With the rapid development of wireless communication systems and also commercialization of fifth generation (5G) technology in the world, the demand for wider bandwidth and high speed data transmission has been increased drastically. To fulfil these demands radio frequency (RF) filters based on acoustic wave technology has drawn great attention as one of the effective solutions for achieving high-performance next-generation (5G) RF filters. To fabricate piezoelectric thin film based high-frequency devices such as surface acoustic wave (SAW), bulk acoustic wave (BAW), thin film bulk acoustic resonator (FBAR) and thin film piezoelectric on substrate (TPoS) resonator, it is required to choose a piezoelectric material with higher acoustic velocity. Among all other piezoelectric materials, Aluminium Nitride (AlN) has been subject of a great deal of interest for the fabrication of acoustic wave resonator devices because of its CMOS process compatibility and unique properties such as wide bandgap (∼6.2 eV), high electrical resistivity (109–1011 Ω.m) and very high acoustic velocity in both longitudinal (∼11,354 ms−1) and transversal (∼5,500 ms−1) modes. The physical properties of AlN films are strongly influenced by the crystallographic orientation, which depends on the process parameters. Differently oriented piezoelectric films exhibit different vibrational modes and acoustic wave properties. Therefore, the orientation of the piezoelectric films strongly affects the device performance. It has been theoretically predicted that the properties of SAW devices with (100) oriented AlN films are better than those with (002) oriented AlN films. It may be noted that, no performance evaluation of electroacoustic devices with (100) oriented AlN film has been reported till today.

By optimizing the sputtering process parameters such as deposition pressure, RF power, substrate temperature, target to substrate distance and N2 concentration, we have been able to grow preferential (100) and (002) oriented AlN thin films on different metal electrodes such as Mo, Ti and BNCD films. We have demonstrated for the first time growth, fabrication and characterisation of preferential (100) oriented AlN thin film based solidly mounted bulk acoustic wave (SMR-BAW) resonators. To fabricate SMR-BAW resonators, preferential (100) oriented AlN thin films were grown on top of Bragg reflector composed of six alternate layers of Ti and Mo to form high-low acoustic impedance layer. The resonance frequency of the measured device was around 5.75 GHz. The measured SMR-BAW resonator exhibit coupling coefficient of 1.4% with -24.3 dB return loss. The acoustic velocity of the fabricated preferential (100) oriented AlN based SMR resonator is ∼6900 m/s. This type of resonator can be a good alternative to SAW and FBAR for 5G applications with CMOS compatible process flow.