Polysilicon resonating beam transducers and method of producing the same

• US 5,188,983 A
• Filed: 01/03/1992
• Issued: 02/23/1993
• Est. Priority Date: 04/11/1990
• Status: Expired due to Term

• Current Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Original Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Inventors: Sniegowski, Jeffry, Guckel, Henry
• Primary Examiner(s): Hearn, Brian E.
• Assistant Examiner(s): Picardat, Kevin M.
• Application Number:
  US07/816,972
• Time in Patent Office: 417 Days
• Field of Search: 437/209, 437/215, 437/219, 73/862.59
• US Class Current: 438/53
• CPC Class Codes: B81C 2203/0145 Hermetically sealing an opening in the lid
• CPC Class Codes: G01L 1/183 by measuring variations of frequency of vibrating piezo-resistive material
• CPC Class Codes: G01L 1/186 optical excitation or measuring of vibrations
• CPC Class Codes: G01L 9/0019 of a semiconductive element
• CPC Class Codes: G01L 9/002 Optical excitation or measuring
• CPC Class Codes: H03H 2009/02511 Vertical, i.e. perpendicular to the substrate plane
• CPC Class Codes: H03H 3/0072 of microelectro-mechanical resonators or networks micromembranes or microbeams B81B2203/01; manufacture of microstructural devices in general B81C
• CPC Class Codes: H03H 9/02259 Driving or detection means
• CPC Class Codes: Y10T 29/49103 Strain gauge making

Abstract

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.

Claims

1. A method of making a resonant beam force transducer comprising the steps of:

(a) forming a beam of polysilicon mounted at its ends above a substrate;

(b) annealing the polysilicon beam so it has zero or low built-in tensile strain;

(c) encapsulating the beam in a polysilicon shell which forms with the substrate a cavity which surrounds the beam, and channels formed in the shell leading from the cavity;

(d) sealing the interior of the cavity from the external atmosphere with a gas which reacts to deposit solid material in the channels to close them and to evacuate the cavity.

2. The method of claim 1 wherein the step of encapsulating the beam in a polysilicon shell involves the further steps of forming a sacrificial layer under the beam and a sacrificial layer over and surrounding the beam, depositing the polysilicon shell over the sacrificial layer surrounding the beam with channels extending through the periphery of the polysilicon layer, applying an etchant to the substrate and the deposited polysilicon layer thereon which etches through the channels in the polysilicon and etches out the sacrificial layers around the beam to expose the beam, removing the etchant and exposing the substrate and polysilicon layer thereon to an oxidizing gas including oxygen for a period of time sufficient to form an oxide of the polysilicon layer in the channels which at least partially closes the channels.

3. The method of claim 2 wherein after the step of applying an etchant to etch the sacrificial layers, the polysilicon beam and the polysilicon shell are maintained in a freezeable liquid so that these structures are not dried out after the sacrificial layers have been removed, then freezing the liquid and sublimating the frozen liquid from the cavity before carrying out the further steps of exposing the substrate and the polysilicon layer to an oxidizing gas.

4. The method of claim 1 wherein the step of forming the polysilicon shell comprises the step of depositing polysilicon by chemical vapor deposition.

5. The method of claim 1 wherein the step of forming the polysilicon beam comprises the steps of depositing polysilicon onto an initially formed sacrificial layer by chemical vapor deposition of polysilicon from a gas, removing the sacrificial layer to leave the free beam mounted at its ends to the substrate, and subsequently annealing the beam at a temperature to provide a low tensile or zero strain within the beam.

6. The method of claim 1 including the further steps of implanting dopants in patterns onto the polysilicon beam to provide conductive paths therein for forming a plate of a capacitor for exciting vibrations of the beam and for providing a piezoresistive conducting path in the beam to allow the vibratory motions of the beam to be detected.