Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
2001-06-15
2004-05-18
Olsen, Allan (Department: 1763)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S039000, C216S055000, C216S062000, C216S079000, C438S050000, C427S527000
Reexamination Certificate
active
06736982
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a micromachined gyroscope and more particularly, to a micromachined gyroscope performing lateral sensing of angular rates, having both driving and sensing oscillation modes, and having all components formed from a single silicon crystal of a single silicon wafer.
BACKGROUND OF THE INVENTION
Gyroscopes are used to measure the angular deviation of a guided missile from its desired flight trajectory; to determine the heading of a vehicle for steering; to determine the heading of an automobile as it turns; to indicate the heading and orientation of an airplane during and after a series of maneuvers; or to stabilize and point radar dishes and satellites. Recently, micromachined gyroscopes are receiving increasing attention because of their low cost, small size and high sensitivity. Micromachined vertical or z-axis gyroscopes are used to counteract the rolling effect on a vehicle, and thus, are a preferred stabilization tool for vehicles such as airplanes, ships, and cars.
One developed micromachined vertical gyroscope utilizes a rapidly spinning, heavy mass. These spinning mass gyroscopes require lubrication and eventually wear out.
Another developed micromachined vertical gyroscope is based on vibration mode, but uses polysilicon technology. All suspension structures of this gyroscope are made of a polysilicon layer. To release the suspension structures a thick sacrificial layer is applied beneath the polysilicon layer. After the polysilicon layer is etched through, the sacrificial layer is removed. There are several problems with this gyroscope.
Although a single crystal silicon wafer may be used as a substrate of the gyroscope, the single crystal silicon wafer with several additional layers thereon is no longer suitable for standard microelectronics processing to realize monolithic integration.
An as-deposited polysilicon layer is in compressive strain. Suspension structures formed from the strain polysilicon layer tend to buckle, causing gyroscope instability or inability to work.
Since the surface of a relatively thick polysilicon layer is quite rough an additional polishing step is required before a planar processing process.
It is impossible to deposit a relatively thick polysilicon layer with device quality. This limits the stiffness of the suspension flexures in the vertical direction and makes electrostatic comb drive levitation more difficult to control.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a micromachined vertical vibrating gyroscope with all suspension structures being of single crystal silicon. The use of single crystal silicon structure eliminates the problems of the prior art caused by utilizing polysilicon as a building material.
It is a further object of the present invention to provide a micromachined vertical vibrating gyroscope with all suspension structures recessed into a processed silicon wafer. These recessed structures are more robust than any structures being out of the plane of the processed silicon wafer.
It is a still further object of the present invention to provide a micromachined vertical vibrating gyroscope capable of being produced by processing a single silicon wafer. Since no other wafer is required, the process is simple and allows batch production.
It is a still further object of the present invention to provide a micromachined vertical vibrating gyroscope capable of being fabricated on a single processing plane all along. This facilitates the use of standard planar processing technologies for integrated circuits.
A still further object of the present invention to provide a micromachined vertical vibrating gyroscope capable of being electronically integrated with other electronics similarly fabricated on the same chip so as to realize monolithic integration.
In accordance with these and other objects, a micromachined vertical vibrating gyroscope is described. The gyroscope consists of three single crystal silicon assemblies: an outer single crystal silicon assembly, an intermediate single crystal silicon assembly, and an inner single crystal silicon assembly. The outer assembly includes a plurality of arc-shaped anchors arranged in a circle and extending from a single crystal silicon substrate coated with an insulating annulus thereon. Each of at least four anchors support a suspension flexure and two suspension fan-shaped stops on its inner edge. The intermediate assembly is a suspension wheel possessing a same center with the circle and having a plurality of combs protruding from its outer edge. The linkage between the outer assembly and the intermediate assembly is realized through the four suspension flexures arranged along two orthogonal axes. The inner assembly is a suspension hub possessing a same center with the circle and no axle at its center. The linkage between the intermediate assembly and the inner assembly is realized through other two flexures arranged along a same axis. The intermediate suspension wheel is driven into rotational vibration by lateral comb capacitors. Each lateral comb capacitor is formed by a combination of a comb protruding from an anchor and a comb protruding from the intermediate wheel. Input angular rates are measured by two vertical capacitors that are formed between the bottom of the inner suspension hub and the interior top surface of the substrate used to support the anchors.
The micromachined vertical vibrating gyroscope is fabricated utilizing a bipolar-compatible process. This process comprises the steps of forming a buried layer, depositing and patterning an insulation layer, and growing an epitaxial layer. The epitaxial layer grown on the buried layer is of single crystal silicon and on the insulation layer is of lateral overgrown single crystal silicon. After converting the buried layer into a porous silicon layer all suspension structures are formed by partially etching into the epitaxial layer and removing the beneath porous silicon layer. The rest of the porous silicon layer is turned into an oxidized porous silicon layer for electrically isolating the anchors located thereon.
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patent: 6602714 (2003-08-01), Tagge et al.
Johnsonbaugh Bruce H.
Olsen Allan
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