Oven-controlled Dual-mode Solidly Mounted Resonator (SMR) Based Biosensor

Acoustic resonators have been widely investigated in the last decade for potential applications in biosensing owing to their compactness, low cost, ultra-high sensitivities, and robustness. These resonators employ piezoelectric films in the thickness range of 0.6 μm - 2 μm, enabling them to operate at high frequencies between 0.8 GHz - 4 GHz. However, at such high frequencies, the environmental and boundary conditions affect these acoustic resonators more compared to resonances that occur at low frequencies. A shift in frequency is observed in these resonators due to changes in environmental temperatures, making it challenging to discern if the resulting shift in resonance is due to a change in temperature or successful biodetection. Therefore, biosensing using these acoustic resonators must be performed in a thermally stable environment to ensure accuracy. The ovenization approach utilized a micro-heater to which a DC voltage was applied to achieve fixed, elevated temperatures of upto 86°C through Joule heating. The simulation and characterization of the micro-heater are also detailed including the fabrication process flow. The pre-fabricated AlN-based solidly mounted resonator (SMR) die was ovenized by bonding them onto the micro-heater. Additionally, the dual-mode configuration of the SMR was utilized which allows temperature measurement using the intrinsic material properties to avoid the use of an external thermometer in order to reduce the circuit complexity. This ovenized SMR structure is capable of performing biosensing at stable, elevated temperatures along with temperature self-sensing. The biosensing experiments were performed by sensing bovine serum albumin (BSA) using these ovenized resonator devices. It was found that these ovenized devices with elevated temperatures displayed a sensitivity of -192 ppm.mL.μg⁻¹ at a BSA concentration of 50 μg.mL⁻¹. This observed sensitivity was approximately three times higher compared to the one observed in devices at room temperature and is due to increased adsorption at higher temperatures as a result of conformational change in the BSA proteins.