P4J-3 Dip-Type Liquid-Phase Sensor Using SH-SAW

This paper describes an SH-SAW based sensor device which can be directly dipped into a liquid. The sensor device is composed of a two-channel SH-SAW delay line on a 36Y-X LiTaO₃. Those IDTs are surrounded with epoxy walls with a 50 mum height and a 160 mum thickness. Since the epoxy walls are constructed by a photo-lithography technique using a thick epoxy based photo resist, the thickness of wall can be minimized and as a result the SH-SAW propagation loss due to the walls can be minimized. Glass covers with an epoxy adhesive are attached onto the walls. Then air-cavities above the IDTs can be achieved and the IDTs can be perfectly isolated from the liquid. In this paper, experimental results of 50 MHz dip-type SH-SAW sensors on 36Y-X LiTaO₃ are presented. Since the SH-SAW which propagates on the un-metallized propagation area in a liquid with a low permittivity attenuates due to surface skimming bulk wave excitation, the obtained permittivity and conductivity are slightly different from the exact values. In order to improve this drawback, a compensation method is presented.