Abstract:
Systems and methods for two degree of freedom dithering for micro-electromechanical system (MEMS) sensor calibration are provided. In one embodiment, a method for a device comprises forming a MEMS sensor layer, the MEMS sensor layer comprising a MEMS sensor and an in-plane rotator to rotate the MEMS sensor in the plane of the MEMS sensor layer. Further, the method comprises forming a first and second rotor layer and bonding the first rotor layer to a top surface and the second rotor layer to the bottom surface of the MEMS sensor layer, such that a first and second rotor portion of the first and second rotor layers connect to the MEMS sensor. Also, the method comprises separating the first and second rotor portions from the first and second rotor layers, wherein the first and second rotor portions and the MEMS sensor rotate about an in-plane axis of the MEMS sensor layer.

Description:
[0001]    This application is a divisional of U.S. application Ser. No. 13/345,132 filed on Jan. 6, 2012, entitled “TWO DEGREE OF FREEDOM DITHERING PLATFORM FOR MEMS SENSOR CALIBRATION”, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The performance of some inertial sensors drift over time. Frequently, performance drifts in MEMS sensors arise due to the interplay between electronic and mechanical components. The electronics of a MEMS inertial sensor drift over time because they provide an analog output that is inherently subject to long term drift. Also, the mechanics of MEMS type inertial sensors cause performance drifts in response to package stresses, thermal expansions, and other mechanical factors. These performance drifts change both the bias and scale factor of the output from the inertial sensor. 
       SUMMARY 
       [0003]    The embodiments of the present disclosure provide systems and methods for a two degree of freedom dithering platform for MEMS sensor calibration and will be understood by reading and studying the following specification. 
         [0004]    Systems and methods for two degree of freedom dithering for micro-electromechanical system (MEMS) sensor calibration are provided. In one embodiment, a method for a device comprises forming a MEMS sensor layer, the MEMS sensor layer comprising a MEMS sensor and an in-plane rotator to rotate the MEMS sensor in the plane of the MEMS sensor layer. Further, the method comprises forming a first and second rotor layer and bonding the first rotor layer to a top surface and the second rotor layer to the bottom surface of the MEMS sensor layer, such that a first and second rotor portion of the first and second rotor layers connect to the MEMS sensor. Also, the method comprises separating the first and second rotor portions from the first and second rotor layers, wherein the first and second rotor portions and the MEMS sensor rotate about an in-plane axis of the MEMS sensor layer. 
     
    
     
       DRAWINGS 
         [0005]    Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
           [0006]      FIG. 1A  is an exploded perspective view of one embodiment of a MEMS sensor in accordance with the present invention. 
           [0007]      FIG. 1B  is a perspective view of an example of a vibratory structure gyroscope implemented as an embodiment of the MEMS sensor in  FIG. 1A . 
           [0008]      FIG. 2  is an illustration of one embodiment of a MEMS sensor with an in-plane rotator in accordance with the present invention. 
           [0009]      FIG. 3  is a perspective view of one embodiment of a MEMS sensor attached to a rotor with accompanying in-plane and out-of-plane rotator in accordance with the present invention. 
           [0010]      FIG. 4  is an illustration of one embodiment of a top view and side view of a MEMS sensor attached to a rotor that is dithered through two degrees of freedom in accordance with the present invention. 
           [0011]      FIG. 5  is an illustration of one embodiment of a fabrication process of a MEMS sensor attached to a two degree of freedom dithering platform in accordance with the present invention. 
           [0012]      FIG. 6  is a flow chart diagram of one embodiment of a method for fabricating a MEMS sensor that is attached to a two degree of freedom dithering platform in accordance with the present invention. 
       
    
    
       [0013]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
       DETAILED DESCRIPTION 
       [0014]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0015]    The embodiments described below relate to a two degree of freedom dithering platform for a MEMS inertial sensor. The dithering platform oscillates the position of a MEMS sensor about two axes. The dithering allows for the correction of a bias and scale factor drift that occurs over time. 
         [0016]      FIG. 1A  is an exploded perspective view of one embodiment of a MEMS sensing device  100 . MEMS sensing device  100  includes a MEMS sensor  102 . In certain embodiments, MEMS sensor  102  functions as a gyroscope and senses rotation about an input axis. Alternatively, MEMS sensor  102  is an accelerometer that senses acceleration along an axis. Further, MEMS sensor  102  includes multiple gyroscopes, accelerometers, and combinations thereof. In certain embodiments, electrical and mechanical factors cause the bias and scale factor of MEMS sensor  102  to drift over time. To calibrate MEMS sensor  102  and correct the bias and scale factor drift, MEMS sensing device  100  includes an in-plane rotator  104  and an out-of-plane rotator  106 . In-plane rotator  104  is a device that dithers the MEMS sensor  102  about an axis that runs through MEMS sensor  102  and is perpendicular to MEMS sensor layer  110  to correct the bias drift. In certain embodiments, in-plane rotator  104  rotationally oscillates MEMS sensor  102  in the plane of MEMS sensor layer  110 . When in-plane rotator  104  oscillates MEMS sensor  102 , in-plane rotator  104  oscillates MEMS sensor  102  up to an angular distance from a non-oscillating position. For example, in at least one exemplary implementation, in-plane rotator  104  rotates MEMS sensor  102  up to 12 degrees from a non-oscillating position. In at least one embodiment, in-plane-rotator  104  is formed as part of MEMS sensor layer  110 . 
         [0017]    To correct the scale factor drift, MEMS sensing device  100  includes an out-of-plane rotator  106 . Out-of-plane rotator  106  rotates MEMS sensor  102  about an axis in the plane of MEMS sensor  102  that extends through MEMS sensor  102 , such that the edges of MEMS sensor  102  oscillate in and out of the plane containing MEMS sensor  102 . In at least one exemplary implementation, out-of-plane rotator  106  rotates MEMS sensor  102  less than one degree about an axis in the plane of MEMS sensor  102 . 
         [0018]    In certain embodiments, out-of-plane rotator  106  is formed as a series of layers that are bonded to MEMS sensor layer  110 . For example, out-of-plane rotator  106  includes first rotor layer  108 , second rotor layer  112 , first rotor controlling layer  116 , and second rotor controlling layer  114 . The first rotor layer  108  and second rotor layer  112  join to MEMS sensor layer  110 . When first rotor layer  108  and second rotor layer  112  are joined to MEMS sensor layer  110 , a first rotor portion of first rotor layer  108  and a second rotor portion of second rotor layer  112  join to MEMS sensor  102 . The combination of first rotor portion and second rotor portion about MEMS sensor  102  form a rotor that is dithered by in-plane rotator  104  and out-of-plane rotator  106 . To control the out-of-plane rotation, first rotor controlling layer  116  and second rotor controlling layer  114  contain electrodes that control the out-of-plane motion of first and second rotor portions. By dithering MEMS sensor  102  in and out of the plane formed by MEMS sensor layer  110 , out-of-plane rotator  106  corrects scale factor drift that occurs over time. Further, the first rotor portion of first rotor layer  108  and the second rotor portion of second rotor layer  112  include capacitive pickoff plates that sense the motion of the gyroscopes. 
         [0019]      FIG. 1B  is a perspective view of an example of a vibratory structure gyroscope  150  implemented as MEMS sensor  102  in  FIG. 1A . In at least one embodiment, gyroscope  150  includes at least one proof mass  152  (also referred to herein simply as “proof mass  152 ”) that is configured to move according to a Coriolis Effect. Gyroscope  150  also includes at least one drive electrode  154  (also referred to herein simply as “drive electrode  154 ”) configured to apply a drive signal to proof mass  152 . In certain implementations, proof mass  152  comprises a generally planar structure having at least one comb  156  (also referred to herein as “proof mass comb  156 ”) on one or more edges of the planar structure. In an example, the proof mass  152  is configured such that a Coriolis Effect moves proof mass  152  in a direction normal to the planar structure; that is, a sense axis  158  of the proof mass  152  is normal to the planar structure. 
         [0020]    In certain embodiments, the drive electrode  154  also includes at least one comb  160  (also referred to herein as “drive comb  160 ”) that is configured to engage with the proof mass  152  and is configured to apply a drive signal to the proof mass comb  156 . In an example, the at least one comb  156  of the proof mass  152  can include a first comb and a second comb, and the at least one comb  160  can include a third comb and a fourth comb. The third comb can be configured to engage with and apply a drive signal to the first comb and the fourth comb can be configured to engage with and apply a drive signal to the second comb. 
         [0021]    In at least one exemplary embodiment, gyroscope  150  includes a first sense electrode  162  and a second sense electrode  163  that are configured to sense movement of the proof mass  152  along the sense axis  158 . In order to sense movement along the sense axis  158 , the proof mass  152  can be disposed between the sense electrodes  162 ,  163 , such that the sense electrodes  162 ,  163  are disposed opposing respective planar sides  164 ,  165  of the proof mass  152 . The sense electrodes  162 ,  163 , therefore, can sense movement of the proof mass  152  along the sense axis  158  by sensing a change in the distance between the proof mass  152  and the sense electrodes  162 ,  163 . This change in distance can be sensed by a change in capacitance between the sense electrodes  162 ,  163 . 
         [0022]      FIG. 2  is an illustration of one embodiment of a MEMS sensor layer  210  with an in-plane rotator in accordance with the present invention. MEMS sensor layer  210  includes MEMS sensor  202 . MEMS sensor  202  is similar to MEMS sensor  102  in  FIG. 1 . The in-plane rotator illustrated in  FIG. 2  is similar to in-plane rotator  104  and rotates MEMS sensor  202  in the plane of MEMS sensor layer  210 . Further, the moving parts of MEMS sensor layer  210  are supported by a supportive substrate  232 . Supportive substrate  232  is similar to first rotor layer  108  or second rotor layer  112 . Further, control signals for the operation of the in-plane rotator and MEMS sensor  202  are received by the in-plane rotator through traces  228  formed in supportive substrate  232 . 
         [0023]    In certain embodiments, traces  228  connect to the in-plane rotator and the in-plane rotator conducts signals received over traces  228  to the MEMS sensor  202  and to in-plane drive electrodes  222 . In at least one particular exemplary implementation, where MEMS sensor  202  is a gyroscope, the in-plane rotator receives signals through traces  228  and conducts the received signals to gyroscope drive electrodes that cause the proof masses of MEMS sensor  202  to oscillate in response to changing electric fields on the gyroscope drive electrodes. Further, traces  228  also connect to the in-plane rotator to provide electrical signals to the in-plane rotator that controls the in-plane rotation of MEMS sensor  202 . 
         [0024]    In the present exemplary embodiment, the in-plane rotator is a circular interdigitated comb drive that includes in-plane drive electrodes  222   a  and  222   b , driven electrodes  220 , torsional suspension  226 , and rotor beam  230 . Traces  228  connect to both in-plane drive electrodes  222  and torsional suspension  226 . In at least one implementation, the traces  228  that connect to torsional suspension  226  conduct control signals to MEMS sensor  202  as described above. Further, traces  228  that connect to in-plane drive electrodes  222  also direct in-plane drive electrodes  222  to create electric fields that cause driven electrodes  220  to move either away or toward in-plane drive electrodes  222 . In at least one implementation, the in-plane drive electrodes  222  and the driven electrodes  220  are placed around MEMS sensor  202  such that in-plane drive electrodes  222  and driven electrodes  220  circularly alternate around MEMS sensor  202 . Because in-plane drive electrodes  222  and driven electrodes  220  circularly alternate around MEMS sensor  202 , the inducement of electric fields by in-plane drive electrodes  222   a  and  222   b  cause driven electrodes  220  to oscillate about an axis that is perpendicular to the plane containing in-plane drive electrodes  222  and extends through MEMS sensor  202 . In certain embodiments, in-plane drive electrodes  222   a  drives the driven electrodes  220  at a different potential than in-plane drive electrodes  222   b . For example, in-plane drive electrode  222   a  and in-plane drive electrode  222   b  alternatively exert a force on driven electrodes  220 . To alternatively exert a force on driven electrodes, a first electrical signal is sent to in-plane drive electrode  222   a  and a second electrical signal is sent to in-plane drive electrode  222   b . The first electrical signal and the second electrical signal are phased such that when in-plane drive electrode  222   a  is exerting a maximum force on driven electrodes  220 , in-plane drive electrode  222   b  is exerting a minimum force on driven electrodes  220  and vice versa. By alternatingly exerting maximum and minimum forces on driven electrodes  220 , in-plane drive electrodes  222   a  and  222   b  cause the circular interdigitated comb drive to dither MEMS sensor  202  in the plane of MEMS sensor layer  210 . Further, the force exerted by in-plane drive electrodes  222   a  and  222   b  on driven electrodes  220  cause the MEMS sensor to oscillatingly dither up to a defined angular distance from a non-oscillating position. In certain implementations, the MEMS sensor is oscillatingly dithered up to 30 degrees from a non-oscillating position. For example, driven electrodes  220  oscillate twelve degrees from a non-oscillating position. 
         [0025]    In certain embodiments, driven electrodes  220  attach to a rotor portion of supportive substrate  232  through rotor beams  230 . The rotor portion of supportive substrate  232  also attaches to MEMS sensor  202  and is detached from a portion of supportive substrate  232  that attaches to in-plane drive electrodes  222 . To support the rotor portion of supportive substrate  232 , driven electrodes  220 , and MEMS sensor  202 , a torsional suspension  226  connects the rotor portion of supportive substrate  232  to the portion of supportive substrate that attaches to in-plane drive electrodes  222 . Further, traces  228  connect to torsional suspension  226  to provide an electrical connection to MEMS sensor  202 . In some embodiments, torsional suspension  26  includes a series of folded beams that expand and contract as the rotor portion of supportive substrate  232  rotates in response to electric fields on in-plane drive electrodes  222 . 
         [0026]    Electrical fields induced on in-plane drive electrodes  222  cause driven electrodes  220  to rotate. Driven electrodes are connected to the rotor portion of supportive substrate  232 . Thus, the electric fields on in-plane drive electrodes  222  cause the rotor portion of supportive substrate  232  to rotate. Because supportive substrate  232  is connected to MEMS sensor  202 , MEMS sensor  202  also rotates in response to the electric fields produced at the in-plane drive electrodes  222 . To maintain the rotor portion of supportive substrate  232  in the same plane of in-plane drive electrodes  222 , torsional suspension  226  connects the rotor portion of supportive substrate  232  to the portion of supportive substrate  232  that is connected to in-plane drive electrodes  222 . Further, when MEMS sensor  202  is a gyroscope, MEMS sensor  202  operates in a vacuum environment. To preserve the vacuum, MEMS sensor layer  210  also includes a hermetic seal  224  around the boundary of MEMS sensor layer  210 . When MEMS sensor layer  210  is bonded to other layers, hermetic seal  224  preserves a vacuum environment around the components of MEMS sensor layer  210 . Alternatively, hermetic seal  224  also preserves a gaseous environment around the components of MEMS sensor layer  210 . 
         [0027]      FIG. 3  is a perspective view of one embodiment of a MEMS device  300  with accompanying in-plane and out-of-plane rotators. MEMS device  300  includes a MEMS sensor  302 , which functions similarly to MEMS sensor  102  in  FIG. 1 . To correct bias and scale factor drift in MEMS device  300 , MEMS sensor  302  is dithered both in the plane of MEMS sensor  302  and out of the plane of MEMS sensor  302 . To dither the MEMS sensor  302  in the plane of MEMS sensor  302 , MEMS device  300  includes in-plane driven electrodes  320 , in-plane drive electrodes  322 , and torsional suspension  326 . In-plane driven electrodes  320  function similarly to driven electrodes  220  in  FIG. 2 , in-plane drive electrodes  322  function similarly to in-plane drive electrodes  222  in  FIG. 2 , and torsional suspension  326  functions similarly to torsional suspension  226  in  FIG. 2 . As described above, in-plane driven electrodes  320  connect to a rotor portion of a supportive substrate. 
         [0028]    In certain embodiments, the supportive substrate includes two layers that encapsulate the MEMS device. For example, a first rotor layer  306  and a second rotor layer  307  encapsulate a MEMS sensor layer and MEMS sensor  302 . First rotor layer  306  includes a rotor portion and a non-rotor portion. In certain implementations, the non-rotor portion attaches to in-plane drive electrodes  322  and torsional suspension  326 . the rotor portion of first rotor layer  306  also connects to torsional suspension  326  while also connecting to in-plane driven electrodes  320  and MEMS sensor  302 . Second rotor layer  307  is similar to first rotor layer  306  and connects to the same components of the MEMS sensor layer. When in-plane drive electrodes  322  cause MEMS sensor  302  to oscillate, the rotor portions of first rotor layer  306  and second rotor layer  307  also oscillate as described above. 
         [0029]    In a further embodiment, the rotor portions of first rotor layer  306  and second rotor layer  307  oscillate out of the plane of the MEMS sensor layer. In one exemplary embodiment, second rotor layer  307  includes out-of-plane driven electrodes  308  formed on the outside surface of the rotor portion of second rotor layer  307 . First rotor layer  306  also has out-of-plane driven electrodes that are similar to out-of-plane driven electrodes  308 . To drive out-of-plane driven electrodes, an out-of-plane driving layer  310  includes driving electrodes that create electric fields that are incident on out-of-plane driven electrodes  308 . The electric fields cause the rotor portions of first rotor layer  306  and second rotor layer  307  to oscillate out of the plane of the MEMS sensor layer. In one implementation, the rotor portions of first rotor layer  306  and second rotor layer  307  oscillate less than 1 degree out-of-plane from the non-oscillating position. 
         [0030]      FIG. 4  is an illustration of one embodiment of a top view  440  and side view  445  of a MEMS sensor  402  attached to a rotor  404  that is dithered through two degrees of freedom in accordance with the present invention. Top view  440  illustrates the in-plane dithering  430  of MEMS sensor  402  and rotor  404 . In-plane dithering  430  corrects the bias drift for measurements from a MEMS sensor. 
         [0031]    In-plane dithering  430  mechanically modulates the input axis of MEMS sensor  402  about an axis orthogonal to the input axis. The input axis of MEMS sensor  401  is rotated through an angle of 2a. The non-oscillating rest position of the gyro input axis is oriented at an angle of θ 0  with respect to the axis of rotation for the MEMS sensing device  100  in  FIG. 1 , where the axis of rotation for MEMS sensing device  100  has a magnitude of Ω 0  that is measured by the sensor. When MEMS sensor  402  is dithered at frequency ω, the input rotation sensed by MEMS sensor  402  can be written as: 
         [0000]      Ω M =Ω 0  cos(θ 0   +a  sin ω t )+ B ( t )
 
         [0000]    where Ω 0  is the magnitude of rotation for MEMS sensing device  100 , θ 0  is the angle between the gyro input axis and the rotation axis for MEMS sensing device  100 , a is the modulation amplitude, and B(t) is a time varying sensor bias. This equation can be rewritten as: 
         [0000]      Ω M =Ω 0  cos θ 0  cos( a  sin ω t )−Ω 0  sin θ 0  sin( a  sin ω t )+ B ( t ).
 
         [0000]    Expanding the cos and sin terms yields: 
         [0000]    
       
         
           
             
               Ω 
               M 
             
             = 
             
               
                 
                   Ω 
                   0 
                 
                  
                 cos 
                  
                 
                     
                 
                  
                 
                   
                     θ 
                     0 
                   
                   ( 
                   
                     1 
                     - 
                     
                       
                         
                           a 
                           2 
                         
                          
                         
                           sin 
                           2 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       2 
                     
                     + 
                     
                       
                         
                           a 
                           4 
                         
                          
                         
                           sin 
                           4 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       
                         4 
                         ! 
                       
                     
                     - 
                     … 
                   
                    
                   
                       
                   
                   ) 
                 
               
               - 
               
                 
                   Ω 
                   0 
                 
                  
                 sin 
                  
                 
                     
                 
                  
                 
                   
                     θ 
                     0 
                   
                   ( 
                   
                     
                       a 
                        
                       
                           
                       
                        
                       sin 
                        
                       
                           
                       
                        
                       ω 
                        
                       
                           
                       
                        
                       t 
                     
                     - 
                     
                       
                         
                           a 
                           3 
                         
                          
                         
                           sin 
                           3 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       6 
                     
                     + 
                     
                       
                         
                           a 
                           5 
                         
                          
                         
                           sin 
                           5 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       
                         5 
                         ! 
                       
                     
                     - 
                     … 
                   
                    
                   
                       
                   
                   ) 
                 
               
               + 
               
                 
                   B 
                    
                   
                     ( 
                     t 
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
         [0032]    Modulating the rotation signal with the dither signal, produces the following: 
         [0000]    
       
         
           
             
               Ω 
               MOD 
             
             = 
             
               
                 
                   Ω 
                   0 
                 
                  
                 cos 
                  
                 
                     
                 
                  
                 
                   
                     θ 
                     0 
                   
                   ( 
                   
                     1 
                     - 
                     
                       
                         
                           a 
                           2 
                         
                          
                         
                           sin 
                           2 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       2 
                     
                     + 
                     
                       
                         
                           a 
                           4 
                         
                          
                         
                           sin 
                           4 
                         
                          
                         ωt 
                       
                       
                         4 
                         ! 
                       
                     
                     - 
                     … 
                   
                    
                   
                       
                   
                   ) 
                 
                  
                 sin 
                  
                 
                     
                 
                  
                 ω 
                  
                 
                     
                 
                  
                 t 
               
               - 
               
                 
                   Ω 
                   0 
                 
                  
                 sin 
                  
                 
                     
                 
                  
                 
                   
                     θ 
                     0 
                   
                   ( 
                   
                     
                       a 
                        
                       
                           
                       
                        
                       sin 
                        
                       
                           
                       
                        
                       ω 
                        
                       
                           
                       
                        
                       t 
                     
                     - 
                     
                       
                         
                           a 
                           3 
                         
                          
                         
                           sin 
                           3 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       6 
                     
                     + 
                     
                       
                         
                           a 
                           5 
                         
                          
                         
                           sin 
                           5 
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       
                         5 
                         ! 
                       
                     
                     - 
                     … 
                   
                    
                   
                       
                   
                   ) 
                 
                  
                 sin 
                  
                 
                     
                 
                  
                 ω 
                  
                 
                     
                 
                  
                 t 
               
               + 
               
                 
                   B 
                    
                   
                     ( 
                     t 
                     ) 
                   
                 
                  
                 sin 
                  
                 
                     
                 
                  
                 ω 
                  
                 
                     
                 
                  
                 t 
               
             
           
         
       
     
         [0000]    The time varying bias is now modulated by ω, removing higher frequency terms results in the following equation: 
         [0000]    
       
         
           
             
               Ω 
               M 
             
             = 
             
               
                 - 
                 
                   Ω 
                   0 
                 
               
                
               sin 
                
               
                   
               
                
               
                 
                   
                     θ 
                     0 
                   
                   ( 
                   
                     
                       
                         1 
                         2 
                       
                        
                       a 
                     
                     - 
                     
                       
                         1 
                         16 
                       
                        
                       
                         a 
                         3 
                       
                     
                     + 
                     
                       
                         1 
                         384 
                       
                        
                       
                         a 
                         5 
                       
                     
                     - 
                     … 
                   
                    
                   
                       
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0033]    Therefore, mechanically modulating the sensor input allows the bias signal to be eliminated from the true rotation signal. Further, mechanically modulating the sensor input also changes the input axis for the MEMS sensing device containing dithered MEMS sensor  402 . In one embodiment, the input axis for the MEMS sensing device  100  in  FIG. 1  is orthogonal to the input axis for the MEMS sensor  402  while being in the same plane as MEMS sensor  402 . 
         [0034]    Further, the following equations illustrate how the frequency of the coriolis force is affected by in-plane dithering  430 . In explaining the following equations the following variables are used:
       A=motor amplitude   ω m =gyro motor frequency   ω d =platform dither   F c =coriolis force   m=proof mass mass   v=velocity of proof mass   Ω=rotation   θ=Angle between motor axis and rotation axis.
 
The equations are as follows:
       
 
         [0000]        F   c =−2 m{circumflex over (Ω)}×{circumflex over (ν)} 
 
         [0000]      {circumflex over (Ω)}=Ω{circumflex over ( y )}
 
         [0000]    The velocity of proof masses in MEMS sensor  402  have components in x and y axes depending on angle θ. This is the dither angle and is a time varying component of cos(ω d t). 
         [0000]      {circumflex over (ν)}= Aω   m  cos ω m   t ·{cos [θ cos(ω d   t )] {circumflex over (x)} +sin [θ cos(ω d   t )] ŷ} 
 
         [0000]        F   c =−2 mAΩω   m  cos ω m   t ·sin [θ cos(ω d   t )]
 
         [0000]      for small θ:
 
         [0000]        F   c =−2 mAΩω   m  cos ω m   t ·[θ cos(ω d   t )+other terms]
 
         [0000]        F   c   =−mAΩω   m θ[cos(ω m +ω d )+cos(ω m −ω d )]
 
         [0000]    Coriolis force is generated at ω m +/−ω d . 
         [0043]    Also, side view  445  illustrates the out-of-plane dithering  431  of MEMS sensor  402  and rotor  404 . Out-of-plane dithering  431  corrects the scale factor drift for measurements from MEMS sensor  402 . To correct for scale factor drift, Ω M , from the equation 
         [0000]    
       
         
           
             
               Ω 
               M 
             
             = 
             
               
                 - 
                 
                   Ω 
                   0 
                 
               
                
               sin 
                
               
                   
               
                
               
                 
                   θ 
                   0 
                 
                 ( 
                 
                   
                     
                       1 
                       2 
                     
                      
                     a 
                   
                   - 
                   
                     
                       1 
                       16 
                     
                      
                     
                       a 
                       3 
                     
                   
                   + 
                   
                     
                       1 
                       384 
                     
                      
                     
                       a 
                       5 
                     
                   
                   - 
                   … 
                 
                  
                 
                     
                 
                 ) 
               
             
           
         
       
     
         [0000]    shown above, is measured by converting the coriolis force caused by the magnitude of rotation Ω 0 . To measure the scale factor, the magnitude of rotations must be known and the resulting signal from the MEMS sensing device is measured. The combination of the magnitude of rotation and output signal allow the scale factor to be measured. 
         [0044]      FIG. 5  is an illustration of one embodiment of a fabrication process of a MEMS sensor attached to a two degree of freedom dithering. To form the MEMS sensor, a rotor layer  574  is formed (shown at  552 ). When rotor layer  574  is formed, recesses  572  are formed in an insulative layer such as glass. For example, when rotor layer is made from insulative glass, recesses  572  are formed in locations that would allow for the free motion of MEMS devices that are in contact with rotor layer  574 . When the recesses  572  are formed, traces  570  are formed on the rotor layer that will provide electrical connections to the MEMS device. Also, the locations on rotor layer  574  that are not recessed are used for connecting to a layer containing a MEMS device. Further, rotor layer is prepared for dithering by partially separating a rotor portion from rotor layer (shown at  554 ). To partially separate the rotor portion, trenches  576  are formed around the boundaries of a rotor portion of rotor layer  574 . For example, a small circle is partially etched around a rotor portion of rotor layer  574 . In some implementations, the rotor portion is not entirely separated from rotor layer  574  to furnish a rigid support structure during the bonding of flexible MEMS structures to rotor layer  574 . 
         [0045]    In certain embodiments, the fabrication process continues by forming MEMS device layer  580  (shown at  556 ). In at least one exemplary implementation, MEMS device layer  580  is formed in an epitaxial layer supported by a semiconductor substrate  578 . For example, a pattern is formed in an epitaxial layer that corresponds with the shape of components in MEMS device layer  580 . The epitaxial layer is then etched to form MEMS device layer  580  supported by semiconductor substrate  578 . 
         [0046]    When MEMS device layer  580  is formed in the epitaxial layer, MEMS device layer  580  is bonded to a first rotor layer  574 , which is formed as described above (shown at  558 ). In at least one exemplary embodiment, to bond MEMS device layer  580  to first rotor layer  574 , the MEMS device layer  580  in the epitaxial layer is flipped and anodically bonded to first rotor layer  574 . When MEMS device layer  580  is bonded to first rotor layer  574 , the semiconductor substrate  578  is removed from the epitaxial layer to free the MEMS device layer  580  such that MEMS devices in MEMS device layer  580  are able to move. In one particular example, a portion of MEMS device layer  580  is bonded to the non-rotor portion of first rotor layer  574 , while a different portion of MEMS device layer  580  is bonded to the rotor portion of first rotor layer  574 . When the first rotor layer  574  is bonded to MEMS device layer  580 , a second rotor layer  575  is bonded to the other side of MEMS device layer  580  in the same way that first rotor layer  574  was bonded to MEMS device layer  580  (shown at  560 ). Further, when both first rotor layer  574  and second rotor layer  575  are bonded to MEMS device layer  580 , out-of-plane driven electrodes  582  are formed on the outside surface of the rotor portions of first rotor layer  574  and second rotor layer  575 . 
         [0047]    Further, to control the out-of-plane dithering, an out-of-plane drive layer  586  is formed (shown at  562 ). To form out-of-plane drive layer  586 , out-of-plane driving electrodes  584  are placed on the inside surface of out-of-plane drive layer  586 . The out-of-plane driving electrodes  584  produce electric fields that cause out-of-plane driven electrodes to dither in and out of the plane containing MEMS device layer  580 . 
         [0048]    When a first out-of-plane drive layer  586  is formed, the rotor portion of first rotor layer  574  is fully separated from the non-rotor portion of first rotor layer  574 . To fully separate the rotor portion from the non-rotor portion, the trench  576  that was formed in first rotor layer  574  is etched entirely through first rotor layer  574 . When the rotor portion is separated from the non-rotor portion, out-of-plane drive layer  586  is anodically bonded to first rotor layer  574  (shown at  564 ). When, out-of-plane drive layer  586  is bonded to first rotor layer  574 , the rotor portion of second rotor layer  575  is fully separated from the non-rotor portion of second rotor layer  575  by extending trench  588  through second rotor layer  575 . When the rotor portion of second rotor layer  575  is separated from the non-rotor portion of second rotor layer  575 . A second out-of-plane drive layer  587  is bonded to second rotor layer  575 . In some embodiments, second out-of-plane drive layer  587  is bonded to second rotor layer  575  in a vacuum environment to seal a vacuum environment within the MEMS sensing device. Alternatively, second out-of-plane drive layer  587  is bonded to second rotor layer in a gaseous environment to seal a gaseous environment within the MEMS sensing device. 
         [0049]      FIG. 6  is a flow chart diagram of one embodiment of a method  600  for fabricating a MEMS sensor that is attached to a two degree of freedom dithering platform. Method  600  begins at  602  where a MEMS sensor layer is formed. In certain embodiments, when a MEMS sensor layer is formed, the MEMS sensor layer includes a MEMS sensor that senses rotation about an input axis; and an in-plane rotator that rotates the MEMS sensor in the plane of the MEMS sensor layer. Method  600  proceeds at  604  where a first rotor layer and a second rotor layer are formed. 
         [0050]    When the first rotor layer and the second rotor layer are formed, method  600  proceeds at  606  where the first rotor layer is bonded to a top surface of the MEMS sensor layer and the second rotor layer is bonded to the bottom surface of the MEMS sensor layer. In at least one implementation, the first rotor layer includes a first rotor portion that is connected to the MEMS sensor. Further, the second rotor layer includes a second rotor portion that is also connected to the MEMS sensor. 
         [0051]    In further embodiments, method  600  proceeds at  608  where the first rotor portion is separated from the first rotor layer and the second rotor portion is separated from the second rotor layer. In at least one implementation, when the first rotor portion and the second rotor portion are separated from their respective rotor layers, the first rotor portion, MEMS sensor, and second rotor portion rotate about an axis in the plane of MEMS sensor layer. For example, first rotor portion and second rotor portion include out-of-plane driven electrodes that respond to electric fields that cause the first rotor potion, MEMS sensor, and second rotor portion to oscillate about the axis in the plane of MEMS sensor layer. Thus, the MEMS sensor is oscillating about two orthogonal axes, which allows for the correction of bias and scale factor drift. 
         [0052]    A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims.