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Radiation Physics and Chemistry 78 (2009) S46–S52

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

E-m

journal homepage: www.elsevier.com/locate/radphyschem

Damage in solids irradiated by a single shot of XUV free-electron laser:Irreversible changes investigated using X-ray microdiffraction, atomic forcemicroscopy and Nomarski optical microscopy

J.B. Pelka a, R. Sobierajski a, D. Klinger a, W. Paszkowicz a, J. Krzywinski a, M. Jurek a, D. Zymierska a,A. Wawro a, A. Petroutchik a, L. Juha b, V. Hajkova b, J. Cihelka b, J. Chalupsky b, T. Burian b, L. Vysin b,S. Toleikis c, K. Sokolowski-Tinten d, N. Stojanovic d, U. Zastrau e, R. London f, S. Hau-Riege f, C. Riekel g,R. Davies g, M. Burghammer g, E. Dynowska a, W. Szuszkiewicz a, W. Caliebe c, R. Nietubyc h

a Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Polandb Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republicc HASYLAB/DESY, Notkestrasse 85, D-226 3 Hamburg, Germanyd Institut fur Exp. Physik, Universitat Duisburg-Essen, Lotharstr. 1, 4748 Duisburg, Germanye Institut fur Optik und Quantenelektronik, FSU Jena, 07743 Jena, Germanyf Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 9455, USAg European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 3 043 Grenoble, Franceh Andrzej Soltan Institute for Nuclear Studies, PL-05400 Swierk, Poland

a r t i c l e i n f o

Keywords:

XUV FEL

Radiation damage

Ablation

Structure modifications

X-ray diffraction

6X/$ - see front matter & 2009 Elsevier Ltd. A

016/j.radphyschem.2009.06.006

ail address: pelkay@ifpan.edu.pl (J.B. Pelka).

a b s t r a c t

The article presents preliminary investigation results on the near-surface damage produced by single

pulses of XUV free-electron laser in the amorphous a-SiO2, the monocrystalline silicon and the epitaxial

films of gold. The irradiation was delivered with single pulses of only 25 fs at a wavelength of 32.5 nm

and of energy up to 10mJ. Structural modifications induced by irradiation were characterized by X-ray

microdiffraction, as well as by the AFM and optical microscopy. Ablation craters of well-defined edges

with smooth interiors and outer embankments surrounding the crater edges were found in the

materials. Polycrystalline phases were revealed in Si and Au film samples, in the embankments and in

central parts of some craters. In a-SiO2, a diffraction pattern typical of an amorphous material was

observed without any traces of irradiation-initiated crystallization. A step-like, complete removal of

gold film was evidenced inside craters, with only small gold residues in central part of craters exposed

to higher fluences.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The VUV- and XUV-FEL beam interaction with matter has beenextensively investigated for the last few years with a specialattention paid to the damage induced in solids. Most of this workis performed with the Free-electron LASer in Hamburg (FLASH),operating in the wavelength range 6–100 nm. The new type ofsource, with the unique combination of XUV quantum energy,extreme short pulse duration and high power/pulse, opens up anaccess to not yet explored phenomena, remaining unattainablewith classical sources. Extensive investigations of these phenom-ena are crucial for understanding the interaction of intense,ultrashort XUV pulses with solids and for refinement oftheoretical models, necessary to predict performance and lifetime

ll rights reserved.

of optical components for the new generation of FELs operating inthe XUV and X-ray region.

The irradiation of solids with short-wavelength femtosecondpulses delivered by the XUV free-electron laser creates states ofstrong electronic excitation with a highly reduced influence ofoptical nonlinearities at frequencies in between the plasmafrequency and the frequency of the inner shell absorption edge.Some experimental results support the opinion, that the absorp-tion depth for most materials can be much larger, than usingfemtosecond optical pulses, boosting creation of well-definedexcitation conditions in relatively large sample volumes (Hau-Riege et al., 2007; Krzywinski et al., 2007).

Recently, pump-and-probe experiments on ablation dynamicsin silicon irradiated with FLASH single pulse at the wavelengthof 32 nm have shown that phase transitions and ablation occuron very short time scales (Stojanovic et al., 2006). The resultsconfirmed also an increased energy deposition depth and theabsence of absorption nonlinearities. A comparison of single-pulse

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Fig. 1. The X-ray reflectometry and diffraction pattern of the Au 90 nm film on

sapphire. The gold film structure showed (111) orientation with respect to

substrate surface. The spectra have been recorded with the diffractometer of W-1.1

beamline at DORIS, Hasylab.

J.B. Pelka et al. / Radiation Physics and Chemistry 78 (2009) S46–S52 S47

femtosecond ablation at the optical wavelength of 520 nm and atthe 32 nm pulse from FLASH, obtained at the same fluences,revealed substantially different depth profiles, indicating involve-ments of various wavelength-dependent damage mechanisms.

Damage processes induced by FLASH pulses lead to theformation of specific morphological structures with sizes onmicrometric and nanometric scales. The absorbed pulse energycan also induce phase transitions changing composition orcrystalline state of near-surface material. Ex-situ investigationof these objects by a number of microscopic and spectroscopictechniques provided significant results contributing to theexplanation of damage mechanisms and refining theoreticalmodels of beam–solid interaction. However, most of thesemethods are unable to probe the crystalline state of the sampleswith the required lateral and in-depth resolution.

To fill this gap, in the present work, we report on preliminaryresults of irradiation-induced near-surface modifications of threematerials: the insulating amorphous a-SiO2, the semiconductingmonocrystalline silicon and epitaxially grown metallic film of golddeposited on sapphire substrate with a thin vanadium buffer. Theinvestigation has been carried-out by combining the traditionalmicroscopic methods providing information on the surfacemorphology with submicrometer beam X-ray diffraction capableto probe the crystalline state with lateral resolution of only250 nm. Interaction of FLASH pulses with solid surfaces atmoderate irradiation intensities (1011–1014 W/cm2) was studied.

Fig. 2. (A) An example of XUV FEL damaged sample surface with two distinct lines

of craters (Nomarski contrast). (B–D) AFM images of craters created by single shots

of FLASH XUV source in a gold film with fluency of 0.28, 0.58 and 1.2 J/cm2,

respectively. Shapes of the left and central craters come from a convolution of a

circular aperture with beam. The crater (D) was created with the beam without any

aperture.

2. Experimental

The 90-nm-thick gold films were deposited on sapphiresubstrates covered with 10 nm vanadium buffer in an UHV MBERiber system by evaporation of V and Au elements in the vacuumof 5�10�10 Torr. The (112 0)-oriented monocrystalline sapphiresubstrate was 0.35 mm thick with both surfaces polished. Thedetails of the deposition procedure have been described else-where (Wawro et al., 2002). In-situ RHEED measurements duringdeposition showed that the gold film and the vanadium bufferwere grown epitaxially. (0 0 1)-oriented silicon samples were cutfrom a 0.28-mm-thick wafer with top layer polished. Amorphousa-SiO2 sample was in form of a disk 20 mm diameter, 2.85 mmthick, with both surfaces polished.

Prior to irradiation, all the samples were tested with opticalNomarski differential interference contrast microscopy (N-DIC)and by X-ray methods at the W-1.1 beamline of DORIS-III storagering in Hasylab, Hamburg. An example of X-ray reflectometry anddiffraction pattern of the gold film on sapphire is shown in Fig. 1.The measurements revealed the gold film structure to be (111)oriented with respect to the substrate surface, with large domainslaterally disordered. Its average roughness was found to be about0.3 nm r.m.s. The roughness of Si and a-SiO2 surfaces was 0.3 and0.5 nm r.m.s, respectively.

The samples were irradiated in an UHV chamber at a beamlineBL2 of the FLASH facility by single pulses at wavelengthof l ¼ 32.570.5 nm with pulse duration of t ¼ 2575 fs. Thepulse energy was up to 10mJ. The beam was focused surface-normal onto the sample using a grazing-incidence carbon-coatedellipsoidal mirror, external to the chamber. The samples weremounted in a high-precision sample manipulator. The entire UHV-system could be translated along the XUV beam axis, whichallowed the adjustment of the sample with respect to the beamfocus (Sobierajski et al., 2005).

The pulse energy was adjusted by a gas attenuator locatedbefore the focusing mirror, and subsequently monitored by a gasdetector (Richter et al., 2003). Due to the self-amplified sponta-neous emission (SASE) process in FLASH, the energy fluctuates

from pulse to pulse with a variance factor of about 2–3 (Saldinet al., 2000; Ayvazyan et al., 2003). As a result, the experimentalscheme had to include irradiation of appropriately more spots justto select the required fluences after the irradiation. In Fig. 2A, apicture of sample surface with two distinct rows (lines) of spotsdamaged by the FLASH single shots is shown.

The resultant fluences on the samples were approximately inthe range 0.1–2 J/cm2. Each single-pulse irradiation was per-formed on a clean, unexposed sample surface at a distance of200–400mm from surrounding irradiated places to avoid over-lapping of defects and strain fields induced in adjacent spots. Thisvalue was found to be sufficient to treat the strain field around anirradiated spot as negligible, even with highest applied fluences(Pelka et al., 2004). The spot sizes, depending on pulse energyand irradiation geometry, changed from few up to few tens ofmicrometers. In Fig. 2B–D, the AFM images of craters in gold filmsproduced by pulses of increased fluence are compared. Shapes ofthe craters (B) and (C) come from convolution of circular aperturewith beam intensity distribution. The crater (D) was created withthe beam without any aperture.

After irradiation, the samples were examined by severaltechniques, including the N-DIC, atomic force microscopy(AFM) as well as by X-ray diffraction and reflectometry at DORISW-1.1 beamline. In order to qualitatively identify the structurallymodified material on the film surface, we adopted at W-1.1 ascanning technique in transmission with a linear beam aperture,50mm wide, formed by a slit system, capable to cover the whole

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row of spots. The detector with a wide acceptance was set at aposition of diffraction peaks from a polycrystalline material.Although the overall intensity of the recorded signal was lowerthan expected, this technique helped to reveal weak traces ofcrystallites in the lines of irradiated places, which might allow theselection of distinct items for further investigation.

The microstructure was probed by X-rays at the ESRF ID-13beamline, exploiting a submicrometer beam of size as lowas �250 nm and of 1 mrad divergence. The wavelength wasl ¼ 0.97626 A. Diffraction patterns were recorded in transmissionmode by a 2D MARCCD-165 detector with 2048�2048 pixels.The sample was step-scanned with step-size of 200–1000 nmalong chosen paths throughout the craters. The sample–detectordistance was 103.9 mm and the detector diameter was 165 mm.We covered the 2y range up to 381, equivalent to d-spacing downto 1.5 A. The experimental scheme of the X-ray microdiffraction atID-13 beamline is shown in Fig. 3. Precise positioning of a crater inthe microfocus beam was accomplished by additional N-DIC

Fig. 3. X-ray microdiffraction at ID-13 bea

Fig. 4. X-ray microdiffraction—explanation of measurement of an exemplary crater pro

pulse fluency was 0.28 J/cm2 and l ¼ 32.5 nm. (I) AFM pattern, (II) a 2D diffraction pat

represent the 111 and 200 reflections of gold. The rectangle C defines the background

thickness variations of the gold film. A dark area inside the ablation crater shown in t

intensity of 111 gold reflection (A) and the integrated background intensity (C) reco

interpretation of the references to colour in this figure legend, the reader is referred to

microscope, which was placed in front of sample when the beamwas closed, and removed before opening it.

3. Results and discussion

In Fig. 4, an example of X-ray microdiffraction and AFM resultsfrom the edge of a single-shot crater formed with the irradiationfluence of 0.28 J/cm2 in the Au film is shown. With thebeam–detector geometry described above, two diffraction peaksof gold (111 and 200, at 2y values of 23.9251 and 27.7021,respectively) and four strongest peaks of sapphire (012, 104, 113,116 at 2y values of 16.1281, 22.0641, 27.0751, 35.4901, respectively)were detectable (according to ICDD PDF-2, 2006). From a seriesof diffraction patterns recorded in a grid at a crater in the goldfilm, a map of background intensity, defined by the rectangularintegration area (Fig. 4II-C), was composed. Due to a highfluorescence of gold atoms at the wavelength applied, the maps

mline, ESRF: an experimental scheme.

duced by the FLASH single-shot on 90-nm-thick Au film deposited on sapphire. The

tern recorded exactly in the embankment at the edge of crater, the A and B rings

intensity integration area. The background intensity distribution map reflects the

he reconstructed 2D map of the crater evidences the lack of gold. (III)-Integrated

rded along a straight path starting inside the crater shown in I (red line). For

the web version of this article.

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excellently reflect a local gold content independently of itscrystalline state with the higher background intensity reflectinga higher gold content.

Fig. 5. A typical pattern of 111 and 200 diffraction peaks of Au, integrated from the

2D patterns recorded at crater embankments: solid line—a pattern with a single

component, crystallite sizes 415 nm, dotted line—the second broad component

indicates an additional component with crystallite sizes o2–4 nm.

Fig. 6. The crater produced with a fluence of 2 J/cm2 at l ¼ 32.5 nm on Si (0 0 1). (A) X

crater profile shown in C. The patterns recorded at the crater embankments show few sp

in the crater and outside was found. (B) AFM pattern, (C) the crater profile along the li

We observed complete, step-like removal of gold in the ablatedcraters on the surface of the films, evidenced both by a relevantbackground intensity drop inside the craters and by the lack ofgold peaks in diffraction signal (see Fig. 4III). The polycrystallinephase was found only in a close vicinity of the crater boundaries innarrow strips �100–500 nm wide, set upright as embankmentsoutlining the crater boundaries. The height of the structures wasup to few micrometers. This was determined by combining themicrodiffraction results and AFM patterns (Fig. 4I). The sizeof gold crystallites in the embankments was estimated fromdiffraction line profiles analysis, and is not smaller than 15 nm(see Fig. 5). In some spots, the profiles showed additional fractionof gold crystallites of sizes below 2–3 nm. In the centers of cratersirradiated with the highest fluences, the diffraction signal fromthe sapphire polycrystalline phase, as well as from small goldresidues was found.

In Fig. 6, a typical crater created on the Si (0 01) surface byirradiation with a fluency of �2 J/cm2 is shown. Here, Si 111 and Si220 reflections at 2y values of 17.91 and 29.461 were recorded.Similarly to craters in gold films, a crystalline phase has beenrevealed in narrow 2–2.5-mm-thick embankments surroundingablation crater edges. The typical size of crystallites, estimatedfrom the diffraction pattern was about 30 nm. Similarly to the goldfilm sample, residues of crystalline material deposits were found inthe central parts of silicon craters irradiated with higher fluencies.

-ray microdiffraction patterns taken from three places indicated by arrows at the

ots of two reflections: 111 and 220 (2y: 17.91 and 29.461). No polycrystalline phase

ne shown in D and (D) N-DIC image of the crater.

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The diffraction pattern depicted in Fig. 7 is a representativeexample for all irradiated spots in the amorphous SiO2. Nocrystalline material was detected by microdiffraction in any pointprobed in the areas of irradiation damage of the material.Moreover, diffraction profiles obtained by the radial integrationover the experimental 2D diffraction patterns did not show anysystematic changes in shape that could suggest, e.g., annealingor other processes leading to a detectable modification ofnanocrystallites in the phase.

The AFM revealed the ablation craters of well-defined edgeswith smooth interiors in all the investigated materials. Pro-nounced structures surrounding the craters were found for thefluences exceeding the ablation threshold. These structuresappeared in two forms: (i) embankments or quasi-continuouswalls, characteristic to spots on gold films (see Fig. 4I) and also athigher fluencies on silicon surface (see Fig. 6); (ii) spikes arrangedin a loop around ablation craters, found on a-SiO2 and on silicon,mostly at intermediate fluences (see Fig. 7). The thickness of thestructures is different in various types of materials with the finestspikes of only 80–120 nm in diameter observed in amorphoussilicon oxide, up to few micrometer wide columns or walls

Fig. 7. A crater produced on amorphous SiO2 by FLASH single-shot with fluency of 1.2 J/

(C) a typical X-ray microdiffraction pattern of irradiated a-SiO2. No diffraction peaks we

the embankment along the path indicated by a green broken line in B. For interpretatio

version of this article.

surrounding some silicon craters. The embankments around goldcraters were found from X-ray microdiffraction to be 100–500 nmwide and more. Comparison with AFM patterns suggests that theycan be formed entirely of the primeval gold film, 100 nm thick.The thin leaflets of this film are located at some craters obliquelyto the sample surface. This occurs in the down part of the cratershown in Fig. 4I.

In the case of optical pulses of longer duration, the edges of theablation craters can show additional near-edge structures. It isknown as ‘‘piston effect’’ and is attributed to the recoil pressure ofthe ablating material. It is assumed, that the piston effect requiressignificant lateral pressure gradients as compared to the dom-inating pressure gradient normal to the surface (Stojanovic et al.,2006). This should favor craters not very shallow compared totheir width.

From the point of view of the ablation mechanisms it isinteresting that while the overall crater is flat, there is apronounced embankment around some craters in the gold film.This situation, illustrated in Fig. 4I, occurs when the fluenceis higher than the ablation threshold for gold (on the order of0.1 J/cm2) but insufficient to ablate the sapphire substrate

cm2 at 32.5 nm wavelength; (A) AFM pattern, (B) a magnified part of embankment,

re found by microdiffraction at any place of the material. (D) The height profile of

n of the references to colour in this figure legend, the reader is referred to the web

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beneath. For the system discussed here, our preliminary resultsindicate that the ablation of the sapphire under the gold filmoccurs far above the fluence of 0.6 J/cm2. This value cannot betaken as ablation threshold for pure sapphire, since a significantpart of pulse energy is deposited in the gold film above thesubstrate. The respective absorption length values at a wavelengthof 32.5 nm are 7.5 nm for gold and 15.6 nm for sapphire (Henkeet al., 1993). In the above mentioned fluency range, the craters inthe gold film are quite shallow, about 100 nm deep, equal to thethickness of the metallic film, with the crater lateral dimensionsof 10mm and higher. Thus, the volume of the material involved inthe ablation is almost independent of the pulse energy for a widerange of fluences. The leaflets at the crater embankmentsare formed exclusively of gold with thickness comparable to thegold film thickness. This fact, together with their obliquearrangement toward the crater interiors observed in some placesat the embankments, suggest that they are formed of a gold filmstripped off and pushed toward the crater edge.

To clarify the above suggestion concerning the mechanismof embankments creation a further investigation is required.However, starting from our present knowledge for all thematerials from the temperature and pressure-dependent mechan-ical stretchability we can draw some conclusions. Assuming that acontinuous wall around the crater is formed during an initial stageof ablation from the stripped near-surface ‘‘skin’’, further devel-opment of the wall structure can depend on the materialsusceptibility to stretching. Qualitatively, for the samples studiedin this work, the highest is the stretchability for the gold and thelowest for the amorphous SiO2 (Li et al., 2004). This is consistentwith our finding that the finest spikes, about 100 nm wide only,are formed in this material. In silicon, which exhibits intermediatestretchability, we find either a quasi-continuous wall, at higherfluences, where the material can be expected to be more plastic(with a higher stretchability), or a loop composed of individualspikes (columns), when stretchability can be expected to be lower.It is interesting that in some cases (not shown here), we observe inan embankment surrounding the silicon crater parts of contin-uous wall and parts composed of individual spikes. This isassociated with the distance from crater edge with continuousparts adhering directly to the edges, and spikes glided a smalldistance out of the crater edge.

Beneath the surfaces of ablation craters, no traces of poly-crystalline phase were found. This can suggest a very thininterface between the crater surface and the unaffected samplematerial that can contain an undetectably small amount ofnanocrystalline or amorphous material.

The observed morphology and structural properties cansuggest that the crystallites at the crater edges are remnants ofa primeval crystalline structure of near-surface layers rather thana newly crystallized material from gold melted by irradiation.This assumption is also supported by the fact that the crystallinematerial occurs on the border between the damaged part insidea crater and the external region of sample surface unaffected byradiation.

4. Conclusions

Near-surface modifications of solid targets exposed to thesingle-shot XUV-FLASH free-electron laser irradiation were stu-died. The applied fluences ranged from less than 0.1 to 2 J/cm2,covering the energy density range from below the ablationthreshold up to far above this threshold for all the materials.The structure and morphology induced on the insulatingamorphous a-SiO2, the semiconducting monocrystalline siliconand epitaxially grown metallic film of gold deposited on sapphire

substrate with a thin vanadium buffer were ex-situ probedwith a number of techniques including the microbeam X-raydiffraction, the AFM and the N-DIC optical microscopy. A variety ofmorphological structures induced in the damaged areas observedby microscopic methods in the samples were associated with thelocal microstructure determined from X-ray diffraction mapsrecorded with a submicrometer beam in a step-scanning mode.

In a-SiO2, a typical diffraction pattern of an amorphousmaterial, free of any traces of irradiation-initiated crystallizationwas evidenced. A step-like, complete removal of the gold filmwas evidenced inside of damaged areas, with only small goldresidues in central part of craters exposed to higher fluencies.A polycrystalline phase of gold was found in thin leafs100–500 nm thick, set upright as walls up to few micrometershigh and outlining the crater boundaries. Beneath the ablationcrater surface no traces of (poly) crystalline phase—the resultssuggest a very thin interface between the crater surface and theunaffected substrate material.

A small residue of polycrystalline phase found in central partsof craters created with highest fluencies in the gold and siliconsamples can be either a debris resultant of ablation processes, or isassociated with a non-linear process leading to the formationof high-density plasma during the pulse, as it was evidenced incase of excitations at optical wavelengths. However, with muchlower contribution. This question needs a further investigation.The supposition that crystallites found in narrow embankmentssurrounding craters on Si and Au surfaces are remains of aprimeval structure that survived irradiation is not excluded fromthe present investigation, though it needs a further study,allowing us to address directly the specific aspects of damageprocesses induced in solids by ultrafast intense XUV pulses thatgenerate structural transformations on the near-surface sampleregion. The results observed in the present experiment support –in the authors opinion – earlier findings assuming a stronglylocalized interaction of XUV-FEL pulse irradiation energy withsolids.

Acknowledgments

This work has been partially supported by the Ministryof Science and Higher Education of Poland, SPB nr. DESY/68/2007, by the Czech Ministry of Education from the NationalResearch Centers program (Projects LC510 and LC528) andprogram INGO (Grant LA08024) and by Academy of Sciencesof the Czech Republic (Grants Z10100523, IAA400100701 andKAN300100702). Irradiation with FLASH has been performedwithin the framework of the Peak-Brightness-Collaboration [projectII-20022049 EC]. Support from the PBC and the operators ofthe FLASH facility are gratefully acknowledged. The synchrotronmeasurements at HASYLAB, Hamburg were supported by the RIAFP6 Contract RII3-CT-2004-506008 of the European Community,the microdiffraction study has been done at the ID-13 beamline atESRF, Grenoble.

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