CMOS compatible polycrystalline silicon–germanium based pressure sensors

This work demonstrates, for the first time, the use of poly-SiGe for the fabrication of both piezoresistive and capacitive pressure sensors at CMOS-compatible temperatures. Despite the low processing temperature (455 °C), a sensitivity of 4.6 mV/V/bar for a membrane of 200 × 200 μm2 is reached by piezoresistor design optimization. The possibility of further enhancing the sensor sensitivity by tuning the piezoresistor's annealing time is investigated, leading to a 30% improvement. Single capacitive pressure sensors with sensitivities up to 73 fF/bar have been successfully fabricated. Annealing tests, performed at a fixed temperature of 455 °C with different annealing times, prove that the presented pressure sensor process flows are compatible with post-processing above 0.13 μm Cu-backend CMOS devices. The increase in metal-to-metal contacts (more than 8% after 6 h annealing), rather than transistor performance or degradation of the metal interconnects, is what limits the post-processing thermal budget.