Pulsed laser deposition of smooth poly(methyl methacrylate) films at 248 nm

Poly(methyl methacrylate) (PMMA) was pulsed laser deposited at a wavelength of 248 nm using different laser fluences between 30 and 9000 mJ/cm2. While the composition of the films obtained after deposition at high laser fluences is close to that of the target, the molecular mass of the film material is reduced to about Mw = 8000 g/mol due to incubation processes independent on the high starting mass of the target. The microstructure of the films strongly depends on the laser fluence. After deposition at laser fluences above 500 mJ/cm2, the illuminated PMMA targets show a rough and molten surface with holes and bubbles resulting from strong subsurface heating. The corresponding PMMA films are very rough and exhibit a high number of droplets. In contrast, at laser fluences close to the deposition threshold of about 60 mJ/cm2 completely smooth films are obtained. Below the deposition threshold, the target performs incubation processes and colour changes occur, but it is only partially molten, and almost no ablation is observed. Only in a narrow fluence range above the deposition threshold (between about 80 and 160 mJ/cm2) a crater with a smooth surface is formed at the target leading to a high ablation rate and to completely smooth films without any droplets.