Three-axis microelectromechanical systems device with single proof mass

• US 9,360,496 B2
• Filed: 10/03/2014
• Issued: 06/07/2016
• Est. Priority Date: 10/03/2014
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Naumann, Michael
• Primary Examiner(s): Caputo, Lisa
• Assistant Examiner(s): WILLIAMS, JAMEL E
• Application Number:
  US14/506,022
• Publication Number:
  US 20160097792A1
• Time in Patent Office: 613 Days
• Field of Search: 735/040.2
• US Class Current: 1/1
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 3/0021 Transducers for transforming electrical into mechanical energy or vice versa dynamo-electric machines H02K99/00; electrostatic machines H02N1/00; piezoelectric devices H10N30/00
• 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: B81B 3/007 For controlling stiffness, e.g. ribs
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 2015/0845 using a plurality of spring-mass systems being arranged on one common planar substrate, the systems not being mechanically coupled and the sensitive direction of each system being different

Abstract

A microelectromechanical systems (MEMS) device, such as a three-axis MEMS device can sense acceleration in three orthogonal axes. The MEMS device includes a single proof mass and suspension spring systems that movably couple the proof mass to a substrate. The suspension spring systems include translatory spring elements and torsion spring elements. The translatory spring elements enable translatory motion of the proof mass relative to the substrate in two orthogonal directions that are parallel to the plane of the MEMS device in order to sense forces in the two orthogonal directions. The torsion spring elements enable rotation of the proof mass about a rotational axis in order to sense force in a third direction that is orthogonal to the other two directions. The translatory spring elements have asymmetric stiffness configured to compensate for an asymmetric mass of the movable element used to sense in the third direction.

Claims

1. A microelectromechanical systems (MEMS) device comprising:

a substrate;

a movable element having a mass that is asymmetric relative to a rotational axis; and

a suspension spring system configured to movably couple said movable element to said substrate, said suspension spring system including a first translatory spring element, a second translatory spring element, and a torsion spring element, wherein said first and second translatory spring elements are configured to facilitate translatory motion of said movable element relative to said substrate, and said torsion spring element is configured to facilitate rotation of said movable element about said rotational axis, wherein;

said first translatory spring element has a first stiffness; and

said second translatory spring element has a second stiffness, said first stiffness being greater than said second stiffness to compensate for moments created by said asymmetric mass of said movable element in response to translatory acceleration relative to said substrate in a first direction substantially parallel to a first axis.

2. The MEMS device of claim 1 wherein:

said translatory motion of said movable element relative to said substrate is in said first direction and in a second direction, said first and second directions being substantially parallel to a surface of said substrate, and said second direction being substantially orthogonal to said first direction;

said rotation of said movable element about said rotational axis comprises rotation about said first axis; and

said first stiffness and said second stiffness define stiffness in said first direction.

3. The MEMS device of claim 1 wherein:

said translatory motion of said movable element relative to said substrate is in said first direction and in a second direction, said first and second directions being substantially parallel to a surface of said substrate, and said second direction being substantially orthogonal to said first direction;

said first direction comprises an X-direction defined by an X-axis;

said second direction comprises a Y-direction defined by a Y-axis;

said first stiffness of said first translatory spring element includes a first X-direction stiffness and a first Y-direction stiffness;

said second stiffness of said second translatory spring element includes a second X-direction stiffness and a second Y-direction stiffness;

said first X-direction stiffness is substantially greater than said second X-direction stiffness; and

said first Y-direction stiffness is substantially equal to said second Y-direction stiffness.

4. The MEMS device of claim 1 wherein said first stiffness is between ten and forty percent greater than said second stiffness.

5. The MEMS device of claim 1 wherein:

said movable element includes a first portion positioned on a first side of said rotational axis and a second portion positioned on a second side of said rotational axis, said first portion having greater mass than said second portion;

said first translatory spring element is adapted to facilitate said translatory motion of said first portion;

said second translatory spring element is adapted to facilitate said translatory motion of said second portion such that said movable element moves substantially linearly in response to said translatory acceleration in said first direction.

6. The MEMS device of claim 1 wherein:

said first translatory spring element has a first end;

said second translatory spring element has a second end; and

said torsion spring has a first torsion spring end, wherein said first end, said second end, and said first torsion spring end are coupled together at a common region suspended above said substrate.

7. The MEMS device of claim 6 further comprising an anchor element coupled to said substrate at said rotational axis, wherein said torsion spring has a second torsion spring end coupled to said anchor element, said first translatory spring element has a third end coupled with a first portion of said movable element, and said second translatory spring element has a fourth end coupled with a second portion of said movable element, said first and second portions being disposed on opposing sides of said rotational axis.

8. The MEMS device of claim 6 further comprising:

a first anchor element coupled to said substrate; and

a second anchor element coupled to said substrate, said first and second anchor elements being disposed on opposing sides of said rotational axis, wherein said first translatory spring element has a third end coupled to said first anchor element, said second translatory spring element has a fourth end coupled to said second anchor element, and said torsion spring element includes a second torsion spring end coupled to said movable element at said rotational axis.

9. The MEMS device of claim 8 wherein said first and second anchor elements are displaced away from said rotational axis by an equivalent distance.

10. The MEMS device of claim 1 further comprising a plurality of travel stops configured to restrain said translatory motion of said movable element.

11. The MEMS device of claim 1 further comprising:

a first electrode coupled to said substrate and positioned proximate said movable element such that a first capacitance between said first electrode and said movable element varies in response to displacement of said movable element along said first direction; and

a second electrode coupled to said substrate and positioned proximate said movable element such that a second capacitance between said second electrode and said movable element varies in response to displacement of said movable element along a second direction, wherein said first and second directions are substantially parallel to a surface of said substrate, and said second direction is substantially orthogonal to said first direction.

12. The MEMS device of claim 11 further comprising a third electrode coupled to said substrate underlying said movable element, wherein a third capacitance between said third electrode and said movable element varies in response to displacement of said movable element about said rotational axis.

13. The MEMS device of claim 1 further comprising outer frame spaced apart from said substrate, said outer frame being coupled to and surrounding said movable element such that said suspension spring system is interposed between said outer frame and said movable element.

14. A microelectromechanical systems (MEMS) device comprising:

a substrate;

a movable element having mass that is asymmetric relative to a rotational axis, said movable element having a first portion positioned on a first side of said rotational axis and a second portion positioned on a second side of said rotational axis, said first portion having greater mass than said second portion; and

a suspension spring system configured to movably couple said movable element to said substrate, said suspension spring system including a first translatory spring element, a second translatory spring element, and a torsion spring element, wherein said first translatory spring element is adapted to facilitate translatory motion of said first portion of said movable element relative to said substrate in a first direction substantially parallel to a first axis, said second translatory spring element is adapted to facilitate said translatory motion of said second portion of said movable element relative to said substrate in said first direction, and said torsion spring element is configured to facilitate rotation of said movable element about said rotational axis, wherein;

said first translatory spring element has a first stiffness; and

said second translatory spring element has a second stiffness, said first stiffness being greater than said second stiffness to compensate for moments created by said asymmetric mass of said movable element in response to translatory acceleration relative to said substrate in said first direction such that said movable element moves substantially linearly in response to said translatory acceleration in said first direction, said first stiffness and said second stiffness defining stiffness in said first direction; and

said rotation of said movable element about said rotational axis comprises rotation about said first axis.

15. The MEMS device of claim 14 wherein:

said translatory motion of said movable element relative to said substrate is in said first direction and in a second direction, said first and second directions being substantially parallel to a surface of said substrate, and said second direction being substantially orthogonal to said first direction;

said first direction comprises an X-direction defined by an X-axis;

said second direction comprises a Y-direction defined by a Y-axis;

said first stiffness of said first translatory spring element includes a first X-direction stiffness and a first Y-direction stiffness;

said second stiffness of said second translatory spring element includes a second X-direction stiffness and a second Y-direction stiffness;

said first X-direction stiffness is substantially greater than said second X-direction stiffness; and

said first Y-direction stiffness is substantially equal to said second Y-direction stiffness.

16. The MEMS device of claim 14 further comprising:

a first electrode coupled to said substrate and positioned proximate said movable element such that a first capacitance between said first electrode and said movable element varies in response to displacement of said movable element along said first direction; and

a second electrode coupled to said substrate and positioned proximate said movable element such that a second capacitance between said second electrode and said movable element varies in response to displacement of said movable element along a second direction, wherein said first and second directions are substantially parallel to a surface of said substrate, and said second direction is substantially orthogonal to said first direction.

17. The MEMS device of claim 14 further comprising a third electrode coupled to said substrate underlying said movable element, wherein a third capacitance between said third electrode and said movable element varies in response to displacement of said movable element about said rotational axis.

18. A microelectromechanical systems (MEMS) device comprising:

a substrate;

a movable element having a mass that is asymmetric relative to a rotational axis; and

a suspension spring system configured to movably couple said movable element to said substrate, said suspension spring system including;

a first translatory spring element having a first stiffness and a first spring end;

a second translatory spring element having a second stiffness and a second spring end, said first and second translatory spring elements being configured to facilitate translatory motion of said movable element relative to said substrate, and said first stiffness being greater than said second stiffness to compensate for moments created by said asymmetric mass of said movable element in response to translatory acceleration relative to said substrate in a first direction substantially parallel to a first axis; and

a torsion spring element having a first torsion spring end, wherein said first end, said second end, and said first torsion spring end are coupled together at a common region suspended above said substrate, and said torsion spring element is configured to facilitate rotation of said movable element about said rotational axis, wherein said rotation of said movable element about said rotational axis comprises rotation about said first axis.

19. The MEMS device of claim 18 further comprising an anchor element coupled to said substrate at said rotational axis, wherein said torsion spring has a second torsion spring end coupled to said anchor element, said first translatory spring element has a third end coupled with a first portion of said movable element, and said second translatory spring element has a fourth end coupled with a second portion of said movable element, said first and second portions being disposed on opposing sides of said rotational axis.

20. The MEMS device of claim 18 further comprising:

a first anchor element coupled to said substrate; and

a second anchor element coupled to said substrate, said first and second anchor elements being disposed on opposing sides of said rotational axis, wherein said first translatory spring element has a third end coupled to said first anchor element, said second translatory spring element has a fourth end coupled to said second anchor element, and said torsion spring element includes a second torsion spring end coupled to said movable element at said rotational axis.