Simulation Tool to Improve Piezoresistive Microsensors Used for Monitoring Ball Bonding

For the first time an integrated piezoresistive microsensor, used for monitoring ball bonding, is rigorously simulated. The sensor is based on a CMOS n + -diffusion and located in situ below a standard bonding pad. It measures a value related to the ultrasound stress which is dissipated during ball bonding. We report on a 2D simulation tool based on the implementation of the governing equation using a finite difference scheme. Using this tool, the sensor sensitivity and the electrical potential distribution are calculated using a previously simulated stress field and the piezoresistive coefficients. The tool takes into account the stress-induced local variations of the conductivity tensor. The simulated sensitivity agrees well with the measured sensitivity. Using our custom program, the geometry parameters of the sensor are optimized, predicting 4.5 times more sensitivity than that of the prototype sensor.