Diaphragm is not only the base of force-sensitive resistor, but also the receiver of external stress, so it is the core part of pressure sensing element. The back of the silicon diaphragm should be machined into a concave shape with a thin middle by mechanical or chemical etching, which is called a silicon cup, and its front should be a piezoresistive full bridge. If the silicon cup is a circular pit, it is called a circular diaphragm. Diaphragm also has square, rectangular and other forms. When there is external force, the stress on the diaphragm is different everywhere. The position and direction of the resistance of the four bridge arms on the template should be determined according to the crystal orientation and stress.
The design and manufacture of the diaphragm determine the performance and range of the sensor. Figure 3.2 shows the oil-filled packaging structure, in which silicone oil is filled between the corrugated diaphragm of the sensor and the chip. At present, the pressure sensor with this structure is quite mature. The measuring range is 0 ~ 100 kPa to 0 ~ 60 MPa, the working temperature is -55℃ ~ 125℃, and the accuracy is 0.5% ~ 0. 1%. It can measure gauge pressure and absolute pressure.
Another packaging form of silicon piezoresistive pressure sensor is to use silicon strain gauge to directly sinter glass powder on metal diaphragm to form sintered pressure sensor. Due to the structural characteristics of the sensor, the elastic element can directly contact with the measured medium, which is easy to miniaturize and suitable for dynamic pressure measurement. The measuring range of the sensor is 0 ~ 100 kPa ~ 0 ~ 80 MPa, the working temperature is -55℃ ~ 125℃, and the accuracy is 0.5% ~ 0. 1%. The natural frequency varies from several thousand Hz to several hundred Hz, which can be used for airflow model test, explosion pressure test and engine dynamic measurement.