Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/710,710, filed Aug. 23, 2005. 
    
    
     BACKGROUND 
     The present invention relates generally to resonating star gyroscopes and fabrication methods relating thereto. 
     Low power vibratory microgyroscopes are needed in numerous consumer applications due to their small size, low power and ease of fabrication. Vibratory gyroscopes, which are based on transfer of energy between two vibration modes of a structure, can operate in either matched-mode or split-mode condition. 
     Under matched-mode condition, the sense mode is designed to have the same (or nearly the same) resonant frequency as the drive mode. Hence, the rotation-induced Coriolis signal is amplified by the Q of the sense mode (which can be high in vacuum). 
     In split-mode condition, the drive and sense modes are separated in resonant frequency. Due to Q amplification, gyroscopes operated under matched-mode configuration offer higher sensitivity and better resolution. 
     Resonant matched devices are themselves broadly classified into two types depending upon the nature of their operating modes. Type I devices rely on non-degenerate vibration modes for driving and sensing. The tuning fork gyroscope is an example of a type I gyroscope. As reported by M. F. Zaman, A. Sharma, B. Amini, F. Ayazi, in “Towards Inertial Grade Microgyros: A High-Q In-Plane SOI Tuning Fork Device”,  Digest, Solid - State Sensors and Actuators Workshop , Hilton Head, S.C., June 2004, pp. 384-385, it is often difficult to achieve and maintain mode matching in these devices. Type II devices on the other hand function with degenerate vibration modes and are invariably easier to match and operate under matched condition. A shell type gyroscope such as the vibrating ring gyroscope disclosed by F. Ayazi and K. Najafi, in “A HARPSS Polysilicon Vibrating Ring Gyroscope”, IEEE/ASME JMEMS, June 2001, pp. 169-179, is an example of a type II gyroscope. 
     The resonating star gyroscope represents a class of type-II vibratory gyroscope that has distinct performance advantages over the existing counterparts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  shows a conceptual illustration of an exemplary resonating star gyroscope; 
         FIG. 2  is a schematic diagram of an exemplary resonating star gyroscope; 
         FIG. 2   a  illustrates a portion of an exemplary gyroscope comprising a solid star-shaped member  12 ; 
         FIG. 3  illustrates an exemplary electrode configuration for the resonating star gyroscope; 
         FIGS. 4   a  and  4   b  illustrate primary degenerate flexural modes of an exemplary resonating star gyroscope; 
         FIGS. 5   a  and  5   b  illustrate higher order flexural modes of an exemplary resonating star gyroscope; 
         FIG. 6  is a schematic illustrating basic characterization electronics for use with the resonating star gyroscope; 
         FIG. 7  illustrates the increased electrode area of an exemplary resonating star gyroscope; 
         FIG. 7   a  shows the encircled portion of  FIG. 7  and illustrates inherent quadrature cancellation provided by the resonating star gyroscope; 
         FIG. 8  illustrates an exemplary multi-shell implementation of a resonating star gyroscope; 
         FIG. 8   a  shows an enlarged view of a section of  FIG. 8 ; 
         FIG. 9  illustrates a HARPSS implementation of the exemplary resonating star gyroscope; 
         FIGS. 10   a - 10   f  illustrate exemplary steps of a HARPSS fabrication process used to fabricate the resonating star gyroscope; 
         FIGS. 11   a  and  11   b  are graphs showing frequency response of the primary flexural modes of an exemplary HARPSS implementation of the resonating star gyroscope before and after mode matching respectively. 
         FIG. 12  is a graph that illustrates sensitivity of the HARPSS fabricated resonating star gyroscope; 
         FIG. 13  illustrates exemplary (100) single crystal silicon on insulator implementation of the exemplary resonating star gyroscope; 
         FIGS. 14   a  and  14   b  are graphs showing frequency response of the higher-order flexural modes of an exemplary (100) single crystalline silicon implementation of the resonating star gyroscope before and after mode matching respectively. 
         FIG. 14   c  is a graph that illustrates the frequency response of a high-Q, higher-order flexural resonant mode of the (100) single crystal silicon on insulator implementation of the resonating star gyroscope; 
         FIG. 15  illustrates an exemplary epitaxial-polysilicon implementation of the exemplary resonating star gyroscope; 
         FIGS. 16   a  and  16   b  are graphs showing frequency response of the higher-order flexural modes of an exemplary epitaxial polysilicon implementation of the resonating star gyroscope before and after mode matching respectively; and 
         FIG. 16   c  is a graph that illustrates sensitivity of the epitaxial polysilicon implementation of the resonating star gyroscope. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are type II resonant matched vibratory gyroscopes  10  and fabrication methods  30  relating thereto. The type II resonant matched vibratory gyroscopes  10  are referred to as resonating star gyroscopes  10 . Referring to the drawing figures,  FIG. 1  shows a conceptual illustration of an exemplary resonating star gyroscope  10 . A schematic diagram of an exemplary resonating star gyroscope  10  is illustrated in  FIG. 2 . 
     As is illustrated in  FIG. 1 , the resonating star gyroscope  10  may be visualized as a merged superposition of two substantially identical square entities  11  that are spatially 45° apart. This provides for pairs of degenerate flexural vibratory modes in the resulting eight-fold star-shaped member  12  or shell  12  ( FIG. 2 ), which is anchored to a substrate  13  comprising a central post  13  or anchor  13 , through flexural springs  14 . It is to be understood, however, that the central post  13  or anchor  13  may be located outside of the shell  12 . In the current design, eight optimally designed springs are used to maintain degeneracy of the resonant modes. Furthermore the star shell or member may also be suspended at alternate points on the periphery using flexural springs or support posts connected at one or several section(s) of the substrate. The physical dimensions of these support structures may/may not be governed by the dimensions of the star-structural material, i.e. the thickness of the support flexures may be less than the actual star shell or member. Rotation-induced Coriolis acceleration causes energy to be transferred between two flexural modes of any degenerate resonant pair. The nodes of each mode are located at the anti-nodes of its degenerate counterpart. 
     The star gyroscope  10  is a fully symmetric and balanced structure that offers differential sensing capability. As is shown in  FIG. 2 , the shell  12  is surrounded by capacitive drive, sense and tuning (balancing) electrodes  15 ,  16 ,  17 . The electrodes  15 ,  16 ,  17  may be separated from the shell  12  by capacitive gaps  18 , although this is not required in all devices. Electrode placement schemes enable frequency matching of both primary and higher-order flexural modes. 
     It is to be understood, however, that the electrodes need not be capacitively coupled to the shell  12 , and may be physically connected in certain embodiments. Electrodes in the form of piezoelectric/piezoresistive material may be deposited along the nodal-points of the star-shaped periphery. Such materials may also act as an anchoring agent to the underlying device substrate. Structural features of the exemplary resonating star gyroscope  10  shown in  FIG. 2  are that the external star-shaped shell  12  is suspended using flexural springs  14  supported at a central anchor  13 , eight flexural springs  14  are used to ensure degenerate resonant flexural mode pairs, the frequency of the structure is dependent on dimensions of the flexural springs  14 , the width of the flexural springs  14  also govern mechanical quality factors of the resonant mode, the central anchor  13  also effects the overall resonant frequency and mechanical quality factor of the flexural modes, the electrodes  15 ,  16 ,  17  (in this case) are distributed around the periphery of the star-shell  12 , the electrodes are electrically isolated from an underlying substrate  20  or structural member  20 , and the resonating star shell  12  is kept at the same potential of the substrate  20 . 
     However, it is to be understood that the resonating star gyroscope  10  need not necessarily embody a shell  12  supported by multiple flexural springs  14  coupled to the support member  13  or central post  13 . The star structure may be fabricated using a solid star-shaped member  12   a  that may or may not employ the flexural springs  14 . A portion of an exemplary gyroscope  10  comprising a solid star-shaped member  12  is illustrated in  FIG. 2   a . An exemplary solid resonating star gyroscope  10  has the solid star-shaped member  12   a  supported at its center of mass or coupled to and supported by the support member  13  using a plurality of flexural springs  14 . The solid star-shaped member  12   a  may have perforations in formed its structure during fabrication to facilitate its release. 
       FIG. 3  illustrates an exemplary electrode configuration for the resonating star gyroscope  10  shown in  FIG. 2 . The electrodes include a drive electrode  15  disposed at 0°, a sense electrode  16  disposed at −45°, a sense electrode  16  disposed at 45°, a sense electrode  16  disposed at 135°, a drive tuning electrode  15   a  disposed at −90°, a sense tuning electrode  16   a  disposed at −135°, a drive monitoring electrode  15   b  disposed at 180°, and two tuning (balancing) electrodes  17  disposed at 157.5° and −157.5°, respectively. 
     The star-shaped shell  12  is electrostatically driven into resonance at the primary flexural mode. When the gyroscope  10  is subjected to rotation, Coriolis force causes energy to be transferred to the secondary degenerate mode located 45° away. This consequential motion is sensed capacitively at the sense electrodes  16 . 
     With regard to the electrode configuration shown in  FIG. 3 , for bare-minimal operation, two electrodes are required—the drive electrode  15  (0°) and the sense electrode  16  (45°). For optimal operation, the operating modes must have the same frequency (mode-matching). To achieve mode-matching quadrature cancellation must be performed. Differential operation (for improved sensitivity) can be achieved using the extra sense-electrodes. The unused electrodes are connected to a polarization voltage (V P ). The star-shell  12  is also maintained at V P . 
       FIGS. 4   a  and  4   b  illustrate primary degenerate flexural modes of an exemplary resonating star gyroscope  10 . As is shown in  FIGS. 4   a  and  4   b , the mode shapes are spatially 45° apart. This is the preferred mode of operation.  FIGS. 5   a  and  5   b  illustrate higher order flexural modes of an exemplary resonating star gyroscope  10 . As is shown in  FIGS. 5   a  and  5   b , the mode shapes are spatially 30° apart. This is a secondary mode of operation. 
       FIG. 6  is an schematic illustrating characterization electronics for use with the resonating star gyroscope  10 . More sophisticated electronics are required to ensure automatic mode-matching, quadrature cancellation, and operation in the closed-loop (to ensure flexible operating bandwidth). 
       FIG. 7  illustrates the increased electrode area of an exemplary resonating star gyroscope  10 .  FIG. 7   a  shows the encircled portion of  FIG. 7  and illustrates that quadrature cancellation is provided by the resonating star gyroscope  10 . 
     In order to increase the effective resonant mass (and consequently decrease the mechanical noise of the gyro  10 ) multiple-shells  12  may be implemented.  FIG. 8  illustrates an exemplary multi-shell implementation of a resonating star gyroscope  10 ,  FIG. 8   a  shows an enlarged view of the portion of  FIG. 8 . 
     HARPSS Implementation 
       FIG. 9  illustrates an exemplary resonating star gyroscope  10  fabricated using a High Aspect Ratio and Poly- and Single-crystalline Silicon (HARPSS) process. In the resonating star gyroscope  10  shown in  FIG. 9 , the substrate  20  is a low resistivity silicon wafer. The structural material is doped trench-refilled polysilicon. The anchor  13  is defined by an etched silicon post. Electrodes  15 ,  16 ,  17  are isolated using a nitride passivation layer. 
     Referring to  FIGS. 10   a - 10   f , they illustrate exemplary steps of a HARPSS fabrication process  30  that may be used to fabricate the resonating star gyroscope  10  shown in  FIG. 9 . The HARPSS fabrication process  30  may be used to fabricate thick polysilicon versions of the resonating star gyroscope  10 . Representative processing steps are also disclosed in a paper by F. Ayazi and K. Najafi, entitled “A High-Aspect Ratio Combined Poly and Single-Crystal Silicon (HARPSS) MEMS Technology”, IEEE/ASME JMEMS, September 2000, pp. 288-294. 
     As is shown in  FIG. 10   a , a nitride isolation layer  21  is formed on a substrate  20 . This layer acts as electrical isolation between the substrate and the device electrode. (A host of other material may be used as a dielectric isolation material). As is shown in  FIG. 10   b , trenches  22  are etched to define the geometry of the resonating star gyroscope  10 . As is shown in  FIG. 10   c , sacrificial oxide  23  is deposited (using a low pressure chemical vapor deposition (LPCVD) system) and doped. The sacrificial oxide layer may also be formed by thermal oxidation of the exposed silicon. The trenches  22  are refilled with LPCVD polysilicon  24 . As is shown in  FIG. 10   d  the polysilicon layer  24  is etched back, the oxide  23  is patterned, and the polysilicon  24  is deposited, doped and patterned. As is shown in  FIG. 10   e , an anisotropic silicon etch is performed and the vibratory structure of the resonating star gyroscope  10  is undercut using an isotropic silicon etch. As is shown in  FIG. 10   f , the sacrificial oxide  23  is then etched  46  in hydrogen fluoride (HF) solution, for example, to release the resonating structure from the substrate  20  and form the gyroscope  10 . 
     As is illustrated by  FIGS. 10   a - 10   f , mechanical structures (resonating star gyroscopes  10 ) are created by refilling trenches  21  with polysilicon  22  deposited over a sacrificial oxide layer  23 . The structural layer of polysilicon  22  is doped to make it conductive. Silicon sense electrodes  16  as tall as the star-shaped shell  12  (ring structure) are released from the substrate  20  using a two-step dry release process. Small high aspect ratio capacitive actuation gaps  24  (1 μm) between the electrodes  15 ,  16 ,  17  and the star-shaped shell  12  enable low voltage operation of the gyroscope  10 . 
       FIGS. 11   a  and  11   b  are graphs showing frequency response and mode matching, respectively, for the primary flexural modes of an exemplary HARPSS implementation of the resonating star gyroscope  10 .  FIG. 12  is a graph that illustrates sensitivity of the HARPSS fabricated resonating star gyroscope  10 . 
       FIG. 13  illustrates an exemplary (100) single crystal silicon on insulator implementation of the resonating star gyroscope  10 . The gyroscope  10  may be fabricated using deep reactive ion silicon etching.  FIGS. 14   a  and  14   b  are graphs showing frequency response and mode matching, respectively, for the higher-order flexural modes of the single crystal silicon on insulator implementation of the resonating star gyroscope  10 .  FIG. 14   c  is a graph that illustrates high-Q, higher-order flexural modes of the (100) single crystal silicon on insulator implementation of the resonating star gyroscope shown in  FIG. 13 . 
       FIG. 15  illustrates an exemplary epitaxial-polysilicon implementation of the exemplary resonating star gyroscope  10 . The gyroscope may be fabricated using deep reactive ion silicon etching.  FIGS. 16   a  and  16   b  are graphs showing frequency response and mode matching, respectively, for the higher-order flexural modes of an exemplary epitaxial-polysilicon implementation of the resonating star gyroscope  10 .  FIG. 16   c  is a graph that illustrates high-Q, higher-order flexural modes of the epitaxial-polysilicon implementation of the resonating star gyroscope  10 . 
     Primary Degenerate Mode Operation 
     A prototype polysilicon resonating star gyroscope  10  was fabricated and tested open loop under vacuum. A sinusoidal drive signal was applied at the drive electrode and output signals, monitored at the 0° and 45° electrodes, were amplified using external amplifiers. The primary flexural mode frequency of the prototype gyroscopes  10  was measured to be 39.6 kHz which is in agreement with ANSYS simulations. Electronic tuning allows compensation of any fabrication imperfections that may cause a frequency separation (˜100-400 Hz) between the two degenerate resonant modes. Frequency splits as great as 430 Hz have been matched by applying less than 11V tuning voltages to the tuning (balancing) electrodes  17 .  FIGS. 11   a  and  11   b  illustrates the two modes before and after balancing, respectively. After balancing, the two peaks merge together and the sense and drive mode frequencies become equal. 
     Table 1 illustrates an exemplary specifications for 1 mm polysilicon resonating star gyroscopes  10 . Rate test from the polysilicon resonating star gyroscopes  10  under matched operation yields an open-loop sensitivity of 1.6 mV/°/s using discrete PCB electronics (C parasitics ˜5 pF), as shown in  FIG. 6 . The measured Q of 1 mm, 65 μm-thick polysilicon resonating star gyroscopes  10  was 1500 under matched mode operation. This low Q-factor is attributed to anchor and bulk TED losses (voids inside polysilicon  22 ) such as is described in a paper by R. Abdolvand, G. K. Ho, A. Erbil, and F. Ayazi, entitled “Thermoelastic Damping in Trench-Refilled Polysilicon Resonators,” Proc. Transducers 2003, pp. 324-327, and can be improved by optimizing the design. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Device Parameter 
                 Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Primary flexural mode frequency 
                 39.6 
                 kHz 
               
               
                   
                 Polarization voltage 
                 4.8 
                 V 
               
               
                   
                 Quality factor 
                 1500 
               
             
          
           
               
                   
                 Mechanical resolution 
                 0.03°/s/√Hz 
               
             
          
           
               
                   
                 Rate sensitivity 
                 1.6 
                 mV/°/s 
               
               
                   
                   
               
             
          
         
       
     
     An epitaxial polysilicon implementation of the resonating star gyroscope  10  also yields primary flexural mode operation.  FIGS. 16   a  and  16   b  illustrates the two modes before and after balancing, respectively. After balancing, the two peaks merge together and the sense and drive mode frequencies become equal. Rate tests from the epitaxial polysilicon implementation of the resonating star gyroscopes  10  under matched operation yields an open-loop sensitivity of 0.5 mV/°/s using discrete PCB electronics, as shown in  FIG. 16   c.    
     The above two implementations are examples of resonating star gyroscope structures fabricated using an isotropic elastic material. Anisotropic (111) silicon may also be utilized to implement resonating star gyroscope  10  (in the primary flexural mode operation). 
     Higher-Order Degenerate Mode Operation 
     A single crystalline silicon (SCS) implementation of the resonating star gyroscope  10  provides for significantly improve quality factor which has been verified by an SOI prototype. The pair of higher-order degenerate modes, shown in  FIGS. 5   a  and  5   b , may also be used to detect rotation. In this degenerate pair the nodes and antinodes are located 30° apart. 
     In order to increase sensitivity and achieve better rate resolutions, it is desirable for the degenerate flexural modes to have high quality factors, greater drive amplitudes and larger mass. In an effort to achieve this, a single crystal silicon (SCS) implementation of the resonating star gyroscope  10  was fabricated. A high Q of 47,000 was measured for the primary flexural mode. However, due to the anisotropic nature of (100) SCS substrate  20 , the primary drive and sense flexural modes occur 3.6 kHz apart (as predicted by ANSYS simulations and verified experimentally). An interesting solution is to operate the gyroscope  10  using its higher-order degenerate flexural modes. As predicted by ANSYS simulations, these higher-order degenerate modes occur within close proximity of one another (&lt;1 kHz) and may be tuned electronically. 
     (100) Single Crystalline Silicon Implementation 
     Single crystal silicon resonating star gyroscopes  10  were fabricated on 40 μm thick low resistivity SOI. Actuation gaps between the electrodes  15 ,  16 ,  17  and the vibrating shell  12  is defined through DRIE trench etching step and is therefore aspect ratio limited. 
     The higher-order flexural mode frequency of the prototype gyroscope  10  was observed at 49.2 kHz as predicted by ANSYS simulations. The frequency split between the two secondary flexural modes is compensated using a similar scheme described to tune the primary order flexural modes of the polysilicon resonating star gyroscope  10 .  FIGS. 14   a  and  14   b  show the two resonant modes before and after balancing, respectively. 
     Wider capacitive gaps  24  (3 μm) reduce device capacitance and consequently increases required operating voltages. Polarization and balance voltages (to compensate 330 Hz frequency split) for the SCS resonating star gyroscope  10  are 20V and 26V respectively. Table II summarizes key parameters of the SCS implementation of the resonating star gyroscope  10  and illustrates exemplary specifications for 1 mm single crystal silicon resonating star gyroscopes  10 . Subsequent testing of other SCS resonating star gyroscopes  10  have yielded quality factors in excess of 100,000 for these higher-order degenerate modes (see  FIGS. 9   a  and  9   b ). 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Device Parameter 
                 Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Flexural mode frequency 
                 49.2 
                 kHz 
               
               
                   
                 Polarization voltage 
                 20 
                 V 
               
               
                   
                 Quality factor 
                 25000 
               
               
                   
                   
               
             
          
         
       
     
     Thus, improved resonating star gyroscopes  10  have been disclosed. Two modes of operation are possible using two distinct fabrication processes. The polysilicon HARPSS implementation of the resonating star gyroscope  10  was used to demonstrate primary degenerate mode operation. The HARPSS fabrication process facilitated high-aspect ratio sense and actuation gaps  24 . This increased the sensitivity and enabled operation at low voltages. The polysilicon resonating star gyroscope  10  demonstrated a sensitivity of 1.6 mV/°/s and has a Brownian noise floor of 0.03°/s/√Hz. The SCS SOI implementation of the resonating star gyroscope  10  exhibited higher-order degenerate mode operation. High-Q and higher frequency resonant modes were achieved in this implementation which improves the Brownian noise floor. 
     Thus, resonating star gyroscopes and fabrication methods relating thereto have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles discussed above. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.

Technology Category: 3