High resolution calorimetric sensing based on Aluminum Nitride MEMS resonant thermal detectors

This paper presents a high temperature resolution (994.5 μK/Hz^1/2 in a 50 Hz measurement bandwidth) micro-calorimetric sensor based on a high frequency (134.5 MHz) Aluminum Nitride (AlN) nano-plate resonator (NPR) overlapped by a freestanding reaction chamber separated by a micro-scale air gap (~50 μm). For the first time, the unique thermal detection capabilities of the AlN NPR technology are exploited to devise a calorimetric sensor with superior performance. Efficient heat transfer from the reaction chamber to the thermal detector is achieved by scaling the air gap between them. By taking advantage of the large thermal resistance (2.64 × 10⁴ K/W) of the AlN NPR and the reduced air gap, high heat transfer efficiency (ratio between the temperature of the resonator and the one of the reaction chamber) of 33% is achieved. The effectiveness of the fabricated prototype is experimentally verified by monitoring an exothermic reaction between Hydrochloric Acid (HCl) and Ammonium Hydroxide (NH₄OH). A high sensitivity of 9.62 kHz/M and detection limit of ~120 μM/Hz^1/2 are achieved for the first device prototype.