KrF-excimer laser pretreatment and metallization of polymers
Metal film adhesion to polymers can be improved by pretreatment with UV-laser radiation before metal deposition. Chemical changes associated with irradiation are investigated for polyimide (PI) and polybutylene terephthalate (PBT) surfaces. Irradiated surfaces are coated with aluminum films < 10 nm in thickness, enabling the effects of irradiation on the metal/polymer interface to be studied. Irradiation is done in air with KrF-excimer laser radiation (lambda = 248 nm) at fluences per pulse < 600 mJ/cm 2 . The threshold fluence epsilon(t) for material removal is determined by profilometry measurements of etched features, and the chemical properties of the polymer and the metal/polymer interface are studied with X-ray photoelectron spectroscopy (XPS). Aluminum films are thermally evaporated in situ in the XPS spectrometer. Irradiation of PI at fluences near epsilon(t)(41 mJ/cm2 ) results in loss of oxygen and opening of the imide ring, resulting in doubly bonded nitrogen species. After evaporation of aluminum the carbonyl (C=O) C1s XPS signal is reduced in intensity, and both Al(0) and Al(3+) are found, the latter being located at the interface. In comparison to unirradiated areas, irradiated areas have more aluminum in total and a higher proportion of interfacial Al(3+) species, indicating an increase in the concentration of metal binding sites. Although for PBT the O to C ratio also decreases with irradiation at fluences near epsilon(t) (38 mJ/cm2), changes in the amounts of Al(0) or Al(3+) for irradiated areas in comparison to unirradiated areas are much smaller than for PI and consist mainly of a slight enhancement of Al(0) for films deposited on irradiated surfaces.