Zinc oxide and aluminium nitrite CMOS-MEMS surface acoustic wave resonator for RF applications

Abstract

The increase in frequency spectrum for wireless communication system has led to the growing interest of thin film electroacoustic technology that scale favourably upon miniaturization. To date, current off chip SAW resonators are made from bulk piezoelectric materials and cannot be easily integrated with silicon substrate. Monolithic CMOS-MEMS integration provides robust platform to realize low power and low cost mass production for a single chip solution. Non-ferroelectric piezoelectric thin films namely Aluminium Nitride and Zinc Oxide are the most promising material for CMOS-MEMS integration due to its silicon compatibility and good piezoelectric properties. This work addresses the implementation of CMOS-MEMS SAW resonators using three different piezoelectric thin film namely Aluminium Nitride (AlN), Zinc Oxide (ZnO) and Al doped Zinc Oxide (AZO). The work begins with finite element modeling using COMSOL Multiphysics to evaluate the performance in terms of resonance frequency, quality factor and electromechanical coupling coefficient. The fabricated devices are based on the optimized simulation results. Post CMOS approach namely piezoelectric thin film deposition, photolithography and wet etching were implemented to fabricate the silicon compatible devices. XRD, AFM and profilometer characterization were conducted to evaluate the quality of piezoelectric thin film. Various issues have been explored such as dependence of RF sputtering power to c-axis orientation and investigation on suitable etchant for AZO thin film. Measurement results revealed AZO thin film enhanced the performance in terms of insertion loss and quality factor compared to ZnO thin film due to improvement in the piezoelectric properties by Al doping. CMOS-MEMS SAW resonator based on AlN thin film demonstrated highest quality factor of 746.8 at 1.040 GHz resonance frequency, giving figure of merit of Q x fS = 7.76 x 1011 Hz. Among the three deposited piezoelectric thin films, AlN thin film has the closest to ideal c-axis orientation which leads to highest piezoelectric properties. This result is comparable to current RF MEMS acoustic wave resonators. This work indicates that AlN, ZnO and AZO piezoelectric thin films have high potential to realize single chip transceiver for the next generation of wireless communication system.