Stress sensor and its operating method

Abstract

To provide a simple stress sensor which can be accurately and easily read, has high sensitivity and resolution, and has improved stability.SOLUTION: A stress sensor circuit 100 includes a substrate 102 and a bridge circuit 110. The bridge circuit is coupled between an output node Vout and a ground node GND, and includes a first branch 112 and a second branch 120, the first branch having a tunable resistor 116 at a first intermediate node 118 and a first resistor 114. The second branch has a variable reference resistor 124 of a value Rref at a second intermediate node 126 and a second resistor 122, where the variable reference resistor is swept through a plurality of discrete values Rref. The bridge circuit also includes an amplifier 140, the amplifier being configured to generate digital voltage output at the output node as a function of mechanical stress applied to the substrate and the value Rref.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The field of the present disclosure generally relates to stress sensors, and more specifically, to stress sensors including bridge circuits configured for stress monitoring and methods of operating the same.

Background Art

[0002] Many electrical systems are implemented in or include electronic circuits fabricated on semiconductor wafers such as silicon wafers. Processing and packaging of semiconductor wafers can apply mechanical stress to the dies cut from the wafers by the cutting operation itself or by subsequent molding processes for encapsulating the dies in packages. Such stress can affect the performance of the electrical system, its circuits, and its structures. For example, post-packaging stress significantly affects the accuracy of analog / digital converters, digital / analog converters, and voltage reference circuits. Such stress also affects the carrier mobility of transistors implemented on the wafer. Thus, by incorporating stress sensors into electronic circuits, it is frequently done to identify mechanical stress acting on larger circuits or system-on-chips (SoCs).

[0003] (1) A simple stress sensor that can be accurately and easily read, (2) has high sensitivity and resolution and improved stability is desired.

Summary of the Invention

[0004] According to one aspect of the present disclosure, a stress sensor circuit is provided. The stress sensor circuit includes a substrate configured to receive mechanical stress and a bridge circuit disposed on the substrate and connected between an output node and a ground node. The bridge circuit includes a first branch having a first resistor of value R1, the first resistor being connected at a first intermediate node to a variable resistor of value R. The bridge circuit also includes a second branch having a second resistor of value R2, the second resistor being connected at a second intermediate node to a variable reference resistor of value Rref, the variable reference resistor being configured to sweep across a plurality of discrete values ​​Rref. The bridge circuit further includes an amplifier having a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node. The amplifier is configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref.

[0005] Another aspect of the present disclosure provides a method for measuring mechanical stress applied to a substrate. The method includes supplying a voltage to a bridge circuit connected between an output node and ground. The bridge circuit includes a first branch having a first resistor of value R1 connected to a variable resistor of value R, and a second branch having a second resistor of value R2 connected to a variable reference resistor of value Rref. The method further includes changing the value Rref by sweeping the variable reference resistor over a plurality of discrete values ​​Rref, and reading a digital voltage output at the output node which varies as a function of the mechanical stress and the value Rref.

[0006] A further aspect of the present disclosure provides a stress sensor system. The system includes a substrate configured to receive mechanical stress and a bridge circuit disposed on the substrate and connected between an output node and a ground node. The bridge circuit includes a first branch having a first resistor of value R1 connected to a variable resistor of value R at a first intermediate node, and a second branch having a second resistor of value R2 connected to a variable reference resistor of value Rref at a second intermediate node. The bridge circuit also includes an amplifier having a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node. The amplifier is configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref. The system also includes a microprocessor connected to the variable reference resistor and the output node. The microprocessor is configured to control the variable reference resistor to sweep across a plurality of discrete values ​​Rref and to read the digital voltage output at the output node.

[0007] The features, functions, and advantages described above can be achieved individually in various embodiments, or in combination with other embodiments, and further details will become apparent by referring to the following description and drawings. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram of one embodiment of a stress sensor circuit. [Figure 2] Figure 1 shows an illustrative graph of the digital voltage output with respect to resistance shift in the stress sensor circuit. [Figure 3] Figure 1 shows an illustrative graph of the digital voltage output as the reference resistance value changes under control in the stress sensor circuit. [Figure 4] This is a flowchart of one embodiment of a method for measuring mechanical stress applied to a substrate. [Figure 5]This is a flowchart of another embodiment of a method for measuring mechanical stress applied to a substrate. [Modes for carrying out the invention]

[0009] In this specification, elements or processes described in the singular form do not necessarily exclude multiple elements or processes unless otherwise explicitly stated. In addition, references to “one embodiment” or “exemplary embodiment” of the present invention are not intended to exclude the existence of other embodiments that similarly incorporate the described features.

[0010] The embodiments of the stress sensors described herein provide bridge circuits that are structurally similar to certain precision voltage reference (PVR) circuits, such as the PVR circuit described in U.S. Patent No. 9,405,305, “Precision Voltage Reference Circuit with Tunable Resistance,” and the stress sensor circuit described in U.S. Patent No. 10,704,969, “Stress Sensor,” both assigned to Boeing Company, Chicago, Illinois, both of which are incorporated herein by reference in their entirety. Such PVR circuits are generally designed to provide a stable voltage output even under aging, temperature changes, and radiation events. Such precision is critical because even slight changes in the voltage reference can lead to errors in acceleration, position, and rotation. For example, vehicles such as long-range guided vehicles like intercontinental missiles and spacecraft have low tolerances for errors in precision and accuracy, and some use inertial pendulum navigation systems, gyroscopic navigation systems, or a combination thereof to meet these requirements.

[0011] The systems and methods of this disclosure improve the stability and resolution of stress sensors, and in particular, enable their output to accommodate undesirable fluctuations in temperature, supply voltage, frequency, and other environmental variables. A single resistive element of a stress sensor is digitally driven between a high and low output (e.g., high output is denoted as "1" and low output as "0"), i.e., between toggle thresholds. The systems and methods of this disclosure utilize the hysteresis and averaging advantages of these stress sensors to achieve high-resolution digital representation of these toggle thresholds. These advantages are achieved without requiring an ADC within the stress sensor. The systems and methods of this disclosure also rely on the characteristics of the voltage output of the stress sensor, as detailed herein, to effectively mimic a digital output, enabling direct supply of the output to a digital device (e.g., a microcontroller, microprocessor, field-programmable gate array (FPGA)).

[0012] The stress sensors described herein can also be implemented in semiconductor wafers incorporated into large mechanical structures, where the mechanical strain of the large mechanical structure is closely linked to the mechanical strain of the semiconductor wafer itself. Several embodiments enable monitoring of localized stresses applied to larger mechanical structures at a microscale. The embodiments of the stress sensors described herein offer higher sensitivity, simpler implementation, and easier operation, such as simpler and more accurate digital readout.

[0013] Figure 1 is a schematic diagram of one embodiment of a stress sensor circuit 100 arranged on a substrate 102. The stress sensor circuit 100 includes a bridge circuit 110 connected between an output node Vout and a ground node GND. The output node and the output voltage present at the output node are interchangeably referred to as Vout in this specification. The bridge circuit 110 includes a first branch 112, sometimes referred to as a tuning branch, which has a resistor 114 with value R1 and a resistor 116 with value R. Within the first branch 112, there is a first intermediate node 118 between resistors 114 and 116. The bridge circuit 110 also includes a second branch 120, sometimes referred to as a variable branch, which has a resistor 122 with value R2 and a variable reference resistor 124 with value Rref. Within the second branch 120, there is a second intermediate node 126 between resistors 122 and 124. In some embodiments, resistors 114 and 122 of the first branch 112 and the second branch 120, as well as resistor 124, are high-precision resistors with temperature stability and radiation stability. In some embodiments, resistors 114 and 122 have equal values, while in other embodiments, resistors 114 and 122 have different values. Although resistors 114 and 122 are shown as having the same orientation, in some embodiments, resistor 122 may be oriented substantially perpendicular to resistor 114.

[0014] The bridge circuit 110 also includes an amplifier 140 connected as a bridge between a first intermediate node 118 and a second intermediate node 126. The amplifier 140 is supplied with a voltage Vdd. The amplifier 140 includes a positive input terminal connected to the first intermediate node 118 and a negative input terminal connected to the second intermediate node 126. The amplifier 140 also includes an output terminal connected to Vout. In some embodiments, the amplifier 140 includes a plurality of metal-oxide-semiconductor field-effect transistors (MOSFETs), thereby providing the amplifier 140 with temperature stability and radiation stability.

[0015] During operation, Vout is divided by the first branch 112 and the second branch 120 based on the values ​​R1 and R and R2 and Rref, respectively. A voltage Vb is generated at the intermediate node 118 and a voltage Va is generated at the second intermediate node 126. The voltage divider formed by the second branch 120 is relatively linear, i.e., Va is linear with respect to Vout. In some embodiments, such as a PVR circuit, the voltage divider formed by the first branch 112 is preferably substantially nonlinear based on the implementation of the resistor 116 and its value R, i.e., Vb is substantially nonlinear with respect to Vout. With such a configuration, the output of the PVR circuit becomes less sensitive to changes in the resistance values ​​R1, R2 and Rref. In other embodiments of the bridge circuit described herein, the voltage divider formed by the first branch 112 is preferably more linear (e.g., slightly nonlinear) based on the implementation of the resistor 116, such that Vout exhibits a larger and steeper sensitivity when the relative resistances R1, R2, and Rref change relative to each other, for example, due to mechanical stress.

[0016] Amplifier 140 operates as a linear, high-gain error amplifier, generating Vout, which is fed back to the branch and functions as a self-reference for the bridge circuit 110. The self-reference of the bridge circuit 110 using amplifier 140 virtually eliminates supply dependency and achieves closed-loop convergence when a starting voltage is applied via a starting circuit (not shown). The starting circuit starts the loop, for example, by raising the voltage Va at the second intermediate node 126 when power is turned on. Amplifier 140 is supplied by a nearly unregulated voltage source and can be implemented with at least 100 dB of power supply rejection (PSR). Furthermore, amplifier 140 operates in the forward path of the closed loop, thereby reducing and / or minimizing the impact of the amplifier's own shift due to, for example, temperature or radiation on the bridge's stability.

[0017] In one embodiment of the stress sensor circuit, the value R (R=1 / CF) of resistor 116 is maintained quasi-constantly. This is achieved by adjusting the frequency F and capacitance C to a constant value and utilizing the small residual nonlinearity of parasitic elements (e.g., junction capacitors of semiconductor switches forming the switched-capacitor equivalent resistance 116 as shown in U.S. Patent No. 10,704,969) to achieve balance in the entire circuit, including the intermediate nodes of the bridge and, consequently, its output (Vout). As a result, the stress sensor circuit 100 is relatively more sensitive to changes in R1, R2, or the difference between them. When any of these values ​​changes, Vout changes relatively abruptly from high output to low output (e.g., from high output set to "1" to low output set to "0"). In some cases, it may be difficult to adjust resistor R to the desired initial state so that the stress sensor circuit 100 has optimal sensitivity. Furthermore, temperature, aging, supply voltage, and frequency change between the initial setup of the stress sensor circuit 100 and the time of reading from the sensor. In such cases, false triggering or mistriggering of the sensor output may occur.

[0018] According to this disclosure, it is realized that the first branch 112 exhibits at least some degree of positive feedback through the amplifier 140. Specifically, when Vout is at a high output, the voltage division across resistors 114 and 116 results in this voltage output contributing to a correspondingly high voltage at the positive terminal of amplifier 140, and vice versa. In an exemplary embodiment, amplifier 140 exhibits a steep shift in the voltage output from high to low in response to a change in the value R1 or R2 (or the difference between them). This voltage output shift is graphically shown in Figure 2 as a voltage output curve 200. Thus, it is further possible to drive Vout to switch between high and low outputs, and between low and high outputs, by changing the resistance value of one of the resistive elements of the bridge circuit (specifically, the value Rref of resistor 124). The result of this positive feedback behavior is also the hysteresis of the sensor loop (for example, the first branch 112, which is looped around amplifier 140, is functionally equivalent to an adjustable hysteresis comparator or adjustable Schmitt trigger), which exhibits opposite output toggle thresholds (during upward and downward sweeps of resistance), occurring not simultaneously but at two different values ​​of the resistive element being changed (specifically, the value Rref of resistor 124). Thus, the voltage output behavior shown in Figure 2 is repeated at two different toggle thresholds, i.e., once as an upward resistance stimulus and once as a downward resistance stimulus (see also Figure 3, which is further described herein).

[0019] As will be further detailed herein, with all other elements fixed and environmental conditions kept as constant as possible, the voltage output can be toggled by varying Rref within a predetermined range of values, including the toggle threshold of the bridge circuit 110, and as a result these toggle thresholds can be easily identified. By repeatedly driving Rref within this range of values ​​in both upward and downward sweeps, a statistical distribution of Rref values ​​(or similarly its driving parameters) at which the Vout output level toggles (from high to low, and from low to high) can be obtained. Averaging can be performed from this statistical distribution, thereby giving the system the advantages of reading stability and noise filtering known in averaging techniques. In particular, by repeatedly performing upward and downward digital "sweeps" of discrete Rref values ​​that cause Vout to switch, more statistical reliability about the precise location of the sensor switching point can be accumulated, even in the presence of random noise.

[0020] Rref is repeatedly increased (i) from its minimum value to its maximum value, passing through a high toggle threshold where the voltage Vout switches from high output to low output, and (ii) decreased from the above maximum value to the above minimum value, passing through a low toggle threshold where the voltage Vout switches from low output to high output. The hysteresis differential voltage response of Vout to the change in Rref, which is further detailed herein, is shown in Graph 300 in Figure 3. The horizontal axis is the resistance value (e.g., the value of Rref), and the vertical axis is the voltage (e.g., the voltage output Vout). Specifically, by changing Rref, Vout is selectively switched between high output (Vref+, e.g., "1") and low output (Vref-, e.g., "0"). As Rref is driven from its minimum to its maximum value, Vout crosses the output curve 302, and the differential input of the hysteresis comparator changes proportionally from β·Vref- to β·Vref+. When Rref reaches a high toggle threshold (e.g., at 306), the output curve 302 reflects the transition of Vout from Vref+ to Vref-. Similarly, as Rref is driven from its maximum to minimum value, Vout crosses the output curve 304, and the differential input of the hysteresis comparator changes proportionally from β·Vref+ back to β·Vref-. When Rref reaches a low toggle threshold (e.g., at 308), the output curve 304 reflects the transition of Vout from Vref- to Vref+. In this embodiment, the coefficient β = R(Vref) / [R1+R(Vref)], where R is determined by the clock frequency F and the fixed capacitance C, as described above. The maximum and minimum values ​​of Rref are set and selected to be higher than the high toggle threshold and lower than the low toggle threshold, respectively (by a predetermined margin that allows for distinction between the actual threshold and noise).

[0021] In exemplary embodiments, the resistor 124 can be implemented as a digitally controlled potentiometer or R-DAC, which may be referred to herein as “R-DAC124”. The R-DAC124 varies the value of Rref using a digital code, also referred to as a digital “word”, where each code represents a unique resistance value (e.g., a combination of internal resistances arranged in series). In exemplary embodiments, the R-DAC124 corresponds to a digital word of at least 8 bits in length. In a particular embodiment, the R-DAC124 includes a 2.5kΩ digital potentiometer with an 8-bit interface that achieves a resolution of 2.5kΩ / 256 = 9.8Ω per LSB (Least Significant Bit), which corresponds to the equivalent sensing resolution in the bridge circuit 110. From this data, the corresponding minimum detectable stress (e.g., expressed as pressure in kPa) can be calculated.

[0022] In some embodiments, the R-DAC 124 can be controlled via an interface by a digital device 150 such as a microcontroller, a microprocessor, a DSP, an FPGA, etc. The digital device 150 is connected to the R-DAC 124 such that the digital device 140 and the R-DAC 124 can communicate. The digital device 150 includes digital memory registers suitable for controlling the R-DAC 124 (e.g., storing the digital word used by the R-DAC 124 to vary the value Rref). In addition or alternatively, the digital device 150 reads and records each digital word for controlling Rref from the R-DAC 124. Further, the digital device 150 reads Vout at the output node of the amplifier 140. Thus, the digital device 150 is configured to read and record the digital word that controls Rref (equivalently, the value of Rref) supplied to the R-DAC 124 when Vout switches from a high output (e.g., 1) to a low output (e.g., 0) and from a low output to a high output. The digital device 150 identifies these Rref values as the corresponding digital words supplied to the R-DAC 124 when the digital output voltage of the sensor toggles, and records these Rref values as high toggle threshold and low toggle threshold, respectively.

[0023] It should be noted that the characteristic response of Vout from a high output to a low output and from a low output to a high output as shown in the graph 300 is in an analog state but is steep enough to be directly identified and read by the digital device 150. Thus, as described above, Vout can be considered functionally equivalent to a digital output. This digital output can be directly obtained using the bridge circuit 11 without performing preconditioning of the signal and without requiring an additional comparator. Thus, the bridge circuit 110 of the present disclosure can be regarded as including a digital input (e.g., the digital word used by the R-DAC 124) and a digital output (e.g., the Vout level directly read by the digital device 150).

[0024] The high and low voltage values ​​are generally referred to as the High or "1" output and the Low or "0" output, respectively, but depend on the specific structure of the bridge circuit 110, the values ​​of the various resistive elements included in the circuit, and the characteristics of the amplifier 140. For example, in one particular embodiment, the high output is between 1.0V and 1.2V, and the low output is between 0.2V and 0.4V. However, this disclosure is preferably generalized to achieve a high-resolution Vout such that the difference between the high and low outputs can be easily identified by the steepness of the shift between them (for example, in the toggle threshold of Rref). Furthermore, the bridge circuit 110 exhibits relatively high noise immunity due to its substantially digital characteristics.

[0025] The value of Rref when the transition of Vout is detected by the digital device 150 (which can be represented by the digital word issued to the R-DAC 124 to control the value of Rref) is affected by thermal noise and other effects, which can be interpreted as an inherent "dither" if they are virtually random. This dither can be advantageously utilized to increase the resolution of the stress sensor circuit 100. In some embodiments, during the operation of the stress sensor circuit 100, artificial (e.g., digital) dither is intentionally introduced. In some such embodiments, this digital dither is introduced in the form of random binary noise generated by a pseudo-random number generator (PRNG) circuit (not shown). Each transition value of Vout is a DC constant value, and since the variable being observed is naturally slow, simple low-pass filtering of this artificial noise can be performed (e.g., using very long averaging), thereby eliminating most of the noise and separating the value of interest. In a particular embodiment, fine dither is introduced in the form of a digital potentiometer in parallel or in series with the resistor Rref. This additional digital potentiometer is modulated by a random digital word that extends the toggle threshold range beyond 1 LSB of the original digital resolution of the R-DAC 124. In combination with the averaging technique, this technique facilitates the identification of the exact toggle threshold Rref (represented by the unique digital word supplied to the R-DAC 124).

[0026] This method for effectively digitizing the stress sensor circuit 100 is, like previous stress sensors, somewhat vulnerable to aging of the circuit and changes in environmental conditions (e.g., temperature). These fluctuations can randomly or systematically affect the toggle threshold Rref. In some embodiments, for example, the toggle threshold Rref can shift by 4-6 LSBs due to changes in the clock frequency F, applied voltage Vdd, and / or temperature. Such shifts can cause false positive triggers in the stress sensor circuit 100, or the shift may be too large to detect changes in mechanical stress that the stress sensor circuit 100 should detect.

[0027] An advantageous realization of this disclosure is that both the high and low toggle thresholds of Rref are similarly affected by these circumstances. Accordingly, the digital device 150 is configured to calculate the difference between the high and low toggle thresholds of Rref by recording their digital word representations. Specifically, the digital device 150 records the high toggle threshold when it detects a transition in Vout from high to low output during an Rref upward sweep, in which Rref increases upward from a minimum to a maximum value. Similarly, the digital device 150 records the low toggle threshold when it detects a transition in Vout from low to high output during the next Rref downward sweep, in which Rref decreases downward from a maximum to a minimum value. The digital device 150 calculates the difference between the high and low toggle thresholds, which is referred to as the differential readout value. In one exemplary embodiment, unlike the large shift of the absolute toggle threshold described above, this differential read value shifts by only 2 LSB even when subjected to large variations in clock frequency F, applied voltage Vdd, and / or temperature.

[0028] Therefore, this differential readout value exhibits higher stability and reliability than the absolute toggle threshold. Consequently, it is possible to achieve more accurate operation of the stress sensor circuit 100 by making it less susceptible to environmental and aging changes. The advantages of averaging described herein with respect to the absolute toggle threshold of Rref can be similarly applied to differential readout values. Specifically, the resolution of the sensor's differential readout value can be increased by repeating upward and downward sweeps any number of times, while introducing variations in noise and / or other parameters, or by utilizing noise and variations that occur naturally within the system.

[0029] Therefore, the stress sensor circuit 100 is a highly sensitive "resistance-to-digital converter (RDC)" circuit that can measure the movement (or difference between them) of resistors 114 and 122 with higher resolution.

[0030] Figure 4 is a flowchart of one embodiment of a method 400 for measuring the mechanical stress applied to a substrate 102 on which a bridge circuit 110 is placed. In 410, a voltage is supplied to the bridge circuit 110 connected between an output node Vout and a ground node GND. The bridge circuit 110 includes a resistor 114 (having a value R1), a variable resistor 116 (having a value R), a resistor 122 (having a value R2), and a variable reference resistor 124 (having a variable value Rref). The equilibrium point of the bridge circuit and the output node voltage change as a function of the mechanical stress on the substrate 102.

[0031] At 420, the value Rref is changed by sweeping a variable reference resistor over multiple discrete values ​​of Rref, and at 430, the digital voltage output at the output node is read. Here, the digital output changes as a function of the mechanical stress applied to the substrate 102 and the value Rref.

[0032] Method 400 may include additional or alternative steps, or may have fewer steps than described above. For example, Figure 5 shows a flowchart of an embodiment of another method 500 for measuring mechanical stress applied to the substrate 102, which includes steps 410, 420, 430 described above, as well as additional steps. Specifically, in 510, the value Rref is changed by increasing the value Rref in an ascending sweep from a first value to a second value (420). Each value may include a multi-bit digital word (e.g., a digital word of 8 bits or more). In 520, the digital voltage at the output node is read by detecting a first transition of the digital voltage output from high level to low level during the ascending sweep (e.g., by a digital device 150) (430). In 530, the multi-bit digital word representation of the value Rref corresponding to the detected first transition is recorded as a high toggle threshold (e.g., by a digital device 150).

[0033] Subsequently, at 540, the value Rref is changed by decreasing it from the second value to the first value in a downward sweep (e.g., by R-DAC124) (420). At 550, the digital voltage at the output node is read by detecting the second transition of the digital voltage output from low level to high level during the downward sweep (e.g., by digital device 150) (430). At 560, the multi-bit digital word representation of the value Rref corresponding to the detected second transition is recorded as a low toggle threshold (e.g., by digital device 150).

[0034] In 570, the difference between the high toggle threshold and the low toggle threshold is recorded as a differential readout (for example, by the digital device 150). As described herein, this method 500 can be repeated many times.

[0035] In one embodiment, Method 400 or Method 500 includes introducing artificial noise into a stress sensor circuit at any or all of its steps, and recording the effect of the artificial noise on the differential readout (e.g., after appropriate filtering).

[0036] The exemplary technical effects of the methods, systems, and apparatus described herein include at least one of the following: (a) improving the sensitivity and stability of the stress sensor circuit compared to known sensing circuits; (b) increasing the resolution of the digital reading from the sensor by applying the benefits of averaging; and (c) realizing these benefits without requiring an ADC.

[0037] The systems and methods described herein are not limited to the specific embodiments described herein, and rather, elements of the steps of the systems and / or methods may be used separately and independently of the other elements and / or steps described herein.

[0038] Furthermore, specific features of various embodiments of this disclosure may be shown in some drawings and not in others, but this is merely for convenience. In accordance with the principles of the present invention, any feature in one drawing may be referenced and / or claimed in combination with any feature in any other drawing.

[0039] Furthermore, this disclosure includes the following appended embodiments.

[0040] Note 1. A stress sensor circuit capable of generating a digital voltage output, A substrate configured to be subjected to mechanical stress, The circuit includes a bridge circuit disposed on the substrate and connected between the output node and the ground node, the bridge circuit being A first branch having a first resistor of value R1, which is connected to a variable resistor of value R at the first intermediate node, A second branch includes a second resistor of value R2, which is connected at a second intermediate node to a variable reference resistor of value Rref, wherein the variable reference resistor is configured to sweep across a plurality of discrete values ​​Rref. A stress sensor circuit, wherein the bridge circuit further includes an amplifier having a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node, the amplifier being configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref.

[0041] Note 2. The stress sensor circuit described in Note 1, wherein the variable reference resistor is a digital potentiometer or R-DAC and is configured to digitally control the value Rref.

[0042] Note 3. The variable reference resistor is the stress sensor circuit described in Note 2, having a resolution of at least 8 bits.

[0043] Note 4. The stress sensor circuit according to any one of Notes 1 to 3, further comprising a digital device connected to the output node and configured to read the digital voltage output at the output node.

[0044] Note 5. The stress sensor circuit as described in Note 4, wherein the variable reference resistor is an R-DAC configured to digitally control the value Rref with an upward sweep from a first value to a second value and a subsequent downward sweep from the second value to the first value.

[0045] Note 6. The stress sensor circuit described in Note 5, wherein the digital device is further configured to detect a first transition of the digital voltage output from a high level to a low level during the upward sweep, and a second transition of the digital voltage output from the low level to the high level during the downward sweep.

[0046] Note 7. The stress sensor circuit described in Note 6, wherein the digital device is further configured to record the digital word representation of the value Rref corresponding to the detected first transition as a high toggle threshold, and the digital word representation of the value Rref corresponding to the detected second transition as a low toggle threshold.

[0047] Note 8. The stress sensor circuit described in Note 7, wherein the digital device is further configured to record the difference between the high toggle threshold and the low toggle threshold as a differential readout value.

[0048] Note 9. The stress sensor circuit as described in Note 8, wherein the R-DAC is further configured to digitally control the value Rref over multiple repetitions of an upward sweep and a downward sweep, and the digital device is further configured to record the difference read values ​​in each pair of upward sweeps and downward sweeps over the multiple repetitions of an upward sweep and a downward sweep.

[0049] Note 10. A stress sensor circuit according to any one of Notes 7 to 9, wherein the first value Rref is greater than or equal to the high toggle threshold, and the second value Rref is less than or equal to the low toggle threshold.

[0050] Note 11. The stress sensor circuit according to any one of Notes 5 to 10, wherein the digital device includes a digital memory register configured to control the R-DAC and determine each value of Rref.

[0051] Note 12. The digital device is a stress sensor circuit as described in any one of Notes 5 to 11, including a microcontroller, microprocessor, or FPGA.

[0052] Appendix 13. A method for measuring the mechanical stress applied to a substrate, A voltage is supplied to a bridge circuit connected between the output node and ground, and the bridge circuit, A first branch having a first resistor of value R1 connected to a variable resistor of value R, A second branch having a second resistor of value R2 connected to a variable reference resistor of value Rref, The value Rref is changed by sweeping the variable reference resistor across multiple discrete values ​​Rref, A method for reading the digital voltage output at the output node, which varies as a function of the mechanical stress and the value Rref.

[0053] Note 14. Changing the above value Rref means The aforementioned value Rref is increased by an upward sweep from the first value to the second value, The method according to Appendix 13, comprising decreasing the value Rref in a subsequent downward sweep from the second value to the first value.

[0054] Note 15. Reading the aforementioned digital voltage output is: The first transition of the digital voltage output from a high level to a low level during the upward sweep is detected, The method according to Appendix 14, comprising detecting a second transition of the digital voltage output from the low level to the high level during the downward sweep.

[0055] Note 16. The digital word representation of the value Rref corresponding to the detected first transition is recorded as the high toggle threshold. The method according to Appendix 15, further comprising recording the digital word representation of the value Rref corresponding to the detected second transition as a low toggle threshold.

[0056] Appendix 17. The method according to Appendix 16, further comprising recording the difference between the high toggle threshold and the low toggle threshold as a differential read value.

[0057] Note 18. Introducing artificial noise into the aforementioned bridge circuit, The method according to Appendix 17, further comprising recording the effect of the artificial noise on the differential readings.

[0058] Note 19. The value Rref is increased or decreased by repeating the upward sweep and downward sweep multiple times. The method according to Appendix 17 or 18, further comprising recording the difference read values ​​in each pair of upward sweeps and downward sweeps in the multiple repetitions of the upward sweep and downward sweep.

[0059] Appendix 20. The method according to any one of Appendix 13 to 19, further comprising connecting a digital device between the variable reference resistor and the output node, wherein the digital device is configured to perform the changing and reading functions.

[0060] Note 21. A substrate configured to be subjected to mechanical stress, A stress sensor system comprising a bridge circuit disposed on the substrate and connected between an output node and a ground node, wherein the bridge circuit is A first branch having a first resistor of value R1, which is connected to a variable resistor of value R at the first intermediate node, A second branch having a second resistor of value R2, which is connected to a variable reference resistor of value Rref at the second intermediate node, The amplifier includes a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node, and is configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref, The stress sensor system further includes the variable reference resistor and a microprocessor connected to the output node, the microprocessor being The variable reference resistor is controlled to sweep across multiple discrete values ​​Rref, A stress sensor system configured to read the digital voltage output at the output node.

[0061] Note 22. The aforementioned microprocessor is The value Rref is digitally controlled by an upward sweep from a first value to a second value and a subsequent downward sweep from the second value to the first value, The stress sensor system described in Appendix 21, further configured to detect a first transition of the digital voltage output from a high level to a low level during the upward sweep, and a second transition of the digital voltage output from the low level to the high level during the downward sweep.

[0062] Note 23. The aforementioned microprocessor is The digital word representation of the value Rref corresponding to the detected first transition is recorded as the high toggle threshold, and the digital word representation of the value Rref corresponding to the detected second transition is recorded as the low toggle threshold, and so on. The stress sensor system described in Appendix 22 is further configured to record the difference between the high toggle threshold and the low toggle threshold as a differential readout value.

[0063] This specification discloses various embodiments, including best modes, by example, and enables a person skilled in the art to implement various embodiments, including the construction and use of any device or system, and the execution of the incorporated methods. The patentable scope is defined by the claims and may include other examples that a person skilled in the art may imagine. Such other examples should be considered to be included in the claims if they have components identical to the language of the claims, or if they include equivalent components that differ only non-essentially from the language of the claims.

Claims

1. A stress sensor circuit capable of generating a digital voltage output, A substrate configured to be subjected to mechanical stress, The circuit includes a bridge circuit disposed on the substrate and connected between the output node and the ground node, the bridge circuit being A first branch having a first resistor of value R1, which is connected to a variable resistor of value R at the first intermediate node, The second branch includes a second resistor of value R2, which is connected at a second intermediate node to a variable reference resistor of value Rref, wherein the variable reference resistor is configured to sweep across a plurality of discrete values ​​Rref. The bridge circuit further includes an amplifier having a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node, wherein the amplifier is configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref. A stress sensor circuit in which the variable reference resistor is configured to digitally control the value Rref by an upward sweep from a first value to a second value and a subsequent downward sweep from the second value to the first value.

2. The stress sensor circuit according to claim 1, wherein the variable reference resistor is a digital potentiometer or R-DAC and is configured to digitally control the value Rref.

3. The stress sensor circuit according to claim 2, wherein the variable reference resistor has a resolution of at least 8 bits.

4. The stress sensor circuit according to any one of claims 1 to 3, further comprising a digital device connected to the output node and configured to read the digital voltage output at the output node.

5. The stress sensor circuit according to claim 4, wherein the variable reference resistor is an R-DAC configured to digitally control the value Rref.

6. The stress sensor circuit according to claim 5, wherein the digital device is further configured to detect a first transition of the digital voltage output from a high level to a low level during the upward sweep, and a second transition of the digital voltage output from the low level to the high level during the downward sweep.

7. The stress sensor circuit according to claim 6, wherein the digital device is further configured to record the digital word display of the value Rref corresponding to the detected first transition as a high toggle threshold, and the digital word display of the value Rref corresponding to the detected second transition as a low toggle threshold.

8. The stress sensor circuit according to claim 7, further configured to record the difference between the high toggle threshold and the low toggle threshold as a differential readout value.

9. The stress sensor circuit according to claim 8, wherein the R-DAC is further configured to digitally control the value Rref over multiple repetitions of an upward sweep and a downward sweep, and the digital device is further configured to record the difference readout values ​​in each pair of upward sweeps and downward sweeps over the multiple repetitions of an upward sweep and a downward sweep.

10. The stress sensor circuit according to any one of claims 7 to 9, wherein the first value is greater than or equal to the high toggle threshold, and the second value is less than or equal to the low toggle threshold.

11. The stress sensor circuit according to any one of claims 5 to 10, wherein the digital device includes a digital memory register configured to control the R-DAC and determine each value of Rref.

12. The stress sensor circuit according to any one of claims 5 to 11, wherein the digital device includes a microcontroller, a microprocessor, or an FPGA.

13. A method for measuring the mechanical stress applied to a substrate, A voltage is supplied to a bridge circuit connected between the output node and ground, and the bridge circuit, A first branch having a first resistor of value R1 connected to a variable resistor of value R, A second branch having a second resistor of value R2 connected to a variable reference resistor of value Rref, The value Rref is changed by sweeping the variable reference resistor over a plurality of discrete values ​​Rref. Each step includes reading the digital voltage output at the output node, which changes as a function of the mechanical stress and the value Rref, The step of changing the value Rref is: The aforementioned value Rref is increased by an upward sweep from the first value to the second value, A method comprising decreasing the value Rref in a subsequent downward sweep from the second value to the first value.

14. Reading the aforementioned digital voltage output means The first transition of the digital voltage output from a high level to a low level during the upward sweep is detected, The method according to claim 13, comprising detecting a second transition of the digital voltage output from the low level to the high level during the downward sweep.

15. The digital word representation of the value Rref corresponding to the detected first transition is recorded as a high toggle threshold, The method according to claim 14, further comprising recording the digital word representation of the value Rref corresponding to the detected second transition as a low toggle threshold.

16. The method according to any one of claims 13 to 15, further comprising connecting a digital device between the variable reference resistor and the output node, wherein the digital device is configured to perform the changing and reading functions.

17. A substrate configured to be subjected to mechanical stress, A stress sensor system comprising a bridge circuit disposed on the substrate and connected between an output node and a ground node, wherein the bridge circuit is A first branch having a first resistor of value R1, which is connected to a variable resistor of value R at the first intermediate node, A second branch having a second resistor with value R2, which is connected to a variable reference resistor with value Rref at the second intermediate node, The amplifier includes a positive input terminal connected to the first intermediate node and a negative input terminal connected to the second intermediate node, and is configured to generate a digital voltage output at the output node as a function of the mechanical stress applied to the substrate and the value Rref, The stress sensor system further includes the variable reference resistor and a microprocessor connected to the output node, the microprocessor being The variable reference resistor is controlled to sweep across multiple discrete values ​​Rref, It is configured to read the digital voltage output at the output node, A stress sensor system in which the microprocessor is configured to digitally control the value Rref in an upward sweep from a first value to a second value and a subsequent downward sweep from the second value to the first value.

18. The stress sensor system according to claim 17, wherein the microprocessor is further configured to detect a first transition of the digital voltage output from a high level to a low level during the upward sweep, and a second transition of the digital voltage output from the low level to the high level during the downward sweep.

19. The aforementioned microprocessor is The digital word representation of the value Rref corresponding to the detected first transition is recorded as the high toggle threshold, and the digital word representation of the value Rref corresponding to the detected second transition is recorded as the low toggle threshold, and so on. The stress sensor system according to claim 18, further configured to record the difference between the high toggle threshold and the low toggle threshold as a differential readout value.

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