Damage to Molecular Solids Irradiated by X-ray Laser Beam

WDS'11 Proceedings of Contributed Papers, Part II, 247 251, 2011. ISBN 978-80-7378-185-9 MATFYZPRESS Damage to Molecular Solids Irradiated by X-ray Laser Beam T. Burian, V. Hájková, J. Chalupský, L. Juha, M. Toufarová, V. Vorlíček Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 8, 182 21. S. P. Hau-Riege Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550-9234, USA. J. Krzywinski, J. D. Bozek, M. Messerschmidt, S. Moeller, C. Bostedt SLAC National Accelerator Lab, 2575 Sand Hill Road, Menlo Park, CA 94025-7015, USA. Abstract. Solids can be efficiently eroded when exposed to a flux of energetic photons. There are two main modes of the erosion depending on the local radiation intensity which can be distinguished and investigated on the irradiated surface. Using a source emitting radiation at low intensity, desorption of the material occurs. Contrary to that, when a source providing high peak power of radiation is used, ablation processes dominate the material erosion. This contribution is focused on investigation of both the desorption and the ablation modes of an organic polymer erosion initiated by the X-ray laser radiation. A solid bulk and a thin layer of PMMA poly(methyl methacrylate) have been irradiated with laser pulses produced by free electron laser system (LCLS, USA). The threshold fluence required for material ablation by a single X-ray laser pulse has been determined for the photon energy of 835 ev. Attention has been also paid to chemical changes and phase transitions occurring in irradiated materials studied by the Raman spectroscopy with a spatial resolution. Introduction Ability of short-wavelength radiation to modify or damage surface of solids is a phenomenon known for decades. The interactions between the EUV (Extreme UltraViolet) and SXR (Soft X-Rays) radiation and the molecular solids were extensively investigated under various conditions [1,2]. Materials were first irradiated with a non-coherent broad-band radiation produced mostly by the laserproduced and the discharge plasmas and at synchrotron radiation facilities. Later on, the high order harmonics and a new generation of synchrotron radiation sources provided the coherent radiation in these spectral regions. Recent advances in development of new type of short-wavelength coherent sources, i.e., the free-electron lasers, opened new fields of interest in the interaction experiments. The peak output power of the new sources is many orders of magnitude higher than that from the sources listed in previous paragraph. Fluences provided by these devices are high enough to exceed the single-shot ablation threshold and cause a massive material removal, i.e., laser ablation. The knowledge of this value is of high importance in numerous branches of science and industry, e.g., optical elements development and protection, EUV and soft x-ray nanopatterning, radiation chemistry, pulsed laser deposition, etc. In this contribution we report results of an irradiation of the poly(methyl methacrylate) - PMMA by a single shot at and above the ablation threshold and by accumulated multiple shots at a fluence below the single-shot ablation threshold. It has been of the highest importance to have an opportunity of comparison of experimental results from irradiation of PMMA with soft x-ray (LCLS) and EUV [2,3] radiation. The results obtained clearly demonstrate the difference between desorption and ablation modes in the material erosion initiated by the energetic photons (for more details about these modes see ref. [3]). Experimental Linac Coherent Light Source (LCLS) is a free-electron laser [4] with an output wavelength tunable from 1.5 nm to 0.15 nm. The wavelengths correspond to an interval of photon energies from 825 to 8250 ev. For our experiment the photon energy of 835 ev was chosen. Pulse duration 247

fluctuated around 100 fs. Average pulse energy of the unattenuated beam was about 1 mj. PMMA was chosen as a material to be irradiated. Samples were mounted on a motorized X-Y-Z stage inside the vacuum chamber placed in an AMO experimental station [5] (Atomic, Molecular and Optical Physics). Incoming laser beam was focused onto the sample surface by two Kirkpatrick-Baez mirrors. Having a focal length of 1.2 m, the focusing system is designed to provide a beam with a diameter less than 3 μm in the tight focus. Energies of the laser pulses were measured by a photo-ionization detector (Gas Monitor Detector). The beam was attenuated by means of gas attenuator and a set of solid beryllium filters with different thicknesses to obtain various levels of fluence. Results and discussion First, a solid bulk of PMMA was used for determining the single-shot ablation threshold. According to the procedure proposed by Chalupsky et al. [6,7], the series of single shots were fired on the PMMA surface at various fluence levels. The irradiated surface was then investigated with a Nomarski optical microscope (also known as Differential Interference Contrast DIC microscope) and the damaged areas were measured as shown in Fig. 1. Plotting the damaged area against the logarithm of the corresponding pulse energy and fitting data with a line, the threshold energy can be determined as shown in Fig. 2. The value of energy threshold was found to be around 126.2 ± 6.5 nj. The threshold fluence is defined as follows: F TH = E TH S BEAM (1) where F TH, E TH and S BEAM correspond to the threshold fluence, the threshold energy and the beam area, a) b) c) Figure 1. (a) As taken and (b) marked DIC image of a crater produced in PMMA by the LCLS beam at a fluence level well above the single-shot ablation threshold in a comparison with (c) the AFM image of the same crater. Figure 2. Dependence of measured damaged area on the corresponding laser pulse energy. 248

respectively. After the threshold energy, we have to determine the beam area to accomplish the successful determination of the threshold fluence. Nevertheless, the effective beam area can be determined from the damaged areas taken at various pulse energies as well. Knowing the threshold energy, the dependence of peak to threshold ratio (E TH / E PULSE ) on the damaged area can be plotted (Fig. 3). An area under the fitting curve corresponds to the effective area (S EFF ) of the beam defined as: S EFF = E TH (2) F PEAK where F PEAK is the peak fluence of the beam. The threshold pulse energy divided by the effective beam area gives the value of single-shot ablation threshold for PMMA around 93.4 ± 5.2 mj/cm 2. The second sample (layer of PMMA, 5 μm in thickness, spin coated on a 10 mm x 20 mm Si slab) was used for irradiation with the accumulations of shots at a fluence below the single-shot ablation threshold at a photon energy of 835 ev. The main goal of this work was to investigate the chemical changes, especially effects of competition between the chain scission and the cross-linking [8], occurring in the irradiated material. The cross-linking is indicated by a formation of double bonds C=C inside the irradiated polymer which is originally aliphatic. Fig. 4 shows schematically the key processes taking place in the X-ray irradiated PMMA. 135μm 2 Figure 3. Dependence of peak to threshold ratio on the damaged area gives the information about effective area of the beam. Figure 4. Formation of a PMMA molecule with a double bond C=C; (a) unaffected PMMA molecule, (b) chain scission caused by incident radiation, (c) and (d) cross-linking. 249

Three sets of craters were made at different fluence levels and with a different number of accumulated pulses. Morphology of these craters was investigated by an AFM working in tapping mode (Veeco D3100 NanoScope Dimension). The effect of laser induced desorption-like damage to PMMA was clearly shown. Comparing Figs 1 and 5, it can be clearly seen that the sub-threshold, multiple-shot damage is of very different kind than the damage caused by the single-shot ablation. The sub-threshold, desorption-like erosion leaves a very clean and smooth surface, without any molten or extruded material, bubbles, etc. A backscattering Raman spectrometer with a spatial resolution of around 4 μm (Renishaw Ramascope) was used to identify and investigate the chemical changes caused in PMMA irradiated by the X-ray laser radiation. Fig. 6 summarizes a comparison of Raman spectra obtained from the irradiated and the unirradiated sites chosen on the PMMA surface. It is clearly visible that a new peak appears at 1650 cm -1 in the Raman spectrum taken in the irradiated area. This peak belongs to the double bonds C=C. The formation of the double bounds is the evidence of cross-linking. a) b) c) Figure 5. AFM profiles of craters in PMMA made with accumulations of 300 shots at different levels of fluence impinging the sample surface: a) 0.4 mj/cm 2, b) 4 mj/cm 2 and c) 40 mj/cm 2. Figure 6. Raman spectra of PMMA irradiated by 300 X-ray laser shots at a fluence level of 40 mj/cm 2. The main difference between the irradiated (black) and the unirradiated (red) area is characterized by a new peak at 1650 cm -1 representing the presence of C=C bonds formed due to the effect of cross-linking. 250

Conclusion An efficient erosion of organic polymer material was observed after the irradiation by X-ray laser pulses. A single shot ablation threshold of 93.4 ± 5.2 mj/cm 2 was determined for PMMA irradiated by photons with energy of 835 ev. For the fluences higher than this threshold value, the material was efficiently eroded by the ablation. This erosion mode can easily be distinguished from the desorptionlike damage induced by the multiple X-ray laser shots fired on the PMMA surface at fluences below the single-shot ablation threshold. The sub-threshold, multiple-shot damage is in PMMA made possible by the photon energy high enough to break covalent C-C bounds in the polymer backbone. It was proven by the Raman spectroscopy that the chain scissions, leading to a polymer decomposition liberating small molecular fragments into the vacuum, is competing with the cross-linking processes making polymer structure less prone to a substantial decomposition. Acknowledgments. This work was supported by the Czech Ministry of Education (Projects LC510, LC528, LA08024, and ME10046), the Czech Science Foundation (P205/11/0571 and P108/11/1312) and the Academy of Sciences of the Czech Republic (Grants Z10100523, IAAX00100903, and KAN300100702). References [1] J. O. Choi et al., Degradation of PMMA by deep ultraviolet, x-ray, electron beam, and proton beam irradiations, J. Vac. Sci. Technol. B, 6(6), pp. 2286 2289 (1988). [2] C. K. Marcia et al., Photodecomposition of PMMA thin films by monochromatic soft x-rays, J. Vac. Sci. Technol. A, 13(4), pp. 1885 1892 (1995). [3] J. Chalupský et al.: Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses, Opt. Express 17(1), pp. 208 217 (2009). [4] P. Emma, et al., First lasing and operation of an ångstrom-wavelength free-electron laser, Nature Photonics 4(9), pp. 641 647 (2010). [5] J. D. Bozek, AMO instrumentation for the LCLS X-ray FEL, Eur. Phys. J., Spec. Top. 169, pp. 129 132 (2009). [6] J. Chalupský et al., Spot size characterization of focused non-gaussian X-ray laser beams, Opt. Express, 18(26), pp. 27836 27845 (2010). [7] J. Chalupský et al., Comparing different approaches to characterization of focused X-ray laser beams, Nuclear Instruments and Methods in Physics Research A 631, pp. 130 133 (2011). [8] E. M. Lehockey et al., Radiation chemistry of PMMA polymer resists, J. Vac. Sci. Technol. A, 6(4), pp. 2221 2225 (1988). 251