ST-quartz/LiTaO3 direct bonding using SiO2 amorphous layers with VUV/O3 pre-treatment for a novel 5G surface acoustic wave device

This paper describes a novel ST-cut quartz (ST-quartz)/LiTaO3 (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 SiO2 (α-SiO2) intermediate layers. The α-SiO2 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/O3). Then they were bonded under pressure of 5 MPa at 200 °C for 15 min in 100 kPa vacuum atmosphere. The tensile strength of 2.9 MPa was achieved in α-SiO2 substrate which is six times stronger than other samples; without intermediate layers or VUV/O3 pre-treatment. In addition, VUV/O3 bonding was compared with Mega-sonic bonding. VUV/O3 treated sample with AIB method slightly increase the bonding strength and achieved the same level of Mega-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 hetero-monocrystalline bonding. This result suggested the proposed ST-quartz/LT direct bonding is a promising technique for future 5G SAW devices.