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

TL;DR

This paper demonstrates a millimeter wave thin-film bulk acoustic resonator using sputtered scandium aluminum nitride, achieving high frequency and quality factor, and discusses fabrication challenges and material quality improvements.

Contribution

It introduces a novel fabrication process for mmWave FBARs with sputtered ScAlN, achieving higher figures of merit than previous AlN/ScAlN resonators.

Findings

Resonator operates at 21.4 GHz with Q of 62

Achieves higher FoM than prior AlN/ScAlN resonators

Identifies bottlenecks in piezoelectric-metal stack quality

Abstract

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 Sc0.3Al0.7N on Al on Si carrier wafer. The resonator achieves electromechanical coupling (k2) of 7.0% and quality factor (Q) of 62 for the first-order symmetric (S1) mode at 21.4 GHz, along with k2 of 4.0% and Q of 19 for the third-order symmetric (S3) mode at 55.4 GHz, showing higher figures of merit (FoM, k2xQ) 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.