Sensor for measuring out-of-plane acceleration

• US 6,792,804 B2
• Filed: 10/19/2001
• Issued: 09/21/2004
• Est. Priority Date: 10/19/2001
• Status: Expired due to Term

• Current Assignee: Kionix Incorporated (ROHM Co., Ltd.)
• Original Assignee: Kionix Incorporated (ROHM Co., Ltd.)
• Inventors: Miller, Scott A., Johnson, Wendy Jo H., Adams, Scott G.
• Primary Examiner(s): KWOK, HELEN C
• Application Number:
  US10/082,064
• Publication Number:
  US 20030106372A1
• Time in Patent Office: 1,068 Days
• Field of Search: 735/143.2, 735/141.8, 735/142.4, 735/143.8, 735/141.6, 735/142.1, 735/143.5, 735/143.6, 361/280, 361/283.1, 361/283.3, 361/283.2
• 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 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends

Abstract

An accelerometer. A silicon wafer is etched to form a fixed portion, a movable portion, and a resilient coupling between, the fixed and movable portions generally arranged in the plane of the wafer, the mass of the movable portion being concentrated on one side of the resilient coupling. One of the fixed and moveable portions of the silicon structure includes a first electrode. The other of the fixed and moveable portions includes a second electrode oriented parallel to the axis of acceleration, and an electrically-conductive layer electrically connected as a third electrode coplanar and mechanically coupled with the second electrode. The second and third electrodes are arranged in capacitive opposition to the first electrode, the capacitance between the first electrode and third electrode increasing as the movable portion moves in a direction along the axis of acceleration relative to the fixed portion and decreasing as the movable portion moves in an opposite direction. A resilient coupling retains the first and third electrodes in capacitive opposition to each other across a capacitance gap while allowing motion of the first electrode relative to the second and third electrodes in response to acceleration along an axis of acceleration perpendicular to the plane of the wafer, and resiliently restores the first electrode to an equilibrium position when the acceleration ceases. The second electrode is in opposition to a majority of the surface area of the first electrode when the electrodes are in the equilibrium position. Capacitance between the first and third electrodes is measured to obtain a measurement of acceleration along the axis.

Claims

1. An accelerometer, comprising:

a monocrystalline silicon wafer etched to form a fixed portion, a movable portion, and a resilient coupling between, the fixed and movable portions generally arranged in the plane of the wafer;

one of the fixed and moveable portions including a first electrode, the other of the fixed cod moveable portions including a second electrode, the other of the fixed and moveable portions comprising an electrically conductive layer mechanically coupled with the second electrode and electrically connected as a third electrode, the second and third electrodes being stacked ins direction parallel to an axis of acceleration and arranged in capacitive opposition to the first electrode;

a resilient coupling designed to retain the first and third electrodes in capacitive opposition to each other across a capacitance gap while allowing motion of the first electrode relative to the second and third electrodes in response to acceleration along an axis of acceleration perpendicular to the plane of the wafer, and to resiliently restore the first electrode to an equilibrium position relative to the second and third electrodes when the acceleration ceases, the first and second electrodes being arranged in direct capacitive opposition and the first and third electrodes being arranged in indirect capacitive opposition, the capacitance between the first electrode and third electrode increasing as the movable portion moves away from the equilibrium position in a direction along the axis of acceleration and decreasing as the movable portion moves in an opposite direction away from the equilibrium position; and

electronics and/or software designed to translate a measurement of capacitance between the first and third electrodes into a measurement of acceleration along the axis of acceleration.

2. An accelerometer, comprising:

a first structure and a second structure, the first and second structures generally arranged in a plane, the first structure comprising a first electrode, the second structure comprising a second and third electrodes, the second and third electrodes being mechanically coupled and stacked in a direction parallel to an axis of acceleration perpendicular to the plane and arranged in capacitive opposition to the first electrode, wherein the first and second electrodes are arranged in direct capacitive opposition and the first and third electrodes are arranged in indirect opposition;

a resilient coupling designed to retain the first and second structures in capacitive opposition to each other across a capacitance gap while allowing motion of the second and third electrodes relative to the first electrode in response to acceleration along the axis of acceleration, and to resiliently restore the electrodes to an equilibrium position when the acceleration ceases; and

electronics and/or software designed to translate a measurement of capacitance between the first and third electrodes into a measurement of the acceleration along the axis.

3. The accelerometer of claim 2, wherein the first structure, second structure and resilient coupling are formed primarily of silicon.

4. The accelerometer of claim 3, wherein the first structure and second structure are formed at least primarily of high aspect ratio beams each having a larger cross-sectional dimension thereof oriented parallel to the axis of acceleration.

5. The accelerometer of claim 3, wherein the first and second electrodes are formed of silicon, and the third electrode is formed as a metal layer on the second structure.

6. The accelerometer of claim 5, wherein the second electrode is connected to a ground potential.

7. The accelerometer of claim 5, wherein the first structure is formed as a high-aspect-ratio beam with a larger cross-sectional dimension of the beam oriented parallel to the axis of acceleration.

8. The accelerometer of claim 3, wherein:

a silicon wafer is etched to form the first structure and the second structure.

9. The accelerometer of claim 8, wherein:

various portions of the second and first structures are electrically isolated from each other by isolation joints formed within the silicon wafer.

10. The accelerometer of claim 8, wherein:

various structures etched from the wafer are released from an underlying substrate of the silicon wafer.

11. The accelerometer of claim 2, wherein:

the electronics and/or software measure differential capacitance between at least two pairs of electrodes, and translate the measured differential capacitance into an expression of acceleration.

12. The accelerometer of claim 2, wherein:

a capacitance between the first and third electrode is at a maximum when the third electrode is displaced from the equilibrium position.

13. The accelerometer of claim 2, wherein:

the resilient coupling is a torsional flexure.

14. The accelerometer of claim 8, wherein:

the resilient coupling is integrally etched from the silicon wafer with the first and second structures.

15. The accelerometer of claim 2, further comprising:

first, second, and third electrodes arranged in first and second regions, such that

motion in a direction perpendicular to the plane results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and

motion in an opposite direction results in decreased capacitance between electrodes in the first region and increased capacitance in the second region.

16. The accelerometer of claim 2, wherein the mass of the movable structure is concentrated on one side of the resilient coupling.

17. A method, comprising the steps of:

applying an acceleration to a fixed structure and a movable structure, the fixed and movable structures generally arranged in a plane perpendicular to an axis of the acceleration, the fixed structure comprising a fixed electrode, the movable structure comprising a movable electrode and a shield electrode, the movable and shield electrodes being mechanically coupled and stacked in a direction parallel to the axis of acceleration and arranged in capacitive opposition to the fixed electrode, wherein the fixed and shield electrodes are arranged in direct capacitive opposition and the fixed and movable electrodes are arranged in indirect opposition;

in response to the acceleration, allowing motion of the movable electrode relative to the fixed electrode, a resilient coupling retaining the fixed and movable electrodes in capacitive opposition to each other across a capacitance gap;

resiliently restoring the fixed and movable electrodes to an equilibrium position when the acceleration ceases; and

measuring capacitance between the movable and fixed electrodes, and translating the measured capacitance into an expression of the acceleration.

18. The method of claim 17, wherein:

the fixed and shield electrodes are formed of silicon, and the moveable electrode is formed as an electrically-conductive layer deposited on the movable structure.

19. The method of claim 17, wherein;

electrodes of movable and fixed structures of the accelerometer are arranged in first and second regions, such that

motion in a direction of the movable structure results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and

motion in an opposite direction of the movable structure results in decreased capacitance between electrodes in the first region and increased capacitance in the second region.

20. The method of claim 17, wherein:

the resilient coupling is a torsional flexure.

21. An accelerometer, comprising:

a fixed portion and a movable portion, the fixed and movable portions generally arranged in a plane;

a resilient coupling designed to allow motion of the movable portion relative to the fixed portion in response to acceleration along an axis of acceleration perpendicular to the plane and to resiliently restore the fixed and movable portions to an equilibrium position when the acceleration ceases;

one of the fixed and moveable portions being electrically connected as a first electrode, the other of the fixed and moveable portions comprising an electrically-conductive layer electrically connected as a second electrode, the first and second electrodes being arranged in capacitive opposition to each other;

electronics and/or software designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration along the axis.

22. The accelerometer of claim 21:

wherein a silicon water is etched to form the fixed portion and the movable portion.

23. The accelerometer of claim 22, wherein:

the mass of the movable portion is concentrated on one side of the resilient coupling.

24. The accelerometer of claim 22, wherein:

the resilient coupling is integrally etched from the silicon wafer with the fixed and movable portions.

25. The accelerometer of claim 22, wherein:

a substantial portion of the movable portion is manufactured by a process including a step of releasing the movable portion from an underlying substrate of the wafer.

26. The accelerometer of claim 22, wherein:

various portions of the movable and fixed portions are electrically isolated from each other by isolation joints formed within the silicon wafer.

27. The accelerometer of claim 22, wherein:

wherein the second electrode is formed as a layer mechanically coupled with and electrically isolated from the movable portion.

28. The accelerometer of claim 21, wherein:

the resilient coupling is formed from a solid of high modulus of elasticity.

29. The accelerometer of claim 21, wherein:

the resilient coupling is a torsional flexure.

30. The accelerometer of claim 21, wherein:

the movable portion includes a Stop designed to engage a floor of the fixed portion to limit excess motion.

31. The accelerometer of claim 21, wherein:

electrodes of the movable and fixed portions are arranged in first and second regions, such that;

motion in a direction of the movable portion results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and

motion in an opposite direction of the movable portion results in decreased capacitance between electrodes in the first region and increased capacitance in the second region.

32. The accelerometer of claim 21, wherein:

the capacitance between the first electrode and second electrode increases as the movable portion moves away from the equilibrium position in a direction along the axis of acceleration and decreases as the movable portion moves in an opposite direction.

33. A method, comprising the steps of:

establishing an electric field between a movable electrode and a fixed electrode of an accelerometer, the movable and fixed electrodes being arranged in capacitive opposition to each other, one of the fixed and moveable electrodes being formed of silicon, the other of the fixed and moveable electrodes being formed as an electrically-conductive layer mechanically coupled with and electrically isolated from a silicon structure and stacked with the silicon structure in a direction of an axis of acceleration,

allowing motion of the movable electrode relative to the fixed electrode in response to an acceleration along the axis of acceleration, and allowing a resilient coupling to restore the fixed and movable electrodes to an equilibrium position when the acceleration ceases;

measuring capacitance between the movable and fixed electrodes, and translating the measured capacitance into an expression of the acceleration.

34. The method of claim 33, wherein:

the electrode formed of silicon is a first silicon electrode; and

the silicon structure on which the conductive-layer electrode is formed is electrically connected as a second silicon electrode, the conductive-layer electrode and second silicon electrode being arranged in capacitive opposition to the first silicon electrode, the second silicon electrode being in opposition to a majority of the surface area of the first silicon electrode when the electrodes are in the equilibrium position.

35. The method of claim 33, wherein:

the silicon structure and one of the fixed and movable electrodes of the accelerometer are formed by etching a silicon wafer.

36. The method of claim 35, wherein:

the resilient coupling is integrally etched from the silicon wafer.

37. An accelerometer, comprising:

a silicon wafer etched to form a fixed portion, a movable portion, and a resilient coupling between, the fixed and movable portions generally arranged in a plane, the resilient coupling designed to allow motion of movable portion relative to the fixed portion perpendicular to the wafer in response to acceleration perpendicular to the wafer and to resiliently restore the fixed and movable portions to an equilibrium position when the acceleration ceases, the mass of the movable portion being concentrated on one side of the resilient coupling;

the fixed portion comprising a fixed electrode and the moveable portion comprising a movable electrode, the electrodes being arranged in indirect capacitive opposition; and

electronics and/or software designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration perpendicular to the wafer.

38. The accelerometer of claim 37, further comprising:

a third electrode mechanically coupled to the movable electrode, the movable electrode and third electrode being arranged in capacitive opposition to the fixed electrode, the third electrode being in direct capacitive opposition to a majority of the surface area of the fixed electrode when the fixed electrode and movable electrode are in the equilibrium position.

39. The accelerometer of claim 37, wherein:

the silicon wafer is etched by a dry-etch process to form the fixed portion and the movable portion.

40. The accelerometer of claim 37, wherein:

electrodes of the movable and fixed portions are arranged in first and second regions, such that;

motion in a direction of the movable portion results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and

motion in an opposite direction of the movable portion results in decreased capacitance between electrodes in the first region and increased capacitance in the second region.

41. The accelerometer of claim 37, wherein:

the resilient coupling is integrally etched from the silicon wafer with the fixed and movable portions.

42. The accelerometer of claim 37, wherein:

the resilient coupling is formed from a solid of high modulus of elasticity.

43. The accelerometer of claim 37, wherein:

the resilient coupling is a torsional flexure.

44. The accelerometer of claim 37, wherein:

the movable portion includes a stop designed to engage a floor of the fixed portion to limit excess motion.

45. The accelerometer of claim 37, wherein:

a substantial portion of the movable portion is manufactured by a process including a step of releasing the movable portion from an underlying substrate of the wafer.

46. A method of detecting acceleration along an axis of acceleration, comprising the steps of:

establishing an electric field between a movable electrode and a fixed electrode of an accelerometer, the movable and fixed electrodes being arranged in capacitive opposition to each other and being mechanically borne on movable and fixed portions, respectively, of a structure etched from a silicon wafer, the fixed and movable portions generally arranged in a plane,

allowing motion perpendicular to the wafer of the movable electrode relative to the fixed electrode in response to an acceleration perpendicular to the wafer, and allowing a resilient coupling to restore the fixed and movable electrodes to an equilibrium position when the acceleration ceases, the mass of the movable portion being concentrated on one side of the resilient coupling; and

measuring capacitance between the movable and fixed electrodes, and translating the measured capacitance into an expression of the acceleration.

47. The accelerometer of claim 46, wherein:

excess motion of the movable portion is limited by urging a stop against a floor of the fixed portion, the stop being cantilevered on an opposing side of the mass concentration relative to the resilient coupling.

48. The accelerometer of claim 46, wherein:

the capacitance between the fixed electrode and movable electrode increases as the movable pardon moves away from the equilibrium position in a direction along the axis of acceleration and decreases as the movable portion moves in an opposite direction.

49. The accelerometer of claim 48, wherein:

the capacitance between the fixed electrode and the movable electrode reaches a maximum when the movable portion has moved from the equilibrium position by a distance of about half the depth of the fixed portion.

50. An accelerometer, comprising:

first, second and third electrodes, second and third electrodes being arranged in capacitive opposition to the first electrode across a capacitance gap;

a resilient coupling designed to allow motion of the first electrode relative to the second and third electrodes along the axis of acceleration in response to acceleration and to resiliently restore the first electrode to an equilibrium position when the acceleration ceases, the second electrode being in opposition to a majority of the surface area of the first electrode when the first, second, and third electrodes are in the equilibrium position; and

electronics and/or software designed to tram late a measurement of capacitance between the first and third electrodes into a measurement of acceleration along the axis.

51. The accelerometer of claim 50:

wherein a silicon wafer is etched to form the first and second electrodes; and

the axis of acceleration is perpendicular to the wafer.

52. The accelerometer of claim 51, wherein the third electrode is formed as an electrically-conductive layer mechanically coupled to the silicon wafer etched to form the second electrode.

53. The accelerometer of claim 51, wherein:

the capacitance between the first electrode and third electrode increases as the movable portion moves away from the equilibrium position in a direction along the axis of acceleration and decreases as the movable portion moves in an opposite direction.

54. The accelerometer of claim 51, wherein:

the third electrode is formed as a layer of electrically-conductive material that is mechanically coupled with and electrically isolated from the silicon wafer etched to form the movable portion.

55. The accelerometer of claim 51, wherein:

various structures etched from the wafer are electrically isolated horn each other by isolation joints formed within the silicon wafer.

56. The accelerometer of claim 51, wherein:

various structures etched from the wafer are released from an underlying substrate of the silicon wafer.

57. The accelerometer of claim 50, wherein:

the second electrode is electrically connected to consume field lines from the capacitance gap.

58. A method, comprising the steps of:

establishing an electric field between first, second and third electrodes of an accelerometer, the second and third electrodes being arranged in capacitive opposition to the first electrode, the first, second and third electrodes being mechanically borne on movable and fixed portions of an accelerometer;

allowing motion, perpendicular to the plane generally containing the second and third electrodes, of the movable portion relative to the fixed portion in response to an acceleration, and allowing a resilient coupling to restore the first, second, and third electrodes to an equilibrium position when the acceleration ceases, the second electrode being in opposition to a majority of the surface area of the first electrode when the first, second, and third electrodes are in the equilibrium position; and

measuring capacitance between the first and third electrodes, and translating the measured capacitance into an expression of the acceleration.

59. The method of claim 58, wherein:

the fixed portion and the movable portion are etched from a silicon wafer.

60. The method of claim 59, wherein:

the first and second electrodes are etched out of silicon.

61. The method of claim 60, wherein:

the third electrode is formed as a layer of electrically-conductive material that is mechanically coupled with and electrically isolated from the silicon etched to form the movable portion.

62. The method of claim 61, wherein:

the first and third electrodes are arranged relative to each other so that motion of the movable portion away from the equilibrium position in one direction increases capacitance between the first and third electrodes, and motion in an opposite direction from the equilibrium position decreases capacitance between the first and third electrodes.

63. The method of claim 58, further comprising the steps of:

measuring differential capacitance between at least two pairs of electrodes; and

translating the measured differential capacitance into an expression of acceleration.

64. The method of claim 58, wherein:

the mass of the movable portion is concentrated on one side of the resilient coupling.