Comparative study of bulk and surface compositions of plasma polymerized organosilicon thin films

•Study of plasma polymerized HMDSO chemical composition under various plasma energy.•Quantitative 29Si solid state NMR for organosilicon thin films.•XPS coupled with DFT calculations for surface characterization.•Films deposited under hard plasma shows a disorder in the bonds formation.•A difference between the chemical composition of the films surface and the bulk. Plasma polymerized organosilicon thin films were deposited in a MW-PECVD using hexadimethylsiloxane (HMDSO) as a precursor. Thin films were synthetized with different plasma conditions ranging from soft (low plasma energy) to hard (high plasma energy) ones. Quantitative 29Si solid state NMR and FTIR spectroscopy were used to characterize the bulk composition. X-ray photoelectron spectroscopy (XPS) coupled with Density Functional Theory (DFT) calculations were used to determine the surface chemical composition. In addition, water contact angle (WCA) measurements were performed in order to complete the surface characterization. The bulk and the surface of the plasma polymerized HMDSO (PP-HMDSO) showed a complex chemical composition. The four main environments M, D, T and Q were present in the films. An additional environment Si(CH2)(CH3)3 denoted S was detected in the PP-HMDSO films. The chemical composition comparison showed a difference between the bulk and the surface of the films. Under soft plasma conditions, a high amount of SiOC3 termination was present in the bulk of the films. While, the SiO2C2 chains were highly present on the surface. On the other hand, under hard plasma conditions, the number of the SiOC3 termination increased on the surface and decreased in the bulk. Under soft plasma conditions, the PP-HMDSO structure was close to PDMS, while under hard plasma conditions, the PDMS similarity was lost.