Differential capacitive sensor and method of making same

• US 20080173091A1
• Filed: 01/18/2007
• Published: 07/24/2008
• Est. Priority Date: 01/18/2007
• Status: Active Grant

• Current Assignee: North Star Innovations Inc. (Wi-LAN Inc.)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Miller, Todd F., Lin, Yizhen, McNeil, Andrew C.
• Granted Patent: US 7,610,809 B2
• Time in Patent Office: Days
• US Class Current: 73/514.32
• 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 differential capacitive sensor (50) includes a movable element (56) pivotable about a rotational axis (60). The movable element (56) includes first and second sections (94, 96). The first section (94) has an extended portion (98) distal from the rotational axis (60). A static layer (52) is spaced away from a first surface (104) of the moveable element (56), and includes a first actuation electrode (74), a first sensing electrode (64), and a third sensing electrode (66). A static layer (62) is spaced away from a second surface (106) of the moveable element (56) and includes a second actuation electrode (74), a second sensing electrode (70), and a fourth sensing electrode (72). The first and second electrodes (64, 70) oppose the first section (94), the third and fourth electrodes (66, 72) oppose the second section (96), and the first and second electrodes (68, 74) oppose the extended portion (98).

Claims

1. A device comprising:

a Micro Electro-Mechanical System (MEMS) sensor, said sensor comprising;

a movable element adapted for motion relative to a rotational axis offset between first and second ends thereof to form a first section between said rotational axis and said first end and a second section between said rotational axis and said second end, said first section including an extended portion spaced away from said rotational axis at a distance approximately equivalent to a length of said second section, and said movable element including a first surface and a second surface;

a first static conductive layer spaced away from said first surface of said moveable element, said first conductive layer including a first actuation electrode and a first sensing electrode; and

a second static conductive layer spaced away from said second surface of said moveable element, said second conductive layer including a second actuation electrode and a second sensing electrode, said first and second actuation electrodes opposing said extended portion.

2. A device as claimed in claim 1 wherein:

said first and second sensing electrodes oppose said first section of said movable element;

said first conductive layer includes a third sensing electrode; and

said second conductive layer includes a fourth sensing electrode, said third and fourth sensing electrodes opposing said second section of said movable element.

3. A device as claimed in claim 2 wherein each of said third and fourth sensing electrodes has a surface area in a range of twenty-five to thirty-five percent of an overall surface area of said movable element.

4. A device as claimed in claim 2 wherein:

said first and third sensing electrodes are symmetrically arranged relative to said rotational axis; and

said second and fourth sensing electrodes are symmetrically arranged relative to said rotational axis.

5. A device as claimed in claim 2 wherein:

said first and third sensing electrodes form respective first and third capacitors with said first and second sections of said movable element; and

said second and fourth sensing electrodes form respective second and fourth capacitors with said first and second sections of said movable element.

6. A device as claimed in claim 1 wherein:

said first and second sensing electrodes oppose said first section of said movable element;

said first actuation electrode is adjacent to said first sensing electrode; and

said second actuation electrode is adjacent to said second sensing electrode.

7. A device as claimed in claim 6 wherein:

said first actuation electrode is electrically isolated from said first sensing electrode; and

said second actuation electrode is electrically isolated from said third sensing electrode.

8. A device as claimed in claim 1 wherein a surface area of each of said first and second actuation electrodes is in a range of twenty to twenty-five percent of an overall surface area of said movable element.

9. A device as claimed in claim 1 wherein voltage is applied to said first and second actuation electrodes to test said sensor.

10. A device as claimed in claim 1 wherein voltage is applied to said first and second actuation electrodes to provide a balance force such that a position of said movable element is substantially constant.

11. A device as claimed in claim 1 wherein said first and second sensing electrodes are adapted to detect movement of said movable element along an axis perpendicular to a plane of said electrodes.

12. A method of fabricating a Micro Electro-Mechanical System (MEMS) sensor comprising:

providing a substrate;

forming a first static conductive layer on said substrate to include a first actuation electrode and a first sensing electrode electrically isolated from said first actuation electrode;

forming a first sacrificial layer on said first conductive layer;

forming a movable element on said first sacrificial layer adapted for motion relative to a rotational axis offset between first and second ends thereof to form a first section between said rotational axis and said first end and a second section between said rotational axis and said second end, said first section including an extended portion spaced away from said rotational axis at a distance approximately equivalent to a length of said second section;

forming a second sacrificial layer on said movable element;

forming a second static conductive layer on said second sacrificial layer to include a second actuation electrode and a second sensing electrode electrically isolated from said second actuation electrode; and

selectively removing said first and second sacrificial layers such that said first and second conductive layers are spaced away from said movable element, and said first and second conductive layers being formed such that said first and second actuation electrodes oppose said extended portion of said movable element.

13. A method as claimed in claim 12 further comprising supporting said movable element on a pivot arranged substantially crosswise to a longitudinal axis of said movable element.

14. A method as claimed in claim 12 further comprising:

forming said first conductive layer to include a third sensing electrode electrically isolated from said first sensing electrode and said first actuation electrode; and

forming said second conductive layer to include a fourth sensing electrode electrically isolated from said second sensing electrode and said second actuation electrode.

15. A method as claimed in claim 14 further comprising:

positioning said first sensing electrode and said first actuation electrode to oppose said first surface at said first section of said movable element;

positioning said second sensing electrode and said second actuation electrode to oppose said second surface at said first section of said movable element;

positioning said third sensing electrode to oppose said first surface at said second section of said movable element; and

positioning said fourth sensing electrode to oppose said second surface at said second section of said movable element.

16. A method as claimed in claim 15 further comprising configuring a surface area of each of said first, second, third, and fourth sensing electrodes to be in a range of twenty-five to thirty-five percent of an overall surface area of said movable element.

17. A method as claimed in claim 15 further comprising configuring a surface area of each of said first and second actuation electrodes to be in a range of twenty to twenty-five percent of an overall surface area of said movable element.

18. A Micro Electro-Mechanical System (MEMS) sensor comprising:

a movable element adapted for motion relative to a rotational axis offset between first and second ends thereof to form a first section between said rotational axis and said first end and a second section between said rotational axis and said second end, said first section including an extended portion spaced away from said rotational axis at a distance approximately equivalent to a length of said second section, and said movable element including a first surface and a second surface;

a first static conductive layer spaced away from said first surface of said moveable element, said first conductive layer including a first actuation electrode, a first sensing electrode, and a third sensing electrode; and

a second static conductive layer spaced away from said second surface of said moveable element, said second conductive layer including a second actuation electrode, a second sensing electrode and a fourth sensing electrode, said first and second actuation electrodes opposing said extended portion, said first and second sensing electrodes opposing said first section, said third and fourth electrodes opposing said second section, and each of said first, second, third, and fourth sensing electrodes having a surface area in a range of twenty-five to thirty-five percent of an overall surface area of said movable element.

19. A sensor as claimed in claim 18 wherein each of said first and second actuation electrodes has a second surface area in a range of twenty to twenty-five percent of an overall surface area of said movable element.

20. A sensor as claimed in claim 18 wherein:

said first and third sensing electrodes are symmetrically arranged relative to said rotational axis; and

said second and fourth sensing electrodes are symmetrically arranged relative to said rotational axis.