Structures for piezoresistive measurement of package induced stress in transfer molded silicon pressure sensors

Abstract

The package induced stress in a transfer molded micromechanical pressure sensor has been measured by use of silicon piezoresistors. Measurements have been carried out by monitoring the individual piezoresistors in the measuring bridge of an existing sensor, and by measurements on a modified sensor structure with specially designed stress sensing structures. The induced difference ( σ x – σ y ) between x- and y- normal stress in the sensor diaphragm has been measured to 50 ± 10 MPa at 25°C. The induced stress creates a thermal zero shift of the sensor signal. By covering the sensor chip with a soft glob top before molding, the stress may be considerably reduced. For glob-topped samples the measured value of ( σ x – σ y ) at 25°C is 20 ± 20 MPa.