Ionised cluster beams as a hardness measurement tool

Z. Insepov *, R. Manory 1, J. Matsuo, I. Yamada

Ion Beam Engineering Experimental Laboratory, Kyoto University Sakyo-ku Kyoto 606-8501 Japan

Abstract

Large cluster ion bombardment has been shown to be a unique tool for generating a variety of bombarding e�ects

over a broad range of acceleration energies. We propose to use the e�ects of the impact of large gas cluster beams on the

surface for a new hardness measurement technique. The cluster impact leaves a crater on the surface, the size of which

varies according to the surface hardness and the cluster parameters (which can be pre-determined). With proper cal-

ibration, this e�ect can be used for determination of hardness. Alternatively, the sputtering yield can be used for the

same purpose. These are entirely surface e�ects and depend on the surface material and not on the substrate. This

measurement method also eliminates the need for indenters that are harder than the material measured. The technique

is particularly useful for measuring hardness of thin ®lm coatings deposited on softer substrates (for which no other

technique is available). Ó 1999 Elsevier Science B.V. All rights reserved.

PACS: 36.40; 07.05.T; 79.20.A; 62.20.Q; 81.40.N

Keywords: Cluster; Crater; Sputtering; Brinell hardness number

1. Introduction

Clusters, or assemblies of atoms, are aggregateswhich can consist of many weakly bound atoms.The cluster beam is formed by supersonic expan-sion of gases through a small nozzle into highvacuum. Electronic ionization of the neutral clus-ters and their electrostatic acceleration lead to theformation of energetic cluster ion beams which can

be used for the bombardment of a target placed inthe same vacuum chamber. Ionized cluster beamsare extracted and particular sizes of cluster ionscan be selected by an E´B mass ®lter. They havebeen used for surface smoothing, shallow implan-tation, and other e�ects [1±7].

A signi®cant e�ect of surface bombardment byheavy particles is crater formation. Craters areknown to form as a result of the impact of largeparticles with a surface [8], and a high degree ofsimilarity has been observed between macro-par-ticles and micro-particles impacting on a surface.Within the obvious di�erence in scale of magni-tude, the e�ect of a meteorite impact is similar inmany ways to the e�ects of small particles im-pacting on a surface with su�cient energy to create

Nuclear Instruments and Methods in Physics Research B 148 (1999) 47±52

* Corresponding author. Tel.: (075)753-5956; fax: (075)751-

6774; e-mail: insepov@kuee.kyoto-u.ac.jp1 Permanent address: Dept. Of Chemical and Metallurgical

Engineering, RMIT, P.O. Box 2476V, Melbourne, Vic. 3001,

Australia.

0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 6 0 - 5

a crater. Various models have been developed forcrater depth calculation based mainly on impactenergy and target properties; a few such theoriesare listed in Refs. [8±10]. The theory suggests thatboth crater formation and the sputtering yield canbe correlated with the Brinell Hardness Number(BHN) which is a measure of a material's resis-tance to localized surface deformation. In theBHN technique, a hard spherical indenter is forcedinto the surface to be tested. After releasing theload, the dimensions of the pit left on the surfaceare measured, and BHN is obtained. Therefore, anew method of hardness measurement can beperformed using cluster ion bombardment bymeasuring crater geometry. The full theoreticalbackground for this method is presented in detailelsewhere [10], but it can be summarized as fol-lows: The crater volume depends on the materialsurface properties, such as sound velocity, com-pressibility, density, as well as on the clusterproperties, such as cluster size, type of cluster at-oms and cluster energy. With proper calibration,the e�ect can be used as an alternative method ofhardness measurement. Crater dimension mea-surements present an advantage over conventionalhardness measurements because they are truesurface e�ects and thus can be useful particularlyfor hard thin ®lms deposited on soft substrates.

2. Correlation between crater size and hardness

The e�ect of crater formation by ion bom-bardment has been rarely reported for monomerions. Merkle and J�ager [11] reported crater for-mation with the probability of about 1% in irra-diation on Au foils by Bi� and Bi�2 ions at energiesbetween 10 and 500 keV. In contrast, for clusterion beams, a relatively large body of results ob-tained from both Molecular Dynamics (MD)simulations [1,9,12] and experiments provides evi-dence that crater formation is a very commonconsequence in this technique because of thephysical e�ects occurring on the surface duringcluster impact [3,4,7]. Upon perpendicular impactof the accelerated cluster ion with the surface, avery high pressure develops in the impact areawhich causes a wave-like motion in the near sur-

face region and ®nally a crater is formed. Thesestages have been described in [1,9,10,13].

The crater formation was studied by MD forArn (n� 200±800) cluster impacts on a Cu(100)surface, for total cluster energies of 6±20 keV.These data were used further as the data points forthe crater depth dependence on cluster energy. Fig.1 shows two crater formed on a Cu surface byclusters with energy of 6 keV (Fig.1a) and 10 keV(Fig. 1b).

Fig. 2 shows the 1/3 power dependence of thecrater depth on the total cluster energy calculatedby MD for impacts of Arn with energy of 6±20keV. The crater depth was de®ned as a distance

Fig. 1. Two snapshots of Arn (n� 236, 370) cluster impacts

with total cluster energies of (a) 6 keV, (b) 10 keV impacting a

Cu(100) surface at t� 20 ps. The Ar atoms are not visible. A

hemispherical crater is formed.

48 Z. Insepov et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 47±52

between the original surface and the bottom of thecrater. Two other dependencies given in this ®gureare: the dashed line, with a� 0.4, and the dottedline, with a� 1/4. As can be seen, the 1/3 powerlaw is the best ®t of these results. The linear cor-relation between the crater depth and the craterdiameter was also obtained by the MD simulation.

Examples of actual craters observed on goldlayers deposited on mica after irradiation bycluster ion beams by scanning tunnelling micros-copy (STM) are shown in Fig. 3(a)±(d) for thecluster sizes of 100, 500, 3000, and 30000 Ar atomswith the same total energy of 150 keV [13,14].

Fig. 4 shows measured values of crater diame-ters produced on gold with an Arn ion clusterbeam of size n� 3000, at increasing accelerationvoltage. The dependence of the crater dimensionon E1=3 is evident. (The cluster energy E is directlyproportional to the acceleration voltage Va.) Theenergy range suitable for crater formation experi-ments varies according to nature of the cluster andthe surface material but would typically be be-tween 20 and 150 keV [13,14].

A correlation between crater depth d, impactenergy E and hardness B can be given by a formulaof the type, d� k(E/B)1=3 [8,10,15]. This depen-

dence is plotted as straight lines in Fig. 5 on adouble logarithmic plot for di�erent cluster ener-gies. In order to calibrate this method againstknown materials we used MD results for the craterformed on Cu (B� 500 MPa) obtained by simu-lation for bombardment at 10 keV which is shownas circle in Fig. 5. The graph presented here can beused to determine hardness when crater dimen-sions can be determined. It is important to notehere, however, that the BHN data currentlyavailable in the literature refer to bulk material,and there are signi®cant, well documented, di�-culties in measuring the hardness of thin ®lms [16].For example, hardness data for gold appears insome sources [17] as 245 MPa ± whereas for thinevaporated ®lms a value of 500 MPa was reported[18]. The experimental results shown in Fig. 3(c)from craters on the Au ®lms bombarded with Arcluster at 150 keV were used to mark a second datapoint on the graph. The STM depth results arealso not very accurate because the tip was notsmall enough to determine the true depth of thecrater. (For this reason the diameter is easier tomeasure with available equipment). The depth wasestimated to be of 22 �A. The hardness value forgold as indicated by the square dot, is well withinthe prediction of the lines of Fig. 5. However, inview of the large discrepancies in hardness data,and the di�culty of obtaining accurate measure-ments of hardness for thin ®lms, we suggest thatthis new method is a better indicator of real near-surface hardness than the existing techniques. Alarger data base of craters will be useful for bettercalibration of this method. It is expected that withimprovements in depth pro®ling technology, betterdepth data can be obtained to improve the accu-racy of these curves.

3. Correlation between sputtering rate and hardness

The nature of the sputtering phenomenon ob-served with cluster ion beam bombardment isdi�erent from that of monomer ion bombardment,as discussed previously [1±7,9,13,14]. The e�ect oflateral sputtering [2±7], for example, is unique tocluster beam sputtering and sputtering yields ob-tained are signi®cantly higher than those observed

Fig. 2. The 1/3 dependence between the crater depth on a

Cu(100) surface and the total cluster energy for 4 cluster en-

ergies: 6, 10, 13 and 20 keV, obtained by MD simulation.

Z. Insepov et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 47±52 49

with monomer ions. Theoretical considerations[10] show a direct correlation between this pa-rameter and crater volume. For the practical pur-pose of measuring hardness the sputtering yieldpresents a signi®cant advantage over crater di-mensions because it is much easier to collect data

for sputtering yield than to measure accurately thecrater dimensions.

However, it should be noted that sputteringyield is more strongly a�ected by phenomena suchas surface oxidation or contamination than iscrater formation. In an attempt to verify if this

Fig. 3. Craters formed by Ar cluster impact. STM images of a gold surface bombarded by Arn at acceleration voltage of 150 kV for

di�erent cluster sizes: (a) n� 100; (b) n� 500; (c) n� 3000; (d) n� 30000.

50 Z. Insepov et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 47±52

method is feasible, sputtering yield data accumu-lated in cluster beam experiments for various ma-terials have been normalized by dividing the

cluster energy by the sputtering yield/atom; theresults are presented in Fig. 6(a) and (b). As canbe seen, many materials can be ®tted to this line.

Problems determining the hardness of thin®lms, which were mentioned above [16], make thismethod more attractive than existing techniquesbecause the elastic contribution from the sub-sur-face after impact is only from a very limited depth-less than two times crater depth (in the order of20±30 �A). Therefore, the method presents signi®-cant practical potential for obtaining true hardnessvalues for thin ®lms.

4. Conclusions

The use of cluster ion beams for surface hard-ness characterization of materials is suggested.Both crater formation and the enhanced sputteringyield are unique e�ects of cluster ion beams whichdepend entirely on the surface properties, leadingto the conclusion that surface hardness could bederived from the measurable properties such ascrater dimension or sputtering yield. This methodneeds further re®ning to be adopted as a mea-surement technique, but it presents a number ofadvantages over existing methods because craterformation and sputtering depend only on the sur-face hardness and not on the substrate hardness

Fig. 6. Brinell hardness data plotted against the total cluster

energy E divided by sputtering yield Y.

Fig. 4. Dependence of crater diameter for Ar3000 impact on a

Au(111) surface on acceleration voltage. The power law d�V1=3a

is obeyed.

Fig. 5. Proposed plot of BHN vs. crater depth for various

cluster beam energies between 20 and 150 keV. The circle is the

MD result for 10 keV Ar236 cluster impact, on a Cu(100) sur-

face. Lines are drown according to the correlation d�(E/B)1=3,

where E is the cluster energy.

Z. Insepov et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 47±52 51

which is signi®cantly di�erent than the bulk, suchas in thin ®lms or alloyed surfaces. In addition,with proper calibration this method also be suitablefor hard coatings deposited on softer substrates.

Acknowledgements

The authors wish to thank T. Seki, T. Aoki, N.Toyoda, and A.J. Perry for their help in thepreparation of this paper.

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