Sputtering and surface modification of thermoplastic polymers with low energy ion beams
Low energy ion beam irradiation proved itself to be suitable technique to alter chemical and physical properties of polymer surfaces. Spin coated thin films and commercial sheets of polymers were irradiated with argon, nitrogen and oxygen ions at energies of 0.5 – 5 keV in the fluence range from 10E12 to 10E16 cm-2. When ion interacts with polymer surface, chain bonds are broken resulting in the formation of smaller molecules, many of which may be volatile that are desorbed. At the same time, formed free radicals participate in different reactions during which the chemistry of polymer surfaces is easily and irreversibly changed resulting in alteration of polymer surface properties. Polymer sputter rates were found to depend on polymer chemistry and ion beam parameters. The sputter rates of polymers decreased strongly with the ion fluence, followed by constant removal rate in the steady-state region. The decrease of typically one order of magnitude was attributed to the chemical changes in polymer structure that occurred in the ion fluence range 10E13 – 10E15 cm-2 depending on polymer chemistry, ion energy and type. Induced changes in the polymer surface layer showed large influence on metal/polymer adhesion properties. Three types of practical adhesion strength, measured using 90° peel test, were observed: (i) adhesion increased with the ion fluence until saturation was reached; (ii) peel strength increased at low ion fluencies, reached maximum and at prolonged treatment decreased; (iii) no improvement in the peel strength on treated polymer surfaces was recorded. XPS analysis of peeled-off surfaces showed that in most cases the failure location changed from interfacial for untreated polymers to cohesive failure in the polymer for treated surfaces. An improvement in the metal polymer adhesion in the ion fluence range 10E13 – 10E16 cm-2 is attributed to the creation of a large density of new adsorption sites resulting in larger contact area and incorporation of chemically active groups that lead to the increased interaction between metal and polymer through metal-oxygen/nitrogen-polymer species formation.