Surface oxygen in plasma polymerized films

Abstract

For plasma polymerized (PP) thin films, many practical optical, electronic, sensing, and bio-applications are closely related to their surface properties. In particular, the surfaces of many PP films have a strong affinity for oxygen and moisture. Therefore, three different types of monomers which do not contain oxygen were selected to fabricate PP films, in order to understand the mechanisms for surface oxygen absorption. These monomers were a hydrocarbon, benzene (B); ferrocene (FC), containing Fe ions which have a great affinity for oxygen; and octafluorocyclobutane (OFCB), containing the strong electronegative (and thus oxygen repellent) element fluorine. X-Ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FTIR) spectroscopy, and electron spin resonance (ESR) were used to explore the chemical composition and structure of the resulting PP films. XPS depth-profiling was also used to investigate the oxygen content in the bulk of the films by analyzing the film surface after various amounts of argon etching. The initial oxygen content on the surface of the PP-FC films was the largest of the three while PP-OFCB films only contained a trace quantity. PP-B films had an intermediate concentration. Affinity for oxygen for this latter film was determined to be due to residual activated species including free radicals and dangling bond sites on the film surface. Depth profiling disclosed little oxygen a short distance into the PP-B films, indicating that the oxygen was attracted after deposition upon exposure to ambient conditions. Although the PP-B film exhibited a high concentration of free radicals as determined by ESR, the dense and crosslinked bulk structure shielded these active centers in the film by prohibiting oxygen diffusion. For the PP-FC films, although a decrease in the amount of oxygen was observed after etching, a substantial concentration of oxygen exists with the depth, indicating incorporation of oxygen during the initial deposition. Because of the chemical nature of fluorine, the as-deposited PP-OFCB films did not exhibit significant affinity towards oxygen. However, a slightly oxygen enriched film surface was present after argon etching due to changes in the surface chemistry and structure. These results demonstrate that the formation and distribution of oxygen on and within the PP films are strongly dependent upon the chemical composition and structure of the films.