Z-AXIS MICROELECTROMECHANICAL DEVICE WITH IMPROVED STOPPER STRUCTURE

• US 20080173959A1
• Filed: 01/15/2008
• Published: 07/24/2008
• Est. Priority Date: 01/19/2007
• Status: Active Grant

• Current Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Simoni, Barbara, Zerbini, Sarah, Merassi, Angelo Antonio
• Granted Patent: US 8,661,900 B2
• Time in Patent Office: Days
• US Class Current: 257/415
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 3/0051 For defining the movement, i.e. structures that guide or limit the movement of an element mechanical arrangements for preventing or damping vibration or shock H01H3/60
• CPC Class Codes: G01P 15/0802 Details
• 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

In a microelectromechanical device, a mobile mass is suspended above a substrate via elastic suspension elements and is rotatable about said elastic suspension elements, a cover structure is set above the mobile mass and has an internal surface facing the mobile mass, and a stopper structure is arranged at the internal surface of the cover structure and extends towards the mobile mass in order to stop a movement of the mobile mass away from the substrate along an axis (z) transverse to the substrate. The stopper structure is arranged with respect to the mobile mass so as to reduce an effect of reciprocal electrostatic interaction, in particular so as to minimize a resultant twisting moment of the mobile mass about the elastic suspension elements.

Claims

1. A microelectromechanical device comprising:

a mobile mass, suspended above a substrate via elastic suspension elements and rotatable about said elastic suspension elements;

a cover structure arranged above said mobile mass and having an internal surface facing said mobile mass; and

a stopper structure arranged at said internal surface of said cover structure and extending towards said mobile mass and designed to stop a movement of said mobile mass away from said substrate along an axis (z) transverse to said substrate, said stopper structure arranged with respect to said mobile mass to reduce an effect of a reciprocal interaction.

2. The device according to claim 1, wherein said stopper structure is arranged to reduce a resultant twisting moment of said mobile mass about said elastic suspension elements due to said interaction, said interaction being of an electrostatic type and resulting from a different electrical potential of the mobile mass and the stopper structure.

3. The device according to claim 2, wherein said stopper structure is arranged so as to make substantially zero said resultant twisting moment.

4. The device according to claim 2, wherein said elastic suspension elements are aligned and define a rotation axis of said mobile mass, and said stopper structure is set in the proximity of said rotation axis.

5. The device according to claim 4, wherein said stopper structure is arranged at and centered with respect to said rotation axis.

6. The device according to claim 4, wherein said stopper structure comprises a first stopper element extending throughout a length of said mobile mass parallel to said rotation axis.

7. The device according to claim 4, wherein said mobile mass internally has a window in which said elastic suspension elements are set, and said stopper structure comprises a first stopper element and a second stopper element set on opposite sides of said window and overlapping opposite sides of said mobile mass along said rotation axis.

8. The device according to claim 2, wherein said mobile mass is unbalanced with respect to a rotation axis defined by said elastic suspension elements and comprises a larger portion and a smaller portion, the smaller portion having dimensions that are smaller than those of said larger portion, set on opposite sides of said rotation axis;

the interaction with said stopper structure designed to generate a first twisting moment of said larger portion and a second twisting moment of said smaller portion which are opposite with respect to said axis of rotation, and said stopper structure configured so as to balance said first and second twisting moments.

9. The device according to claim 8, wherein said stopper structure comprises a first portion overlapping said larger portion and a second portion overlapping said smaller portion of said mobile mass, which are set on opposite sides with respect to said rotation axis;

said first and second portions of said stopper structure set at the same distance from said rotation axis and having equal areas of overlap with said mobile mass or else set at a different distance from said rotation axis and having different areas of overlap with said mobile mass, said different areas of overlap being a function of a respective distance from said rotation axis.

10. The device according to claim 8, wherein said stopper structure comprises a first stopper element and a second stopper element, which are equal to one another and are set on opposite sides of, and at a same distance from, said rotation axis.

11. The device according to claim 8, wherein said stopper structure comprises a first stopper element and a second stopper element, overlapping opposite sides of said mobile mass with respect to said rotation axis for an entire extension of said sides;

said first stopper element overlapping said smaller portion of said mobile mass and having an amount of overlap in a direction transverse to said rotation axis greater than a corresponding amount of overlap of said second stopper element with said larger portion.

12. The device according to claim 8, wherein said stopper structure comprises a first stopper element and a second stopper element, extending in a direction transverse to said rotation axis and overlapping opposite sides of said mobile mass, for an entire extension of said sides.

13. The device according to claim 12, wherein said first and second stopper elements have a first width (L₁) at said smaller portion and a second width (L₂) at said larger portion of said mobile mass, said first width (L₁) being greater than said second width (L₂).

14. The device according to claim 1, wherein said mobile mass extends mainly in a plane (xy) and is arranged so as to rotate out of said plane about said elastic suspension elements;

said stopper structure designed to limit a movement of said mobile mass in a direction transverse to said plane (xy).

15. The device according claim 1, wherein said stopper structure and said cover structure are formed in a same wafer of material, and said stopper structure comprises a downward projecting portion of said cover structure towards said mobile mass.

16. The device according to claim 1, moreover comprising a further mobile mass suspended above said substrate, and a corresponding further stopper structure, said stopper structures arranged in a corresponding way relative to the respective mobile masses to reduce an effect of a reciprocal interaction, and said further mobile mass turned upside down with respect to said mobile mass in a direction (y) transverse to a direction of extension of said elastic suspension elements;

wherein first electrodes facing corresponding portions of said mobile mass and further mobile mass, respectively, are electrically connected to one another.

17. The device according to claim 16, wherein said mobile masses are unbalanced with respect to the respective elastic suspension elements, and comprise a respective larger portion and a respective smaller portion, the smaller portion having dimensions that are smaller than those of said respective larger portion, said smaller and larger portions set on opposite sides of the respective elastic suspension elements;

said first electrodes facing a respective larger portion of said mobile masses, and moreover second electrodes are provided that face a respective smaller portion of said mobile masses, and are electrically connected to one another.

18. An electronic apparatus, comprising:

a microelectromechanical device according to claim 1, in particular said microelectromechanical device comprising one of;

an accelerometer, a gyroscope, and a microphone.

19. A process for manufacturing a microelectromechanical device, comprising:

forming a mobile mass, suspended above a substrate via elastic suspension elements and rotatable about said elastic suspension elements;

coupling, above said mobile mass, a cover structure having an internal surface facing said mobile mass; and

forming a stopper structure at said internal surface of said cover structure and extending towards said mobile mass and configured to stop a movement of said mobile mass away from said substrate along an axis (z) transverse to said substrate,

forming a stopper structure comprises arranging said stopper structure with respect to said mobile mass so as to reduce an effect of a reciprocal interaction.

20. The process according to claim 19, wherein arranging said stopper structure with respect to said mobile mass comprises reducing a resultant twisting moment of said mobile mass about said elastic suspension elements due to said interaction, said interaction being of an electrostatic type and being due to a different electrical potential of said mobile mass and said stopper structure.

21. The process according to claim 20, wherein arranging said stopper structure with respect to said mobile mass comprises making substantially zero said resultant twisting moment.

22. A device, comprising:

a mobile mass suspended above a substrate on an elastic suspension system to rotate about the elastic suspension system, the mobile mass having a larger portion on one side of the elastic suspension system and a smaller portion on an opposite side of the elastic suspension system to have a twisting moment about a rotational axis of the elastic suspension system; and

a cover structure arranged above the mobile mass and having an internal surface facing the mobile mass, the cover structure comprising a stopper structure extending toward the mobile mass and separated therefrom by a distance that stops a movement of the mobile mass away from the substrate along an axis substantially perpendicular to the substrate, the stopper structure configured to have a greater area of overlap with the smaller portion of the mobile mass than with the larger portion of the mobile mass, resulting in an electrostatic force between the stopper structure and the mobile mass to counteract the twisting moment of the mobile mass.

23. The device of claim 22, wherein the stopper structure comprises a first stopper element extending throughout a length of the mobile mass and parallel to the rotational axis.

24. The device of claim 23, wherein the mobile mass comprises a window in which the elastic suspension system has elements set therein, and the stopper structure comprises the first stopper element and a second stopper element set on opposite sides of the window and overlapping opposite sides of the mobile mass along the rotational axis.

25. The device of claim 24, wherein the first and second stopper elements have a first width at the smaller portion of the mobile mass and a second width at the larger portion of the mobile mass, the first width being greater than the second width.

26. The device of claim 22, wherein the stopper structure and the cover structure are formed from the same material and the same wafer of the material.