Nuclear Instruments and Methods in Physics Research B 241 (2005) 496–500

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Computer simulation of surface modification with ion beams

Z. Insepov a,*, A. Hassanein a, D. Swenson b, M. Terasawa c

a Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USAb Epion Corporation, 37 Manning Road, Billerica, MA 01821, USA

c University of Hyogo, 3-1-2 Kouto, Kamigori-cho, Ako-gun, Hyogo, 678-1205, Japan

Available online 18 August 2005

Abstract

Interactions of energetic ions with various solid targets including silicon and a few metal surfaces were studied bycomputer simulation and verified by experiment. Surface sputtering and modification for collisions of Arn (n � 100)cluster ions, with kinetic energies of 12–54 eV/atom, and slow highly charged ions (HCI), with potential energies of80–3500 eV, have been simulated. Various energy transfer mechanisms of the ion energy into the solid target, suchas shock wave generation, hollow atom formation, Coulomb explosion, charge screening and neutralization were stud-ied. Atomistic molecular dynamics (MD), as well as a phenomenological surface dynamics methods were employed andthe results of the simulations were compared with the experimental data.� 2005 Elsevier B.V. All rights reserved.

PACS: 32.80.Rm; 68.49.Sf; 79.20.�m; 96.35.Gt; 81.16.Nd

Keywords: Gas cluster ion beam; Highly charged ions; Sputtering; Extreme ultra-violet lithography

1. Introduction

Mitigation of high-voltage breakdowns is amajor concern in development of higher field RFcavities for the next generation accelerators [1].All existing techniques for electrode surface prepa-ration and conditioning fail to provide adequatecorrection and passivation of atomic scale defects

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.nimb.2005.07.061

* Corresponding author. Tel.: +1 630 252 5049; fax: +1 630252 3250.

E-mail address: insepov@anl.gov (Z. Insepov).

and asperities. As a result, RF cavities must invari-ably be operated at much lower potentials thanwould otherwise be possible. Gas cluster ion beam(GCIB) processing of RF cavities could signifi-cantly reduce the size and cost of high-energy par-ticle accelerators by allowing reliable operation athigher acceleration gradients [2].

An emerging EUV-lithography technologybased on gas-discharge produced plasma (GDPP)and laser produced plasma (LPP) needs highly sta-ble and durable surfaces of condenser optics andsurfaces facing the plasma. Erosion of reflecting

ed.

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metal and silicon surfaces due to the highlycharged Xe10+ ions (HCI) that are generated inplasma impedes the development of future EUV-lithography sources. Mitigation of this effect re-quires materials of high stability and toughness [3].

In the bombardment of slow HCIs into solids,the sputtering yield of secondary ions increases sig-nificantly with the charge state of the ions for insu-lator materials such as SiO2, LiF and UO2 [4–8].Similar phenomena have also been observed forsilicon and electrically conductive materials likeCu, Al, Ni [9]. A crater formation is reported onmica surface bombarded with Xe44+ ions [10].These phenomena are not yet well understoodand therefore, theoretical and computer simula-tion approaches are much needed to advance ofthe experimental work and the development ofimportant industrial applications.

2. Computational models

For simulation of the surface smoothing by gascluster irradiation and sputtering of the surfacesby a HCI collision, a classical molecular dynamics(MD) with constant pressure was implemented[11]. A rough Cu(100) surface was prepared byplacing a few pyramidal hills on the top of an idealCu(100) surface. Simulations of HCI collisionswere modeled by using flat silicon and metal sur-faces. Interactions between metal ions in the targetwere modeled via an embedded-atom model(EAM) potential [12], and silicon atoms interactthrough the Stillinger–Weber potential [13].

Neutral Xe atoms and Xe+ ions interacting withthe surface via a Buckingham and Ziegler–Biersack–Littmark (ZBL) potentials were devel-oped with the parameters from [14]. Ar–Ar andAr–Cu interactions and other details of the simula-tion model for energetic gas cluster ion impacts ona Cu surface are discussed elsewhere [15].

Surface smoothening by cluster ion irradiationwas obtained by calculation of a cluster ion dosedependence of the average heights hzii and thestandard deviation Ra for the topmost surfaceatoms.

A simplified model was used to study the relax-ation of HCI approaching a conductive surface

[16]. The dynamics of HA formation was studiedvia visualization of the events by recording moviesat various energies and charge states q [17].

During the impact on the target surface, HCItransfers its potential energy into electronic de-grees of freedom thus leading to a high-densityionization of local target atoms on the surface. Amechanism for the energy transfer of such an elec-tronic excitation into atomic motion has been dis-cussed based on the ‘‘Coulomb explosion’’ or‘‘thermal spike’’ models [18]. Shock waves gener-ated by the Coulomb explosion would lead to for-mation of a nanocrater on the surface and anenhancement in sputtering [19].

The characteristic charge neutralization time ofa HA in the target are very short (typically <10 fs)[20,21]. This time was approximated by the Max-well relaxation time: s = e/r, where e is the electri-cal permittivity of the target material and r is theconductivity. This formula gives an estimate for Sisn � 1 ps. Therefore, charge neutralization in Sicould be neglected because this time is much long-er than the interaction time between Coulombcharges. However, the neutralization times aremuch shorter for conductive targets. For example,gold and aluminum have the time of about 0.3 fs,copper – 0.2 fs, nickel – 0.8 fs and tungsten –5 fs. During the simulation, the number of chargeswas decreased in accordance with a radioactive de-cay law: N(t) = N0exp(�t/s). Such a decay neutral-ization law was first implemented for modeling ofinteraction of swift ions with targets in [18].

The dynamics of particle ejection from the sur-face and crater formation on the surface were sim-ulated by MD method. The sputtering yield Y isdefined as a number of target atoms removed fromthe surface with one highly charged ion impact.

3. Surface smoothing by GCIB

The surface sputtering and modification pro-cesses were simulated for Arn (n = 92) cluster ionimpacts, with kinetic energies of 12, 27 and54 eV/atom, on a rough Cu surface containingfrom 5 to 10 pyramidal hills, with the heights ofabout 3–7 nm, that were placed on the top of aCu(100) surface; and the simulation was followed

Fig. 2. Dose dependence of the average surface roughness formultiple Ar92 cluster ion impacts, with energies of 12, 27 and54 eV/atom obtained by MD simulation.

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up to 50 cluster impacts on the same surface. Themaximum ion dose in simulation was of about1015 ions/cm2 and the total simulation time wasof about 1 ns.

Fig. 1 shows evolution of a rough Cu surfacebuilt by placing five hills, with the average heightsof 3.6 nm, on the top of a Cu(100) surface duringirradiation with 54 eV/atom cluster ions with thefollowing ion doses: (a) 0, (b) 1.1 · 1013, (c)3.3 · 1013 and (d) 4.4 · 1013 ions/cm2. This figureshows that small hills disappear at lower doses.Our results have shown that one of the reasonsfor the small hill removal is that these small intru-sions on the surface gain a much higher tempera-ture compared to the rest of the surface whichleads to a local melting and eventual smootheningof the surface.

Fig. 2 shows the dose dependence of the surfaceaverage heights of a rough Cu surface irradiatedwith multiple cluster collisions calculated in thiswork. This figure compares the residual surfaceroughness of the initially rough Cu surfaces irradi-

Fig. 1. Evolution of a rough Cu surface built by placing five hills, withduring irradiation with 54 eV/atom cluster ions with the following ioncm2.

ated with 50 Arn (n = 92) clusters with differentenergies per atom: 12, 27 and 54 eV/atom, whichcorrespond to typical energies in experiments withGCIB treatment. This figure shows that the rough-ness decreases rapidly for the clusters with energy

the average heights of 3.6 nm, on the top of a Cu(100) surfacedoses: (a) 0, (b) 1.1 · 1013, (c) 3.3 · 1013 and (d) 4.4 · 1013 ions/

Z. Insepov et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 496–500 499

of >27 eV/atom. However, Fig. 2 shows that evenat a lowest cluster ion energy of 12 eV/atom, thesurface roughness could still be efficiently reduced,but at a higher ion dose. This is a ‘‘soft surfacetreatment’’ that has been verified by experimentwith removal of narrow asperities on a Cu surface.Our finite-difference simulation based on the phe-nomenological surface (continuum) dynamicsequations that were deduced from the Mullins–Herring theory [21,22] has also revealed an impor-tant result that there exist an optimum cluster ionsize and cluster energy that could smooth the sur-face up to a lowest Ra level of below 1 A.

4. Sputtering yields by highly charged ions

The Coulomb explosion induced sputtering byXeq+ ion bombardment on a Si, Cu and Ni sur-faces, assuming that the total potential energy ofincident Xeq+ ion can be consumed to produce sin-gly charged ions in the target. Finally, the sputter-ing yields as a function of the potential energy ofXeq+ were studied.

Strong dependence of the sputtering yield onthe HCIs potential energy is found in MD studyand this finding agrees with experiment. Fig. 3shows comparison of the results calculated in thiswork with those obtained in experiment [9] for

Fig. 3. Comparison of the slopes of calculated by MD in thiswork and experimental sputtering yields obtained for a highlycharged Xeq+ ion bombarding a Si(100) surface with a kineticenergy of 1 keV. Total sputtering yields for CsI, LiF, SiO2,GaAs were drawn versus the potential energies of projectile.Kinetic energies were constant within each data set [20].

the sputtering yield of a highly charged Xeq+ ion,with a kinetic energy of 1 keV, bombarding aSi(100) surface. All existing experimental dataon the system Xeq+/Si are given in arbitrary units[9]. Therefore, Fig. 4 compares the slope of ourcalculated sputtering yields data to that of theexperimental yields of Si surfaces bombarded withXeq+ HCIs [9].

Fig. 4 shows the calculated sputtering yield datafor Xeq+ sputtering of a Cu and Ni (100) surfaces.This figure also shows three data sets obtained inexperiment [9] for potential sputtering of Cu, Niand Al surfaces by a slow highly charged Xeq+

ion (q < 54). Experiment shows that there aretwo distinct regions with different slopes: (i) asmall energy region, with an energy below10 keV, where the sputtering yield does not dependstrongly on the HCI energy and (ii) a region withan E2 dependence, at HCI energies higher than10 keV. As the experimental data are given in rel-ative units, comparison is made using the slope.Fig. 4 also shows that our charge neutralizationmodel gives a good agreement for the low HCI en-ergy sputtering yields on both Cu and Ni surfaces.However, when the HCI energy becomes high, thecharge neutralization dynamics should certainlybe different and the screening length should bemodified for example, by including an energydependence.

Fig. 4. Comparison of the slopes of calculated by MD in thiswork and experimental sputtering yields obtained for a highlycharged Xeq+ ion bombarding Cu and Ni surfaces with akinetic energy of 100 eV. The experimental sputtering yields forCu, Ni and Al are in relative units [9].

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5. Summary

Surface smoothing with Arn (n = 92) clusterions was modeled for a Cu surface containing5–10 pyramidal hills and irradiation was imple-mented by bombarding this surface with up to50 clusters. Our simulation shows that the surfaceroughness drops well below of the initial valueand the rate of the smoothing depends on thecluster energy. Our simulation results were com-pared with experimental GCIB treatment resultswith 30 and 5 keV cluster ion beams and thiscomparison shows that simulation and experi-ment agree well. Molecular dynamics models ofhollow atom formation, charge neutralization,electric field screening, surface sputtering and cra-ter formation were developed and implemented tostudy the mechanisms of Si(100) surface erosionby highly charged Xeq+ (q < 54) ion bombard-ment. Our result have shown excellent agreementbetween the slopes of the calculated and experi-mental sputtering yield dependences on the HCIpotential energy.

Surface erosion of a Si, Cu and Ni (100) sur-faces by low energy HCI bombardments has beenmodeled by studying interactions of slow Xeq+

ions and the results of this simulation were com-pared with the experimental data. Our results haverevealed that the sputtering yield of HCI on Cuand Ni does not depend strongly on the energy,for a low HCI energy, in accordance withexperiment.

Acknowledgements

This work is partially supported by Intel, Inter-national Sematech Corporations and the DOESBIR program.

References

[1] J. Norem, Z. Insepov, I. Konkashbaev, Nucl. Instr. andMeth. A 537 (2005) 510.

[2] Z. Insepov, D. Swenson, A. Kirkpatrick, A. Hassanein,ANL 2003 RF vacuum Breakdown Workshop, October2003, ANL.

[3] J.P. Allain, A. Hassanein, T. Burtseva, A. Yacout, Z.Insepov, S. Taj, B.J. Rice, SPIE Proc. 5374 (2004) 112.

[4] M. Sporn, G. Libiseller, T. Niedhart, M. Schmidt, F.Aumayr, H.P. Winter, P. Varga, Phys. Rev. Lett. 79 (1997)945.

[5] P. Varga, T. Neithart, M. Sporn, G. Libiseller, M. Schmid,F. Aumayr, H.P. Winter, Phys. Scr. T 73 (1997) 307.

[6] T. Niedhart, F. Pichler, F. Aumayr, H.P. Winter, M.Schmidt, P. Varga, Phys. Rev. Lett. 74 (1995) 5280.

[7] T. Neidhart, F. Pichler, F. Aumayr, H.P. Winter, M.Schmidt, P. Varga, Nucl. Instr. and Meth. B 98 (1996) 465.

[8] T. Schenkel, A.V. Barnes, A.V. Hamza, D.H. Schneider,Phys. Rev. Lett. 80 (1998) 4325.

[9] T. Sekioka, M. Terasawa, T. Mitamura, M.P. Stockli, U.Lehnert, C. Fehrenbach, Nucl. Instr. and Meth. B 146(1998) 172, 182 (2001) 121.

[10] D.C. Park, R. Bastasz, R.W. Schmieder, M.P. Stoeckli,J. Vac. Sci. Technol. B 13 (1995) 941.

[11] H.J.C. Berendsen, J.P.M. Postma, W.F. van Gunsteren, A.DiNola, J.R. Haak, J. Chem. Phys. 81 (1984) 3684.

[12] V. Rosato, M. Guillope, B. Legrand, Phil. Mag. A 59(1989) 321.

[13] F.H. Stillinger, T.A. Weber, Phys. Rev. B 31 (1985) 5262.[14] J.F. Ziegler, J.P. Biersack, U. LittmarkThe Stopping and

Range of Ions in Solids, Vol. 1, Pergamon Press, NewYork, 1985, p. 35.

[15] Z. Insepov, R. Manory, J. Matsuo, I. Yamada, Phys. Rev.B 61 (2000) 8744.

[16] J. Burgdorfer, P. Lerner, F.W. Meyer, Phys. Rev. A 44(1991) 5674.

[17] M. Terasawa, Z. Insepov, T. Sekioka, A.A. Valuev, T.Mitamura, Nucl. Instr. and Meth. B 212 (2003) 436.

[18] E.M. Bringa, R.E. Johnson, Phys. Rev. Lett. 88 (2002)1655011.

[19] H.-P. Chen, J.D. Gillaspy, Phys. Rev. B 55 (1997) 2628.[20] T. Schenkel, A.V. Barnes, A.V. Hamza, J.C. Banks, B.L.

Doyle, D.H. Schneider, Phys. Rev. Lett. 81 (1998) 2590.[21] W. Mullins, J. Appl. Phys. 30 (1959) 77.[22] C. Herring, J. Appl. Phys. 21 (1950) 301.