Silicon Multi-Stage Current-Mode Piezoresistive Pressure Sensor with analog temperature compensation

Pressure measurement is a key part of many commercial and industrial systems. Piezoresistive pressure sensors are simpler to integrate with electronics, they are inherently shielded from RF noise and their response is more linear while compared to capacitive pressure sensors. And piezoresistive devices have always dominated the pressure sensor market. The analyticals that are typically used to model the diaphragm of the pressure sensor have been analysed by many researchers. To optimize the pressure sensor for parameters like linearity and sensitivity, the Finite Element Method (FEM) is incorporated. The selection of appropriate parameters of piezoresistors such as the shape and the position of the piezoresistor on the pressure sensor diaphragm, thickness of diaphragm are important. This study shows the scope of using analytical solutions and design techniques for a piezoresistive pressure sensor.

A nickel–chromium (Ni–Cr) piezoresistive pressure sensor is presented, which has the advantages of low-cost and easy fabrication processes. In the designed sensor, Ni–Cr alloy (80:20 wt%), which can be fabricated using simple processes, is used as the strain-detecting material with a smaller but acceptable gauge factor. Eutectic-bonding technology, based on silver–tin (Ag–Sn) alloy, which contains 3.5 wt% Ag and has 20 µm thickness, is used as an alternative and easy bonding choice to complete the vacuum package. Normally, Ag–Sn alloy is mostly obtained by the electrochemical deposition method. However, here the Ag–Sn solder film is directly used as a bonding material. It reduces the fabrication difficulty of eutectic bonding of the proposed pressure sensor. By studying the processes of annealing of Ni–Cr fabrication and eutectic bonding with Ag–Sn, the authors complete the alloy piezoresistive atmosphere pressure sensor. Bonding quality is evaluated by inspection through the deflection of a diaphragm of silicon with more than 95% of the area successfully bonded. The pressure–voltage characteristic test results suggest a precision within 0.3% in square fitting. The temperature coefficient offset is 620 ppm/(°C free space optic (FSO)).

At present, the silicon piezoresistive pressure sensor is a mature technology in the industry, and its measurement accuracy is more rigorous in many advanced applications. Micro piezoresistive pressure sensor is fabricated by a MEMS process, and its main operational principle is that the external pressure loading causes the deflection, strain, and stress which occur on the silicon membrane. The environmental temperature, the humidity, and the pressure will reduce its performance. Moreover, the drifts of output voltage in the same temperature caused by the residual stress on the aluminum trace under thermal cycle loading also influence the performance of the piezoresistive pressure sensor. This is called the thermal hysteresis phenomenon, and the variation of output voltage is called the thermal hysteresis voltage. Among the critical issues of silicon piezoresistive pressure sensor, the hysteresis phenomenon should be deeply paid attention to obtain better sensor accuracy. For this reason, this research would like to investigate the effect of the external loading on the output voltage of a pressure sensor. Based on the process of pressure sensors, this research adopts the concept of pseudo temperature combined with a finite element method (FEM) to obtain the thermal hysteresis voltage. After several numerical analysis of thermal hysteresis voltage, the experiment is then performed to validate the simulation results. After being validated with the experimental results, the variation of thermal hysteresis voltage under a different trace layout is analyzed. Based on simulation results of a different trace layout, it is found that the trace line layout plays an important role in the thermal hysteresis performance of a pressure sensor, which indicates that the longer aluminum trace increases the hysteresis voltage. The uniform and symmetrical layout of aluminum trace can reduce the variation of resistance change and decrease the hysteresis voltage. Furthermore, the trace layout which is far away from the silicon membrane and piezoresistance is suggested to reduce the hysteresis voltage.

SummaryVoltage references are typically designed by combining a complimentary to absolute temperature (CTAT) and proportional to absolute temperature (PTAT) voltage references. However, in contrast to conventional voltage references, the proposed circuit generates a voltage reference by subtracting two PTAT voltages, resulting in an extremely low process‐dependent voltage reference. A novel subtractor is also proposed for subtracting PTAT voltages, which is used to generate the reference voltage. The temperature coefficient of the proposed voltage reference is further improved using a novel PTAT slope compensation technique. The proposed voltage reference is a MOS‐only ultra‐low power circuit that works with sub‐bandgap supply voltage. Simulation results using UMC 0.18 m technology MOSFETs show that the proposed circuit has a line sensitivity of 0.5%/V, a temperature coefficient of 21.37 ppm/° C, a power consumption of 15.6 nW, and an output voltage variation of only 0.22% due to process variations.

Traditional bandgap-based current references display various limitations. This includes large temperature coefficients (TCs) across process corners, necessitating costly trimming cycles, and area inefficiency for low current values (<10 μA). This work addresses these shortcomings by generating a complementary to absolute temperature (CTAT) current that reproduces the temperature dependence of the proportional to absolute temperature (PTAT) current including its curvature over process corners, eliminating multi-temperature TC trimming while retaining a single-point room-temperature calibration to set the absolute current. The design provides this matching PTAT/CTAT feature by relying on the temperature-stable negative TC of the electron mobility to generate the CTAT current instead of the nonlinear and process-sensitive diode-based CTAT generation. Moreover, thanks to the mobility-based CTAT generation, high-resistivity undoped poly resistors with a strong negative TC can be used, enabling area-efficient CTAT current generation in the sub-μA to single-digit μA range. To validate the proposed approach, we fabricated a prototype design in a 130nm SOI CMOS technology, occupying an active area of only 0.016 mm2 and operating over a wide supply range from 1.6V to 3V. Measurements from 10 chips yield a mean current of 1.1 μA (σ/μ ≈ 4.4%) close to the design target of 1.2 μA. Experimental results further demonstrate an average TC of 110 ppm/∘C from 0 ∘C to 100 ∘C without multi-temperature trimming, an average room-temperature supply line sensitivity of 1000 ppm/V, and a load sensitivity better than 250 ppm/V.

Sensitivity analysis of packaging effect of silicon-based piezoresistive pressure sensor

High reliability of piezoresistive pressure sensors by wafer to wafer direct bonding at room temperature

In this paper, a Bandgap reference (BGR) with high power supply rejection ratio (PSRR) and low temperature coefficient (TC) is presented. On the basis of first-order temperature-compensated BGR, a complementary curvature-compensation technique (CCCT) is realized by using the saturation region current characteristics of PMOS transistors with proportional to absolute temperature (PTAT) bias voltage. Besides, in addition to the effect of cascode structure and the isolation of the current amplifier, the PSRR of proposed BGR is further improved by adopting self-bias power supply with feedback. The presented BGR is implemented in a standard 0.35 μm BCD process. The post-simulation results show that the TC of the BGR is 4.740 ppm/∘C with a 2.50V average output voltage, and the PSRR is up to -78.5dB.

Design of piezoresistive pressure sensor for enhancing stress of MEMS cantilever

Two original superior-order curvature-corrected voltage references was presented. In order to improve the temperature behavior of the circuits, double differential structures was used, implementing the linear and the superior-order curvature corrections. An original CTAT (complementary to absolute temperature) voltage generator was proposed, using exclusively MOS transistors biased in weak inversion for a low power operation of the voltage reference. The superior-order curvature-correction was implemented by taking the difference between two gate-source voltages of subthreshold-operated MOS transistors, biased at drain currents having different temperature dependencies: PTAT (proportional to absolute temperature) and PTAT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The SPICE simulations confirm the theoretical estimated results, showing temperature coefficients under 11 ppm/K for an extended input range 173K < T < 423K and for a supply voltage of 2.5V and a current consumption of about 1muA

Surface micromachining is becoming increasingly popular for microelectromechanical systems (MEMS) and a new application for this process technology is pressure sensors. Uncompensated surface micromachined piezoresistive pressure sensors were fabricated by Sandia National Labs (SNL). Motorola packaged and tested the sensors over pressure, temperature and in a typical circuit application for noise characteristics. A brief overview of surface micromachining related to pressure sensors is described in the report along with the packaging and testing techniques used. The electrical data found is presented in a comparative manner between the surface micromachined SNL piezoresistive polysilicon pressure sensor and a bulk micromachined Motorola piezoresistive single crystal silicon pressure sensor.

This paper focuses the structural design and optimisation of the micro piezoresistive pressure sensor to enhance the sensitivity and linearity. Finite element method (FEM) is adopted to optimize the sensor parameters, such as the resistor location and number of turns. An absolutely type pressure sensor with 150kPa full scale span (FSS) is built based on the optimal design programme as well as the bulk- micromachined process. For an 1150-mum-wide 30-mum-thick square-shape pressure sensor, experimental results show that the sensitivity of 2.3mV/V/10kPa, linearity error of 0.57% FSS and pressure hysteresis of 0.04% FSS are achieved. The well compatibility between the results of simulation and testing certifies the validity of the optimized scheme.

Flexible piezo-resistive pressure sensor based on conducting PANI on paper substrate

Wearable devices have developed significantly due to the use and development of various materials such as organic materials and carbon nanotubes, and the demand for wearables is also growing. The piezoresistive pressure sensor has been developed using conductive fabric coated with a simple solution coating process. The fabric has an open structure with the interconnected top and lower layers that can be utilized as pressure sensors and can detect physical movements. The piezoresistive pressure sensor has excellent resistance linearity in the low pressure to 10kPa range, as well as consistent resistance to strain up to 20% when stretched. The pressure sensor array can detect pressure-induced local deformation and will be expected to contribute considerably to the development of wearable devices with a simplified process.

In this study, a variety of piezoelectric pressure transducer designs and mounting configurations were compared for measuring in-cylinder pressure on a heavy-duty single-cylinder diesel engine. A unique cylinder head design was used which allowed cylinder pressure to be measured simultaneously in two locations. In one location, various piezoelectric pressure transducers and mounting configurations were studied. In the other location, a Kistler water-cooled switching adapter with a piezoresistive pressure sensor was used. The switching adapter measured in-cylinder pressure during the low pressure portion of the cycle. During the high pressure portion of the cycle the sensor is protected from the high pressure and high temperature gases in the cylinder. Therefore, the piezoresistive sensor measured in-cylinder pressure highly accurately, without the impacts of short term thermal drift, otherwise known as thermal shock. Additionally, the piezoresistive sensor is an absolute pressure sensor which does not require a baseline or “pegging” on every engine cycle. With this measurement setup, the amount of thermal shock and induced measurement variability was accurately assessed for the piezoelectric sensors. Data analysis techniques for quantifying the accuracy of a piezoelectric cylinder pressure measurements are also presented and discussed. It was observed that all the piezoelectric transducers investigated yielded very similar results regarding compression pressure, start of combustion, peak cylinder pressure, and the overall heat release rate shape. Differences emerged when studying the impact of the transducer mounting (e.g., recessed vs. flush-mount). Recessed-mount transducers tended to yield a more accurate measurement of the cycle-to-cycle variability when compared to the baseline piezoresistive sensor. This is thought to be due to reduced levels of thermal shock, which can vary from cycle-to-cycle.