Flip-chip packaging of piezoresistive barometric pressure sensors
To miniaturize piezoresistive barometric pressure sensors we have developed a package using flip-chip bonding. However, in a standard flip-chip package the different coefficients of thermal expansion (CTE) of chip and substrate and strong mechanical coupling by the solder bumps would lead to stress in the sensor chip which is not acceptable for piezoresistive pressure sensors. To overcome this problem we have developed a new ultra low stress flip-chip packaging technology. In this new packaging technology for pressure sensors first an under bump metallization (UBM) is patterned on the sensor wafer. As the next step solder bumps are deposited. After wafer-dicing the chips are flip-chip bonded on copper springs within a ceramic cavity. As sources of residual stress we identified the copper springs, the UBM and the solder bumps on the sensor chip. Different CTEs of the silicon chip and the UBM/solder lead to creep strain in the aluminum metallization between UBM and chip. As a consequence a temperature hysteresis can be measured.