LASER CLEANING OF ABLATION DEBRIS FROM CO2-LASER-ETCHED VIAS IN POLYIMIDE

CO2-laser-drilled vias in polyimide-based flex circuits generate subst antial surface debris, requiring new approaches to reduce or eliminate the debris and therefore do away with wet chemical or plasma cleaning steps. A dry laser cleaning process based on a wavelength-tunable CO2 laser is shown for the first time to effectively remove the ablation debris. Other techniques based on gas flow, pressure control, or ultra violet lasers, were found ineffective due to the presence of both mass ive (> 10 mu m) fibrous debris and submicron (< 500 nm) soot. The debr is-removal process is driven by disparate mechanisms. The soot is ejec ted in only similar to 5 laser pulses by rapid thermal expansion of th e laser-heated polyimide substrate. The removal of fibrous debris deve lops over many more pulses and involves Fresnel diffraction, surface-r ippling phenomena, and multipulse ablation of the debris fragments. Th e fastest debris cleaning time of 2.5 s per via was provided by the 9R 12 laser line at 20 Hz and 0.6 J/cm(2) fluence. (C) 1998 Elsevier Scie nce B.V.