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.

Description:
[0001]    This application 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, and U.S. Provisional Application No. 62/004,264, filed May 29, 2014. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The present invention relates to MEMS force sensing dies used for converting force into strain, which is sensed by piezoresistive strain gauges. 
       BACKGROUND 
       [0003]    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. 
         [0004]    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 
       [0005]    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. 
         [0006]    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. 
         [0007]    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 
         [0008]    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: 
           [0009]      FIG. 1  is an isometric view of the MEMS force sensor. 
           [0010]      FIG. 2  is a top view of the MEMS force sensor. 
           [0011]      FIG. 3  is a side view of the MEMS force sensor. 
           [0012]      FIG. 4  is a bottom view of the MEMS force sensor. 
           [0013]      FIG. 5  is a top view of a wafer section comprising a 2×2 array of MEMS force sensors. 
           [0014]      FIG. 6  is an isometric view of the MEMS force sensor with a machined cap. 
           [0015]      FIG. 7  is a top view of the MEMS force sensor with a machined cap. 
           [0016]      FIG. 8  is an isometric view of the MEMS force sensor with a retaining wall. 
           [0017]      FIG. 9  is a top view of the MEMS force sensor with a retaining wall. 
           [0018]      FIG. 10  is an isometric view of the MEMS force sensor with a retaining wall and corner flexures. 
           [0019]      FIG. 11  is a top view of the MEMS force sensor with a retaining wall and corner flexures. 
           [0020]      FIG. 12  is an isometric view of the MEMS force sensor according to another exemplary embodiment. 
           [0021]      FIG. 13  is a top view of the MEMS force sensor according to another exemplary embodiment. 
           [0022]      FIG. 14  is a side view of the MEMS force sensor according to another exemplary embodiment. 
           [0023]      FIG. 15  is a bottom view of the MEMS force sensor according to another exemplary embodiment. 
           [0024]      FIG. 16  is an isometric bottom view of the MEMS force sensor according to another exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    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. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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. 
         [0030]    The present invention relates to a microelectromechanical system (“MEMS”) force sensor device 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. 
         [0031]    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. 
         [0032]    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. 
         [0033]    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. 
         [0034]    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&#39;s  15  yield strength. 
         [0035]    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. 
         [0036]    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. 
         [0037]    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. 
         [0038]    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. 
         [0039]    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. 
         [0040]    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. 
         [0041]      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. 
         [0042]    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. 
         [0043]    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.