Quantification of polyimide carbonization after laser ablation

Polyimide was irradiated with a XeCl excimer laser (308 nm) and the ablated area and its surrounding were studied using transmission electron microsco py (TEM) and confocal Raman microscopy. Ring-like structures surrounding th e ablated area were detected at all fluences. At fluences lower than 250 mJ /cm(-2) the formation of conical structures was observed within the irradia ted area. The width of the rings increases with fluence and only slightly w ith the number of pulses. The rings consist mainly of polycrystalline carbo n with a relatively high bond angle disorder, with thickness decreasing rad ially from the crater edge. The thickness of the deposited carbon was deter mined from TEM analysis and calculated from the intensity ratios of Raman b ands assigned to carbon and polyimide using a two layer model. Comparing th e two results an estimate of the absorption coefficient of the deposited ca rbon could be obtained. On top of the cone structures carbon was detected w ith a higher degree of crystallinity and lower bond angle disorder as compa red to the material deposited outside the crater. With energy dispersive x- ray analysis, calcium could be detected on top of the cones. Therefore, it can be assumed that the Ca impurities are causing the cone structures. The higher crystallinity of the carbon inside the irradiated area is probably d ue to a tempering-like process on top of the Ca compound which is heated up on laser irradiation or to a mixture of growth mechanisms similar to the on es suggested for the formation of carbon nanotubes on metal particles and c arbon nanohorns without metal catalysis. (C) 2000 American Institute of Phy sics. [S0021- 8979(00)04919-7].