MEMS capacitive bending and axial strain sensor
A three-dimensional micro-electro-mechanical-systems (MEMS) capacitive bending and axial strain sensor capacitor is described. Two independent comb structures, incorporating suspended polysilicon interdigitated fingers, are fabricated simultaneously on a substrate that can displace independently of each other while attached to a substrate undergoing bending or axial deformation. A change in spacing between the interdigitated fingers will output a change in capacitance of the sensor and is the primary mode of operation of the device. On the bottom and to the end of each comb structure, a glass pad is attached to the comb structure to allow for ample surface area for affixing the sensor to a substrate. During fabrication, tethers are used to connect each comb structure to maintain equal spacing between the fingers before attachment to the substrate. After attachment, the tethers are broken to allow independent movement of each comb structure.
1. A micro electro-mechanical system (MEMS) capacitive strain sensor, comprising:
(a) a first comb structure, wherein the first comb structure comprises:
(i) an anchored end,
(ii) a free end, and
(iii) a first plurality of spaced apart members located between the anchored end and the free end;
(b) a second comb structure, wherein the second comb structure comprises:
(i) an anchored end,
(ii) a free end, and
(iii) a second plurality of spaced apart members located between the anchored end and the free end, wherein the first and second pluralities of spaced apart members are arranged such that the first and second comb structures together form a capacitor to provide a capacitance value; and
(c) a substrate, wherein the substrate defines a first plane wherein the anchored end of the first comb structure and the anchored end of the second comb structure are mounted to the substrate such that the free ends of the first and second comb structures are movable independently relative to each other;
wherein the first and second comb structures are configured move with the substrate and along second plane that is parallel to the first plane to provide a change in the capacitance value in response to one or both of axial strain or bending strain on the substrate;
wherein the first and second comb structures are configured to move relative to the substrate and along a third plane that is not parallel to the first plane to provide a change in the capacitance value in response to bending strain on the substrate.
2. The MEMS capacitive strain sensor as recited in claim 1 , wherein the free end of the first comb structure is positioned proximate to the anchored end of the second comb structure, wherein the free end of the second comb structure is positioned proximate to the anchored end of the first comb structure.
3. The MEMS capacitive strain sensor as recited in claim 1 , further comprising:
(a) a first pad positioned between the anchored end of the first comb structure and the substrate, wherein the first pad elevates the first comb structure over the substrate; and
(b) a second pad positioned between the anchored end of the second comb structure and the substrate, wherein the second pad elevates the second comb structure over the substrate.
4. A micro electro-mechanical system (MEMS) capacitive strain sensor, comprising:
(a) a substrate, wherein the substrate defines a first plane; and
(b) a dual comb structure having a first end and a second end, wherein the second end is opposite to the first end, wherein the dual comb structure defines a second plane, wherein the second plane is parallel to the first plane, wherein the dual comb structure comprises:
(i) a first comb structure, wherein the first comb structure comprises:
(A) a first set of fingers, and
(B) a first anchor portion located at the first end of the dual comb structure, wherein the first anchor portion secures the first comb structure to the substrate, and
(ii) a second comb structure, wherein the second comb structure comprises:
(A) a second set of fingers, wherein the first and second comb structures are arranged such that the first set of fingers and the second set of fingers are interdigitated, wherein the interdigitated fingers form a capacitor configured to provide a capacitance value, and
(B) a second anchor portion located at the second end of the dual comb structure, wherein the second anchor portion secures the second comb structure to the substrate,
wherein the first and second comb structures are configured such that the spacing between the interdigitated fingers changes in response to one or both of axial strain or bending strain in the substrate to provide a change in the capacitance value in response to one or both of axial strain or bending strain in the substrate;
wherein the first and second comb structures are configured such that the first comb structure and the second comb structure are movable along the second plane to change the spacing between the interdigitated fingers in response to one or both of axial strain or bending strain in the substrate;
wherein the first and second comb structures are configured such that the first comb structure and the second comb structure are movable along a third plane to change the spacing between the interdigitated fingers in response to bending strain in the substrate, wherein the third plane is perpendicular to the first and second planes.
5. The MEMS capacitive strain sensor as recited in claim 4 , wherein the first comb structure further comprises a first free end opposite to the first anchor portion, wherein the first free end is positioned at the second end of the dual comb structure, wherein the second comb structure further comprises a second free end opposite to the second anchor portion, wherein the second free end is positioned at the first end of the dual comb structure, such that the first comb structure is configured to move relative to the substrate in a manner independent from movement of the second comb structure relative to the substrate.