Design and Fabrication of AlN Thin Film based Bulk Acoustic Wave Filters Targeting the 2.3 GHz frequency band

The explosive growth of smart home and Internet of Things (IoT) devices has induced significant spectral congestion and put forward higher requirements for the high-performance radio frequency (RF) filters. Film bulk acoustic wave resonators (FBARs), especially those based on aluminum nitride material, play a pivotal role in the realm of high-performance RF filters for wireless communication systems. In this work, we fabricate an AlN piezoelectric thin film through physical vapor deposition (PVD) on an AlN seed layer, achieving high crystalline quality with a full width at half maximum (FWHM) of 1.72° and a highly oriented (002) crystalline structure. Leveraging this optimized film, we demonstrate a FBAR operating at around 2.3 GHz with an impressively high quality factor (Q) exceeding 1700. Through optimizing the area of each resonator within the filter, the proposed filter effectively optimizes signal transmission and demonstrates significant performance improvements in minimizing the insertion loss and strengthening the out-of-band rejection. The implemented filter design achieves superior spectral performance with a minimum insertion loss of -1.11 dB, a -3 dB bandwidth ranging from 2.320 GHz to 2.409 GHz, and the out-of-band rejection below -36 dB. Owing to the optimized resonator geometry design, the FBAR filters we developed exhibit excellent passband behavior and thus pave the way for more stable and efficient wireless communication systems.