Piezoresistive stress sensors

Piezoresistive stress sensors make use of the material property in which the bulk resistivity is a fimction of the mechanical stresses applied to the material. The sensor is fabricated by placing a piezoresishve device on a thin silicon membrane, supported by a thicker silicon rim as shown in Figure 3.32. The thin membrane is fabricated by etching away the bulk silicon on... 

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). 

Among voltammetric MEMS biosensors, a piezoresistive microcantilever is a good example. The general configuration of a piezoresistive microcantDever sensor is shown in Fig. 4. The stress caused by biochemical reactions is concentrated on the surface of the microcantUever. Piezoresistors are usually placed on the microcantilever to the rim, where the stress is maximal. When the microcantilever is bent due to biochemical reaction, the resistivity of the microcantilever is changed. And by measuring the change, the concentration of the biochemical is detected [7]. 

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. 

Fig. 7.3.1 shows the principle of the piezoresistive sensor. Diffused resistors (gages) are formed on the thin-walled section called the diaphragm. An applied pressure is detected via the piezoresistive effect, which is the change in electrical resistance when a stress is applied to the diaphragm. The sensitivity is determined by the material, diameter, and thickness of the diaphragm. The thin-film piezoresistive sensor offers low sensitivity because the piezoresistive coefficient of thin-film silicon is less than one-third of that of single-crystal silicon. 

As is evident from equation 4, Nip can be made zero by making both (<4 — and (rrxx — ffyy) zero. Fig. 7.3.19 shows the result of FEM stress analysis by sensor device simulation. With the piezoresistive gages arranged concentrically with the inner circumference of the metal stem, both n xx — other hand, since TNS is proportional to the temperature characteristics of (o, /x — [Pg.328]

Piezoresistive sensors. To measure the pressure, the resistance change to stress (the piezoresistance effect) may be employed. When silicon is stressed, the resulting strain breaks the cubic symmetry of the underlying crystal structure. The band structure of silicon is very sensitive to its crystal structure and, as a result, the consequent modification causes changes in the resistivity of the material (holes in the case of p " material). This change is... 

Stress measurement 20 channels, multi-channel data acquisition card of high speed, piezoresistive and piezoelectric sensors. 

The sensor arrays considered in this paper are fabricated using a piezoresistive material. Piezoresistive materials are materials which exhibit a change in its resistivity upon an application of a mechanical stress on its surface. This phenomenon is called piezoresistivity. It has been found that the contact resistance could be described by the following equation [10] ... 

Table 28.1 A number of mechanical properties as stimulus also called measurands, device types for mechanical sensors and some mechanisms often employed for the measurand in question. Piezoresistance is discussed further in the text. Magnetostriction is deformation induced by field, and inverse magnetostrictive effect is when magnetic properties are modified by the application of stress, resonant sensors have a vibrating solid that is sensitive for a given measurand... 

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