Low-Residual-Stress Amorphous Film for LiTaO₃/Quartz or LiNbO₃/Quartz Bonding toward 5G Surface Acoustic Wave Devices

Abstract

LiTaO 3 (LT)/quartz or LiNbO 3 (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-residualstress amorphous film deposition for LT/quartz or LN/quartz bonding was studied. The residual stresses of amorphous SiO 2 and Al 2 O 3 films deposited by ion beam sputtering, electron cyclotron resonance sputtering, and atomic layer deposition (ALD) were evaluated. The LT substrate with an amorphous Al 2 O 3 film deposited by ALD (ALD-Al 2 O 3) had the minimum warpage of 0.152 µm and residual stress of 127.3 MPa. The ALD-Al 2 O 3 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-Al 2 O 3 film. A maximum LN/quartz bonding strength of 3.7 MPa was achieved with the ALD-Al 2 O 3 film. These results indicate that ALD-Al 2 O 3 films are promising materials for LT/quartz or LN/quartz SAW substrates in 5G devices.

Modeling and Optimization of CMOS Compatible Various ZnO/SiO2/Si Multilayer Structure for SAW Devices Using FEM

“…Recently, fabrication of pitch of the IDTs pattern for a maximum frequency of operation is limited by the photolithography resolution. Hence, varying the piezoelectric substrate materials with high acoustic velocity is the key to achieve high-frequency SAW devices [4].…”