Low-Residual-Stress Amorphous Film for LiTaO<sub>3</sub>/Quartz or LiNbO<sub>3</sub>/Quartz Bonding toward 5G Surface Acoustic Wave Devices

LiTaO3 (LT)/quartz or LiNbO3 (LN)/quartz bonded surface acoustic wave (SAW) substrates with an amorphous intermediate layer have been proposed for high-frequency communication. Requirements for 5G mobile communication are high-performance SAW substrates with a large SAW velocity, a small temperature coefficient of frequency, and a large electromechanical coupling factor. Reduction of the residual stress of the amorphous intermediate layer is expected to improve the bonding strength and SAW characteristics of the bonded substrate. In this report, a method of low-residual-stress amorphous film deposition for LT/quartz or LN/quartz bonding was studied. The residual stresses of amorphous SiO2 and Al2O3 films deposited by ion beam sputtering, electron cyclotron resonance sputtering, and atomic layer deposition (ALD) were evaluated. The LT substrate with an amorphous Al2O3 film deposited by ALD (ALD-Al2O3) had the minimum warpage of 0.152 µm and residual stress of 127.3 MPa. The ALD-Al2O3 film formed with near-identical thicknesses on both sides of the LT substrate simultaneously, which is likely the source of the low residual stress of the ALD-Al2O3 film. A maximum LN/quartz bonding strength of 3.7 MPa was achieved with the ALD-Al2O3 film. These results indicate that ALD-Al2O3 films are promising materials for LT/quartz or LN/quartz SAW substrates in 5G devices.