Influence of Ar+ ion irradiation on the properties of plasma polymerized acetylene films

Plasma polymerized acetylene is an amorphous material whose properties can be significantly changed upon ion irradiation. In this work, the modification of the structural, optical, electrical and mechanical properties of PPA films by 150-keV Ar+ ions is investigated. The ion fluence varied from 1018 to 1021 m−2 at a constant current of 100 μA. Structural and optical properties were analyzed using infrared and ultraviolet-visible spectroscopies, respectively. From oxygen plasma etching experiments, further structural information was obtained. A two-point probe was employed to measure the electrical resistivity. Hardness measurements were made using a nanoindenter. The infrared spectra showed depletion of hydrogen and increased concentration of carbon double bonds with increasing ion fluence. As a consequence, the optical, electrical and mechanical properties were also modified. The optical transmission in the UV and visible regions were diminished with increasing ion fluence, reflecting a narrowing of the optical gap. The electrical resistivity decreased with increasing fluence; a decrease of 11 orders of magnitude with respect to the pristine film was measured for the highest fluence (1021 m−2). A strong increase of hardness was observed as the ion fluence was increased. A hardness of 10.6 GPa was measured after irradiation at the fluence of 1021 m−2. The oxygen-etching rate decreased significantly as the fluence was increased. Both the hardness and etching results strongly indicate that a highly cross-linked structure was formed at high ion fluences.