Growth Mechanism During Initial Stages of Molecular Layer Deposition of Polyurea

Molecular layer deposition (MLD) is of interest as a technique for growing ultrathin polymeric films for various applications. However, the basic surface adsorption and reaction process are still not well understood, leading to challenges in achieving high growth rates and good reproducibility. This study uses in situ Fourier-transform infrared spectroscopy in combination with in situ ellipsometry to better understand the initial stages of MLD of polyurea. 1,4 diisocyanatobutane (DICB) and diethylene triamine (DETA) were used as MLD precursors, and polyurea was grown through the reaction between an isocyanate group supplied by DICB and an amine group supplied by DETA. The growth per cycle was fit from ellipsometry measurements to be 0.022 nm/cycle. Film growth was also confirmed by infrared spectroscopy, which showed isocyanate groups being alternately added and consumed during the DICB and DETA half-cycles. Amide I/II modes increased in absorbance with every half-cycle as polyurea linkages were formed between incoming molecules and the growing film. We show that although some surface reactive sites are terminated during film deposition, new sites are introduced by precursor physisorption. Furthermore, we show that different precursors have different rates of double reaction and physisorption, highlighting the complex interactions between the molecular precursors and the growing film. DICB has a high rate of double reactions and a low rate of physisorption, as evidenced by the loss of isocyanate sites throughout initial stages of growth. To maintain linear film growth, DETA physisorbs into the film, thereby adding new amine sites for incoming molecules to react with. These results provide useful insight into mechanisms that control the MLD of thin films.