Ageing Effects on Sputtered YSZ Thin Films and Gold Electrodes for Electrochemical Sensor Applications

Abstract

8mol% Yttria Stabilized Zirconia (8YSZ) is one of the most studied solid electrolytes. With increasing demand to miniaturize electrochemical sensors and solid oxide fuel cells using 8YSZ thin film electrolytes, it is vital to study the ageing effects of 8YSZ thin films. Although ageing effects in bulk YSZ had been previously studied, no work has been reported on YSZ thin films. This paper reports the fabrication, characterization of RF sputtered, polycrystalline 8YSZ thin films and the effect of prolonged heat treatment on the ionic conductivity and surface morphology. The films are annealed at 10000C for three hours before starting the aging experiment. With electrochemical sensor applications in mind, the ageing temperature was chosen to be closer to exhaust gas temperature (i.e, 6500C). The results show that there is no significant variation in the ionic conductivity and activation energy values of 8YSZ thin films (250 nm) before and after 15000 minutes of heat treatment at 6500C. The conductivity of YSZ film is constant at 0.057Scm-1 throughout the course of high temperature soaking in spite of significant grain growth. The grain size of the film is found to increase from 15nm in as deposited film to 60nm in aged film. The activation energy value before and after heat treatment was 0.98eV. After 15000 minutes, the YSZ film started developing cracks in the vicinity of thick film gold electrode and the conductivity values deteriorated. However, no cracks are present in the (bare) YSZ sample without any electrodes. Hence the conductivity deterioration is attributed to the cracks on YSZ triggered by the presence of thick film gold electrode. YSZ samples are aged with three types of thin film gold electrodes on them i) 200nm Au without Ti adhesion layer ii) 200nm Au with 10nm Ti adhesion layer and ii) 200nm Au with 10nm Ti adhesion layer in 45nm trench on YSZ. The electrodes without adhesion layer completely de-wetted itself in 48 hours. Ti/Au layers showed re-crystallization induced porosity after 48 hours and gradually disintegrated around 100hours. In the third type where Ti/Au layers were deposited on a 45nm trench on 250nm YSZ thin film. This buried electrode seemed more promising and continuous up to at least 15000 minutes. Figure 1