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

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Fabrication of RR-P3HT-Based TFTs Using Low-Temperature PECVD Silicon Nitride Passivation Sarswati Koul, Yuri Vygranenko, Flora Li, Andrei Sazonov, and Arokia Nathan Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada 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-SiNx films by plasma-enhanced chemical vapor deposition (PECVD) at 75oC 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-SiNx interface. INTRODUCTION Organic semiconductor materials are receiving considerable attention in view of its great promise for low-cost large-area flexible displays [1,2]. Organic thin film transistors (OTFTs) with field-effect mobility up to 0.01-0.1 cm2/Vs have been reported using regioregular poly(3hexylthiophene) (RR-P3HT) as the channel material [3]. Variations in the extracted mobility values of RR-P3HT OTFTs are attributed to the different processing techniques that are used for the semiconductor layer deposition [4]. Depending on the deposition method, films of varying degrees of molecular ordering are formed. Since the electrical properties and intrinsic carrier mobility depend critically on the molecular ordering and extent of π-π stacking within the polymeric backbone, a deviation in the mobility of P3HT films is observed [5,6]. In addition to the quality of the polymer thin film, the device structure, contact resistance, choice of material, encapsulation, and optimization of the dielectric-semiconductor interfaces are critical to device performance [7-9]. The most common gate dielectric materials for OTFTs are SiO2, Al2O3, Ta2O5 and PVP (poly-vinylphenol) [10]. Polymeric gate dielectric material often exhibits hysteresis leading to a continuous shift in threshold voltage, thus undermining device stability. Coop