Low-cost lithographically patterned source/drain bottom contacts for high mobility p-type organic thin film transistors
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Low-cost lithographically patterned source/drain bottom contacts for high mobility p-type organic thin film transistors Robert Mueller1, Steve Smout1, Myriam Willegems1, Jan Genoe1, and Paul Heremans1,2 1 imec, Kapeldreef 75, B-3001 Leuven, Belgium 2 KULeuven, Dpt ESAT, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium ABSTRACT Short channel organic thin film transistors in bottom-gate, bottom contact configuration use typically gold metallization for the source and drain contacts because this metal can easily be cleaned from photoresist residuals by oxygen plasma or ultraviolet-ozone and allows also surface modification by self-assembled monolayers (e.g. thiols). Alternative low-cost bottom contact metallization for high performance short-channel organic thin film transistors are scarce because of the incompatibility of the bottom contact material with the cleaning step. In this work a new process flow, involving a temporary thin aluminum protection layer, is presented. Short channel (3.4 μm) pentacene transistors with lithographical defined and thiol modified silver source/drain bottom contacts (25 nm thick, on a 2 nm titanium adhesion layer) prepared according to this process achieved a saturation mobility of 0.316 cm2/(V.s), and this at a metal cost below 1% of the standard 30 nm thick gold metallization. INTRODUCTION During the past decades organic electronics evolved from a laboratory curiosity towards a technology for innovative products such as radio-frequency identification tags (RFID-tags) [1] and microprocessors [2] on flexible foil. In order to achieve sufficiently high frequencies the organic thin film transistors (OTFTs) need to fulfill several requirements. (i) Since the operation frequency of the OTFTs is proportional to μ/L2 [3], where μ is the mobility and L the channel length of the OTFT, a sufficient high mobility must be achieved at short channel length, (ii) the uniformity of the transistors must be high; this means that the standard deviation of the oncurrent to their average and the standard deviation of the threshold voltage are low [4], and (iii) the process flow must be compatible with high yield industrial processes. For pentacene OTFT based circuits as used in [1,2] these requirements are fulfilled for a bottom-gate bottom contact configuration with self-assembled monolayer modified lithographic defined gold bottom contacts. Due to usage of a gold layer (typical thickness: 30 nm) for source/drain metallization low cost applications seem to be precluded because the gold cost alone might already attain 25% of the target price [5]. Although a large variety of conducting materials has been evaluated for pentacene transistors with bottom contact configuration [5] (Fig. 1) a suitable gold replacement for lithographical defined short channels is not straightforward. Indeed, many results reported so far were achieved with large channels OTFTs, mostly prepared by shadow mask technologies. Shadow mask technologies however are currently not suitable for electronic circuits with short channel bottom c
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