Ferroelectric Aluminum Scandium Nitride Thin Film Bulk Acoustic Resonators with Polarization‐Dependent Operating States

Abstract

Given the recent discovery of ferroelectricity in highly doped AlN films and their widespread use in bulk acoustic wave (BAW) filters, the ferroelectric and acoustic performance of AlScN‐based devices at two distinct polarization states, i.e., nitrogen (N)‐polar and metal‐polar states, is investigated. Unreleased metal−ferroelectric−metal (MFM) capacitors, as well as suspended MFM structures in the form of thin‐film bulk acoustic resonators (FBARs), are fabricated based on Mo/AlScN/Mo‐sputtered films with ≈30% Sc concentration. The hysteresis polarization−electric field (P−E) loops and internal resistance of the MFM capacitors are characterized. The FBAR devices are characterized at the two polarization states, where the polarization inversion is accompanied by the modulation of thin‐film stress and resistance. Furthermore, the as‐fabricated N‐polar FBAR shows a fundamental thickness‐mode resonance frequency of 3.17 GHz, with a quality factor (QBode) of 572 and a high effective electromechanical coupling coefficient (kt2) of 11.4%. The FBAR frequency response changes when the polarization is switched from N‐polar to metal‐polar. Herein, the first investigations of electric and acoustic properties of N‐polar and metal‐polar AlScN‐based devices are demonstrated, paving the way for the realization of high‐kt2‐reconfigurable acoustic filters for next‐generation communication systems operating at the super‐high‐frequency (SHF) range.