Near-threshold ultraviolet-laser ablation of Kapton film investigated by x-ray photoelectron spectroscopy
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K.C. Yung Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China
C.S. Xie The State Key Laboratory of Plastic Forming Simulation and Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan (430074), People’s Republic of China (Received 9 July 2002; accepted 8 October 2002)
Near-threshold ultraviolet-laser (355 nm) ablation of 125-m thick Kapton films was investigated in detail using x-ray photoelectron spectroscopy. Different from the irradiation at higher fluences, the contents of the oxygen, amide group, and C–O group on the ablated surface increased with an increase in the pulse number, whereas the carbon contents decreased, although the contents of the nitrogen and the carbonyl group (C⳱O) decreased slightly. This implied that there was no carbon-rich residue on the ablated surface. Near the ablation threshold, only photolysis of the C–N bond in the imide rings and the diaryl ether group (C–O) took place due to a low surface temperature rise, and the amide structure and many unstable free radical groups were created. Sequentially, the oxidation reaction occurred to stabilize the free radical groups. The decomposition and oxidation mechanism could explain the intriguing changes of the chemical composition and characteristics of the ablated surface. In addition, the content of the C–O group depended on the opposite factors: the thermally induced decomposition of the ether groups and the pyrolysis of the Caryl–C bond. Upon further irradiation, the cumulative heating may induce the breakage of the Caryl–C bond and enhance the oxidation reaction, resulting in an increase of the content of the C–O group.
I. INTRODUCTION
Since ultraviolet (UV) pulsed laser ablation of polymers was first reported,1,2 it has been studied extensively. These studies clearly show that ablation caused clean etching of polymers with excellent size resolution. Until now, UV pulsed laser ablation of polymers, as a dominant microvia technology, is a routine part in the manufacture of the high-density printed boards and substrates.3 Despite the large number of experimental and theoretical studies, the mechanisms involved in UV pulsed laser ablation of polymers—photothermal4–8 versus photochemical9–12—are still often controversial. Still other researchers have concluded that photothermal and photochemical decomposition both contribute, with each
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 1, Jan 2003
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dominating depending on the ablating conditions and chemical structure of polymers. 13–17 During laser ablation of polymers, the composition and chemical structure of the ablated surface is of potential use for design of UV laser tools for drilling. Srinivasan12 has repeatedly emphasized the importance of looking into the composition and characteristics of the ablated surface
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