Abstract:
A microelectromechanical (“MEMS”) load sensor device for measuring a force applied by a human user is described herein. In one aspect, the load sensor device has a contact surface in communication with a touch surface which communicates forces originating on the touch surface to a deformable membrane, on which load sensor elements are arranged, such that the load sensor device produces a signal proportional to forces imparted by a human user along the touch surface. In another aspect, the load sensor device has an overload protection ring to protect the load sensor device from excessive forces. In another aspect, the load sensor device has embedded logic circuitry to allow a microcontroller to individually address load sensor devices organized into an array. In another aspect, the load sensor device has electrical and mechanical connectors such as solder bumps designed to minimize cost of final component manufacturing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/668,135, filed on Jul. 5, 2012, entitled “Microelectromechanical Load Sensor and Methods of Using Same,” the disclosure of which is expressly incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the technical field of touch interfaces based on microelectromechanical (“MEMS”) load sensors that are used as input devices for data processing systems. 
     BACKGROUND 
     A variety of known MEMS devices are designed to measure applied load and produce an output differential voltage signal proportional to the applied load. These known devices, such as conventional piezoresistive, piezoelectric, and capacitive MEMS force sensors, pressure sensors, and strain gauges, utilize the unique electromechanical properties of materials such as silicon and lead zirconate titanate. 
     However, there is a need in the pertinent art for an interface device that is capable of receiving and recognizing a range of human user actions. There is also a need in the pertinent art for an interface device that is capable of tolerating a force from a user without being damaged or causing injury to the user. 
     SUMMARY 
     Described herein are MEMS load sensor devices and, more specifically, MEMS load sensor devices designed to measure an applied load and produce an output signal. More specifically, the MEMS load sensor devices described herein are configured to measure forces originating from human user actions such as pressing a button or a touch surface. The MEMS load sensor devices can be configured as a component of a force sensitive touch interface for providing touch input into a data processing system, for example. The MEMS load sensor devices can include a substrate, a deformable membrane, a load sensor element configured to produce a signal when deformed, an overload protection portion, and a means to communicate load sensor signals to an electrical circuit. 
     An example MEMS load sensor device can include a substrate defining a deformable membrane, a mesa and an overload protection portion. The mesa can be configured to receive and transfer an applied force to the deformable membrane, and the deformable membrane can be configured to deform in response to the applied force. The MEMS load sensor device can also include at least one load sensor element formed on the deformable membrane. The load sensor element can be configured to change at least one electrical characteristic based on an amount or magnitude of the applied force. Additionally, a height of the mesa can be greater than a height of the overload protection portion. 
     Optionally, the mesa defines a contact surface for receiving the applied force. The contact surface can have at least one of a substantially square, rectangular, rounded, circular or elliptical shape. 
     Alternatively or additionally, the mesa can optionally be arranged in a central portion of the substrate. Alternatively or additionally, the overload protection portion can optionally be arranged in a peripheral portion of the substrate. 
     Optionally, the MEMS load sensor device can further include a touch surface fixed to at least a portion of the mesa. The touch surface can be configured to receive and transfer the applied force to the mesa. Additionally, a gap can be arranged between the touch surface and the overload protection portion when the touch surface is fixed to the mesa. The gap can limit an amount of deflection of the deformable membrane and can prevent the deformable membrane from mechanically failing under an excessive applied force. 
     Alternatively or additionally, the MEMS load sensor device can optionally include one or more electromechanical connectors for connecting the MEMS load sensor device to an external circuit. 
     Optionally, the deformable membrane, the mesa and the overload protection portion can be formed by removing a portion of the substrate. For example, the substrate can have a first surface and a second surface opposite to the first surface. The deformable membrane, the mesa and the overload protection portion can be formed using a deep reactive ion etching technique on the second surface. Additionally, the MEMS load sensor device can optionally include one or more electromechanical connectors for connecting the MEMS load sensor device to an external circuit formed on the first surface. 
     Optionally, the load sensor element can be a piezoresistive element. Additionally, the MEMS load sensor device can include a plurality of piezoresistive elements electrically connected in a Wheatstone bridge circuit. Additionally, the MEMS load sensor device can optionally include an activation circuit for supplying a voltage to the Wheatstone bridge circuit and for communicating a differential output voltage from the Wheatstone bridge circuit. For example, the activation circuit can include one or more output signal traces for communicating the differential output voltage from the Wheatstone bridge circuit to a signal bus and one or more voltage supply traces for connecting the Wheatstone bridge circuit to an external voltage source. The MEMS load sensor device can also include a row trace and a column trace for addressing the MEMS load sensor device and a logical gate having an input and output and one or more switches connected to the output of the logical gate. The input of the logical gate can be connected to the row and column traces, and each switch can be configured to electrically connect an output trace and the signal bus or a voltage supply trace and the external voltage source. 
     Also described herein is a method of manufacturing a MEMS load sensor device and optionally electrically and mechanically attaching the MEMS load sensor device to a separate circuit substrate. For example, the method includes the steps to manufacture the MEMS load sensor device and also optionally includes the steps to attach the MEMS load sensor device to the separate circuit substrate. The steps include micromachining the mechanical elements such as the deformable membrane, mesa, and overload protection portion, forming (e.g., by ion implantation) a load sensor element such as a piezoresistive element, metallization steps to form traces to connect the piezoresistive element to other electrical circuit elements on the MEMS load sensor device, steps to embed complementary metal-oxide-semiconductor (“CMOS”) circuitry to activate the MEMS load sensor device and to connect the MEMS load sensor device output to a signal bus of a separate circuit, steps to add electrical and mechanical connectors, and post processing steps to electrically and mechanically attach the MEMS load sensor device to a separate circuit substrate. 
     An example method for manufacturing a MEMS load sensor device can include providing a substrate having a first surface and a second surface opposite to the first surface, forming at least one load sensor element on the first surface of the substrate and etching the second surface of the substrate to form a deformable membrane, a mesa and an overload protection portion. The mesa can be configured to receive and transfer an applied force to the deformable membrane, and the deformable membrane can be configured to deform in response to the applied force. Additionally, the load sensor element can be provided on the deformable membrane and can be configured to change at least one electrical characteristic based on an amount or magnitude of the applied force. A height of the mesa can be greater than a height of the overload protection portion. 
     Optionally, in the method above, etching the second surface of the substrate to form a deformable membrane, a mesa and an overload protection portion can further include applying a layer of photoresist over the second surface of the substrate, irradiating a portion of the layer of photoresist with ultraviolet light through a mask and removing the irradiated portion of the layer of photoresist to expose a portion of the second surface of the substrate. Thereafter, the method can further include etching the exposed portion of the second surface of the substrate to form the deformable membrane, the mesa and the overload protection portion. For example, the second surface of the substrate can be etched using a deep ion etching technique. 
     Alternatively or additionally, in the method above, forming at least one load sensor element on the first surface of the substrate can further include applying a layer of silicon oxide over the first surface of the substrate, applying a layer of photoresist over the layer of silicon oxide, irradiating a portion of the layer of photoresist with ultraviolet light through a mask and removing the irradiated portion of the layer of photoresist to expose a portion of the layer of silicon oxide. Thereafter, the method can further include etching the exposed portion of the layer of silicon oxide to expose a portion of the first layer of the substrate and forming the load sensor element on the exposed portion of the first layer of the substrate. 
     Optionally, the load sensor element can be a piezoresistive element formed using a deposition, diffusion, or ion implantation technique. 
     Alternatively or additionally, the method can optionally include forming at least one electrical trace on the first surface of the substrate. The electrical trace can be electrically connected to the load sensor element. 
     Alternatively or additionally, the method can optionally further include forming one or more electromechanical connectors for connecting the MEMS load sensor device to an external circuit. 
     Optionally, the mesa defines a contact surface for receiving the applied force. The contact surface can have at least one of a substantially square, rectangular, rounded, circular or elliptical shape. 
     Alternatively or additionally, the mesa can optionally be arranged in a central portion of the substrate. Alternatively or additionally, the overload protection portion can optionally be arranged in a peripheral portion of the substrate. 
     Optionally, the method can further include fixing a touch surface to at least a portion of the mesa. The touch surface can be configured to receive and transfer the applied force to the mesa. Additionally, a gap can be arranged between the touch surface and the overload protection portion when the touch surface is fixed to the mesa. The gap can limit an amount of deflection of the deformable membrane and can prevent the deformable membrane from mechanically failing under an excessive applied force. 
     Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is an isometric view of an example load sensor device. 
         FIG. 2  is a side view of the load sensor device of  FIG. 1 . 
         FIG. 3  is a top view of the load sensor device of  FIG. 1 . 
         FIG. 4  is a bottom view of the load sensor device of  FIG. 1 . 
         FIG. 5  is a cross sectional view of the load sensor device of  FIG. 1  along the line L of  FIG. 3 . 
         FIG. 6  is a cross sectional view of the load sensor device along the line L of  FIG. 3 , with a force F applied showing concentration of stresses within the device. 
         FIG. 7  is a cross sectional view of the load sensor device of  FIG. 1  when electrically and mechanically connected to a circuit substrate and a touch surface. 
         FIG. 8  is an electrical schematic diagram of the electrical circuits embedded within the load sensor device of  FIG. 1 . 
         FIG. 9  is an electrical schematic diagram of multiple load sensor devices of  FIG. 1  in an independently addressable array. 
         FIG. 10  illustrates the process steps to manufacture the load sensor device of  FIG. 1 . 
         FIG. 11  illustrates the process steps to electrically and mechanically attach the load sensor device of  FIG. 1  to a separate circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the” include plural referents unless the context clearly dictates otherwise. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. The terms “optional” or “optionally” used herein mean that the subsequently described feature, event or circumstance may or may not occur, and that the description includes instances where said feature, event or circumstance occurs and instances where it does not. While implementations will be described with respect to a MEMS load sensor device and method of manufacturing the same, it will become evident to those skilled in the art that the implementations are not limited thereto. 
     Described herein is an example MEMS load sensor device (e.g., load sensor device) for measuring a force applied to a least a portion thereof. In one aspect, as depicted in  FIGS. 1-6 , the load sensor device  10  includes a substrate  61  defining a deformable membrane  13 , a mesa  12  and an overload protection portion  15 . It should be understood that the overload protection portion as used herein can include a range of structures such as a ring, for example. The substrate  61  can optionally be a silicon substrate. As shown in  FIGS. 1 and 2 , the substrate  61  can have a first surface  61 A and a second surface  61 B opposite the first surface  61 A, and the mesa  12  and the overload protection portion  15  can optionally be arranged in a central portion  61 C and a peripheral portion  61 D of the substrate  61 , respectively. As discussed in more detail below, the deformable membrane  13 , the mesa  12  and the overload protection portion  15  can be formed from the substrate  61  using an etching process. Additionally, at least one load sensor element (e.g., piezoresistive elements  14  and  82  discussed below) can be formed on the deformable membrane  13 . Optionally the load sensor element can be a piezoresistive, piezoelectric or capacitive element. The load sensor element can be configured to change at least one electrical characteristic (e.g., resistance, charge, capacitance, etc.) based on an amount or magnitude of the applied force. Optionally, the load sensor element can output a signal proportional to the amount or magnitude of the applied force. 
     Additionally, the mesa  12  can define a contact surface  11 , for example along the top surface of the mesa  12 , for receiving an applied force F and transmitting the force F to the deformable membrane  13 . It is contemplated that the contact surface  11  can have any shape, such as, for example and without limitation, a substantially square shape, a substantially round, circular or elliptical shape as depicted in  FIGS. 1-3 , a substantially rectangular shape, and the like. Additionally, this disclosure contemplates that the contact surface  11  can have shapes other than those described herein, and therefore, this disclosure should not be limited to the shapes described herein and/or shown in the figures. It is further contemplated, with reference to  FIG. 3 , that the contact surface  11  can be affixed below a touch surface  71  in order to receive the reaction force F transmitted through the touch surface  71  as depicted in  FIG. 7 . For example, as discussed below, at least a portion of the contact surface  11  can be bonded to at least a portion of the touch surface  71  using an adhesive. As the deformable membrane  13  deforms, a strain gradient forms within the substrate  61 . The concentration of stresses in the load sensor device  10  in response to the applied force F is illustrated in  FIG. 6 . The strain gradient imparts a localized strain on the piezoresistive elements  14  and  82  (e.g., the load sensor elements). As the piezoresistive elements  14  and  82  experience strain, their respective resistivities change, such that a Wheatstone bridge circuit, e.g., the Wheatstone bridge circuit  81  of  FIG. 8 , including two piezoresistive elements  14  and two oppositely arranged piezoresistive elements  82  (or stationary resistors) becomes unbalanced and produce a differential voltage across the positive signal terminal  83  and the negative signal terminal  84 . This differential voltage is directly proportional to the applied force F on the contact surface  11 . In addition, the load sensor device  10  can include one or more electromechanical connectors  75  for electrically and mechanically connecting the load sensor device  10  to a separate circuit substrate. 
     As discussed above, the load sensor device  10  can include a touch surface (e.g., touch surface  71 ), for example, fixed to the contact surface  11  of the mesa  12 . In an additional aspect, the load sensor device  10  incorporates an upper air gap  73  between the overload protection portion  15  and the touch surface  71 . As shown in  FIG. 7 , the upper air gap  73  exists because the height of the mesa  12  is greater than the height of the overload protection portion  15 . The difference in height between the mesa  12  and the overload protection portion  15  can be selected or engineered to be less than the maximum vertical deflection of the deformable membrane  13  before it yields or fails due to an excessive applied force. As the force F is applied to the touch surface  71 , the deformable membrane  13  deforms, and when the force F reaches a threshold, the touch surface  71  comes into contact with the overload protection portion  15 . At this point, the deformable membrane  13  no longer deforms linearly with applied force F. In this way the upper air gap  73  and the overload protection portion  15  work together to prevent the load sensor device  10  (e.g., the deformable membrane  13 ) from mechanically failing under excessive applied force F. 
     Alternatively or additionally, the load sensor device  10  optionally incorporates a lower air gap  74  between a lower surface  16  of the load sensor device  10  and an upper surface of a separate circuit substrate  72  as shown in  FIG. 7 . As the force F is applied to the touch surface  71 , the deformable membrane  13  deforms, allowing the touch surface  71  to move closer to and, as the force F becomes sufficiently large, come in contact with the overload protection portion  15 . Alternatively or additionally, as the force F becomes sufficiently larger and the deformable membrane  13  deforms lower, the lower surface  16  of the load sensor device  10  comes into contact with an upper surface  76  of the separate circuit substrate  72 . Once force F reaches an upper threshold and the touch surface  71  is in contact with the overload protection portion  15  and the lower surface  16  of the load sensor device  10  comes into contact with the upper surface  76  of the separate circuit substrate  72 , the deformable membrane  13  no longer deforms linearly with applied force F. In this way the upper air gap  73 , the lower air gap  74 , and the overload protection portion  15  work together to prevent the load sensor device  10  (e.g., the deformable membrane  13 ) from mechanically failing under excessive applied force F. Electromechanical connectors  75 , such as a solder joints or wire bonds, can be provided. The electromechanical connectors  75  are used to electrically and mechanically connect the load sensor device  10  to the separate circuit substrate  72 . 
     Referring now to  FIG. 8 , in an additional aspect, the load sensor device  10  includes and/or incorporates electrical circuitry  80  to activate the load sensor device  10  and electrically connect the load sensor device  10  to a separate circuit signal bus, for example, through electromechanical connectors such as electromechanical connectors  75  of  FIGS. 2 ,  4 ,  5  and  7 . Optionally, the electromechanical connectors are solder joints or wire bonds. As discussed above, the electrical circuitry  80  can include the Wheatstone bridge circuit  81 . In one embodiment, the electrical circuitry  80  includes of an activation circuit  86  including an X row signal trace  90  and a Y column signal trace  91  and a logical gate (e.g., AND gate  89 ). The row and column signal traces  90  and  91  can be used to individually address the load sensor device  10 . For example, when both X and Y signals are logic high, the AND gate  89  is enabled, which closes a plurality of switches  87 . The electrical circuitry  80  can also include one or more voltage supply traces for electrically connecting the Wheatstone bridge circuit  81  to an external voltage source  88  and one or more output signal traces for electrically connecting the positive and negative terminals  83  and  84  to the separate circuit signal bus. Each of the switches, respectively, connects the external voltage source  88  to the Wheatstone bridge circuit  81  or connects the positive signal terminal  83  and the negative signal terminal  84 , respectively, to the separate circuit signal bus. In this way, an array of load sensor devices  10  can be placed into an independently addressable array  90 , wherein the sensor value of each individual load sensor device  10  can be read independently, for example, by a microcontroller. An independently addressable array  90  including a plurality of load sensor device  10  is depicted in  FIG. 9 . 
     Referring now to  FIG. 10 , a method for manufacturing a MEMS load sensor device such as the load sensor device  10  discussed above is described herein. In the described method, step  110  involves growing or depositing a layer of silicon oxide  151  onto a surface of a bare silicon wafer  150 . Optionally, the layer of silicon oxide is grown or deposited on a surface of a silicon wafer on which electrical circuitry (e.g., the electrical circuitry  80  discussed above) for activating the load sensor device is already embedded through a separate CMOS semiconductor fabrication process. It should be understood that the silicon wafer can have an upper surface (or a first surface as used herein) and a lower surface (or a second surface as used herein) opposite to the upper surface. The surface of the silicon wafer on which the layer of silicon oxide is grown or deposited in step  110  can be the upper surface of the silicon wafer. In step  111 , a layer of photoresist  152  is applied or deposited onto at least a portion of the upper surface of the silicon wafer, for example, over the layer of silicon oxide. Thereafter, ultraviolet light is cast through a mask to weaken portions of the layer of photoresist. The layer of photoresist is then developed and at least a portion of the layer of silicon oxide  151 A is exposed, such that the exposed portion can be removed with an etchant as in step  112 . In step  113 , one or more piezoresistive elements  153  (e.g., load sensors elements) are formed, for example, by a deposition, diffusion or ion implantation process. The piezoresistive elements  153  can be the piezoresistive elements  14  and  82  discussed above, for example. 
     In step  114 , the layer of silicon oxide  151  is etched completely. In step  115 , the silicon wafer is annealed and an additional layer of silicon oxide  154  is formed, for example, on the upper surface of the silicon wafer over the piezoresistive elements  153 . Additionally, a layer of silicon oxide  155  is also formed over on the opposite surface of the silicon wafer. It should be understood that the opposite surface of the silicon wafer as used herein can be the lower surface of the silicon wafer. In step  116 , an additional layer of photoresist  156  is applied onto at least a portion of the upper surface of the silicon wafer, for example, over the layer of silicon oxide  154 . Thereafter, ultraviolet light is cast through a mask, which weakens portions of the layer of photoresist  156 . The layer of photoresist is then developed in order to expose at least a portion of the layer of silicon oxide  154 A covering the piezoresistive elements  153 . In step  117 , the exposed portion of the layer of silicon oxide is etched completely to expose the piezoresistive elements  153 , e.g., an upper portion of the piezoresistive elements  153 . In step  118 , a conductive metal  157  (e.g., an electrical trace), such as aluminum, is sputtered onto the upper surface of the silicon wafer, for example, over the exposed piezoresistive elements, to form electrical connections between the piezoresistive elements  153  and the electromechanical connectors (discussed below). In step  119 , an additional layer of photoresist  158  is applied onto at least a portion of the upper surface of the silicon wafer, for example, over the conductive metal. Thereafter, ultraviolet light is cast through a mask, which weakens at least a portion of the layer of photoresist. The layer of photoresist is then developed in order to expose at least a portion of the conductive metal  157 A to be removed. In step  120 , the exposed portion of the conductive metal is etched completely to leave the remaining conductive metal forming one or more portions of the connecting circuitry (e.g., one or more portions of the electrical circuitry  80  discussed above). 
     In step  121 , a passivation layer  159  is deposited or applied to protect the piezoresistive elements  153  and conductive metal  157 . In step  122 , an additional layer of photoresist  160  is applied to the upper surface of the silicon wafer, for example, over the passivation layer. Thereafter, ultraviolet light is cast through a mask to weaken at least a portion of the layer of photoresist. The layer of photoresist is then developed in order to expose at least a portion of the passivation layer  159 A to be removed. In step  123 , the exposed portion of the passivation layer is etched completely to leave a portion of the conductive metal  157 B underneath exposed for electrical contact. 
     In step  124 , the silicon wafer is inverted to expose the lower surface of the silicon wafer. As discussed above with regard to step  115 , the lower surface of the silicon wafer has the layer of silicon oxide  155  formed thereon. In addition, an additional layer of photoresist  161  is applied to the lower surface of the silicon wafer, for example, over a portion of the layer of silicon oxide. Thereafter, ultraviolet light is cast through a mask to weaken at least a portion of the layer of photoresist. The layer of photoresist is then developed in order to expose at least a portion of the layer of silicon oxide  155 A formed over the lower surface of the silicon wafer. In step  125 , the exposed portion of the layer of silicon oxide is etched completely to expose a portion of the lower surface of the silicon wafer  150 A. In step  126 , the exposed portion of silicon wafer is etched to form the height offset between a mesa and overload protection portion (e.g., the mesa  12  and overload protection portion  15  discussed above) to provide overload protection. In step  127 , an additional layer of photoresist  162  is applied onto at least a portion of the lower surface of the silicon wafer. Thereafter, ultraviolet light is cast through a mask to weaken at least a portion of the layer of photoresist. The layer of photoresist is then developed in order to expose at least a portion of the lower surface of the silicon wafer  150 B. In step  128 , the silicon on the lower surface of the silicon wafer is etched away using a deep reactive ion etching process to form an integrated mesa, contact surface, deformable membrane and overload protection portion (e.g., the mesa  12 , contact surface  11 , deformable membrane  13  and overload protection portion  15 ). In step  129 , the layer of silicon oxide on the upper surface of the mesa is etched completely to leave exposed bare silicon. The silicon wafer can then optionally be inverted and electromechanical connectors (e.g., electromechanical connectors  75  discussed above) such as solder bumps, wire bonds, etc. are attached to the load sensor device, e.g., to the same surface of the bare silicon wafer on which the piezoresistive elements were formed. The load sensor device is then ready for a separate manufacturing process to be attached to an electrical circuit. 
     Referring now to  FIG. 11 , the steps to attach a load sensor device (e.g., load sensor device  10  of  FIGS. 1-7 ) to a separate circuit substrate (e.g., separate circuit substrate  72  of  FIG. 7 ) and complete a touch solution are illustrated. In this final manufacturing process, the load sensor devices are first bonded to the substrate, such as FR 4 , of a separate circuit. Then, the electrical and mechanical connections are formed through a process such as reflow soldering or wire bonding. Finally, a touch surface (e.g., touch surface  71  of  FIG. 7 ) is affixed to each load sensor device (e.g., to the contact surface  11  of the mesa  12  of each load sensor  10  of  FIGS. 1-7 ) in the finished component using adhesive, and the entire assembly is cured to form a finished touch surface component. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.