Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-12-06
2001-02-27
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06192757
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to monolithic micromechanical devices and, more particularly, to monolithic micromechanical devices which include suspended microstructures and which may include circuitry.
BACKGROUND OF THE INVENTION
The construction of microsensors on microchips is of great interest in many industries because of its potential to reduce size and cost of devices which require the sensing of environmental or other conditions. An accelerometer is just one example of a type of sensor which has wide application possibilities.
Acceleration sensors are known for measuring force or mass or to operate control systems responsive to acceleration. For instance, acceleration sensors may be used in automotive vehicles to deploy air bags responsive to a particular threshold deceleration rate of a vehicle. Acceleration sensors may also be used in the automotive industry as part of active suspension systems in which microcontrollers adjust suspension components responsive to the vertical acceleration of the wheels.
An accelerometer comprises an acceleration-sensing element, or transducer, which is commonly interfaced to resolving circuitry for producing a useful output signal from the output of the transducer. Though the term “accelerometer” is sometimes used to refer to the sensor (or transducer) itself, the term is used herein to denote a complete system including a transducer as well as the resolving circuitry.
Many commercially available accelerometers employ transducers comprising a mechanical or electromechanical element (e.g., piezoelectric, piezoresistive or strain gauge).
Acceleration sensing microstructures embodied on silicon chips have been suggested in the prior art. For instance, U.S. Pat. No. 4,711,128 issued to Boura discloses an acceleration sensor formed by micromachining a fine monocrystal wafer. The sensor comprises a flat mobile mass suspended above the rest of the structure by means of two thin parallel strips situated on each side of the mass. The mass comprises at least one mobile capacitive plate which is disposed between two other capacitive plates which are not part of the suspended microstructure but are fixed on the structure. The mobile plates are charged to a voltage relative to the stationary plates. When the sensor is subjected to an acceleration, the mobile plates move relative to the fixed plates causing a change in capacitance between the mobile plates and each of the fixed plates. The change in capacitance is observed by observing the voltage between the mobile plate and the fixed plates and is a direct indication of the distance of movement of the fixed plate which, in turn, is a measurement of the acceleration.
U.S. Pat. No. 4,705,659 issued to Bernstein et al. teaches a technique of fabricating a free standing thin or thick structure such as an acceleration sensor, including the steps of providing a substrate, forming a layer of carbon on the substrate and depositing a film of polycrystalline material over the layer of carbon. The sandwich structure is heated in an oxidizing ambient to cause the oxidation of the carbon layer, leaving the polysilicon material as a free-standing film.
The prior art, however, does not teach a monolithic accelerometer in which the acceleration sensor as well as the resolving circuitry for producing a useful output are embodied on a single chip or a technique for making such a monolithic accelerometer. Thus, prior art accelerometers require a separate chip or other means containing circuitry for resolving the output of the sensor into a usable signal.
SUMMARY OF THE INVENTION
In accordance with the invention, a monolithic capacitance-type microstructure is provided. The microstructure comprises a semiconductor substrate including a surface, a plurality of posts extending from the surface of the substrate, a bridge suspended from the posts, the bridge including an element that is movable with respect to the surface of the substrate, and an electrically-conductive substantially stationary element anchored to the substrate and positioned relative to the movable element such that the movable element and the substantially stationary element form a capacitor. Preferably, the movable element is laterally movable with respect to the surface of the substrate.
The bridge may include a central beam and a plurality of generally parallel electrically conductive movable fingers extending transversely from the beam. The substantially stationary element may comprise a plurality of generally parallel electrically-conductive, substantially stationary fingers. Each of the substantially stationary fingers may correspond to one of the movable fingers and be positioned relative to the corresponding movable finger such that the movable finger and the corresponding substantially stationary finger form a capacitor, with movement of the movable fingers relative to the substantially stationary fingers altering the capacitance between each of the movable fingers and the corresponding substantially stationary finger.
According to a further aspect of the invention, a monolithic micromechanical apparatus may comprise a monolithic capacitance-type microstructure, as described above, and circuitry disposed on the substrate and operationally coupled to the movable fingers and the substantially stationary fingers for processing a signal based on a relative positioning of the movable fingers and the substantially stationary fingers. In a preferred embodiment, the monolithic capacitance-type microstructure comprises an acceleration sensor.
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Brokaw A. Paul
Core Theresa A.
Sherman Steven J.
Tsang Robert W. K.
Analog Devices Inc.
Chapman John E.
Wolf Greenfield & Sacks P.C.
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