Insight into plasma polymerization with a significant contribution of etching to the deposition process

Abstract

Plasma polymer deposition processes are well-studied and optimized on flat substrates. Understanding the role of substrate geometry is crucial for optimizing deposition on non-planar substrates. We investigated the altered transport of film-forming species into two 3D geometries, cavities with a slit opening and a cavity with an undercut, to assess the contribution of ions to the deposition and etching and to estimate the sticking coefficient of depositing species for the CO₂/C₂H₄/Ar gas mixture. Profilometry and ellipsometry were employed to obtain film thicknesses. It revealed a significant extension of the deposition inside the cavities attributed to film-forming species with a low sticking coefficient. These depositing species contain less oxygen because a spatially resolved ATR-FTIR analysis revealed an increasing proportion of hydrocarbons further inside the cavity. Inside the cavities with a slit, the film thickness exceeded its value on the flat Si surface outside. This difference indicated that ions responsible for etching collide during their flight toward the growing film inside the slit. However, it also suggests that some ionic species contribute to the deposition because directional species become more prominent under the slit than outside due to the geometrical shielding of thermalized species represented by the structure’s angular aperture. Monte Carlo simulations inside the cavity with different slits confirmed that diffusion alone did not explain the experimentally obtained profiles, as the model considering thermalized deposition and etching particles is not able to reproduce the narrow film thickness profiles obtained in the cavities with a slit. The model with directional particles demonstrated significantly better agreement.