Sponsored Student: Flexographic Patterning in Vacuum of Sacrificial Oil for Aluminium Contacts of Organic Transistors

Combining vacuum deposition of functional materials with patterning is crucial for low cost roll-toroll production of devices such as transistors, organic displays and photovoltaics. An investigation into flexography printing in vacuum to create organic thin film transistors is presented. Specifically the rotary flexographic process has been optimised to pattern a sacrificial oil onto a plastic web, on top of which aluminium is thermally evaporated to form the source and drain of common gate top contact transistors. This deposition process requires a lower latent heat of vaporisation of the oil than the thermally evaporated metal, in order that the patterned oil is evaporated as the metal vapour condenses on the substrate. Using a custom built flexographic printer the oil was loaded onto the relief pattern of a silicone plate roller in ambient conditions. Under vacuum the plate roller/process drum nip force was varied to remove killer defects and achieve the best line edge, assessed using image analysis of the patterned metal stripes. This aluminium metallisation process was applied to organic thin film transistors with a molybdenum tri-oxide carrier injection layer. The results demonstrate the applicability of this flexographic process to metallizing any pattern on vacuum processed high end applications.