Self-aligned flexible organic thin-film transistors with gates patterned by nano-imprint lithography
Many applications that rely on organic electronic circuits still suffer from the limited switching speed of their basic elements - the organic thin film transistor (OTFT). For a given set of materials the OTFT speed scales inversely with the square of the channel length, the parasitic gate overlap capacitance, and the contact resistance. For maximising speed we pattern transistor channels with lengths from 10 mu m down to the sub-micrometre regime by industrially scalable UV-nanoimprint lithography. The reduction of the overlap capacitance is achieved by minimising the source-drain to gate overlap lengths to values as low as 0.2 mu m by self-aligned electrode definition using substrate reverse side exposure. Pentacene based organic thin film transistors with an exceptionally low line edge roughness <20 nm of the channels, a mobility of 0.1 cm(2)/Vs, and an on-off ratio of 10(4), are fabricated on 4 '' x 4 '' flexible substrates in a carrier-free process scheme. The stability and spatial distribution of the transistor channel lengths are assessed in detail with standard deviations of L ranging from 185 to 28 nm. Such high-performing self-aligned organic thin film transistors enabled a ring-oscillator circuit with an average stage delay below 4 mu s at an operation voltage of 7.5 V.