Atmospheric-pressure plasma oxidation of aluminum for large-area electronics

Abstract

We developed an atmospheric pressure plasma (APP) process for forming metal oxides and self-assembled monolayers (SAMs) without a vacuum process. SAMs are frequently utilized for surface modification. In particular, a combination of aluminum oxide (AlOx) and alkylphosphonic acid is used as the gate dielectric in flexible electronics, with a low operating voltage of 2 V because of its simple fabrication process at room temperature in air. However, when using a combination of SAMs and AlOx as the gate dielectric, it is necessary to form a metal oxide with sufficient thickness using a vacuum plasma process to reduce the leakage current, which is not suitable for use in flexible large-area electronics. In this study, we resolved this problem by utilizing an APP process and substrate heating instead of the vacuum plasma process. Heating a substrate above 100 °C forms a sufficient AlOx layer. The leakage current of plasma-processed AlOx with SAMs was reduced by three orders of magnitude compared to that of natural AlOx with SAMs. X-ray photoelectron spectroscopy revealed a time dependence of the thickness of AlOx. An exposure time of 100 s was sufficient to form an AlOx layer with a thickness of 4 nm. An organic transistor with this APP treatment of AlOx and SAMs showed a mobility of 1.4 cm2/V s and an on/off ratio of 104 within 2 V. We also fabricated an inverter and a ring oscillator circuit with an inverter gain of more than 300 and an oscillation frequency of 40 Hz.