Dry process technology for high frequency SAW devices

Dry etching for fine patterns in high-frequency SAW (surface acoustic wave) devices is described. Several dry-etching processes are discussed in the connection with the following subjects: etching anisotropy, selectivity to photoresists and substrates (LiNbO₃, LiTaO₃, and quartz), and substrate damage. For the etching of fine Al electrodes in high-frequency SAW devices, reactive ion etching (RIE) is chosen, because RIE is the most cost-effective among the anisotropic dry-etching processes and is adequate for electrodes with low aspect ratios. Problems associated with the RIE were poor reproducibility (for example, induction of Al corrosion) and damage of the substrates. These problems are related to residual Cl from chlorinated etching gases and to Li contained in the substrates, e.g. LiNbO₃ and LiTaO₃. The defects caused by the corrosion are removed mainly by CF₄ post-sputter treatment just after RIE without exposing the substrates to the air. A damage-free process condition is obtained by measuring SAW velocity dispersion, etch depth, substrate roughness, and RHEED (reflection of high-energy electron diffraction)