MEMS magnetic field sensor based on silicon bridge structure

A MEMS piezoresistive magnetic field sensor based on a silicon bridge structure has been simulated and tested.The sensor consists of a silicon sensitivity diaphragm embedded with a piezoresistive Wheatstone bridge,and a ferromagnetic magnet adhered to the sensitivity diaphragm.When the sensor is subjected to an external magnetic field, the magnetic force bends the silicon sensitivity diaphragm,producing stress and resistors change of the Wheatstone bridge and the output voltage of the sensor.Good agreeme...     

Noise due to Brownian motion in ultrasensitive solid-state pressure sensors

Noise due to Brownian motion of diaphragm in ultrasensitive solid-state capacitive and piezoresistive pressure sensors operating at sub-millimeters of mercury pressures in a gaseous ambient is considered. The statistical properties and spectral characteristics of the noise are obtained as functions of the diaphragm dimensions, temperature, and applied pressure. The results show that the Brownian equivalent pressure noise is substantially less than has been previously reported and is well below 1µmHg for most practical cases of interest. Thus it is not a limiting factor in setting device performance when compared to circuit noise sources.     

SENSIM: A simulation program for solid-state pressure sensors

A simulation program is described which is capable of calculating the output response of silicon piezoresistive or capacitive pressure sensors as a function of both pressure and temperature. A thermoelastic plane-stress formulation is used in calculating the stress and deflection of the transducer diaphragm. Both analytical and finite-difference solution methods are available, depending on the sensor structure. Diaphragm thickness taper, oxide and package stress, and rim effects are simulated. For capacitive structures, the program accurately predicts the diaphragm deflection and pressure sensitivity as a function of pressure and temperature. Stepped diaphragm structures are shown to be capable of improving pressure sensitivity by as much as 50 percent. The package-induced thermal drift for electrostatically sealed glass-silicon devices is typically less than 0.05 mmHg/°C.     

Temperature sensitivity in silicon piezoresistive pressure transducers

The various mechanisms responsible for temperature sensitivity in silicon piezoresistive pressure sensors are described. As a representative transducer, a full-bridge device having a 1-mm-square 23-µm-thick diaphragm is used. The 200 Ω/square, 2K-Ω bridge resistors produce a pressure sensitivity of 13.3 µV/V.mmHg with a temperature coefficient of -1300 ppm/°C. Variability in this sensitivity is most strongly influenced by the diaphragm thickness and the absolute resistor tolerance. A new technique-the electrochemical EDP etch-stop-is found to offer significant advantages over alternative schemes for diaphragm formation. Temperature sensitivity in electrostatically-bonded, vacuum-sealed devices is dominated by resistor match, with oxide stress and junction leakage current playing relatively minor roles over the -40 to + 180°C temperature range. While individual pressure trims for offset and sensitivity will continue to be required, individual temperature trims may be eliminated in these devices for many applications as increasingly precise resistor processes are used.     

压阻式MEMS压力传感器的原理与分析

本文介绍了一种压阻式MEMS压力传感器的工作原理,该装置由玻璃支座、单晶硅衬底及PZT薄膜构成,压阻薄膜连接组成惠斯登电桥,以取得更高的电压灵敏度和低温度敏感性。使用有限元分析软件ANSYS对单晶硅衬底的轴对称模型进行了仿真分析,以达到优化设计的目的。     

Simulations for thermal warpage and pressure nonlinearity of monolithic CMOS pressure sensors

The operation function of a piezoresistive pressure sensor utilizes a voltage output to detect the magnitude of pressure. The basic design concept for monolithic pressure sensors is to fabricate a standard submicron CMOS process with appropriate modifications to integrate on-chip signal conditioning circuits with anisotropic-etched piezoresistive sensing elements. In this study, thermal stress simulations with applied pressure loadings are used to estimate the electromechanical behavior of a new monolithic sensing element concept design. The major tasks are to predict the ripple deformation of a silicon diaphragm due to the thermal residual stresses from multiple passivation layers and estimate the pressure nonlinearities on the transducer. More detailed approaches with design and performance concerns are also discussed.     

Fabrication of catheter-tip and sidewall miniature pressure sensors

Silicon-diaphragm miniature pressure sensors, which use the piezoresistive effect, were developed for biomedical applications. We fabricated two types of sensors; that is, a catheter-tip (1.2-mm outside diameter, 0.17 mm thick) and a sidewall (1.4 × 3.45 × 0.22 mm) sensor, both having a thin circular diaphragm. Their diaphragms, 10 µm in thickness and 0.55 mm in diameter, were formed by an electrochemical etching method. Since the stability of pressure sensors is the most important requirement for precise pressure measurements, attractive approaches have been investigated to improve stability. The major instabilities of the present miniature pressure sensors are electrical drifts caused by leakage currents and thermal disturbances related to packaging stress. A shield and a guard plate can prevent the device from leakage currents. A thick supporting rim structure of the sensor and mounting on a stainless steel support with elastic material contribute to eliminate the packaging stress. For the purpose of easy lead attachment to the catheter-tip sensor, we use a unique structure having deep contact holes in deposited thick polysilicon layer (0.05 mm thick). Experimental results are as follows: Initial drift after power up was improved to about one tenth. Thermal disturbances, as temperature zero shift, thermal transient response, and temperature cycle hysteresis were greatly reduced. Low-temperature zero shift of 0.2 mmHg/°C was obtained using a simple temperature compensation method. Long-term drift was 0.6 mmHg/day. The catheter of 1.8-mm outer diameter having two sidewall sensors has been satisfactorily used for the study of urodynamics.     

Scaling limits in batch-fabricated silicon pressure sensors

The scaling properties of silicon capacitive and piezoresistive pressure sensors are described. An evaluation of the various noise mechanisms and pressure offsets in the scaled devices is presented, including Brownian noise, electrical noise, electrostatic pressure variations and pressure offset due to resistor mismatch. The analysis of diaphragm deflection includes the effects of intrinsic stress and the transition from plate theory to membrane theory. Both ultraminiature and ultrasensitive sensors are considered. Ultraminiature piezoresistive sensors with diaphragms measuring 100 µm in length and resolving 1 mmHg should be possible using present technology as well as ultrasensitive capacitive sensors that resolve 1 µmHg.     

Low-pressure sensor with bossed diaphragm

A novel design of piezoresistive, low-pressure sensors is presented. This design exhibits a number of advantages compared to conventional ones. The main objective of this development was realizing a sensor with high sensitivity, high pressure overload, and low non-linearity. This paper describes the theory of designing a piezoresistive low-pressure sensor. Through the application of a square diaphragm bossed in the center, a piezoresistive low-pressure sensor for the pressure range of ±10 kPa could be realized, exhibiting excellent sensitivity and low non-linearity of 35 mV/VF.S.O. and <|±0.05|%, respectively.     

Microelectromechanical System for Controlling Flow Past an Airfoil: Pressure Transducers

A technology is developed for making single-chip diaphragm pressure sensors with polycrystalline-silicon piezoresistive elements on a monocrystalline-silicon substrate. It allows one to produce piezoresistive elements with a conductivity–temperature characteristic that neutralizes the temperature dependence of piezoresistive sensitivity. Sensors with a 1.8 × 1.8-mm² diaphragm are designed and fabricated by the above technology for pressures ranging from 1 to 10⁵ Pa, showing a maximum sensitivity of 10^–6 Pa^–1. The sensors are tested in aerodynamic experiments on monitoring the flow past a model wing of finite span at angles of attack close to the stall angle.     

四端硅压力传感器输出电压的解析模型

本文用摄动法求解了横向压阻效应四端硅压力传感器输出电压的两维偏微分方程,导出了器件的输出电压与器件尺寸的关系;证明了随应力而变的输出电压最大值在器件横向L/2处,并给出了最大输出电压的解析表达式.这些解析公式得到的计算结果,都和数值解、实验数据相符合,说明了得到的公式具有高的精度.用所给的解析表达式可以很方便地进行器件设计和模拟.     

Monolithic polycrystalline-silicon pressure transducer

A monolithic pressure transducer using polycrystalline silicon for both the diaphragm material and an integral piezoresistor has been fabricated. The device can be made with good repeatability and with easily varied diaphragm thickness. Electrical-output linearity is very good over a pressure range of 0?11 cm Hg for a 2.4 ?m diaphragm having an area of 0.00136 cm2.     

Mechanism of the Ultradeep Anisotropic Chemical Etching of Si(100) in the Microfabrication of Piezoresistive Pressure Sensors

An experimental examination of a thin diaphragm for integrated piezoresistive pressure sensors is reported. The diaphragm is fabricated by ultradeep anisotropic chemical etching of monocrystalline silicon. The mechanism of the process is investigated by exploring the morphology of the etched surface. Process-induced pyramidal hillocks, pits, and trenches are measured.     

Solid-state digital pressure transducer

The solid-state digital transducer represents a new standard in precision pressure transduction for the next generation of airborne sensors. The pressure transducer produces pulse train outputs as a measure of pressure by developing strains in a silicon diaphragm which incorporates piezoresistive sensing elements. These elements are distributed resistance-capacitance (RC) networks which are diffused into a diaphragm surface as the control elements of phase shift oscillators. By this approach, a digital (frequency) signal generated at the source can be transmitted without noise and distance limitations and the need for precision analog to digital conversion is eliminated. The device exhibits advantages in the areas of reliability, accuracy, size and cost over present day analog devices. The solid-state digital pressure transducer is being developed to meet the requirements of supersonic and subsonic air data applications when coupled with a high-performance air data computer. This application requires low hysteresis with repeatability and stability which are the main features of the solid-state pressure transducer. Other possible applications are FM data acquisition systems and industrial robots.     

In-Vitro and In-Vivo Trans-Scalp Evaluation of an Intracranial Pressure Implant at 2.4 GHz

Elevation of intracranial pressure is one of the most important issues in neurosurgery and neurology in clinical practice. The prevalent techniques for measuring intracranial pressure require equipments that are wired, restricted to a hospital environment, and cause patient discomfort. A novel method for measuring the intracranial pressure is described. A wireless completely implantable device, operating at an industrial-scientific-medical band of 2.4 GHz, has been developed and tested. In-vitro and in-vivo evaluations are described to demonstrate the feasibility of microwave pressure monitoring through scalp, device integrity over a long period of time, and repeatability of pressure measurements. A distinction between an epidural and sub-dural pressure monitoring techniques is also described. Histo-pathological results obtained upon a long-term device implantation favor the utilization of the sub-dural pressure monitoring method. On the other hand, in-vivo studies illustrate a maximum pressure reading error of 0.8 mm middot Hg obtained for a sub-dural device with a capacitive microelectromechanical system sensor compared to 2 mm middot Hg obtained for an epidural device with a piezoresistive sensor.     

Simulation of circular silicon pressure sensors with a center bossfor very low pressure measurement

A characteristics simulation method of circular silicon-diaphragm piezoresistive pressure sensors was developed to obtain accurate sensors for very-low-pressure measurement. The anisotropic stress-strain relationship of a silicon single-crystal plate and the nonlinear characteristics of silicon piezoresistive gauges were considered. Nonlinear deflection and strain formulas of circular silicon diaphragm sensors with a center boss and sensors with a center boss and ribs were derived by taking the effects of the large deflection and the support stiffness of the diaphragms into account. Based on these considerations, the characteristics of the sensors were simulated. The simulated results show good agreement with the observed results and indicate that output voltage can be greatly increased while maintaining low nonlinearity even in the low-pressure range by narrowing the rib width and thinning the diaphragm thickness of sensors with a center boss and ribs. This is because the rib strain that produces output voltage is increased while maintaining small deflection by using this type of sensor     

A monolithic capacitive pressure sensor with pulse-period output

A new microminiature monolithic capacitive pressure transducer (CPT I) is 20 times more sensitive than piezoresistive strain-gauge pressure transducers, requires one percent of the power, and can be batch fabricated through current integrated circuit technology. A second device (CPT II) incorporates bipolar signal-processing electronics on the same silicon chip to produce a low-duty-cycle pulse-mode output with period related to pressure. This output format helps to re-solve the problem of shunting between leads, which is one of the principal causes of long-term drift in piezoresistive transducers designed for implantable medical applications. Because this device uses capacitance change as a transductional mechanism rather than piezo-resistivity, it is not susceptible to drift caused by temperature variations in the piezoresistive coefficient. Optimization for totally implantable biomedical applications places special emphasis on small size, high sensitivity, improved long-term baseline stability, and greatly reduced power consumption. These properties are also important for a wide range of pressure-sensing applications-from automotive to general industrial use.     

Review: Semiconductor Piezoresistance for Microsystems

Piezoresistive sensors are among the earliest micromachined silicon devices. The need for smaller, less expensive, higher performance sensors helped drive early micromachining technology, a precursor to microsystems or microelectromechanical systems (MEMS). The effect of stress on doped silicon and germanium has been known since the work of Smith at Bell Laboratories in 1954. Since then, researchers have extensively reported on microscale, piezoresistive strain gauges, pressure sensors, accelerometers, and cantilever force/displacement sensors, including many commercially successful devices. In this paper, we review the history of piezoresistance, its physics and related fabrication techniques. We also discuss electrical noise in piezoresistors, device examples and design considerations, and alternative materials. This paper provides a comprehensive overview of integrated piezoresistor technology with an introduction to the physics of piezoresistivity, process and material selection and design guidance useful to researchers and device engineers.     

Intelligent differential pressure transmitter with multiple sensorformed on a (110)-oriented circular silicon diaphragm

A multiple piezoresistive gauge sensor was developed for application to intelligent differential pressure transmitters. The sensor can measure differential pressure, static pressure, and temperature. Three piezoresistive gauges are positioned on a (110)-oriented circular monocrystalline silicon diaphragm. Proper dimensional design and optimal gauge positioning maximize the output and minimize crosstalk. Using data maps, three voltage outputs are combined by a microprocessor unit to yield a compensated sensor output. An experimental sensor was fabricated and the compensation scheme was proved useful. The sensor accuracy was within ±0.1% of the full scale in the pressure range of ±80 kPa. The zero and span shifts were less than 0.25% for the temperature range of -20-60°C, and zero shift was less than 0.1% for the static pressure change of 15 MPa     

压阻式压力传感器灵敏度与线性度的仿真方法

灵敏度和线性度是压力传感器最重要的两个性能指标。为了制作出能够满足实际应用需求的压力传感器,必需探索出一种压力传感器灵敏度和线性度的有效仿真方法。提供了一种基于对压阻式压力传感器薄膜表面应力的有限元分析(FEA)和路径积分的仿真方法,从而实现了在满量程范围内不同压力值下对传感器电压输出值的精确估计,在此基础上对压力传感器的灵敏度和线性度进行了有效仿真。实验结果验证了该方法的精确性:传感器样品的灵敏度测试值为42.462～44.460 mV/MPa,与仿真值之间的相对误差控制在3.3%以下,同时得到非线性低于0.16%的良好线性度以满足应用需求。