Patent Publication Number: US-11653568-B2

Title: Integrated circuit stress sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/958,530 filed on Jan. 8, 2020 and to U.S. Provisional Patent Application Ser. No. 62/983,794 filed on Mar. 2, 2020, both of which are hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This relates generally to a torque sensor and more specifically, to an integrated circuit stress sensor. 
     BACKGROUND 
     Various types of torque sensors exist for measuring and recording torque on a rotating shaft. Many existing torque sensor systems are often costly and/or include components measure an effect from the shaft, such as electric, magnetic or optical effects, that result in systems becoming large and bulky. For example, a Hall-based magnetic system requires either a magnetized shaft or magnets attached to the shaft, which are prone to interference from magnetic fields. Optical systems are complex to build and often require a special housing. A micro-electromechanical system (MEMS) torque sensor uses piezoresistors externally attached to an integrated circuit (IC) chip to measure bending and/or torsion of a shaft. The MEMS torque sensor, however, requires an external standalone readout and signal conditioning circuit mounted to a printed circuit board, which creates a bulky system. 
     Other torque sensor systems include strain gauges that are attached to a rotating shaft of a structure. The strain gauge may include metal strips or wires that attach to the shaft via an adhesive. The torque sensor system also includes signal conditioning circuitry and a power supply mounted to the shaft. When the shaft is torqued, the strain gauges stretch and send signals to the signal conditioning circuitry. Thus, the strain gauge torque sensor system also is a bulky system requiring several separate components separated from each other mounted to the shaft. 
     SUMMARY 
     In one example, an integrated circuit is described that includes a semiconductor substrate. The integrated circuit includes a first strain-sensitive sensor on or in the substrate that has a first sensing axis extending in a first direction parallel with a surface of the substrate, and a second strain-sensitive sensor on or in the substrate that has a second sensing axis extending in a second direction parallel with the surface of the substrate and perpendicular to the first direction. A third strain-sensitive sensor is on or in the substrate and has a third sensing axis extending in a third direction parallel with the surface of the substrate and neither parallel nor perpendicular to the first and second directions. 
     In another example, a system is described that includes an integrated circuit (IC) including strain-sensitive sensors on or in a semiconductor substrate. The substrate has a crystal orientation and each of the strain-sensitive sensors have a respective sensing axis oriented at an angle relative to the crystal orientation of the substrate. The IC includes a sense circuit configured to determine a change in resistance for each of the respective strain-sensitive sensors responsive to deformation of the substrate. A communication device is coupled to the IC, where the communication device is configured to wirelessly communicate data representative of the deformation of the substrate responsive to the change in resistance. A controller is coupled to the IC and the communication device and is configured to control communication the communication device. 
     In still yet another example, an integrated circuit (IC) is described that includes strain-sensitive sensors on or in a semiconductor substrate, where the substrate has a crystal orientation. The strain-sensitive sensors include a first strain-sensitive sensor on or in the substrate to have a first sensing axis extending in a first direction parallel to a surface of the substrate and to the crystal orientation of the substrate. A second strain-sensitive sensor is on or in the substrate and has a second sensing axis extending in a second direction transverse to the crystal orientation of the substrate and parallel to the surface of the substrate, where the second direction is perpendicular to the first direction. A third strain-sensitive sensor is on or in the substrate and has a third sensing axis extending in a third direction parallel with the surface of the substrate that is neither parallel nor perpendicular to the first and second directions. A sense circuit is coupled to each of the first, second and third strain-sensitive sensors, the sense circuit is configured to provide respective sense signals, where the sense signals are representative of a change in resistance of the respective first, second, and third strain-sensitive sensors responsive to deformation of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a block diagram of an example integrated circuit. 
         FIG.  1 B  is a block diagram of a sensing circuit from the IC of  FIG.  1   . 
         FIG.  2 A  depicts a piezoresistive sensor showing stress applied to the sensor from different directions. 
         FIG.  2 B  is a resistor model showing series resistors responsive to stress applied to the sensors in  FIG.  2 A . 
         FIG.  3    is an example readout circuit for sensing normal stress from a pair of piezoresistive sensors. 
         FIG.  4    is an example graph of resistance versus stress, such as provided by the readout circuit of  FIG.  3   . 
         FIG.  5    is another example readout circuit for sensing shear stress from a pair of piezoresistive sensors. 
         FIG.  6    is an example graph of resistance versus stress, such as provided by the readout circuit of  FIG.  5   . 
         FIG.  7    is an exploded view of an example system on chip circuit implementing a torque sensor. 
         FIG.  8    is an exploded view of another example system on chip circuit implementing a torque sensor. 
         FIG.  9    is a perspective view of an example torque sensor system mounted on a surface of a mechanical structure. 
         FIGS.  10 - 13    depict schematic views of piezoresistive sensors oriented at different sensing angles for sensing stress by the example torque sensor system of  FIG.  5   . 
         FIG.  14    is a perspective view of an example torque sensor system mounted perpendicular to a surface of a mechanical structure. 
         FIGS.  15 - 18    depict schematic views of piezoresistive sensors oriented at different sensing angles for sensing stress by the example torque sensor system of  FIG.  10   . 
         FIG.  19    is a cross section view of an example torque sensor system. 
         FIGS.  20  and  21    are cross-sectional views illustrating the example torque sensor system of  FIG.  19    mounted to a mechanical structure for sensing normal stress applied to the structure. 
         FIGS.  22  and  23    are cross-sectional views illustrating the example torque sensor system of  FIG.  19    mounted to a mechanical structure for sensing torsional stress applied to the structure. 
         FIG.  24    depicts the torque sensor of  FIG.  19    mounted to a shaft showing power supply and communications. 
         FIGS.  25  and  26    are cross-sectional and top views of another example torque sensor system. 
         FIG.  27    is a cross-sectional view illustrating the example torque sensor system of  FIGS.  25  and  26    mounted to a mechanical structure for sensing torsional stress applied to the structure  FIGS.  25  and  26   . 
         FIG.  28    is a cross section of an example reference resistor in a semiconductor substrate. 
         FIG.  29    is a cross section of an example sensing resistor in a semiconductor substrate. 
         FIG.  30    is an example mounting method for mounting the torque sensor to a mechanical structure. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is an integrated circuit (IC) that includes strain-sensitive sensors that are configured to work in conjunction with a sense circuit to measure stress components (e.g., normal and shear) responsive to mechanical force applied to the IC. The IC is adapted to be coupled to a mechanical structure (e.g., shaft, beam, feather key, leadframe, etc.) to measure torque or other stress applied to the mechanical structure. Strain-sensitive sensors are formed on or in a surface of a substrate of the IC with respective sensing axes oriented in different directions with respect to a crystal orientation of the substrate. The strain-sensitive sensors thus may be configured to measure deformation in the substrate responsive to a mechanical force applied to or otherwise experienced by a mechanical structure to which the IC is coupled via an adhesive, clamp, metallic joint, cohesive joint, positive or non-positive fit joint. The strain-sensitive sensors may include circuit components or devices formed on or in the semiconductor substrate. Examples of strain-sensitive components are active and passive components including bipolar transistors, complementary metal oxide semiconductor (CMOS) transistors, metal lines, piezoresistors and the like. The strain-sensitive components are described in examples below mainly as piezoresistive sensors. However, other types of strain-sensitive components may be implemented in the ICs and SoCs described herein. 
     As a further example, the piezoresistive sensors are coupled to circuitry, which may be integrated in the IC with the sensors. For example, the piezoresistive sensors have respective resistances values that change responsive to the mechanical forces (e.g., shear and normal forces) experienced by the substrate. The circuitry thus can be configured to determine an indication of normal and shear forces based on the measured resistance values of the piezoresistive sensors. The resistance values of the respective piezoresistive sensors can also be used to correct the impact of the forces on associated circuitry in the IC. In an example, the IC includes a temperature sensor integrated into the substrate adjacent the piezoresistive sensors. The temperature sensor can provide a temperature signal representative of the substrate temperature. The circuitry may be configured to adjust the measured resistance values responsive to the temperature signal to reduce the temperature impact on the measured resistance values so as to also reduce the temperature impact on the determined indication of forces. 
       FIG.  1 A  is a block diagram of an integrated circuit (IC)  100 . The IC  100  is adapted to couple to a mechanical structure (e.g., shaft, beam, feather key, leadframe, etc.). The IC  100  includes a substrate  102  having a contact surface and an opposing mounting surface  104 . The contact surface of the IC is adapted to couple the IC  100  to the mechanical structure, such as by an adhesive, clamp, metallic joint, cohesive joint, positive fit, non-positive fit. The substrate  102  is made from a semiconductor material, such as silicon, germanium, gallium arsenide. The substrate has a crystal orientation (axis) that can extend in any direction (e.g., x-direction, y-direction, z-direction, or a combination thereof) based on the material and formation of the substrate  102 . In the example of  FIG.  1 A , the crystal orientation of the substrate  102  is represented by a crystal (or central) axis  106  extending in the x-direction. 
     As an example, the substrate  102  is a semiconductor wafer formed from a single crystal rod such that the substrate surface is oriented relative to a crystallographic plane. Miller indices, indicated by curly brackets { }, are used to determine the respective plane in a cubic crystal. For example the substrate may be a p-type (or n-type) semiconductor substrate that is cut in the {100} plane. 
     The example IC  100  further includes piezoresistive sensors formed on or in the surface  104  of the substrate  102 . For example, a first piezoresistive sensor  108  has a sensing axis  110  extending in a first direction parallel with the mounting surface  104  of the substrate  102  and parallel with the crystal axis (e.g., the [100] or another crystal axis). A second piezoresistive sensor  112  has a sensing axis  114  extending in a second direction parallel with the mounting surface  104  of the substrate  102  and perpendicular to the first direction of the first piezoresistive sensor  108 . A third piezoresistive sensor  116  has a sensing axis  118  extending in a third direction parallel with the mounting surface  104  of the substrate  102  and that is neither parallel nor perpendicular to the first and second directions of the first and second piezoresistive sensors  108 ,  112  respectively. In an example, the sensing axis  118  may be oriented at an angle that is approximately half-way between the angle between the first and second directions of respective sensing axes  110  and  114  (e.g., about 45° from the axes  110  and  114 ). In another example, the sensing axis  118  of the third piezoresistive sensor  116  may be oriented at other angles (e.g., 15°, 30°, 60°, 75°, etc.) with respect to the respective sensing axes  110  and  114 . The arrangement of the piezoresistive sensors  108 ,  112 ,  116  on or in the substrate  102  can be such that the orientations of the respective sensing axis can be parallel, perpendicular, or oriented at other angles (e.g., 15°, 30°, 60°, 75°, etc.) to the crystal axis of wafers cut in a different crystal plane (e.g., the [110] crystal, the [111] crystal). 
     The piezoresistive sensors  108 ,  112 ,  116  are coupled to a sense (readout) circuit  120 . For example, each of the piezoresistive sensors  108 ,  112 ,  116  has input and output terminals coupled to respective outputs and inputs of the sense circuit  120 . The sense circuit  120  is configured to measure a change in resistance of each of the respective piezoresistive sensors  108 ,  112 ,  116 . The change in resistance is responsive to deformation of the substrate  102  from its normal (rest) condition, such as may be caused by mechanical force (e.g., compression, tension and/or shear forces) applied to the substrate  102 . The sense circuit  120  provides output signals (sense signals) representative of longitudinal (e.g., compression and/or tension), normal and shear forces responsive to measured changes in the resistance of the respective piezoresistive sensors  108 ,  112 ,  116 . As described herein, the particular forces being measured by each piezoresistive sensors  108 ,  112 ,  116  depends on the orientation of the sensing axis of respective piezoresistive sensors with respect the orientation of the crystal axis  106  of the substrate  102 . 
     As a further example, the sensing axis  110  of the first piezoresistive sensor  108  is oriented parallel with the orientation of the crystal axis  106  of the substrate  102  (e.g., parallel to the [100] crystal axis). The sensing axis  114  of the second piezoresistive sensor  112  is oriented perpendicular with the orientation of the crystal axis  106  of the substrate  102  (e.g., perpendicular to the [100] crystal axis—or parallel to the [010] axis). In this configuration, the piezoresistive sensor  108  is configured so the change in resistance in the first and second piezoresistive sensors  108  is representative of longitudinal (e.g., compression and tension) forces responsive to the force applied to the substrate  102 . The piezoresistive sensor  112  is configured so the change in resistance in the first and second piezoresistive sensors  108  is representative of normal forces responsive to the force applied to the substrate  102 . Similarly, the sensing axis  118  of the third piezoresistive sensor  116  is oriented neither parallel nor perpendicular to the orientation of the crystal axis  106  (e.g., it is at angle of about ±45° relative to the [100] crystal axis). The third piezoresistive sensor  116  is thus configured such that the change in resistance in the third piezoresistive senor  116  corresponds to shear forces responsive to the force applied to the substrate  102 . 
     To enable the sense circuit  120  to measure the longitudinal, normal and shear forces, as described above, the orientation of the crystal axis  106  of the substrate  102  is oriented either parallel (or transverse) to a longitudinal axis of the mechanical structure to which the IC  100  is mounted. Thus, to measure the longitudinal normal forces applied to the mechanical structure, the sensing axis  110 ,  114  of the first and second piezoresistive sensor  108 ,  112  are oriented parallel and perpendicular to the longitudinal axis of the mechanical structure respectively. To measure the shear force, the sensing axis  118  of the third piezoresistive sensor  116  is neither parallel nor perpendicular (e.g., at an angle of about ±45°) to the longitudinal axis of the mechanical structure. 
     Although, the first, second and, third piezoresistive sensors  108 ,  112 ,  116  are configured to exhibit a change in resistance to mechanical forces along their sensing axes, the piezoresistive sensors  108 ,  112 ,  116  also experience a resistance change due to other forces. For example, the resistance of the respective piezoresistive sensors  108 ,  112 ,  116  is also responsive to forces normal to their respective sensing axes. In an example, each of the piezoresistive sensors  108 ,  112 ,  116  is formed on or in the substrate  102  as a plurality of piezoresistor elements coupled in series between respective terminals. For example, each of the piezoresistive elements is implemented as a doped silicon resistor (e.g., P-type or N-type) depending on the doping of the semiconductor substrate. The plurality of piezoresistor elements includes a first (main) set of piezoresistor elements formed in the substrate having sensing axes parallel to a desired (main) sensing axis for the respective piezoresistive sensor. For example, the main sensing axis is determined based on the longitudinal direction of the piezoresistor element relative to the crystal axis of the semiconductor substrate. A second (e.g., compensation) set of one or more piezoresistor elements are formed in the substrate having sensing axes transverse to the desired (main) sensing axis for the respective piezoresistive sensor  108 ,  112 ,  116 . The second set of piezoresistor element(s) in a respective piezoresistive sensor  108 ,  112 ,  116  is configured to provide a resistance that cancels out changes in resistance due to forces normal to the desired (main) sensing axis of the respective piezoresistive sensor. 
     As a further example, the resistance of the first and second piezoresistive sensors  108 ,  112  is also responsive to shear forces applied at angles that are neither longitudinal nor normal to the sensing axis of the respective. The change in resistance to the first and second piezoresistive sensors  108 ,  112  due to such shear forces can skew the measured resistance of the respective piezoresistive sensors by introducing a change in resistance responsive to the shear forces. This further can result in an inaccurate force being determined along the respective axes  110  and  114 . To compensate for resistance variations due the shear forces and to correct the final value of the measured force, the measured shear force due to the change in resistance of the third piezoresistive sensor  116  is used to compensate for the shear force experienced by the first and second piezoresistive sensors  108 ,  112 . In general, with four differently oriented resistors (as an example at an orientation with respect to the crystal axis of approximately 0°, 90°, +45°, −45°, respectively) one could correct the output of each of the sensors by the output of the other sensors for their inaccuracies if normal and shear stress are present at different directions at the same time. 
     As an example, the IC  100  includes other circuitry  123  implemented on the IC that includes a controller. The controller can be implemented as state machine, a processor core or microcontroller. The controller is configured to determine a value representative of longitudinal and normal force along axes  110 ,  114  responsive to the change of resistance for respective piezoresistive sensors  108 ,  112  (e.g., determined by sense circuit  120 ). The controller is also configured to determine a value representative of shear force along axis  118  responsive to the change of resistance for the sensors  116  (e.g., determined by sense circuit  120 ). In an example, controller is further configured to use the shear force value (due to the third piezoresistive sensor  116 ) to compensate for the shear forces experienced by the first and second piezoresistive sensors  108 ,  112 . As a result, the final values of longitudinal and normal force determined by the controller responsive to a change in resistance of the respective first and second piezoresistive sensors  108 ,  112  may be provided to include only force components aligned with or along the respective sensor axes  110 ,  114 . 
     Similarly, the third piezoresistive sensor  116  includes an arrangement of one or more piezoresistors having a resistance configured to change responsive to shear forces applied to the substrate  100  along the axis  118 . The resistance of the third piezoresistive sensor  116 , however, also changes responsive to longitudinal and normal forces applied to the substrate  100 . The change in resistance of the third piezoresistive sensor  116  due to the longitudinal and normal forces can skew the measured resistance of the respective piezoresistive sensor  116  by introducing a change in its resistance responsive to the longitudinal and normal forces. The influence of the longitudinal and normal forces on the resistance of the piezoresistive sensor  116  can result in an inaccurate shear force being determined along the respective axis  118 . The longitudinal and normal forces determined responsive to changes in resistance of the first and second piezoresistive sensors  108 ,  112  can be used to compensate for the longitudinal and normal forces experienced by the third piezoresistive sensor  116 . For example, the controller is further configured to use the longitudinal and normal force values (from piezoresistive sensors  108 ,  112 ) to compensate for the longitudinal and normal forces experienced by the third piezoresistive sensor  116 . As a result, in such example, the final value of the shear force determined by the controller responsive to a change in resistance of the third piezoresistive sensors  116  may be determined to include only the force component aligned with or along the respective sensor axis  118 . 
     As a further example and still referring to the example of  FIG.  1 A , the IC  100  includes a temperature sensor  122  formed on or in the substrate surface  104 . The temperature sensor has an output coupled to an input of the other circuitry  123 , and is configured to provide a temperature signal representative of a temperature of the substrate  102 . A change in temperature of the IC  100  and hence, the substrate  102  can affect resistance of the piezoresistors in each of the respective piezoresistive sensor  108 ,  112 ,  116 , which may affect the resistance values determined by the sense circuit  120 . Thus, the temperature of the substrate  102  is monitored and a change in substrate temperature is used by other circuitry to correct the normal and shear force values measured by the sense circuit  120 . In addition, temperature compensation of a mismatch between thermal coefficients of expansion of the sensor substrate and the mechanical structure is also used to correct the normal and shear force values measured by the circuit  120 . 
     The other circuitry  123  can also include an arrangement of components depending on the functionality of the IC  100 . For example, the other circuitry includes inductors, capacitors, antennas, A/D converters, microcontrollers, etc. formed on the mounting surface  104  of the substrate  102 , such as for implementing a system on chip (SoC). In an example, the SoC described herein is a multi-chip module (MCM). The MCM can include circuitry/components having different technologies/functions. For example, one or more ICs of the MCM includes strain-sensitive sensors and sensing circuitry/components configured to measure strain. Thus, the strain sensing IC can include analog circuitry and components. One or more other ICs of the MCM includes circuitry (e.g., microcontroller, state machine, processing core(s)) and components configured to perform processing and computing and related control functions. The ICs in the MCM can be coupled together through electrically conductive traces, wires or the like, such as to communicate data and instructions. 
     In another example, some of the circuitry  123  is be implemented on a circuit board to which the IC or SoC is coupled. The other circuitry  123  in the SoC (or MCM) may also be configured to establish a wired or wireless link (e.g., an inductive link, near field communications (NFC), Bluetooth, etc.) for power transfer from external circuitry to the IC  100 . The other circuitry  123  further can be used to establish a wired or wireless communication channel for communicating data between the IC and a remote system. For example, the wireless link may utilize the wireless communication channel to calibrate the circuitry  120  or  123  and/or may to communicate sensor readout values to an external reader system. 
       FIG.  1 B  is a block diagram of an example of resistance stress sensing circuitry that includes the piezoresistive sensor  108  and the sense circuit  120  of  FIG.  1 A . Accordingly, the description of  FIG.  1 B  also refers to  FIG.  1 A . As shown in the example of  FIG.  1 B , the first piezoresistive sensor  108  includes a pair of piezoresistors  124  and  130 . For example, the piezoresistor  124  includes terminals  126  and  128 . The terminal  128  is coupled to an output of the sense circuit  120  and the terminal  126  is coupled to a first input of the sense circuit  120 . The piezoresistor  130  includes terminals  132  and  134 . The terminal  134  is coupled to an output of the sense circuit  120  and the terminal  132  is coupled to a second input of the sense circuit. As described herein, each of the piezoresistors  124  and  130  may include one or more piezoresistor elements coupled between respective terminals  126 ,  128  and  132 ,  134 , in which the piezoresistor elements are formed in the substrate  102  to provide sensitivity to forces applied to substrate. For example, the sensitivity is along the sensing axis [110] or along another sensing axis. The output terminal  126  of the first piezoresistor  123  is coupled to a first input  136  of the sense circuit  120  and the output terminal  132  of the second piezoresistor  130  is coupled to a second input  138  of the sense circuit  120 . 
     In the example of  FIG.  1 B , the piezoresistor  124  is sensing resistor configured to provide a variable resistance (R_SENSE) between terminals  126  and  128 . For example, the resistance R_SENSE is variable responsive to longitudinal forces applied to the substrate along the axis  110 . The piezoresistor  130  is a reference resistor configured to provide a fixed resistance (R_REF) between terminals  132  and  134 . The sense circuit  120  is configured to provide first and second input signals (e.g., a DC input voltage) to the respective terminals  128  and  134 . For example, the first and second input signals may be the same (e.g., where R_SENSE=R_REF) or the first and second input signals may be different (e.g., where R_SENSE≠R_REF). The inputs  136  and  138  of the sense circuit  120  receives output signals from the respective piezoresistors  124 ,  130  based on the input signals provided at  128  and  134 . The sense circuit  120  is configured to provide the sense signal representative of a difference between the signals received at  134  and  136 . Because the resistance R_REF of piezoresistor  130  remains fixed, the sense signal thus is representative of the change in resistance of the piezoresistor  124  responsive to force applied to the substrate  102 . As another example, R_REF is dependent on stress differently as compared to R_SENSE. Thus, the sense signal (difference) is also representative of the change in stress. As described herein, the sense signal then can be used (e.g., by other circuitry  123 ) to determine stress components (e.g., longitudinal, normal and shear components) based on the change in resistance measured by the sense signal. 
     As a further example, the piezoresistor  124  is arranged in a lateral plane parallel to the mounting surface of the substrate  102 . The piezoresistor  130  is formed in the substrate  102  having its sensitivity axis in a direction perpendicular to the lateral plane. Additionally, the reference piezoresistor  130  can be configured to have the same temperature dependency as the associated sensing piezoresistor  124  by forming the respective resistors to have substantially the same dopings. This configuration helps to ensure the sensing and reference piezoresistors  124  and  130  have the same temperature coefficients and respond to temperature changes in substantially the same way, which improves the accuracy in the change in resistance measured over temperature. In another example, different piezoresistors are combined in series where each has a different doping and different material to achieve a total temperature coefficient (TC) which is similar to the TC of R_SENSE and, at the same time, having a different stress coefficient. In this example, the TC cancels, but the stress does not. In still another example, R_REF has the same doping in silicon but has a different orientation (e.g., vertical instead of parallel to the surface). 
     Each of the second and third piezoresistive sensors  112 ,  116  may be configured similarly to the sensor  108  for sensing resistance (responsive to applied mechanical forces) along respective axes  114  and  118 . Also, each of the second and third piezoresistive sensors  112 ,  116  can be coupled to a respective instance of the sense circuit  120  for generating sense signals representative of the change in resistance of the respective piezoresistive sensors  112 ,  116  responsive to force applied to the substrate  102 . 
       FIGS.  2 A and  2 B  depict example of single piezoresistor  200  showing sensitivity to different mechanical stresses. The piezoresistor  200  is useful example of a piezoresistor (e.g., variable piezoresistor  124  of  FIG.  1 B ) that may be implemented to form piezoresistive sensors  108 ,  112  and  116 . In the example of  FIGS.  2 A and  2 B , the forces experienced by the piezoresistor  200  are shown only as longitudinal and transverse (e.g., normal) stresses (in the x- and y-directions). The example single piezoresistor  200  includes resistor components shown as R 1 -R 5 . The five resistor elements R 1 -R 5  are coupled together in series. As described herein, each of the resistors R 1 -R 5  has a first sensitivity in a longitudinal direction and a second sensitivity in the transverse direction, in which the first sensitivity each of the resistor elements are formed on the substrate with axes of sensitivity oriented relative to the crystal axis so that the combined resistance of R 1 -R 5  (e.g., shown as R_SENSE in  FIG.  1 B ) is sensitive to only one direction of stress, namely the longitudinal direction. Each of the resistors R 1 -R 4  are formed in the substrate to have a sensitivity axis parallel to the longitudinal axis. The resistor R 5  is formed in the substrate to have a sensitivity axis transverse (normal) to the longitudinal axis. Thus, the piezoresistor  200  is a single-direction (e.g., uniaxial) sensing resistor formed in a semiconductor substrate with a sensing axis having a specific orientation relative to the crystal axis of the substrate. 
     In the example of  FIGS.  2 A and  2 B , the piezoresistor  200  is a longitudinal stress sensor that provides a variable resistance that is responsive to only longitudinal stress (along) (in the y-direction). The piezoresistor  200  is internally configured to cancel the effects of transverse stress (σtrans). As an example, each resistor element R 1 -R 4  of the piezoresistor  200  has a sensitivity to transverse stress (σtrans) that is approximately 1%/100 MPA, which totals 4%/100 MPA for the combination of R 1 -R 4 . The resistor element R 5  of piezoresistor  200  has an individual sensitivity to transverse stress (σtrans) that is approximately negative 4%/100 MPa. For example, the equation below demonstrates the total sensitivity to transverse (normal) stress (σtrans) in the x-direction of 0%/100 MPa. 
                 dR     t   ⁢   o   ⁢   t           R   0     ⁢   d   ⁢   σ       =           1   5     ⁢     (         +   1     ⁢   %     +     1   ⁢   %     +     1   ⁢   %     +     1   ⁢   %     -     4   ⁢   %       )         100   ⁢         MPa       =     0   ⁢     %   /   100     ⁢         MPa             
Thus, the piezoresistor  200  has no sensitivity to transverse stresses in the x-direction.
 
     As described, the piezoresistor  200  is configured to be sensitive to longitudinal stress (along) in the y-direction. R 1 -R 4  are formed in the substrate to provide current flow through piezoresistor elements in a direction extending along or parallel to the y-direction (e.g., the longitudinal sensing axis). In the example of  FIG.  2 B , the piezoresistor  200  has a net sensitivity of −3%/100 MPa in the longitudinal direction. For example, each resistor element R 1 -R 4  has an individual sensitivity of about negative 4%/100 MPA to longitudinal stress (along) in the y-direction. Additionally, the resistor element R 5  of piezoresistor  200  has an individual sensitivity to longitudinal stress (along) that is approximately positive 1%/100 MPa. For example, the equation below demonstrates the total sensitivity to longitudinal (normal) stress (along) in the y-direction of approximately −3%/100 MPa. 
                 dR     t   ⁢   o   ⁢   t           R   0     ⁢   d   ⁢   σ       =           1   5     ⁢     (         -   4     ⁢   %     -     4   ⁢   %     -     4   ⁢   %     -     4   ⁢   %     +     1   ⁢   %       )         100   ⁢         MPa       =       -   3     ⁢     %   /   100     ⁢         MPa             
Therefore, the overall sensitivity of the example piezoresistor  200  to longitudinal stress is approximately −3%/100 MPa.
 
     In this example, in order to cancel stresses in the x-direction so that the piezoresistor  200  is a y-normal stress sensor, the ratio of resistor components in the y-direction to the x-direction is 4:1. The ratio however, can change (e.g., 1.5:1, 2:1, 3:1, 5:1, etc.) based on the type of crystal substrate, the dimensions of the respective resistor element the mounting orientation of the IC package, the type of application, etc. Also, in other examples, different numbers of resistor elements may be used to form the piezoresistor  200 . 
       FIG.  3    is an example of a stress sensing circuit  300  for an IC having a piezoresistive sensor configured to be responsive to normal or longitudinal stress, such as due to force applied parallel or normal to the crystal axis of the substrate. The circuit  300  includes a voltage source  301  having outputs coupled to respective terminals  302  and  304  of first and second piezoresistors  306 ,  308 . Each of the piezoresistors  306 ,  308  has another terminal coupled to electrical ground. An amplifier (e.g., an operational amplifier)  310  has first and second inputs  312  and  314  and an output  316 . The first input  312  is coupled to the inputs  302  of piezoresistor  306 , and the second input  314  is coupled to the inputs  304  of piezoresistor  308 . The piezoresistors  306 ,  308  form a piezoresistive sensor, such as to implement piezoresistive sensors  110  and  112  in the IC  100  of  FIG.  1   . 
     As an example, the voltage source  301  is configured to provide a DC voltage (VDD) that provides constant current, shown as  320  and  322 , to each of the piezoresistors  306 ,  308 . The current through the piezoresistors  306 ,  308  thus provides a voltage across each piezoresistor  306 ,  308  that varies based on its resistance. The amplifier  310  is configured to provide a sense signal at the output  316  representative of a difference between the voltage at inputs  302  and  304 . For example, one of the piezoresistors  308  is configured as described with respect to  FIGS.  2 A and  2 B  to have a highest sensitivity to stress in the longitudinal direction. The other piezoresistor  306  has a fixed resistance (no stress dependency). As described herein, both piezoresistors  306 ,  308  have the same temperature dependence. The sense circuit  300  thus outputs a sense signal (a voltage signal) that is representative of a difference in resistance between the first and second piezoresistors  306 ,  308 . In an example, the sense signal at  316  is proportional to longitudinal stress applied to the piezoresistive sensor  318 , which includes piezoresistors  306 ,  308 . 
       FIG.  4    is a graph showing a change in the resistance value of the piezoresistors  312 ,  314  as the stress (σ long ) increases from 0 MPa to 100 MPa. For example, the difference in resistance of the piezoresistors  312 ,  314  changes by approximately −7% as the force increases from 0 MPa to 100 MPa. As shown in  FIG.  4   , in this example the change in resistance is inversely linearly proportional to longitudinal stress. In other examples, the change in resistance may also be directly proportional based on the sign of the longitudinal and transverse stress coefficients (see  FIG.  2 B ), which in turn depends on the type of material, type of doping, etc. As described above, the sense circuit  300  outputs the voltage that is proportional to the change in resistance responsive to longitudinal stress. 
       FIG.  5    is an example of another stress sensing circuit  500  for an IC having piezoresistive sensor  502  configured to be responsive to shear stress (σ shear ) due to components of mechanical force applied parallel to the crystal axis of the substrate. The circuit  500  is the same as the circuit  300  except for the orientation of respective resistor elements (e.g., doped silicon resistors) in the piezoresistive sensor  502 . The circuit  500  includes a sense circuitry  504  including a voltage source  506  having outputs coupled to respective terminals  508  and  510  of first and second piezoresistors  512 ,  514  of the piezoresistive sensor  502 . Each of the piezoresistors  512 ,  514  has another terminal coupled to electrical ground. An amplifier  516  has first and second inputs  518  and  520  coupled to respective inputs  508  and  510 . Amplifier  516  also includes an output  522 . 
     Similar to as described with respect to  FIG.  3   , the voltage source  506  is configured to provide a DC voltage (VDD) that provides constant current, shown as  524  and  526 , to each of the piezoresistors  512 ,  514 . The current through the piezoresistors  512 ,  514  thus causes a voltage across each piezoresistor  512 ,  514  that varies proportional to its resistance. Unlike the example of  FIG.  3   , each of the piezoresistors  512  and  514  is formed in the substrate with a highest sensitivity to shear stress. For example, piezoresistor  512  has an axis of highest sensitivity oriented at approximately −45 degrees relative to the crystal axis and piezoresistor  514  has an axis of highest sensitivity oriented at approximately +45 degrees relative to the crystal axis. Thus, in this example, the piezoresistors  512  and  514  are oriented with orthogonal axes of highest sensitivity (90 degrees apart). 
     The amplifier  516  is configured to provide a sense signal at the output  522  representative of a change in resistance between piezoresistors  512 ,  514 . The amplifier  516  of the sense circuit  504  thus outputs a sense signal (a voltage signal) that is representative of a difference in resistance between the first and second piezoresistors  512 ,  514 . In an example, the sense signal at  522  is proportional to shear stress applied to substrate of the IC implementing the piezoresistive sensor  502 . 
       FIG.  6    is a graph showing a change in the resistance value of the piezoresistors  512 ,  514  as the stress (σ shear ) increases from 0 MPa to 110 MPa. For example, the difference in resistance of the piezoresistors  512 ,  514  changes by approximately −X as the force increases from 0 MPa to 100 MPa. As shown in  FIG.  6   , in this example the change in resistance is inversely linearly proportional to shear stress. In other examples, the change in resistance may also be directly proportional based on the sign of the longitudinal and transverse stress coefficients (see  FIG.  2 B ), which in turn depends on the type of material, type of doping, etc. As described above, the sense circuit  500  outputs the voltage that is proportional to the change in resistance responsive to shear stress. 
       FIGS.  7  and  8    show examples of SoCs  700 ,  800  that may be implemented using the IC  100  to provide respective stress sensing systems. The SoC is adapted to be coupled to mechanical structure to measure mechanical stress of the structure, including normal stress (in one or more directions) and shear stress. As described herein, the IC includes a substrate  102  having integrated with piezoresistive sensors (e.g., sensors  108 ,  112 ,  116 ) formed on or in a substrate surface. The piezoresistive sensors are configured to change resistance responsive to a force applied to the IC  100 . The IC  100  also includes circuitry (e.g., sense circuit  120 ) configured to measure a change in resistance that is representative of (e.g., proportional to) stress components (e.g., normal and/or shear stress). In another example, the SoC  700 ,  800  is implemented as a multi-chip module (MCM) that includes multiple ICs (including one or more instance of the IC  100 ) configured to perform respective functions, as described herein. 
     In the example of  FIG.  7   , the SoC  700  includes other components and/or circuitry  702  coupled to the IC  100 . The SoC  700  includes a packaging material (e.g., epoxy molding compound, magnetic molding compound, polyimide, metal, plastic, glass, ceramic, etc.)  704  that encapsulates the IC  100  and the components/circuitry  702  mounted therein. The other components and/or circuitry  702  can include inductors, capacitors, antennas, A/D converters, microcontrollers, and the like. The other components and/or circuitry  702  is either integrated into the substrate  102  and part of the IC or mounted to a mounting surface  706  of the substrate  102  via interconnects (e.g., solder bumps)  708 . In the example of  FIG.  7   , an antenna  710  is shown as being mounted to the mounting surface  706  of the substrate  102 . Alternatively, the antenna  710  can be an external antenna. For example, the SoC  700  can include one or more pins  712  configured to couple such external (or other external components) to the substrate  102 . Thus, the other components/circuitry can be either inside the package  700  or outside connected through a set of pins coupled to the internal circuitry and/or components. The pins  712  can provide communication (e.g., signal readout, analog/digital signals, sensor signals, etc.) to and from external components. The SoC  700  can include a coupling layer (e.g., metal layer, such as including a leadframe).  714  attached to a contact surface (surface opposite that of the mounting surface  706 ) of the IC for the purpose of attaching the SoC  700  to a respective surface  716  of a mechanical structure  718 . Alternatively, the coupling layer  714  can be omitted and the contact surface of the substrate  102  can mount directly to the prepared surface  716  of the mechanical structure  718 . 
       FIG.  8    is another example of an SoC  800  that includes the IC  100  having a monolithic, single crystal substrate  102 , as described above. The SoC  800  include similar components of the SoC  700  of  FIG.  7   . Thus, the description of  FIG.  8    also refers to  FIG.  7   . The SoC  800  further includes an interposer layer (e.g., printed circuit board (PCB))  802  mounted on the mounting surface  706  of the substrate  102  via the interconnects  708 . The other components/circuitry  702  and antenna  710  are mounted to the interposer (PCB)  802  and coupled to the substrate  102  through interconnects formed in the interposer  802 . In another example, the other components/circuitry  702  and the antenna  710  along with the interposer  802  can be an additional PCB board with its own function, integrated into one package. 
     As a further example,  FIGS.  9 - 13    illustrate an example mounting orientation for an SoC  900 . The SoC  900  may be implemented by SoC  700 ,  800 , which includes the IC  100 , as described above.  FIG.  9    illustrates the SoC  900  mounted to a surface of a mechanical structure (e.g., a shaft, beam, feather key or the like)  902 . In an example, more than one SoC may be mounted to the mechanical structure  902 . Alternatively, as described herein, the IC  100  itself may be used as the SoC and mounted directly to the mechanical structure  902 . The SoC or IC may be coupled to the mechanical structure  902  through an adhesive, clamp, metallic joint, feather key. 
       FIGS.  10 - 13    illustrate the orientation of an example SoC  900  (or IC  100 ) having an arrangement of piezoresistive sensors when coupled to a mechanical structure to measure the mechanical stress of the mechanical structure, as described herein. In the examples of  FIGS.  10 - 13   , the piezoresistive sensors implemented on the IC  100  each has a sensing axis having a respective orientation relative to the crystal axis of the semiconductor substrate  102 . In another example, the arrangement of the piezoresistive sensors can be implemented on the IC  100  so that the respective sensing axis of the piezoresistive sensors are oriented relative to the crystal axis of wafers cut in different respective crystal planes (e.g., cut in the [100], [110], [111] or other crystal plane). Thus, depending on the orientation of the crystal axis of the substrate, the IC  100  and SoC  900  can be coupled to the mechanical structure at an orientation to align the highest sensitivity axes of respective piezoresistive sensors with the force components to be measured on the mechanical structure  902 . For example, each of the normal and shear piezoresistive sensors would be aligned so as to measure longitudinal, normal and shear forces applied to or experienced by the mechanical structure. An aligned marking may be printed on the IC  100  or SoC such as to show the direction of longitudinal sensing (e.g., parallel with the crystal axis of the substrate  102 ). In another example, the SoC  900  is implemented as an MCM that includes multiple ICs (including one or more instances of the IC  100 ) configured to perform respective functions, such as described herein. 
       FIGS.  10  and  11    illustrate an example IC (or SoC)  1000  coupled to a surface of a mechanical structure  1002 . In the example of  FIGS.  10  and  11   , the IC  1000  includes piezoresistive sensors  1004 ,  1006  and  1008  formed on a [100] semiconductor substrate having crystal axis extending through the IC shown at  1010 . For example, the piezoresistive sensors  1004 ,  1006  and  1008  may be implemented by piezoresistive sensors  108 ,  112 ,  116 , each including a respective pair of piezoresistors, as described herein. 
       FIG.  10    shows the orientation of normal piezoresistive sensors  1004  and  1006  having respective longitudinal sensing axes  1012  and  1014  in the x-direction and y-direction, respectively. For example, piezoresistive sensor (having a variable resistance R 0° )  1004  is an x-normal stress sensor having its longitudinal sensing axis oriented parallel with respect to the x-direction parallel to the axis  1010 . The other normal piezoresistive sensor (having a variable resistance R 90° )  1006  is an y-normal stress sensor having its longitudinal sensing axis oriented parallel with respect to the y-direction perpendicular to the axis  1010 . 
       FIG.  11    separately shows piezoresistive sensor  1008  as including two piezoresistors, shown as R 45° , R −45° . The piezoresistive sensor  1008  is a shear stress sensor having a sensing axis  1016  oriented neither parallel nor perpendicular with respect to the axis  1010 . In an example, the piezoresistive sensor  1008  includes piezoresistors, shown as R 45° , R −45° , which are oriented at approximately +45° and −45° relative to the axis  1010  (in the x-direction) of the substrate of the IC  1000 . 
     In the example of  FIGS.  10  and  11   , the axis  1010  of the substrate of the IC  1000  is parallel with a longitudinal axis  1018  of the mechanical structure  1002 . Therefore, the sensing axis  1012  of the piezoresistor  1004  is parallel with a longitudinal axis  1018  of the mechanical structure  1002  and the sensing axis  1014  of the piezoresistor  1006  is perpendicular with the longitudinal axis  1018  of the mechanical structure  1002 . Additionally, the sensing axes  1016  of the piezoresistive sensor  1008  are oriented at ±45° relative to the longitudinal axis  1018  of the mechanical structure  1002 . The piezoresistors  1004 ,  1006 , which have sensing axes  1012 ,  1014  parallel and perpendicular, respectively, to the longitudinal axis  1018  of the mechanical structure  1002 , are configured to measure normal forces along the surface of the mechanical structure. The piezoresistive sensor  1008  is configured to measure shear forces along the surface of the mechanical structure. 
       FIGS.  12  and  13    illustrate an example IC (or SoC)  1200  coupled to a surface of a mechanical structure  1202 . In the example of  FIGS.  12  and  13   , the IC  1200  includes piezoresistive sensors  1204 ,  1206  and  1208  formed on a [110] semiconductor substrate having a [100] crystal axis extending through the IC  1200  shown at  1210 . For example, the piezoresistive sensors  1204 ,  1206  and  1208  may be implemented by piezoresistive sensors  108 ,  112 ,  116 , each including a respective pair of piezoresistors, as described herein. 
       FIG.  12    shows the orientation of normal piezoresistive sensors  1204  and  1206  having respective longitudinal sensing axes  1212  and  1214  that are offset from the substrate orientation (substrate axis)  1210  by approximately 45°, as shown. For example, piezoresistive sensor (having a variable resistance R −45° )  1204  has its longitudinal sensing axis  1212  oriented at −45° relative to the substrate orientation and along [100] of the substrate and parallel to the axis  1210 . Thus, the piezoresistive sensor  1204  is configured to sensing stress in a direction that is parallel to an axis  1218  of the mechanical structure  1202 . The other normal piezoresistive sensor (having a variable resistance R 45° )  1206  is a y-normal stress sensor having its longitudinal sensing axis  1214  oriented at +45° relative to the crystal axis and along [010] of the crystal and perpendicular to the axis  1210 . The piezoresistive sensor  1206  is thus configured to sensing normal stress in a direction that is perpendicular to the axis  1218  mechanical structure. 
       FIG.  13    separately shows piezoresistive sensor  1208  as including two piezoresistors, shown as R 90° , R 0° . The piezoresistive sensor  1208  is a shear stress sensor having its resistors oriented parallel and perpendicular with respect to the substrate orientation and +/−45° with respect to the axis  1210 . In an example, the piezoresistive sensor  1208  includes piezoresistors, shown as R 0° , R 90° , which are oriented at approximately 45° and −45° relative to the axis  1210 , namely along [110] and [−110] of the substrate of IC  1200 . 
     In the example of  FIGS.  12  and  13   , the axis  1210  of the substrate for the sensors  1204 ,  1206  is aligned parallel with a longitudinal axis  1218  of the mechanical structure  1202  by rotating the IC  1200  by 45°, as shown (compared to  FIGS.  10  and  11   ). The rotation of the IC  1200  results in the sensing axis  1212  of the piezoresistor  1204  being parallel with the longitudinal axis  1218  of the mechanical structure  1202  and the sensing axis  1214  of the piezoresistor  1206  being perpendicular with the longitudinal axis  1218  of the mechanical structure  1202 . Additionally, a sensing axes  1216  of the piezoresistive sensor  1208  are oriented at ±45° relative to the longitudinal axis  1218  of the mechanical structure  1202 . The piezoresistors  1204 ,  1206  thus are configured to measure normal forces along the surface of the mechanical structure. The piezoresistive sensor  1208  is configured to measure shear forces along the surface of the mechanical structure  1202 . 
       FIGS.  14 - 18    illustrate another example mounting orientation for a SoC  1400 . The SoC  1400  may be implemented according to the example SoCs  700 ,  800  of  FIGS.  7  and  8   , as described above, each of which includes the IC  100 .  FIG.  14    illustrates the SoC  1400  mounted in a cutout or slit  1402  perpendicular to a surface of a mechanical structure  1404  (e.g., a shaft, beam or the like). In another examples, the SoC  1400  is mounted on a sidewall of a carrier material (e.g. on the side of a feather key structure), which can be mounted in a keyway or indentation. 
       FIGS.  14 - 18    illustrate orientations of the SoCs and respective piezoresistive sensors with respect to the mechanical structure to measure longitudinal, normal and shear mechanical forces. As described herein, the piezoresistive sensors implemented on the IC  100  each has a sensing axis having a respective orientation relative to the crystal axis of the semiconductor substrate  102 . Thus, depending on the orientation of the crystal axis of the substrate, the IC  100  and SoC  1400  should be coupled to the mechanical structure  1404  at an orientation to align the highest sensitivity axes of respective piezoresistive sensors with the force components to be measured on the mechanical structure  1404 . For example, each of the normal and shear piezoresistive sensors would be aligned so as to measure longitudinal, normal and shear forces applied to or experienced by the mechanical structure  1404 . An aligned marking may be printed on the IC  100  or SoC such as to show the direction of longitudinal sensing (e.g., parallel with the crystal axis of the substrate  102 ). In an example, the SoCs of  FIGS.  14 - 18    are implemented as a MCMs that include multiple ICs (including one or more strain sensing IC) configured to perform respective functions, such as described herein. 
       FIGS.  15  and  16    illustrate an example IC (or SoC)  1500  coupled to in a slot  1501  of a mechanical structure  1502 . The IC  1500  includes piezoresistive sensors  1504 ,  1506  and  1508  formed on a [100] semiconductor substrate having crystal axis extending through the SoC shown at  1510 . For example, the piezoresistive sensors  1504 ,  1506  and  1508  may be implemented by piezoresistive sensors  108 ,  112 ,  116 , each including a respective pair of piezoresistors, as described herein. 
       FIG.  15    shows the orientation of normal piezoresistive sensors  1504  and  1506  having respective longitudinal sensing axes  1512  and  1514  in the x-direction and y-direction, respectively. For example, piezoresistive sensor (having a variable resistance R 0° )  1504  is an x-normal stress sensor having its longitudinal sensing axis oriented parallel with respect to the x-direction parallel to the axis  1510 . The other normal piezoresistive sensor (having a variable resistance R 90° )  1506  is an y-normal stress sensor having its longitudinal sensing axis oriented parallel with respect to the y-direction perpendicular to the axis  1510 . 
       FIG.  16    separately shows piezoresistive sensor  1508  as including two piezoresistors, shown as R 45° , R −45° . The piezoresistive sensor  1508  is a shear stress sensor having a sensing axis  1516  oriented neither parallel nor perpendicular with respect to the axis  1510 . In an example, the piezoresistive sensor  1508  includes piezoresistors, shown as R 45° , R −45° , which are oriented at approximately +45° and −45° relative to the axis  1510  (in the x-direction) of the substrate of IC  1500 . 
     In the example of  FIGS.  15  and  16   , the axis  1510  of the substrate of IC  1500  is parallel with a longitudinal axis  1518  of the mechanical structure  1502 . Therefore, the sensing axis  1512  of the piezoresistor  1504  is parallel with the longitudinal axis  1518  of the mechanical structure  1502  and the sensing axis  1514  of the piezoresistor  1506  is perpendicular with the longitudinal axis  1518  of the mechanical structure  1502 . Additionally, the sensing axes  1016  of the piezoresistive sensor  1508  are oriented at ±45° relative to the longitudinal axis  1518  of the mechanical structure  1502 . Thus, the piezoresistors  1504 ,  1506  are configured to measure normal forces and the piezoresistive sensor  1508  is configured to measure shear forces along the mechanical structure within the substrate. This translates to measuring torque and bending of the mechanical structure, respectively 
       FIGS.  17  and  18    illustrate an example IC (SoC)  1700  coupled in a slot  1701  of a mechanical structure  1702 . In the example of  FIGS.  17  and  18   , the IC  1700  includes piezoresistive sensors  1704 ,  1706  and  1708  formed on a [110] semiconductor substrate having a crystal axis extending through the IC  1700  shown at  1710 . For example, the piezoresistive sensors  1704 ,  1706  and  1708  may be implemented by piezoresistive sensors  108 ,  112 ,  116 , each including a respective pair of piezoresistors, as described herein. 
       FIG.  17    shows the orientation of normal piezoresistive sensors  1704  and  1706  having respective longitudinal sensing axes  1712  and  1714  that are offset from the substrate orientation  1710  by approximately 45°, as shown. For example, piezoresistive sensor (having a variable resistance R −45° )  1704  has its longitudinal sensing axis  1712  oriented at −45° relative to the substrate orientation and along [100] of the crystal and parallel to the axis  1710 . Thus, the piezoresistive sensor  1704  is configured to sense normal stress in the substrate which is representative for torque and/or bending in the mechanical structure  1702 . The other normal piezoresistive sensor (having a variable resistance R 45° )  1706  is a y-normal stress sensor having its longitudinal sensing axis  1714  oriented at +45° relative to the substrate orientation (along [010] of the crystal) and perpendicular to the axis  1710 . The piezoresistive sensor  1706  is thus configured to sensing normal stress in a direction that is perpendicular to the axis  1718  of the mechanical structure  1702 . 
       FIG.  18    separately shows piezoresistive sensor  1708  as including two piezoresistors, shown as R 90° , R 0° . The piezoresistive sensor  1708  is a shear stress sensor having its resistors oriented parallel and perpendicular with respect to the substrate orientation and +/−45° with respect to the axis  1710 . In an example, the piezoresistive sensor  1708  includes piezoresistors, shown as R 0° , R 90° , which are oriented at approximately 90° and 0° relative to the substrate orientation and +/−45° relative to the axis  1710 , namely along [010] and [100] of the substrate of IC  1700 . 
     In the example of  FIGS.  17  and  18   , the crystal axis  1710  on the substrate of IC  1700  is aligned parallel with a longitudinal axis  1718  of the mechanical structure  1702  by rotating the IC  1700  by 45°, as shown (compared to  FIGS.  15  and  16   ). The rotation of the IC  1700  results in the sensing axis  1712  of the piezoresistor  1704  being parallel with the longitudinal axis  1718  of the mechanical structure  1702  and the sensing axis  1714  of the piezoresistor  1706  being perpendicular with the longitudinal axis  1718  of the mechanical structure  1702 . Additionally, a sensing axes  1716  of the piezoresistive sensor  1708  are oriented at ±45° relative to the longitudinal axis  1718  of the mechanical structure  1702 . The piezoresistors  1704 ,  1706  thus are configured to measure normal forces and the piezoresistive sensor  1708  is configured to measure shear forces along the mechanical structure  1702 . 
       FIG.  19    illustrates an example stress sensing system  1900  that includes an IC  1902 . In an example, the stress sensing system  1900  The IC  1902  includes a substrate and an arrangement of piezoresistive sensors, such as sensors  108 ,  112 ,  116  or otherwise described herein. The IC  1902  may be configured to measure stress along multiple directions, including normal stress along perpendicular sensing axes and shear stress. The stress sensing system  1900  thus may be referred to as a torque sensor system. 
     In the example of  FIG.  19   , the torque sensor system  1900  includes a printed circuit board (PCB)  1904  coupled to a mounting surface  1906  of the IC  1902  through interconnects (e.g., solder balls)  1908 . The torque sensor system  1900  can include a coupling (metal) layer  1910  coupled to a contact surface  1912  of the IC  1902 , in which the coupling layer  1910  is adapted to be coupled to a mechanical structure. Alternatively, the contact surface  1912  can be adapted to attach directly to the mechanical structure, such as by an adhesive, clamp, metallic joint, keyed joint. A communication device (e.g., transponder coil)  1914  is attached to an opposite side of the PCB  1904  and is configured to provide a wireless interface to transfer power and data between the IC  1902  and external electronic circuitry. The system  1900  may also include a microcontroller (e.g., one or more of microprocessor, digital signal processor, FPGA or the like)  1916  attached to the opposite side of the PCB  1904 . For example, the microcontroller is configured to function as readout circuit to convert the measured change in resistance (from piezoresistive sensors) into respective stress and/or torque measurements. The microcontroller  1916  can also further process signals from the IC such as to implement additional temperature compensation or determine a condition of the mechanical structure to which the system  1900  is coupled. The microcontroller  1916  can also control the communication device  1914 , such as to communicate sensor data between the IC  1902  and external circuitry through wires or through a wireless communication protocol (e.g., NFC, ZigBee, Bluetooth, etc.). 
       FIGS.  20  and  21    illustrate an example application of the torque sensor system  1900  of  FIG.  19    coupled to a mechanical structure  2000 . The mechanical structure  2000  includes a fixed support  2002  at one end, and a flexible support  2004  at an opposite end. In an alternative example, both ends of the mechanical structure  2000  are fixed. A metal beam cantilever  2006  has a fixed end  2008  secured in the fixed support  2002  and a movable end  2010  disposed in the flexible support  2004 . Alternatively, the metal beam  2006  can be fixed along a side surface of the mechanical structure  2000 . The torque sensor system  1900  is coupled to a surface of the metal beam cantilever  2006  through the coupling layer  1910 , such as described herein. The IC  1902  is located on the cantilever surface adjacent the fixed end  2002  of the cantilever  2006 . In an example, when a normal force NF is applied to the mechanical structure  2000 , the movable end  2010  of the metal beam cantilever  2006  is deflected as shown in  FIG.  21   . The deflection causes the metal beam cantilever  2006  to bend thereby imparting stress on the contact surface of the IC  1902  of the torque sensor system  1900  responsive to the bending. As described above, the stress can be determined by measuring the change in resistance of the piezoresistors implemented on the IC  1902  of the torque sensor system  1900 . 
       FIGS.  22 ,  23  and  24    illustrate another example application of the torque sensor system  1900  of  FIG.  19    coupled to another mechanical structure  2200 . As shown in  FIGS.  22  and  23   , the torque sensor system  1900  is mounted in a cut-out area  2202  of the mechanical structure  2200  and oriented to detect stresses along respective sensing axes (see, e.g.,  FIGS.  14 - 18   ). The mechanical structure  2200  includes a movable shaft  2204 , a fixed support  2206  at one end, and a flexible support  2208  at an opposite end. A metal beam cantilever  2210  has a fixed end  2212  secured in the fixed support  2206  and another end  2214  disposed in the flexible support  2208  that is moveable responsive to applied torque to the shaft  2204 . The torque sensor system  1900  is coupled to the metal beam cantilever  2210  via the coupling layer  1910 . As shown in  FIG.  23   , when a torque force TF (see  FIG.  23   ) is applied to the mechanical structure  2200 , the torque force twists the shaft  2204  about its longitudinal axis. The installation of the sensor torque system  1900  to extend along a radial direction of the shaft  2204  (see, e.g.,  FIG.  14   ) translates the torque into respective stress components similar to the stress on the metal beam cantilever in the example of  FIGS.  20  and  21   . For example responsive to the torque force TF, the end  2214  of the metal beam cantilever  2008  deflects, which causes the metal beam cantilever  2008  to bend thereby inflicting stress on the surface of the IC substrate that is coupled to the cantilever. As described above, the stress can be determined by measuring the change in resistance of the piezoresistors implemented on the IC  1902  in the torque sensor system  1900 . 
       FIG.  24    illustrates an example of wireless communication between the torque sensor system  1900  and external system. For example, a transmitter coil (e.g., antenna)  2216  is wound around a housing  2218  of the movable shaft  2204 . The housing  2218  thus remains static, as compared to the movable shaft  2204 , responsive to force applied to the movable shaft  2204  An electric field  2220  is generated by the communication device (e.g., transponder coil)  1914  transfers data signals  2222  to the transmitter coil  2216 , which in turn communicates to the external reader systems. Additionally, external circuitry may be configured to provide a wireless power signal can be provided to the transmitter coil  2216  that is received by the communication device  1914 . The received power signal can be harvested, such as by converting it to electrical energy for storage in a battery (or other energy storage element) implemented on the PCB  1904 . Alternatively, the communication device (e.g., transponder coil)  1914  can be wound around the movable shaft  2204  and the transmitter coil  2216  can be placed on the shaft housing  2218 . In still yet another configuration, the communication device (e.g., transponder coil)  1914  can be wound around the movable shaft  2204  and the transmitter coil  2216  can be wound around the shaft housing  2218 . In addition, the wireless communication system can include multiple antennas (e.g. four transmitter antennas statically placed in 90° orientation to the shaft axis on the shaft housing and one transponder antenna freely rotating on the shaft or vice versa). Still further, there can be multiple transmitter and transponder antennas placed on the shaft inside the housing. The antennas do not need to be wound entirely around either shaft or shaft housing. 
       FIGS.  25  and  26    illustrate another example torque sensor system  2500  that includes an IC, such as the IC  100  described herein. The torque sensor system  2500  includes a substrate  2502  having a sensor area  2504 , which includes an arrangement of piezoresistive sensors as described herein. In the example of  FIGS.  25  and  26   , the substrate  2502  (e.g., a monolithic, single crystal substrate) functions as the carrier (i.e., the metal beam in the above examples) in a mechanical structure. The system  2500  can also include a printed circuit board (PCB)  2506  attached to a mounting surface  2508  of the substrate  2502  through interconnects (e.g., solder balls)  2510 . Polymer pillars  2512  are provided between the PCB  2506  and the substrate  2502  to maintain a uniform height of the PCB  2506  with respect to a surface of the substrate  2502 . A coupling layer (e.g., lead-frame)  2514  is attached to a contact surface  2516  of the substrate  2502 . The coupling layer  2514  couples the substrate  2502  to the mechanical structure. In the example of  FIGS.  25  and  26   , a communication device (e.g., transponder/power coil)  2518  is coupled to an opposite side of the PCB  2506  and provides an interface to transfer power and/or data between the system  2500  and external circuitry. A microcontroller  2520  may also be coupled to the opposite side of the PCB  2506 . As described the microcontroller can be configured to function as a readout circuit, to further process sense signals, and/or control can communication between the system  2500  and external circuitry through a wireless protocol. In another example, some or all of the circuitry coupled to the PCB  2506  may be implemented in the IC or otherwise coupled to the IC in an SoC. 
       FIG.  27    illustrates an example application of the torque sensor system  2500  of  FIG.  25    coupled to a mechanical structure  2700 . The mechanical structure  2700  includes a housing  2702 . The housing  2702  includes a fixed support  2706  at one end of the housing  2702  that secures the coupling layer  2514  and one end of the substrate  2502 . The housing  2702  also includes a non-fixed support  2708  at an opposite end of the housing  2702 . The coupling layer  2514  and an opposite end of the substrate  2502  are disposed in the non-fixed support  2708 . The non-fixed support  2708  allows for movement of the carrier (i.e., the substrate  2502 ) in a thrust direction TD but not in a direction perpendicular to a plane of the substrate  2502 . Thus, the torque sensor  2500  thus may be configured to measure stresses responsive to any force that displaces the substrate  2502  in a thrust direction. 
       FIG.  28    is a cross-sectional view of an example reference piezoresistor  2800 . The piezoresistor  2800  is a useful example of the reference piezoresistor  130  shown in  FIG.  1 B . The piezoresistor  2800  can be an n-type or alternatively a p-type resistor, depending on the type of dopant used to form the semiconductor substrate. A deep well  2801  is implanted with the opposite conductivity type of dopant into a doped substrate  2802 . The substrate  2802  can include an epitaxial layer (not specifically shown). The deep well  2801  forms a buried layer and is highly doped to promote current flow and exhibit low resistance. Trenches  2804  are side wall doped deep trenches contacting opposite ends of the deep well  2801 , and are highly doped for horizontal current flow and lower doped for vertical current flow. This causes the trenches  2804  to have a first piezo-resistive coefficient for current flow in lateral directions and a second, higher piezo-resistive coefficient for current flow in vertical directions. 
     Still referring to  FIG.  28   , wells  2806  are implanted into the surface of substrate  2802  to contact the trenches  2804 , followed by the implantation of a well  2808  having a second opposite conductivity. A dielectric layer  2810  is then formed to cover the surface of substrate  2802 . Contacts  2812  having the first conductivity type (e.g., N-type) are implanted in the wells  2806  (e.g., N-wells), and a contact  2814  having the second opposite conductivity (e.g., P-type) is implanted in the well  2808  having the second conductivity. An intermediate dielectric  2816  is deposited over the dielectric layer  2810 , and vias  2818  are formed through the intermediate dielectric layer  2816  to the contacts  530  and the contact  525 . A metallization layer  2820  is formed over vias  2818 . During operation, as shown in  FIG.  28   , current  2822  flows from one well  2806  via the trenches  2804  and the deep well  2800  and up through the other well  2806 . 
       FIG.  29    is a cross-sectional view of an example sensing piezoresistor  2900 . The piezoresistor  2900  is a useful example a resistor element of the sensing piezoresistor  124  shown in  FIG.  1 B . For example, one or more sensing piezoresistor  2900  may be used to implement the sensing piezoresistor  124  and combined with the piezoresistor  2800  to form a piezoresistive sensor. 
     In the example of  FIG.  29   , the sensing piezoresistor  2900  is formed on a substrate  2801  (e.g., on the same substrate as piezoresistor  2800 ). The sensing piezoresistor  2900  may be formed as P-type of N-type diffusion resistor on a top surface of the substrate and be oriented to a particular crystal axis (e.g., oriented along [100], [010] or [110]). A buried layer  2904  is formed on in the substrate  2902 . For example, the buried layer  2904  is formed by implanting dopants of a first conductivity type (e.g., N-type or P-type dopants). An epitaxial layer  2906  is formed over the buried layer  2904 . Another buried layer  2908  is formed into the epitaxial layer  2906 . For example, the buried layer  2908  is formed by implanting dopants having the opposite conductivity type as the buried layer  2904 . A doped well region  2910  is formed in the buried layer  2908 , such as by implanting dopants having the same conductivity type as the buried layer  2908  in which it is formed. Additional doped well region  2912  are formed on opposite sides of the well region  2910 . The well regions are formed by implanting dopants having the opposite conductivity as the well region  2910 , thus forming respective junctions between the respective well regions  2912  and  2910 . For example, an N-type piezo resistor may be formed by implant N-type dopants to form well  2912  and P-type dopants for the well  2910 . 
     A shallow trench isolation structure  2914  may be formed around the well region  2912 . A dielectric layer  2916 , which includes n respective contacts  2918 , is formed over the well region  2912 . Vias  2920  are formed through the dielectric layer  2916  to the respective contacts  2918 . A metallization layer  2922  is formed over the vias  2920 . Current  2924  flows from the n-contact  2918  at one end of the dielectric layer  2916  along junction between layers  2910  and  2912  and to the other n-contact  2918  at an opposite end of piezoresistor  2900 . 
     In the examples described herein, a normal piezoresistive sensor (e.g., sensor  108  and/or  112 ) includes an arrangement of sensing resistors  2900  formed in a lateral plane parallel to the mounting surface of the substrate  2801 . The resistances from the sensing resistors are compared to the resistance from the reference resistor  2800 , which is in a vertical direction perpendicular to the lateral plane. The reference resistor can have the same temperature dependency as the associated sensing resistors  2900  by forming the resistors in the same substrate and with substantially the same doping concentrations. This ensures the sensing and reference resistors have the same temperature coefficient and respond to temperature changes in substantially the same way, preventing confusion of different temperature responses for actual stress on the sensing resistor. 
       FIG.  30    is a perspective view of an example mounting assembly  3000  that includes an IC  3002  attached to a surface  3004  of a mechanical structure (e.g., shaft, beam, etc.)  3006 . The IC  3002  includes a substrate  3008  and an arrangement of piezoresistive sensors, such as sensors  108 ,  112 ,  116  or otherwise described herein, disposed on a mounting surface  3010  of the substrate  3008 . An interconnect  3012  is disposed between a contact surface (surface opposite that of the mounting surface  3010 ) of the substrate  3008  and the surface  3004  of the mechanical structure  3006 . To enhance the interconnection between the IC  3002  and the mechanical structure  3006 , prior to disposition of the interconnect  3012 , one or both surfaces may be prepared through processing. The type of preparation used to prepare respective surfaces can vary depending on the material properties of the surface as well as the form of cohesive interconnect  3012 . The preparation techniques may include texturing, patterning, cleaning, striping, etc. or any combination thereof. 
     One example of an interconnect  3012  between the substrate  3008  and the mechanical structure  3006  is a cohesive interconnect. A cohesive interconnect is an interconnection between two materials (e.g., metals) where the two materials essentially become one integrated joint. Examples of some different types of cohesive interconnects  3012  include, adhesives (e.g., epoxy), nanowires, welds (e.g., formed by welding process such as spot welding, ultrasonic welding or laser welding), and sinters (e.g., copper or silver sintering). 
     Another example of an interconnect  3012  includes a form fit interconnect where the interlocking of two connection members creates a positive connection. Form fitting connections have a tight tolerance where the connection members cannot become disengaged. Examples of form fitting interconnects include screws, clamps, press-fit hooks/bolts, tongue and groove and the like. 
     Still another example of an interconnect  3012  includes traction or friction connections where the static friction between adjacent surfaces of two connecting members prevents movement between the two connection members. For example, the mutual displacement of the two connection members is prevented so long as a counter-force caused by the static friction between the connecting members is not exceeded. 
     Some examples of the mounting methods described above may include silicon to metal (e.g., steel) bonding by induction heating. In the case of induction heating, the interconnect  3012  can include solder added to the contact surface of the substrate and/or to the prepared surface of the mechanical structure. Heat is generated to an isolated area several micrometers below the surface of the mechanical structure. A bond is then formed by a low temperature eutectic solder with adhesion layers of for example gold, silver, or nickel. 
     Another example includes laser micro-welding, such as where a lead frame of the IC is welded to the mechanical structure (e.g., a stainless steel shaft). In this example, the IC is either partially encapsulated by a material that can be welded to the mechanical structure or has a coupling layer (e.g., metal base layer) that can be welded to the mechanical structure to form the interconnect  3012 . 
     Still yet another example of the interconnect  3012  includes plastic deformation and cold metal welding of plain (e.g., non-patterned) or patterned metal. In this example, both the contact surface of the substrate (or the coupling layer) and the prepared surface of the mechanical structure are patterned to increase the bonding. More specifically, sealing rings are patterned on both the contact surface and the prepared surface. When attached, the metal rings overlap and undergo plastic deformation. The metal rings can be cold welded to create metal-to-metal bonding and sealing between the respective parts. 
     Another example of the interconnect  3012  includes ultrasonic welding in which high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When applied to metals, a notable characteristic of ultrasonic welding is that the temperature stays well below the melting point of the involved materials thus preventing any unwanted properties or reactions that might arise from high-temperature exposure of the materials. 
     For each of example interconnect  3012  described herein, when attaching the IC to the mechanical structure, physical (e.g., electrical, mechanical and/or thermal) properties of the interconnect between the adjoining surfaces (i.e., the contact surface of the substrate and the prepared surface of the mechanical structure can be configured based on the application of the IC sensor. For example, in regards to mechanical properties, the mechanical connection between the joining surfaces can be configured as a stronger connection or a weaker connection based on the application. A strong joint connection increases the sensitivity of the sensors by transferring more stress from the surface of the mechanical structure to the sensor. As a result, a stronger mechanical connection may increase transfer of stress force such as in applications where sensitivity is crucial. Conversely, a weaker mechanical connection may decrease the amount of stress transfer to the sensor, which results in a decreased sensitivity of the sensor. 
     The mechanical properties of the interconnect  3012  can also be configured to be an isotropic or anisotropic. For the example of an anisotropic interconnect, the interconnect  3012  is configured to transfer stress to the sensor along one or more particular directions such that the sensor is more sensitive to stresses in respective direction in relation to other directions. In contrast, an isotropic interconnect  3012  may uniformly transfer stress from all directions from the mechanical structure to the sensor. 
     The electrical properties of the interconnect  3012  may also be configurable. For example, the interconnect may be formed of an electrically conductive material, an electrically insulating material or have a resistivity or dielectric permittivity configured based on the application. 
     The thermal properties of the interconnect  3012  may also be configurable. For example, interconnect may be formed of a material having a thermal conductivity to control thermal transfer between the two joining surfaces. The interconnect  3012  can have a high thermal conductivity for applications where it is desirable to expose the IC to the temperature of the mechanical structure (e.g., when the IC or SoC also includes a temperature sensor). Conversely, the interconnect  3012  can be formed of a material having a low thermal conductivity for applications where it is desirable to insulate the sensor from the temperature of the mechanical structure to which it is attached. In addition to thermal conductivity, the interconnect  3012 , the substrate and the mechanical structure should be formed of materials having the same or similar thermal expansion coefficient. This ensures that the substrate and hence the IC expands and contracts at similar rates to that of the mechanical structure. 
     In this application, the term “couple” or “couples” means either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. For example, if device A generates a signal to control device B to perform an action: in a first example, device A is coupled to device B; or in a second example, device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B, so device B is controlled by device A via the control signal generated by device A. 
     In this description, the recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, then X may be a function of Y and any number of other factors. 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.