Laser cleaning of ablation debris from CO2-laser-etched vias in polyimide

CO2-laser-drilled vias in polyimide-based flex circuits generate substantial 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 ultraviolet lasers, were found ineffective due to the presence of both massive (> 10 mu m) fibrous debris and submicron (< 500 nm) soot. The debris-removal process is driven by disparate mechanisms. The soot is ejected in only similar to 5 laser pulses by rapid thermal expansion of the laser-heated polyimide substrate. The removal of fibrous debris develops over many more pulses and involves Fresnel diffraction, surface-rippling phenomena, and multipulse ablation of the debris fragments. The fastest debris cleaning time of 2.5 s per via was provided by the 9R12 laser line at 20 Hz and 0.6 J/cm(2) fluence. (C) 1998 Elsevier Science B.V.