Interaction of nanosecond EUV pulses with organic polymers

Complete text of publication follows. Interaction of nanosecond pulses of extreme ultraviolet (EUV) with organic polymers have been studied. The EUV pulses were produced from a compact laser plasma EUV source based on a gas puff target formed by pulsed injection of a small amount of gas under high-pressure into a laser focus region. The use of the gas puff target instead of a solid target allows for efficient generation of EUV radiation without debris production. EUV radiation in the wavelength range of about 5 to 50 nm was produced by irradiation of xenon or krypton gas puff target with a commercial Nd:YAG laser operating at 10 Hz and delivering 4 ns pulses of energy up to 0.8 J per pulse. The time duration of the EUV pulses was about 3 ns. The source was equipped with a grazing incidence axisymmetrical ellipsoidal mirror to focus EUV radiation in the relatively broad spectral range with the strong maximum near 10 nm. The size of the focal spot is about 1.3 mm in diameter with the maximum fluence up to 70 mJ/=cm2. The experiments on interaction of the EUV pulses of various polymers have been performed. Modification of polymer surfaces was achieved, primarily due to direct photo-etching with EUV photons and formation of micro- and nanostructures onto the surface. The mechanism of the interaction is similar to the UV laser ablation where energetic photons cause chemical bonds of the polymer chain to be broken. However, because of very low penetration depth of EUV radiation, the interaction region is limited to a very thin surface layer (< 100 nm). This makes it possible to avoid degradation of bulk material caused by deeply penetrating UV radiation. The results of the studies should be applicable in biomedical engineering. In the case of EUV irradiation of PMMA at relatively low fluence characteristic nodules of the size 100-200 nm in diameter were observed on the surface. It was suggested that dry desorption of polymer fragments which can be trapped several nanometers under the surface may form bubbles of nanometer size that lead to the nodule formation. The phenomenon is somewhat similar to the formation of nanovoids in silica in result of interaction of low-energy femtosecond laser pulses. Computer simulation performed using the hydrodynamic code have shown that simultaneous absorption of few EUV photons in PMMA can lead to formation of the nodules. Acknowledgements. The research was supported by the Ministry of Science and Higher Education under the EUREKA project E 3892 ModPolEUV, and the EC's 7. Framework Program (LASERLAB-EUROPE, ELI-PP and COST Action MP0601).