Surface acoustic wave resonators for parametric mode coupling in MEMS

Abstract

Microelectromechanical systems (MEMS) sensors offer practical advantages such as miniaturization, low power consumption, and low cost. However, as devices based on physical microstructures, MEMS sensors are subject to technical limitations and noise. In particular, noise represents a fundamental limit that determines the performance of MEMS sensors in the detection of small forces and masses. Meanwhile,the field of cavity optomechanics has established the development of high-precisionmeasurement techniques, extending beyond fundamental scientific research; these concepts have also been investigated for practical sensing applications.In particular, cavity optomechanics utilizes parametric coupling for precisionsensing and noise reduction. Inspired by this framework, the thesis research explores whether the principles of cavity optomechanics can be adapted to MEMS sensors with a new integrated approach to extend existing sensing strategies in MEMS devices.Parametric coupling lies at the heart of cavity optomechanics, where it establishes an energy exchange between high-frequency optical modes and low-frequency mechanical modes. However, translating cavity optomechanical techniques into MEMS sensor platforms presents challenges. The need for bulky and alignment sensitiveoptical components makes their integration into compact MEMS architectures difficult or even impractical.To address this limitation, a compact MEMS architecture based on a surface acoustic wave (SAW) resonator is designed to serve as a GHz mechanical cavity,thereby replacing the high-frequency optical mode and introducing a purely mechanical framework for parametric coupling. This proposed system consists of akHz microcantilever acting as a low-frequency mechanical oscillator, with a highfrequency SAW cavity formed on its surface. To realize this platform, GHz SAW resonators are first developed and characterized, focusing on their resonance frequency and quality factor as key performance parameters. The strain induced by cantileverde formation is then analyzed, and its effect on the SAW resonance is quantified. Finally,a MEMS device with an integrated a GHz SAW cavity on a kHz microcantileveris fabricated, enabling coupling between the two mechanical modes. Based on a theoretical framework adapted from cavity optomechanics, the coupling strength in the proposed system is extracted and compared with previously reported values.The SAW resonators operate at resonance frequencies of approximately 1.2 GHz and show quality factors on the order of 103 , resulting in a Qf product on the order of1012 under ambient conditions. The GHz SAW devices demonstrate a strain responsivity of up to 114.99 Hz/με, quantifying the strain-induced frequency shift. Whenboth modes are simultaneously driven, distinct parametric sidebands are observed in the SAW spectrum at frequency offsets of ωc ± Ωm. In the latter system, the coupling strength g0 is estimated to be on the order of 10−3 Hz.The presented platform represents a purely mechanical implementation of parametric coupling analogous to cavity optomechanics within a CMOS-compatibleMEMS architecture. The frequency upconversion of kHz mechanical motion to GHzsidebands enables RF-domain readout without optical components under ambient conditions, introducing a frequency-domain transduction strategy that may be relevant for sensing under noise-limited conditions. Although demonstrated in the weak-coupling regime, further improvements in quality factor and operation at cryogenic temperatures may enhance the achievable coupling strength to exploit this mechano-mechanical approach for sensing applications at the quantum limit in MEMS.