STICTION RESISTANT MEMS DEVICE AND METHOD OF OPERATION

• US 20140217929A1
• Filed: 02/06/2013
• Published: 08/07/2014
• Est. Priority Date: 02/06/2013
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Lin, Yizhen, McNeil, Andrew C., Schlarmann, Mark E.
• Granted Patent: US 9,190,937 B2
• Time in Patent Office: Days
• US Class Current: 318/116
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 2203/058 Rotation out of a plane parallel to the substrate
• CPC Class Codes: B81C 1/00976 Control methods for avoiding stiction, e.g. controlling the bias voltage
• CPC Class Codes: H02N 1/006 of the gap-closing type H02N1/004 takes precedence

Abstract

A MEMS device (20) includes a movable element (20) suspended above a substrate (22) by a spring member (34) having a spring constant (104). A spring softening voltage (58) is applied to electrodes (24, 26) facing the movable element (20) during a powered mode (100) to decrease the stiffness of the spring member (34) and thereby increase the sensitivity of the movable element (32) to an input stimulus (46). Upon detection of a stiction condition (112), the spring softening voltage (58) is effectively removed to enable recovery of the movable element (32) from the stiction condition (112). A higher mechanical spring constant (104) yields a stiffer spring (34) having a larger restoring force (122) in the unpowered mode (96) in order to enable recovery from the stiction condition (112). A feedback voltage (56) can be applied to feedback electrodes (28, 30) facing the movable element (32) to provide electrical damping.

Claims

1. A microelectromechanical systems (MEMS) device comprising:

a substrate;

a movable element positioned in spaced apart relationship above a surface of said substrate;

at least one spring member interconnecting said movable element with said substrate, said at least one spring member enabling motion of said movable element, and said at least one spring member exhibiting a first stiffness;

at least one electrode facing said movable element; and

a voltage source in communication with said at least one electrode, said voltage source being configured to apply a spring softening voltage to said at least one electrode to shift a stiffness of said spring member from said first stiffness to a second stiffness, said second stiffness being less than said first stiffness.

2. A MEMS device as claimed in claim 1 wherein said first stiffness of said at least one spring member is at least two times greater than a predetermined operational stiffness for said spring member.

3. A MEMS device as claimed in claim 2 wherein said second stiffness is approximately equivalent to said predetermined operational stiffness, said predetermined operational stiffness defining a sensitivity of said movable element to a magnitude of an input stimulus.

4. A MEMS device as claimed in claim 1 wherein said first stiffness of said at least one spring member is in a range of two to six times greater than a predetermined operational stiffness for said spring member.

5. A MEMS device as claimed in claim 1 wherein said movable element is configured to move in response to an input stimulus while said spring softening voltage is applied to said at least one electrode.

6. A MEMS device as claimed in claim 1 further comprising:

a detection circuit in electrical communication with said at least one electrode, said detection circuit being configured to detect a stiction condition of said movable element in response to an output signal from said at least one electrode; and

a multiplexor circuit in communication with said at least one electrode and said movable element, said multiplexor circuit being configured to set each of said at least one electrode and said movable element to an equivalent voltage potential to effectively remove said spring softening voltage in response to said stiction condition.

7. A MEMS device as claimed in claim 1 further comprising:

a sense circuit in electrical communication with said at least one electrode for generating an electrical output signal in response to an input stimulus imposed on said movable element;

at least one force feedback electrode facing said movable element; and

an electrostatic force feedback circuit interposed between said sense circuit and said at least one force feedback electrode for controlling a feedback voltage applied to said at least one force feedback electrode based on said electrical output signal.

8. A MEMS device as claimed in claim 7 further comprising:

a detection circuit in electrical communication with said at least one electrode, said detection circuit being configured to detect a stiction condition of said movable element in response to an output signal from said at least one electrode; and

a multiplexor circuit in communication with said at least one electrode, said at least one force feedback electrode, and said movable element, said multiplexor circuit being configured to set each of said at least one electrode, said at least one force feedback electrode, and said movable element to an equivalent voltage potential to effectively remove said spring softening voltage in response to said stiction condition.

9. A MEMS device as claimed in claim 1 wherein:

said movable element is adapted for motion relative to a rotational axis positioned between first and second ends of said movable element, a first section of said movable element is formed between said rotational axis and said first end, a second section of said movable element is formed between said rotational axis and said second end, said first section having a first mass that is greater than a second mass of said second section; and

said at least one electrode includes a first electrode facing said first section of said movable element and a second electrode facing said second section of said movable element, and said spring softening voltage is applied to each of said first and second electrodes.

10. A method of operating a microelectromechanical systems (MEMS) device that includes a movable element positioned in spaced apart relationship above a surface of a substrate, at least one spring member interconnecting said movable element with said substrate, at least one electrode facing said movable element, wherein said at least one spring member enables motion of said movable element, said at least one spring member exhibits a first stiffness, and said method comprises:

applying a spring softening voltage to said at least one electrode to shift a stiffness of said spring member from said first stiffness to a second stiffness, said second stiffness being less than said first stiffness;

detecting a stiction condition of said movable element from an output signal provided from said at least one electrode, said stiction condition indicating that said movable element has moved beyond an allowable movement range; and

removing said spring softening voltage from said at least one electrode in response to said stiction condition to shift said stiffness of said spring member from said second stiffness to said first stiffness to thereby enable said movable element to return to a first position within said allowable movement range.

11. A method as claimed in claim 10 wherein said second stiffness is approximately equivalent to a predetermined operational stiffness.

12. A method as claimed in claim 11 wherein said first stiffness of said at least one spring member is at least two times greater than said predetermined operational stiffness for said spring member, said predetermined operational stiffness defining a sensitivity of said movable element to a magnitude of an input stimulus.

13. A method as claimed in claim 10 further comprising sensing an input signal as a function of an input stimulus imposed on said movable element, said sensing operation being performed while said spring softening voltage is applied to said at least one electrode.

15. A method as claimed in claim 10 wherein said MEMS device further includes a sense circuit in electrical communication with said at least one electrode, at least one force feedback electrode facing said movable element, and an electrostatic force feedback circuit interposed between said sense circuit and said at least one force feedback electrode, and said method further comprises:

generating, at said sense circuit, an electrical output signal in response to an input stimulus imposed on said movable element;

controlling, at said force feedback circuit, a feedback voltage based on said electrical output signal; and

supplying said feedback voltage to said at least one force feedback electrode from said force feedback circuit.

16. A method as claimed in claim 15 wherein said removing operation comprises setting each of said at least one electrode, said at least one force feedback electrode, and said movable element to an equivalent voltage potential.

17. A method as claimed in claim 10 further comprising:

following said removing operation, detecting return of said movable mass to said first position; and

reapplying said spring softening voltage to said at least one electrode to shift said stiffness of said spring member from said first stiffness to said second stiffness.

18. A method as claimed in claim 10 wherein said movable element is adapted for motion relative to a rotational axis positioned between first and second ends of said movable element, a first section of said movable element is formed between said rotational axis and said first end, a second section of said movable element is formed between said rotational axis and said second end, said first section having a first mass that is greater than a second mass of said second section, said at least one electrode includes a first electrode facing said first section of said movable element and a second electrode facing said second section of said movable element, and said applying operation applies said spring softening voltage to each of said first and second electrodes.

14. A method as claimed in 10 wherein said detecting operation comprises determining that said output signal exceeds an overload threshold value.

19. A microelectromechanical systems (MEMS) device comprising:

a substrate;

a movable element positioned in spaced apart relationship above a surface of said substrate;

at least one spring member interconnecting said movable element with said substrate, said at least one spring member enabling motion of said movable element, and said at least one spring member exhibiting a first stiffness;

at least one electrode facing said movable element;

at least one force feedback electrode facing said movable element;

a voltage source in communication with said at least one electrode, said voltage source being configured to apply a spring softening voltage to said at least one electrode to shift a stiffness of said spring member from said first stiffness to a second stiffness, said second stiffness being less than said first stiffness;

a sense circuit in electrical communication with said at least one electrode for generating an output signal in response to an input stimulus imposed on said movable element;

an electrostatic force feedback circuit interposed between said sense circuit and said at least one force feedback electrode for controlling a feedback voltage applied to said at least one force feedback electrode based on said output signal;

a detection circuit in electrical communication with said sense circuit, said detection circuit being configured to detect a stiction condition of said movable element in response to said output signal; and

a multiplexor circuit in communication with said at least one electrode, said at least one force feedback electrode, and said movable element, said multiplexor circuit setting said at least one electrode, said at least one force feedback electrode, and said movable element to an equivalent voltage potential to effectively remove said spring softening voltage in response to said stiction condition.

20. A MEMS device as claimed in claim 19 wherein said first stiffness of said at least one spring member is at least two times greater than a predetermined operational stiffness for said spring member, said second stiffness is approximately equivalent to said predetermined operational stiffness, and said predetermined operational stiffness defines a sensitivity of said movable element to a magnitude of said input stimulus.