Abstract:
A microgyroscope has a baseplate made of the same material as the rest of the microgyroscope. The baseplate is a silicon baseplate having a heavily p-doped epilayer covered by a thick dielectric film and metal electrodes. The metal electrodes are isolated from the ground plane by the dielectric. This provides very low parasitic capacitive coupling between the electrodes. The thick dielectric reduces the capacitance between the electrodes and the ground plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 60/143,200, filed Jul. 9, 1999. 
    
    
     ORIGIN OF INVENTION 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) in which the Contractor has elected to retain title. 
    
    
     TECHNICAL FIELD 
     This invention relates to vibratory gyroscopes, and more particularly to silicon micromachined vibratory gyroscopes. 
     BACKGROUND 
     Multi-axis sensors are highly desirable for inertial sensing of motion in three dimensions. Previously, such sensors were constructed of relatively large and expensive electromagnetic and optical devices. More recently, micromechanical sensors have been fabricated using semiconductor processing techniques. Microelectrical mechanical or “MEMS” systems allow formation of physical features using semiconductor materials and processing techniques. These techniques enable the physical features to have relatively small sizes and be precise. Specifically, micromechanical accelerometers and gyroscopes have been formed from silicon wafers by using photolithographic and etching techniques. Such microfabricated sensors hold the promise of large scale production and therefore low cost. 
     The integration of three gyroscopic sensors to measure the rotation rates about the three separate axes coupled with three accelerometric sensors to measure the acceleration along the three axes on a single chip would provide a monolithic, six degree-of-freedom inertial measurement system capable of measuring all possible translations and orientations of the chip. There has been some difficulty in constructing a high-performance, or sensitive vibratory rate gyroscope to measure the rotation about the axis normal to the plane of the silicon chip, i.e., the Z-axis. 
     In a vibratory gyroscope, the Coriolis effect induces energy transfer from the driver input vibratory mode to another mode which is sensed or output during rotation of the gyroscope. Silicon micromachined vibratory gyroscopes are integratable with silicon electronics. These devices are capable of achieving high Q factors, can withstand high g shocks due to their small masses, are insensitive to linear vibration and consume little power. However, most of these micromachined gyroscopes have a very small rotation response, since their input and output vibration modes have different mode shapes and resonant frequencies. The use of different resonant modes also makes these devices very temperature sensitive due to the different temperature dependency of each of the modes. These devices usually have very high resonant frequencies resulting in low responsitivity, since the Coriolis induced response is inversely proportional to the resonant frequency of the structure. Finally, due to the small mass of the structure, thermal noise limits the ultimate performance and use of microgyroscopes. For these reasons, micromachined vibratory gyroscopes have not been used for precision navigation and attitude control applications, but have been employed primarily for automotive applications in which extreme low cost is a major driving factor and performance is set at a lower premium. 
     Traditional baseplates for the microgyroscopes were constructed using either quartz or ceramic. Although these materials were helpful in reducing parasitic capacitance, the thermal expansion coefficient for these materials is different than the rest of the microgyroscope. Thus, under a variety of temperatures, the stress on the microgyroscope varies. What is needed is a baseplate having a similar thermal expansion coefficient as the rest of the microgyroscope while reducing the effects of parasitic capacitance. 
     SUMMARY 
     The present invention is a microgyroscope having a baseplate made of the same material as the rest of the microgyroscope. The baseplate is a silicon baseplate having a heavily p-doped epilayer covered by a thick dielectric film and metal electrodes. The metal electrodes are isolated from the ground plane by the dielectric. This provides very low parasitic capacitive coupling between the electrodes. The thick dielectric reduces the capacitance between the electrodes and the ground plane. 
    
    
     DESCRIPTION OF DRAWINGS 
     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
     FIG. 1 is a side view of the components of a microgyroscope constructed according to one embodiment of the present invention. 
     FIG. 2 is a side view of a bonded microgyroscope according to one embodiment of the present invention. 
     FIG. 3 is a side view of the baseplate of the microgyroscope according to one embodiment of the present invention. 
     FIG. 4 is a side view of the microgyroscope including the baseplate of FIG. 3 according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a side view of a microgyroscope  100  constructed according to one embodiment of the present invention. The microgyroscope  100  detects forces in the x-direction  105 , the y-direction  110 , and in the z-direction  115 . Vertical capacitors  130  surround the vertical post  120 . The vertical capacitors  130  provide electrostatic actuation of the vertical post  120  and allow for capacitive detection of the motions of the vertical post  120 . 
     A first section  205  of the microgyroscope  100  is made from a first silicon wafer and a second section  210  of the microgyroscope is made from a second silicon wafer. The first section  205  of the microgyroscope  100  includes a first portion  220  of the vertical post  120  and first portions  230  of the vertical capacitors  130 . The second section  210  of the microgyroscope  100  includes a second portion  240  of the vertical post  120 , second portions  245  of the vertical capacitors  130 , and a baseplate  135 . Although the invention is described with the vertical capacitors  130  being constructed of first portions  230  and second portions  245 , it can be appreciated that the first portions  230  and second portions  245  may be electrically isolated to create even more independent vertical capacitors  130 . 
     To construct the microgyroscope  100 , the first section  205  is positioned above the section  210 . The first section  205  is lowered as indicated by reference numeral  200  onto the second section  210 . The first and second sections  205 ,  210  are then bonded together using standard bonding techniques such as metal-to-metal bonding. The first and second sections  205 ,  210  are bonded in a position so that the first portion  220  and the second portion  240  of the vertical post  120  are aligned to form one continuous vertical post  120 . The first portions  230  of the vertical capacitors  130  are aligned with the second portions  245  of the vertical capacitors  130  on the baseplate  135 . Of course, the first portions  235  and the second portions  230  may be electrically isolated. Thus, once the first section  205  is connected to the second section  210 , the vertical post  120  is positioned with the microgyroscope  100 . 
     FIG. 2 shows a side view of the completed microgyroscope  100  according to one embodiment of the present invention. Once constructed, the vertical post  120  is supported by a series of flanges  150 . The flanges  150  allow the vertical post  120  to rest upon the baseplate. As can be seen in FIG. 2, the flanges  150  are positioned at approximately the mid-point of the vertical post  120 . Because the vertical post  120  is connected to the baseplate  135  via the flanges  150  at approximately the mid-point, the vertical post  120  is free to move in a rocking motion in the x-direction  105  and the y-direction  110 . Under input rotation, the Coriolis force causes the vertical post  120  to move in the orthogonal direction to the drive motor. The rotation rate sensitivity is proportional to the input rotation rate, the drive amplitude, and the quality factor of the resonator. 
     Because each portion of the microgyroscope  100  is constructed from a silicon wafer, the performance variations from device to device is reduced. Further, the behavior of each portion of the microgyroscope  100  under varying temperature conditions is more consistent. 
     FIG. 3 is a side view of the baseplate  135  of the microgyroscope  100  according to one embodiment of the present invention. The baseplate  135  comprises a silicon substrate  305 , a silicon p+ epilayer  310 , a dielectric layer  315 , and metal electrodes  320 . The silicon substrate  305  is grounded and covered by the heavily p-doped epilayer  310 . The epilayer  310  is covered by the thick dielectric layer  315 . The metal electrodes  320  are formed on the dielectric layer  315 . 
     The dielectric layer  315  may be composed of silicon dioxide, silicon nitride, or other known dielectric materials. The dielectric layer  315  provides isolation of the metal electrodes  320  from the ground plane. The dielectric layer  315  is generally about 3 to 4 microns thick, and may be thicker than 4 microns to enhance the isolation. Because of the isolation provided by the dielectric layer, there is very little capacitive coupling between the metal electrodes  320 . 
     FIG. 4 is a side view of the microgyroscope  100  including the baseplate  135  of FIG. 3 according to one embodiment of the present invention. In the fully assembled gyroscope  100 , the vertical post  120  rests on a clover-leaf structure  330 . The metal electrodes  320  provide electrostatic drive and sense control. The metal electrodes  320  are electrically isolated from each other and other electrical interference by the dielectric layer  315  and the substrate  305 . 
     Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.