Monolithic micromechanical vibrating beam accelerometer with trimmable resonant frequency

• US 5,760,305 A
• Filed: 02/20/1996
• Issued: 06/02/1998
• Est. Priority Date: 10/17/1990
• Status: Expired due to Term

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Greiff, Paul
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): MOLLER, RICHARD ALAN
• Application Number:
  US08/602,512
• Time in Patent Office: 833 Days
• Field of Search: 73/514.36, 73/514.38, 73/514.15, 73/514.17, 73/514.18, 73/514.32, 73/504.02, 73/504.12, 257/417, 257/419, 156/628.1, 156/630.1, 156/633.1, 156/644.1, 156/647.1, 156/657.1, 156/659.11, 156/662.1, 156/651.1, 437/228, 437/233, 437/238, 437/921, 216/2, 216/33, 216/41, 216/62, 216/79
• US Class Current: 73/514.15
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: G01P 2015/0831 the mass being of the paddle type having the pivot axis between the longitudinal ends of the mass, e.g. see-saw configuration

Abstract

A monolithic, micromechanical vibrating beam accelerometer with a trimmable resonant frequency is fabricated from a silicon substrate which has been selectively etched to provide a resonant structure suspended over an etched pit. The resonant structure comprises an acceleration sensitive mass and at least two flexible elements having resonant frequencies. Each of the flexible elements is disposed generally collinear with at least one acceleration sensitive axis of the accelerometer. One end of at least one of the flexible elements is attached to a tension relief beam for providing stress relief of tensile forces created during the fabrication process. Mass support beams having a high aspect ratio support the mass over the etched pit while allowing the mass to move freely in the direction collinear with the flexible elements. Also disclosed is a method for fabricating such an accelerometer. Further disclosed is an alternative embodiment of the aforementioned accelerometer characterized by a low profile, and alternative planar processing methods for fabrication of these and other embodiments.

Claims

1. A semiconductor transducer comprising:

a planar surface oxide or nitride layer on a first planar semiconductor substrate;

a second planar semiconductor substrate having a first planar portion, and a second planar portion bonded to said planar surface oxide or nitride layer;

said first planar portion having an etched pattern of a mass suspended at one or more discrete selected locations from said second planar portion and having at least one flex portion;

said second planar portion having respective supports for said first planar portion in said one or more discrete locations and at least one discrete sense and drive electrode underlying said at least one flex portion,

said semiconductor transducer produced by a fabrication process including the steps of;

providing said first planar semiconductor substrate having said planar surface oxide or nitride layer as an etch resistant first surface;

providing said second planar semiconductor substrate with a precursor portion and said second planar portion, and bonding said second planar portion to said etch resistant first surface of said first planar semiconductor substrate;

removing said precursor portion;

selectively diffusing said second planar portion to define a plurality of electrodes and supports;

growing on said second planar portion a first epitaxial layer;

aperturing said first epitaxial layer above said support defining diffusions;

growing on said first epitaxial layer a second epitaxial layer;

removing selected portions of said second epitaxial layer to define a suspended portion; and

removing said first epitaxial layer.

2. The semiconductor transducer of claim 1 wherein said at least one flex portion is suspended above respective sense and drive electrodes.

3. A semiconductor transducer comprising:

a planar surface oxide or nitride layer on a first planar semiconductor substrate;

a second planar semiconductor substrate having a first planar portion, and a second planar portion bonded to said planar surface oxide or nitride layer;

said first planar portion having an etched pattern of a mass suspended at one or more discrete selected locations from said second planar portion and having at least one flex portion; and

said second planar portion having respective supports for said first planar portion in said one or more discrete locations and having at least one discrete sense and drive electrode underlying said at least one flex portion,

said semiconductor transducer produced by a fabrication process including the steps of;

providing said first planar semiconductor substrate having said planar surface oxide or nitride layer as an etch resistant first surface;

providing said second planar semiconductor substrate with said precursor portion and a second planar portion, and bonding said second planar portion to said etch resistant first surface of said first planar semiconductor substrate;

removing said precursor portion;

selectively diffusing said second planar portion to define a plurality of doped electrodes and supports, and undoped regions;

growing on said second planar portion an epitaxial layer;

etching a pit in said epitaxial layer over each of selected support defining diffusion in said second portion;

selectively diffusing said epitaxial layer to define a suspended portion;

removing selected portions of said epitaxial layer; and

removing said undoped regions of said second portion.

4. The semiconductor transducer of claim 3 wherein said at least one flex portion is suspended above respective sense and drive electrodes.

5. A semiconductor transducer comprising:

a planar surface oxide or nitride layer on a first planar semiconductor substrate;

a second planar semiconductor substrate having a first planar portion, and a second planar portion bonded to said planar surface oxide or nitride layer;

said first planar portion having an etched pattern of a mass suspended at one or more discrete selected locations from said second planar portion and having at least one flex portion; and

said second planar portion having respective supports for said first planar portion in said one or more discrete locations and having at least one discrete sense and drive electrode underlying said at least one flex portion,

said semiconductor transducer produced by a fabrication process including the steps of;

providing said first planar semiconductor substrate having said planar surface oxide or nitride layer as an etch resistant first surface;

providing said second planar semiconductor substrate with said precursor portion and a second planar portion, and bonding said second planar portion to said etch resistant first surface of said first planar semiconductor substrate;

removing said precursor portion;

etching at least one via through said second planar portion and said etch resistant first surface into said first planar semiconductor substrate;

selectively diffusing said second planar portion to define a plurality of doped electrodes and supports, and undoped regions;

growing on said second planar portion an epitaxial layer;

etching a pit in said epitaxial layer over each of selected support defining diffusions in said second planar portion;

selectively diffusing said epitaxial layer to define a suspended portion;

removing selected portions of said epitaxial layer; and

removing said undoped regions of said second planar portion.

6. The semiconductor transducer of claim 5 wherein said at least one flex portion is suspended above respective sense and drive electrodes.

7. A semiconductor transducer comprising:

a planar surface oxide or nitride layer on a first planar semiconductor substrate;

a second planar semiconductor substrate having a first planar portion, and a second planar portion bonded to said planar surface oxide or nitride layer;

said first planar portion having an etched pattern of a mass suspended at one or more discrete selected locations from said second planar portion and having at least one flex portion; and

said second planar portion having respective supports for said first planar portion in said one or more discrete locations and having at least one discrete sense and drive electrode underlying said at least one flex portion,

said semiconductor transducer produced by a fabrication process including the steps of;

providing said first planar semiconductor substrate having said surface oxide or nitride layer as an etch resistant first surface;

providing said second planar semiconductor substrate with a precursor portion and said second planar portion, and bonding said second planar portion to said etch resistant first surface of said first planar semiconductor substrate;

removing said precursor portion;

etching at least one via through said second planar portion and said etch resistant first surface into said first planar semiconductor substrate;

selectively diffusing said second planar portion to define a plurality of doped electrodes and supports, and undoped regions;

growing on said second planar portion a first epitaxial layer;

etching an aperture in said first epitaxial layer over each of selected support defining diffusions in said second planar portion;

growing on said first epitaxial layer a second epitaxial layer;

selectively diffusing said second epitaxial layer to define a suspended portion;

removing selected portions of said second epitaxial layer; and

removing said undoped regions of said second planar portion and said first epitaxial layer.

8. The semiconductor transducer of claim 7 wherein said at least one flex portion is suspended above respective sense and drive electrodes.