Abstract:
In accordance with one embodiment, a single chip combination inertial and pressure sensor device includes a substrate, an inertial sensor including a movable sensing structure movably supported above the substrate, and a first fixed electrode positioned adjacent to the movable sensing structure, and a pressure sensor including a gap formed in the sensor at a location directly above the movable sensing structure, and a flexible membrane formed in a cap layer of the device, the flexible membrane defining a boundary of the gap and configured to flex toward and away from the gap in response to a variation in pressure above the flexible membrane.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/886,124 filed Oct. 3, 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 
         [0005]    “c r ” is the effective damping constant of the resonator, 
         [0006]    “η” is the transduction efficiency, 
         [0007]    “g” is the gap between electrodes, 
         [0008]    “A” is the transduction area, and 
         [0009]    “V” is the bias voltage. 
         [0010]    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. 
         [0011]    Traditionally, out-of-plane electrodes are not fully utilized because of the difficulty in reliably fabricating such devices. For example, packaging is difficult for out-of-plane devices because out-of-plane electrodes are easily damaged during packaging processes. MEMS resonators incorporating an out-of-plane electrode are particularly challenging because such devices require a vacuum encapsulation process. 
         [0012]    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. 
         [0013]    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. 
       SUMMARY 
       [0014]    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 pressure sensor and fabricate an inertial sensor below a pressure sensor. In this way, the pressure sensor is realized within the membrane layer of the device. In some embodiments, the movable portion of the inertial sensor is used as a lower electrode. 
         [0015]    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. Additionally, by fabricating the pressure sensor within the membrane layer above the inertial sensor, there is a reduction in the overall sensor area. Such inertial sensing structures have the same design flexibility as current inertial sensors. 
         [0016]    In one embodiment, a single chip combination inertial and pressure sensor device includes a substrate, an inertial sensor including a movable sensing structure movably supported above the substrate, and a first fixed electrode positioned adjacent to the movable sensing structure, and a pressure sensor including a gap formed in the sensor at a location directly above the movable sensing structure, and a flexible membrane formed in a cap layer of the device, the flexible membrane defining a boundary of the gap and configured to flex toward and away from the gap in response to a variation in pressure above the flexible membrane. 
         [0017]    In one or more embodiments the first fixed electrode is located in the substrate directly beneath the movable sensing structure, and the movable sensing structure is configured as an electrode in the pressure sensor. 
         [0018]    In one or more embodiments the movable sensing structure is formed in a device layer, and the device further includes a second fixed electrode formed in the device layer at a first location adjacent to the movable sensing structure, and a third fixed electrode formed in the device layer at a second location adjacent to the movable sensing structure. 
         [0019]    In one or more embodiments the device includes a support post extending from the substrate to the cap layer through a hole in the movable sensing structure. 
         [0020]    In one or more embodiments a fourth fixed electrode is defined in the cap layer at a location directly between the movable sensing structure and the flexible membrane, the flexible membrane is configured as a movable electrode, the support post extends into the cap layer, and the support post is electrically isolated from the fourth fixed electrode by a non-conductive cap. 
         [0021]    In one or more embodiments the device includes a first connector extending between the first fixed electrode and an upper surface of the cap layer, and a nitride spacer within the cap layer, the nitride spacer electrically isolating the first connecter within the cap layer. 
         [0022]    In one or more embodiments the first connector extends through a first buried oxide layer located between the substrate and the device layer, and the first connector extends through a second buried oxide layer located between the device layer and the cap layer. 
         [0023]    In one or more embodiments the device includes a second connector extending from the second fixed electrode to the upper surface of the cap layer, wherein the nitride spacer includes a first side portion in direct contact with the first connector and a second side portion in direct contact with the second connector. 
         [0024]    In one or more embodiments the device includes a piezoresistive element attached to the flexible membrane. 
         [0025]    In one embodiment a method of forming a single chip combination inertial and pressure sensor device includes providing a wafer including a buried oxide layer between a handle layer and a device layer, forming a first trench through the device layer and the buried oxide layer to define a sensing structure within the device layer, forming a first oxide portion within the first trench and on an upper surface of the handle layer, forming a first cap portion on an upper surface of the first oxide portion, defining a first membrane portion within the first cap portion directly above the defined sensing structure, releasing the defined sensing structure through vent holes formed through the first membrane portion, forming a second cap portion on an upper surface of the first cap portion, and defining a second membrane portion within the second cap portion directly above the released sensing structure. 
         [0026]    In one or more embodiments a method includes forming at least one second trench through the device layer and the buried oxide layer to define at least one electrode within the device layer, and filling the at least one second trench with a first nitride portion prior to forming the first trench, wherein forming the first cap portion includes forming a portion of at least one first connector for the defined at least one electrode by forming the first cap portion within trenches in the first oxide portion. 
         [0027]    In one or more embodiments a method includes forming a third trench through the first oxide portion, the handle layer, and the buried oxide layer, the third trench separated within the device layer from the at least one electrode by the first nitride portion, and filling the third trench with a polysilicon deposition, thereby forming a portion of a second connector to an electrode within the handle layer. 
         [0028]    In one or more embodiments a method includes forming a fourth trench through the first oxide portion, the handle layer, and the buried oxide layer, the third trench located within an outer perimeter of the defined sensing structure within the device layer, and filling the fourth trench with a polysilicon deposition, thereby forming a post extending upwardly from the handle layer, forming a second oxide portion on an upper surface of the second cap portion at a location directly above the filled further trench, forming an upper cap layer on an upper surface of the second cap portion, etching the second oxide portion through vent holes formed through the second cap portion, and sealing the vent holes in the second cap portion. 
         [0029]    In one or more embodiments a method includes forming a fifth trench through the first cap portion to the first oxide portion, and forming a nitride cap, the nitride cap extending within the fifth trench and directly above the post thereby electrically isolating the post within the first cap portion. 
         [0030]    In one or more embodiments a method includes forming a piezoresistor on an upper surface of the upper cap layer. 
         [0031]    The 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 disclosure is not limited to the use of a poly-crystalline silicon membrane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      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, integrated in a vertical way on top of each other; 
           [0033]      FIGS. 2-14  depict side cross-sectional views of a fabrication process for the sensor device of  FIGS. 1 ; and 
           [0034]      FIG. 15  depicts a side cross-sectional view of a sensor device similar to that of  FIG. 1  wherein the sensing member of the inertial sensor functions as a fixed electrode for a pressure sensor. 
       
    
    
     DESCRIPTION 
       [0035]    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. 
         [0036]      FIG. 1  depicts a combination inertial and pressure sensor  100  including an inertial sensor  102  (e.g., an accelerometer and/or a gyroscope with one or multiple sensing axes) and a pressure sensor  104  on a single chip either on top of each other or in a stacked configuration. The combination inertial and pressure sensor  100  includes a handle layer  106 , a buried oxide layer  108 , and a device layer  110 . An oxide layer  112  separates the device layer  110  from a cap layer  114 . A remainder  116  of a passivation layer is located above the cap layer  114 . 
         [0037]    The inertial sensor  102  includes a lower fixed sensor electrode  120  which is in electrical communication with an electrode pad  122  on the upper surface of the cap layer  114  through a connector  124 . The connector  124  is electrically isolated from the cap layer  114  by a spacer  126 . The inertial sensor  102  further includes two fixed in-plane electrodes  130  and  132  within the device layer  110 . The fixed in-plane electrode  130  is electrically isolated from the device layer  110  by a spacer  134  and the fixed in-plane electrode  132  electrically isolated from the device layer  110  by a spacer  136 . The fixed in-plane electrode  130  is in electrical communication with a fixed electrode pad  138  on the upper surface of the cap layer  114  through a connector  140  which is electrically isolated from the cap layer  114  by a spacer  142 . The fixed in-plane electrode  132  is in electrical communication with a fixed electrode pad  144  on the upper surface of the cap layer  114  through a connector  146  which is electrically isolated from the cap layer  114  by a spacer  148 . 
         [0038]    The inertial sensor  102  further includes a sensing structure  150 . The sensing structure  150  is electrically isolated from the handle layer  106  by a gap  152 . A gap  154  electrically isolates the sensing structure  150  from the cap layer  114 . The sensing structure  150  is in electrical communication to a connector pad (not shown) through the device layer  110  in a manner similar to the fixed in-plane electrodes  130 / 132  via anchoring points on which the sending structure is suspended. 
         [0039]    The pressure sensor  104  includes a fixed electrode  160  and a membrane  162 . The fixed electrode  160  is supported by a support post  164  which extends from the handle layer  106 . The support post  164  extends through the sensing structure  150  but is not physically connected to the sensing structure  150 . Thus, the sensing structure  150  is free to move to provide in-plane as well as out-of-plane sensing. The support post  164  is electrically isolated from the cap layer  114  by a nitride cap  166 . 
         [0040]    The membrane  162  in some embodiments functions as a movable electrode. In other embodiments, a piezoresistor (not shown) is positioned on the membrane. In embodiments with a piezoresistor, the nitride cap  166  may be omitted. 
         [0041]    The inertial sensor  102  and the pressure sensor  104  of the combination inertial and pressure sensor  100  function in substantially the same manner as other known sensors. Unlike other sensors, the inertial sensor  102  and the pressure sensor  104  are formed using a single process flow while providing a substantially smaller footprint. One exemplary process for forming the combination inertial and pressure sensor  100  is discussed with reference to  FIGS. 2-21 . 
         [0042]    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 trenches  208 / 210  which extend to the handle layer  202 . The trenches  208 / 210  are then filled with trench nitride portions  212 / 214  and a plurality of trenches  216  and a trench  218  are etched ( FIG. 3 ). The trench  218  in one embodiment defines a cylinder, with the thickness of the trench selected to provide sufficient space between the support post  164  and the sensing structure  150  (see  FIG. 1 ) to allow for the desired in-plane movement of the sensing structure  150 . Likewise, the diameter of the trench or trenches  216  is selected to provide sufficient space between the sensing structure  150  and the in-plane electrodes  130 / 132 . 
         [0043]    The trench portions  216  and  218  are then filled with trench oxide portions  220 / 222 , respectively, as shown in  FIG. 4  using a conformal oxide deposition. 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). 
         [0044]    Trenches  226 / 228  are 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 ). Epi-polysilicon deposition is used to fill the trenches  226 / 228  with epi-polysilicon deposits  230 / 232 , respectively as shown in  FIG. 6 . Trenches  234 ,  236 , and  238  are etched through the oxide layer  224 . 
         [0045]    Referring to  FIG. 7 , an epi-polysilicon deposition is used to fill the trenches  234 ,  236 , and  238  with lower middle contact portions  242 ,  244 , and  246 . The epi-polysilicon deposition further results in a lower cap layer portion  250  above the oxide layer  224 . A number of trenches  252  are then etched into the cap layer portion  250  ( FIG. 8 ). The trenches  234 ,  236 , and  238  are filled with nitride and a nitride layer is formed on the upper surface of the lower cap portion  250  and then patterned and etched resulting in the configuration of  FIG. 9 .  FIG. 9  thus includes a number of nitride portions  254 , nitride gaskets  256 , and a nitride cap  258 . After forming the nitride portions  254 , nitride gaskets  256 , and a nitride cap  258 , vent holes  260  are formed and an HF vapor etch release is performed which releases sensing structure  262 . 
         [0046]    A clean high temperature seal is then performed in an epi-reactor to seal the vent holes  260  and to form a middle cap portion  262  which extends above the nitride gaskets  256  and the nitride cap  258  as shown in  FIG. 10 . An oxide remnant  264  is also shown in  FIG. 10 . The oxide remnant is formed by depositing and patterning an oxide layer on the upper surface of the middle cap portion  262 . 
         [0047]    An upper cap portion  266  (see  FIG. 11 ) is then formed on the upper surface of the oxide remnant  264  and the middle cap portion  262 , and trenches  268  are formed. The trenches  268  each extend to a respective one of the nitride gaskets  256  ( FIG. 12 ). The trenches directly adhere to the perimeter of the oxide remnant  264  (full enclosure of the perimeter). The configuration of  FIG. 13  is then achieved by filling the trenches  268  with nitride portions  270  while forming a nitride layer  272  on the upper surface of the upper cap portion  266 . Trenches  274  to the buried oxide remnant  264  are formed by first etching through the nitride layer  272  and then etching through a portion of the upper cap portion  266 . 
         [0048]    A vapor etch is performed to remove the buried oxide remnant  264  and an epi-seal is performed to seal the trenches  274 . The nitride layer  272  is then patterned and etched resulting in the configuration of  FIG. 14  wherein the membrane  276  is positioned above a gap  278  formed by etching the buried oxide remnant  264 . Connector pads are then added as desired resulting in the configuration of  FIG. 1 . 
         [0049]    In some embodiments the above described process is modified by omitting the formation of the support post  164  and the gap  278 . The resulting configuration is shown in  FIG. 15  which depicts a combination inertial and pressure sensor  300  including an inertial sensor  302  and a pressure sensor  304 . The combination inertial and pressure sensor  300  includes a handle layer  306 , a buried oxide layer  308 , and a device layer  310 . An oxide layer  312  separates the device layer  310  from a cap layer  314 . A remainder  316  of a passivation layer is located above the cap layer  314 . 
         [0050]    The inertial sensor  302  includes a lower fixed sensor electrode  320  which is in electrical communication with a lower electrode pad  322  on the upper surface of the cap layer  314  through a connector  324 . The connector  324  is electrically isolated from the cap layer  314  by a spacer  326 . The inertial sensor  302  further includes two fixed in-plane electrodes  330  and  332  within the device layer  310 . The fixed in-plane electrode  330  electrically isolated from the device layer  310  by a spacer  334  and the fixed in-plane electrode  332  electrically isolated from the device layer  310  by a spacer  336 . The fixed in-plane electrode  330  is in electrical communication with a fixed electrode pad  338  on the upper surface of the cap layer  314  through a connector  340  which is electrically isolated from the cap layer  314  by a spacer  342 . The fixed in-plane electrode  332  is in electrical communication with a fixed electrode pad  344  on the upper surface of the cap layer  314  through a connector  346  which is electrically isolated from the cap layer  314  by a spacer  348 . 
         [0051]    The inertial sensor  302  further includes a sensing structure  350 . The sensing structure  350  is electrically isolated from the handle layer  306  by a gap  352 . A gap  354  electrically isolates the sensing structure  350  from the cap layer  314 . The sensing structure  350  is in electrical communication to a connector pad (not shown) through the device layer  310  in a manner similar to the fixed in-plane electrodes  330 / 332 . The fixed in-plane electrodes and the movable sensing structure  350  form together e.g. an in-plane accelerometer. 
         [0052]    The pressure sensor  304  includes a membrane  362 . The gap  354  allows the membrane to move. Thus, a piezoelectric element is provided on the membrane  362  in some embodiments. In other embodiments, the sensing structure  350  serves as a “fixed” electrode. Input from the lower electrode pad  322  in such embodiments is used to compensated for movement of the sensing structure  350  in the z-axis so that the sensing structure  350  is in effect “fixed” with respect to the movable electrode formed by the membrane  362 . 
         [0053]    The above described processes may be further modified in a number of ways to provide additional features. For example, 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 5-20 μm thick, while the inertial sensor functional layer is preferably 10-40 μm thick. 
         [0054]    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. 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). 
         [0055]    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. 
         [0056]    The above described embodiments thus provide a pressure sensor within an epi-polysilicon encapsulation layer. The pressure sensor in some embodiments is a capacitive pressure sensor with the epi-polysilicon encapsulation layer functioning as a movable electrode. The above described embodiments provide a low-cost combination sensor with a small footprint. 
         [0057]    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.