Silicon micromachined accelerometer/seismometer and method of making the same

• US 6,196,067 B1
• Filed: 05/05/1998
• Issued: 03/06/2001
• Est. Priority Date: 05/05/1998
• Status: Expired due to Fees

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Martin, Richard D., Pike, W. Thomas
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): MOLLER, RICHARD ALAN
• Application Number:
  US09/072,793
• Time in Patent Office: 1,036 Days
• Field of Search: 73/514.32, 73/514.17, 73/514.16, 73/862.52, 73/862.61, 73/862.626
• US Class Current: 73/514.32
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass
• CPC Class Codes: G01P 2015/0842 the mass being of clover leaf shape

Abstract

A silicon-based microaccelerometer for seismic application is provided using a low-resonant frequency (10 Hz), large proof mass (1 gram), and high Q suspension to achieve high sensitivity of less than 1 ng with a bandwidth a 0.05 to 50 Hz. The proof mass is cut away from a planar substrate in the form of a disk using abrasive cutting, which disk closely fits but does not touch a surrounding angular frame. The spring of the microaccelerometer between the angular frame and the proof mass is provided from two continuous, 3 microns thick membranes. The fixed capacitive electrodes are provided on separate, subsequently bonded substrates, and movable capacitive plates are provided on the membranes. By fabricating capacitive plates on the separate substrates, the gap between the fixed and movable capacitive plates in the differential capacitive sensor is closely controlled. The use of continuous membranes for the spring produces a shock resistant, robust sensor.

Claims

1. A micromachined differential capacitive sensor comprising:

a proof mass micromachined in a first silicon substrate in which said proof mass is defined by a predetermined mass;

a frame adjacent to said proof mass;

at least one peripheral spring connected between said frame and said proof mass to suspend said proof mass substantially without tilt;

at least one movable capacitive plate carried by said proof mass;

at least one fixed capacitive plate; and

at least one separate substrate upon which said fixed capacitive plate is separately fabricated, said separate substrate being assembled on said frame and on at least one side of said proof mass thereby opposing said proof mass, said fixed capacitor plate on said substrate being positioned at a predetermined gap away from said movable plate, said fixed capacitive plate being disposed in an etched cavity defined in said separate substrate prior to assembly of said separate substrates opposing said proof mass and frame, said predetermined gap having an initial size defined as the distance between said fixed capacitor plate and said movable plate when said fixed capacitor plate and said movable plate are both at rest with respect to each other,

wherein said proof mass is a cylindrical disk and wherein said spring is at least one continuous membrane extending between said frame and said proof mass.

2. The sensor of claim 1 wherein said proof mass has an upper and lower surface, and wherein said spring comprises two continuous membranes extending across said upper and lower surface of said proof mass and between said proof mass and said frame.

3. A micromachined differential capacitive sensor comprising:

a proof mass micromachined in a first silicon substrate in which said proof mass is defined by a predetermined mass;

a frame adjacent to said proof mass;

at least one peripheral spring connected between said frame and said proof mass to suspend said proof mass substantially without tilt;

at least one movable capacitive plate carried by said proof mass;

at least one fixed capacitive plate; and

at least one separate substrate upon which said fixed capacitive plate is separately fabricated, said separate substrate being assembled on said frame and on at least one side of said proof mass thereby opposing said proof mass, said fixed capacitor plate on said substrate being positioned at a predetermined gap away from said movable plate, said fixed capacitive plate being disposed in an etched cavity defined in said separate substrate prior to assembly of said separate substrates opposing said proof mass and frame, said predetermined gap having an initial size defined as the distance between said fixed capacitor plate and said movable plate when said fixed capacitor plate and said movable plate are both at rest with respect to each other,

wherein said peripheral spring is an integral membrane and said proof mass has a mass of at least one gram.

4. A micromachined differential capacitive sensor comprising:

a proof mass micromachined in a first silicon substrate in which said proof mass is defined by a predetermined mass;

a frame adjacent to said proof mass;

at least one peripheral spring connected between said frame and said proof mass to suspend said proof mass substantially without tilt;

at least one movable capacitive plate carried by said proof mass;

at least one fixed capacitive plate; and

at least one separate substrate upon which said fixed capacitive plate is separately fabricated, said separate substrate being assembled on said frame and on at least one side of said proof mass thereby opposing said proof mass, said fixed capacitor plate on said substrate being positioned at a predetermined gap away from said movable plate, said fixed capacitive plate being disposed in an etched cavity defined in said separate substrate prior to assembly of said separate substrates opposing said proof mass and frame, said predetermined gap having an initial size defined as the distance between said fixed capacitor plate and said movable plate when said fixed capacitor plate and said movable plate are both at rest with respect to each other,

wherein said spring and said proof mass are micromachined elements and wherein said spring has a stiffness relative to said mass of said proof mass so that said sensor has a resonant frequency of approximately 10 Hz, and

wherein said peripheral spring is an integral membrane and said proof mass has a mass of at least one gram.

5. A differential capacitive sensor comprising:

a proof mass micromachined in a first silicon substrate;

a frame adjacent to said proof mass;

at least one peripheral spring connected between said frame and said proof mass to suspend said proof mass substantially without tilt;

at least one movable capacitive plate carried by said proof mass;

at least one fixed capacitive plate; and

at least one separate substrate upon which said fixed capacitive plate is separately fabricated, said separate substrate being assembled on said frame and on at least one side of said proof mass thereby opposing said proof mass, said fixed capacitor plate on said substrate being positioned at a predetermined gap away from said movable plate, said fixed capacitive plate being disposed on said separate substrate prior to assembly of said separate substrates opposing said proof mass and frame, said predetermined gap having an initial size defined as the distance between said fixed capacitor plate and said movable plate when said fixed capacitor plate and said movable plate are both at rest with respect to each other,

wherein said proof mass is a circular disk having an upper and lower surface, and wherein said frame comprises an angular ring with said peripheral spring comprising an upper and lower continuous membrane extending across said upper and lower surface of said proof mass between said angular ring and said circular disk-shaped proof mass.

6. The sensor of claim 5 wherein said membranes are formed on separate wafers from said proof mass and wherein said membranes are wafer-bonded to said frame and proof mass.

7. A differential capacity of sensor comprising:

an angular ring-shaped frame;

a circular cylindrical proof mass of at least one gram disposed within said angular ring-shaped frame;

a pair of continuous membranes disposed between said frame and said proof mass, said proof mass being coupled to said pair of continuous membranes and opposing upper and lower surfaces of said proof mass being coupled to said membranes at a central location of said proof mass;

movable capacitive plates disposed on said membranes; and

an upper and lower substrate having fixed capacitive plates disposed within cavities defined therein opposing said movable capacitive plates on said membranes,

whereby a robust, compact sensor is provided with high shock resistance and small electrode spacing for increased sensitivity.