Design, Fabrication, and Characterization of Aluminum Scandium Nitride-Based Thin Film Bulk Acoustic Wave Filter

The demand for filters with a large fractional bandwidth, up to 5%, still puts a great challenge to the typical aluminum nitride (AlN) thin films-based bulk acoustic wave (BAW) filters in an acoustic-only approach. This paper presents the design, fabrication, and characterization of film bulk acoustic wave resonator (FBAR) and filters using aluminum scandium nitride (Al0.8Sc0.2 N) thin films. The fundamental thickness-mode resonance of the FBAR is measured at 4.235 GHz with an excellent effective electromechanical coupling factor ( $K_{eff}^{2}$ ) of 14%. The filters employ a modified structure with additional $\vphantom {^{\int }}$ molybdenum (Mo) layers formed on the connection strips between each adjacent resonator. The proposed filtering topologies including ladder topology, lattice topology, and ladder-lattice topology are constructed. Results show that the modified connection strips can decrease the magnitude impedance at series resonant frequency and improve the insertion loss (IL) of the filters. The fabricated ladder filter demonstrates a high center frequency of 4.25GHz, low IL of 1.804 dB, wide bandwidth of 189 MHz, and out-of-band rejection of 30 dB, holding a great potential for the 5G wideband applications. [2022-0201]