Simulation and experimental measurements of high-overtoned bulk acoustic resonators (H-BAR)

Abstract

This paper presents experimental and simulation studies on High overtone bulk acoustic resonators (HBAR). The devices are based on a thin piezoelectric layer of Aluminum Nitride (AlN) deposited on a bulk substrate. A high order longi- tudinal wave is excited by the AlN layer and propagating in the substrate. In order to validate the computations, resonators have been fabricated and tested experimentally. Low measurements have been also performed to check the Q factor limitations. The comparison between calculations and measurements are presented in the paper. The potential applications of such resonators in term of microwave stabilized sources or filters are discussed. I. INTRODUCTION During the past twenty years, several acoustic devices have been investigated for filters and sources applications. Different kinds of resonators using SAW, SSBAW, BAW have been considered with variable interest considering their skills (i.e performance, cost, complexity, etc). Nowadays the frequency range of sources and filters tend to overpass the Gigahertz. The classical technics of lithography are rising their upper limits in this range of frequencies. Consequently, surface wave seem to be unappropriated for high frequency applications. Even if traditional bulk resonators such as Quartz Crystal Microbalance (QCM) are frequency limited by practical usable minimum thickness, improvements of thin piezoelectric layer sputtering have grown the interest for thin Film Acoustic Bulk Resonators (FBAR) (1). Several approaches have been explored in FBAR fabrication like membrane structures or solidly mounted resonators. This kind of resonators offers a large variety of substrates employing integrated circuit type wafer scale processing. Filters with low losses and large band- pass have been achieved with FBARs but the quality factor is rather low due to piezoelectric quality of thin deposited films . In the 80's, several papers have emphasized the possibility of HBAR to present high Q at frequencies of several GHz. High overtone resonators consist of a piezoelectric film acting as a transducer to generate a standing wave resonance in a several wavelenghts thick substrate. The transducer itself occupies a small fraction of the resonator volume and most of the energy is stored in the substrate. The quality factor mainly depends on the acoustic losses of the substrate. Resonances occur at frequencies where the total resonator thickness corresponds to one or more half wavelengths. To reach high frequencies, high order resonances are used. In order to control the working fre- quency, the response has to be filtered because each resonance occur every fundamental frequency of the resonator given by: f0 = vl 2e (1) where vl is the velocity of the longitudinal wave in the substrate and e the thickness of the resonator. As shown in Eq.1, the frequency shift between two consecutive resonances is reduced if the substrate is thinned. An original method of filtering has been developed by using a FBAR ladder filter (2). In this study, HBARs have been realized using C-axis normal aluminum nitride (AlN) films for transduction, quartz has been used for substrate material.