Area-Selective Molecular Layer Deposition of Polyimide on Cu through Cu-Catalyzed Formation of a Crystalline Interchain Polyimide

Novel area-selective molecular layer deposition (AS-MLD) of polyimide (PI) on Cu versus native SiO₂ was studied. By use of 1,6-diaminohexane (DAH) and pyromellitic dianhydride (PMDA) as precursors, PI films can be selectively deposited on the Cu surface at 200–210 °C with a rate around 7.8 Å/cycle while negligible growth takes place on SiO₂. The selectivity was successfully demonstrated also on Cu/SiO₂ patterns at 200 °C; after 180 MLD cycles, around 140 nm thick PI was deposited on Cu regions while <10 nm thick PI was measured on SiO₂ regions. By contrast, at 170 °C, similar growth rates were measured on Cu (4 Å/cycle) and native SiO₂ (5 Å/cycle). To understand the origin of the selectivity, properties of PI film grown on Cu at the selectivity enabling temperature of 200 °C have been thoroughly compared with those grown on both Cu and native SiO₂ at the nonselective temperature of 170 °C. Significant differences in crystallinity, surface morphology, and chemical structure suggest that the PI films grown on Cu at 200 °C have a novel crystalline interchain PI structure. According to this finding, the selectivity mechanism is proposed to derive from a new MLD reaction mechanism catalyzed by copper ions at 200–210 °C that leads to the formation of new crystalline polymer species. Additionally, it is suggested that small amounts of Cu ions can float on the surface of the growing PI film and thereby constantly catalyze the MLD reaction, as evidenced by XRD, XPS, and ToF-ERDA. The high catalytic efficiency has been also proven by using successfully an ultrathin Cu layer, nominally 0.1 nm, as a seed layer to initiate the MLD growth.