Study of Low-residual Stress Amorphous Film Deposition Method for LiTaO₃/Quartz or LiNbO₃/Quartz Bonding toward 5G Surface Acoustic Wave Devices

LiTaO₃ (LT) or LiNbO₃ (LN)/Quartz bonded substrates with an amorphous intermediate layer were proposed to achieve both a large surface acoustic wave (SAW) velocity and a smaller temperature coefficient of frequency. Residual stress reduction of the amorphous film is expected to improve the bonding strength of a SAW substrate. In this report, we studied a method of low-residual stress amorphous film deposition for LT or LN/Quartz bonding. The residual stress of the LT substrate with an amorphous SiO₂ or Al₂O₃ film deposited by ion beam sputtering, electron cyclotron resonance sputtering, and atomic layer deposition was evaluated. The LT substrate with the amorphous Al₂O₃ film deposited by ALD had the minimum warpage (-0.152 μm) and residual stress (127.3 MPa). The residual stress of the Al₂O₃ film deposited by ALD might be reduced because almost the same thickness of the Al₂O₃ film was deposited on both sides of the LT substrate at the same time. The maximum bonding strength of 3.7 MPa was achieved in the substrate with the Al₂O₃ film deposited by ALD. From these results, LT or LN/Quartz substrates with the Al₂O₃ film deposited by ALD are promising materials to reduce residual stress toward SAW devices for 5G mobile communication.