Fyzikální ústav Akademie věd ČR

Die chemischen Wirkungen der Röntgenstrahlen kommen auf ganz andere Weise zustande als die chemischen Wirkungen des Lichtes. [J. Eggert: Lehrbuch der physikalischen Chemie in elementarer Darstellung. Sechste Auflage, S. Hirzel, Leipzig; Seite 654]

Although the decomposition of solid structures into tiny pieces induced by high-energy XUV/X-ray photons, resulting in material removal due to rapid evaporation of the low-molecular weight fragments, was first studied more than twenty years ago these processes were investigated only infrequently until now. Current research activities are not yet very extensive, although there are at least five strong sources of motivation to conduct systematic study in this field:

1) Estimating and minimizing damages to surfaces of highly irradiated XUV/X-ray optical elements developed and used for the guiding and focusing of short-wavelength laser beams as well as those used for long-term irradiation with high repetition rate sources,
2) durability assessments of materials suggested for the first walls of ICF reactors and optical elements exposed to intense X-ray radiation in a laser-plasma interaction chamber,
3) diffraction-limited ultrastructuring and patterning of solid surfaces for fabrication of microelectronic and micromechanical elements and devices,
4) determination of radiation field characteristics: imaging of spatial energy distribution in a focused beam ablatively imprinted on the irradiated material and determination of pulse energy content, and
5) production of very dense plasmas with low electron temperatures, i.e. T_e ~ 10eV (WDM – warm dense matter).

The XUV/X-ray sources used for materials removal emit at both low peak power (synchrotron radiation sources, rotating anode sources, etc.) and high peak power (free-electron lasers - FELs, and various sources based on hot dense plasmas):

With low-peak-power sources, materials are removed by photon-induced desorption of material components from the irradiated sample surface. Each XUV/X-ray photon carries enough energy to break any chemical bond. This energy is also usually higher than the cohesive energy of any crystal. Therefore, the photons absorbed in a near-surface region may create small fragments of a sample material, which are emitted into the vacuum. It is necessary to underline, that in the case of low-peak-intensity irradiation, material is removed from the surface and a very thin near-surface layer only.

Quite a different situation is expected if a high-peak-power source delivers a single high-energy pulse to the sample. The sample is then exposed to a high local dose of radiation (given by the energy content of the pulse and the absorption length of the radiation in the irradiated material) in a short period of time (given by the pulse length), i. e. at a very high dose rate. This means that a large number of events that cause radiation-induced structural decomposition (i. e. polymer chain scissions, etc.), occur almost simultaneously in a relatively thick layer of irradiated material. Since a portion of the radiation energy absorbed in the material will be thermalized, the sudden heating of the layer, which is also heavily chemically altered by the radiation, must be taken into account. The overheated, fragmented region of the sample represents a new phase, which tends to blow off into the vacuum. These particular processes, as well as specific features of short-wavelength ablation, with respect to ablation induced by conventional UV-Vis-IR sources, represent the subject of our research.

Ablation characteristics, i.e. thresholds, etch (ablation) rates, and ablated structure quality, often differ dramatically with conventional UV-Vis-IR lasers, depending on whether the radiation energy is delivered to the material surface in either short (typically nanosecond) or ultra-short (typically femtosecond) pulses. Does such a difference also exist for lasers operating in spectral regions with l < 100 nm? This question can now be answered due to the availability of XUV/X-ray lasers with pulse durations ranging from tens of femtoseconds to several nanoseconds.

These sources, which only recently became available, are promising tools for applications in the field of nano-patterning of solids, as they will enable the printing of features with dimensions comparable to the wavelength. A key advantage of XUV/X-ray lasers for nano-structure fabrication is the unique combination of exceptionally short wavelength, spatial coherence, and high peak power. The ablation threshold for processing materials makes it necessary for XUV/X-ray sources to deliver enough fluence and thus sufficiently high power to the irradiated surface area. Although non-coherent sources developed for EUV lithography can also pattern material surfaces at sub-10-nm resolutions, they cannot directly produce three-dimensional structures using only a few shots in a single processing step.

We are conducting research in all the five areas identified above. Detailed description of obtained results can be found in selected references listed below.

References:
1. L. Juha, J. Krása, A. Präg, A. Cejnarová, D. Chvostová, K. Rohlena, K. Jungwirth, J. Kravárik, P. Kubeš, Yu. L. Bakshaev, A. S. Chernenko, V. D. Korolev, V. I. Tumanov, M. I. Ivanov, A. Bernardinello, J. Ullschmied, F. P. Boody: Ablation of poly(methyl methacrylate) by a single pulse of soft X-rays emitted from Z-pinch and laser-produced plasmas, Surf. Rev. Lett. 9, 347 (2002).
2. L. Juha, M. Bittner, D. Chvostová, V. Létal, J. Krása, Z. Otčenášek, M. Kozlová, J. Polan, A. R. Präg, B. Rus, M. Stupka, J. Krzywinski, A. Andrejczuk, J. B. Pelka, R. Sobierajski, L. Ryc, J. Feldhaus, F. P. Boody, M. E. Grisham, G. O. Vaschenko, C. S. Menoni, J. J. Rocca: XUV-laser induced ablation of PMMA with nano-, pico-, and femtosecond pulses, J. Electron. Spectrosc. Rel. Phenom. 144-147, 929 (2005).
3. L. Juha, M. Bittner, D. Chvostova, J. Krasa, M. Kozlova, M. Pfeifer, J. Polan, A. R. Präg, B. Rus, M. Stupka, J. Feldhaus, V. Letal, Z. Otcenasek, J. Krzywinski, R. Nietubyc, J. B. Pelka, A. Andrejczuk, R. Sobierajski, L. Ryc, F. P. Boody, H. Fiedorowicz, A. Bartnik, J. Mikolajczyk, R. Rakowski, P. Kubat, L. Pina, M. Horvath, M. E. Grisham, G. O. Vaschenko, C. S. Menoni, J. J. Rocca: Short-wavelength ablation of molecular solids: pulse duration and wavelength effects, J. Microlith. Microfab. Microsyst. 4, 033007 (2005).
4. L. Juha, M. Bittner, D. Chvostova, J. Krasa, Z. Otcenasek, A. R. Präg, J. Ullschmied, Z. Pientka, J. Krzywinski, J. B. Pelka, A. Wawro, M. E. Grisham, G. Vaschenko, C. S. Menoni, and J. J. Rocca: Ablation of organic polymers by 46.9-nm laser radiation, Appl. Phys. Lett. 86, 034109 (2005).
5. A. Bartnik, H. Fiedorowicz, R. Jarocki, L. Juha, J. Kostecki, R. Rakowski, M. Szczurek: Strong temperature effect on X-ray photo-etching of polytetrafluoroethylene using a 10 Hz laser-plasma radiation source based on a gas puff target, Appl. Phys. B 82, 529 (2006).
6. T. Mocek, B. Rus, M. Kozlová, M. Stupka, A. R. Präg, J. Polan, M. Bittner, R. Sobierajski, L. Juha: Focusing a multimillijoule soft x-ray laser at 21 nm, Appl. Phys. Lett. 89, 051501 (2006).
7. N. Stojanovic, D. von der Linde, K. Sokolowski-Tinten, U. Zastrau, F. Perner, E. Foerster, R. Sobierajski, R. Nietubyc, M. Jurek, D. Klinger, J. Pelka, J. Krzywinski, L. Juha, J. Cihelka, A. Velyhan, S. Koptyaev, V. Hajkova, J. Chalupsky, J. Kuba, T. Tschentscher, S. Toleikis, S. Duesterer, H. Redlin: Ablation of solids using a femtosecond extreme ultraviolet free electron laser, Appl. Phys. Lett. 89, 241909 (2006).
8. S. P. Hau-Riege, H. N. Chapman, J. Krzywinski, R. Sobierajski, S. Bajt, R. A. London, M. Bergh, C. Caleman, R. Nietubyc, L. Juha, J. Kuba, E. Spiller, S. Baker, R. Bionta, K. Sokolowski-Tinten, N. Stojanovic, B. Kjornrattanawanich, E. Gullikson, E. Ploenjes, S. Toleikis, T. Tschentscher: Subnanometer-scale measurements of the interaction of ultrafast soft X-ray free-electron-laser pulses with matter, Phys. Rev. Lett. 98, 145502 (2007).
9. S. P. Hau-Riege, R. A. London, R. M. Bionta, M. A. McKernan, S. L. Baker, J. Krzywinski, R. Sobierajski, R. Nietubyc, J. B. Pelka, M. Jurek, L. Juha, J. Chalupský, J. Cihelka, V. Hájková, A. Velyhan, J. Krása, J. Kuba, H. Wabnitz, K. Tiedtke, S. Toleikis, T. Tschentscher, M. Bergh, C. Caleman, K. Sokolowski-Tinten, N. Stojanic, U. Zastrau: Damage threshold of inorganic solids under free-electron-laser irradiation at 32.5 nm wavelength, Appl. Phys. Lett. 90, 173128 (2007).
10. J. Chalupský, L. Juha, J. Kuba, J. Cihelka, V. Háková, S. Koptyaev, J. Krása, A. Velyhan, M. Bergh, C. Caleman, J. Hajdu, R. M. Bionta, H. Chapman, S. P. Hau-Riege, R. A. London, M. Jurek, J. Krzywinski, R. Nietubyc, J. B. Pelka, R. Sobierajski, J. Meyer-ter-Vehn, A. Krenz-Tronnier, K. Sokolowski-Tinten, N. Stojanovic, K. Tiedtke, S. Toleikis, T. Tschentscher, H. Wabnitz, U. Zastrau: Characteristics of focused soft X-ray free-electron laser beam determined by ablation of organic molecular solids, Opt. Express 15, 6036 (2007).
11. J. Krzywinski, R. Sobierajski, M. Jurek, R. Nietubyc, J. B. Pelka, L. Juha, M. Bittner, V. Letal, V. Vorlicek, A. Andrejczuk, J. Feldhaus, B. Keitel, E. Saldin, E. Schneidmiller, R. Treusch, M. Yurkov: Conductors, semiconductors and insulators irradiated with short-wavelength free-electron laser, J. Appl. Phys. 101, 043107 (2007).
12. T. Mocek, B. Rus, M. Kozlová, J. Polan, P. Homer, L. Juha, V. Hájková, J. Chalupský: Single-shot soft x-ray laser-induced ablative microstructuring of organic polymer with demagnifying projection, Opt. Lett. 33, 1087 (2008).
13. J. Chalupský, L. Juha, V. Hájková, J. Cihelka, L. Vyšín, J. Gautier, J. Hajdu, S. P. Hau-Riege, M. Jurek, J. Krzywinski, R. A. London, E. Papalazarou, J. B. Pelka, G. Rey, S. Sebban, R. Sobierajski, N. Stojanovic, K. Tiedtke, S. Toleikis, T. Tschentscher, C. Valentin, H. Wabnitz, and P. Zeitoun: Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses, Opt. Express 17, 208 (2009).
14. T. Mocek, J. Polan, P. Homer, K. Jakubczak, B. Rus, I. J. Kim, C. M. Kim, G. H. Lee, C. H. Nam, V. Hájková, J. Chalupský, L. Juha: Surface modification of organic polymer by dual action of XUV/Vis-NIR ultrashort pulses, J. Appl. Phys. 105, 026105 (2009).
15. A. J. Nelson, S. Toleikis, H. Chapman, S. Bajt, J. Krzywinski, J. Chalupsky, L. Juha, J. Cihelka, V. Hajkova, L. Vysin, T. Burian, M. Kozlova, R.R. Fäustlin, B. Nagler, S.M. Vinko, T. Whitcher, T. Dzelzainis, O. Renner, K. Saksl, A. R. Khorsand, P. A. Heimann, R. Sobierajski, D. Klinger, M. Jurek, J. Pelka, B. Iwan, J. Andreasson, N. Timneanu, M. Fajardo, J.S. Wark, D. Riley, T. Tschentscher, J. Hajdu, R. W. Lee: Soft x-ray free electron laser microfocus for exploring matter under extreme conditions, Opt. Express 17, 18271 (2009).
16. J. Chalupský, V. Hájková, V. Altapova, T. Burian, A. J. Gleeson, L. Juha, M. Jurek, H. Sinn, M. Störmer, R. Sobierajski, K. Tiedtke, S. Toleikis, Th. Tschentscher, L. Vyšín, H. Wabnitz, J. Gaudin: Damage of amorphous carbon induced by soft x-ray femtosecond pulses above and below the critical angle, Appl. Phys. Lett. 95, 031111 (2009).
17. B. Nagler, U. Zastrau, R. Fäustlin, S. M. Vinko, T. Whitcher, A. J. Nelson, R. Sobierajski, J. Krzywinski, J. Chalupsky, E. Abreu, S. Bajt, T. Bornath, T. Burian, H. Chapman, J. Cihelka, T. Döppner, S. Düsterer, T. Dzelzainis, M. Fajardo, E. Förster, C. Fortmann, E. J. Galtier, S. H. Glenzer, S. Göde, G. Gregori, V. Hajkova, P. Heimann, L. Juha, M. Jurek, F. Y. Khattak, A. R. Khorsand, D. Klinger, M. Kozlova, T. Laarmann, H. J. Lee, R. Lee, K.-H. Meiwes-Broer, P. Mercere, W. J. Murphy, A. Przystawik, R. Redmer, H. Reinholz, D. Riley, G. Röpke, F. Rosmej, K. Saksl, R. Schott, R. Thiele, J. Tiggesbäumker, S. Toleikis, T. Tschentscher, I. Uschmann, H. J. Vollmer, J. Wark: Turning solid aluminium transparent by intense soft X-ray photo-ionization, Nature Physics 5, 693 (2009).
18. L. Juha, V. Hájková, J. Chalupský, V. Vorlíček, A. Ritucci, A. Reale, P. Zuppella, M. Störmer: Radiation damage to amorphous carbon thin films irradiated by multiple 46.9 nm laser shots below the single-shot damage threshold, J. Appl. Phys. 105, 093117 (2009).
19. A. R. Khorsand, R. Sobierajski, E. Louis, S. Bruijn, E. D. van Hattum, R. W. E. van de Kruijs, M. Jurek, D. Klinger, J. B. Pelka, L. Juha, T. Burian, J. Chalupsky, J. Cihelka, V. Hajkova, L.Vysin, U. Jastrow, N. Stojanovic, S. Toleikis, H. Wabnitz, K. Tiedtke, K. Sokolowski-Tinten, U. Shymanovich, J.Krzywinski, S. Hau-Riege, R. London, A. Gleeson, E.M. Gullikson, F. Bijkerk: Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure, Opt. Express 18, 707 (2010).
20. S. M. Vinko, U. Zastrau, S. Mazevet, J. Andreasson, S. Bajt, T. Burian, J. Chalupsky, H. N. Chapman, J. Cihelka, D. Doria, T. Döppner, S. Düsterer, T. Dzelzainis, R. R. Fäustlin, C. Fortmann, E. Förster, E. Galtier, S. H. Glenzer, S. Göde, G. Gregori, J. Hajdu, V. Hajkova, P. A. Heimann, R. Irsig, L. Juha, M. Jurek, J. Krzywinski, T. Laarmann, H. J. Lee, R.W. Lee, B. Li, K.-H. Meiwes-Broer, J. P. Mithen, B. Nagler, A. J. Nelson, A. Przystawik, R. Redmer, D. Riley, F. Rosmej, R. Sobierajski, F. Tavella, R. Thiele, J. Tiggesbäumker, S. Toleikis, T. Tschentscher, L. Vysin, T. J. Whitcher, S. White, J. S. Wark: Electronic structure of an XUV photogenerated solid-density aluminum plasma, Phys. Rev. Lett. 104, 225001 (2010).