Temperature Dependence of the Growth per Cycle during Molecular Layer Deposition of Polyurea on Plasma Deposited SiO2

In molecular layer deposition (MLD) the growth per cycle (GPC) depends on process conditions with no well-defined temperature window. During MLD of polyurea using toluene diisocyanate (TDIC) and ethylendiamine (ED) as the MLD precursors, the GPC decreased with deposition temperature from 0.22 nm at 0 C to 0.022 nm at 45 C. While we observed reaction of toluene diisocyanate with the underlying SiO2 at 45 C, at 22 C, in situ infrared spectroscopy indicates that the MLD film is not anchored to the substrate via covalent bonds. TDIC and ED displayed different temperature-dependent interactions with the growing polyurea film during MLD. Specifically, at lower temperatures, ED was more likely to be added to the film via physisorption, and as a consequence, TDIC was more likely to double react with both of its functional groups to -NH2 sites within the film. While the lower GPC at higher temperature is driven predominantly by a decrease in ED physisorption, the rate of TDIC double reaction must also decrease due to the lower density of available reactive sites. As a result, at higher temperatures, both ED physisorption and TDIC double reactions are suppressed, and the ratio of polyurea linkages formed in each half-cycle was closer to unity. While the GPC was the highest at 0 C, we also show that the films deposited at a lower temperature were unstable both in vacuum and the ambient, and showed up to ~15% decrease in thickness, most likely due to the desorption of physisorbed precursor molecules.