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 a nd the availability of reliable 308 nm optics has motivated the develo pment 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 stron gly promote laser ablation at low fluences in the case of chlorobenzen e (CB), but not in the case of N-methyl 2-pyrrolidinone (NMP). At low laser fluences, many laser pulses may be required before significant n eutral particle emissions are observed - an incubation effect. Scannin g electron microscope observations indicate that the onset of emission coincides with the rupture of a thin surface layer, presumably deplet ed of solvent during film manufacture. The depleted layer would be rel atively impervious to volatile fragments produced in the bulk. When th is 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. (C) 1998 Elsevier Science B.V.