A CMOS MEMS capacitive resonant sensor array utilizing a PLL-based oscillator loop

While resonant sensing has been applied to detect various physical or chemical quantities, many reported sensing systems are still bulky and expensive as they rely on external instruments to conduct measurements. For practical applications, it is often desirable to have a low-cost miniaturized sensor chip with multiple sensing devices to validate a sensing event. This work presents the first attempt to implement a capacitive resonant sensor array in a CMOS (complementary metal oxide semiconductor) process. Damping loss of all sensors is compensated by placing them in an oscillator loop that contains a phase-locked loop (PLL) driving circuit. Inclusion of a PLL is beneficial for sustaining the oscillation of various resonant frequencies due to manufacturing tolerance and detected quantity within the PLL capture range. The fabricated micromechanical resonators have a resonant frequency near 200 kHz. To evaluate sensor performance, parylene-D thin film was deposited on microstructures and the corresponding frequency shift of twelve devices was 33.87 kHz in average. The resolution for mass detection was estimated to be 23 pg.