Damage free Al reactive-ion-etching for high frequency SAW devices

The RIE (reactive ion etching) of Al is studied for the fabrication of fine electrodes in high-frequency SAW (surface acoustic wave) devices on substrates such as LiNbO₃ and LiTaO₃ , using a discharge excited at 13.56 MHz in gases containing BCl ₃. Sputter treatments with CF₄, cleaning of the chamber, and postcleaning the samples just after RIE are shown to improve reproducibility of the etching markedly by removing defects connected with Cl and Al. Analyses of H-atom emission from Ar and Cf₄ discharges show the effect of treatments that remove Cl and H₂O. Damage of substrates surfaces is investigated by measuring the dispersion of SAW velocity, reflected high-energy electron diffraction, etch depth, and surface roughness on the substrates. These measurements reveal the existence of a threshold for reactive ions. Below the threshold, although reactive ions slightly etch the amorphous surface layer caused by polishing, the damage is negligible for SAW propagation. The distribution of Al etch rate is equalized, which is different from the case of Si substrates. By using RIE with the abovementioned condition, precise, fine Al electrodes are replicated for high-frequency SAW devices on substrate composed of Li compounds