Investigation of substrate-molecule and molecule-molecule interactions on the chemical vapor deposition of conformal thin films
Authors:   , Rumrill Shayna M , Palmese Giuseppe , Lau Kenneth K. S. 
Published: February 2025

Ultrathin conformal films, films that are less than 100 nm thick, are important in applications such as surface energy modification, air-gap fabrication in micro- and nano-electromechanical systems (MEMS/NEMS), low dielectric coatings, and protective coatings with good mechanical, thermal, and chemical stability. To achieve such films, precise control over the process of thin film formation is required. Solvent-based methods suffer from surface tension, diffusion, and wettability effects that lead to poor conformality and defects such as de-wetting with pinhole formation in planar films, bridging between nano-features, and unwanted pore-filling or capillary collapse in micro- and nano-trenches. Additionally, solvents can be damaging to underlying substrates and are environmentally harmful. In this thesis, chemical vapor deposition (CVD) is demonstrated as a one-step, solvent-free method for depositing ultrathin conformal films that overcomes solvent-based processing constraints. The focus on the studies shown here is how specific substrate-molecule and molecule-molecule interactions affect the deposition and formation of ultrathin, conformal films, where previously these interactions have not been considered or fully explored in CVD literature. First, iCVD is used to deposit a hydrophilic polymer, poly(hydroxyethyl methacrylate) (PHEMA), onto a hydrophobic substrate of polytetrafluoroethylene (PTFE). The large interfacial energy difference -- from the surface energy difference between the two polymers -- causes PHEMA to prefer to de-wet and bead up on the PTFE, rather than spread conformally. The iCVD process is carefully controlled to achieve polymer growth rates sufficiently fast to cause dense nucleation of PHEMA on the PTFE surface, which quickly grows laterally and coalesces into ~10 nm thick films. This is the opposite of conventional iCVD depositions where very slow polymer growth rates are used to form conformal films. This highlights the substrate-dependence on film formation, where a large surface-energy difference between the film and substrate will cause de-wetting and formation of non-conformal films due to vertical preferential growth that does not fully cover the underlying substrate below 100 nm of total film thickness. Second, the effect of crystal structure orientation during the formation of the polymer on ultrathin film conformality is studied using the cationic CVD of the linear homopolymer polyoxymethylene (POM). In CVD, polymer crystallization happens simultaneously with polymer growth. Polymer chains can be arranged in either highly oriented, extended chain crystal structures (ECC), or random coils as layered lamellae in folded chain crystal structures (FCC). Bulk polymer melts of POM are found to be FCC when cooled slowly and ECC when cooled quickly. This is expected to translate to CVD as slow and fast deposition rates, which are a measure of polymer growth and therefore crystallization rates. This is believed to be a result of slow crystallization allowing the polymer chains to relax into an FCC conformation, while rapid crystallization locks into place the ECC conformation. Third, the effect of 2D network formation during CVD deposition was studied using a thermal batch CVD process. A fully organic two-dimensional polymer (2DP) has yet to be reported in CVD literature. A dicyanate ester monomer, 1,4-dicyanatobenzene, was synthesized. This monomer thermally polymerizes into a highly aromatic network that is expected to yield a 2DP. Monomer and substrate heating temperature were varied to observe adsorption and annealing behaviors of the polymer film, where slow adsorption followed by annealing lead to extended ultrathin films. This is believed to be because the slow adsorption results in low nucleation, allowing film to grow across the surface in large sheets. Additionally, a Cu substrate surface was explored as a catalyst and was found to reduce the thermal energy needed to achieve good polymer yields. Overall, this thesis demonstrates the highly controllable CVD process that can deposit ultrathin conformal films while overcoming substrate-molecule and molecule-molecule interactions that can affect film formation and achieve coatings for applications in surface energy modification, electronics, and durable coatings.