High Power Laser Science and Engineering, (2014), Vol. 2, e26, 5 pages.
© Author(s) 2014. The online version of this article is published within an Open Access environment subject to the conditions of the
Creative Commons Attribution licence <http://creativecommons.org/licences/by/3.0>.
doi:10.1017/hpl.2014.31
EUV ablation of organic polymers at a high fluence
Chiara Liberatore
1,2
, Klaus Mann
3
, Matthias M
¨
uller
3
, Ladislav Pina
2
, Libor Juha
1
, Jorge J. Rocca
4
,
Akira
Endo
1
, and Tomas Mocek
1
1
HiLASE Centre, Institute of Physics ASCR, v.v.i., Za Radnicí 825, 25241 Dolní B
ˇ
re
ˇ
zany, Czech Republic
2
Czech Technical University, Th
´
akurova 2077/7, 160 00 Prague 6, Czech Republic
3
Laser Laboratorium G
¨
ottingen (LLG), 37077 G
¨
ottingen, Germany
4
Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, CO 80523, USA
(Received 3 March 2014; revised 9 May 2014; accepted 3 July 2014)
Abstract
A preliminary investigation on short-wavelength ablation mechanisms of poly(methyl methacrylate) (PMMA) and poly
(1,4-phenylene ether ether-sulfone) (PPEES) by extreme ultraviolet (EUV) radiation at 13.5 nm using a table-top laser-
produced plasma from a gas-puff target at LLG (G
¨
ottingen) and at 46.9 nm by a 10 Hz desktop capillary discharge
laser operated at the Institute of Physics (Prague) is presented. Ablation of polymer materials is initiated by photo-
induced polymer chain scissions. The ablation occurs due to the formation of volatile products by the EUV radiolysis
removed as an ablation plume from the irradiated material into the vacuum. In general, cross-linking of polymer
molecules can compete with the chain decomposition. Both processes may influence the efficiency and quality of
micro(nano)structuring in polymer materials. Wavelength is a critical parameter to be taken into account when an EUV
ablation process occurs, because different wavelengths result in different energy densities in the near-surface region of
the polymer exposed to nanosecond pulses of intense EUV radiation.
Keywords: EUV ablation; organic polymer; photo-erosion mechanisms; wavelength effect
1. Introduction
Laser ablation is an efficient removal of a material from
its surface and near-surface region irradiated by an in-
tense laser beam at a fluence above the single-shot ab-
lation threshold
[1, 2]
. Due to its potential use in surface
patterning/structuring and other applications (e.g., pulsed
laser deposition (PLD)), ablation has attracted attention from
the advent of laser technology in the sixties. Among the
lasers widely utilized to initiate material ablation, ultravi-
olet (UV) lasers are often applied because of their lim-
ited penetration depth and ability to induce photochemical
ablation while avoiding thermal artefacts
[3]
. It is known
that photochemical, non-thermal decomposition plays an
important role when the wavelengths decrease. Some ab-
lation studies at shorter wavelengths in the vacuum UV
(VUV) region have been conducted, for example, with F
2
excimer lasers (λ = 157 nm
[4]
) and radiation generated
by four-wave-sum-frequency mixing of a frequency-doubled
Nd:YAG laser (λ = 125 nm
[5]
). Laboratory-scaled laser
sources operating at even shorter wavelengths have recently
Correspondence to: C. Liberatore, HiLASE Centre, Institute of Physics
ASCR, v.v.i., Za Radnicí 825, 25241 Dolní B
ˇ
re
ˇ
zany, Czech Republic.
Email: liberatore@fzu.cz
become operational in the extreme UV (EUV) spectral range
(λ<100 nm)
[6–8]
.
The EUV sources utilized to induce material erosion emit
at both low peak power and high peak power. In principle,
this results in different modes of material removal
[9, 10]
.
With low-peak-power sources, materials are removed by
photo-induced desorption of their volatile fragments from
the irradiated sample surface. Each EUV photon carries
enough energy to break any chemical bond. This energy is
also usually higher than the cohesive energy of any solids.
Therefore, the energetic photons absorbed by the photo-
effect in atoms located in a near-surface region may create
small fragments of a sample material, which are desorbed
into the vacuum.
Quite a different situation is expected if a high-peak-power
source delivers a single high-energy pulse to the material.
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 duration), i.e., a very
high dose rate. This means that a large number of events
that cause radiation-induced structural decomposition (i.e.,
organic polymer chain scissions, etc.) occur almost simul-
taneously in a relatively thick layer of irradiated material.
1
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