Abstract:
A drive frequency tunable MEMS sensor in one embodiment includes a mass, a mass drive component configured to drive the mass within a plane, a plurality of non-linear springs supporting the mass a first tuner operably connected to the plurality of non-linear springs for modifying the stress condition of the plurality of non-linear springs in response to a trim voltage, and a trim circuit electrically coupled with the first tuner for providing the trim voltage.

Description:
BACKGROUND 
       [0001]    This invention relates to semiconductor devices and particularly to devices incorporating sensor elements. 
       Background 
       [0002]    In the past, micro-electromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. Consequently, MEMS based sensors, such as accelerometers, gyroscopes and pressure sensors, have been developed for use in a wide variety of applications. 
         [0003]    MEMS gyroscopes can be configured as rotating mass devices or as vibration devices, both of which rely upon the Coriolis effect to generate an angular change in the device that is measured by the sensor. Linearly oscillating devices have various advantages over devices which rely upon a rotating mass. In a linear device, a mass supported by springs, referred to as the Coriolis mass, is induced to a linear oscillatory movement. As the MEMS device is rotated, the Coriolis mass rotates with respect to a sense electrode. The out of plane rotation of the Coriolis mass thus changes a capacitance between the sense electrode and the Coriolis mass which provides an indication of the angular rate of rotation. 
         [0004]    Because linear devices incorporate an oscillating mass, such devices exhibit a resonant drive frequency (f dr ). The f dr  is a function of the various components of the sensor which affect the movement of the Coriolis mass in the drive plane. Accordingly, the f dr  is a function of various mechanical springs which support the Coriolis mass as well as the component used to impart the oscillatory motion. Typically, oscillation of the Coriolis mass is effected by a comb finger device. 
         [0005]    In comb finger drives, plates on the Coriolis mass are interlaced with plates on a base component. As voltage is applied to the base component plates, the Coriolis mass is displaced. The force of the comb finger drive on the Coriolis mass is relatively constant throughout the oscillatory movement of the mass. Accordingly, the effect of the comb finger drive on the f dr  is negligible. Thus, the f dr  of a linear MEMS gyroscope is primarily a function of the mechanical springs in the drive plane. 
         [0006]    Because the Coriolis mass is oscillating as the mass rotates out of plane when the sensor is subjected to a rotation, the out of plane movement of the Coriolis mass which is detected by the sensor (detection movement) also exhibits a resonant frequency (f det ). The f det  is a function of various mechanical springs which support the Coriolis mass through the out of plane rotation as well as the force exerted on the Coriolis mass by the detection component. As noted above, the detection component is typically an electrode which, with the Coriolis mass, forms a capacitor. The force of the electrode on the Coriolis mass, which has a spring constant with a sign opposite to the sign of the mechanical springs, is a function of the voltage applied to the electrode. The effect of the electrode on the Coriolis mass is not negligible. 
         [0007]    Matching the f dr  with the f det  assists in optimizing the signal to noise ratio (SNR) of a MEMS sensor. Manufacturing processes, however, do not allow sufficient control over the various forces within the sensor device to provide the desired consistency between the resonant frequencies. Accordingly, because the effect of the detection electrode on the Coriolis mass can easily be modified by modifying the applied voltage, MEMS devices are typically fabricated with a mechanical component of the f det  higher than the mechanical component of the f dr . Subsequently, the electrical component of the f det  can be modified to tune the f det  to the f dr . 
         [0008]    Various approaches to tuning the f det  using a trim or electrode voltage (U DF ) have been developed. In one approach, separate sense (detection) and trim electrodes are provided and a constant U DF  is applied to the trim electrodes. Alternatively, a trim voltage may be applied as a time multiplexed signal to the same electrodes used to detect the out of plane movement of the Coriolis mass. 
         [0009]    While the modification of the f det  with the various known approaches is effective, the various approaches described above require the addition of voltage on the detection side of the device. Additional voltage on the detection side of the sensor increases the complexity of conditioning required on the output of the sensor to allow detection of the out of plane movement of the Coriolis mass. For example, applying a pulsed U DF  results in additional noise. Additionally, because the application of U DF  to an electrode positioned beneath the Coriolis mass results applies an attractive force to the Coriolis mass, the possibility of parasitic mechanical mode vibrations in the Coriolis mass is increased. 
         [0010]    A sensor capable of sensing the out of plane movement of a Coriolis mass is useful. The ability to match the f det  with the f dr  of such a system would be useful. A device which allows matching of the f det  with the f dr  without applying additional voltage to the detection side of the device would be beneficial. 
       SUMMARY 
       [0011]    In accordance with one embodiment, a drive frequency tunable MEMS sensor includes a mass, a mass drive component configured to drive the mass within a plane, a plurality of non-linear springs supporting the mass a first tuner operably connected to the plurality of non-linear springs for modifying the stress condition of the plurality of non-linear springs in response to a trim voltage, and a trim circuit electrically coupled with the first tuner for providing the trim voltage. 
         [0012]    In accordance with another embodiment, a method of operating a micro-electromechanical systems (MEMS) device includes supporting a mass above a substrate with a plurality of non-linear springs, driving the mass within a plane, providing a trim voltage to a first tuner, and modifying the stress condition of the plurality of non-linear springs with the first tuner. 
         [0013]    In yet another embodiment, a micro-electromechanical systems (MEMS) device includes a substrate, an electrode on the substrate, a mass positioned above the electrode, a plurality of non-linear springs supporting the mass above the electrode, a drive device configured to drive the mass along a plane in response to a drive signal, and at least one tuner physically connected to at least one of the plurality of non-linear springs for modifying the stress of the at least one of the plurality of non-linear springs responsive to a trim signal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  depicts a top plan view of a MEMS sensor configured to tune the resonant drive frequency of a linearly oscillating Coriolis mass using tuners that include interleaved fingers positioned between springs used to support the Coriolis mass above a sense electrode; 
           [0015]      FIG. 2  depicts a partial perspective view of the device of  FIG. 1  showing one of the tuner systems in the trim system including a tuner, two non-linear springs and an anchor; 
           [0016]      FIG. 3  depicts a force/displacement plot of a linear and non-linear spring; and 
           [0017]      FIG. 4  depicts a top plan view of a MEMS sensor configured to tune the resonant drive frequency of a linearly oscillating Coriolis mass using tuners that include interleaved fingers positioned between springs positioned within an opening in the Coriolis mass. 
       
    
    
     DESCRIPTION 
       [0018]    A micro-electromechanical systems (MEMS) device  100  is depicted in  FIG. 1 . The device  100  includes a substrate  102 , drive electronics circuit  104 , sense electronics circuit  106 , and trim electronics circuit  108 . The drive electronics circuit  104  is connected to drive devices  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124 . 
         [0019]    The drive devices  110 ,  112 ,  114 , and  116  include comb fingers which are interleaved with comb fingers on a Coriolis mass  126  which is positioned above a sense electrode  128 . The sense electrode  128  is connected to the sense electronics circuit  106 . The drive devices  118 ,  120 ,  122 , and  124  include comb fingers which are interleaved with comb fingers on a Coriolis mass  130  which is positioned above a sense electrode  132 . The Coriolis mass  130  is connected to the mass  126  by a coupling spring  134  and the sense electrode  132  is connected to the sense electronics circuit  106 . 
         [0020]    Each corner of the mass  126  is supported by a respective tuner system  140 ,  142 ,  144 , or  146  and each corner of the mass  130  is supported by a respective tuner system  148 ,  150 ,  152 , or  154 . The tuner system  140  includes a tuner  160  which is connected through a spring  162  to the mass  126 . A spring  164  is located between the tuner  160  and an anchor  166 . The anchor  166  is fixedly attached to the substrate  102  and to the spring  164  to support the mass  126 . 
         [0021]    Similarly, the tuner system  142  includes a tuner  170  which is connected through a spring  172  to the mass  126 . A spring  174  is located between the tuner  170  and an anchor  176 . The anchor  176  is fixedly attached to the substrate  102  and to the spring  174  to support the mass  126 . Additionally, the tuner system  144  includes a tuner  180  which is connected through a spring  182  to the mass  126 . A spring  184  is located between the tuner  180  and an anchor  186 . The anchor  186  is fixedly attached to the substrate  102  and to the spring  184  to support the mass  126 . Finally, the tuner system  146  includes a tuner  190  which is connected through a spring  192  to the mass  126 . A spring  194  is located between the tuner  190  and an anchor  196 . The anchor  196  is fixedly attached to the substrate  102  and to the spring  194  to support the mass  126 . 
         [0022]    The mass  130  is similarly supported above the electrode  132 . Specifically, the tuner system  148  includes a tuner  200  which is connected through a spring  202  to the mass  130 . A spring  204  is located between the tuner  200  and an anchor  206 . The anchor  206  is fixedly attached to the substrate  102  and to the spring  204  to support the mass  130 . 
         [0023]    Similarly, the tuner system  150  includes a tuner  210  which is connected through a spring  212  to the mass  130 . A spring  214  is located between the tuner  210  and an anchor  216 . The anchor  216  is fixedly attached to the substrate  102  and to the spring  214  to support the mass  130 . Additionally, the tuner system  152  includes a tuner  220  which is connected through a spring  222  to the mass  130 . A spring  224  is located between the tuner  220  and an anchor  226 . The anchor  226  is fixedly attached to the substrate  102  and to the spring  224  to support the mass  130 . Finally, the tuner system  154  includes a tuner  230  which is connected through a spring  232  to the mass  130 . A spring  234  is located between the tuner  230  and an anchor  236 . The anchor  236  is fixedly attached to the substrate  102  and to the spring  234  to support the mass  130 . 
         [0024]      FIG. 2  depicts a partial perspective view of the device  100  showing the tuner system  140 . The spring  162  and the spring  164  in this embodiment are generally ovoid shaped springs. Other symmetrical spring forms may be used if desired. The springs  162  and  164  are connected to a floating portion  250  of the tuner  160 . The floating portion  250  includes a number of comb fingers  252  which are interleaved with comb fingers  254  located on base portions  256  and  258  of the tuner  160 . The base portions  256  and  258  are fixedly attached to the substrate  102 . In this embodiment, the base portions  256  and  258  are made of a conductive material and are electrically coupled to the trim electronics circuit  108  by a trace  260 . The trim electronics circuit  108  and the tuner systems  140 ,  142 ,  144 ,  146 ,  148 ,  150 ,  152 , and  154  comprise the trim system for the device  100 . 
         [0025]    In operation, the drive electronics circuit  104  ( FIG. 1 ) selectively applies a drive signal to the drive devices  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124 . In response the drive devices  110 ,  112 ,  114 , and  116  drive the mass  126  into a linear oscillation toward and away from the mass  130 . Additionally, the drive devices  118 ,  120 ,  122 , and  124  drive the mass  130  into a linear oscillation toward and away from the mass  126 . To a large extent, operation of the device  100  is similar to other gyroscope devices. The primary difference between other gyroscopes and the device  100  is the use of the trim system to modify the f dr  of the device  100 . 
         [0026]    Specifically, the trim electronics circuit  108  provides a trim signal to the tuner systems  140 ,  142 ,  144 ,  146 ,  148 ,  150 ,  152 , and  154 . With reference to  FIG. 2  and the tuner system  140 , a trim voltage is supplied through the trace  260  to the base portions  256  and  258  of the tuner  160 . The voltage is passed by the base portions  256  and  258  to the fingers  254 , resulting in a force exerted on the fingers  252  of the floating portion  250 . The exerted force causes the floating portion  250  to be biased in the direction of the arrow  262 . Accordingly, the spring  164  is biased toward the anchor  166  placing the spring  164  into compression. Additionally, the spring  162  is biased away from the mass  126 , placing the spring  162  in expansion. 
         [0027]    Biasing of the spring  162  causes force from the tuner  160  to be transferred through the mass  126  to each of the other tuner systems  142 ,  144 , and  146 . Additionally, the tuner systems  148 ,  150 ,  152 , and  154  are connected to the mass  126  through the mass  130  and the coupling spring  134 . The mass  126  and the mass  130  are not affected positionally by the trim system, however, because the trim system forces are balanced. By way of example, the bias exerted on the mass  126  by the compression of the spring  162  is countered by an equal but opposite force exerted on the mass  126  by the spring  172  in the tuner system  142  as the spring  172  is placed into compression in a manner substantially identical to the foregoing description of the tuner system  140 . 
         [0028]    Accordingly, the tuner systems  140 ,  142 ,  144 ,  146 ,  148 ,  150 ,  152 , and  154  are used to pre-stress the springs  162 ,  164 ,  172 ,  174 ,  182 ,  184 ,  192 ,  194 ,  202 ,  204 ,  212 ,  214 ,  222 ,  224 ,  232 , and  236 . The amount of pre-stress applied is selected to tune the f dr  to the f det . Modification of the f dr  is enabled in the foregoing embodiment by selection of springs which exhibit a non-linear displacement per unit of applied force. 
         [0029]    By way of example,  FIG. 3  depicts a plot  270  of the relationship between the amount of force applied to a selected spring and the resulting displacement of the selected spring. The line  272  depicts the force/displacement relationship of an exemplary linear spring. The spring constant for a given spring is related to the first derivative of the force/displacement curve for the spring. Thus, because the line  272  is linear, the spring constant for the linear spring does not change as the spring is stressed. 
         [0030]    An exemplary non-linear spring, however, generates the force/displacement curve  274 . Accordingly, the derivative of the curve  274  at the location  276  is relatively small while the derivative of the curve  274  at the location  278  is relatively large. Thus, the spring constant for a non-linear spring can be modified by controlling the pre-stress applied to the spring. 
         [0031]    Therefore, by selecting and pre-stressing non-linear springs used to support a Coriolis mass above an electrode, the f dr  of a device may be modified to match the f det  of the device. If needed, additional tuning freedom may be provided by the inclusion of more than one tuner in a tuner system, each of the tuners separated from other tuners in the tuner system by a spring. The springs used to separate the tuners may be non-linear springs if desired. 
         [0032]    If desired, the location of the tuning system may be modified. By way of example,  FIG. 4  depicts a MEMS device  300  that is substantially similar to the MEMS device  100  of  FIG. 1  with the exception of the tuning system placement. Specifically, in the MEMS device  300 , each of the masses  302  and  304  include an opening  306  and  308 , respectively. Within the openings  306  and  308  are located tuner systems  310  and  312 . The tuner systems  310  and  312  are identical, are more fully described with reference to tuner system  312 . 
         [0033]    The tuner system  312  includes an anchor  314  and two tuners  316  and  318 . Each of the tuners  316  and  318  are identical to the tuners  160 ,  170 ,  180 ,  190 ,  200 ,  210 ,  220  and  230 . The tuner  316  is connected to one end of the opening  308  through a spring  320  and to the anchor  314  through a spring  322 . The tuner  318  is connected to the end of the opening  308  opposite to the end connected to the spring  322  along the oscillation axis of the mass  304  through a spring  324 , and to the anchor  314  through a spring  326 . 
         [0034]    Operation of the MEMS device  30  is similar to the operation of the MEMS device  100 . The mass  302  and the mass  304  are not affected positionally by the trim system, however, because the tuner systems  310  and  312  are located on the oscillation axis of the masses  302  and  304 . Thus, the trim system forces are balanced. 
         [0035]    While the invention 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 invention are desired to be protected.