Orthogonal Processing: A Novel Photolithographic Patterning Method for Organic Electronics

We describe the discovery and evaluation of a semi-perfluoroalkyl resorcinarene photoresist which is processable in hydrofluoroether solvents (HFEs). This novel, high-performance imaging material is specifically designed to be orthogonal to the vast majority of organic conductors and semiconductors, and hence enable their photolithographic patterning. Furthermore, this material paves the way for the multilevel patterning of organic electronic materials, as demonstrated by the fabrication of overlaid patterns of a polyfluorene and a transition metal complex [Ru(bpy)3](PF6)2. Summary of Research: Organic electronics is emerging as a promising technology to enable mechanically flexible devices through solution processing of organic electronic materials [1]. As with traditional electronics, organic devices require active functional materials to be tailored into micro-patterned and multi-layered device components. While the former relies on photolithographic patterning techniques, the latter is restricted from adopting such robust, high-resolution and high-throughput techniques because of the chemical compatibility issue between organic electronic materials and patterning agents [2]. By carefully selecting less damaging processing solvents and designing new patterning materials processable in such solvents, we can enable the use of conventional lithography with sensitive organic materials. In our search for universal, material-friendly solvents, we have identified highly fluorinated liquids (fluorous solvents), which are in general poor solvents for nonfluorinated organic materials. Among the variety of fluorous solvents, segregated hydrofluoroethers (HFEs) are attractive because of their nonflammability, zero ozone-depletion potential and low toxicity for humans [3,4]. By combining these benign solvents with specifically tailored photoresist materials, we are able to apply conventional lithography to patterning organic materials. Figure 1: Acid-catalyzed deprotection of the resorcinarene imaging material. The next stage is to design photoresist materials which can be processed in HFEs. A design motif was hinted by molecular resists, which are amorphous small molecules with mono-disperse molecular weight distribution. Recently we have reported that a resorcinarene material possesses excellent patterning properties under conventional lithographic conditions [5]. The same molecular framework was adopted, to which four semi-perfluoroalkyl chains and eight acidcleavable tert-butoxycarbonyl (tBoc) groups were appended. The resulting resorcinarene (RF-CalixtBoc) in Figure 1 is able to form a negative tone image by transformation into an insoluble form upon an