Piezoresistive pressure sensor

Dynamic explosion detectors use a piezoresistive pressure sensor installed behind the large-area, gas-tight, welded membrane. To ensure optimum pressure transference from the membrane to the active sensor element, the space between the membrane and the sensor is filled with a special, highly elastic oil. The construc tion is such that the dynamic explosion detec tor can withstand overpressures of 10 bar without any damage or effect on its setup characteristic. The operational range is adjustable between 0 and 5 bar abs. Dynamic explo-... 

Fig. 5.1.16 Piezoresistive pressure sensor made by silicon fusion-bonding (SFB) to obtain small chip size and anodic bonding to form a sealed vacuum cavity...

The sensitivity of a piezoresistive pressure sensor depends on the piezoresistive coefficient. Silicon crystal face selection and gage layout on the crystal face are important because of the anisotropy of the piezoresistive effect. Silicon (100) and (110) are often used with P-type diffused resistors to achieve a desired sensitivity. The next consideration is the thermal stress effect originating from the silicon crystal face. Fig. 7.3.5 shows the stress-distribution maps for silicon (100) and silicon (110) by the finite element method (FEM). 

To develop any sensor with the potential to replace the piezoresistive pressure sensor, it is essential to impart the ability to detect the same physical quantity from very low to high pressure, to employ micromachining technology, and to produce a simple one-chip package [11]. If these goals can be achieved, the prospect of combining reduced cost with a many-fold increase in the number of pressure sensor applications per vehicle may no longer be a dream. 

Samaun, Wise K D and Angell J B 1973 An IC piezoresistive pressure sensor for... 

A low-cost, low-power-consuming micromachined flow and pressure sensing device has also been reported. This sensor can be used for monitoring pressure and flow rate in clean fluids without particles and without the tendency to coat the channel. The pressure is measured with capacitive or piezoresistive pressure sensors and the flow rate is computed from the pressure drop over a well-defined, hydraulic resistance. Although this device is yet to be used for process analysis in industrial stream, it will gain more use in this area in the future. 

Figure 4.30 Stretch-based (a) piezoresistive pressure sensor (FSA, n.d.) and (b) capacitive...

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

W. Romes, J. Muchow, S. Finkbeiner, J. Franz, O. Schatz, H.-P. Trah, Simulation of nonideal behaviour in integrated piezoresistive silicon pressure sensors, Proc. 2nd Int. Conf Modeling and Simulation of Microsystems, MSM99, Computational Publications, Cambridge MA, USA, April 1999, 624-627. 

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

This can further be enhanced by the so-called lock-in technique, in which modulated signals are used, which after demodulation separate the desired signal from the noise. Both techniques are described here by examples one is a monolithic pressure sensor with moderate piezoresistive bridge signals and the other is a signal evaluation circuit for a high-pressure sensor with very small sensor signal levels. 

Tab. 7.3.1 provides a summary of the principles and features of the pressure sensors. The pressure sensors are broadly divided into three groups according to which principle they employ piezoresistive sensors, capacitive sensors, and piezoelectric sensors. 

Fig. 7.3.4 Automotive applications of DENSO pressure sensors (Si piezoresistive sensors)...

The automotive applications of semiconductor pressure sensors utilizing the large piezoresistive coefficient of silicon have been greatly expanded, owing to the following three advantages. 

Fiber optic intravascular pressure sensors can be made in sizes comparable to piezoresistive ones, but at a lower cost. The fiber optic device determines the diaphragm displacement optically by measuring the varying reflection of light from the back of the deflecting diaphragm. These optical devices are inherently safer electrically, but lack a convenient way to measure relative pressure without an additional lumen either atmospherically vented or connected to a second pressure sensor. 

Figure 1 illustrates the operational principle of hydrogel-based sensors. Pressure sensor chips with a flexible thin silicon bending plate and with an integrated piezoresistive Wheatstone bridge inside this plate have been employed as... 

Piezoresistive sensors automatically normalized to ambient pressure are used as clinical pressure sensors today. They have to be protected against water by use of a filter in the sensing line. The installation is set up in the sidestream by means of a thin hose as the sensing line in which only a negligible flow occurs. Electronic ventilators utilize additional pressure sensors for device control. 

Body movement, posture Piezoresistive strain/ pressure sensors, accelerometers, gyroscopes, optical fibre sensors Body kinematics Dependent on motion to be analysed... 

The resistive-type sensors measure strain-related mechanical properties such as strain, force, acceleration, and pressure-utilizing piezoresistive properties of sensitive film resistors [2,14,38,39]. These sensors are also incorporated as biosensors for detecting biomolecules. The electrode configuration is the same as in electrochemical transducers, but it measures the change of resistance when biomolecules interact with the biologically sensitive elements [2]. The piezoresistive microcantilever sensors are also extensively studied for biosensing. The piezoresistive sensors. 

Pressure sensor selection guidelines can be found in Ref. 3. Pressure sensors have various transducers connected to them to convert the measurement into a usable signal. Strain gauges and piezoresistive and piezoelectric pressure transducers are commonly used to produce the measured values. 

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