Bias stress stabilities of PMMA-passivated indium-gallium-zinc-oxide thin-film transistors after 100 °C steam exposure

Bias stress stabilities of the polymethyl methacrylate (PMMA)-passivated IGZO thin-film transistors (TFTs) after being exposed in a normal and harsh (100 °C steam) environment were studied, in order to comprehensively evaluate protection effects of PMMA. In a normal environment, the PMMA-passivated TFTs exhibited normal switching characteristics and electrical stabilities. However, the switching characteristics and bias stress stabilities were changed after being exposed on 100 °C steam. There were negative V_th shifts on the transfer curves of the steam-exposed IGZO TFTs. Our XPS analysis revealed that the negative ΔV_th was related to the steam-induced H₂O molecules throughout the IGZO films, which acted as electron donors to introduce more electrons in the front channel. Under PBS, the steam-exposed IGZO TFTs showed an abnormal negative V_th shift while the un-exposed IGZO TFTs showed negligible V_th shift. This abnormality was ascribed to the electrons released from steam-induced H₂O molecules, which render the conductive path more easily opened. Under NBS, the steam-exposed IGZO TFT presented larger negative V_th shift than the un-exposed TFT. This result was interpreted in terms of the steam-induced donor states (H₂O molecules) near or at channel/insulator interface. Under PBTS and NBTS, the changes in V_th for steam-exposed TFTs were similar to those for un-exposed TFTs. Such a similarity indicates that steam exposure had no effects on NBTS and PBTS stabilities. It was understood in terms that the steam-induced H₂O⁺ recombined with the electrons released from the steam-induced H₂O molecules under bias stress, forming H₂O to compensate the thermally-induced H₂O adsorption. Our results suggest that one-micron-thick PMMA passivation layer enabled to protect IGZO TFTs from H₂O in a normal environment, but it provided inadequate protection in a harsh environment. Therefore, a thicker PMMA passivation layer should be considered.