FORMATION OF INTRINSIC SURFACE-DEFECTS DURING 248 NM PHOTOABLATION OF POLYIMIDE

Photoablation of polymers by pulsed excimer laser radiation is commonly believed to be a controlled layer-by-layer removal process. A mass spectrometer was used to monitor neutral species ejected from polyimide samples in vacuum by 248 nm laser radiation. For fluences close to threshold, the ablation rate starts to drop after the first 100-200 pulses and eventually falls almost to zero. the falloff in etch rate is accompanied by a dramatic slowdown in the product translational energy distributions and by the appearance of conical defects on the sample surface. The number of cones is approximately the same for samples irradiated in air or vacuum for the same number of pulses, proving that ablation debris is not the initiating factor. It is argued that carbon enrichment at the sample surface initiates cone formation by locally shifting the ablation threshold to higher values. In effect, the polymer surface becomes ''radiation hardened.''