Thermally Modulated CMOS-Compatible Particle Sensor for Air Quality Monitoring

Abstract

Combating the health effects of particulate matter (PM) pollution requires affordable and reliable real-time air quality monitoring. The potential for large-scale manufacturing of acoustic-wave-based sensors makes them an interesting option for low-cost, low-power particle sensing applications. This article demonstrates a solidly mounted resonator (SMR) PM sensor with improved sensitivity through thermal modulation of the device. A novel, complementary metal oxide semiconductor (CMOS)-compatible SMR with an integrated microheater was designed, manufactured, and tested. In simulations, it was found that particle deposition increases both the heat loss and the thermal time constant of SMR. The effect of this on the resonant frequency shift of the device caused by particle deposition is investigated closely in this work. The sensitivity of the devices to particle deposition was tested experimentally with and without temperature modulation by placing the device in a test chamber and allowing the randomized settling of aerosolized particles on its surface. The unmodulated sensor demonstrated a particle mass sensitivity of ~40 Hz/ng while the mass sensitivity of the temperature-modulated device was shown to improve by a factor of nearly <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 5$ </tex-math></inline-formula> to 190 Hz/ng. Temperature modulation also improved the detection limit from 100 to 50 ng. Further experiments were conducted by adding an impactor mechanism to have a more controlled measurement setup. To this effect, a thermophoretic particle deposition mechanism was added to the device to enhance its performance. It was demonstrated that the repeatability of measurements was significantly improved, making the device a promising low-cost technology for air quality monitoring.