Abstract:
In one embodiment, the process flow for a capacitive pressures sensor is combined with the process flow for an inertial sensor. In this way, an inertial sensor is realized within the membrane layer of the pressure sensor. The device layer is simultaneously used as z-axis electrode for out-of-plane sensing in the inertial sensor, and/or as the wiring layer for the inertial sensor. The membrane layer (or cap layer) of the pressure sensor process flow is used to define the inertial sensor sensing structures. Insulating nitride plugs in the membrane layer are used to electrically decouple the various sensing structures for a multi-axis inertial sensor, allowing for fully differential sensing.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/862,370 filed Aug. 5, 2013, the entire contents of which is herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This disclosure relates generally to wafers and substrates such as those used for microelectromechanical system (MEMS) devices or semiconductor devices. 
       BACKGROUND 
       [0003]    Electrostatic MEMS resonators have been a promising technological candidate to replace conventional quartz crystal resonators due to the potential for smaller size, lower power consumption and low-cost silicon manufacturing. Such devices typically suffer, however, from unacceptably large motional-impedance (R x ). MEMS devices operating in the out-of-plane direction, i.e., a direction perpendicular to the plane defined by the substrate on which the device is formed, have the advantage of large transduction areas on the top and bottom surfaces, resulting in a reduction in motional-impedances. Consequently, out-of plane devices have received an increasing amount of attention resulting in significant advances in areas such as digital micro-mirror devices and interference modulators. 
         [0004]    The potential benefit of out-of-plane electrodes is apparent upon consideration of the factors which influence the R x . The equation which describes R x  is as follows: 
         [0000]    
       
         
           
             
               
                 R 
                 x 
               
               = 
               
                 
                   c 
                   r 
                 
                 
                   η 
                   2 
                 
               
             
             ; 
           
         
       
       
         
           with 
         
       
       
         
           
             η 
             = 
             
               
                 V 
                  
                 
                   
                     ∂ 
                     C 
                   
                   
                     ∂ 
                     g 
                   
                 
               
               = 
               
                 
                   
                     ɛ 
                     0 
                   
                    
                   AV 
                 
                 
                   g 
                   2 
                 
               
             
           
         
       
     
         [0000]    wherein “c r ” is the effective damping constant of the resonator, 
         [0005]    “η” is the transduction efficiency, 
         [0006]    “g” is the gap between electrodes, 
         [0007]    “A” is the transduction area, and 
         [0008]    “V” is the bias voltage. 
         [0009]    For in-plane devices, “A” is defined as H×L, with “H” being the height of the in-plane component and “L” being the length of the in-plane component. Thus, η is a function of H/g and H/g is constrained by the etching aspect ratio which is typically limited to about 20:1. For out-of-plane devices, however, “A” is defined as L×W, with “W” being the width of the device. Accordingly, η is not a function of the height of the out-of-plane device. Rather, η is a function of (L×W)/g. Accordingly, the desired footprint of the device is the major factor in transduction efficiency. Out-of-plane devices thus have the capability of achieving significantly greater transduction efficiency compared to in-plane devices. 
         [0010]    The encapsulation of the inertial sensor is a standard process, e.g., performed by waferbonding. This is needed in order to protect the sensor structure from environmental influences and in order to provide an optimal operation pressure. Accelerometers typically have a higher pressure (&gt;10 mbar) in order to provide sufficient damping. Gyroscopes have a lower pressure (&lt;10 mbar) in order to operate efficiently. The encapsulation process is not further described herein or shown in the figures. 
         [0011]    Additionally, MEMS sensors are generally fabricated using dedicated process flows for each sensor with each sensor on a unique chip. For example, a pressure sensor is fabricated with a completely different process flow than an inertial sensor and, as a result, it is difficult to fabricate both sensors on a single chip. 
         [0012]    What is needed is a device that is fabricated using commonly understood fabrication steps that combines multiple sensing devices of different types on a single chip. It would be beneficial if the device could be realized using a single fabrication process on one single chip. 
       SUMMARY 
       [0013]    The disclosure advantageously combines the process flows for multiple MEMS sensors, so that they are fabricated on a single chip. For example, some embodiments use the process flow for a capacitive pressures sensor and fabricate an inertial sensor in parallel with fabrication of the pressure sensor. In this way, an inertial sensor is realized within the membrane layer of the pressure sensor. The device layer is simultaneously used as z-axis electrode for out-of-plane sensing in the inertial sensor, and/or as the wiring layer for the inertial sensor. The membrane layer (or cap layer) of the pressure sensor process flow are used to define the inertial sensor sensing structures. Insulating nitride plugs in the membrane layer are used to electrically decouple the various sensing structures for a multi-axis inertial sensor, allowing for fully differential sensing/readout. Additionally, by fabricating the inertial sensor within the membrane layer, there is a reduction in the overall sensor area. Such inertial sensing structures have the same design flexibility as current inertial sensors, such as allowing for large lateral gaps without additional process steps. 
         [0014]    Other process flows may be adapted to the membrane layer of a pressure sensor process flow, allowing for many sensors to be integrated into a single process flow and a single chip. Additionally, the present disclosure is not limited to a capacitive pressure sensor process flow and can also be applied to other transducers, such as piezo-resistive transducers. Further, the present disclosure is not limited to the use of a poly-crystalline silicon membrane. 
         [0015]    In one embodiment A MEMS sensor device includes a handle layer, a device layer located above the handle layer and in some embodiments spaced apart from the handle layer by a buried oxide layer, a first fixed electrode defined in the device layer, a second fixed electrode defined in the device layer and electrically isolated from the first fixed electrode, a cap layer located above the device layer, a first movable electrode defined in the cap layer at a location directly above the first fixed electrode, a second movable electrode defined in the cap layer, at least partially directly above the second fixed electrode, and electrically isolated from the first movable electrode, a first void located directly between the first fixed electrode and the first movable electrode, the first void sealed from an atmosphere above the cap layer, and a second void located directly between the second fixed electrode and the second movable electrode, the second void open to the atmosphere. 
         [0016]    In some MEMS sensor devices, a first portion of the cap layer supporting the first movable electrode has a first minimum thickness, a second portion of the cap layer supporting the second movable electrode has a second minimum thickness, and the first minimum thickness is less than the first minimum thickness. 
         [0017]    In some of the above embodiments, a maximum thickness of the first movable electrode is substantially identical to a maximum thickness of the second movable electrode. 
         [0018]    In some of the above embodiments, the first void includes a first void portion positioned directly between the first fixed electrode and the first movable electrode, and a second void portion extending upwardly from the first void portion into the cap layer. 
         [0019]    In some of the above embodiments, a maximum thickness of the first movable electrode is less than a maximum thickness of the second movable electrode. 
         [0020]    In some of the above embodiments, an uppermost surface of the first movable electrode is below an uppermost surface of the second movable electrode. 
         [0021]    In some of the above embodiments, the first movable electrode is defined at least in part by a first nitride portion extending vertically upwardly from a portion of the first void, and a second nitride portion extending horizontally from the first nitride portion. 
         [0022]    In some of the above embodiments, the first movable electrode is further defined at least in part by a third nitride portion extending vertically upwardly from the second nitride portion. 
         [0023]    In some of the above embodiments, the second movable electrode includes a first movable electrode portion directly above the second fixed electrode, a second movable electrode portion horizontally offset from the second fixed electrode, and a nitride spacer electrically isolating the first movable electrode portion from the second movable electrode portion, the MEMS sensor device further including a third fixed electrode defined in the cap layer adjacent to the second movable electrode portion. 
         [0024]    In some of the above embodiments, the first fixed electrode and the first movable electrode are configured as a pressure sensor, and the second fixed electrode and the second movable electrode are configured as a gyroscope sensor. 
         [0025]    In some of the above embodiments, the first fixed electrode and the first movable electrode are configured as a pressure sensor, and the second fixed electrode and the second movable electrode are configured as an accelerometer sensor. 
         [0026]    In accordance with another embodiment, a method of forming a MEMS sensor device, includes defining a first fixed electrode in a device layer located above a handle layer, defining a second fixed electrode in the device layer at the same time that the first fixed electrode is defined, defining a first movable electrode in a cap layer above the device layer, the first movable electrode at a location directly above the first fixed electrode, defining a second movable electrode in the cap layer, the second movable electrode at least partially directly above the second fixed electrode, and electrically isolated from the first movable electrode, releasing the first movable electrode from the device layer by forming a first void, sealing the first void from an atmosphere above the cap layer, releasing the second movable electrode from the device layer by forming a second void, and opening the second void to the atmosphere. 
         [0027]    In some of the above embodiments, releasing the second movable electrode occurs during the same process step as releasing the first movable electrode. 
         [0028]    In some of the above embodiments, releasing the second movable electrode occurs during a process step subsequent to releasing the first movable electrode. 
         [0029]    In some of the above embodiments, a method includes isolating a first portion of the second movable electrode from a second portion of the second movable electrode, wherein the first portion is directly above the second fixed electrode, and defining a third fixed electrode in the cap layer at a location adjacent to the second portion. 
         [0030]    In some of the above embodiments, defining a first movable electrode includes forming a lower nitride portion within the cap layer, the lower nitride portion extending upwardly from a buried oxide layer, forming a gasket portion above the lower nitride portion, and forming an upper nitride portion within the cap layer, the upper nitride portion extending upwardly from the gasket. 
         [0031]    In some of the above embodiments, a method includes etching a portion of the cap layer directly above the defined first movable electrode to provide a reduced thickness membrane in which the first movable electrode is defined. 
         [0032]    In some of the above embodiments, releasing the first movable electrode from the device layer includes forming a first void portion located directly between the defined first fixed electrode and the defined first movable electrode, and forming a second void portion extending upwardly from the first void portion within the cap layer. 
         [0033]    In some of the above embodiments, a method includes defining a third fixed electrode in the device layer at the same time that the first fixed electrode is defined, and defining a third movable electrode in the cap layer, the third movable electrode at least partially directly above the third fixed electrode, and electrically isolated from the second movable electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  depicts a side cross-sectional view of a sensor device incorporating in-plane and out-of-plane inertial sensors and a pressure sensor on a single chip; 
           [0035]      FIGS. 2-21  depict side cross-sectional views of a fabrication process for the sensor device of  FIG. 1 ; 
           [0036]      FIG. 22  depicts a side cross-sectional view of a sensor device similar to that of  FIG. 1  wherein the pressure sensor membrane has a boss-like formation, thereby reducing the effective thickness of the pressure sensor membrane; 
           [0037]      FIG. 23  depicts a side cross-sectional view of a sensor device similar to that of  FIG. 1  wherein the wide etched portion that reveals the pressures sensor is etched deeper so that the pressure sensor membrane has a reduced thickness; and 
           [0038]      FIG. 24  depicts a modified process step, wherein a wide etched portion above the pressure sensor has been etched using the thin oxide layer as an etch stop. 
       
    
    
     DESCRIPTION 
       [0039]    For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art which this disclosure pertains. 
         [0040]      FIG. 1  depicts an inertial and pressure sensor  100  including a handle layer  102 , a buried oxide layer  104 , and a device layer  106 . An oxide layer  108  separates the device layer  106  from a cap layer  110 . A remainder of a passivation layer  112  is located above the cap layer  110 . 
         [0041]    Within the device layer  106 , a lower pressure sensor electrode  114  is defined by two etch portions  116  and  118 . While the description discusses etch portions  116  and  118  separately, it is understood that in at least some embodiments, the etch portions  116  and  118  are a single etch portion bounding the electrode  114 . The lower pressure sensor electrode  114  is isolated from the cap layer  110  by an etched portion  120  of the oxide layer  108 . The etched portions  116 ,  118 , and  120  are released through vent holes that are sealed by the cap layer  110  as discussed in more detail below. Sealing of the vent holes provides a closed void including the etched portions  116 ,  118 , and  120 . 
         [0042]    An upper pressure sensor electrode  122  is located above the lower pressure sensor electrode  114  and electrically and mechanically isolated from the lower pressure sensor electrode by the etched portion  120 . The upper pressure sensor electrode  122  is isolated from the rest of the cap layer  110  by two spacers  124  and  126 . The spacers  124  and  126  extend from the upper surface of cap layer  110  to the etched portion  120  and include upper and lower nitride portions,  128  and  130 , with a nitride gasket portion  132  between the upper and lower nitride portions,  128  and  130 . While the description discusses spacers  124  and  126  separately, it is understood that in at least some embodiments, the spacers  124  and  126  are a single spacer bounding the electrode  122 . 
         [0043]    Spacers  134  and  136 , extending from the passivation layer  112  to the oxide layer  108 , electrically isolate a connector  138  in the cap layer  110  from the rest of the cap layer  110 . The connector  138  is in electrical communication with an in-plane electrode portion  140 , which is further connected to the lower pressure sensor electrode  114 . Spacer  134  is formed like spacers  124  and  126 . Spacer  136  is formed having an upper nitride portion  142 , and two lower nitride portions  144  with a nitride gasket portion  146  between the upper and lower nitride portions,  142  and  144 . A connector  147  extends from the passivation layer  112  to the handle layer  102 . The connector  147  is defined in the cap layer  110  by the spacer  136  and in the device layer by an isolation post  148 . Isolation post  148  extends from oxide layer  108 , beneath spacer  136 , through the device layer  106 , through the buried oxide layer  104 , and into the handle layer  102 . 
         [0044]    Within the device layer  106 , a lower inertial sensor electrode  150  for out-of-plane sensing is defined by two etched portions  152  and  154 . While the description discusses etch portions  152  and  154  separately, it is understood that in at least some embodiments, the etch portions  152  and  154  are a single etch portion bounding the electrode  150 . The lower inertial sensor electrode  150  is electrically isolated from the cap layer  110  by etched portions  156  and  158  of the oxide layer  108 , except by a contact portion  160 . The etched portions  152 ,  154 ,  156 , and  158  are released through vent holes. The vent holes remain open, thereby creating a void which is open, i.e., in fluid communication with the atmosphere above the cap layer. It is furthermore possible, to release said etched portions after the trenches  180  etc. are made with a separate release etching process. This procedure has the advantage, that the sacrificial layer can then also be used as an etch-stop for the trenches  180 , etc. 
         [0045]    A sensing structure  162  for out-of-plane sensing (similar structures can be used for in-plane sensing: counter electrode is then also in-plane similar to sensing structure) is located above the lower inertial sensor electrode  150 , and electrically and mechanically isolated from the lower inertial sensor electrode  150  by etched portion  156 . The sensing structure  162  is mechanically and electrically isolated from a portion of the cap layer  110  on one side by etched portion  166 , which extends completely from the upper surface of the cap layer  110  to the etched portion  156 . The etched portion  166  and a spacer  168 , which is formed like the spacers  124  and  126 , electrically isolate a connector  170  in the cap layer  110  from the rest of the cap layer  110 . The connector  170  is in electrical communication with the lower inertial sensor electrode  150 , via the contact portion  160 . A spacer  172 , which consists of a nitride portion extending from the upper surface of the cap layer  110  to the etched portion  156 , electrically isolates the sensing structure  162  from a sensing structure  176 , while providing a mechanical coupling thereof. This allows fully differential capacitive sensing/readout. 
         [0046]    The sensing structure  176  provides in-plane or out-of plane sensing and is located in the cap layer  110  and is isolated from the device layer  106  by etched portion  156 . The sensing structure  176  is mechanically and electrically isolated on one side from the rest of the cap layer  110  by etched portion  180 , which extends completely from the upper surface of the cap layer  110  to the etched portion  156 . The etched portion  180  and a spacer  183 , which is formed like the spacers  124  and  126 , electrically isolate an upper inertial sensor electrode  184  in the cap layer  110  from the rest of the cap layer  110 . 
         [0047]    A wide etched portion  188  exposes the upper surface of the cap layer revealing sensing structures  162  and  176 . A wide etched portion  190  exposes the upper surface of the cap layer revealing the upper pressure sensor electrode  122 . A bond pad  192  is connected through the passivation layer  112  to the upper inertial sensor electrode  184 . A bond pad  194  is connected through the passivation layer  112  to the connector  170 , which is further connected to the lower inertial sensor electrode  150 . A bond pad  196  is connected through the passivation layer  112  to the connector  138 , which is further connected to the lower pressure sensor electrode  114 . A bond pad  198  is connected through the passivation layer to the connector  147 . 
         [0048]    A process for forming a sensor such as the inertial and pressure sensor  100  is discussed with reference to  FIGS. 2-21 . Referring initially to  FIG. 2 , an SOI wafer  200  including a handle layer  202 , a buried oxide layer  204 , and a device layer  206  is provided and etched with a trench  208  to define a connector  210 . The trench  208  is then filled with a trench nitride portion  212  and trenches  214  and  216  are etched ( FIG. 3 ). The trenches  214  define lower electrode  218  for out-of-plane inertial sensing while the trenches  216  define a lower pressure electrode  220 . 
         [0049]    The trench portions  214  and  216  are then filled with a trench oxide portion  222  as shown in  FIG. 4  using a conformal oxide deposition. The filled trenches define the fixed electrodes in the device layer. While only two are formed in this embodiment, additional electrodes may be defined for particular applications. Oxide deposition further results in an oxide layer  224  on the upper surface of the device layer  206 . The oxide layer  224  may be planarized by any desired technique such as chemical mechanical polishing (CMP). 
         [0050]    A trench  226  is etched through the oxide layer  224 , the device layer  206 , and the buried oxide layer  204  to expose the upper surface of the handle layer  202  ( FIG. 5 ). An epi-polysilicon deposition  228  fills the trench  226  ( FIG. 6 ) and additional trenches  230 ,  232 ,  234 ,  236 , and  238  are etched through the oxide layer  224 . 
         [0051]    Referring to  FIG. 7 , an epi-polysilicon deposition fills the contact openings  230 ,  232 ,  234 ,  236 , and  238  with lower middle contact portions  242 ,  244 ,  246 ,  248 , and  250 . The epi-polysilicon deposition further results in a lower cap layer portion  254  above the oxide layer  224 . A number of trenches are then etched into the cap layer portion  254 . A trench  256  is etched after CMP of the lower cap layer portion  254  ( FIG. 8 ). The trench  256  extends from the upper surface of the lower cap layer portion  254  to the upper surface of the oxide layer  224  to define an upper electrode  258  for pressure sensing. Trenches  260 ,  262 , and  264  are similarly etched to define an in-plane inertial sensing portion  266  between trenches  260  and  262 , and to define an out-of-plane inertial sensing portion  268  between trenches  262  and  264 . 
         [0052]    Trench  270  is similarly etched to define a middle contact portion  272 , which is connected with lower middle contact portion  250  (see  FIG. 7 ). Finally, trench  274  is similarly etched to define a middle contact portion  276 , which is connected to lower contact portion  228  (see  FIG. 6 ). Trenches  256 ,  260 ,  262 ,  264 ,  270 , and  274  are then filled with a low stress nitride to create trench nitride portions  278 ,  280 ,  282 ,  284 ,  286 , and  288 , as shown in  FIG. 9 . 
         [0053]    Referring to  FIG. 10 , after vent holes  290  and  292  are formed, an HF vapor etch release is performed which releases the in-plane inertial sensing portion  266  and the out-of-plane inertial sensing portions  268 . The upper pressure sensor electrode  122  is also released by the vapor etch. This step thus sets the corresponding gaps between sensing electrodes and structures. A clean high temperature seal is then performed in an epi-reactor to seal the vent holes  290 / 292 . Alternatively, the vent holes  290 / 292  may be sealed using oxide, nitride, silicon migration, etc. The resulting configuration is shown in  FIG. 11  wherein lower cap layer portion  254  has been reformed after the HF vapor release. 
         [0054]    A low stress nitride layer  292  is then deposited on the upper surface of the lower cap layer portion  254  ( FIG. 12 ), and then planarized. The nitride layer  292  is patterned and etched resulting in the configuration of  FIG. 13 . In  FIG. 13 , remainders of the nitride layer  292  form gaskets  294 ,  296 , and  298 . Additional remainders of the nitride layer  292  form a gasket  300  for the upper electrode  258 , and gaskets  302  and  304  for the middle contact portion  272  and the middle contact portion  276 . The lateral extent of gaskets  294 ,  296 ,  298 ,  300 ,  302 , and  304  when viewed in the cross-section may be selected to provide the desired isolation characteristics for the components defined thereby. 
         [0055]    An epi-polysilicon layer is then formed on the upper surface of the lower cap layer portion  254  and the upper surface of the gaskets  294 ,  296 ,  298 ,  300 ,  302 , and  304  to form an upper cap layer portion  306  ( FIG. 14 ). The upper cap layer portion  306  may be planarized if desired. 
         [0056]    Trenches  308 ,  310 ,  312 ,  314 ,  316 , and  318  are then etched as depicted in  FIG. 15 . The trench  308  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  294  which acts as an etch stop. The trench  310  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  296  which acts as an etch stop. The trench  312  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  298  which acts as an etch stop. The trench  314  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  300  which acts as an etch stop. The trench  316  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  302  which acts as an etch stop. The trench  318  extends from the upper surface of the upper cap layer portion  306  to the upper surface of gasket  304  which acts as an etch stop. 
         [0057]    A nitride passivation layer  320  is then deposited on the upper surface of the upper cap layer portion  306  as shown in  FIG. 16 . The deposited nitride also fills the trenches  308 ,  310 ,  312 ,  314 ,  316 , and  318  with passivation portions  322 ,  324 ,  326 ,  328 ,  330 , and  332 . While not shown in  FIG. 16 , in some embodiments a gasket such as the gasket  300  is also provided for the passivation portion  300 . 
         [0058]    Contact openings  334 ,  336 ,  338 , and  340  are then etched through the nitride passivation layer  320  ( FIG. 17 ). As shown in  FIG. 19 , a metal layer  342  is then deposited on the passivation layer  320 . The metal layer  342  also fills the contact openings  334 ,  336 ,  338 , and  340  with metal contact portions  344 ,  346 ,  348 , and  350 . The metal layer  342  is then patterned and etched resulting in the configuration of  FIG. 19 . In  FIG. 19 , a remainder of the metal layer  342  forms a bond pad  352  for the in-plane inertial sensing portion  266 . A remainder of the metal layer  342  forms a bond pad  354  for the out-of-plane inertial sensing portion  268 . A remainder of the metal layer  342  forms a bond pad  356  for the middle contact portion  272 . A remainder of the metal layer  342  forms a bond pad  358  for the middle contact portion  276 . 
         [0059]    The passivation layer  320  is then patterned and etched resulting in the configuration of  FIG. 20 . In  FIG. 20 , wide etched portion  360  substantially reveals the upper surface of the in-plane inertial sensing portion  266  and the out-of-plane inertial sensing portion  268 . There is also a wide etched portion  362  completely revealing the upper electrode  258  and some of the surrounding portions of the upper cap layer portion  306 . 
         [0060]    Referring to  FIG. 21 , trenches  364 ,  366 ,  368 , and  370  are etched entirely through upper cap layer portion  306  and the lower cap layer  254  portion. The structure of  FIG. 21  is the same as the structure of the inertial and pressure sensor  100  of  FIG. 1 . 
         [0061]    The above described process may be modified in a number of ways to provide additional features. For example, in embodiments wherein the release of layer  224  happens in the inertial sensor region after the steps discussed with respect to  FIG. 21 , the trench for passivation portion  324  (when there is no gasket associated with the passivation portion  324 ) can be formed in one single step along with the trenches described with respect to  FIG. 15 . 
         [0062]    As another example, in some embodiments the passivation layer  320  is not etched as discussed above with respect to  FIG. 20 . Rather, the passivation layer  320  is left in place in order to protect the pressure sensor, and/or to be used as a hardmask for the inertial sensor patterning discussed above with respect to the trenches of  FIG. 21 . 
         [0063]    Moreover, it is sometimes beneficial to have a large structural layer thickness for an inertial sensor area and a lower structural thickness in a pressure sensor area in order to have good sensitivity for both sensors. A pressure sensor membrane is preferably 8-12 μm thick, while the inertial sensor functional layer is preferably 10-40 μm thick.  FIG. 22  depicts an inertial and pressure sensor  400  wherein the effective thickness of a pressure sensor membrane  401  has been reduced. 
         [0064]    The inertial and pressure sensor  400  is similar to the inertial and pressure sensor  100 , shown in  FIG. 1 , and shares all the structural features of the inertial and pressure sensor  100 . The difference between the inertial and pressure sensor  400  and the inertial and pressure sensor  100  is that the inertial and pressure sensor  400  has an etched portion  420  (which is corresponds with the etched portion  120  of the inertial and pressure sensor  100 ) that further comprises a raised portion  421 . The raised portion  421  of the etch portion  420  is located at the outer edge  423  of the etched portion  420  and protrudes into the cap layer  410  (which corresponds with the cap layer  110  of the inertial and pressure sensor  100 ). The raised portion  421 , as depicted, protrudes roughly halfway through the cap layer  410 , but may be designed to modify the effective thickness of the pressure sensor membrane as desired. The reduced thickness of the cap layer  410  about the electrode in the membrane (cap layer  410 ) effectively reduces the membrane thickness. The raised portions  21  are readily formed simply by etching the cap layer portion  254  (at  FIG. 7 ) and depositing additional oxide in the trenches. The remaining process steps are substantially identical. 
         [0065]      FIG. 23  depicts an alternative embodiment in which the effective thickness of the pressure sensor membrane has again been reduced.  FIG. 23  depicts an inertial and pressure sensor  600  wherein the effective thickness of a pressure sensor membrane  601  has been reduced by etching a deeper wide etched portion  690  (which corresponds with wide etched portion  190  of sensor  100 ). The inertial and pressure sensor  600  is otherwise similar to the inertial and pressure sensor  100 , shown in  FIG. 1 , and shares all the other structural features of the inertial and pressure sensor  100 . 
         [0066]    The inertial and pressure sensor  600  can be fabricated substantially in the same manner as the inertial and pressure sensor  100 . The main difference is that during the deposition of the EPI membrane-layer (i.e., between the configuration of  FIGS. 13 and 14 ), a thin layer of oxide is deposited. This is used as a trench stop during the trench of the inertial sensor structures resulting in the configuration of  FIG. 24  which shows an inertial and pressure sensor  600  with the thin layer of oxide  709 . Using this modification, the membrane thickness can be reduced to the required thickness. The oxide layer  709  is removed during the release etch of the sensor structures in embodiments wherein the release of oxide layer  224  happens in the inertial sensor region after the steps discussed above with respect to  FIG. 21 , or during a separate release/etch step. Therefore, the process is modified to include only additional steps for oxide deposition and patterning. 
         [0067]    In accordance with the above described embodiments, a pressure sensor and an accelerometer are fabricated on a single chip. In some embodiments, a pressures sensor and a gyroscope are fabricated on a single chip. The device of  FIG. 1  can thus be configured as a pressure sensor and an accelerometer, or as a pressure sensor and a gyroscope. By providing another structure substantially like the structure to the left of the sensor electrodes  114  and  122 , additional degrees of sensing are possible. Thus, in other embodiments, a 7-degree of freedom combination chip is fabricated having a pressure sensor, a 3-axis accelerometer, and a 3-axis gyroscope. In some embodiments, the chip has magnetometer functionality (e.g. hall sensor). 
         [0068]    In some embodiments, a 10-degree of freedom combination chip is fabricated having a 3-axis magnetometer is realized on an application-specific integrated circuit (ASIC), as an add-on chip, or as a Lorentz-force magnetometer in the same process flow as the inertial sensor part on the same MEMS chip. 
         [0069]    In some embodiments, encapsulation is done using wafer-bonding (metallic, eutectic, SLID, glass-frit) with an Si-cap wafer. In other embodiments, the Si-cap wafer of a wafer-bonding encapsulation is an ASIC chip. 
         [0070]    In some embodiments, the chip has a bare-die packaging, and is an ASIC with through-silicon-vias. In some embodiments, the pressure sensor area is not covered by the cap wafer. In some embodiments, the cap wafer has an access port etched into it within the area of the pressure sensor in order to have a pressures port. In some embodiments, the chip has a membrane recess using an oxide layer within the EPI membrane layer as an etch stop. In some embodiments, the membrane has a boss formation using an oxide block formation prior to the membrane deposition. 
         [0071]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.