MICROELECTROMECHANICAL Z-AXIS DETECTION STRUCTURE WITH LOW THERMAL DRIFTS

• US 20110023604A1
• Filed: 07/29/2010
• Published: 02/03/2011
• Est. Priority Date: 07/31/2009
• Status: Active Grant

• Current Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Coronato, Luca, Simoni, Barbara, Cazzaniga, Gabriele
• Granted Patent: US 8,434,364 B2
• Time in Patent Office: Days
• US Class Current: 73/514.32
• CPC Class Codes: G01C 19/5712 the devices involving a micromechanical structure
• CPC Class Codes: G01C 19/5755 the devices having a single sensing mass
• CPC Class Codes: G01C 19/5769 Manufacturing; Mounting; Housings
• CPC Class Codes: G01P 15/02 by making use of inertia forces using solid seismic massesG01P15/14 takes precedence
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• 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 MEMS detection structure is provided with: a substrate having a top surface, on which a first fixed-electrode arrangement is set; a sensing mass, extending in a plane and suspended above the substrate and above the first fixed-electrode arrangement at a separation distance; and connection elastic elements that support the sensing mass so that it is free to rotate out of the plane about an axis of rotation, modifying the separation distance, as a function of a quantity to be detected along an axis orthogonal to the plane. The MEMS detection structure also includes: a coupling mass, suspended above the substrate and connected to the sensing mass via the connection elastic elements; and an anchoring arrangement, which anchors the coupling mass to the substrate with a first point of constraint, set at a distance from the axis of rotation and in a position corresponding to the first fixed-electrode arrangement.

Claims

1. A MEMS detection structure, comprising:

a substrate having a top surface;

a first fixed-electrode arrangement on the top surface of the substrate;

a sensing mass extending in a plane and suspended above said substrate and said first fixed-electrode arrangement at a separation distance;

connection elastic elements configured to support said sensing mass and allow the sensing mass to rotate out of said plane about an axis of rotation and modify said separation distance as a function of a quantity to be detected along an orthogonal axis orthogonal to said plane;

a coupling mass suspended above said substrate and connected to said sensing mass via said connection elastic elements; and

an anchoring arrangement configured to anchor said coupling mass to said substrate, the anchoring arrangement being connected to said substrate at a first point of constraint set at a distance from said axis of rotation and in a position corresponding to said first fixed-electrode arrangement.

2. The structure according to claim 1, wherein said plane is defined by a first axis and by a second axis, which are mutually orthogonal;

and said first point of constraint is set within an envelope region of said first fixed-electrode arrangement, with respect to said plane.

3. The structure according to claim 2, wherein said anchoring arrangement is configured to maintain said coupling mass substantially still with respect to said quantity to be detected, and to generate a displacement of said coupling mass along said orthogonal axis in response to a displacement of said substrate along said orthogonal axis at said first point of constraint;

and wherein said connection elastic elements are configured to generate a displacement of said sensing mass in response to the displacement of said coupling mass.

4. The structure according to claim 3, wherein said anchoring arrangement is configured to define a hinge element at said first point of constraint.

5. The structure according to claim 3, wherein the position of said first point of constraint substantially corresponds to a median point of said envelope region, along one of said first and second axes.

6. The structure according to claim 3, wherein said anchoring arrangement includes further points of constraint configured to anchor said coupling mass to said substrate and generate displacements of said coupled mass along said orthogonal axis in response to displacements of said substrate along said orthogonal axis;

said coupling mass being configured to be displaced in space according to a plane interpolating said displacements along said orthogonal axis.

7. The structure according to claim 3, wherein the position of said first point of constraint is such as to reduce a mean deviation between the displacement of said sensing mass and the displacement of said substrate along said orthogonal axis in a region surrounding said first point of constraint, following upon a deformation of said substrate.

8. The structure according to claim 7, wherein the position of said first point of constraint is such as to minimize said mean deviation.

9. The structure according to claim 7, wherein said first axis is substantially parallel to said axis of rotation, and wherein said envelope region has coordinates along said second axis, referred to said axis of rotation, comprised between a first minimum value and a first maximum value, and coordinates along said first axis comprised between a second minimum value and a second maximum value;

and wherein said first point of constraint is set at a first distance from said axis of rotation along said second axis, between said minimum value and said maximum value, and coordinate at a second distance along said first axis comprised between said second minimum value and said second maximum value.

10. The structure according to claim 9, wherein said first fixed-electrode arrangement has a substantially rectangular envelope region in said plane, and wherein said anchoring arrangement includes a second point of constraint configured to anchor said coupling mass to said substrate and set within said envelope region;

said first and second points of constraint being set along said first axis in a way symmetrical with respect to a median axis of said envelope region, parallel to said second axis and crossing said first axis at a point of origin having a zero coordinate value along said first axis; and

wherein the coordinate of said first point of constraint along said first axis satisfies in absolute value the following relation;

0<

a<

w

where w is the absolute value of said second minimum value and of said second maximum value.

11. The structure according to claim 1, wherein said anchoring arrangement includes an anchoring element rigidly coupled on said substrate, and a supporting elastic element configured to connect said coupling mass to said anchoring element;

said supporting elastic element having, with respect to said connection elastic elements, a greater stiffness, in such a way as to be substantially rigid in regard to said quantity to be detected along said orthogonal axis.

12. The structure according to claim 11, wherein said anchoring element is at least in part set within an envelope region of said first fixed-electrode arrangement with respect to said plane.

13. The structure according to claim 11, wherein said plane is defined by a first axis and by a second axis, which are mutually orthogonal;

and wherein said supporting elastic element comprises a first torsional spring having a rectilinear extension along said second axis and being connected to said coupling mass, and a second torsional spring having a rectilinear extension along said first axis and being connected to said anchoring element;

said first torsional spring and second torsional spring having one end in common, in a position that defines said first point of constraint.

14. The structure according to claim 1, wherein said sensing mass has a through opening, and said coupling mass and said anchoring arrangement are set within said through opening;

and wherein said connection elastic elements extend within said through opening and are aligned to define said axis of rotation.

15. The structure according to claim 1, further comprising a second fixed-electrode arrangement set on said substrate on an opposite side of said first fixed-electrode arrangement with respect to said axis of rotation;

said sensing mass being set above said substrate in such a way as to form a first detection capacitor and a second detection capacitor respectively with said first fixed-electrode arrangement and said second fixed-electrode arrangement, the detection capacitors having respective capacitance values that vary in a differential way as a function of said quantity to be detected.

16. A microelectromechanical sensor, comprising:

a read-interface circuit; and

a MEMS detection structure electrically coupled to the read-interface circuit and including;

a substrate having a top surface;

a first fixed-electrode arrangement on the top surface of the substrate;

a sensing mass extending in a plane and suspended above said substrate and said first fixed-electrode arrangement at a separation distance;

connection elastic elements configured to support said sensing mass and allow the sensing mass to rotate out of said plane about an axis of rotation and modify said separation distance as a function of a quantity to be detected along an axis orthogonal to said plane;

a coupling mass suspended above said substrate and connected to said sensing mass via said connection elastic elements; and

an anchoring arrangement configured to anchor said coupling mass to said substrate, the anchoring arrangement being connected to said substrate at a first point of constraint set at a distance from said axis of rotation and in a position corresponding to said first fixed-electrode arrangement.

17. The sensor according to claim 16, wherein said MEMS detection structure and said read-interface circuit form a z-axis accelerometer, and said quantity to be detected is an acceleration directed along said axis orthogonal to said plane.

18. The sensor according to claim 16, wherein said anchoring arrangement is configured to maintain said coupling mass substantially still with respect to said quantity to be detected, and to generate a displacement of said coupling mass along said orthogonal axis in response to a displacement of said substrate along said orthogonal axis at said first point of constraint;

and wherein said connection elastic elements are configured to generate a displacement of said sensing mass in response to the displacement of said coupling mass.

19. An electronic device, comprising:

a microprocessor unit; and

a microelectromechanical sensor that includes;

a read-interface circuit electrically coupled to the microprocessor unit; and

a MEMS detection structure electrically coupled to the read-interface circuit and including;

a substrate having a top surface;

a first fixed-electrode arrangement on the top surface of the substrate;

a sensing mass extending in a plane and suspended above said substrate and said first fixed-electrode arrangement at a separation distance;

connection elastic elements configured to support said sensing mass and allow the sensing mass to rotate out of said plane about an axis of rotation and modify said separation distance as a function of a quantity to be detected along an axis orthogonal to said plane;

a coupling mass suspended above said substrate and connected to said sensing mass via said connection elastic elements; and

an anchoring arrangement configured to anchor said coupling mass to said substrate, the anchoring arrangement being connected to said substrate at a first point of constraint set at a distance from said axis of rotation and in a position corresponding to said first fixed-electrode arrangement.

20. The electronic device according to claim 19, wherein said MEMS detection structure and said read-interface circuit form a z-axis accelerometer, and said quantity to be detected is an acceleration directed along said axis orthogonal to said plane.

21. The electronic device according to claim 19, wherein said anchoring arrangement is configured to maintain said coupling mass substantially still with respect to said quantity to be detected, and to generate a displacement of said coupling mass along said orthogonal axis in response to a displacement of said substrate along said orthogonal axis at said first point of constraint;

and wherein said connection elastic elements are configured to generate a displacement of said sensing mass in response to the displacement of said coupling mass.

22. A process, comprising:

manufacturing a MEMS detection structure, the manufacturing including;

forming a first fixed-electrode arrangement on a top surface of a substrate;

forming a sensing mass extending in a plane and suspended above said substrate and said first fixed-electrode arrangement at a separation distance;

forming connection elastic elements configured to support said sensing mass and allow the sensing mass to rotate out of said plane about an axis of rotation and modify said separation distance as a function of a quantity to be detected along an orthogonal axis orthogonal to said plane;

forming a coupling mass suspended above said substrate and connected to said sensing mass via said connection elastic elements; and

forming an anchoring arrangement configured to anchor said coupling mass to said substrate, the anchoring arrangement being connected to said substrate at a first point of constraint set at a distance from said axis of rotation and in a position corresponding to said first fixed-electrode arrangement.

23. The process according to claim 25, wherein said determining comprises determining in a numeric and iterative manner the position of said first point of constraint in such a way as to minimize said mean deviation.

24. The process according to claim 23, wherein determining the position of said first point of constraint comprises:

determining an expression of the displacement of said substrate along said orthogonal axis, considering deformations of said substrate up to a cubic order;

determining an expression of the displacement of said sensing mass along said orthogonal axis, imposing that said displacement of said sensing mass approximates said displacement of said substrate at said first point of constraint;

determining an expression of the difference between the displacement of said sensing mass and the displacement of said substrate;

integrating said difference over an area of extension of said first fixed-electrode arrangement so as to obtain an expression of said mean deviation; and

iteratively determining a position of said first point of constraint that minimizes said expression of said mean deviation.

25. The process according to claim 22, comprising:

determining the position of said first point of constraint in such a way as to reduce a mean deviation between a displacement of said sensing mass and a displacement of said substrate along said orthogonal axis in a region surrounding said first point of constraint, in response to a deformation of said substrate

26. The process according to claim 25, further comprising determining at least one of an offset drift and a sensitivity drift of said MEMS detection structure in the detection of said quantity to be detected as a function of the deformation of said substrate, and wherein said determining the position of said first point of constraint comprises determining iteratively the position of said first point of constraint that minimizes said at least one of the offset drift and the sensitivity drift.