Fabrication of RR-P3HT-based TFTs using low-temperature PECVD silicon nitride passivation

Abstract

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a commercially available semiconducting polymer. Its high processability makes it favorable for fabrication of organic thin film transistors (OTFTs). Depending on the processing technique and device configuration, the field effect mobility of this polymer ranges from 0.01 to 0.1 cm²/Vs. The mobility also shows a correlation with the choice of gate dielectric material. The most commonly reported dielectric materials for OTFTs are SiO₂, Al₂O₃ and Ta₂O₅. In this work, we report a new fully encapsulated top-gate RR-P3HT-based TFT structure with a-SiNx implemented as the gate dielectric and passivation material. The fabrication process enables realization of discrete transistors and transistor circuits through four consecutive photolithographic steps. The process is compatible for various substrates including Corning glass, Si wafers, and any appropriate plastic substrates. This paper addresses a number of critical technological issues such as substrate surface treatment to improve film adhesion, optimal spin coating conditions for uniform polymer film formation, preparation of device quality a-SiN_x films by plasma-enhanced chemical vapor deposition (PECVD) at 75°C substrate temperature, and a tailored etch process for patterning of the polymer film. Current-voltage characteristics of the fabricated transistors are analyzed to evaluate the quality of the polymer/a-SiN_x interface.