Low-temperature bonding of a LiNbO₃ waveguide chip to a Si substrate in ambient air for hybrid-integrated optical devices

We report the low-temperature bonding of a lithium niobate (LiNbO ₃) chip with gold (Au) thin film to a silicon (Si) substrate with patterned Au film for hybrid-integrated optical devices. The bonding was achieved by introducing the surface activation by plasma irradiation into the flip-chip bonding process. After the Au thin film (thickness: 500 nm) on the LiNbO₃ chip (6 mm by 6 mm) and the patterned Au film (thickness: 2 μm) on the Si substrate (12 mm by 12 mm) were cleaned by using argon (Ar) radio-frequency (RF) plasma, Au-Au bonding was carried out in ambient air with applied static pressure (̃50 kgf). The LiNbO₃ chips were successfully bonded to the Si substrates at relatively low temperature (< 100 °C). However, when the bonding temperature was increased to be greater than 150 °C, the LiNbO₃ chips cracked during bonding. The tensile strength (calculated by dividing the total cross-sectional area of the initial, undeformed micropatterns) of the interface was estimated to be about 70 MPa (bonding temperature: 100 °C). It was sufficient for use in optical applications. These results show the potential for producing highly functional optical devices and for low-cost packaging of LiNbO₃ devices.