Monolithic micromechanical vibrating string accelerometer with trimmable resonant frequency

• US 5,408,119 A
• Filed: 10/17/1990
• Issued: 04/18/1995
• 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): Sotomayor, John B.
• Application Number:
  US07/599,131
• Time in Patent Office: 1,644 Days
• Field of Search: 357/26, 257/417, 257/419, 257/420, 73/505, 73/517 R
• US Class Current: 257/417
• 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
• CPC Class Codes: Y10S 438/978 forming tapered edges on substrate or adjacent layers

Abstract

A monolithic, micromechanical vibrating string 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 colinear 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 colinear with the flexible elements. Also disclosed is a method for fabricating such an accelerometer with high aspect ratio tension relief and mass support beams.

Claims

1. A monolithic micromechanical vibrating accelerometer with a trimmable resonant frequency, for detecting acceleration along at least one acceleration sensitive axis comprising:

a silicon substrate in which has been selectively etched a pit over which is suspended a non-etched silicon resonant structure comprising an acceleration sensitive mass and at least two flexible elements having a resonant frequency, said at least two flexible elements generally equally suspending said mass above said etched pit;

each of said flexible elements disposed generally colinear with said at least one acceleration sensitive axis, and having first and second ends, the first flexible element having its first end attached to a first side of said mass and the second flexible element having its first end attached to a second side of said mass diametrically opposed from said first side; and

said non-etched silicon resonant structure further comprising means for establishing a trimmable resonant frequency for said flexible elements, including at least a first opening having a predetermined length and width located proximate the second end of at least one of said first and second flexible elements, said at least a first opening including one edge located a predetermined distance from said second end of said at least one of said first and second flexible elements, said predetermined distance from said second end of said at least one of said first and second flexible elements to said one edge of said first opening defining a first tension relief beam, said first tension relief beam attached to the second end of said at least one of said first and second flexible elements, said first tension relief beam flexible in a direction parallel to said acceleration sensitive axis, for deflecting a first distance under longitudinal tension from said at least one of said first and second flexible elements and for providing stress relief of longitudinal tensile forces between said flexible elements and said tension relief beam, thereby establishing a trimmable resonant frequency for said flexible elements.

2. The accelerometer of claim 1 wherein said non-etched silicon resonant structure further includes a second opening having a predetermined length and width, located proximate the second end of the other of said first and second flexible elements, one edge of said second opening located a predetermined distance from the second end of the other of said first and second flexible elements, said predetermined distance from said second end of said other of said flexible elements to said one edge of said second opening defining a second tension relief beam deflecting a first distance under longitudinal tension from said other of said first and second flexible element, and to which is attached the second end of said other of said first and second flexible element.

3. The accelerometer of claim 2 in which said means for establishing a trimmable resonant frequency includes first and second regions of removable non-etched silicon resonant structure adjacent said first and second openings, for facilitating removal of a portion of said first and second regions for increasing at least one of the length and width of said first and second openings.

4. The accelerometer of claim 3 wherein said first and second regions of removable non-etched silicon resonant structure have a thickness which is less than the thickness of the surrounding non-etched silicon resonant structure, for permitting removal of at least a portion of said first and second regions of removable non-etched silicon resonant structure by a trimming instrument, for enlarging said first and second openings and for increasing said first distance of deflection of said first and second tension relief beams thereby trimming the frequency of said flexible elements.

5. The accelerometer of claim 4 wherein said trimming instrument includes a laser.

6. The accelerometer of claim 2 wherein said tension relief beams have a high height-to-width ratio.

7. The accelerometer of claim 6 wherein said height-to-width ratio is at least 2-to-1.

8. The accelerometer of claim 1 further including drive means, operative for causing said flexible elements to resonate at a resonant frequency.

9. The accelerometer of claim 8 wherein said drive means include buried electrodes and bridge electrodes.

10. The accelerometer of claim 9 wherein said bridge electrodes are electrically isolated from said silicon substrate.

11. The accelerometer of claim 1 further including sense means for measuring the resonant frequency of said flexible elements.

12. The accelerometer of claim 11 wherein said sense means includes a buffer/amplifier.

13. The accelerometer of claim 1 wherein said mass includes a silicon mass on which is disposed a counter weight.

14. The accelerometer of claim 13 in which said counter weight includes gold.

15. The accelerometer of claim 1 further including means for supporting said mass.

16. The accelerometer of claim 15 wherein said means for supporting the mass includes first and second silicon beams disposed generally perpendicular to said acceleration sensitive axis, and having first and second ends, the first support beam having its first end attached to one side of said mass and its second end attached to said substrate, the second support having its first end attached to an opposite side of said mass and its second end attached to said substrate, for allowing said mass to move along said acceleration sensitive axis and generally preventing movement of said mass transverse to the direction of said acceleration sensitive axis.

17. The accelerometer of claim 16 wherein said support beams have a high height-to-width ratio.

18. The accelerometer of claim 17 wherein said high height-to-width ratio is at least 2-to-1.

19. The accelerometer of claim 1 further including heater means for keeping said substrate and said resonant structure at a generally constant temperature.

20. The accelerometer of claim 19 further including temperature sensing means, for monitoring the temperature of said substrate and said resonant structure, and operative for selectively enabling and disabling said heater means at preselected temperatures.

21. The accelerometer of claim 1 further including electrical isolation means, for electrically isolating said mass from said flexible elements.

22. The accelerometer of claim 21 wherein said electrical isolation means includes a dielectric lap joint.

23. The accelerometer of claim 1 further including third and fourth flexible elements each having a first end attached on opposite sides of said mass, and disposed generally colinear with a second acceleration sensitive axis which is generally perpendicular to the first acceleration sensitive axis.

24. A monolithic micromechanical vibrating accelerometer, for detecting acceleration along at least one acceleration sensitive axis comprising:

a silicon substrate in which has been selectively etched a pit over which is suspended a non-etched silicon resonant structure comprising an acceleration sensitive mass and at least two flexible elements having a resonant frequency, said at least two flexible elements generally equally suspending said mass above said etched pit;

each of said flexible elements disposed generally colinear with said at least one acceleration sensitive axis, and having first and second ends, the first flexible element having its first end attached to a first side of said mass, and the second flexible element having its first end attached to a second side of said mass diametrically opposed from said first side;

means for electrically isolating said mass from said flexible elements;

said non-etched silicon resonant structure further comprising means for establishing a trimmable resonant frequency for each of said flexible elements, including first and second openings having a predetermined length and width located proximate the second end of said first and second flexible elements respectively, each of said first and second openings including one edge located a predetermined distance from said second end of said first and second flexible elements, said predetermined distance from said second end of said first and second flexible elements to said one edge of said first and second openings defining first and second tension relief beams, said first tension relief beam attached to the second end of said first flexible element and said second tension relief beam attached to the second end of said second flexible element, said first and second tension relief beams flexible in a direction parallel to said acceleration sensitive axis, for deflecting a first distance under longitudinal tension from said first and second flexible elements, and for providing stress relief of longitudinal tensile forces between said flexible elements and said tension relief beam, thereby establishing a trimmable resonant frequency for said flexible elements; and

said means for establishing a trimmable resonant frequency for said flexible elements including first and second regions of removable non-etched silicon resonant structure adjacent said first and second openings, said first and second regions of removable non-etched silicon resonant structure having a thickness which is less than the thickness of the surrounding non-etched silicon resonant structure, for permitting removal of at least a portion of said first and second removable regions by a trimming instrument, for increasing the length and width of said first and second openings and for increasing said first distance of deflection of said first and second tension relief beams thereby trimming the frequency of said flexible elements.

25. A monolithic micromechanical vibrating accelerometer, for detecting acceleration along at least one acceleration sensitive axis comprising:

a silicon substrate in which has been selectively etched a pit, over which is suspended a non-etched silicon resonant structure comprising an acceleration sensitive mass and at least two flexible elements having a resonant frequency, said at least two flexible elements generally equally suspending said mass above said etched pit;

each of said flexible elements disposed generally colinear with said at least one acceleration sensitive axis, and having first and second ends, the first flexible element having its first end attached to a first side of said mass, and the second flexible element having its first end attached to a second side of said mass diametrically opposed from said first side;

means for electrically isolating said mass from said flexible elements;

means for supporting the mass including first and second silicon beams disposed generally perpendicular to said acceleration sensitive axis, and having first and second ends, the first support beam having its first end attached to one side of said mass and its second end attached to a first region of said substrate, the second support beam having its first end attached to an opposite side of said mass and its second end attached to a second region of said substrate diametrically opposed from said first region, for allowing said mass to move in a direction parallel to said acceleration sensitive axis and generally preventing movement of said mass transverse to the direction of said acceleration sensitive axis;

said non-etched silicon resonant structure further comprising means for establishing a trimmable resonant frequency for each of said flexible elements, including first and second openings having a predetermined length and width located proximate the second end of said first and second flexible elements respectively, each of said first and second openings including one edge located a predetermined distance from said second end of said first and second flexible elements, said predetermined distance from said second end of said first and second flexible elements to said one edge of said first and second openings defining first and second tension relief beams, said first tension relief beam attached to the second end of said first flexible element and said second tension relief beam attached to the second end of said second flexible element, said first and second tension relief beams flexible in a direction parallel to said acceleration sensitive axis, for deflecting a first distance under longitudinal tension from said first and second flexible elements and for providing stress relief of longitudinal tensile forces between said flexible elements and said tension relief beams, thereby establishing a trimmable resonant frequency for said flexible elements; and

said means for establishing a trimmable resonant frequency for said flexible elements including first and second regions of removable non-etched silicon resonant structure adjacent said first and second openings, said first and second regions of removable non-etched silicon resonant structure having a thickness which is less than the thickness of the surrounding non-etched silicon resonant structure, for permitting removal of at least a portion of said first and second removable regions by a trimming instrument, for increasing the length and width of said first and second openings and for increasing said first distance of deflection of said first and second tension relief beams thereby trimming the frequency of said flexible elements.