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.