Sensitization of PMMA to laser ablation at 308 nm

Pure polymethylmethacrylate PMMA.is highly resistant to ablation at 308 nm. The value of PMMA in lithography and semiconductor packaging and the availability of reliable 308 nm optics has motivated the development of dopants to facilitate PMMA ablation at 308 nm. We investigate the laser ablation of solvent cast PMMA films with and without pyrene, a typical dopant. The presence of residual solvent is shown to strongly promote laser ablation at low fluences in the case of chlorobenzene CB., but not in the case of N-methyl 2-pyrrolidinone NMP.. At low laser fluences, many laser pulses may be required before significant neutral particle emissions are observed – an incubation effect. Scanning electron microscope observations indicate that the onset of emission coincides with the rupture of a thin surface layer, presumably depleted of solvent during film manufacture. The depleted layer would be relatively impervious to volatile fragments produced in the bulk. When this layer ruptures, volatile fragments escape and can be detected. Thus, the ablation behavior depends not only on the choice of dopant, but on the choice of solvent and the details of film manufacture. q1998 Elsevier Science B.V