Abstract:
A ring resonator gyro comprises a folded cylinder suspension that supports a cylindrical ring resonator. The folded cylinder suspension comprises an inner cylinder, an outer cylinder, and an annulus. The inner cylinder, the outer cylinder, and the annulus are concentric. A top edge of the inner cylinder is coupled with an inner edge of the annulus. A top edge of the outer cylinder is coupled with an outer edge of the annulus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application contains subject matter that is related to the subject matter of the following applications, which are assigned to the same assignee as this application. The below-listed applications are hereby incorporated herein by reference in their entireties:  
         [0002]     “OSCILLATION OF VIBRATING BEAM IN A FIRST DIRECTION FOR A FIRST TIME PERIOD AND A SECOND DIRECTION FOR A SECOND TIME PERIOD TO SENSE ANGULAR RATE OF THE VIBRATING BEAM,” by Stewart, Application No. 60/549,709, filed Mar. 3, 2004.  
         [0003]     “REAL TIME BIAS ESTIMATOR,” by Lee, application Ser. No. ______, filed ______.  
         [0004]     “COPLANAR PROOFMASSES EMPLOYABLE TO SENSE ACCELERATION ALONG THREE AXES,” by ______, application Ser. No. ______, filed Dec. 13, 2004.  
         [0005]     “RING RESONATOR GYROSCOPE WITH CYLINDRICAL RING SUSPENSION,” by Stewart, application Ser. No. ______, co-filed herewith. 
     
    
     BACKGROUND  
       [0006]     An electromechanical system in one example measures a parameter. The electromechanical system may comprise a micro-electromechanical system (“MEMS”) accelerometer or gyroscope that measures the parameter. For example, the accelerometer measures an acceleration and the gyroscope measures an angular rate (e.g., rotation). The gyroscope in one example comprises a vibrating ring with high Q degenerate fundamental modes of vibration. For example, high Q vibrating rings require little energy to sustain vibration. The vibrating ring in one example is employable for high performance closed loop angular rate sensing. The vibrating ring in another example is employable for lower performance open loop angular rate sensing. The mathematical model of the symmetrical vibrating ring is in many aspects similar to a vibrating ring or hemispherical resonator gyroscope (“HRG”). The analytical similarity to the hemispherical resonator gyroscope indicates that the vibrating ring gyroscope has the potential of achieving similar performance.  
         [0007]     Drive components coupled with the vibrating ring cause a first oscillation of the vibrating ring. An angular rate of the vibrating ring and the first oscillation induce a Coriolis force on the vibrating ring. For example, the angular rate is about the longitudinal axis of the vibrating ring. The Coriolis force causes a second oscillation of the vibrating ring. The second oscillation is substantially perpendicular to the first oscillation. Feedback components in one example provide feedback on a magnitude of the first oscillation to the drive components for regulation of the first oscillation. Pickoff sensor components sense the second oscillations and apply control signals to null the pickoff signal. The control signals are a measure of the magnitude and polarity of the angular rate of the vibrating ring.  
         [0008]     Small, low cost, low power navigation-grade inertial systems are needed to enable new applications such as personal navigation of individual soldiers and the guidance and control of air, ground and under water autonomous vehicles in GPS denied environments. Micro-electromechanical systems inertial systems are currently in development that promise to provide small, low cost, low power inertial systems for tactical grade applications such as guided munitions. Current tactical-grade MEMS inertial systems have gyro bias uncertainty in the range of 20-50 degrees per hour and angle random walk of 0.02 degrees per root hour. Future, small, low cost, low power navigation-grade inertial systems require lower gyro bias uncertainty and angle random walk.  
         [0009]     Currently, a manufacturer of inertial sensors performs calibration of the inertial sensors with thermal modeling at a system level. The inertial system performance may be limited by one or more of: instability of the inertial sensors&#39; bias and scale factor, non-repeatability of the thermal model, or hysteretic and thermal gradient induced errors that can not be modeled.  
       SUMMARY  
       [0010]     The invention in one implementation encompasses a ring resonator gyro. The ring resonator gyro comprises a folded cylinder suspension that supports a cylindrical ring resonator. The folded cylinder suspension comprises an inner cylinder, an outer cylinder, and an annulus. The inner cylinder, the outer cylinder, and the annulus are concentric. A top edge of the inner cylinder is coupled with an inner edge of the annulus. A top edge of the outer cylinder is coupled with an outer edge of the annulus.  
         [0011]     Another implementation of the invention encompasses a method. A first circular cavity is patterned and etched through a device layer of an SOI wafer and into a handle layer of the SOI wafer to a depth that is less than a thickness of the handle layer. A sacrificial layer is deposited or grown on the SOI wafer and in the first circular cavity. The sacrificial layer is removed from a surface of the device layer. A layer of epi-poly silicon is deposited on the SOI wafer and in the first circular cavity. A portion of the layer of epi-poly silicon comprises a folded cylinder suspension. A ring resonator is patterned and etched through the layer of epi-poly silicon and device layer down to a first insulator layer of the SOI wafer. A second circular cavity is patterned and etched through the handle layer to the first insulator layer of the SOI wafer. The first circular cavity and the second circular cavity are concentric. The sacrificial layer is etched surrounding the folded cylinder suspension and first insulator layer of the SOI wafer to release the ring resonator, the folded cylinder suspension, and a center mount.  
         [0012]     A further implementation of the invention encompasses a method. A first circular cavity is patterned and etched through a handle layer to a first insulator layer of an SOI wafer. A sacrificial layer is deposited or grown on the SOI wafer and in the first circular cavity. The sacrificial layer is removed from a surface of the handle layer. A layer of epi-poly silicon is deposited on the SOI wafer and in the first circular cavity. A portion of the epi-poly silicon comprises a folded cylinder suspension. A ring resonator is patterned and etched through the layer of epi-poly silicon and handle layer down to the first insulator layer of the SOI wafer. A plurality of windows are patterned and etched through the device layer to the first insulator layer of the SOI wafer. The plurality of windows are concentric with the first circular cavity. The plurality of windows expose the sacrificial layer around the folded cylinder suspension and the first insulator layer of the SOI wafer. The sacrificial layer is etched around the folded cylinder suspension and the first insulator layer of the SOI wafer to release the ring resonator, the folded cylinder suspension, and a center mount.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]     Features of example implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:  
         [0014]      FIG. 1  is a representation of an exploded, perspective view of one implementation of an apparatus that comprises a ring resonator gyro with folded cylinder suspension.  
         [0015]      FIG. 2  is a representation of a partial, cross-sectional side view of one implementation of the ring resonator gyro of  FIG. 1 .  
         [0016]      FIG. 3  is a representation of a partial, cross-sectional side view of another implementation of the ring resonator gyro of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0017]     A navigation-grade inertial system in one example comprises relatively lower performance gyroscopes configured in a “self-calibrating” inertial system. In a further example, the inertial system is configured such that all of the inertial sensors lie in a single plane. This configuration does not require a sensor mounting block to orient the sensors to form an orthogonal three axis coordinate frame and does not require out-of-plane interconnections.  
         [0018]     The self-calibration of the sensors in one example allows for an increase in accuracy of navigation grade inertial sensors by an order of magnitude without the use of a global positioning system (GPS). In a further example, the self-calibration of the sensors reduces gyro bias performance requirements of the sensors by two orders of magnitude for miniature, low power, low cost navigation-grade inertial systems.  
         [0019]     To maintain the “self-calibrating” feature, a Z axis gyro in one example is a Class II Coriolis vibratory gyro, such as a vibrating ring gyro. The hemispheric resonator gyro (HRG) is an example of this class of gyro. Current HRGs, while demonstrating navigation-grade performance, do not satisfy the low cost requirement.  
         [0020]     A low cost MEMS equivalent of the hemispheric resonator gyro (HRG) in one example may be operated in a self-calibrating mode and when integrated with a planar accelerometer and vibrating ring gyro in one example will form a very low cost, low power navigation-grade inertial system.  
         [0021]     A cylinder suspension of a ring resonator gyro in one example has a non-linear spring stiffness for in-plane deflections of the ring. The in-plane stiffness of the suspension increases with the deflection of the ring which puts the fibers of the suspension in tension at the antinodes of the ring&#39;s vibration pattern. In the case of the cylinder suspension, electrical contact to the ring is made through the top cover which in one example employs a lead to be connected to the top of the gyro chip while the remaining electrical contacts to the drive/sense electrodes are made through the bottom cover. By making all of the electrical contacts through the bottom cover the gyro can directly interface both electrically and mechanically with the surface to which it mounts without additional wire bonds or leads.  
         [0022]      FIG. 1  is a cutaway exploded view of one implementation of a ring resonator gyro  100  with folded cylinder suspension  102  ( 202  and  302 ).  FIGS. 2 and 3  are partial cross sections of the gyro showing two example configurations of the ring resonator  104  ( 204  and  304 ), ring resonator center mount  106  ( 206  and  306 ), spacer  108  ( 208  and  308 ), drive/sense electrodes  110  ( 210  and  310 ) and sealing ring  112  ( 212  and  312 ). In  FIG. 2  the ring resonator  204 , ring resonator center mount  206 , drive/sense electrodes  210  and sealing ring  212  are formed in the device layer  214  of an SOI wafer  216 . The spacer  208  is formed in a handle layer  218  of the SOI wafer  216 . The folded cylinder suspension  202  is formed by deposition of a material such as epi-poly silicon in a cavity formed in both the device and handle layers  214  and  218 , respectively, of the SOI wafer  216 . The cavity is lined with a sacrificial layer such as a silicon dioxide which is subsequently removed to release the ring resonator  204  and folded cylinder suspension  202 . In this configuration the sacrificial layer and the epi-poly silicon deposition must uniformly bridge the interface between the device and handle layers  214  and  218  of the SOI wafer  216 . In  FIG. 3  the ring resonator  304 , ring resonator center mount  306 , drive/sense electrodes  310  and sealing ring  312  are formed in the handle layer  318  of an SOI wafer  316 . The spacer  308  is formed in the device layer  314 . This configuration allows the ring resonator  304  to have a higher aspect ratio providing more mass for a given resonant frequency and increased out-of-plane stiffness.  
         [0023]     The folded cylinder suspension  102  substantially reduces the non-linear spring stiffening effect associated with the single cylinder suspension in much the same manner that the folded beam suspension substantially reduces the same effect in current MEMS gyros. Since both ends of the beam or cylinder originate from a common line or plane the center of the folded beam or cylinder is free to accommodate the lateral in plane motion without introducing tension in the beam or cylinder. An additional feature of the folded cylinder suspension is the increased compliance of the suspension for a given SOI thickness.  
         [0024]     The electronics to operate the ring resonator gyro  100  in one example are identical to that used to operate a hemispheric resonator gyro except for the changes necessary to accommodate the difference in resonant frequency and pickoff and forcer scaling.  
         [0025]     The following fabrication sequence describes example steps to fabricate a single ring resonator sensor chip of the configuration shown in  FIG. 2 . Multiple chips in one example will be fabricated on a single silicon wafer. The number of chips will depend on the wafer diameter. 
        1. Using deep reaction ion etching such as the Bosch process, pattern and etch a circular cavity through the device layer  214  of the SOI wafer  216  and into the handle layer  218  to a depth that is less than the thickness of the handle layer  218 . The width of the circular cavity is determined by the wall thicknesses of the sacrificial layers and the folded cylinder suspension design.     2. Deposit or grow a sacrificial layer such as silicon dioxide on the SOI wafer and in the circular cavity.     3. Remove the sacrificial layer from the surface of the device layer and deposit epi-poly silicon on the SOI wafer  216  and in the circular cavity.     4. Pattern and etch the ring resonator  204  and drive/sense electrode patterns through the epi-poly silicon and device layer  214  down to the insulator of the SOI wafer  216 .     5. Pattern and etch a circular cavity through the handle layer  218  to the insulator layer of the SOI wafer  216 . The cavity is concentric with the cavity etched from the device side and exposes the sacrificial layer surrounding the epi-poly silicon folded cylinder suspension and the insulator layer of the SOI wafer  216 .     6. Etch the sacrificial layer surrounding the folded cylinder suspension  202  and insulator layer of the SOI wafer  216  to release the ring resonator  204  and suspension  202 .     7. Deposit or grow a dielectric insulator layer  220  and  222  on both the top and bottom cover wafers  120  and  122 .     8. Bond the bottom wafer to the device layer of the SOI wafer and bond the top cover  120  to the handle layer  218  of the SOI wafer  216 .     9. Form vias  116  in the bottom cover  122  to make electrical contact with the drive/sense electrodes  210  and the ring resonator center mount  206 , for example, with an ohmic contact  224 .        
 
         [0035]     The following fabrication sequence describes example steps to fabricate a single ring resonator sensor chip of the configuration shown in  FIG. 3 . Multiple chips in one example will be fabricated on a single silicon wafer. The number of chips will depend on the wafer diameter. 
        1. Using deep reaction ion etching such as the Bosch process, pattern and etch a circular cavity through the handle layer  318  to the insulator layer of the SOI wafer  316 . The width of the circular cavity is determined by the wall thicknesses of the sacrificial layers and the folded cylinder suspension design.     2. Deposit or grow a sacrificial layer such as silicon dioxide on the SOI wafer  316  and in the circular cavity.     3. Remove the sacrificial layer from the surface of the handle layer  318  and deposit epi-poly silicon on the SOI wafer  316  and in the circular cavity.     4. Pattern and etch the ring resonator  304  and drive/sense electrode patterns  310  through the epi-poly silicon and handle layer  318  down to the insulator of the SOI wafer  316 .     5. Pattern and etch multiple windows through the device layer  314  to the insulator layer of the SOI wafer  316 . The windows are concentric with the cavity etched from the handle side and expose the sacrificial layer surrounding the epi-poly silicon folded cylinder suspension  302  and the insulator layer of the SOI wafer  316 .     6. Etch the sacrificial layer surrounding the folded cylinder suspension  302  and insulator layer of the SOI wafer  316  to release the ring resonator  304  and suspension  302 .     7. Deposit or grow a dielectric insulator layer on both the top and bottom cover wafers  120  and  122 .     8. Bond the bottom wafer to the handle layer  318  of the SOI wafer  316  and bond the top cover to the device layer  314  of the SOI wafer  316      9. Form vias  116  in the bottom cover  122  to make electrical contact with the drive/sense electrodes  310  and the ring resonator center mount  306 , for example, through an ohmic contact  320 .