Millimeter Wave Thin-Film Bulk Acoustic Resonator in Sputtered Scandium Aluminum Nitride

This work reports a millimeter wave (mmWave) thin-film bulk acoustic resonator (FBAR) in sputtered scandium aluminum nitride (ScAlN). This paper identifies challenges of frequency scaling sputtered ScAlN into mmWave and proposes a stack and new fabrication procedure with a sputtered Sc0.3 Al0.7 N on Al on Si carrier wafer. The resonator achieves electromechanical coupling ( ${k} ^{2}$ ) of 7.0% and quality factor ( ${Q}$ ) of 62 for the first-order symmetric (S1) mode at 21.4 GHz, along with ${k} ^{2}$ of 4.0% and ${Q}$ of 19 for the third-order symmetric (S3) mode at 55.4 GHz, showing higher figures of merit (FoM, ${k} ^{2} \cdot {Q}$ ) than reported AlN/ScAlN-based mmWave acoustic resonators. The ScAlN quality is identified by transmission electron microscopy (TEM) and X-ray diffraction (XRD), identifying the bottlenecks in the existing piezoelectric-metal stack. Further improvement of ScAlN/AlN-based mmWave acoustic resonators calls for better crystalline quality from improved thin-film deposition methods. [2023-0151]

I. Introduction

Radio frequency (RF) acoustic devices are widely used as sub-6 GHz front-end filters [1], [2], [3], [4]. Acoustic resonators, i.e., key building blocks for filters, piezoelectrically convert the electromagnetic (EM) energy to mechanical vibrations and efficiently store energy at resonances. Such transduction offers two key advantages over EM counterparts, namely, miniature footprints and better frequency selectivity [2]. Among different piezoelectric RF acoustic platforms, current commercial FBARs have been dominated by sputtered aluminum nitride (AlN) and scandium aluminum nitride (ScAlN) since they possess good acoustic properties high-quality factor (), and electromechanical coupling () and their well-established microfabrication process that can be integrated into semiconductor industries effortlessly [2], [5].