OPTICAL ELLIPSOMETRY AND ELECTRON-SPECTROSCOPY STUDIES OF COPPER OXIDATION RELATED TO COPPER ON PRINTED-CIRCUIT BOARDS

The aim of the present work was to investigate the oxidation of pure copper samples and of those cut out from circuit boards by using optical ellipsometry XPS and AES, in order to establish whether ellipsometry alone could be used as a simple, economical means for routine analysis at the wiring line of circuit boards. Samples of pure copper, with a polished surface or after treatment in an ENTEK solution, were oxidized for different lengths of time at 180-degrees-C, the temperature at which circuit boards are heated before the soldering and wiring process. The oxide growth rate is constant for thicknesses of the oxide layer up to approximately 100 angstrom. The thicknesses measured by XPS/AES depth profiles agree well with those measured by ellipsometry provided that two oxide films, Cu2O and CuO, are present for oxidation times greater than 5 mins. XPS measurements confirm that Cu2O forms during the early stages of oxidation, while CuO begins to form at later stages. An empirical correlation between the experimental ellipsometric and Auger spectroscopy data on 50 samples cut out from circuit boards, before and after the wiring process, permitted the oxide thickness and the quality of the copper surface of the circuit boards to be established by ellipsometry alone. An ellipsometer was installed on the assembly line of the circuit boards and is used for routine analysis of the copper surface of the boards.