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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 the XUV (λ < 100 nm) spectral region? This
question can now be answered due to the availability of XUV lasers
with pulse durations ranging from tens of femtoseconds to several
nanoseconds.
These sources, whichonly recently became available,
are promising tools for appli cations 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 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 sources
to deliver enough fluence and thus sufficiently high power
to the irradiated surface area. Although non-coherent sources developed
for XUV 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. Recently
it has been demonstrated that intense XUV radiation can directly
produce sub-100-nm structures quite easily. Grating-like structures
(i.e. LIPSS – laser-induced periodic surface structures) with
a spatial period about 70 nm have already been produced in
amorphous carbon (a-C; [8]) and poly(methyl methacrylate) –
PMMA [6] surfaces irradiated with FEL radiation at 98 nm
and 86 nm, respectively.
Recent publications
[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,
347–352 (2002).
[2] L. Juha, J. Krása,
A. Cejnarová, D. Chvostová, V. Vorlíček,
J. Krzywinski, R. Sobierajski, A. Andrejczuk, M. Jurek,
D. Klinger, H. Fiedorowicz, A. Bartnik, M. Pfeifer,
P. Kubát, L. Pína, J. Kravárik,
P. Kubeš, Yu. L. Bakshaev, A. S. Chernenko,
V. D. Korolev, M. I. Ivanov, M. Scholz,
L. Ryc, J. Feldhaus, J. Ullschmied, F. P. Boody:
Ablation of various materials with intense XUV radiation, Nucl.
Instrum. Meth. Phys. Res. A507, 577–581
(2003).
[3] H. Fiedorowicz, A. Bartnik,
L. Juha, K. Jungwirth, B. Kralikova, J. Krasa,
P. Kubat, M. Pfeifer, L. Pina, P. Prchal, K. Rohlena,
J. Skala, J. Ullschmied, M. Horvath, J. Wawer:
High-brightness laser plasma soft X-ray source using a double-stream
gas puff target irradiated with the Prague Asterix Laser System
(PALS), J. Alloys Comp. 362, 67–70 (2004)
[4] H. Fiedorowics, A. Bartnik, M. Bittner,
L. Juha, J. Krasa, P. Kubat, J. Mikolajczyk,
R. Rakowski, Micromachining of organic polymers by direct photo-etching
using a laser plasma X-ray source, Microelectronic Eng. 73–74,
336–339 (2004).
[5] L. Juha, J. Krzywinski, A. Andrejczuk,
M. Bittner, F. P. Boody, D. Chvostova, J. Feldhaus,
M. E. Grisham, J. Krasa, V. Letal, C. S. Menoni,
J. B. Pelka, Z. Otcenasek, J. J. Rocca, R. Sobierajski,
G. O. Vaschenko, Comparing etch rates of poly(methyl methacrylate)
ablated by nanosecond and femtosecond pulses of XUV-laser radiation,
HASYLAB Annual Report 2003 – Part I., 241–242
(2003)
[6] L. Juha, J. Krzywinski,
A. Andrejczuk, M. Bittner, J. Feldhaus, J. B. Pelka,
R. Sobierajski, B. Steeg, A. Wawro, Laser-induced
periodic surface structures (LIPSS) produced by short-wavelength,
fast beam of TTF1 FEL, HASYLAB Annual Report 2003 –
Part I., 763–764 (2003)
[7] M. Bittner, L. Juha, D. Chvostova,
V. Letal, J. Krasa, Z. Otcenasek, M. Kozlova,
J. Polan, A. R. Praeg, B. Rus, M. Stupka,
J. Krzywinski, A. Andrejczuk, J. B. Pelka, R. Sobierajski,
J. Feldhaus, F. P. Boody, M. E. Grisham,
G. O. Vaschenko, C. S. Menoni, J. J. Rocca,
Comparing ablation induced by fs, ps and ns XUV-laser pulses, Proceedings
of the High-Power Laser Ablation 2004, April 25–30, 2004 Taos USA,
Proc. SPIE 5448, 827–836 (2004).
[8] B. Steeg, L. Juha, J. Feldhaus,
S. Jacobi, R. Sobierajski, C. Michaelsen, A. Andrejczuk,
J. Krzywinski: Total reflection amorphous carbon mirrors for
VUV Free Electron Laser, Appl. Phys. Lett. 84,
657–659 (2004).
[9] 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, H. Fiedorowicz, A. Bartnik, J. Mikolajczyk,
R. Rakowski, L. Pína, M. E. Grisham,
G. O. Vaschenko, C. S. Menoni, J. J. Rocca:
Short-wavelength ablation of solids: pulse duration and wavelength
effects, Proc. SPIE 5534, 95–107 (2004).
[10] 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).
Networking
[A] inland
- IP-ASCR staff members participating in
the project: V. Létal, J. Polan,
A. R. Präg, B. Rus, J. Skála,
M. Stupka
- J. Heyrovský Institute of Physical Chemistry ASCR,
Prague: P. Kubát
- Institute of Plasma Physics ASCR, Prague: J. Ullschmied
- Czech Technical University, Prague: P. Kubeš, J.
Kuba
[B] abroad
- Institute of Physics, Polish Academy of
Sciences, Warsaw, Poland: J. Krzywinski
- HASYLAB / DESY, Hamburg, Germany: J. Feldhaus
- Argonne National Laboratory, Argonne, IL, USA: S. G. Biedron,
S. V. Milton, J. F. Moore
- Military University of Technology, Warsaw, Poland: H. Fiedorowicz
- Russian Research Center “Kurchatov Institute”, Moscow, Russia:
V. D. Korolev
- Institute of Plasma Physics and Laser Microfusion, Warsaw,
Poland: M. Scholz
- NSF ERC for Extreme Ultraviolet Science and Technology,
Colorado State University, Fort Collins, USA: J. J. Rocca,
G. Vaschenko
- Groupe Sources Cohérentes X, Laboratoire d’Optique
Appliquée (LOA), ENSTA – Ecole Polytechnique, Centre
de l’Yvette, Palaiseau Cedex, France: S. Sebban
- Service des Photons des Atomes et des Molécules (SPAM),
CEA Saclay, France: H. Merdji, S. Guizard
[C] R&D enterprises, industry
- GKSS-Forschungszentrum Geeshacht GmbH,
Geeshacht, Germany: M. Störmer
- Ion Light Technologies GmbH, Bad Abbach, Germany: F. P. Boody
- Reflex sro, Prague: L. Pína
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