# Patent

## Title
Miniaturized and ruggedized wafer level MEMs force sensors

## Abstract

Described herein is a miniaturized and ruggedized wafer level MEMS force sensor composed of a base and a cap. The sensor employs multiple flexible membranes, a mechanical overload stop, a retaining wall, and piezoresistive strain gauges.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 371 of International Application No. PCT/US2015/011144,filed Jan. 13, 2015, which claims the benefit of U.S. Provisional Application No. 61/926,472,filed Jan. 13, 2014, U.S. Provisional Application No. 61/937,509, filed Feb. 8, 2014, U.S. Provisional Application No. 61/983,553, filed Apr. 24, 2014, and U.S. Provisional Application No. 62/004,264, filed May 29, 2014.


STATEMENT REGARDING FEDERALLY FUNDED RESEARCH
This invention was made with government support under SBIR Award No. 1353450 awarded by the U.S. National Science Foundation. The government has certain rights in the invention.


FIELD OF TECHNOLOGY
The present invention relates to MEMS force sensing dies used for converting force into strain, which is sensed by piezoresistive strain gauges.
BACKGROUND
Current technology MEMS force dies are based on linking the applied force to the center of a sensing diaphragm comprising four piezoresistive strain gauges. The contact pads are positioned around the diaphragm, which makes current force dies relatively large. In addition, current MEMS force dies are fragile, lack the robustness of other force sensing technologies, such as force sensitive resistors, and are susceptible to debris from the external environment.
Accordingly, there is a need in the pertinent art for a small, low-cost, silicon force sensor that may be sealed and that is robust against mechanical overload.
SUMMARY
The present invention pertains to a microelectromechanical (“MEMS”) force sensor comprising multiple compact sensing elements positioned on the periphery of the die. Each sensing element is comprised of a flexure and a piezoresistive strain gauge. In one exemplary embodiment, four sensing elements may be employed in each force die, although additional or fewer sensing elements may also be used. The small sensing element reduces die size and the peripheral layout allows retaining walls to be included, which prevents dicing debris from entering the die and clogging the overload stop. In addition, in one embodiment, the peripheral layout allows the die to be fully sealed against debris from the external environment.
The dies may be manufactured by bonding a cap (typically Pyrex) wafer to a base (typically silicon) wafer. The sensing elements may be formed by etching flexures on the top side of the silicon wafer. Some flexures may also require etching grooves or slots on the bottom side. Piezoresistive strain gauges may also be diffused on the flexures and interconnected to the contact pads on the bottom of the die.
The bond between the base and cap wafers includes a gap produced by protrusions sculptured either on the top of the base and/or on the bottom of the cap. In exemplary embodiments, after the Pyrex wafer is bonded, release slots are etched on the periphery of base. In some embodiments, the slots release a retaining wall, designed to prevent debris from entering the air gap, from the rest of the base. The protrusions and retaining wall deflect with force, straining the piezoresistive strain gauges and producing an output signal proportional to the force. The gap may be designed to limit the displacement of the cap in order to provide force overload protection.



BRIEF DESCRIPTION OF THE FIGURES
These and other features of the preferred embodiments of the invention will become more apparent in the detailed description in which reference is made to the appended drawings wherein:
FIG. 1 is an isometric view of the MEMS force sensor.
FIG. 2 is a top view of the MEMS force sensor.
FIG. 3 is a side view of the MEMS force sensor.
FIG. 4 is a bottom view of the MEMS force sensor.
FIG. 5 is a top view of a wafer section comprising a 2×2 array of MEMS force sensors.
FIG. 6 is an isometric view of the MEMS force sensor with a machined cap.
FIG. 7 is a top view of the MEMS force sensor with a machined cap.
FIG. 8 is an isometric view of the MEMS force sensor with a retaining wall.
FIG. 9 is a top view of the MEMS force sensor with a retaining wall.
FIG. 10 is an isometric view of the MEMS force sensor with a retaining wall and corner flexures.
FIG. 11 is a top view of the MEMS force sensor with a retaining wall and corner flexures.
FIG. 12 is an isometric view of the MEMS force sensor according to another exemplary embodiment.
FIG. 13 is a top view of the MEMS force sensor according to another exemplary embodiment.
FIG. 14 is a side view of the MEMS force sensor according to another exemplary embodiment.
FIG. 15 is a bottom view of the MEMS force sensor according to another exemplary embodiment.
FIG. 16 is an isometric bottom view of the MEMS force sensor according to another exemplary embodiment.



DETAILED DESCRIPTION
The present invention can be understood more readily by reference to the following detailed description, examples, drawings, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, and, as such, can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.
The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.
As used throughout, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a force sensor” can include two or more such force sensors unless the context indicates otherwise.
Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
The present invention relates to a microelectromechanical system (“MEMS”) force sensor device 10 for measuring a force applied to at least a portion thereof. In one aspect, as depicted in FIGS. 1-3, the force sensor device comprises a base 11 and a cap 12 adhered at the surfaces formed by at least one rigid boss 13 in the base 11. A contact surface 14 exists along the top surface of the cap 12 for receiving an applied force F and transmitting the force F through the at least one rigid boss 13 to at least one flexure 15. The base 11 comprises an air gap 16 between the base 11 and cap 12 wherein the thickness of the air gap 16 is determined by the breaking deflection of the at least one flexure 15, such that the air gap 16 between the base 11 and the cap 12 will close and stop further deflection before the at least one flexible membrane 15 is broken.
It is contemplated that the air gap 16 formed between the base 11 and cap 12 could collect debris during the process of dicing the device. To mitigate this effect, the base comprises a shelf 17 that is etched significantly below the air gap 16. The shelf 17 creates a distance between the dicing interface at the edge of the base 11 and the air gap 16 where debris will tend to collect, creating a channel for water to carry away debris and preventing mechanical interference with the functional range of the device.
Referring now to FIGS. 3 and 4, the side and bottom views of the device are shown, respectively. The force sensor device comprises at least one deposited or implanted piezoresistive element on the bottom surface 18 of the base 11. As strain is induced in the at least one flexure 15 proportional to the force F, a localized strain is produced on the piezoresistive elements 19 (depicted schematically), such that the piezoresistive elements 19 experience compression, depending on their specific orientation. As the piezoresistive elements compress and tense, their resistivity changes in opposite fashion, such that a Wheatstone bridge circuit containing four piezoresistive elements 19 (two of each orientation relative to strain) becomes unbalanced and produces a differential voltage across the positive signal terminal SPOS and the negative signal terminal SNEG. This differential voltage is directly proportional to the applied force F on the contact surface 14, and is measured through electrical terminals 20 that are connected to external circuitry.
Referring now to FIG. 5, the top view of an undiced section of a wafer is shown. The wafer section comprises two dicing lanes 21 to separate a 2×2 array of devices. The at least one rigid boss are supported on two sides by bridges 22 which prevent the at least one flexure 15 from bending under the pressure of the bonding process that attaches the base 11 to the cap 12. Without the bridges 22, the at least one rigid boss 13 would only be supported by the at least one flexure 15. This would deform under pressure, causing the air gap 16 to close and resulting in the cap 12 bonding to the entire base 11, effectively eliminating the functional range of the device. The bridges 22 are placed in the dicing lanes 21 such that they will be removed during dicing to release the at least one rigid boss 13 and allow it to move with applied force F.
Referring still to FIG. 5, the wafer section comprises etched holes 23 at the edges of the at least one flexure 15. The holes 23 are etched into the base prior to dicing such that when the wafer is diced, the blade does not come into contact with the at least one flexure 15. This technique allows for smooth surface edges to be achieved by etching processes, which in turn increases the at least one flexure's 15 yield strength.
Referring now to FIGS. 6 and 7, in another embodiment, the force sensor device comprises a base 11 and a cap 12 adhered at the surfaces formed by at least one rigid boss 13 in the base 11. A contact surface 14 exists along the top surface of the cap 12 for receiving an applied force F and transmitting the force F through the at least one rigid boss 13 to at least one flexure 15. The base 11 comprises an air gap 16 between the base 11 and cap 12 wherein the thickness of the air gap 16 is determined by the breaking deflection of the at least one flexure 15, such that the air gap 16 between the base 11 and the cap 12 will close and stop further deflection before the at least one flexible membrane 15 is broken.
It is contemplated that the air gap 16 formed between the base 11 and cap 12 could collect debris during the process of dicing the device. To mitigate this effect, the cap comprises quarter circle machined holes 24. The holes 24 create a distance between the dicing interface at the edge of the base 11 and the air gap 16 where debris will tend to collect, creating a channel for water to carry away debris and preventing mechanical interference with the functional range of the device.
Referring now to FIGS. 8 and 9, in yet another embodiment, the force sensor device comprises a base 11 and a cap 12 adhered at the surfaces formed by at least one rigid boss 13 and retaining wall 25 in the base 11. A contact surface 14 exists along the top surface of the cap 12 for receiving an applied force F and transmitting the force F through the at least one rigid boss 13 and retaining wall 25 to at least one flexure 15. The base 11 comprises an air gap 16 between the base 11 and cap 12 wherein the thickness of the air gap 16 is determined by the breaking deflection of the at least one flexure 15, such that the air gap 16 between the base 11 and the cap 12 will close and stop further deflection before the at least one flexible membrane 15 is broken.
It is contemplated that the air gap 16 formed between the base 11 and cap 12 could collect debris during the process of dicing the device. To mitigate this effect, the base comprises a retaining wall 25. The retaining wall 25 is released and allowed to move with respect to the rest of the base 11 due to slots 26 etched through the base. The cap 12 and the base 11 are sealed together at the retaining wall 25 in order to prevent debris from entering the air gap 16 during dicing.
Referring now to FIGS. 10 and 11, in yet another embodiment, the force sensor device comprises a base 11 and a cap 12 adhered at the surfaces formed by at least one rigid corner 27 and retaining wall 25 in the base 11. A contact surface 14 exists along the top surface of the cap 12 for receiving an applied force F and transmitting the force F through the at least one rigid corner 27 and retaining wall 25 to at least one flexure 15. The base 11 comprises an air gap 16 between the base 11 and cap 12 wherein the thickness of the air gap 16 is determined by the breaking deflection of the at least one flexure 15, such that the air gap 16 between the base 11 and the cap 12 will close and stop further deflection before the at least one flexible membrane 15 is broken.
It is contemplated that the air gap 16 formed between the base 11 and cap 12 could collect debris during the process of dicing the device. To mitigate this effect, the base comprises a retaining wall 25. The retaining wall 25 is released and allowed to move with respect to the rest of the base 11 due to slots 26 etched through the base. The cap 12 and the base 11 are sealed together at the retaining wall 25 in order to prevent debris from entering the air gap 16 during dicing.
FIG. 12 illustrates an isometric view of the MEMS force sensor according to another exemplary embodiment. In particular, FIG. 12 illustrates a microelectromechanical system (“MEMS”) force sensor device 110 for measuring a force applied to at least a portion thereof. In one aspect, as depicted in FIGS. 12-14, the force sensor device includes a base 111 and a cap 112 adhered at the surfaces formed by at least one rigid boss 113 and an outer wall 114 in the base 111. The surfaces adhered between the base 111 and the cap 112 form a sealed cavity 115. A contact surface 116 exists along the top surface of the cap 112 for receiving an applied force F and transmitting the force F through the at least one rigid boss 113 and outer wall 114 to at least one flexure 117. The sealed cavity 115 may include an air gap 118 between the base 111 and cap 112 wherein the thickness of the air gap 118 may be determined by the breaking deflection of the at least one flexure 117, such that the air gap 118 between the base 111 and the cap 112 will close and stop further deflection before the at least one flexure 117 is broken.
Referring now to FIGS. 14 and 15, the side and bottom views of the device are shown, respectively. The force sensor device includes at least one deposited or implanted piezoresistive element on the bottom surface 119 of the base 111. As strain is induced in the at least one flexure 117 proportional to the force F, a localized strain is produced on the piezoresistive elements 120 (depicted schematically), such that the piezoresistive elements 120 experience compression, depending on their specific orientation. As the piezoresistive elements compress and tense, their resistivity changes in opposite fashion, such that a Wheatstone bridge circuit containing four piezoresistive elements 120 (two of each orientation relative to strain) becomes unbalanced and produces a differential voltage across the positive signal terminal SPOS and the negative signal terminal SNEG. This differential voltage is directly proportional to the applied force F on the contact surface 116, and may be measured through electrical terminals 121 that are connected to external circuitry. The electrical terminals 121 may comprise solder bumps to allow flip-chip assembly.
Referring now to FIG. 16, an isometric view of the bottom of the device is shown. The force sensor device may include grooves 122 sculptured into the bottom surface of the base 111. The grooves 122 may serve to reduce the amount of force absorbed by the peripheral flexures around the outer electrical terminals 121 and increase the amount of force absorbed by one or more center flexures, thereby increasing the strain in the piezoresistive elements 120 and improving overall sensitivity of the force sensor device.

## Claims

1. A force sensor, comprising:
a base comprising at least one rigid boss and an outer wall,
a cap attached to the base at a surface defined by the at least one rigid boss and the outer wall, wherein the cap and the base form a sealed cavity,
at least one flexure formed in the base and around the at least one rigid boss to convert force applied to the cap into strain,
a gap between the base and the cap that narrows with the application of the force to the cap such that the at least one flexure will not deform beyond its breaking point, and
a sensor configured to sense strain on a bottom surface of the base.

2. The force sensor of claim 1, wherein the at least one flexure is formed in the base and around the at least one rigid boss by etching.

3. The force sensor of claim 1, wherein the sensor comprises at least one of a piezoresistive, piezoelectric, or capacitive transducer.

4. The force sensor of claim 1, wherein the sensor is configured to produce an electrical voltage, current, or charge signal proportional to the strain.

5. The force sensor of claim 1, further comprising a plurality of electrical terminals formed on the bottom surface of the base, wherein the electrical signal produced by the sensor is routed to the electrical terminals.

6. The force sensor of claim 5, wherein the electrical terminals comprise solder bumps.

7. The force sensor of claim 1, wherein the base further comprises grooves to focus mechanical strain on the at least one flexure.

8. The force sensor of claim 1, wherein the sealed cavity is formed by at least one surface adhered between the cap and the base.

9. The force sensor of claim 8, wherein the base further comprises a cavity formed by etching the at least one flexure, wherein the cavity is bounded by a surface of at least one of the at least one flexure, the at least one rigid boss, or the outer wall.

10. The force sensor of claim 9, wherein the cavity is bounded by a respective surface of each of the at least one flexure, the at least one rigid boss, and the outer wall.

11. The force sensor of claim 9, wherein the cavity is sealed between the base and the cap.

12. The force sensor of claim 11, wherein the cap comprises a contact surface and a surface opposite to the contact surface, and wherein the sealed cavity is further bounded by the surface opposite to the contact surface.

13. The force sensor of claim 8, wherein the sealed cavity is sealed from an environment external to the force sensor.