250 Nuclear Instruments and Methods in Physics Research B15 (1986) 250-253

North-Holland. Amsterdam

ARTEFACTS IN RBS ANALYSIS OF LASER TREATED SURFACES

Laser treatment of deposited films often result in globules on the surface which may influence the solute depth profiles, as

measured by Rutherford backscattering spectrometry (RBS). We show that the globules generally have poor adhesion to the substrate and can be easily removed using techniques commonly employed for testing adhesion of thin films, such as a scotch tape test, or mild

abrasion test using a cotton bud or tip of a soft wooden piece. The effectiveness of this approach is demonstrated for Zn films on Al

(treated with a 12 ns fwhm Nd: glass laser pulse). and Sb films on Al (treated with a 100 ns fwhm CO, laser pulse). The solute depth

profiles, both before and after removal of globules from the laser treated surfaces, have been measured by employing RBS of He+

ions and dramatic differences between the two cases have been observed. Laser treated surfaces are also characterized by optical

microscopy and the topography is correlated with the RBS depth profiles, Results are presented for laser heating and liquid phase

diffusion analysis of the solute depth profiles obtained after scotch tape and abrasion tests.

1. Introduction

Laser treatment of deposited films is by now a well established technique to produce metastable surface al- loys [l]. The laser treated region often has a nonuniform surface topography. In particular, globules of the film material on the surface have been seen after laser treat- ment [2]. The presence of such globules or similar surface structures may affect the solute depth profiles as measured by Rutherford backscattering spectrometry (RBS). Such an effect has indeed been demonstrated, for example, in the case of annealing studies of lead films on silicon [3]. Some attempts have been made in the past to remove these globules by dissolving any unreacted material on the surface of the sample in a suitable solvent. This method is not foolproof as the underlying alloyed region may also be attacked by the solvent. We have noticed that these globules generally have poor adhesion to the substrate, and can therefore be conveniently removed by techniques commonly em- ployed for testing adhesion of thin films, such as a scotch tape test, or mild abrasion test using a cotton bud or tip of a soft wooden piece. It is expected that the part of film material which has actually diffused in to the matrix becomes an integral part of the substrate without a sharp interface, and thus cannot be easily removed during such an adhesion test. In this paper we demonstrate the effectiveness of this approach for two different cases: a) zinc films on aluminium substrates treated with 12 ns fwhm Nd: glass laser pulses, and b) antimony films on aluminium substrates treated with 100 ns fwhm CO, laser pulses. Removal of the globules is confirmed by optical microscopy. Dramatic dif- ferences are seen between RBS depth profiles obtained

0168-583X/86/$03.50 %” Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

before and after removal of the globules. The solute depth profiles obtained after removal of the globules are analysed using laser heating and liquid phase diffusion calculations. Results of such analysis are also presented for both Zn and Sb films.

2. Experimental

Well annealed, high purity (99.999%) polycrystalline aluminium substrates (- 0.5 mm thick) were electro- polished in an electrolyte containing 90% methanol and 10% perchloric acid to get a mirror finish. Zinc or antimony films of thickness - 9000 A and 2000 A, respectively, were vacuum deposited onto these polished aluminium substrates. The Zn films were irradiated in air using a Nd : glass laser (A = 1.06 pm) with single

pulses (12 ns fwhm) of peak energy densities from 2.0 to 6.5 J/cm’. The laser treatment of Sb films was carried out using single pulses (100 ns fwhmf from a CO, laser (A = 10.6 pm). The energy density on the target was approximately 10 J/cm’ in this case. These laser treat- ments were carried out at the Laser Division, BARC.

The RBS measurements were carried out using He+ ions of 2.0 MeV and 3.4 MeV to obtain the solute depth profiles. A well collimated beam of 0.5 mm diameter was incident normal to the sample. The backscattered ions were detected at 165’ using a surface barrier detec- tor and the usual electronics. Several RBS measure- ments were carried out across a diameter of the laser treated region on all the specimens. Almost all the RBS spectra showed a sharp surface peak, followed by a deep tail. Such profiles could arise due to the presence of globules on the surface of the samples. The samples

Kuldeep. A.K. Jam / ArreJuc~rs ,,I RBS unulvvu 251

Here therefore examined under an optical microscope, \5 hich in fact revealed the presence of surface artefacts. A scotch tape test was first attempted to remove these artefacts. The samples were re-examined under micro- scope to confirm removal of these artefacts. In the case 01‘ Zn films, the scotch tape test was found adequate. However, for the case of Sb, the scotch tape test was

found ineffective and an abrasion test using either a c,)tton bud, or the tip of a soft wooden piece was n.:cessary. The RBS depth profiles were obtained again a’ter these tests and compared with the depth profiles o I the as-laser-treated surfaces. The results are pre- srnted in the next section.

3. Results and discussion

A typical depth profile of Zn. as measured on an a>-laser-treated sample in the centre of the laser spot h.Lving energy density of 6.5 J/cm* is shown in fig. 1 by onen circles. A sharp surface peak followed by a flat profile extending to depths >- 2 pm is seen. Such a profile however may not represent the true depth profile alId might result from presence of globules, or ripples on the surface of the sample. The observation of laser treated region under optical microscope reveals a com- plex pattern of interconnected globules or islands on the surface of the sample (see fig. 2a). These globules were removed from the surface by using a scotch tape (see

12 ns Nd :Glass 6.5 J/cm2

Laser

CLL 0 o as Laser treated 1 . after scotch Tape

Test

Fig. 1. RBS depth profiles of Zn measured on a laser treated

sample. before (0) and after (0) scotch tape test. The continu-

OUI curve is the Zn profile calculated on the basis of liquid

phase diffusion. For the case of as-laser-treated surface, the

experimental points represent an “apparent depth profile” due

to presence of globules.

fig. 2b). The RBS depth profile after this removal is

shown by filled circles in fig. 1. A dramatic change in the shape of backscattering spectrum after scotch tape test (see corresponding depth profile in fig. 1. filled circles) confirms that the deep tail in the earlier RBS depth profile was indeed due to presence of globules. This is further supported by the optical micrograph taken after this test (see fig. 2b) which shows a relatively

smooth surface devoid of the globule-like structure. This new profile (filled circles) therefore represents the actual intermixing of Zn and Al. The total amount of Zn remaining after scotch tape test is much smaller than the original film thickness, and is also small compared to the detector resolution. The profile however has a width much larger than the resolution. Thus the ob- served profile after scotch tape test does correspond to a diffused layer. This depth profile has been analysed on the basis of laser heating and liquid phase diffusion calculations with a moving melt front. Laser heating calculations were performed to obtain the melt kinetics.

Fig. 2. Optical micrographs taken near centre of a laser spot

(6.5 J/cm’) on Zn deposited Al sample (a) before. and (b) after

scotch tape test.

VI. RBS AND RESONANT SCATTERING

252 Kuldeep, A. K. Jain / Artefwts in R BS analysis

Reflection coefficients of 0.69 and 0.94 [4,5] were used for Zn and Al, respectively, for the Nd: glass laser wavelength. Our calculations yield a melt duration of 30 ns and a maximum melt depth of 0.35 pm (in the substrate) for 6.5 J/cm2 incident energy density. For the purpose of liquid phase diffusion calculations, the initial film thickness was assumed to be equal to the Zn amount retained after scotch tape test. This is because it is only this amount which was actually available for diffusion, while the rest amount was used in forming globules. The calculated profile was folded with a Gaussian detector resolution function (fwhm - 550 A) and then compared with the experimental data. A liquid phase diffusivity, D, of Zn in Al has been estimated to

be - 1 x lo-’ cm2 sP1 from such a fit to the experi- mental data (filled circles in fig. 1). The calculated profile is also shown in fig. 1 by a solid curve. The quality of this fit is quite satisfactory and the depth profile can be considered consistent with diffusion in liquid phase.

The Sb depth profile of as-laser-treated sample at energy density of - 10 J/cm2 is shown in fig. 3 by open circles. Optical microscopy on this sample also shows presence of globules (see fig. 4a). These globules were found to have much better adhesion to the substrate than in the case of Zn films, and could not be removed using a scotch tape. The surface after a scotch-tape test remained practically the same (although the untreated film was almost completely removed) and the RBS depth profile also showed only a marginal deviation (see triangles in fig. 3) from the as-laser-treated profile. It was therefore necessary to carry out a more stringent

DEPTH (U m 1

Fig. 3. RBS depth profiles of Sb measured on i) as-laser-treated

sample (0), ii) after scotch tape test (A), and iii) after mild abrasion test (0). The solid curve is the Sb profile calculated on

the basis of liquid phase diffusion. For the case of as-laser-

treated surface. the experimental points represent an “apparent

depth profile” due to presence of globules.

adhesion test than this. Use of hard abrasive tools was ruled out due to the extremely soft nature of the sub-

strates used, and also danger of damage to the underly- ing alloyed region. Mild abrasion tests were therefore performed on some samples using a cotton bud, and on some samples using the tip of a soft wooden piece. Both of these were found to be equally effective. The surfaces on all the samples were found to be thoroughly cleaned up after abrasion test and looked almost like a bare aluminium substrate. On microscopic examination, the surface showed very fine scratches (see fig. 4b) which have resulted mainly due to the very soft nature of the well annealed aluminium substrate. RBS analysis on such a surface however revealed that all the solute has not been removed. The Sb depth profile after an abra- sion test is shown by filled circles in fig. 3. It is seen that

this profile is much narrower, and is devoid of any deep tails.

Fig. 4. Optical micrographs showing (a) as-laser-treated surface in the case of Sb films, and (b) after abrasion test.

Kuldeep. A. K. Join / Arte/um rn RBS uno!ws 253

Another interesting effect seen in the case of Sb films

was enhancement of adhesion of globules to the sub- strates by the analysing He+ beam used for RBS. Thus on the regions where RBS was done on as-laser-treated surface. the globules were not removed even after an abrasion test. This effect is similar to the ion beam induced enhancement in thin film adhesion observed in recent years [6-E]. It should be noted that this effect I gecessitated RBS depth profiling (after abrasion test) to lie carried out at regions slightly away from the original RBS spots to avoid the complications due to globules.

There is a possibility that the observed profile after i brasion test (filled circles in fig. 3) may be due to removal of the surface as a whole and may represent only the tail of the initial profile (triangles). However, I he peak concentration of Sb profile (filled circles) is ifround 25 X 10”’ at./cm3 which corresponds to the

concentration at - 2200 k in the profile (triangles)

csbtaind after scotch tape test. Thus. the maximum surface removal could only be by - 2200 A. In such a case one should still see a deep tail in the profile after abrasion test, if the original profile is genuine. Since no such tail is seen in this profile. it is clear that the deep tail in the as-laser-treated profile was caused by glob-

ules only.

The profile after the abrasion test (filled circles in fig. 3) has also been analyzed on the basis of liquid phase diffusion calculations similar to the case of Zn films described earlier. We have used reflection coeffi- c,ents of 0.72 and 0.98 [4] for Sb and Al. respectively, ft,r the CO, laser wavelength in our heating calcula- tons, which yield a melt duration of 200 ns and a

maximum melt depth of 1.2 pm for 10 J/cm* energy density. The full curve in fig. 3 is the least squares fit to the data (filled circles), giving an effective diffusion coefficient of - 8.5 X 10m5 cm2 s-‘. Unlike in the case of Zn films, the fit in the case of Sb films is not very satisfactory. One plausible reason for this failure could be the oversimplification of the actual physical diffusion srtuation in our model. For example. the globules may

act as localized thick diffusion sources for liquid phase diffusion, whereas we have assumed a laterally homoge- ncous (and much thinner) diffusion source in our calcu- lations,

4. Conclusions

We have shown that globules on the surface of laser treated films give rise to spurious deep tails in the solute depth profiles as measured by RBS. Microscopic ex- amination of the surface is essential to establish reliabil- ity of these depth profiles. We have also shown that the globules can be removed by either a scotch tape or a mild abrasion test, facilitating reliable depth profiling on laser treated surfaces. It may be added here that in addition to causing problems in depth profiling by RBS. these globules may also prove a nuisance in other char- acterization techniques as well, for example in sample thinning for TEM investigations. The method proposed here for removal of the globules should be useful even for such studies.

We thank T.P.S. Nathan, K. Rustagi, P.K. Gupta and Sucharita Dutta of the Laser Division, BARC for providing laser treatment facilities. We also thank P.K. Bhattacharya, M.J. Kansara, V.P. Salvi and S.K. Sharma for assistance during RBS work and the Van de Graaff operation staff for skillful operation of the accelerator.

References

PI PI

[31

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WI

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VI. RBS AND RESONANT SCATTERING