Abstract:
Methods and apparatus are provided for an accelerometer. The apparatus includes first, second, and third substrates. The first substrate includes the first plate of a first capacitor. The second substrate includes a moveable mass that is coupled to the second substrate by at least one spring. The moveable mass is the second plate of the first capacitor and the first plate of a second capacitor. The third substrate includes the second plate of the second capacitor. The moveable mass is prevented from moving in any direction where the at least one spring is inelastically flexed. The first substrate couples to the second substrate. The third substrate couples to the second substrate. The method includes forming a moveable mass in a substrate. The moveable mass is formed having a plurality of springs coupling the moveable mass to the substrate. The moveable mass is released using a dry etch.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to micromachined structures, and more particularly relates to a single axis accelerometer for use in a medical device.  
       BACKGROUND OF THE INVENTION  
       [0002]     A pulse generator is one of many medical devices that are implantable in a patient and provide a therapy that is dependent on the current status of the patient. For example, a pacemaker is a widely used medical device that includes a pulse generator for providing stimulus to cardiac tissue. The amount of stimulus provided corresponds to the activity level of the patient. A patient that is sleeping requires lower stimuli than a person that is active and in motion. One method for determining the status of the patient is to use an accelerometer.  
         [0003]     An accelerometer measures changes in a patient&#39;s physical activity. The physical changes are detected by the accelerometer and algorithmically interpreted by circuitry within the pulse generator to produce a modified therapy that is correct for the current activity level. The accelerometer is placed within the implantable medical device. One type that has been successfully implemented in a pulse generator is a single axis accelerometer that measures both dynamic and static acceleration (e.g. gravity) in a single direction. Measurement in all three dimensions is achieved by using three single axis accelerometers respectively mounted to detect in the x, y, and z axis.  
         [0004]      FIG. 1  is an isometric exploded view of a known accelerometer  10 . Accelerometer  10  comprises a substrate  20 , a substrate  30 , and a substrate  40 . In general, substrates  20 ,  30 , and  40  are made of silicon. A moveable mass  50  is centrally located in substrate  30 . The moveable mass is coupled to the main body of substrate  30  by flexures  60 . For reference, the x, y, and z directions are shown relative to substrates  20 ,  30 , and  40 . Flextures  60  allow moveable mass  50  to move in the z-direction. A lower surface of substrate  20  couples to an upper surface of substrate  30 . An upper surface of substrate  40  couples to a lower surface of substrate  30 . Accelerometer  10  is sealed from an external environment when substrates  20 ,  30 , and  40  are coupled together.  
         [0005]     Moveable mass  50  is a conductive element. An upper surface of moveable mass  50  is spaced a predetermined distance from a conductive surface on the lower surface of substrate  20  forming a first capacitor. Similarly, a lower surface of moveable mass  50  is spaced a predetermined distance from a conductive surface on the upper surface of substrate  40  forming a second capacitor. The value of both the first and second capacitor changes as the moveable mass  50  moves. In an embodiment of accelerometer  10 , the values of the first and second capacitors are used differentially such that the difference in capacitor values is detected. For example, moving mass  50  moves closer to the conductive surface on the lower surface of substrate  20  increasing the value of the first capacitor. Conversely, moveable mass  50  is moving farther away from the conducting surface of the upper surface of substrate  40  decreasing the value of the second capacitor. The difference between the first and second capacitors values is detected and corresponds to the movement induced in moving mass  50 .  
         [0006]     In general, moveable mass  50  is formed from the material comprising substrate  30 . A wet etch is used to separate moveable mass  50  from substrate  30 . The wet etch process leaves a substantial distance between moveable mass  50  and substrate  30 . Flexures  60  are designed to flex which allows movement of moveable mass  50  in the z-direction. Flexures  60  are not flexible in the y-direction and may crack or fracture under conditions of high g-force in the y-direction. For example, dropping accelerometer  10  can produce movement in the y-direction where moveable mass  50  hits a sidewall of substrate  30 . The distance between moveable mass  50  and the sidewall of substrate  30  is such that sufficient movement is generated to stress flexures  60  into cracking or fracturing.  
         [0007]      FIG. 2  is an isometric view of a prior art accelerometer  100  coupled to a substrate  160 . Accelerometer  100  includes substrates  110 ,  120 , and  130  coupled together similar to that described in  FIG. 1 . An end cap  140  and an end cap  150  are coupled to substrates  110 ,  120 , and  130 . End caps  140  and  150  are primarily used for providing interconnection and physically fastening to substrate  160 . Substrates  110 ,  120 , and  130  have exposed interconnect (not shown) that abuts and couples to interconnect (not shown) on end cap  150 . Interconnect  180  is coupled to the exposed interconnect (not shown) on substrates  110 ,  120 , and  130 . The three separate interconnects comprising interconnect  180  correspond to the terminals of two capacitors with one terminal common to both capacitors. The common terminal is the moveable mass in accelerometer  100 .  
         [0008]     End caps  140  and  150  increase the size, add complexity, and cost to the manufacture of accelerometer  100 . Interconnect  180  aligns with and couples at a right angle to interconnect  170  on substrate  160 . Solder or an adhesive epoxy is used to electrically couple interconnect  180  to interconnect  170 . Although not shown, interconnect  170  typically couples to other circuitry (not shown) coupled to substrate  160 .  
         [0009]     Accordingly, it is desirable to provide a more reliable accelerometer. In addition, it is desirable to provide an accelerometer that is simple to manufacture and lower cost. It would be of further benefit if the accelerometer had a small footprint and was easily coupled to a substrate. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     Methods and apparatus are provided for an accelerometer. The apparatus includes a first substrate, a second substrate, and a third substrate. The first substrate corresponds the first plate of a first capacitor. The second substrate is bonded to the first substrate. The second substrate includes a moveable mass that is coupled to the second substrate by at least one spring. The moveable mass corresponds to the second plate of the first capacitor and the first plate of a second capacitor. The third substrate is bonded to the second substrate. The third substrate corresponds to the second plate of the second capacitor. The moveable mass is prevented from moving in any direction that inelastically flexes the at least one spring. The method includes providing a first and third semiconductor substrate. A moveable mass is formed in a second semiconductor substrate. The moveable mass is formed having a plurality of springs coupling the moveable mass to the second semiconductor substrate. The moveable mass is released using a dry etch. The moveable mass is limited from moving in any direction that inelastically flexes the plurality of springs. The first semiconductor substrate is coupled to the second semiconductor substrate. The third semiconductor substrate is coupled to the second semiconductor substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0012]      FIG. 1  is an isometric exploded view of a prior art accelerometer;  
         [0013]      FIG. 2  is an isometric view of a prior art accelerometer coupled to a substrate;  
         [0014]      FIG. 3  is a schematic representation of an accelerometer in accordance with the present invention;  
         [0015]      FIG. 4  is a side profile of an accelerometer coupled to a substrate in accordance with the present invention;  
         [0016]      FIG. 5  is a top view of a middle substrate of an accelerometer in accordance with the present invention;  
         [0017]      FIG. 6  is a magnified top view of an over travel stop in accordance with the present invention;  
         [0018]      FIG. 7  is a top view of an alternate version of a middle substrate  410  in accordance with the present invention;  
         [0019]      FIG. 8  is a magnified top view of an over travel stop for the alternate version of the middle substrate in accordance with the present invention;  
         [0020]      FIGS. 9-11  are cross-sectional views of a substrate illustrating wafer process steps to form a top cap of an accelerometer in accordance with the present invention;  
         [0021]      FIGS. 12-15  are cross-sectional views of a substrate illustrating wafer process steps to form a bottom cap of an accelerometer in accordance with the present invention;  
         [0022]      FIGS. 16-21  are cross-sectional views of a substrate illustrating wafer process steps to form a moveable mass of an accelerometer in accordance with the present invention;  
         [0023]      FIGS. 22-23  are cross-sectional views of a middle substrate coupled to top cap in accordance with the present invention; and  
         [0024]      FIGS. 24-26  are cross-sectional views of a top cap, middle substrate, and a bottom cap coupled together in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.  
         [0026]      FIG. 3  is a schematic representation of an accelerometer in accordance with the present invention. The accelerometer is a three terminal device having a terminal  210 , a terminal  220 , and a terminal  230 . The accelerometer includes a variable capacitor  240  and a variable capacitor  250 . Capacitor  240  has a first capacitor plate coupled to terminal  210  and a second capacitor plate coupled to terminal  220 . Capacitor  250  has a first capacitor plate coupled to terminal  220  and a second capacitor plate coupled to terminal  230 .  
         [0027]     In an embodiment of the accelerometer according to the present invention, the first capacitor plate of capacitor  240  and the second capacitor plate of capacitor  250  have a fixed position in the accelerometer. The second capacitor plate of capacitor  240  and the first capacitor plate of capacitor  250  are in common and together form a moveable mass within the accelerometer. The moveable mass is placed between the first capacitor plate of capacitor  240  and the second capacitor plate of capacitor  250  such that capacitors  240  and  250  have approximately the same capacitance under quiescent conditions. Movement of the accelerometer places the moveable mass closer to one of the fixed position capacitor plates and farther from the other thereby changing the capacitance value of capacitors  240  and  250 . The change in capacitance is detectable and corresponds to the movement of the accelerometer.  
         [0028]     An accelerometer is a micromachined element having moving parts. Shock can be imparted to the micromachined element before assembly and after the medical device is implanted in the patient. For example, it is possible for the medical device to be dropped by improper handling prior to implantation. The medical device would not have an outward signs of damage and could still work when enabled. The micromachined element could be stressed or fractured depending on the angle in which the medical device was dropped and the force of the shock. Moreover it is also not possible to look at the accelerometer and see any damage that may have occurred since it is a sealed device. This would be a long-term reliability problem for the medical device. The accelerometer described herein limits movement of the accelerometer to a distance where the springs elastically flex and do not reach a point where cracking or fracturing occurs.  
         [0029]     Another factor that determines usability is the size of the accelerometer and how the device is assembled with other components of the medical device. The therapies being provided by implantable medical devices are becoming more complex. The complexity may include adding many outputs to the medical device. It is highly beneficial under this scenario to make the medical device as small as possible to allow placement near the targeted tissue where the therapy is provided. This allows the wiring size and length to be reduced as well as the miniaturization of any other delivery system required for the device. The accelerometer described herein minimizes the footprint and simplifies physical and electrical coupling to a substrate of the medical device.  
         [0030]      FIG. 4  is a side profile of an accelerometer  310  coupled to a substrate  320  in accordance with the present invention. Accelerometer  310  is a variable capacitor accelerometer having all electrical contact areas on a single major surface. In an embodiment of accelerometer  310 , electrical contact areas are on a major surface of substrate  360 . In general, each electrical contact area is a metal pad. The electrical contact areas correspond to the terminals of the variable capacitors. Accelerometer  310  includes substrates  340 ,  350 , and  360  coupled together.  
         [0031]     In one embodiment, accelerometer  310  is coupled to substrate  320  that is part of an implantable medical device. An example of an implantable medical device that may use accelerometer  310  is a pacemaker, defibrillator, or a pulse generator. Substrate  320  includes interconnect that couples to other circuits (not shown) of the medical device. Solder bumps  330  are formed on the electrical contact areas on substrate  360 . Accelerometer  310  is placed such that solder bumps  330  align to, and contact corresponding electrical contact areas on substrate  360 . The solder of solder bumps  330  is reflowed electrically and physically coupling accelerometer  310  to substrate  320 . Flip chip mounting accelerometer  310  on substrate  320  allows a smaller implantable medical device to be made which is highly beneficial to the patient.  
         [0032]      FIG. 5  is a top view of a middle substrate  350  of an accelerometer  310  corresponding to  FIG. 4 . Middle substrate  350  includes a moveable mass  320  supported by springs  330 . Springs  330  allow moveable mass  320  to move in the z-direction. Springs  330  rigidly hold moveable mass  320  to prevent movement in the x and y direction. Mechanical failure of the accelerometer can occur if movement of moveable mass  320  stressed springs  330  to a point of fracture or cracking. An example of a shock that can stress springs  330  is dropping a component such as a medical device in which the accelerometer is used.  
         [0033]     In this embodiment, movement of moveable mass  320  in the x-direction is not a substantial problem. Springs  330  have sufficient strength and are anchored securely to middle substrate  350  to prevent movement in the x-direction. Moveable mass  320  can move in the y-direction under some circumstances. Springs  330  will fracture if moveable mass  320  is allowed to move a sufficient distance in the y-direction. To prevent fracture under any conditions, the movement of moveable mass  320  is limited in the y-direction by over travel stops  342 . Over travel stops  342  are formed on moveable mass  320  and extend outward to limit movement of moveable mass  320  in the y-direction. Over travel stops  342  come in contact with and engage against middle substrate  350  as moveable mass  320  moves in the y-direction, preventing further movement of moveable mass  320  in the y-direction. The amount of travel allowed in the y-direction is an amount that bends springs  330  elastically allowing springs  330  to return to their original position when normal operating conditions resume. Under normal operation of the accelerometer, over travel stops  342  are spaced away from middle substrate  350  and do not impair movement of moveable mass  320  in the z-direction.  
         [0034]      FIG. 6  is a magnified top view of an over travel stop  342  in accordance with the present invention. The magnified top view corresponds to the upper left corner of middle substrate  350  of  FIG. 5 . Over travel stop  342  is a semicircular shaped extension that extends outward from moveable mass  320  in the y-direction. A spacing  352  is the distance moveable mass  320  is allowed to travel in the y-direction before over travel stop  342  engages against middle substrate  310 . As mentioned  5 previously, spacing  352  is chosen to stress springs  330  elastically. Over travel stop  342  does not impede movement of moveable mass  320  in the z-direction.  
         [0035]      FIG. 7  is a top view of an alternate version of a middle substrate  450  in accordance with the present invention. A moveable mass  420  is coupled to middle substrate  450  by springs  430  in a manner similar to a trampoline. Moveable mass  420  moves in the z-direction and has two springs on each side. Over travel stops  442  are spaced a predetermined distance from moveable mass  420  and are formed in middle substrate  450 . In an embodiment of middle substrate  450  according to the present invention, over travel stops  442  are placed in each corner of middle substrate  450  adjacent each corner of moveable mass  420  to prevent movement in the both the x and y direction. Over travel stops  442  limit movement in the x and y direction to a distance where springs  430  elastically flex and are prevented from fracturing. Under normal operating conditions, over travel stops  442  are not engaged against and therefore do not contact moveable mass  420 .  
         [0036]      FIG. 8  is a magnified top view of an over travel stop  442  in accordance with an embodiment of the present invention. The magnified view corresponds to the upper right corner of middle substrate  450  of  FIG. 7 . Over travel stop  442  is formed as a right angle corresponding to a corner of moveable mass  420 . A spacing  452  is the distance moveable mass  420  is allowed to travel in the x or y-direction before moveable mass  420  contacts over travel stop  442 . As mentioned previously, spacing  452  is chosen to stress springs  430  elastically. Over travel stop  440  does not  2 impede movement of moveable mass  420  in the z-direction.  
         [0037]      FIGS. 9-11  are cross-sectional views of a substrate illustrating wafer process steps to form a top cap of an accelerometer in accordance with an embodiment of the present invention. Substrate  500  is n-type silicon having a &lt;100&gt; crystal orientation. In an embodiment of the top cap according to the present invention, an oxide layer is first formed on substrate  500  followed by an overlying silicon nitride layer. The combination of the oxide layer and the silicon nitride layer is denoted as oxide/nitride layer  510 . Oxide/nitride layer  510  acts as a protective layer on both surfaces of substrate  500  for subsequent processing steps. In an embodiment of the wafer process flow, the oxide layer is formed approximately 300 angstroms thick and the silicon nitride layer is formed approximately 1200 angstroms thick.  
         [0038]     Referring to  FIG. 10 , a layer of photoresist is formed and patterned on a first side of substrate  500  exposing portions of oxide/nitride layer  510  of  FIG. 9 . The exposed silicon nitride layer is removed revealing the underlying oxide layer. The oxide layer is then removed exposing substrate  500 . An oxide layer  520  is then formed in areas where substrate  500  is exposed. In an embodiment of the wafer process flow, oxide layer  520  is approximately 8000 angstroms thick. Oxide/nitride layer  510  is a non-conductive layer. Oxide/nitride layer  510  prevents the moveable mass of the accelerometer from shorting to substrate  500  should contact occur.  
         [0039]     Referring to  FIG. 11 , a wafer thinning process is applied to substrate  500 .  
         [0040]     Material is removed from a second side of substrate  500 . The second side is a backside on which no wafer process steps have been performed. In an embodiment of the wafer process, substrate  500  is thinned to approximately 13 mils. Photoresist is then applied to the second side of substrate  500  and patterned to expose portion of the backside surface of substrate  500 . An etch is then performed to remove material from the backside surface of substrate  500  forming a trench  530  which is used later for alignment.  
         [0041]      FIGS. 12-15  are cross-sectional views of a substrate  600  illustrating wafer process steps to form a bottom cap of an accelerometer in accordance with the present invention. Substrate  600  is n-type silicon having a &lt;100&gt; crystal orientation. An oxide layer is first formed overlying substrate  600 . A silicon nitride layer is then formed overlying the oxide layer. The combination of the oxide layer and the silicon nitride layer is denoted as oxide/nitride layer  615 . In an embodiment of the wafer process flow, the oxide layer is formed approximately 300 angstroms thick and the silicon nitride layer is formed approximately 1200 angstroms thick.  
         [0042]     Photoresist is then applied on the wafer and patterned exposing portions of the silicon nitride layer. The exposed silicon nitride layer is then removed revealing the underlying oxide layer. The exposed oxide layer is then removed revealing substrate  600 . The area where substrate  600  is exposed is indicated by openings  610 . The remaining photoresist is then removed.  
         [0043]     Referring to  FIG. 13 , an etch step is performed on substrate  600  in areas corresponding to openings  610  of  FIG. 12 . Trenches  620  are created by the silicon etch. In an embodiment of the wafer process, a TMAH etch is performed. The TMAH is a timed etch where the depth of trenches  620  corresponds to the size of opening  610  and the length of time of the etch. In an embodiment of the wafer process, the deeper trenches are formed to a depth of approximately 20 microns ±5 microns. The smaller trench will be used as an alignment aid in subsequent fabrication steps.  
         [0044]     Referring to  FIG. 14 , oxide/nitride layer  615  is removed from both surfaces of substrate  600 . The first side of substrate  600  is the surface where trenches  620  of  FIG. 13  are formed. After oxide/nitride layer  615  is removed, a new oxide layer is grown on both sides of substrate  600 . A layer of silicon nitride is then formed overlying the layer of oxide on both sides of substrate  600 . The combination of the oxide layer and the silicon nitride layer is denoted as oxide/nitride layer  625 . In an embodiment of the wafer process, the layer of oxide is approximately 300 angstroms thick and the silicon nitride layer is approximately 1200 angstroms thick. Oxide/nitride layer  625  aligns with the moveable mass of the accelerometer is non-conductive thereby prevents shorting the moveable mass to substrate  600 . A layer of photoresist is formed overlying the first side of substrate  600  and patterned exposing areas of the silicon nitride layer. The exposed silicon nitride is removed on both the first and second sides of substrate  600  revealing the underlying oxide layer. The oxide layer is then removed exposing areas of substrate  600  on the first side indicated by openings  630 . The second side of substrate  600  is also exposed.  
         [0045]     Referring to  FIG. 15 , an oxide layer  640  is grown in openings  630  of  FIG. 14  and the backside of substrate  600 . In an embodiment of the wafer process, oxide layer  630  is approximately 8000 angstroms thick.  
         [0046]      FIGS. 16-21  are cross-sectional views of a substrate  700  illustrating wafer process steps to form a moveable mass of an accelerometer in accordance with the present invention. Substrate  700  is a middle substrate for the accelerometer. Photoresist is applied on a first side of the substrate  700  and patterned exposing a portion of substrate  100 . The exposed portion of substrate  100  is etched forming a trench  710  that is used for alignment in subsequent steps.  
         [0047]     Referring to  FIG. 17 a  layer an oxide layer is formed on both sides of substrate  700 . A silicon nitride layer is then formed overlying the oxide layer on both sides of substrate  700 . In an embodiment of the wafer process, the oxide layer is approximately 300 angstroms thick and the silicon nitride layer is approximately 1200 angstroms thick. The combination of the oxide layer and the silicon nitride layer is indicated by oxide/nitride layer  720 . Oxide/nitride layer  720  denotes the combination of the oxide layer and the silicon nitride layer.  
         [0048]     Photoresist is applied on the first side of substrate  700  and patterned to expose areas of oxide/nitride layer  720 . The exposed silicon nitride layer is removed revealing the underlying oxide layer. The exposed oxide layer is then removed revealing substrate  700 . The exposed areas of substrate  700  are identified by openings  730 . A p-implant is deposited into substrate  700  through openings  730 . In an embodiment of the wafer process, a p-type material comprising BF2@3E15, 60 KeV is implanted through openings  730 . The p-implant is driven in to a depth of approximately 0.4 microns in a subsequent thermal cycle. The p-implant is an etch stop for a subsequent step of the wafer process.  
         [0049]     Referring to  FIG. 18 , the p-implants driven in substrate  700  as described in  FIG. 17  are shown as p-wells  735 . Oxide/nitride layer  720  of  FIG. 17  is then removed from the first and second side of substrate  700 . An oxide layer  740  is formed on both sides of substrate  700 . In an embodiment of the wafer process, oxide layer is approximately 500 angstroms thick and is used to protect the wafer surface prior to forming an epitaxial layer.  
         [0050]     Referring to  FIG. 19 , oxide layer  740  of  FIG. 18  is removed and an epitaxial layer  750  is formed on the first side of substrate  700 . In an embodiment of the wafer process, epitaxial layer  750  is an n-type epitaxial layer and is formed having a thickness of approximately 20 microns. An oxide layer is then formed overlying epitaxial layer  750  and on the second side of substrate  700 . A silicon nitride layer is then formed overlying the oxide layer on both sides of substrate  700 . Oxide/nitride layer  760  denotes the combination of the oxide layer and the silicon nitride layer. In an embodiment of the wafer process, the oxide layer has a thickness of 300 angstroms and the silicon nitride layer has a thickness of 1200 angstroms.  
         [0051]     Referring to  FIG. 20 , a wafer thinning process is performed on the second side of substrate  700 . In an embodiment of the wafer process, substrate  700  is thinned to approximately 13 mils thick.  
         [0052]     An oxide layer is then formed on the second side of substrate  700 . A silicon nitride layer is then formed overlying the oxide layer on the second side of substrate  700 . Oxide/nitride layer  770  denotes a combination of the oxide layer and the silicon nitride layer. In an embodiment of the wafer process, the oxide layer is formed approximately 300 angstroms thick and the silicon nitride layer is formed approximately 1200 angstroms thick.  
         [0053]     Photoresist is formed and patterned on the second side of substrate  700  exposing portions of oxide/nitride layer  770 . In particular, the moveable mass of the accelerometer is being defined. The exposed silicon nitride layer is then removed on the second side of substrate  700  revealing the underlying oxide layer. The exposed oxide layer is then removed revealing substrate  700 . The exposed areas of substrate  700  is denoted by openings  780 . Some of openings  780  align with p-wells  735  on the first side of substrate  700 .  
         [0054]     Referring to  FIG. 21 , an etch is performed to remove exposed silicon from the second side of substrate  700  through openings  780  of  FIG. 20 . The etch is terminated when p-wells  735  are reached. As mentioned previously, p-wells  735  act as an etch stop for this etch step. A substantial amount of substrate  700  is removed through openings  780  almost reaching the first side of substrate  700 . In an embodiment of the wafer process, the etch is a TMAH etch. The etched areas are denoted as trenches  790 . Oxide/nitride layer  760  and  770  of  FIG. 20  are then removed respectively exposing the underlying epitaxial layer  750  and the second side of substrate  700 .  
         [0055]      FIGS. 22-23  are cross-sectional views of a middle substrate coupled to a top cap in accordance with the present invention. The top cap and middle substrate respectively correspond to substrate  500  disclosed in  FIGS. 9-11  and substrate  700  disclosed in  FIGS. 16-21 . Substrate  500  and substrate  700  are aligned to one another using target fields  810 . The target fields  810  are trenches formed in the surface of each substrate during previous wafer processing steps when substrates  500  and  700  were individual wafer processed. In an embodiment of the wafer process, substrate  500  is coupled to substrate  700  in a fusion bond. Oxide layer  520  overlying substrate  500  is in contact with the surface of substrate  700 . A heating step fuses oxide layer  520  to substrate  700  that permanently holds substrates  500  and  700  together.  
         [0056]     An oxide layer  820  is formed overlying epitaxial layer  750 . Oxide layer  820  is also formed overlying substrate  500 . In an embodiment of the wafer process, oxide layer  820  is formed having a thickness of approximately 8000 angstroms. A photoresist layer is then formed overlying oxide layer  820  on substrate  700  and patterned. The photoresist layer is patterned to create a middle via. An exposed area of oxide layer  820  is removed by etching revealing the underlying epitaxial layer  750 . The exposed epitaxial layer  750  and the underlying substrate  700  are then etched. In an embodiment of the wafer process, a TMAH etch is used to form the middle via. The etch stops upon reaching oxide/nitride layer  510  and oxide layer  520 . A trench  830  is formed that couples through substrate  700 .  
         [0057]     Referring to  FIG. 23 , oxide layer  820  shown in  FIG. 21  is removed from substrate  500  and substrate  700 . An oxide layer  840  is then formed overlying substrate  500  and substrate  700 . In an embodiment of the wafer process oxide, layer  840  is a dielectric oxide approximately 400-500 angstroms thick. Oxide layer  840  also overlies an exposed surface of substrate  700  in trench  830 .  
         [0058]     Photoresist is formed on substrate  700  and patterned to release a moveable mass  850 . The pattern includes over travel stops (not shown) to limit movement of moveable mass  850 . Exposed areas of oxide layer  840  are then etched to reveal the underlying epitaxial layer  750 . Spacing between moveable mass  850  and substrate  700 , and more particularly to the over travel stops is important. A dry etch is used to etch exposed areas of epitaxial layer  750 , and the underlying p-wells  735  of  FIG. 21  which releases moveable mass  850 . Epitaxial layer  750  coupling moveable mass  850  to substrate  700  forms springs  860 . The dry etch controls spacing between moveable mass  850  and substrate  700  that limits movement of moveable mass  850  to a distance where springs  860  flex elastically to movement in all directions thereby preventing fracturing of springs  860  under a high shock condition. In other words, moveable mass  850  cannot move in a manner that would inelastically flex springs  860  and damage the accelerometer. Oxide layer  840  overlying substrate  700  is then removed.  
         [0059]      FIGS. 24-26  are cross-sectional views of a top cap, middle substrate, and a bottom cap coupled together in accordance with the present invention. The top cap and middle cap respectively corresponds to substrates  500  and  700  of  FIGS. 22-23 . The bottom cap corresponds to substrate  600  of  FIGS. 12-15 . Substrate  600  is aligned to substrate  700  using target fields  910 . The target fields  910  are trenches formed in the surface of each substrate during previous wafer processing steps. In an embodiment of the wafer process, substrate  600  is coupled to substrate  700  in a fusion bond. Oxide layer  640  overlying substrate  600  is in contact with the surface of substrate  700 . A heating step fuses oxide layer  640  to substrate  700  which permanently holds substrates  600  and  700  together.  
         [0060]     Material is removed from substrate  600  in a thinning process. In an embodiment of the wafer process, substrate  600  is thinned to approximately 10 mils. An oxide layer  920  is formed on the surface of substrate  600 . In an embodiment of the wafer process, oxide layer  920  is a passivation layer. Photoresist is formed overlying substrate  600  and patterned. The pattern exposes areas of oxide layer  920  corresponding to locations of vias through substrate  600 . Exposed areas of oxide layer  920  are removed revealing underlying substrate  600 . An etch is then performed on exposed substrate  600 . In an embodiment of the wafer process, a TMAH etch is performed that etches through substrate  600  forming trenches  930 . The etch is stopped upon reaching oxide layer  640  or oxide/nitride layer  625 .  
         [0061]     Referring to  FIG. 25 , photoresist is formed overlying oxide layer  940  and patterned. In the patterning process, photoresist is removed and trenches  930  are thoroughly cleaned out. Exposed areas of oxide layer  940  are then removed. In particular, contacts are being formed to couple to substrates  500 ,  600 , and  700 . Oxide/nitride layers are then removed in trenches  930  exposing portions of epitaxial layer  750  of substrate  700  and substrate  500 . An implant is then performed to lower the contact resistance in coupling to substrates  500 ,  600 , and  700 . A metal layer  950  is then formed that overlies oxide layer  940  and contacts substrates  500 ,  600 , and  700 . Photoresist is then formed overlying metal layer  950  and patterned. Exposed portions of metal layer  950  are etched away. Metal layer  950  is patterned to define individual electrical contacts of the accelerometer. In an embodiment of the accelerometer, three electrical contact areas are defined corresponding to two terminals of two capacitors and a common terminal between the two capacitors. Moveable mass  850  is the common terminal of the two capacitors.  
         [0062]     Referring to  FIG. 26 , a passivation layer  960  is formed overlying metal layer  950  and substrate  600 . Passivation layer  960  protects the accelerometer from an external environment. Photoresist is then formed overlying passivation layer  960  and patterned. The pattern corresponds to electrical contact regions on substrate  600 . Exposed areas of passivation layer  960  are removed exposing underlying metal layer  950 . Solder bumps  970  are formed on the exposed areas of metal layer  950 . As shown, three electrical contact regions are solder bumped that couple to substrate  500 , substrate  600 , and substrate  700 . The electrical contact regions are approximately planar to one another and are accessible from the surface of substrate  600  allowing flip chip mounting to another substrate which minimizes the footprint of the accelerometer.  
         [0063]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.