CMU MEMS Laboratory Publication Abstract


in Symposium Gyro Technology 1997 (SGT), pp. 3.0-3.8, September 16-17, 1997, Stuttgart, Germany.
Micromechanical Vibratory Rate Gyroscopes Fabricated in Conventional CMOS
M. S. Kranz and G. Fedder
This paper provides an overview of two microelectromechanical vibratory rate gyroscope designs implemented using a conventional CMOS process. This process uses the metallization and dielectric layers as both electrical interconnect and as structural layers. The gyroscope designs take advantage of the multiconductor structural layers, and of the ability to integrate electronics along with the mechanical structure. The microelectromechanical systems are fabricated using the three metal Hewlett-Packard 0.5 micron n-well process available through the MOS Implementation Service (MOSIS). To produce a suspended microelectromechanical structure, the metal layers, as well as the dielectric layers between them, combine to form laminated structural elements. The top metal layer is used to mask a series of dry etch steps. The microstructural sidewalls are formed by directionally etching the top oxide layers down to the substrate. The mechanical structure is then released from the substrate by isotropically etching the exposed silicon. The gyroscope designs involve a central mass, comb finger actuators, comb finger capacitive position sensors, and mechanical springs. Comb finger actuators apply an electrostatic force to the proof mass, creating oscillation along the x-axis. This central mass, when placed in a rotating inertial reference frame, experiences a Coriolis force orthogonal to the axis of excitation, with a frequency equal to that of the excitation frequency. The Q factor of the system at the frequency of excitation provides a gain in the resulting deflection of the mass. The y-axis deflection is sensed with a pair of comb finger capacitors which form a differential capacitive voltage divider. A unity-gain buffer detects the divider's voltage. The three-fold symmetric gyroscope design shown places the central mass within a completely symmetric spring structure. Because of this symmetry, the resonant frequencies of the normal mode oscillations along each gyro axis are inherently matched. The gain in the system due to the Q-factor is then maximized. This design is expected to resonate at a frequency of 11kHz and have a driven displacement of 2 microns when excited by a sinusoidal voltage with an amplitude of 30V at the resonant frequency. A second gyroscope design, the elastically gimbaled gyroscope, takes advantage of the multiconductor microstructures available in the CMOS fabrication process. This design has the central mass, a set of sensing comb fingers, and a set of springs nested within a outer cage. The entire structure (cage, inner mass, etc.) is then suspended by springs and driven by a set of comb drives. The distinguishing characteristic of this design is that the oscillation of the inner proof mass is decoupled from the oscillation of the outer cage. This decoupling results in a more linear system in that the motion one axis does not affect the spring constant along the other axis. However, the drawback is that matching the resonant frequencies becomes more difficult. The fabrication of this structure is possible because of the flexibility offered by the laminated structural elements. Having multiple conductors embedded within a single structure removes the restriction that an entire moveable structure must be at a single voltage. This gyroscope design is expected to resonate at 7kHz and have a driven displacement of 2 microns when excited by a voltage of 30V. The expected sensitivity of this gyroscope is 40mV/deg/sec. An initial design for the mechanical structure of the nested gyroscope has been fabricated. An SEM of this device is shown in Fig.4. Currently, second generation gyroscope designs are being fabricated. We will report on the latest fabrication and test results in the final paper.
© 1997 Deutsche Gesellschaft für Ortung und Navigation (DGON).
Full paper not available from outside CMU

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