A compact and high optical transmission SAW touch screen with ZnO thin-film piezoelectric transducers

A compact and high optical transmission touch screen has been developed using surface acoustic waves (SAWs) oscillated by ZnO thin-film piezoelectric transducers. It has a 98% optical transparency and a narrow frame of 1.4 mm. Therefore, it is ideally suitable for mobile devices, such as PDAs or cellular phones. Utilizing a single-phase transducer (SPT) constructed with a ZnO piezoelectric thin film sandwiched between a lower planar electrode and a comb-type electrode enabled us to reduce the ZnO film thickness to 1/20. The peculiarity of the ZnO transducer is using the local maximum of the electromechanical coupling factor with the SPT structure. It has been found that the hillocks on the lower electrode extremely affects the insertion loss and their suppression can improve the ZnO transducer quality. It is very helpful to introduce our own index, the area ratio of hillocks, to determine whether an aluminum alloy is suitable for the ZnO thin-film transducer or not. This paper describes the ZnO thin film we developed for the new SAW touch screen.