To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

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This Letter reports a new bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric film which has the potential application in 5G wireless communication. The single crystal AlN is deposited on SiC substrate by MOCVD and the air-cavity structure BAW device is fabricated. Testing results show that the fabricated resonators have Q-factor up to 837 and electromechanical coupling coefficient up to 7.2% with resonant frequency 3.2 GHz. The ladder-type filters are also developed on the same wafer, which have a center frequency of 3.38 GHz and 3 dB bandwidth 160 MHz, minimum insertion loss of 1.5 dB, and out-of-band rejection above 31dB. High performance of the filters comes from low defects of the single crystal AlN film, which demonstrates promising potential of single crystal AlN filters in 5G and future 6G applications.