Ultrathin adhesive layer between LiNbO₃ and SiO₂ for bonded LNOI waveguide applications

A RT wafer bonding method for waveguide applications was investigated using an ultrathin Fe intermediate adhesive layer between LiNbO₃ (LN) and SiO₂ wafers. Here we focus on the optimal amounts of Fe in this layer to minimize the propagation losses of the resultant LN on-insulator (LNOI) waveguide. A sub-nanometer-thick Fe-containing intermediate layer exhibited strong bonding strength (surface energy: >1 J m^-2) at RT, which may be sufficient for device applications. The influence of the Fe intermediate layer on the propagation loss of light through the LNOI waveguide was also investigated using numerical calculations. The present study is expected to be a significant contribution to the development of fabrication techniques for waveguides composed of various materials to be bonded by this RT bonding method using a metal intermediate adhesive layer.