A simulation program is developed which is capable of calculating the output responses of piezoresistive pressure sensors as a function of pressure and temperature. Analytical models based on small and large deflection theories have been applied to predict the sensitivity and linearity of pressure sensors. Surfacemicromachined diaphragms with square or circular shapes, fabricated by a low pressure chemical vapor deposition sealing process, are designed and tested to verify the program. They are made of polysilicon and have a standard width (diameter) of 100 m and thickness from 1.5 to 2.2 m. Various parameters of the piezoresistive sensing resistors, including length, orientation, and dopant concentration, have been derived and constructed on top of the diaphragms. For a 100m-wide 2m-thick square-shape pressure sensor, calculated and experimental results show that sensitivity of 0.24 mV/V/(lbf/in2) is achieved. Experimentally, a maximum linearity error of 0.1% full-scale span) is found on a 100m-wide 2.2m-thick square-shape pressure sensor. Both sensitivity and linearity are characterized by the diaphragm thickness and the length of the sensing resistors. 
15 Figures and Tables
Fig. 1. Process flow of a surface-micromachined pressure sensor.
Fig. 2. SEM microphoto of a fabricated micropressure sensor with 100- m-wide square-shape diaphragm.
Fig. 3. SEM microphoto showing the cross-sectional view of a cleaved cavity. The depth of the cavity is about 2 m.
Fig. 4. Most sensitive positions for square and circular diaphragms.
Fig. 5. Sensitivity analysis of a square-shape diaphragm.
Fig. 6. Dimensionless deflection-load diagram for clamped circular plates.
Fig. 7. Calculated x-directional strain distributions by linear theory, Föppl’s method, Hook’s method, and ANSYS simulation for a square diaphragm, 100 m in width, 2 m in thickness, and under a pressure of 100 lbf/in2.
Fig. 8. Simulation result of linearity error with respect to input pressure by using the small deflection theory.
Fig. 9. Linearity error for a 100- m-wide square diaphragm with a thickness of 2 m and various resistor lengths.
Fig. 10. Linearity error for a 100- m-wide square diaphragm with a resistor length of 10 m and various diaphragm thickness.
Fig. 11. Linearity error for 100 m in diameter, circular diaphragm with 2 m in thickness, and various resistor lengths.
Fig. 12. Linearity error for 100 m in diameter, circular diaphragm with resistor length of 10 m, and various diaphragm thickness.
Fig. 13. Measured (symbols) and simulated (solid lines) linearity error of square-shape pressure sensors with different diaphragm thickness.
Fig. 14. Simulation result of temperature effects on square- and circular-shape (100 m wide and 2 m thick) pressure sensors.
Fig. 15. Experimental (symbols) and simulation (lines) results of a square-shape (100 m wide and 2 m thick) pressure sensor under different temperature and pressure.
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