Polysilicon Nanofilm Pressure Sensor

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Abstract
PS nanofilm PS of 80nm is used to calculate its pressure properties by measuring its sensitivity. It is found that better performance is seen due to good temp and piezoresistive characteristics. The sensitivity is measured from temp 0-200C and it is noted that sensitivity at full scale pressure 0.6MPa and temperature 0C is 23.00 Mv/v/mpA and is 18.27 mV/V/MPa at 200C and the temperature coefficient of sensitivity (TCS) is -0.098%/C without any compensation and temperature coefficient of offset is -0.017%/C. Intro

4 piezoresistors made of 80 nm polysilicon nanofilm are connected in a wheatstone bridge structure. Sometimes separation by implantation of oxygen (SIMOX) technology is used to make the piezoresistors to reduce the leakage current of pn-junction at high temperature but polysilicon material is cheaper so it’s widely used. The pressure sensor in this paper have high sensitivity and low temperature coefficient. The complete fabrication process will be described in this paper.

Temperature characteristics of offset and sensitivity
2.1. Structure of the pressure sensor
The structure of the polysilicon nanofilm pressure sensor is shown in Fig. 1. The basic principle of the pressure sensor is the four polysilicon nanofilm piezoresistors, deposited on the oxidized micromachining silicon diaphragm, are connected into Wheatstone bridge configuration. For constant voltage VB supply, the bridge output V0 is: equation(1)

where Ri (i = 1–4) is the resistance of bridge resistors.
Thermal drift for offset
In Eq. (1), the offset disappears if R1R3 is equal to R2R4. Nonetheless, R1R3 is not generally equal to R2R4. The pressure sensor, thus, has an offset: equation(2)
V0=S0K0VB
Turn MathJaxon

where S0 = R1R3 − R2R4, K0 = (R1 + R2)(R3 + R4).
The temperature coefficient of offset (TCO) is [17]:
equation(3)
∂V0∂T=VBK0∂S0∂T−VBS0K20∂K0∂T=V0S0∂S0∂T−V0K0∂K0∂T≈V0S0∂S0∂T=VBK0[(α1+α3)R01R03−(α2+α4)R02R04] Turn MathJaxon

where αi...
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