23P145-R
Precise nanostructuring of metal surfaces by low intense picosecond soft X-ray
laser pulse. Observation and modeling.
低強度ピコ秒軟 X 線レーザーによる金属表面での高精度ナノ構造形
成：観測とモデル化
Tatiana Pikuz1,2, Anatoly Faenov1,2, Masaki Ishino1, Sergei Starikov2,3, Vladimir Stegailov2,3, Genry
Norman2,3, Vladimir Fortov2, Igor Skobelev2, Satoshi Tamotsu4, Yoshiaki Kato5, Mamoko Tanaka1,Noboru Hasegawa1, Masaharu Nishikino1, Toshiyuki Ohba1, Takeshi Kaihori1, Yoshihiro Ochi3,Takashi Imazono1, Masaki Kando1, Tetsuya Kawachi1
1Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai Kizugawa, Kyoto 619-0215, Japan
2
Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow 125412, Russia
3Moscow Institute of Physics and Technology, Moscow region, Russia
4
Graduate School of Humanities and Science, Nara Women’s University, Japan
4
The Graduate School for the Creation of New Photonics Industries, Hamamatsu, Shizuoka 431-1202, Japan

We show experimentally the possibility of the precise (~ 10 – 40 nm) nanostructuring of aluminum and gold surfaces byultra low (~ 10 - 30 mJ/cm2) fluencies of single picosecond soft X-ray laser (SXRL) pulse. The atomistic model ofablation is developed that reveals the ultra-low threshold fluency values of this process to be an effect of the highelectronic pressure build-up and the comparatively low electron-ion energy relaxation rates. Our modeling show that
relatively slow electron-ionic relaxation in such metals results in maintaining of the high electron pressure in the nearsurface region for several picoseconds, that is sufficiently long for the development of the hydrodynamic response thatcauses the negative pressure region formation and the nanostructuring of a thin surface layer. Calculated ablation depth
as a function of irradiation fluency is in good agreement with the experimental data presented as well as with theexisting data on optical ablation. Our results demonstrate that tensile stress created in metals by short X-ray pulse canproduce spallative ablation of metals even for drastically small X-ray fluencies, which open new opportunities formaterial nano processing.

1. Introduction
Laser ablation has many technological applications in material microprocessing and fabrication ofnanostructures. Recently it was demonstrated strong
lowering of ablation threshold of dielectrics in thecase of its irradiation by picosecond XRL pulses [1].
In new investigations, we found that the surface
modification of metal aluminum and gold surfacesby the irradiation of the SXRL pulses could beobserved already at fluencies ~ 14 and ~ 21 mJ/cm2 ,correspondently.

2. Experimental set up.
The spatially coherent SXRL pulse was generated
from the silver (Ag) plasma mediums using oscillator -amplifier configuration with Ag double targets
and had the wavelength of 13.9 nm, the bandwidth< 10−4, the duration time of ~ 7 ps, and the beam
divergence of 0.35 mrad (H) x 0.3 mrad (V) [2].
The SXRL pulse was focused on the sample surface
by using a spherical Mo/Si multilayer coated mirror
having a radius of 1000 mm in curvature. Theoutput energy of the SXRL pulse was varied in each
shot, but the average was estimated to be about
200-300 nJ. The total energy of the SXRL beam on
the sample surface after passing through the 0.2 μmFig. 1. The schematic diagram of experimental setup (a)and the AFM images of the Au surface of the target afterirradiation by a single shot of the SXRL beam at 0.5 mm
out of the best focusing position (b) and near it (c). A
magnified box shows a SEM image. Red and black lines
show the profiles of AFM images.

Zr filter and focusing by mirror was ~ 48-72 nJ.
The Al plate with a thickness of 2.5 mm or the Au
plate with a thickness of 0.05 mm was used as thetarget samples. Both Al and/or Au samples weremounted on the surface of LiF crystal and such
sandwich of targets was mounted on the holder

Fig.2 The SEM a) and AFM b) images of the Al surface after
irradiation by a single shot of the SXRL beam.

having two movable directions (Fig. 1a). After theirradiation, the Al and Au surfaces were examinedusing optical, SEM and AFM microscopes (Seeimages in Fig. 1 b),c) and 2).
2. Experimental results and modeling.
It was found that the nanoscale modification ofsurfaces already occurred at very low fluence of ~14 and ~ 21 mJ/cm2 for Al and Au, correspondently.
The typical obtained size of surface irregularities isin the order of 50 – 200 nm for both targets, butmorphological of surface nanostructuring is rather
different for them. The conical structures with adiameter 50-100 nm are formed at Al surface inareas of lower fluence region and with increasing ofthe SXRL fluence up to 30 mJ/cm2 or higher holes
with micron size and submicron deepness mainly
produced. At Au surfaces the ripple-like nanostructures with depth of only ~ 15-20 nm were mainly
formed. With increasing of SXRL fluence on the Au
surface the depth of surface modification is growth,Fig. 3. Ablation patterns of Au given by our atomistic
model: (a) and (b) —  = 7 ps at Fabs = 65 mJ/cm2 andFabs = 130 mJ/cm2; (c) and (d) —  = 0.1 ps at Fabs = 65mJ/cm2 and Fabs = 150 mJ/cm2.

Fig.4. The dependence d(Fabs) for Au target: 1 — X-raypulses (this work); 2 and 3 — optical pulses; 4 — MD
previous results; 5 - 8 — our MD results with different
Ce(Te) (see for details [3]).

but not so significantly as in the case of irradiationof Al targets. The differences in surface behaviors
may be attributed to the difference of the melting
points of materials or electron conductivity.
The atomistic model of ablation was developed[3]. It was shown that the ultra - low threshold
SXRL beam fluence values of metals ablation to bean effect of the high electronic pressure build-up
and the comparatively low electron-ion energy
relaxation rate [3]. Results of modeling of ablation
patterns developing at different time after SXRL
surface irradiation are given by our atomistic model
are presented in Fig.3. As could be seen from Fig. 4
the calculated ablation depth as a function ofirradiation fluence is in a rather good agreementwith the experimental data.
Finally we could conclude that the results wedemonstrated in this work will be important not
only for a pursuit of surface nanostructuring andablation processes, but also for future application of
SXRL beam such as micromachining.
Acknowledgments
This work was supported by the RussianFoundation for Basic Research (grants 10-02-91174- GFEN-a and 10-02-00345-a), by the RAS
Presidium Program of Basic Research 2 and 21 andby the U.S. Department of Energy Contract No.
DE-AC5206NA25396.
References[1] A.Ya. Faenov, N.A. Inogamov, V.V. Zhakhovskii etal., Appl. Phys. Lett. 94, 231107 (2009)[2] Y. Ochi, T. Kawachi, N. Hasegawa et al., Jpn. J. Appl.
Phys. 48, 120212 (2009).
[3] S.V. Starikov, V.V. Stegailov, G.E. Norman et al.,
JETP Letters, 93, 642 (2011)