Long-term drift mechanism of Ta2O5 gate pH-ISFETs

In this paper, the long-term drift mechanism of Ta2O5 gate pH-ISFET has been investigated by new long-term drift-testing protocol of pH-ISFET (IMCS7). Ta2O5 gate films of pH-ISFETs were grown from pentaethoxytantalum [Ta(OC2H5)5] by using plasma-enhanced chemical vapor deposition (PECVD). The thermal treatments were carried out in dry O2, or O3, after Ta2O5 gate film deposition, and in-depth distributions of the additive fluorine (F) in Ta2O5 gate film were also tested. Any thermal treatments in dry O2 or O3 gas cannot reduce the light-induced drifts of the Ta2O5. In the samples that had the impurity F located on the top of the surface of the Ta2O5 film, large light-induced drifts and long-term drifts were observed. The main origin of light-induced drifts is located on the interface between electrolyte and the surface of Ta2O5 films. No long-term drift of the Ta2O5 gate films will be attained by designing the ultra-thin layer on the surface of the Ta2O5 films: the low-temperature process and the reduction of impurities in Ta2O5 films.