Tri-layer PEALD ZnO thin film transistors and circuits

In oxide semiconductors, defect chemistry and hydrogen can influence free carrier concentration and these materials can have strong interactions with the atmosphere and contaminents.[1,2] An inverted staggered structure is commonly used for oxide TFTs and a high-quality passivation layer is needed to provide protection and minimize back channel surface charge changes.[2,3] Negative shifts in turn-on and threshold voltage after passivation with inorganic thin films have been reported by several groups.[4,5] We have previously reported weak oxidant plasma enhanced atomic layer deposition (PEALD) ZnO TFTs with an ALD-based Al₂O₃ passivation layer [6]. Before passivation the TFT has a turn-on voltage near 0 V, but significant hysteresis (often > 0.5 V). A 32 nm thick Al₂O₃ layer deposited by ALD eliminates the hysteresis, but causes a negative shift in turn-on and threshold voltage (~3 V). A 32 nm thick Al₂O₃ layer deposited by PEALD also removes the hysteresis, but shifts the device turn-on and threshold voltage negative by more than 10 V [6]. We have developed a tri-layer process for bottom-gate, top contact TFTs. An Al₂O₃-ZnO-Al₂O₃ tri-layer is deposited sequentially at 200°C and provides effective passivation and reduced turn-on voltage shift.