Investigation on Alumina Passivation for Improved IGZO TFT Performance

Abstract

A study on the influence of the back-channel interface on the operation of bottom-gate indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) is presented. TFTs without any passivation material deposited typically exhibit best-case initial results. However a passivation layer is required for device stability and process integration. The surface of IGZO is very sensitive to deposition conditions of passivation materials; in general additional oxygen vacancies are created which renders the IGZO conductive. Annealing in an oxidizing ambient may reverse this effect and result in devices which exhibit appropriate semiconducting properties. The impact of passivation using alumina deposited via electron beam evaporation and atomic layer deposition (ALD) has been investigated. A decrease in TFT channel mobility is attributed to the IGZO/alumina back-channel interface degradation associated with the alumina passivation process. Low temperature ALD (e.g. T ≤ 150°C) has been explored to minimize this degradation effect. Pre-passivation and post-passivation anneal treatments have also been explored to better understand the mechanisms involved. Improvements in the back-channel interface were shown to depend on the thickness and deposition method of the alumina passivation layer. Temperature-dependent measurements reveal differences in electrical characteristics including bias-stress stability. Modifications in the passivation and annealing procedures and process integration details have resulted in a marked improvement in the performance of passivated devices.