FABRICATION OF FLEXIBLE FIELD-EFFECT TRANSISTORS ON 25 μm PEN FOIL

Abstract

The development of high performance thin-film transistors on flexible plastic substrates is of great importance for the manufacturing and industrialization of flexible electronic devices, such as flexible displays. Here we present different approaches to fabricate bottom-gate field-effect transistors on 25 µm heat stabilized polyethylene naphthalate (PEN) foil using photolithography and solution processing. The flexibility and dimensional instability of the substrate constituted the main challenge during manufacturing. We developed a novel method to handle the foils during processing by reversibly attaching them onto 6 inch Si wafers. The so-called foil-on-carrier assembly showed excellent flatness (about 1 µm) and good dimensional stability and tolerance towards the different processing steps up to 160°C. Transistors were made either using an organic polymer or an inorganic oxide as gate dielectric material. Source and drain gold electrodes were patterned using standard photoresist patterning followed by wet etching, or by lift-off. The feature sizes in the transistors were downsized from 5 µm to sub-micrometer to improve the performance. In order to achieve structures with high resolution, all functional layers were patterned using a commercial wafer stepper (PAS5500/100D). The two dielectric materials used and the two ways of making the source-drain resulted in four different metal-insulator-metal (MIM) stacks. The MIM stacks were characterized by optical microscopy, scanning electron microscopy and four point resistance measurements. Registration accuracies below 0.5 µm were found over the whole wafer. Transistor fabrication was finished by depositing pentacene derivatives from solution.