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

This work describes sputtered scandium aluminum nitride (ScAlN) thin-film bulk acoustic resonators (FBAR) at millimeter wave (mmWave) with high quality factor (Q) using platinum (Pt) electrodes. FBARs with combinations of Pt and aluminum (Al) electrodes, i.e., Al top – Al bottom, Pt top – Al bottom, Al top – Pt bottom, and Pt top – Pt bottom, are built to study the impact of electrodes. The FBAR with Pt top – Pt bottom achieves electromechanical coupling (k²) of 1.8% and Q of 94 for third-order symmetric (S3) mode at 61.6 GHz, confirming FBAR can achieve a Q approaching 100 with optimized fabrication and acoustic design.

INTRODUCTION

Radio frequency (RF) piezoelectric devices are key technology for sub-6 GHz RF filtering resolution [1]–[4]. Electrically coupled piezoelectric resonators transform electromagnetic (EM) waves into mechanical vibrations at mechanical resonances [5], [6]. Such conversion presents two advantages over EM equivalents: efficient frequency selectivity and miniature footprints [2]. One commercially successful technology is thin-film bulk acoustic wave resonators (FBAR) in sputtered scandium aluminum nitride (ScAlN) and aluminum nitride (AlN), thanks to high electromechanical coupling (k²), quality factor (Q), and well-established CMOS compatible microfabrication [2], [3], [7]. As wireless communication advances into the millimeter-wave (mmWave) bands exceeding 30 GHz, there is a chance to scale the frequency of ScAlN/AlN devices while preserving high performance [3].