Passive Alignment and Mounting of LiNbO $_3$ Waveguide Chips on Si Substrates by Low-Temperature Solid-State Bonding of Au

In this study, passive alignment and mounting of lithium niobate (LiNbO₃) chips, with a large mismatch in the coefficient of thermal expansion with most semiconductors, are demonstrated for hybrid-integrated optical devices. LiNbO₃ chips were aligned passively using the visual index alignment method and were subsequently bonded on the Si substrates by low-temperature solid-state bonding with Au microbumps, which allow for electrical connections and heat dissipation. Au-Au bonding was carried out at 100°C in ambient air after surface activation by argon RF plasma. The vertical bonding accuracy was determined by assessing the height variations of the Au microbumps due to the plastic deformation in the bonding process. The bonding accuracies in the horizontal and vertical directions were estimated to be within ±1 μm. Average excess loss due to misalignment between titanium-diffused single-mode LiNbO₃ waveguides and V-groove-guided single-mode fibers was about 0.5-dBm per interface (wavelength: 1.55 μm).