Loss reduction of longitudinal-type leaky SAW by reverse proton exchange

To reduce the attenuation of the longitudinal-type leaky surface acoustic wave (LLSAW), we proposed a layered structure of air, a bulk LiNbO₃ (LN) layer, and an elastically softened LN substrate. When the layered structure was applied to X-36°Y LN with a large coupling factor K² for the LLSA W, the calculated attenuation for the metallized surface decreased at a certain depth of the bulk layer. To realize such a layered structure, a reverse proton exchange (RPE) process was applied to X-36°Y LN. A "good region" existed on the RPE wafer, in which the value of K² for the Rayleigh-type SAW was recovered to approximately 80% of that of the virgin wafer. In comparison with the virgin wafer, the propagation loss of the good region on the RPE wafer was decreased to 2/5, the insertion loss was decreased drastically, and the resonance properties were improved markedly. When the Li⁺ concentration in the proton source was increased from 1.0 to 2.0 mol%, the area of the good region on the RPE was increased from 1.0 to 2.0 mol%, the area of the good region on the RPE wafer increased fourfold.