The propagation properties and resonance characteristics of leaky surface acoustic waves (LSAWs) and longitudinal-type LSAWs (LLSAWs) on a LiTaO3 (LT) thin plate bonded to an AT-cut quartz substrate were investigated experimentally. For the LSAWs and LLSAWs, the bonded structures of 36°Y-cut X-propagating LT (36°YX-LT)/AT-cut 90°X-propagating quartz (AT90°X-quartz) and X-cut 31°Y-propagating LT (X31°Y-LT)/AT-cut 45°X-propagating quartz (AT45°X-quartz) were fabricated, respectively. For the LSAW on 36°YX-LT/AT90°X-quartz, the electromechanical coupling factor (K2) of 11.1% was obtained at an LT thin-plate thickness of 0.25 wavelength, whereas K2 for a single LT substrate was measured to be 5.7%. For the LLSAW on X31°Y-LT/AT45°X-quartz, K2 increased from 2.8% for the single LT substrate to 7.2% at an LT thin-plate thickness of 0.14 wavelength. Furthermore, K2 of approximately 12% and the temperature coefficient of frequency (TCF) of 0 ppm/°C were theoretically obtained simultaneously for the LSAW on 36°YX-LT/AT-cut 90°X-quartz at a certain thin-plate thickness.

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This paper describes a novel ST-cut quartz (STquartz) /LiTaO 3 (LT) direct bonding for surface acoustic wave (SAW) devices of next 5G mobile communication. The ST-quartz and LT were bonded to fabricate temperature compensated piezoelectric substrates using amorphous SiO 2 (α-SiO 2 ) intermediate layers. The α-SiO 2 thin layer was prepared on each substrate by ion beam sputtering (IBS) to realize highly active bonding interfaces and treated by vacuum ultraviolet irradiation in the presence of oxygen gas (VUV/O 3 ). Then they were bonded under pressure of 5 MPa at 200 °C for 15 min in 100 kPa vacuum atmosphere. The tensile strength of 2. MPa was achieved in α- SiO 2 substrate which is six times stronger than other samples; without intermediate layers or VUV/O 3 pre-treatment. In addition, VUV/O 3 bonding was compared with Mega-sonic bonding. VUV/O 3 treated sample with AIB method slightly increase the bonding strength and achieved the same level ofMega-sonic bonding sample with AIB. Hence, it is indicated that AIB method could prepare the considerably activated surface even using low vacuum condition and affect effectively to heteromonocrystalline bonding. This result suggested the proposed STquartz/LT direct bonding is a promising technique for future 5G SAW devices.