Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies

Abstract

The formation of C₂ in the ablation of polymethyl methacrylate (PMMA) by 193- and 248-nm laser pulses was studied by laser-induced fluorescence. The formation of C₂ is observable at a fluence at 248 nm that is well below the fluence threshold where significant etching takes place. The opposite is true at 193 nm. The velocity distribution of the diatomic form was also measured and found to approach but not exactly fit a Maxwell–Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. The other products of the laser ablation of PMMA at 193 or 248 nm are methyl methacrylate (MMA) and a solid that is a low-molecular-weight fraction of PMMA. Although the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilog plot of the mass of material removed versus the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm² at 193 nm; 600–2000 mJ/cm² at 248 nm), which corresponds to rapid etching, is identified as ablative photodecomposition. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably because of the secondary absorption of the incoming photons by the products. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. It is suggested that ablative photodecomposition involves both a one-photon process that produces MMA and low-molecular-weight polymeric fragments and a many-photon process that gives rise to products such as C₂ with high translational energy. In order to establish the role of the many-photon process, samples of PMMA doped with acridine were exposed to laser pulses. The presence of as little as 2% of acridine lowers the threshold for significant etching at 248 nm from 400 to 90 mJ/cm². The absorption characteristics of the doped polymer at this wavelength show that the acridine molecules absorb >10 photons each, whereas the absorption of the polymer changes little. The drop in the threshold energy for etching should therefore be attributed to the many-photon decomposition of the acridine and the consequent ablation of the polymer as a whole.