Removal of copper oxide from copper surfaces using Q-switched Nd:YAGradiation at 1064 nm, 532 nm, and 266 nm,

摘要

Abstract: During electronic device fabrication it is necessary to remove the oxides from copper surfaces prior to soldering in order to improve the surface wetability and achieve a good quality solder joint. The usual method of achieving this is by using acids in a flux. The work reported here explores the possibility of removing these oxides by laser cleaning using the harmonics of a Q-switched Nd:YAG laser, a technique which could be incorporated into a industrial laser soldering process. The effect of Q-switched Nd:YAG radiation (5 - 10 ns pulses), at 1064 nm, 532 nm and 266 nm, on the oxidized surface of a copper alloy foil is studied with increasing fluence. In order to successfully compare the effect of increasing fluence at the three wavelengths each area treated was only subjected to one laser pulse. The laser treated surfaces were characterized using optical microscopy, SEM, and surface analysis performed by static secondary ion mass spectrometry (SSIMS). SSIMS and SNMS (secondary neutral mass spectrometry) with mechanical depth profilometry were used to characterize the oxide layer. The reflectivity of the oxidized plates for the three wavelengths was ascertained using a reflectivity spectrometer. Successful cleaning was achieved at all wavelengths, above certain threshold values which defined the lower end of the process operating window for single pulse operation. The threshold for the cleaning process decreased with laser wavelength. Surface melting was evident at the lowest fluences examined for all the wavelengths ($LS .5 J/cm$+2$/). This value is well below the lower end of the process windows of all wavelengths. Microscopic `explosive' features were found at the onset of copper oxide removal possibly resulting from ionization or a plasma induced shock waves. There was some possible evidence of mechanical effects at 1064 nm and 532 nm. Large amounts of sputtered debris was found around the 266 nm craters. A SSIMS analysis was performed on the 532 nm spots. The SSIMS plots of the ratio of the CuO$PLU@/Cu$PLU counts versus laser fluence decrease sharply at the cleaning threshold to less than half the value for an untreated plate. SSIMS analysis below this point indicates a laser material interaction involving further oxidation of the copper oxides (predominantly Cu$- 2$/O) to black copper oxide (CuO), rather than material removal. This is also evident from the change in color of the copper surface. Continued increasing of the fluence eventually led to craters with irregular surfaces and large borders which would be unacceptable from the point of view of surface damage at 1064 nm and 532 nm. This defined the upper end of the process window. At 266 nm, the laser spots produced at the highest fluences used did not differ as significantly from those within the process window below this value.!13