Intra-Grain and Oligo-Grain Top-Contact Organic Thin film Transistors
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Intra-Grain and Oligo-Grain Top-Contact Organic Thin film Transistors Stijn Verlaak, Stijn De Jonge, Stijn Noppe, Dimitri Janssen, Soeren Steudel, Stijn De Vusser and Paul Heremans Polymer and Molecular Electronics group, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. ABSTRACT We describe and demonstrate a micromachined shadowmask that allows the realization of intra-grain and oligo-grain top-contact organic thin film transistors (OTFTs). First experimental results of OTFT’s show that for small channel lengths, grain boundary barriers indeed appear to dominate the output characteristics of OTFTs. INTRODUCTION Organic thin film transistors (OTFTs) offer a potentially low-cost, large-area technology for low-end applications. Sublimed organic small-molecule thin films offer advantages as active layer in OTFTs compared to solution-processed films. Small-molecule thin films can have higher charge-carrier mobilities due to their crystalline nature and often are of higher purity. However, polycrystalline films have traps at grain boundaries and at the interface with the gate dielectric. Both trap distributions can be expected to depend on growth conditions, providing for a complex relation between morphology and OTFT properties like mobility, threshold voltage, onset voltage and subthreshold slope. Intragrain transistors and transistors having only a limited number of grain boundaries in their channel are a useful tool to study this relation. While photolithographically defined bottom-contacts can easily reach channel lengths comparable to or smaller than the grain size of typical organic thin films, the growth of the organic thin film on top of those contacts is often disturbed by the presence of the contacts. Moreover, contact effects are usually more pronounced than for contacts grown on top of the organic thin films due to the smaller effective contact area of the bottom-contacts [1]. We evaporated metal contacts on top of organic thin films through a micromachined shadowmask to make transistors having channel lengths down to 400nm. This top-contact approach allows to study the properties of intragrain and oligograin transistors without influencing film growth and with little contact effects. The technology to make those micromachined shadowmask will be presented here, together with the first results on organic thin films. EXPERIMENT The fabrication of the shadowmask using standard micromachining techniques is explained in figure 1. 500nm SiO2 is grown on a silicon wafer with (100) surface. Next, 150 nm LPCVD Si3N4 is deposited on both sides of the wafer. This nitride will grow almost free of stress on the oxide, apart from some residual tensile stress. On the backside of the wafer large rectangular areas, having a width approximately twice the wafer thickness, are defined approximately parallel to the main crystal axes using photolithography. In those areas, the nitride is removed by CF4 plasma. In a KOH-solution, a trench is etched fully through the silicon wafer selectively along the (111) surfaces, using the
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