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Cleaning of copper traces on circuit boards with excimer laser radiation

: Wesner, D.A.; Mertin, M.; Lupp, F.; Kreutz, E.W.


Applied surface science 96/98 (1996), S.479-483 : Ill., Lit.
ISSN: 0169-4332
Fraunhofer ILT ()
copper trace; excimer laser radiation

Cleaning of Cu traces on circuit boards is studied using pulsed excimer laser radiation (pulse width ca. 20 ns, wavelength 248 run), with the goal of improving the properties of the Cu surface for soldering and bonding. Traces with well-defined oxide overlayers are cleaned by irradiation in air using <= 10 (exp 3) laser pulses at fluences per pulse of <= 2 J cm(-2). After treatment the surface morphology is analyzed using optical microscopy, optical profilornetry, and scanning electron microscopy, while the chemical state of the surface is investigated with X-ray photoelectron (XPS) spectroscopy. Ellipsometry is used to determine the oxide overlayer thickness. Prior to cleaning samples exhibit a contamination overlayer about 15-25 nm in thickness containing Cu2O and C. Cleaning reduces the overlayer thickness to <= 10 nm by material removal. The process tends to be self-limiting, since the optical reflectivity of the oxidized Cu surface for laser radiation is smaller than that of the c leaned surface. Additionally, the interaction with the laser radiation results in surface segregation of a minor alloy component out of the bulk (e.g. Zn), which may help to passivate the surface for further chemical reactions.