Highly Doped Single Crystal Al₁-ₓScₓN Bulk Acoustic Resonators for High-Frequency and Wideband Applications

This work reports a super high-frequency (SHF) bulk acoustic wave (BAW) resonator, utilizing a single crystal aluminum scandium (Sc) nitride (Al $_{{1}-{x}}$ Sc_xN) piezoelectric film with 30% Sc concentration, fabricated by a novel cavity-embedded process and exhibiting an effective electromechanical coupling coefficient ( ${k} ^{{2}}_{\text {eff}}$ ) of 21% for Al $_{{1}-{x}}$ Sc_xN BAW resonators operating above 4 GHz. A novel two-step method was proposed to obtain high crystalline quality Al_0.7Sc_0.3N films, characterized by the full-width at half-maximum (FWHM) of 0.73° in (0002) X-ray diffraction (XRD) and the absence of abnormal oriented grains (AOGs). The resonator unit, designed without lateral airgaps, doubles the thermal conductivity and significantly reduces the maximum stress of the suspended film stack by an order of magnitude. The novel cavity-embedded fabrication process combined with the lateral-airgapless device design offers excellent mechanical stability and manufacturing feasibility of BAW filters. The fabricated 4.39-GHz single crystal Al_0.7Sc_0.3N BAW resonators exhibit exceptional ${k} ^{{2}}_{\text {eff}}$ , leveraging the high crystallinity of Al_0.7Sc_0.3N film, surface-intact device design, and feasible fabrication process. Additionally, a 4.43-GHz ladder-typed filter was demonstrated with a −3-dB fractional bandwidth of 9.0%. This work paves the path toward the development of the next-generation high-frequency and wideband acoustic filters for emerging 5G and Wi-Fi wireless communications.