Piezoresistive sensor element and piezoresistive pressure sensor with minimized long-term drift

Abstract

The invention relates to a piezoresistive sensor. In particular, the present disclosure relates to piezoresistive sensor elements and piezoresistive pressure sensors that detect mechanical stresses acting on a membrane. The piezoresistive sensor element (100) comprises a substrate which, in operation, is subjected to a mechanical stress in response to a measured variable to be measured, a first array of at least four sensitive varistors (114), where the sensitive varistors (114) are arranged on the substrate and connected to form a first Wheatstone bridge for generating a first bridge signal, and a second array of at least four sensitive varistors (114) arranged on the substrate and connected to form a second Wheatstone bridge for generating a second bridge signal. A second array of at least four insensitive varistors (122), where the insensitive varistors (122) are connected to form a second Wheatstone bridge for generating a second bridge signal, where the stress sensitivity of the sensitive varistors (114) is higher than the stress sensitivity of the insensitive varistors (122), and wherein an output signal of the piezoresistive sensor element (100) is generated based on a difference of the first bridge signal and the second bridge signal.

Description

Technical Field

[0001] The present disclosure relates to piezoresistive sensors. In particular, the present disclosure relates to piezoresistive sensor elements and piezoresistive pressure sensors that detect mechanical stress acting on a membrane. Background Art

[0002] The piezoresistive sensor element according to the present disclosure is based on the physical principle of piezoresistance, which will be briefly outlined below.

[0003] Piezoresistive sensors are one of the first microelectromechanical systems (MEMS) devices and constitute a considerable market share of MEMS sensors in the market today. In particular, silicon piezoresistors have been widely used in various sensors including pressure sensors, accelerometers, cantilever force sensors, inertial sensors and strain gauges. A detailed overview is given, for example, in Barlian, A. Alvin & Park, Woo-Tae & Mallon, Joseph R., Jr. & Rastegar, Ali J. & Pruitt, Beth L. (2009): "Review: Semiconductor Piezo-resistance for Microsystems", Proceedings of the IEEE. Institute of Electrical and Electronics Engineers, 97, pp. 513-552, DOI: 10.1109 / JPROC.2009.2013612.

[0004] It is known that the resistance (R) of a homogeneous material is a function of its size and resistivity (ρ),

[0005]

[0006] Where l is the length and A is the average cross-sectional area.

[0007] The change in resistance due to applied stress is a function of geometry and resistivity change. The cross-sectional area of ​​a bulk material decreases in proportion to the longitudinal strain by its Poisson's ratio, ν, which for most metals is in the range of 0.20 to 0.35. For anisotropic silicon, the effective directional Poisson's ratio is in the range of 0.06 to 0.36. The isotropic lower and upper limits of ν are -1.0 and 0.5. The so-called gauge factor (GF) of a strain gauge is defined as

[0008]

[0009] Where ε is the strain and Δr / R is the fractional change in resistance with strain. The change in resistance is due to the geometric effect of the material (1+2ν) and the fractional change in resistivity with strain (Δρ / ρ).

[0010]

[0011] The geometric effect alone provides a GF of about 1.4 to 2.0, and the resistivity change Δρ / ρ for metals is small - about 0.3. However, for silicon and germanium in certain directions, Δρ / ρ is 50 to 100 times larger than the geometric term. For semiconductors, elasticity and piezoresistivity are directionally dependent under a given direction of load (stress, strain) and field (potential, current). For sensors according to the present disclosure, stress-induced resistivity changes (the so-called piezoresistive effect) are responsible for the generation of the electrical output signal.

[0012] In the following, some basics about the symbol and the underlying principles of piezoresistance in semiconductors will be discussed.

[0013] Since this is generally known, the Miller indices can be used to describe the crystal structure. Crystals have a periodic arrangement of atoms arranged in one of 14 lattice types, and a full review is available elsewhere. The Miller indices specify crystal planes by n-tuples. The direction index [hkl] denotes the vector perpendicular to the plane described by (hkl), and t denotes the family of planes that are equivalent to (hkl) by symmetry. The indices with angle brackets like hkl denote all directions that are equivalent to [hkl] by symmetry. In hexagonal crystals, as found in most silicon carbide polytypes, the Bravais-Miller index scheme is generally employed, in which four indices are used to denote the directions corresponding to the four major crystallographic axes (a 1 , a 2 , a 3 and c) the reciprocal of the intercept. Axis a 1 、a 2 and a 3 In the same plane and spaced 120° apart from each other, and c is perpendicular to the plane defined by (a 1 , a 2 , a 3 )The A plane defined by the triplet.

[0014] Crystalline silicon forms a covalently bonded diamond-cubic structure with a lattice constant of The diamond cubic structure is equivalent to two interpenetrating face-centered cubic (FCC) lattices, in which the basic atoms are offset by 1 / 4a in three orthogonal directions. Compared with the regular face-centered cubic (FCC) lattice (74% packing density), the diamond cubic lattice of silicon is relatively sparse (34% packing density). Commonly used wafer surface orientations in micromachining include (100), (111) and (110).

[0015] Lithography and etching techniques can create devices in various directions to obtain desired material properties. For example, <111> The piezoresistors with the (100) orientation will have the highest piezoresistance sensitivity in the pressure sensor. <110> The piezoresistors are aligned because they have high equal and opposite longitudinal and transverse piezoresistance coefficients.

[0016] To define the stress state of a unit element, nine components σ must be specified: ij , as given in the following matrix:

[0017]

[0018] The first index i indicates the direction of the applied stress, while j indicates the direction of the force or stress. If i = j, the stress is perpendicular to the specified surface, while i ≠ j indicates shear stress on surface i. According to the static equilibrium requirement that the sum of forces and moments is zero, the stress tensor is always symmetrical, that is, σ ij =σ ji , so the stress tensor contains only six independent components.

[0019] Strain ε ij is also directional. For isotropic homogeneous materials, stress is related to strain via Hooke's law, σ = εE. Although "effective" values ​​of Young's modulus and Poisson's ratio in a single direction are often used for simple loading cases, tensors are needed to fully describe the stiffness of anisotropic materials such as silicon. Stress and strain are related via the elastic stiffness matrix C, where σ ij =C ijkl *εKL, or equivalently via the inverse compliance matrix S, where ε ij =S ijkl *σKL:

[0020]

[0021] The folding notation reduces each pair of subscripts to a single number: 11→1, 22→2, 33→3, 23→4, 13→5, 12→6, e.g., σ 11 to 1 , ε 12 to ε 6 、c 1111 to c 11 and 2323 to 44 .

[0022] Single crystal germanium and silicon, both of which have a diamond lattice crystal structure, were the first materials to be widely used as piezoresistors. The piezoresistance coefficient is used to describe the correlation between the electric field components, the current density, and the stress. The piezoresistance coefficient (π) requires four subscripts because they involve two second-order tensors of stress and resistivity. The first subscript refers to the electric field component (measuring the potential), the second subscript refers to the current density (the current), and the third and fourth subscripts refer to the stress (stress has two directional components). For simplicity, the subscripts of each tensor are also folded, for example, π 1111 →π 11 , π 1122 →π 12 , π 2323 →π 44 For fixed voltage and current orientation (ω) as a function of stress (λ), these relationships can be generalized:

[0023]

[0024] These coefficients were determined for relatively lightly doped silicon and germanium samples with resistivities in the range of 1.5-22.7 Ω-cm (e.g., 7.8 Ω-cm for p-type silicon). Current commercial and research practice uses doping levels that are several orders of magnitude higher. Higher concentrations have slightly lower piezoresistance coefficients, but much lower temperature coefficients of resistance and sensitivity. For example, doping levels that result in resistivities in the range of 0.005-0.2 Ω-cm are known. (100) samples along the <100> and <110> The piezoresistance coefficients in the crystal directions. The longitudinal and transverse coefficients for the fundamental crystal axes are determined directly. The shear piezoresistance coefficients are derived. From these measurements and taking into account the crystal symmetry, the piezoresistance tensor for 7.8 Ω-cm silicon is fully characterized according to Smith, CS (1979). (1954) Piezoresistance effect in germanium and silicon. Physical Review, 94, 42-49, as follows:

[0025]

[0026] For a p-type piezoresistor implanted on (100) silicon along the 100 crystal direction, the lateral and longitudinal piezoresistance coefficients can be defined and approximated as follows:

[0027]

[0028] Therefore, the resulting resistance change is

[0029]

[0030] Fig.10 It is shown that when a pressure of 1 bar is applied from the back, Fig. 9 The stress value s of the film 202 is shown as l -st In this example, the membrane has a dimension of 390 μm in the x-direction, a dimension of 390 μm in the y-direction, and a thickness of 6 μm.

[0031] It should be noted that a distinction must be made between the terms stress and strain.

[0032] Mechanical stress is created when a material is placed under pressure or has a mechanical load applied to it. When a solid is under stress, it has the ability to deform. This deformation is called strain. Stress is the pressure per unit area of ​​a material, and the strain created is the deformation that occurs due to this stress. Strain and stress are strongly intertwined, as strain only occurs as a result of stress. Stress is defined as the force per unit area that is created within a material due to an externally applied force, uneven heating, or sustained deformation. The unit of stress is Nm -2 (Pa). Strain, on the other hand, is defined as the amount of deformation a body undergoes in the direction of the force compared to its original dimensions. Strain defines the relative change in the shape of an object.

[0033] The piezoresistors used in piezoresistive sensor elements according to the present disclosure are responsive to the stresses they experience.

[0034] Furthermore, it is known in the art to use the concept of a Wheatstone bridge to ensure accurate and offset-free measurements. Fig.11 A schematic representation of a MEMS pressure sensor 200 comprising four stress-sensitive piezoresistors R1, R2, R3, R4 is illustrated. A deflectable membrane (also referred to as a diaphragm) 202 is surrounded by a relatively stiff frame 204. Pressure acting on the membrane 202 induces stress in the membrane, which is sensed by the four stress-sensitive piezoresistors R1, R2, R3, R4.

[0035] These piezoresistors are arranged in a Wheatstone bridge circuit, such as Fig.12 When no pressure is applied, the output voltage Vout is 0V because all piezoresistors are equal to R 0 (1+αΔT). When pressure is applied, the diaphragm is under stress. The active piezoresistors located in the high stress areas on the diaphragm are strained, thus changing their value to R 0 (1+αΔT+π l Δσ l +π t Δσ t ), where is the temperature coefficient of resistance, π l and π t are the longitudinal and transverse piezoresistance coefficients respectively. l , and Δσ tare the changes in temperature, longitudinal stress, and transverse stress, respectively. For example, in the case where the pressure comes from the top of the membrane, the active vertical resistors (R1 and R3) experience mainly longitudinal stress, and the active parallel resistors (R2 and R4) experience mainly transverse stress. In other words, the resistors are subject to both stresses. Fig.10 The sum of these two stresses is plotted in Figure 1, showing why the values ​​of two of these resistors increase while the values ​​of the other two decrease when the membrane is exposed to stress. The longitudinal tensile stress increases the resistivity of the p-type resistor, while the transverse stress has the opposite effect. Therefore, Vout changes accordingly due to these changes. Assuming ΔR1 = ΔR3 and ΔR2 = ΔR4, the Vout expression for the full bridge is given by the following expression:

[0036]

[0037] Since this is generally known, by using a Wheatstone bridge configuration, many disturbances acting uniformly on all four resistors can be eliminated. Examples of such disturbances include process variations such as implant dose, line width, and variations in resistors due to temperature.

[0038] However, it has been found that conventional piezoresistive sensors suffer from slow long-term drift effects caused, for example, by electrostatic accumulation (e.g., in a covering gel or plastic cover) and / or environmental effects and / or process issues (such as ionic contamination, material impurities, electromigration, stress relief, stress in material layers, process residual stress, thermal coefficient (TC) mismatch, TC variation, etc.) Stress isolation may cause such drift, but this effect is not considered in the present disclosure because there is a concept to limit such drift effects.

[0039] Slow drifts of the offset caused by external disturbances cannot be distinguished from slow changes in the measured variable. Especially for the application of MEMS pressure sensors, this disadvantage leads to functional safety problems. In particular, the relevant standards in the application field of the automotive industry, or safety in medical applications, such as breathing equipment in the context of the present disclosure, the term long-term drift relates to time spans between several months and about two years, preferably about one year.

[0040] Some conventional sensor concepts deal with this problem using buried field shields to compensate for the effects of any ionic contamination due to the manufacturing process. In addition, it is known to use polysilicon or metal field shields to eliminate external electric fields. Some conventional sensors include a sensor housing with a metal cap with a cover shield to avoid static charge accumulation. However, these countermeasures complicate the manufacturing process and generally do not achieve satisfactory results.

[0041] In addition, EP 3287758 B1 discloses a differential pressure sensor that can provide common-mode corrected differential pressure readings. The differential pressure sensor includes, for example, two pressure sensing diaphragms. The pressure sensor can be configured so that the first diaphragm measures the pressure difference between two sections of the fluid. The pressure sensor can also be configured so that the second diaphragm measures the common-mode error experienced by the die when the differential pressure is read by the first diaphragm. The electrical connector can be configured so that the pressure differential outputs a common-mode error-corrected differential pressure reading based on the readings of the first diaphragm and the second diaphragm. In particular, the second diaphragm includes at least one pressure-sensitive electrical element that exhibits a changing resistance in response to a deflection of the diaphragm of the second pressure sensing die, and the changing resistance represents the common-mode error of the second pressure sensing die.

[0042] European patent application 22198016.2 relates to another differential pressure sensor. According to this document, a drift signal of a drift pressure sensor (i.e., a signal generated by a drift sensing unit formed on or in a symmetrical diaphragm) can additionally be used to trigger a warning signal. In more detail, by comparing the drift signal with a predetermined threshold, it is possible to estimate whether the differential pressure sensor (in particular, the signal generated by the differential sensing unit formed on the differential diaphragm) has deteriorated.

[0043] This achieves the following technical effect: the differential pressure sensor can have a longer life, and in particular the pressure sensor does not need to be replaced after a predefined fixed time, but can be replaced on demand (ie when a warning signal sensed by a drifting pressure sensor is output).

[0044] However, there remains a need for a piezoresistive sensor which allows particularly precise and long-term stable measurements while being able to be produced in a cost-effective manner. Summary of the invention

[0045] The above mentioned objects are solved by the subject matter of the independent claims. Advantageous embodiments of the disclosure are subject matter of the dependent claims.

[0046] The present disclosure is based on the concept of providing a second Wheatstone bridge in addition to the main bridge for monitoring and compensating for drift effects caused by the environment and / or process. In particular, the piezoresistive sensor element according to the present disclosure includes a substrate, which is subjected to mechanical stress in operation in response to a measured variable to be measured, and a first array of at least four sensitive piezoresistive resistors, wherein the sensitive piezoresistive resistors are arranged on the substrate and are connected to form a first Wheatstone bridge for generating a first bridge signal. In addition, a second array of at least four input-sensitive piezoresistors is provided, wherein the input-sensitive piezoresistors are connected to form a second Wheatstone bridge for generating a second bridge signal, and wherein the stress sensitivity of the sensitive piezoresistors is higher than the stress sensitivity of the input-sensitive piezoresistors, and wherein the output signal of the piezoresistive sensor is generated based on the difference between the first and second bridge signals.

[0047] This solution has the advantage that by subtracting the output signals of the two bridges, long-term drifts can be compensated, while the sensitivity of the main bridge remains unaffected. Thus, the reliability and safety of the sensor are enhanced.

[0048] According to an advantageous example of a piezoresistive sensor element, the substrate comprises silicon, wherein each of the piezoresistors comprises a doped region ion-implanted into the silicon material. Silicon is a mature material in the semiconductor industry and offers the potential to manufacture the sensor element and further electronic components using established CMOS processes. As described in the theoretical section above, single crystal silicon also exhibits a strong piezoresistive effect, particularly when ion implantation is used to manufacture the piezoresistors.

[0049] However, it is clear that other materials showing the piezoresistive effect, such as germanium or silicon carbide, may also be used in conjunction with the present disclosure.In addition, for manufacturing the varistors, other known techniques may also be employed, such as diffusion, epitaxy or deposition of doped polysilicon layers.

[0050] When a piezoresistive sensor element is fabricated using a single crystal material, such as single crystal silicon, the piezoresistivity may be anisotropic. Advantageously, each of the sensitive piezoresistors may be oriented along a first crystal orientation of the substrate and each of the corresponding insensitive piezoresistors may be oriented along a second crystal orientation of the substrate, the first crystal orientation being different from the second crystal orientation and causing a higher stress sensitivity than the second crystal orientation.

[0051] For example, the substrate may be made of p-type silicon with a (100) plane forming the outer surface, wherein the sensitive piezoresistors are arranged along the vertical direction. Directions and / or And wherein the insensitive varistors are arranged along a vertical direction. direction and / or a

[010] direction. With this configuration, particularly high sensitivity and precision can be achieved.

[0052] Alternatively, the substrate may also be made of n-type silicon with a (100) plane forming the outer surface, wherein the sensitive piezoresistors are arranged along the vertical direction. Directions and / or direction and / or a

[010] and wherein the insensitive varistor is arranged along a vertical direction. Directions and / or direction.

[0053] In order to ensure that the elements of the second Wheatstone bridge experience the same disturbances as the varistors of the first Wheatstone bridge so that all long-term drift effects are matched as closely as possible, each of the insensitive varistors can be arranged in close proximity to a corresponding sensitive varistor so as to be subjected to substantially the same stress as the corresponding sensitive varistor.

[0054] According to an advantageous example of the disclosed piezoresistive sensor element, the insensitive piezoresistors are arranged to comprise an angle of 45° to each sensitive piezoresistors.

[0055] According to another advantageous example, the piezoresistive sensor element further comprises a signal processing unit for evaluating the first bridge signal and the second bridge signal and for generating a sensor output signal. Therefore, there is no need to involve an external signal processing unit and connecting leads for compensation calculations, thereby improving accuracy and reliability.

[0056] The most accurate and compact architecture is achieved when the signal processing unit is monolithically integrated with the piezoresistors on the same chip (also called die).

[0057] The influence of temperature effects can be monitored using the inherent temperature coefficient of resistance (TCR) of the second Wheatstone bridge or by additionally providing a temperature sensor (such as a temperature diode) arranged within the second Wheatstone bridge. Such a temperature diode is also called a thermal diode. The function of a thermal diode is based on the properties of an electrical diode to linearly change the voltage across it according to the temperature.

[0058] As mentioned above, the architecture proposed in the present disclosure aims to eliminate the adverse effects of long-term drift effects due to electrostatic accumulation or external or process-induced ionic influences. In order to enhance the impact of these influences on the second Wheatstone bridge, the passivation layer present on the piezoresistive sensor element can be at least partially removed in the area above the insensitive piezoresistors.

[0059] The present disclosure exemplarily relates to a piezoresistive pressure sensor comprising at least one piezoresistive sensor element according to the above principles, wherein the substrate comprises a deflectable membrane (which may also be referred to as a diaphragm) which in operation deflects in response to a pressure to be measured.

[0060] It is clear that the ideas of the present disclosure may also be applicable to other piezoresistive sensors, such as force and inertial sensors. For example, cantilever sensors, strain gauges, accelerometers, and gyroscopes may be equipped with piezoresistive sensor elements according to the present disclosure. However, piezoresistive pressure sensors are some of the most reported and developed micro-machined devices. Therefore, the following detailed description will focus on piezoresistive pressure sensors, which typically measure the deformation of a thin circular or rectangular membrane (diaphragm) under an applied external pressure. The membrane may be made of the same material as the wafer substrate (silicon, diamond, etc.) or a CVD-based thin film (oxide, nitride, etc.). Integrated varistors are formed by dopant diffusion, ion implantation, or doped epitaxy. However, it is clear that other materials that show the piezoresistive effect (e.g., germanium or silicon carbide) may also be used in conjunction with the present disclosure.

[0061] According to an advantageous example, the membrane is surrounded by a frame having a higher stiffness than the membrane, and wherein the insensitive piezoresistors are arranged on the frame. Thus, the internally insensitive piezoresistors are decoupled from the deformation of the membrane. In this case, the insensitive piezoresistors can be arranged close to the periphery of the membrane or further away from the membrane. However, the insensitive piezoresistors are arranged on the deflectable membrane itself so as to be subjected to exactly the same stresses and environments as the sensitive piezoresistors.

[0062] According to an advantageous example, the membrane has a rectangular profile and the sensitive piezoresistors have an elongated shape and wherein a first pair of sensitive piezoresistors are arranged along opposite sides of the profile of the deflectable membrane and a second pair of sensitive piezoresistors are arranged at an angle to other opposite sides of the profile of the deflectable membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings are incorporated into the specification and form a part of the specification to illustrate several embodiments of the present disclosure. These drawings, together with the specification, are used to explain the principles of the present disclosure. The drawings are only for the purpose of illustrating how to make and use the preferred and alternative examples of the present disclosure, and should not be interpreted as limiting the present disclosure to only the embodiments shown and described. In addition, several aspects of the embodiments may form solutions according to the present disclosure alone or in different combinations. Therefore, the embodiments described below may be considered alone or in any combination thereof. As shown in the accompanying drawings, further features and advantages will become apparent from the following more specific description of various embodiments of the present disclosure, in which the same reference numerals represent the same elements, and in which:

[0064] Figure 1 is a schematic top view of a piezoresistive pressure sensor according to a first example;

[0065] Figure 2 yes Figure 1 A schematic cross-sectional view of a piezoresistive pressure sensor is shown;

[0066] Figure 3 is a schematic diagram showing the crystal orientation of a piezoresistors according to an advantageous example;

[0067] Figure 4 is a schematic representation of electronic signal processing according to an advantageous example;

[0068] Figure 5 is a schematic top view of a piezoresistive pressure sensor according to a second example;

[0069] Figure 6 is a schematic cross-sectional view of a piezoresistive pressure sensor according to another example;

[0070] Figure 7 is a schematic top view of a piezoresistive pressure sensor according to another example;

[0071] Figure 8 is a schematic representation of electronic signal processing according to another advantageous example;

[0072] Fig. 9 is a schematic top view of a diaphragm region of a conventional piezoresistive pressure sensor;

[0073] Fig.10 yes Fig. 9 Schematic diagram of stress distribution in a piezoresistive pressure sensor;

[0074] Fig.11 is a schematic top view of a conventional piezoresistive pressure sensor;

[0075] Fig.12 Yes Fig.11 The circuit diagram of the piezoresistors of the Wheatstone bridge is shown in FIG. DETAILED DESCRIPTION

[0076] Reference will now be made to the drawings and first to Figure 1 and Figure 2 The present disclosure is explained in more detail.

[0077] Figure 1 is a schematic top view of a piezoresistive pressure sensor 100 according to a first example of the present disclosure. Figure 2 It is along Figure 1 Corresponding cross-sectional view along cutting line II-II.

[0078] According to this particular example, the piezoresistive pressure sensor 100 has a rectangular profile. Of course, the principles of the present disclosure can also be applied to sensor structures with different profiles (e.g., circular profiles). The piezoresistive pressure sensor 100 includes a deflectable membrane 102, which is also called a diaphragm. The membrane 102 is surrounded and supported by a thicker and therefore stiffer frame 104.

[0079] The membrane 102 and the frame 104 define a gap 106 in which the pressure to be measured is allowed to act on a first (inner) side 108 of the membrane. A second side 110 of the membrane is oriented outwardly.

[0080] When a pressure difference is established between the first side 108 and the second side 110 of the membrane 102, mechanical stress is generated. The membrane is deflectable and the maximum stress occurs at the edges. This can be, for example, Fig.10 Thus, the maximum stress is generated in the area 112 defined by the diaphragm 102. The frame 104 is where the stress caused by the pressure is the smallest.

[0081] According to the present disclosure, a set of four sensitive varistors 114 are arranged on the diaphragm 102 around the periphery of the diaphragm 102. According to the illustrated example, each sensitive varistor 114 includes one or more (e.g., two) ion-implanted resistor regions 116 and conductive connecting leads 118 that interconnect the resistor regions 116 and allow interconnection and signal processing circuitry between the sensitive varistors 114. Bonding pads 120 may be provided for electrical connection of the die, such as by wire bonding, solder bumps in flip chip technology, etc. In Figure 1 and Figure 2 In FIG. 1 , only one of the pads 120 is shown in order to keep the schematic diagram simple.

[0082] In operation of the pressure sensor 100, the sensitive piezoresistors 114 change their resistivity with the greatest possible sensitivity in response to mechanical stresses in the membrane. The four piezoresistors 114 are interconnected to form a Fig.12 The Wheatstone bridge circuit is shown in FIG.

[0083] According to the present disclosure, in addition to these sensitive varistors 114, a set of insensitive varistors 122 is provided for compensating the effects of long-term drift. The insensitive varistors 122 react to mechanical stresses without sensitivity or with only very low sensitivity. Therefore, these insensitive varistors 122 essentially only sense the drift causing the effect. The input sensitive varistors 122 are connected to form a second Wheatstone bridge, and the output signal of the second Wheatstone bridge can be subtracted from the output signal of the first Wheatstone bridge in order to produce an accurate and drift-compensated sensor signal.

[0084] The insensitive varistors 122 may each include one or more interconnected ion-implanted resistive regions 124 .

[0085] like Figure 2 As schematically shown in FIG. 1 , the connecting lead 118 may, for example, include a diffused conductor 126 or a deposited conductive layer, such as a metal layer 128 .

[0086] Figure 1 and Figure 2 The example shown in shows a piezoresistive pressure sensor 100, which is based on p-type single crystal silicon. Figure 1 It is derived that the resistance region 124 of the stress-input-sensitive varistor 122 is arranged at an angle of 45 degrees compared to the resistance region 116 of the stress-sensitive varistor 114. Figure 3 Explain the reason for this arrangement.

[0087] Specifically, in Figure 3 In the present invention, the orientation of the ion-implanted resistive region 116 of the stress-sensitive piezoresistor 114 and the ion-implanted resistive region 124 of the stress-insensitive piezoresistor 122 is compared with the piezoresistance coefficient in the (100) plane of a p-type silicon substrate, as in, for example, Barlian, A. Alvin & Park, Woo-Tae & Mallon, Joseph R., Jr. & Rastegar, Ali J. & Pruitt, Beth L. (2009): "Review: Semiconductor Piezo-resistance for Microsystems", Proceedings of the IEEE. Institute of Electrical and Electronics Engineers, 97, pp. 513-552, DOI: 10.1109 / JPROC.2009.2013612.

[0088] The ion implanted resistance region 116 of the stress sensitive varistor 114 is arranged along the substrate 110 . Direction and This allows these piezoresistors to exhibit the greatest possible stress sensitivity. In contrast, the ion-implanted resistor regions 124 of the stress-insensitive piezoresistors 122 are arranged along the substrate 124 . direction and the

[010] direction and therefore exhibit no or only negligible stress sensitivity.

[0089] Although not shown in the figures, it should be noted that for n-type silicon, the sensitive varistors must be arranged along the surface of the substrate. direction direction and / or

[010] while the insensitive varistors are arranged along the vertical direction. Directions and / or To achieve similar results.

[0090] Figure 4An example of a sensor element 100 is shown which integrally comprises a first Wheatstone bridge 130 with sensitive piezoresistors, a second Wheatstone bridge 132 with insensitive piezoresistors and an integrated signal processing unit 134. The integrated signal processing unit 134 is operable to calculate a drift-compensated sensor output signal. Two temperature sensors (particularly temperature diodes) 136 are provided for sensing the temperature of the sensor element 100 so that further compensation calculations can be performed, thereby making the output signal even more accurate.

[0091] All components may be integrated into one ASIC, for example together with an electronic control circuit (ECU) 140 .

[0092] according to Figure 1 and Figure 2 In the example shown in , each insensitive piezoresistor 122 is placed near a corresponding sensitive piezoresistor 114 . However, this need not necessarily be the case. The insensitive piezoresistors 122 may alternatively be located away from the sensitive piezoresistors 114 . Figure 5 An example of such an arrangement is depicted in . Here, the insensitive varistor 122 is arranged in the peripheral region of the frame 104. Also, the resistance region 124 of the stress-insensitive varistor 122 is arranged at a 45 degree angle compared to the resistance region 116 of the stress-sensitive varistor 114. Thus, the stress sensitivity of the insensitive varistor 122 is zero or negligible compared to the stress sensitivity of the sensitive varistor 114.

[0093] It has been found that the long-term drift effects to be addressed according to the present disclosure are partially shielded by the passivation layer that is usually applied. The passivation comprises, for example, a stack of a silicon dioxide layer followed by a silicon nitride layer. Figure 6 The exemplary arrangement shown takes this discovery into account.

[0094] according to Figure 6 , the passivation layer 138 is partially removed to enhance the effect of drift. In particular, the passivation layer 138 is removed from the area where the insensitive varistor 122 is located to facilitate the effect of drift.

[0095] Furthermore, if the depth of the piezoresistors is close to the thickness of the membrane 102, the back side 108 of the membrane 102 may also be the cause of drift due to charge or due to leakage currents. Figure 6 As shown, the insensitive varistors 122 can be arranged on the frame 104 to make them as insensitive to stress as possible, or on the membrane 102 to mimic the situation of the stress-sensitive varistors 114 .

[0096] exist Figure 6In FIG. 1 , reference numeral 122 denotes a stress-insensitive resistor, while reference numeral 114 denotes a stress-sensitive varistor. Another possibility is to have resistor 122 with passivation on the film (ie, the same arrangement as resistor 114, but in a stress-insensitive orientation), but this arrangement will be less favorable.

[0097] Figure 7 14 shows a further advantageous example of a piezoresistive pressure sensor 100. The perimeter of the membrane is here marked by a dashed line 142. Again, the resistance area of ​​the stress-insensitive piezoresistors 122 is arranged at an angle of 45 degrees compared to the resistance area of ​​the stress-sensitive piezoresistors 114. Thus, the stress sensitivity of the insensitive piezoresistors 122 is zero or negligible compared to the stress sensitivity of the sensitive piezoresistors 114.

[0098] Figure 8 Another example of an electronic circuit is shown that may be used to calculate a compensated sensor output signal from the output signals of a first Wheatstone bridge 130 with sensitive piezoresistors and a second Wheatstone bridge 132 with insensitive piezoresistors.

[0099] Compensation using an ASIC (such as the dual channel 24-bit resistive sensor signal conditioner with analog and digital outputs ZSSC3281 sold by Renesas Electronics) is a mathematical operation using the outputs of the stress sensitive bridge and the stress insensitive bridge. One example is to subtract the stress insensitive output (i.e., drift) from the stress sensitive output. Another example is to use the output of the stress insensitive bridge to trigger a warning if it exceeds a certain amount of drift.

[0100] Reference numerals list

[0101] Reference numerals describe 100 Piezoresistive sensor element; pressure sensor 102 Diaphragm 104 frame 106 Hollow 108 The first side of the membrane is oriented toward the void 110 The second side of the membrane, oriented toward the outside of the sensor 112 Maximum stress area 114 Sensitive Varistor 116 Ion implantation resistor area 118 Connecting leads 120 Contact pads 122 Insensitive Varistor 124 Ion implantation resistor area 126 Diffused conductor 128 Depositing metal layers 130 The first Wheatstone bridge 132 Second Wheatstone Bridge 134 Signal Processing Unit 136 Temperature Sensor 138 Passivation 140 ECU 142 Membrane periphery 200 Conventional piezoresistive pressure sensor 202 Diaphragm 204 frame

Claims

1. A piezoresistive sensor element (100), comprising: a substrate which, in operation, is subjected to mechanical stresses in response to a measurand to be measured, a first array of at least four sensitive piezoresistors (114), wherein the sensitive piezoresistors (114) are arranged on the substrate and connected to form a first Wheatstone bridge for generating a first bridge signal, a second array of at least four input sensitive varistors (122), wherein the input sensitive varistors (122) are connected to form a second Wheatstone bridge for generating a second bridge signal, wherein the sensitive piezoresistors (114) have a stress sensitivity that is higher than the stress sensitivity of the insensitive piezoresistors (122), and wherein an output signal of the piezoresistive sensor (100) is generated based on a difference between the first bridge signal and the second bridge signal.

2. The piezoresistive sensor element of claim 1, wherein the substrate comprises silicon, and wherein each of the piezoresistors (114, 122) comprises a doped region ion-implanted into the silicon material.

3. A piezoresistive sensor element according to claim 1 or 2, wherein each of the sensitive piezoresistors (114) is oriented along a first crystal orientation of the substrate, and each of the corresponding insensitive piezoresistors (122) is oriented along a second crystal orientation of the substrate, the first crystal orientation being different from the second crystal orientation and causing a higher stress sensitivity than the second crystal orientation.

4. A piezoresistive sensor element according to any one of the preceding claims, wherein the substrate is made of p-type silicon, wherein the (100) plane forms the outer surface, and wherein the sensitive piezoresistors (114) are arranged along Directions and / or direction, and wherein the insensitive varistor (122) is arranged along Directions and / or Direction layout.

5. A piezoresistive sensor element according to any one of the preceding claims, wherein the substrate is made of n-type silicon, wherein the (100) plane forms the outer surface, and wherein the sensitive piezoresistors (114) are arranged along Directions and / or Directions and / or direction, and wherein the insensitive varistor (122) is arranged along Directions and / or Direction layout.

6. A piezoresistive sensor element according to any of the preceding claims, wherein each of the insensitive piezoresistors (122) is arranged in close proximity to a corresponding sensitive piezoresistors (114) so ​​as to be subjected to substantially the same stress as the corresponding sensitive piezoresistors (114).

7. The piezoresistive sensor element according to any of the preceding claims, wherein the insensitive piezoresistors (122) are arranged to include an angle of 45 degrees with each of the sensitive piezoresistors (114).

8. The piezoresistive sensor element according to any of the preceding claims, further comprising a signal processing unit (134) for evaluating the first bridge signal and the second bridge signal and for generating the sensor output signal.

9. The piezoresistive sensor element according to claim 8, wherein the signal processing unit (134) is monolithically integrated with the piezoresistors (114, 122).

10. The piezoresistive sensor element according to any of the preceding claims, further comprising a temperature sensor (136) arranged in the second Wheatstone bridge.

11. The piezoresistive sensor element according to any of the preceding claims, further comprising at least one passivation layer (138), wherein the passivation layer (138) is at least partially removed in the region above the insensitive piezoresistors (122).

12. Piezoresistive pressure sensor, comprising: The piezoresistive sensor element (100) according to any one of the preceding claims, The substrate comprises a deflectable membrane (102) which, in operation, deflects in response to a pressure to be measured.

13. The piezoresistive pressure sensor according to claim 12, wherein the diaphragm (102) is surrounded by a frame (104), the frame (104) having a higher stiffness than the diaphragm (102), and wherein the insensitive piezoresistors (122) are arranged on the frame (104).

14. A piezoresistive pressure sensor according to claim 12 or 13, wherein the membrane (102) is surrounded by a frame (104), the frame (104) having a higher stiffness than the membrane (102), and wherein the insensitive piezoresistors (122) are arranged on the deflectable membrane (102).

15. A piezoresistive pressure sensor according to any one of claims 12 to 14, wherein the membrane (102) has a rectangular profile and the sensitive piezoresistors (114) have an elongated shape, and wherein a first pair of the sensitive piezoresistors (114) are arranged along opposite sides of the profile of the deflectable membrane and a second pair of the sensitive piezoresistors (114) are arranged at an angle to other opposite sides of the profile of the deflectable membrane.

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