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 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 the 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 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 in both vacuum and the ambient and showed up to ∼15% decrease in thickness, most likely due to the desorption of physisorbed precursor molecules.
Scholl et al. (Sun,) studied this question.