabilization ( 01 S) for phones and cameras and wearable electronics there has been a growing interest in utilizing such technology for more advanced applications traditionally catered to by much larger, more expensive higher grade non-MEMS sensors. These applications include single and multiple axis devices for industrial applications, Inertial Measurement Units (IMUS) for navigation systems and Attitude Heading Reference Systems (AHRS), control systems for unmanned air, ground and sea vehicles and for precise personal indoor and even GPS-denied navigation. They also may include healthcare/medical and sports performance monitoring and advanced motion capture systems for next generation virtual reality. These advanced applications require lower bias drift and higher sensitivity specifications well beyond existing consumer-grade MEMS inertial sensors on the market. In order to expand these markets, the higher performance specifications must also be developed and addressed by producing a low cost and small size sensor and/or a MEMS inertial sensor-enabled system. 
     In general, a MEMS device must interact with a particular aspect of its environment while being protected from damage. For example, a micro mirror has to interact with light and an electrical addressing signal, while being protected from moisture and mechanical damage. An accelerometer has to be free to move in response to accelerated motion, but be protected from dirt and moisture, and perhaps also be kept under vacuum or low pressure to minimize air damping. 
     In many applications, MEMS devices are sensitive to variations in ambient pressure. In some cases, such as for pressure sensors, this sensitivity is desirable. However, in many other applications, sensitivity to outside pressure is undesirable as it interferes with the parameter actually being measured. This can be particularly problematic in the case of capacitive sensors which can respond to outside pressure or other external forces, such as those exerted during wire bonding if the device is not packaged carefully. 
     To illustrate the undesirable effects of pressure sensitivity, consider the particular example of a capacitive inertial sensor. The earliest forms of MEMS inertial sensors were accelerometers etched in bulk silicon wafers. These accelerometers consist of a large proof mass suspended from a thin compliant beam or spring. The mass and spring move in response to acceleration, and the movement is detected capacitively using the mass and cap (or caps) as capacitor plates. The change in position of the proof mass relative to the cap electrode is proportional to the acceleration being experienced by the proof mass. However, if the pressure outside the package is different from the pressure inside, the top electrode can flex, adding a non-inertial error term to the measurement. The amount of flex is proportional to Pw 4 /t 3  where “P” is the pressure differential across the thickness of the cap and “w” and “t” are the width and thickness respectively of the unsupported portion of the cap. 
     Early accelerometers minimized the effects of pressure by using very thick cap electrodes and operating at atmospheric pressure. However, in state-of-the-art motion sensors, it is desirable to minimize the height of the packaged MEMS so that it can fit in consumer applications such as cell phones. Additionally, in order to use the same technology to fabricate a resonant gyroscope, the package must be at vacuum to minimize the damping of the proof mass motion. 
     Surface micromachining has helped to alleviate the pressure sensitivity of the cap and has helped to reduce the chip size. Surface micromachining techniques include the use of thin films to form MEMS structures. In particular, in surface micromachined inertial sensors, polycrystalline silicon is used to form the springs and proof mass in a single layer. With this arrangement, the proof mass moves laterally in response to x and y acceleration, and the motion is detected with comb capacitors. Since the capacitive detection between the proof mass and the cap is removed, they are less sensitive to outside pressure. However, since the MEMS material is deposited using thin film processes, the mechanical polysilicon films tend to be thin, on the order of a few microns rather than the hundreds of microns of bulk micromachined sensors. Thus, the proof mass and electrode area are small, reducing sensitivity and increasing mechanical noise. 
     In order to improve the performance of the described MEMS sensors, it is desirable to mitigate or reduce the pressure sensitivity of MEMS devices. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, a micro-electro-mechanical system (MEMS) device is provided. The MEMS device includes a top cap wafer, a bottom cap wafer and a MEMS wafer disposed between the top cap wafer and the bottom cap wafer. The top cap wafer, the bottom cap wafer and the MEMS wafer define sidewalls of a cavity or chamber. A MEMS structure is housed within the cavity and can move relative to the top and bottom caps. At least one electrode is provided in one of the top cap wafer, the MEMS wafer and the bottom cap wafer. This at least one electrode is operatively coupled to the MEMS structure to detect or induce a movement of the MEMS structure. A support structure extends through the cavity from the top cap wafer to the bottom cap wafer to prevent bowing in the top cap and/or bottom cap wafer(s). 
     In some embodiments, the support structure comprises a cap portion formed within the top cap wafer, a core portion formed within the MEMS wafer and a base portion formed within the bottom cap wafer. 
     In some embodiments, the top cap wafer, the bottom cap wafer and the MEMS wafer are made of electrically-conductive material. 
     In some embodiments, the top cap wafer, the bottom cap wafer and the MEMS wafer are made of silicon-based material. 
     In some embodiments, the support structure is electrically conductive. 
     In some embodiments, the top cap wafer has inner and outer sides, the MEMS wafer has first and second sides and the bottom cap wafer has inner and outer sides. The inner sides of the top and bottom cap wafers are electrically bonded to the first and second side of the MEMS wafer, respectively. 
     In some embodiments, the MEMS wafer is a silicon-on-insulator (SOI) wafer comprising a device layer, an insulating layer and a handle layer. 
     In some embodiments, the support structure includes a conducting shunt extending from the device layer to the handle layer, through the insulating layer. p In some embodiments, the support structure passes through the MEMS structure without interfering with movement of the MEMS structure. 
     In some embodiments, the MEMS structure is a suspended proof mass, preferably suspended by four flexural springs. 
     In some embodiments, at least one of the cap portion and the base portion is delimited by insulated closed-loop channels etched through the corresponding top or bottom cap wafer. 
     In some embodiments, the core portion is spaced away from the MEMS structure and surrounded by a clearance gap etched through the MEMS wafer. 
     In some embodiments, the cap wafer and the bottom cap wafer respectively include electrical contacts electrically connected to the support structure for transmitting electrical signals between the respective electrical contacts of the bottom cap and top cap wafers via the support structure. 
     In some embodiments, the MEMS device includes at least one additional support structure extending though the cavity from the top cap wafer to the bottom cap wafer. 
     In accordance with an aspect of the invention, a method for manufacturing a MEMS device is also provided. The method includes the steps of:
         providing a top cap wafer and a bottom cap wafer having respective inner and outer sides, patterning in the top and bottom cap wafers respective cap and base portions of a support structure to be formed and respective top and bottom sidewalls of a cavity to be formed, and at least one electrode in one of the top and bottom cap wafers;   providing a MEMS wafer having first and second sides, and patterning on one of the first and second sides at least a part of a MEMS structure and a part of a core portion of the support structure;   bonding the side of the MEMS wafer patterned in the previous step to the inner side of one of the top and bottom cap wafers by aligning the corresponding cap or base portion to the part of the core portion of the MEMS wafer;   patterning on the other side of the MEMS wafer the remaining part of the MEMS structure, the remaining part of the core portion, and lateral sidewalls of the cavity; and   bonding the side of the MEMS wafer patterned the previous step to the inner side of the other one of the top and bottom cap wafers by aligning the top, lo bottom and lateral sidewalls to form the cavity housing the MEMS structure therein, the at least one electrode being operatively coupled to the MEMS structure, and by aligning the corresponding cap or base portion of said other cap wafer with the remaining part of the core portion, the support structure extending through the cavity from the bottom cap wafer to the top cap wafer to prevent bowing in the top cap and bottom cap wafers.       

     In some embodiments of the method, the top, bottom and MEMS wafer are electrically conductive, and the bonding steps are made with a conductive bond. 
     In some embodiments of the method, the cap and base portions are formed by etching trenches in the respective inner sides and at least partially through the top and bottom cap wafers, and by filling the trenches with an insulating material or an insulating lining followed by a conductive fill. 
     In some embodiment, the method includes a step of removing a portion of the outer sides of the top and bottom cap wafers to isolate the at least one electrode and the cap and base portions. 
     In some embodiments, the method includes a step of forming first and second electrical contacts on the outer sides of the top and bottom cap wafers, respectively, the first electrical contact being electrically connected to the cap portion and the second electrical contact being electrically connected to the bottom cap portion. 
     In some embodiments, the method includes the patterning a clearance gap within parts of the MEMS structure to form the support structure, such that after completing the MEMS device, the support structure passes through the MEMS structure. 
     In some embodiments of the method, the MEMS wafer is an SOI wafer with an insulating layer separating a device layer from a handle layer, the method comprises forming a conducting shunt between the device and handle layers in said part of the core portion. 
     Advantageously, some embodiments of the MEMS device and of the method take advantage of larger masses and electrode areas available through micromachined inertial sensors. Some embodiments of the present invention also allow minimizing the height of the MEMS device. Some embodiments of the present invention allow mitigating or reducing the pressure sensitivity of a MEMS packaging, and in particular, they take advantage of the larger masses and electrode areas available with bulk micromachined inertial sensors without having to worry about errors introduced by cap electrode flexing due to pressure variations. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       It should be noted that the appended drawings illustrate only exemplary embodiments of the invention and should therefore not be considered limiting of its scope, as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic perspective view of a MEMS device, according to a possible embodiment.  FIG. 1A  is a schematic cross-sectional view of the MEMS device of  FIG. 1 , taken along line A-A 
         FIG. 2  is a schematic exploded perspective view of the MEMS device of  FIG. 1 .  FIG. 2A  is a schematic exploded cross-sectional view of the MEMS device of  FIG. 1 .  FIG. 2B  is a schematic exploded cross-sectional view of a MEMS device according to another possible embodiment. 
         FIG. 3  is a schematic top view of the MEMS wafer of the MEMS device of  FIG. 1 . 
         FIG. 4  is a schematic top view of a top cap wafer of the MEMS device of  FIG. 1 .  FIG. 4A  is a cross-sectional view of the top cap wafer of  FIG. 4 , during the manufacturing process.  FIG. 4B  is another cross-sectional view of the top cap wafer of  FIG. 4 , during another step of the process, according to a possible embodiment. 
         FIG. 5A  is a cross-sectional view of the bottom cap wafer of the MEMS device of  FIG. 1 , during the manufacturing process, according to a possible embodiment. 
         FIG. 6  is a schematic top view of a MEMS wafer of the device of  FIG. 1 .  FIG. 6A  is a cross-sectional view of the MEMS wafer of  FIG. 6 , during the manufacturing process.  FIG. 6B  is another cross-sectional view of the MEMS wafer of  FIG. 6 , during another step of the process, according to a possible embodiment. 
         FIG. 7  is a schematic top view of the MEMS wafer of the device of  FIG. 1 .  FIG. 7A  is a cross-sectional view of the MEMS wafer of  FIG. 7 , during the manufacturing process. 
         FIG. 8  is a schematic exploded view of the top cap wafer of  FIG. 4  and of the MEMS wafer of  FIG. 7 .  FIG. 8A  is a cross-sectional view of the top cap and MEMS wafers of the MEMS device of  FIG. 1 , during the manufacturing process, showing the bonding of the top cap wafer to the MEMS wafer. 
         FIG. 9  is a schematic top view of the MEMS wafer and of the top cap wafer of the MEMS device of  FIG. 1 , during a possible step of the manufacturing process.  FIG. 9A  is a cross-sectional view of the MEMS wafer bonded to the top cap wafer shown in  FIG. 9 . 
         FIG. 10  is a schematic exploded perspective view of the bottom cap wafer and of the MEMS wafer bonded to the top cap wafer of the device of  FIG. 1 , during a possible step of the manufacturing process.  FIG. 10A  is a cross-sectional view of the top and bottom cap wafers bonded to the MEMS wafer of  FIG. 10 . 
         FIG. 11  is a schematic perspective view of the device of  FIG. 1 , during the manufacturing process.  FIG. 11A  is a cross-sectional view of the device of  FIG. 1 , during a possible manufacturing step. 
         FIG. 12  is a schematic perspective view of the device of  FIG. 1 .  FIG. 12A  is a cross-sectional view of the device of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
     The present invention provides a micro-electro mechanical system (MEMS) device, such as a sensor or an actuator, whose architecture includes a support structure that enables a thin cap to be used as part of the MEMS device. The support structure advantageously allows minimizing the sensitivity of the cap to pressure or other forces. The present invention also provides a method for manufacturing such a MEMS device. In an exemplary embodiment, the support structure allows to reduce or prevent flexure of cap electrodes and pressure sensitivity in a three-dimensional (3D) motion sensor, which can include one or several pendulous proof mass or masses. The support structure is preferably fabricated using a 3D packaging architecture which can also provide, in addition to mechanical support, isolated electrical pathways through the package. The support structure can include a three-dimensional through-chip-via, so as to provide an access extending through the several wafer(s) forming the MEMS device to route electrical signals through the MEMS device. Throughout the description, the term MEMS encompasses devices such as, but not limited to, accelerometers, gyroscopes, pressure sensors, magnetometers, microphones, actuators, micro-fluidic, micro-optic devices and the like. The MEMS wafer may also include microelectronic circuits such as power amplifiers, detection circuitry, GPS, microprocessors, and the like. 
     In the present description, the terms “top” and “bottom” relate to the position of the wafers as shown in the figures. Unless otherwise indicated, positional descriptions such as “top”, “bottom” and the like should be taken in the context of the figures and should not be considered as being limitative. The top cap wafer can also be referred as a first cap wafer, and the bottom cap wafer can be referred as a second cap wafer. The terms “top” and “bottom” are used to facilitate reading of the description, and persons skilled in the art of MEMS know that, when in use, MEMS devices can be placed in different orientations such that the “top cap wafer” and the “bottom cap wafer” are positioned upside down. In this particular embodiment, the “top” refers to the direction of the device layer. 
     Referring to  FIGS. 1 and 1A , a possible embodiment of a micro-electro-mechanical system (MEMS) device  10  is shown. The MEMS device can also be referred to as a MEMS package. The MEMS device  10  includes a top cap wafer  12  and a bottom cap wafer  14  and a MEMS wafer  16  disposed between the top and bottom cap wafers  12 ,  14 . The top cap wafer  12  has inner and outer sides  121 ,  122 , the MEMS wafer  16  has first and second sides  161 ,  162  and the bottom cap wafer  14  has inner and outer sides  141 ,  142 . The inner sides  121 ,  141  of the top and bottom cap wafers  12 ,  14  are preferably electrically bonded to the first and second side  161 ,  162  of the MEMS wafer  16 , respectively. The top cap wafer  12 , the bottom cap wafer  14  and the MEMS wafer  16  define sidewalls  124 ,  164 ,  144  of a cavity or chamber  31 . The three wafers  12 ,  16 ,  14  are bonded together, preferably under vacuum, to provide a hermetically sealed cavity  31 . A MEMS structure  17  is housed within the cavity  31  and can move relative to the top and/or bottom caps  12 ,  14 . A MEMS device or package configured as such thus includes elements surrounding and/or protecting MEMS structure such as a sensor or an actuator. In this particular embodiment, the MEMS device  10  is a motion sensor, and the MEMS structure  17  is a bulk proof mass suspended by flexible springs (not visible in this cross-section, but shown in  FIG. 3 ) which are themselves connected to an outer frame formed at least partially by the MEMS wafer  16 . At least one electrode is provided in one of the top cap wafer  12 , the MEMS wafer  16  and the bottom cap wafer  14 . The electrode(s) is/are operatively coupled to the MEMS structure  17  to detect or induce a movement of the structure. By operatively coupled, it is meant that the electrode is capacitively, electrically and/or magnetically connected or linked to the MEMS structure  17 . Typically, the MEMS device  10  will include several electrodes located within the top, bottom or MEMS wafer so as to be able to detect a movement of the structure  17 . In the present embodiment, the MEMS device  10  includes five top electrodes  13  (identified in  FIG. 1 ) and five bottom electrodes  15  (best shown in  FIG. 2 ). The electrodes  13 ,  15  form capacitors with the MEMS structure  17 . 
     The MEMS device  10  also includes a support structure  48 , or post, extending through the cavity  31  from the top cap wafer  12  to the bottom cap wafer  14  to prevent bowing in the top cap  12  and/or the bottom cap  14  wafers. The support structure  48  spans the height of the MEMS device  10 , from bottom cap  14  to top cap  12  and can, when necessary, penetrate the MEMS movable structure  17 , preferably without inhibiting its motion. In the present embodiment, insulated vias or channels etched within the different wafer layers allows the creation of a mechanical support  48  which prevents the top and bottom caps  12 ,  14  from deforming and/or flexing. This support structure  48  is preferably formed of a conducting material, such that the support structure  48  can transmit, control and/or inhibit flow of current passing through it, which may or may not be desirable according to different applications in which the MEMS device  10  is used. The support structure  48  can thus be used for both its mechanical and electrical properties. 
     Referring to  FIGS. 2 and 2A , the support structure  48  preferably includes a cap portion  48   a  formed within the top cap wafer  12 , a core portion  48   b  formed within the MEMS wafer  16  and a base portion  48   c  formed within the bottom cap wafer  14 . At least one of the cap portions  48   a,    48   c  is delimited by an insulated closed-loop channel  29  etched through the cap wafer  12  and/or  14 . The cap and base portions  48   a,    48   b  are preferably isolated from the remainder of the wafer caps. In the present case, the channels  29  are lined with an insulating material  30 , such as silicon dioxide (SiO 2 ) or any other suitable material. The channel may also be optionally filled with a conducting material  32  including one of metal (e.g., copper), silicon and polysilicon. Alternatively, the channel  29  could be filled only with an insulating material. The core portion  48   b  is spaced away from the MEMS structure  17 , and surrounded by a gap  50  etched through the MEMS wafer  16 . 
     The top cap wafer  12 , the bottom cap wafer  14  and the MEMS wafer  16  are preferably made of electrically-conductive material, such as a silicon-based material. The MEMS wafer  16  is preferably a silicon-on-insulator (SOI) wafer, which includes a device layer  20 , an insulating layer  24  and a handle layer  22 . In this case, the support structure  48  may include conducting shunts  34  (or electrical SOI vias) extending from the device layer  20  to the handle layer  22  through the insulating layer  24 , making the core portion  48   b  electrically conductive over its entire length. The support structure  48  passes through the MEMS structure  17  without interfering with the movement of the MEMS structure  17 . In the present embodiment, the support or post  48  is centered within the MEMS structure  17 , which in this embodiment consists in a proof mass  17 . However, in other embodiments, it is possible for the support structure to be located off-center relative to the MEMS structure  17 , for example it could be located near one side of the MEMS structure  17 . 
     The sense electrodes  13 ,  15  are isolated by insulating channels and sense capacitor gaps  38  are provided in both the top and bottom caps  12 ,  14 . The inertial sensor&#39;s MEMS structure  17  consisting of a proof mass and suspension spring (not visible in this cross-section) is fabricated in the device layer  20  of the SOI wafer  16 . Various insulated conducting pathways can be provided in the MEMS device. The insulated conducting pathways can be referred to as three-dimensional though-chip-vias (3DTCVs). The pathways are constructed by aligning feedthrough structures on each level of the MEMS device. Sections of the pathways are thus provided in the MEMS wafer to conduct electrical signals between the top and bottom caps. Some of the pathways can, for example, pass through the support structure  48 . Conducting shunts or plugs  34  can be provided through the insulating layer  24  (typically buried oxide (BOx)) in the MEMS wafer  16  between the device layer  20  and handle layer  22 , in select places to provide a conducting path from the bottom cap to the top cap. Where an insulating mechanical support is required, the conducting plugs  34  can be omitted. 
     When the sense capacitor gaps  38  are etched in the top and bottom caps  12 ,  14 , silicon is left unetched at the desired location of the support structure to form cap  48   a  and base  48   c  portions of the support structure. Additionally, a clearance gap  50 , which serves to separate the support structure  48  from the surrounding MEMS structure  17 , is etched around the core portion of the support  48   b  in the MEMS wafer. The clearance gap  50  can be provided in many shapes and can for example be annular in shape. While it is preferable to provide at least one support in the center of the MEMS structure, as illustrated in  FIG. 2A , it is also possible to form one or more supports  48  in other locations, as per  FIG. 2B , which shows an alternate embodiment of a MEMS device  100 . The MEMS device can include at least one additional support structure  48 ′ extending though the cavity  31  from the top cap wafer  12  to the bottom cap wafer  14 . The support(s)  48 ,  48 ′ can also have other configurations and/or shapes; however, a circular pillar shape is preferred, since it allows for providing a uniform clearance gap  50  around the support  48  for the proof mass to move. Of course, other shapes are possible, such as a rectangular post for example. 
     It is desirable to minimize the thickness of the caps to more easily fabricate through-cap structures such as electrical vias and electrodes and to reduce the overall height of the completed device. For example, the top cap wafer can have a thickness on the order of 100 um to 200 um, while the MEMS wafer has a thickness between 50 and 700 um, and therefore the proof mass will also typically measure between 50 and 700 um in thickness. As the ratio of the cap width to cap thickness increases, the device becomes more and more sensitive to pressure. However, by placing a support structure or post  48 , preferably at the center of the device, the device can be made about sixteen times less sensitive to pressure due to the width 4  (w 4 ) dependence. By placing additional supports between the center and edge, preferably symmetrically, the sensitivity can be reduced even further. 
       FIG. 3  shows the first face  161  of the MEMS wafer  16 , with a portion of the support structure passing through the proof mass  17 , suspended by four flexural springs  27 . The core  48   b  and clearance gap  50  comprise a 3DTCV with the core  48   b  forming the central support between the caps. The clearance gap  50 , which in this embodiment as of annular shape, is preferably wide enough to not interfere with the motion of the proof mass  17 . For the inertial sensor described here in operation, the full scale angular translation of the proof mass is typically around 4 mrad. As an example, motion of a thick proof mass with a thickness on the order of 400 μm may result in a lateral translation of less than 2 μm, i.e. much less than the typical minimum 20-50 μm width required to etch a channel through a silicon wafer. Additionally, in order to avoid disturbing the proof mass  17  electrostatically, it is preferable to include a conducting plug through the insulating layer (typically buried oxide) to control the potential of the support and avoid charging effects. 
     Manufacturing Method 
     Fabrication of the present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by this specification, drawings, and the appended claims. 
     Referring to  FIGS. 4 to 12 , and broadly described, the method includes the steps of providing a top cap wafer  12  and a bottom cap wafer  14  and then patterning: cap and base portions  48   a,    48   c  of the support structure; top and bottom sidewalls  124 ,  144  of the cavity  31 , and at least one electrode  13  and/or  15  in one of the top and bottom cap wafers. The method also includes a step of providing a MEMS wafer  16  and patterning on one of its sides at least a part of a MEMS structure  17  and a part of a core portion  48   b  of the support structure. The method includes bonding the mentioned side of the MEMS wafer to the inner side of one of the top and bottom cap wafers  12 ,  14  by aligning the corresponding cap or base portion  48   a  or  48   c  of the support structure with the part of the core portion  48   b  of the support structure previously patterned in the MEMS wafer. The method then includes patterning the other side of the MEMS wafer with the remaining part of the MEMS structure  17 , the remaining part of the core portion  48   b,  and lateral sidewalls  164  of the cavity. The method then includes bonding this other side of the MEMS wafer to the other cap wafer  12 ,  14 , by aligning the top, bottom and lateral sidewalls to form the cavity, housing the MEMS structure  17  therein. With the top and bottom cap wafers  12 ,  14  bonded to the MEMS wafer  16 , the electrode(s) are operatively coupled to the MEMS structure  17 , and by aligning the cap or base portion  48   c,    48   a  with the remaining part of the core portion  48   b,  the support structure  48  extends through the cavity  31  from the bottom cap wafer  12  to the top cap wafer  14 , advantageously preventing bowing in the top cap  12  and bottom cap  14  wafers. Possible implementations of each step of the method will be described in more detail below. 
     Referring to  FIGS. 4 and 4A-4B , the capacitor gap  38  is first etched into the top cap wafer  12 . An island of silicon is left at the location of the cap portion  48   a  of the future support structure, the cap portion  48   a  having the same height as the cap periphery, so that the cap portion  48   a  will contact the MEMS wafer during wafer bonding. The boundary of the top cap wafer  12  is then patterned with the electrode pattern, to delineate the different electrodes  13 . The cap portion is thus formed by etching trenches  28  on the inner side  121  of the top cap wafer  12  and at least partially through wafer  12 . The trenches  28  are then lined with an insulating material  30 , followed by a conductive fill  32 . Alternatively, the trenches  28  can be filled with an insulating material  30  only. 
     Referring to  FIG. 5A , these steps are repeated on a second or bottom cap wafer  14  to form the bottom cap electrode pattern, with electrodes  15  and the support base portion  48   c.    
     Referring to  FIGS. 6 and 6A-6B , the MEMS wafer  16  is provided. In the present embodiment, the MEMS wafer  16  is a SOI wafer with an insulating layer  24  separating a device layer  20  from a handle layer  22 . Trenches  28  are etched on the first side  161  (corresponding to the side of the device layer  20 ) through the insulating layer  24  or slightly into the SOI Handle layer  22 . The trench is filled with a conductive material  32 , such as metal, doped polycrystalline silicon (polysilicon), or other conducting material. In this way an electrical path  34  is formed vertically between the SOI Device and Handle layers  20 ,  22  at desired spots. 
     Referring to  FIGS. 7 and 7A , the side  161  is then patterned with trenches to delimit a portion of the MEMS structure  17  and a portion  48   b ′ of the core portion of the support structure. The trenches delimiting the portion  48   b ′ of the core portion of the support structure form a portion  50   a  of the annular clearance gap. Other elements can also be patterned in order to define any other desired MEMS structures. Such other structures can include springs  27  and the top of the proof mass  17 , leads, and feedthroughs, delimited by trenches  28  in the SOI device layer  20 . 
     Referring to  FIGS. 8 and 8A , the top cap wafer  12  is then aligned and bonded to the side of the MEMS wafer patterned in the previous step, which in this case corresponds to the SOI device layer  20 . The cap portion of the support structure  48   a  is thereby aligned and bonded to the portion  48   b ′of the core portion of the support structure which has been partially etched in the SOI device layer  22 . Additionally, the electrodes  13  on the top cap  12  are aligned to the relevant electrodes  19  in the MEMS wafer  16 . The wafer bonding process used provides a conductive bond, and may include processes such as fusion bonding, gold thermocompression bonding, or gold-silicon eutectic bonding. 
     Referring to  FIGS. 9 and 9A , the other side of the MEMS wafer  16 , corresponding in this case to the SOI handle layer  22 , is next patterned with trenches  28  to form the portion  50   b  of the clearance gap, defining the remainder of the core portion  48   b  of the support structure in the proof mass  17 . Trenches  28  are also formed to delimit the lateral sides of the proof mass  17 , as well as any additional MEMS structures, such as peripheral 3DTCV feedthroughs. 
     Referring to  FIG. 10 , the bottom cap wafer  14  is next bonded to the backside of the MEMS wafer  16 , i.e. in this case to the SOI handle layer  22 , again using a conductive a wafer bonding method. 
     As best shown in  FIG. 10A , the support structure base  48   c  is thereby bonded to the rest of the support structure  48   a,    48   b  for forming the conductive mechanical support  48 , from the bottom cap  14 , through the MEMS structure  16 , and to the top cap  12 . In the same way, conductive paths can be provided from the bottom electrodes, through the bottom cap wafer, handle feedthroughs, conducting shunts, and SOI device layer  20  to the top cap wafer  12 . At this point, the cavity  31  and the MEMS structure  17  are hermetically sealed between the cap wafers  12 ,  14 . The electrodes  13 ,  15 ,  19  are aligned and the top cap and bottom cap are supported by the central support structure  48  and by the outer frame of the device. However, the electrodes in the top and bottom caps are still shorted by the silicon of the top and bottom cap wafer extending beyond the insulated trenches. 
     Referring to  FIGS. 11 and 11A , the present method includes a step of removing a portion of the outer sides  122 ,  142  of the top and bottom cap wafers  12 ,  14  to isolate the electrodes and the cap and base portions of the support structure  48 . More specifically, both cap wafers  12 ,  14  are ground and polished to expose the insulated channels. The support cap and base portions  48   a,    48   c,  as well as electrodes are thereby electrically isolated except for the connections to the top cap pads through the feedthroughs and silicon vias. Both outer surfaces  122 ,  142  are passivated with an insulating oxide layer  40  to protect them. 
     Referring to  FIGS. 12 and 12A , first and second electrical contacts  42 ,  43  are formed on the outer sides  122 ,  142  of the top and bottom cap wafers  12 ,  14 , the first electrical contact  42  being electrically connected to the cap portion  48   a  of the support structure and the second electrical contact  43  being electrically connected to the bottom cap portion  48   c  of the support structure. More specifically, openings are made in the insulating layer  40  and a metallic layer  41  is deposited and patterned in predetermined locations, for forming leads and electrical pads. A passivating layer  45  can then be applied, and openings are made in the passivating layer to expose the electrical contacts  42 ,  43  (typically bond pads). Additional contacts  47  can be formed, in electrical connection with the electrodes  13  or  15 , or peripheral feedthroughs. In this way, electrical connections from the top, sides, and bottom of the MEMS are accessible from the top of the MEMS device  10  for wire bonding, flip chip bonding, or wafer bonding. 
     Of course, other processing steps may be performed prior, during or after the above described steps. The order of the steps may also differ, and some of the steps may be omitted or combined. 
     The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.