Initiated chemical vapor deposition of polymeric thin films : mechanism and applications

Abstract

Initiated chemical vapor deposition (iCVD) is a novel technique for depositing polymeric thin films. It is able to deposit thin films of application-specific polymers in one step without using any solvents. Its uniqueness of in situ surface polymer synthesis distinguishes iCVD from conventional processes such as spin-on deposition and plasma-enhanced chemical vapor deposition. It allows engineering polymers to be made with specific microscale properties translating to well-defined macroscale behaviors. In this thesis work, two application-specific polymers based on poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(cyclohexyl methacrylate) (PCHMA) were synthesized using iCVD. PHEMA thin films with specific degrees of cross-linking leading to well-defined structural, thermal, wetting, and swelling properties were made in a single vacuum step by simply adjusting chamber conditions. Cross-linked PCHMA thin films were synthesized for use as sacrificial layers for microfabrication. Such films of engineering polymers cannot be made using conventional methods. A study of the polymerization mechanism was included to serve as a groundwork for increased understanding of iCVD as a thin- film deposition method.

(cont.) Growth rates and molecular weights, crucial parameters for polymeric thin films, were found to be highly dependent on the surface concentrations of monomers, leading to the conclusion that polymer formation occurs predominantly on the surface of the substrate. This conclusion also infers that controlling the surface concentrations of monomers can lead to copolymers/terpolymers with well-defined compositions, which was demonstrated in the iCVD of PHEMA-based thin films. iCVD therefore can be extended to complex polymer systems with multiple monomeric building blocks. Photo- initiatied chemical vapor deposition (piCVD) using a volatile photoinitiator is introduced for the first time in this thesis. piCVD possesses all the benefits of iCVD over conventional processes but uses a photochemical initiation mechanism that simplifies chamber design and potentially allows self-patterning during deposition.