Correlation between adhesion of diamond-like carbon film on LiTaO3 substrate and SAW velocity

Coating diamond-like carbon (DLC) film to LiTaO3 substrate is theoretically expected to stiffen the substrate surface so as to accelerate the surface acoustic wave (SAW) at the surface due to its high elastic constant and low mass density. However, it has been reported that DLC film on LiTaO3 substrate led to a reduction of SAW velocity probably due to poor adhesion between the DLC film and LiTaO3 substrate. In this study, the adhesion strength between DLC film and LiTaO3 substrates has been enhanced by introducing a SiO2 interlayer. Scratch test shows that relatively thick DLC films with an appropriated thickness SiO2 interlayer exhibit a good adhesion. The increase in SAW velocity correlates with the adhesion strength of the DLC film to the LiTaO3 substrate, indicating that the adhesion plays an important role of joining the two materials together. In addition, with an improved adhesion, SAW phase velocity in the DLC/SiO2/LiTaO3 structure increases with increasing DLC film thickness and with decreasing SiO2 interlayer thickness.