Force transducers are formed of a beam of polysilicon which is mounted at its ends to a silicon substrate and is encapsulated within a polysilicon shell which defines, with the substrate, a cavity around the resonating beam. The cavity is sealed off from the atmosphere and evacuated to maximize the Q of the resonating beam. The beam is produced by deposition of polysilicon in such a way that, combined with subsequent annealing steps, the beam is in zero or low tensile strain. Resonant excitation of the beam may be accomplished in various ways, including capacitive excitation, and the vibratory motion of the beam may be detected utilizing an implanted resistor which is piezoresistive. Formation of the beam is carried out by depositing the beam on a sacrificial layer and surrounding it in a second sacrificial layer before the encapsulating polysilicon shell is formed. The sacrificial layers are etched out with liquid etchant which passes through channels in the periphery of the shell. Following etching, the interior of the cavity surrounding the beam is maintained in a wash liquid so that the beam is not deflected toward any of the adjacent surfaces, and the wash liquid is removed by freezing and su This invention was made with United States government support awarded by the National Science Foundation (NSF), Grant # EET-88-15285. The United States Government has certain rights in this invention.

Polysilicon resonating beam transducers and method of producing the same

Force transducers are formed of a beam of polysilicon which is mounted at its ends to a silicon substrate and is encapsulated within a polysilicon shell which defines, with the substrate, a cavity around the resonating beam. The cavity is sealed off from the atmosphere and evacuated to maximize the Q of the resonating beam. The beam is produced by deposition of polysilicon in such a way that, combined with subsequent annealing steps, the beam is in zero or low tensile strain. Resonant excitation of the beam may be accomplished in various ways, including capacitive excitation, and the vibratory motion of the beam may be detected utilizing an implanted resistor which is piezoresistive. Formation of the beam is carried out by depositing the beam on a sacrificial layer and surrounding it in a second sacrificial layer before the encapsulating polysilicon shell is formed. The sacrificial layers are etched out with liquid etchant which passes through channels in the periphery of the shell. Following etching, the interior of the cavity surrounding the beam is maintained in a wash liquid so that the beam is not deflected toward any of the adjacent surfaces, and the wash liquid is removed by freezing and sublimation. The interior surfaces of the cavity and the outer surfaces of the beam are passivated and the channels leading into the cavity may be sealed by oxidation in an oxidizing atmosphere, which also results in consumption of oxygen within the cavity.

Polysilicon resonating beam transducers

The sensor according to the present invention provides a device for measuring the magnitude of an applied load as a shift in resonant frequency of a mechanical resonator caused by load-induced strains on the resonator. The sensor includes a substrate, generally constructed of a semiconductor material, e.g. silicon, a diaphragm substantially supported along its outer periphery by the substrate, a boss abutting a region of the diaphragm remote from the outer periphery of the diaphragm, at least one resonator in the form of a beam having one end integral to the diaphragm proximate the region of the boss and the other end integral to the diaphragm remote from the boss, a hermetic seal for enclosing the resonator, and an exciter/detector for measuring changes in the natural frequency of the resonator due to an applied load. Preferably, the sensor includes a differential resonator configuration in which the resonators are positioned rectilinearly with respect to each other and are covered by the same hermetic seal.

Resonant mechanical sensor

A silicon substrate (112) having {100} nominal crystalline planes is anisotropically etched to form a pair of V-shaped grooves (18,20) along the top planar surface (14) and a first plate (16) between the grooves. The top planar surface is then doped to form a conductor region including the first plate 16). A substantially uniform layer (26) of a selectively etchable material, such as silicon oxide, is then grown over the grooved top planar surface. A layer (24) of doped silicon is grown over the silicon oxide layer to define a pair of V-shaped members opposite the pair of grooves. The silicon layer (24) is then partially etched to form a second plate (22) connected to the silicon layer (24) through a pair of V-shaped members (28,30). Both the second plate (22) and the pair of V-shaped members (28,30) are then suspended over the first capacitive plate (16) by etching a portion of the selectively etchable layer(26). Electronic circuitry (32) is then coupled to both the doped silicon layer and the doped substrate to detect changes in capacitance between the first and second plates (16,22) in response to an applied force, such as airflow, to be measured.

Method for fabricating a silicon force transducer

The method comprises the steps of providing a substrate wafer (10); depositing a first layer of resist (12) upon the substrate wafer (10); removing selected areas of the first resist layer (12), thereby to provide first etch windows; forming first cavities (16) in the substrate wafer (10) by a first etching process through the first windows; bonding a relatively thick membrane wafer (24) to the substrate wafer (10), thereby covering the cavities (16); polishing the surface of the relatively thick membrane wafer (24) thereby to produce a relatively thin membrane (24a); depositing a second layer of resist (33) on the relatively thin membrane (24a); removing selected areas of the second deposited resist layer, thereby to provide second etch windows (40); etching away the relatively thin membrane (24a) in the region of the second etch windows (40) until the first cavities (16) are exposed, thereby to form in the relatively thin membrane (24a) a free standing resonator structure (18 ) suspended on a plurality of compliant beam mounts (38) extending from a fixed portion (50).

A method of manufacturing a motion sensor

Vibratory transducer and method of making the same

A vibration transducer contains a vibration bar which consists of an n-type layer and is fixed at at least one end and is shaped by selective etching of an n-type layer which has been produced by local doping of a silicon monocrystal with foreign atoms. Furthermore, the vibration transducer contains devices for allowing the vibration bar to vibrate as well as devices for tapping off the vibration of the vibration bar. The vibration transducer is capable of measuring pressure, temperature, density etc. by detecting a change in the resonance frequency of the vibration bar. The vibration bar can be finely etched independently of its doping density in that it is formed in an aqueous alkaline solution with the application of a negative direct voltage or a pulsating voltage to a p-type layer and of a positive direct voltage or a pulsating voltage to an n-type layer. The vibration and tap-off devices for the vibration bar can be readily provided by constructing a diode and a transistor, enclosing the vibration bar, in the vibration transducer.

Yasushi Higashino

Papers

Vibratory transducer and method of making the same

Vibration transducer and method for manufacturing it