Patent ID: 12203819

DETAILED DESCRIPTION

The present disclosure 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 disclosure 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 is provided as an enabling teaching. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made, while still obtaining beneficial results. It will also be apparent that some of the desired benefits can be obtained by selecting some of the features without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations may be possible and can even be desirable in certain circumstances, and are contemplated by this disclosure. Thus, the following description is provided as illustrative of the principles 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 sensing element” can include two or more such force sensing elements unless the context indicates 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.

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 disclosure relates to the TCO compensation layer for MEMS force sensor and strain gauge. Three different types of MEMS force sensor's TCO can be compensated with this layer. The force sensor can also be reconfigured as strain gauge if the force is applied through the package substrate. The root cause of the TCO is illustrated and the material property and dimension effect to the TCO is also illustrated.

FIG.1illustrates the force sensor102(e.g., a MEMS force sensor) mounted on a package substrate108. The package substrate108can be a printed circuit board (PCB), a flexible printed circuit board (FPC), or a co-fired ceramic. It should be understood that PCBs, FPCs, and co-fired ceramics are provided only as example package substrates. The combined force sensor and package substrate is shown by reference number101inFIG.1. The force sensor102can include a dielectric layer103, a sensor substrate104, and a piezoresistive sensing element105, The sensor substrate, which can also be referred to as a sensor die, can be made of a semiconductor material such as silicon or gallium arsenide, for example. As shown inFIG.1, the sensor substrate104has a top surface104A and a bottom surface1048, which is opposite to the top surface104A, The piezoresistive sensing element105is arranged on the bottom surface1048. Optionally, in some implementations, a plurality of piezoresistive sensing elements105can be arranged on the sensor substrate104. This disclosure contemplates that the piezoresistive sensing element(s)105can be diffused, deposited, or implanted on the bottom surface104B. The dielectric layer103can then be arranged (e.g., deposited) over the bottom surface104B to electrically isolate the piezoresistive sensing element(s)105.

The piezoresistive sensing element105can change an electrical characteristic (e.g., resistance) in response to deflection of the sensor substrate104. For example, the piezoresistive sensing element105can sense strain on the bottom surface104A of the sensor substrate104. The change in electrical characteristic can be measured as an analog electrical signal. In one implementation, the piezoresistive sensing element105can optionally be a piezoresistive transducer. For example, as strain is induced in the sensor substrate104proportional to a force “F” applied to the force sensor102, a localized strain is produced on the piezoresistive transducer such that the piezoresistive transducer experiences compression or tension, depending on its specific orientation. As the piezoresistive transducer compresses and tenses, its resistivity changes in opposite fashion. Accordingly, a Wheatstone bridge circuit including a plurality (e.g., four) piezoresistive transducers (e.g., two of each orientation relative to strain) becomes unbalanced and produces a differential voltage (also sometimes referred to herein as an “analog electrical signal”) across the positive signal terminal and the negative signal terminal. This differential voltage is directly proportional to the applied force “F” on the force sensor102. This differential voltage can be received at and processed by digital circuitry. For example, digital circuitry can be configured to, among other functions, convert the analog electrical signal to a digital electrical signal. Although piezoresistive transducers are provided as an example sensing element, this disclosure contemplates that the sensing element(s) can be any sensor element configured to change at least one electrical characteristic (e.g., resistance, charge, capacitance, etc.) based on an amount or magnitude of an applied force and can output a signal proportional to the amount or magnitude of the applied force. Other types of sensing elements include, but are not limited to, piezoelectric or capacitive sensors. Additionally, application of force “F” to the force sensor102is provided only as an example. This disclosure contemplates that force can be applied to other sides of the force sensor including, but not limited to, via the package substrate108, which is arranged below the force sensor. Such application of force can produce a localized strain in the sensors. Example MEMS force sensors using piezoresistive sensing elements are described in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016 and entitled “Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342, issued Nov. 15, 2016 and entitled “Wafer Level MEMS Force Dies;” U.S. Pat. No. 9,902,611, issued Feb. 27, 2018 and entitled “Miniaturized and ruggedized wafer level mems force sensors;” and U.S. Patent Application Publication No. 2016/0363490 to Campbell et al., filed Jun. 10, 2016, now U.S. Pat. No. 10,466,119, and entitled “Ruggedized wafer level mems force sensor with a tolerance trench,” the disclosures of which are incorporated by reference in their entireties.

As shown inFIG.1, the force sensor102can also include metal layer(s)106. The metal layers106can be made of any suitable conductive material, including but not limited to, aluminum, copper, or gold, for example. The metal layers106can provide electrical connection between the force sensor102and the package substrate108. For example, the force sensor102can be electrically and mechanically coupled to the package substrate108through solder bumps107provided on the bottom surface of the force sensor102, The solder bumps107are connected to the force sensor102at the metal layers106, which provide the electrical connection to the force sensor102such that an electrical signal can be transferred from the force sensor102to the package substrate108. It should be understood that solder bumps107are provided inFIG.1only as an example mechanism for mechanically and electrically connecting the force sensor102to the package substrate108.

The force sensor102can also include a compensation layer109(also sometimes referred to herein as “TCO compensation layer”). The compensation layer109can be formed from materials including, but not limited to, polymer, polyimide, resin, polycarbonate, acrylonitrile butadiene styrene (ABS), silicon oxide, glass, or combinations thereof. As shown inFIG.1, the TCO compensation layer109can be arranged on the top surface104A of the sensor substrate104. This disclosure contemplates that the force “F” can be applied to the force sensor102via the TCO compensation layer109, e.g., the TCO compensation layer109can be disposed on the top surface of the force sensor. As described herein, the compensation layer109has a thermal coefficient of expansion (TCE) that is different than a TCE of the sensor substrate104. In some implementations, the TCE of the compensation layer109can be less than the TCE of the sensor substrate104. In other implementations, the TCE of the compensation layer109can be greater than the TCE of the sensor substrate104. This disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer109can be selected to reduce TCO. Alternatively or additionally, this disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer109can be selected to minimize TCO. Optionally, the above characteristics of the compensation layer109can be selected to make TCO zero. Optionally, the above characteristics of the compensation layer109can be selected to make TCO a minimum but non-zero value based on design limitations (e.g., commercial, engineering, etc.). In other words, it should be understood that it may not be desirable or possible to reduce TCO to zero for every force sensor design. The effect of compensation layer stiffness on TCO is described below with regard toFIG.5. The effect of compensation layer thickness on TCO is described below with regard toFIG.6. Alternatively, as described above, the force “F” can be applied to the package substrate108. In this implementation, the force “F” can be applied via a TCO compensation layer and the TCO compensation layer can be designed as described above.

FIG.2illustrates the force sensor102(e.g., a MEMS force sensor) mounted on a package substrate108. The package substrate108can be a printed circuit board (PCB), a flexible printed circuit board (FPC), or a co-fired ceramic. It should be understood that PCBs, FPCs, and co-fired ceramics are provided only as example package substrates. The combined force sensor and package substrate is shown by reference number201inFIG.2. The force sensor102can include a dielectric layer103, a sensor substrate104, and a piezoresistive sensing element105. The sensor substrate, which can also be referred to as a sensor die, can be made of a semiconductor material such as silicon or gallium arsenide, for example. As shown inFIG.2, the sensor substrate104has a top surface104A and a bottom surface104B, which is opposite to the top surface104A. The piezoresistive sensing element105is arranged on the bottom surface104B. Optionally, in some implementations, a plurality of piezoresistive sensing elements105can be arranged on the sensor substrate104. This disclosure contemplates that the piezoresistive sensing element(s)105can be diffused, deposited, or implanted on the bottom surface104B. The dielectric layer103can then be arranged (e.g., deposited) over the bottom surface104B to electrically isolate the piezoresistive sensing element(s)105. Piezoresistive sensing elements are described above with regard toFIG.1and are therefore not described in further detail below.

As shown inFIG.2, the force sensor102can include a piezoelectric sensor in addition to the piezoresistive sensing element(s)105. This disclosure contemplates that the force sensor102can include a plurality of piezoelectric sensors. A piezoelectric sensor can include a piezoelectric sensing element210arranged between opposing electrodes. InFIG.2, the piezoelectric sensing element210is sandwiched between piezoelectric electrode211and metal layer106(e.g., the opposing electrodes). Piezoresistive and piezoelectric sensing elements can be used together in MEMS force sensors. For example, piezoresistive sensing elements are useful for sensing static forces applied to the force sensor102, while piezoelectric sensing elements are useful for sensing dynamic forces acting on the force sensor102. Thus, both piezoresistive and piezoelectric sensors can be used in conjunction to detect both static and dynamic forces. As described above, the piezoelectric sensing element210is located between piezoelectric electrode211and metal layer106. The piezoelectric sensing elements105can be configured to convert a change in strain to an analog electrical signal that is proportional to the change strain on the bottom surface1043. The piezoelectric sensing elements210sense dynamic forces applied to the force sensor102. Additionally, the electrical signals detected by the piezoresistive and piezoelectric sensing elements can be routed to digital circuitry. For example, the digital circuitry can be configured to, among other functions, convert the analog electrical signals to a digital electrical output signal. The use of both piezoresistive and piezoelectric sensing elements in a MEMS force sensor is described in detail in WO2018/148510, published Aug. 16, 2018 and entitled “INTEGRATED PIEZORESISTIVE AND PIEZOELECTRIC FUSION FORCE SENSOR,” the disclosure of which is expressly incorporated herein by reference in its entirety.

As shown inFIG.2, the force sensor102can also include metal layer(s)106. The metal layers106can be made of any suitable conductive material, including but not limited to, aluminum, copper, or gold, for example. The metal layers106can provide electrical connection between the force sensor102and the package substrate108. For example, the force sensor102can be electrically and mechanically coupled to the package substrate108through solder bumps107provided on the bottom surface of the force sensor102. The solder bumps107are connected to the force sensor102at the metal layer106, which provides the electrical connection to the force sensor102such that an electrical signals can be transferred from the force sensor102to the package substrate108. It should be understood that solder bumps107are provided inFIG.2only as an example mechanism for mechanically and electrically connecting the force sensor102to the package substrate108.

The force sensor102can also include a compensation layer109(also sometimes referred to herein as “TCO compensation layer”). The compensation layer109can be formed from materials including, but not limited to, polymer, polyimide, resin, polycarbonate, acrylonitrile butadiene styrene (ABS), silicon oxide, glass, or combinations thereof. As shown inFIG.2, the TCO compensation layer109can be arranged on the top surface104A of the sensor substrate104. This disclosure contemplates that the force “F” can be applied to the force sensor102via the TCO compensation layer109, e.g., the TCO compensation layer109can be disposed on the top surface of the force sensor. As described herein, the compensation layer109has a thermal coefficient of expansion (TCE) that is different than a TCE of the sensor substrate104. In some implementations, the TCE of the compensation layer109can be less than the TCE of the sensor substrate104. In other implementations, the TCE of the compensation layer109can be greater than the TCE of the sensor substrate104. This disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer109can be selected to reduce TCO. Alternatively or additionally, this disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer109can be selected to minimize TCO. Optionally, the above characteristics of the compensation layer109can be selected to make TCO zero. Optionally, the above characteristics of the compensation layer109can be selected to make TCO a minimum but non-zero value based on the design limitations (e.g., commercial, engineering, etc.). In other words, it should be understood that it may not be desirable or possible to reduce TCO to zero for every force sensor design. The effect of compensation layer thickness on TCO is described below with regard toFIG.6. The effect of compensation layer stiffness on TCO is described below with regard toFIG.5. Alternatively, as described above, the force “F” can be applied to the package substrate108. In this implementation, the force “F” can be applied via a TCO compensation layer and the TCO compensation layer can be designed as described above.

FIG.3illustrates another force sensor302(e.g., a MEMS force sensor) mounted on a package substrate310. The package substrate310can be a printed circuit board (PCB), a flexible printed circuit board (FPC), or a co-fired ceramic. It should be understood that PCBs, FPCs, and co-fired ceramics are provided only as example package substrates. The combined force sensor and package substrate is shown by reference number301inFIG.3. The force sensor302can include a dielectric layer303, a sensor substrate304, a bond oxide layer307, a cap substrate308, and a piezoresistive sensing element305. The sensor substrate, which can also be referred to as a sensor die, can be made of a semiconductor material such as silicon or gallium arsenide, for example. As shown inFIG.3, the sensor substrate304has a top surface304A and a bottom surface304B, which is opposite to the top surface304A. The piezoresistive sensing element305is arranged on the bottom surface304B. Optionally, in some implementations, a plurality of piezoresistive sensing elements305can be arranged on the sensor substrate304. This disclosure contemplates that the piezoresistive sensing element(s)305can be diffused, deposited, or implanted on the bottom surface304B. The dielectric layer303can then be arranged (e.g., deposited) over the bottom surface304B to electrically isolate the piezoresistive sensing element(s)305. Piezoresistive sensing elements are described above with regard toFIG.1and are therefore not described in further detail below.

As described above, the force sensor302can include the sensor substrate304and the cap substrate308. The sensor substrate304and the cap substrate308can be bonded together via the bonded oxide layer307. It should be understood that the bonded oxide layer307is only provided as an example mechanism for bonding the sensor substrate304and the cap substrate308. For example, this disclosure contemplates bonding the substrates using other techniques known in the art including, but not limited to, silicon fusion bonding, anodic bonding, glass frit, thermo-compression, and eutectic bonding. The cap substrate308can optionally be made of glass (e.g., borosilicate glass) or semiconductor (e.g., silicon). The internal surfaces between the sensor substrate304and the cap substrate308form a sealed cavity350. The sealed cavity350can be formed by etching a trench from the sensor substrate304and then sealing a volume between the bonded sensor substrate304and cap substrate308. For example, the volume is sealed between the sensor substrate304and the cap substrate308when adhered together, which results in formation of the sealed cavity350. Example MEMS force sensors having a sealed cavity are described in U.S. Pat. No. 9,902,611, issued Feb. 27, 2018 and entitled “Miniaturized and ruggedized wafer level mems force sensors;” and U.S. Patent Application Publication No. 2016/0363490 to Campbell et al., filed Jun. 10, 2016, now U.S. Pat. No. 10,466,119, and entitled “Ruggedized wafer level mems force sensor with a tolerance trench,” the disclosures of which are incorporated by reference in their entireties. The force sensor302therefore has a sealed cavity350that defines a volume entirely enclosed by the sensor substrate304and the cap substrate308. The sealed cavity350can be sealed from the external environment.

As shown inFIG.3, the force sensor302can also include metal layer(s)306. The metal layers306can be made of any suitable conductive material, including but not limited to, aluminum, copper, or gold, for example. The metal layers306can provide electrical connection between the force sensor302and the package substrate310. For example, the force sensor302can be electrically and mechanically coupled to the package substrate310through solder bumps309provided on the bottom surface of the force sensor302. The solder bumps309are connected to the force sensor302at the metal layer306, which provides the electrical connection to the force sensor302such that an electrical signal can be transferred from the force sensor302to the package substrate310. It should be understood that solder bumps309are provided inFIG.3only as an example mechanism for mechanically and electrically connecting the force sensor302to the package substrate310.

The force sensor302can also include a compensation layer311(also sometimes referred to herein as “TCO compensation layer”). The compensation layer311can be formed from materials including, but not limited to, polymer, polyimide, resin, polycarbonate, acrylonitrile butadiene styrene (ABS), silicon oxide, glass, or combinations thereof. As shown inFIG.3, the TCO compensation layer311can be arranged on the top surface304A of the sensor substrate304. This disclosure contemplates that the force “F” can be applied to the force sensor302via the TCO compensation layer311, e.g., the TCO compensation layer311can be disposed on the top surface of the force sensor. As described herein, the compensation layer311has a thermal coefficient of expansion (TCE) that is different than a TCE of the sensor substrate304. In some implementations, the TCE of the compensation layer311can be less than the TCE of the sensor substrate304. In other implementations, the TCE of the compensation layer311can be greater than the TCE of the sensor substrate304. This disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer311can be selected to reduce TCO. Alternatively or additionally, this disclosure contemplates that the thickness, the stiffness, the TCE, and/or the combination of thickness, stiffness, and TCE of the compensation layer311can be selected to minimize TCO. Optionally, the above characteristics of the compensation layer311can be selected to make TCO zero. Optionally, the above characteristics of the compensation layer311can be selected to make TCO a minimum but non-zero value based on the design limitations (e.g., commercial, engineering, etc.). In other words, it should be understood that it may not be desirable or possible to reduce TCO to zero for every force sensor design. The effect of compensation layer thickness on TCO is described below with regard toFIG.6. The effect of compensation layer stiffness on TCO is described below with regard toFIG.5. Alternatively, as described above, the force “F” can be applied to the package substrate310. In this implementation, the force “F” can be applied via a TCO compensation layer and the TCO compensation layer can be designed as described above.

Referring now toFIG.4, deformation of an example MEMS force sensor due to an increase in temperature is shown. Such deformation has been exaggerated by 5000 times to provide clear visual examination. InFIG.4, a force sensor401is mounted to a package substrate403through solder bumps402. It should be understood that the force sensor401can be similar to one of the force sensors shown inFIG.1-3with the exception of including a compensation layer (e.g., compensation layer109or311inFIGS.1-3). In other words, the force sensor401can optionally include a sensor substrate, sensing element(s), a dielectric layer, and/or a cap substrate but does not include a TCO compensation layer. This disclosure contemplates that the bottom of the package substrate404is fixed for the purposes of the simulation, which mimics the actual operation conditions. InFIG.4, a temperature raise (e.g., an increase in temperature) is applied to the model for simulation from zero stress condition for TCO simulation. As shown inFIG.4, the package substrate403experiences thermal expansion as a result of the increase in temperature. Additionally, the deformation of the package substrate403is transferred to the force sensor401through the solder bumps402. The deformation shown inFIG.4causes negative TCO. Although deformation causing negative TCO is shown inFIG.4, this disclosure contemplates that deformation causing positive TCO can occur, for example, due to decreases in temperature.

Referring now toFIG.5, a graph illustrating normalized TCO versus normalized Young's modulus for an example compensation layer is shown. InFIG.5, TCO is normalized to the specific material and dimensions of an example MEMS force sensor.FIG.5illustrates the effect of varying the Young's modulus on TCO of the compensation layer. In the examples described herein, Young's modulus is provided as an example measure of the stiffness of the compensation layer material. Young's modulus is a known property that defines the relationship between stress and strain of a material. This disclosure contemplates that other measures of stiffness can be used. In some implementations, the TCE of the compensation layer is selected to be within the same order of magnitude of the TCE of the package substrate. In some implementations, the TCE of the compensation layer is selected to be about equal to the TCE of the package substrate. Optionally, the respective TCE of both the compensation layer and the package substrate can be larger than the TCE of silicon. As shown inFIG.5, by increasing the Young's modulus of the compensation layer, the TCO increases linearly and proportionally to the Young's modulus of the compensation layer. Additionally, at a particular Young's modulus, the TCO crosses over from negative to positive value. InFIG.5, this occurs where the normalized Young's modulus is about 2.0. Accordingly, by setting the Young's modulus appropriately, e.g., between about 1.5 to 2 for the example shown inFIG.5, the TCO can be tuned to zero.

In some cases, neither the Young's modulus nor the TCE of the compensation layer can be chosen without limitation. This is because both the Young's modulus and TCE are material properties, and there may be limitations (e.g., commercial, engineering, etc.) on the materials used. As described below, the thickness of the compensation layer affects the TCO. Referring now toFIG.6, a graph illustrating normalized TCO versus thickness of an example compensation layer is shown. In the example ofFIG.6, the Young's modulus and TCE are set based on the material of the compensation layer.FIG.6illustrates the relationship between the TCO and thickness of the compensation layer. At some value, the TCO crosses from negative to positive value. InFIG.6, the TCO crosses over from negative to positive value around the normalized value of 1.9. Accordingly, by selecting a thickness of the compensation layer, the TCO can be reduced to zero at that specific thickness.

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.