Influence of nitrogen implantation on the properties of polymer films deposited in benzene glow discharges

Abstract

Investigations of the structural, optical and electrical properties of plasma-polymerized benzene films were carried out as a function of the ion fluence. The films were obtained from rf (40 MHz, 70 W) plasmas of benzene and afterwards implanted with 170 keV molecular nitrogen ions (N 2 +) with fluences Φ ranging from 10 17 to 10 21 m −2. Infrared (IR) and ultraviolet–visible (UV–VIS) spectroscopy were used to analyze the chemical and the electronic structure of the films, respectively. Further structural informations were obtained from etching experiments using an oxygen plasma. Electrical resistivity measurements were performed by the two-point probe technique. With rising ion fluence, the IR spectra of the implanted films reveal an increasing loss of hydrogen, the formation of new single and condensed aromatic structures and oxygen-containing functional groups. The latter are formed from reactions between the free radicals created upon ion implantation and oxygen from ambient air. The optical gap E G, calculated from the UV–VIS spectra of the films, showed a decrease from 1.6 to 0.9 eV as Φ increased from 0 to 10 19 m −2. The electrical resistivity ρ of the films decreases significantly with the fluence. The decreases in E G and ρ are attributed to an increase in the overlap of the π-orbitals. The plasma etching experiments show that at low ion fluences, polymer chain degradation prevails. The highly chemically resistant polymer surface developed under high fluences suggests a strongly cross-linked structure.