Pretreatment by laser radiation and its effect on adhesion in the metallization of poly(butylene terephthalate)

Chemical and morphological changes resulting from pretreatment with excimer-laser radiation were investigated for poly(butylene terephthalate) (PBT) surfaces. The irradiated surfaces were coated with aluminum or copper films of thickness less than or equal to 10 nm, enabling the effects of irradiation on the metal/polymer interface to be studied. In addition, adhesion was tested for electroplated nickel films on irradiated surfaces. Irradiation was carried out in air with KrF-excimer laser radiation (lambda = 248 nm) at fluences per pulse in the range epsilon = 3-600 mJ/cm(2). The threshold fluence for material removal, epsilon(t) = 38 +/- 12 mJ/cm(2), was determined by profilometry measurements; the morphology of the irradiated surfaces was determined by optical and scanning electron microscopy; and the chemical properties of the polymer and the metal/polymer interface were studied with X-ray photoelectron spectroscopy (XPS). Depending on the irradiation conditions (fluence, number of laser pulses), various surface morphologies were found, including slightly discolored surfaces, structured surfaces (ripples), and surfaces with indications of melting and resolidification. The existence regions in irradiation-parameter space for these various structures were estimated. Irradiation at fluences near epsilon(t) resulted in oxygen depletion and bond scission. At higher fluences, the O/C ratio decreased continuously with irradiation, reaching a value similar to 30% less than that of the unirradiated surface for irradiation at epsilon = 401 mJ/cm(2). After deposition of an aluminum overlayer 3 nm in thickness, the ester (O=C-O) C 1s XPS core level was reduced in intensity, suggesting an aluminum binding site on the carbonyl (O=C) oxygen. Both Al-0 and Al3+ were found, the latter being located at the interface. In comparison with unirradiated areas, irradiated areas exhibited only a slightly higher amount of Al-0 species, indicating no irradiation-induced enhancement of interfacial metal binding sites. Deposition of a copper overlayer 1 nm in thickness caused a uniform attenuation of all C 1s core level peaks and only CuO was found for both unirradiated and irradiated surfaces, indicating a weak interaction between the metal and the polymer. Electroplated nickel films exhibited enhanced adhesion in irradiated areas for pretreatments that produced structured surfaces (epsilon = 150 mJ/cm(2), N = 50 pulses, roughness R-z similar to 12 mu m), and also for pretreatments for which no strong irradiation-induced surface structures were found (epsilon = 3.6-6.4 mJ/cm(2), N = 1800-3600 pulses).