Closed-Loop Frequency Tracking of a Surface-Micromachined Thermal Sensor for Hydrogen Detection

Hydrogen detection with high sensitivity and fast response is essential for the safe operation of hydrogen-based technologies. This work presents a surface-micromachined thermal MEMS sensor that exploits the thermophysical properties of the surrounding gas for hydrogen detection. The sensor comprises a honeycomb-shaped n-doped polysilicon heater suspended on an 80 nm-thick perforated membrane above a 1 µm gas gap, providing high thermal isolation and low thermal mass. A lumped-element model was introduced to describe the static and dynamic behavior of the sensor. The miniaturization enables dynamic operation in the kilohertz range using the 3ω method. A closed-loop phase-locked approach was implemented to continuously adjust the excitation frequency to a defined phase of the thermal response, using the resulting operating frequency as the measurement parameter. A noise-equivalent 3σ hydrogen detection limit of approximately 6 ppm was determined from the Allan deviation at an integration time of 20 s. However, the sensor response exhibits a pronounced pressure dependence due to rarefaction effects in the micrometer-scale gas gap, which modify the effective thermal conductivity and thus the dynamic thermal response. These results demonstrate the potential of high-bandwidth thermal MEMS sensors for sensitive hydrogen detection while highlighting the need to account for pressure-dependent heat transfer at microscale dimensions.