Low temperature bonding of LiNbO₃ waveguide chips to Si substrates in air

This paper describes the low-temperature bonding of a lithium niobate (LiNbO₃) waveguide chip to a silicon (Si) substrate for integrated optical systems. The bonding was achieved by introducing the surface activation by plasma irradiation into the flip-chip bonding process. After the surfaces of the Au thin films (thickness: 100 nm) of the LiNbO₃ chip and the Si substrate were cleaned using an Ar radio frequency (RF) plasma, Au-Au bonding was carried out only by contact in ambient air with applied static pressure. The bonded chips fractured at bonding temperature higher than 150°C because of the coefficient of thermal expansion (CTE) mismatch. The LiNbO₃ chips were successfully bonded to the Si substrates at relatively low temperature (100°C). The die-shear strength of the LiNbO₃ chip was estimated to be more than 12 kg (3.8 MPa), the upper limit of our shear testing equipment.