Surface analytical applications of ion-induced secondary emission target currents

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Surface Science 180 (1987) L113-L118 North-Holland, Amsterdam

SURFACE

SCIENCE

LETTERS

SURFACE ANALYTICAL SECONDARY EMISSION I. Oxidation of In and Cu M.A. KAROLEWSKI Department Received

APPLICATIONS OF ION-INDUCED TARGET CURRENTS

and R.G. CAVELL

of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 11 August

1986; accepted

for publication

23 October

1986

Measurements of ion-induced secondary emission target currents afford a sensitive and rapid means of following the adsorption process. The method has been applied in this instance to monitor 0, uptake by polycrystalline In and Cu substrates.

In a series of papers [l-5] Rhead and co-workers have described a new method of surface characterization which is based upon the measurement of electron beam induced secondary electron crystal currents. Chadwick et al. [6,7] subsequently demonstrated that the same measurement was applicable to crystal currents generated by X-ray irradiation. In this letter we propose and evaluate a third crystal current method based on the phenomenon of ion-induced secondary electron emission which is suitable for vacuum systems equipped with facilities for ion bombardment. In conformity with previous usage we shall refer to this technique as measurement of the ion-induced crystal current, or ICC [l-7]. In the first instance we consider the application of the ICC method to oxidation studies of metals. Polycrystalline (polyx) In and Cu substrates were chosen as these are readily cleaned and display extremes of oxidation behavior. In addition both systems, O/In and O,/Cu, have been previously characterized by SIMS and other surface techniques [8-121. These experiments were performed on a VG SIMS spectrometer (base pressure 10 - lo mbar) equipped with an (AG61) Arf ion source which could be rastered across the specimen surface. Polyx In and Cu substrates (5N purity) were mechanically and chemically polished, then cleaned in UHV by numerous cycles of ion bombardment and heating. However, chemisorption experiments were performed at room temperature on unannealed surfaces to avoid recontamination while the specimen cooled. The cleanliness of the In and Cu surfaces was assessed by SIMS. With the target bias adjusted for optimum sensitivity the following were the principal ionic species observed for 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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M.A. Karolewski,

R.G. CurdI / Measurement

of ICC. I

AICC PA

1

1

0

,

100

I

I

200

300

I

400 02

exposure

I 500 L

Fig. 1. ICC and SIMS Om intensity variations with O2 exposure on polyx In surface. Ar’. I, < 3 nA cm *. ti(incident) = 45”. Target bias zero.

E, = 2 keV

clean specimens (expressed as counts per second per nA of incident Art at 2 keV). Polyx In: In+ < 40; F- < 13; O-, C;, Cl-< 4. Polyx Cu: Cu+ < 3; were made in each case C, < 30; 0-, F-, S, Cl- < 3. ICC measurements a digital picoammeter using primary Ar + currents of ,< 1 nA by connecting ( + 0.5 PA) between the specimen and ground. The samples were exposed to 0, via a nozzle at room temperature and 0, partial pressures ranging from lo-’ to 1O-5 mbar. Relative 0, exposures are expressed as langmuirs (1 L = lop6 mbar s). The Ar partial pressure during analysis was (5-10) X lo-” mbar. The ICC variations recorded when clean polyx In was exposed to 0, are displayed in fig. 1. Also shown is the SIMS O- yield measured in the same experiment. The ICC showed a net decrease after 0, exposure which corresponds to a reduction in the secondary electron yield for oxidized surface. At low oxygen exposures (< 200 L) the ICC was found to vary in a non-linear way with 0, exposure, thereby lending a markedly sigmoid appearance to the uptake isotherm. This behaviour (which was quantitatively reproducible) we ascribe to the preliminary stage of the In oxidation: namely, the nucleation of oxide islands upon the metal surface [ll]. The shape of the isotherm suggests that

M.A. Karolewski,

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of ICC. I

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the presence of these nuclei serves to facilitate subsequent oxidation, presumably by weakening the local intermetallic bonding in the vicinity of the oxide-metal selvedge [13,14]. The product of the In oxidation has been no precursor chemisorbed oxygen species is characterized by XPS as In,O,: observed [ll]. The linear development of the ICC curve between 200-1000 L coincides with the lateral expansion of In,O, nuclei to overlap the entire metal substrate. With further 0, exposure the ICC continues to fall, albeit less rapidly, due to penetration of oxygen into the bulk metal [ll]. The ICC shift showed no sign of abating at 20000 L exposure. The SIMS O- intensity (fig. 1) did not mirror the ICC shift precisely but qualitative similarities are evident. Comparison with the XPS data of Hewitt and Winograd [ll] shows that in this particular system the ICC shift reflects quite faithfully the surface oxygen uptake. This similarity should not be presumed on for other systems, however. The functional dependence of the SIMS O- yield recorded in fig. 1 differs from that reported by Hewitt and Winograd [ll], probably as a result of the zero target bias used in this work. The reproducibility of the ICC shift for O/In (and also O,/Cu) between different experimental runs was found to be around +3% of the shift, which may be attributed either to the limitations of the picoammeter or small variations in surface cleanliness. The oxidation of clean and contaminated polyx Cu surfaces was examined by monitoring ICC’s in a similar way as for In. The adsorption of residual gases from our UHV chamber prior to oxidation led to a continuous fall in the ICC with time..Subsequent exposure to 0, then resulted in a further fall in ICC but of reduced magnitude compared to clean Cu. These effects are illustrated in fig. 2. The AICC curves are readily interpreted as being the result of primary shifts induced by adsorption of carbonaceous contaminants (confirmed by SIMS), with further O,-induced shifts superposed. There exists an inverse (though non-linear) relationship between the contamination-induced AICC and the subsequent O,-induced AICC. In contrast to In, bulk oxidation of Cu does not take place in the pressure regime under consideration here. Indeed, the saturation coverage of chemisorbed oxygen on clean polyx Cu is probably no more than 0.5-0.75 ML [9]. Consequently 0, exposures above lo3 L produce no measurable variation in the ICC. Clean Cu is therefore an ideal substrate for probing the influence of primary ion energy and angle of incidence on the observed saturation AICC. The O,-induced saturation AICC on polyx Cu showed a linear increase with energy. There was, however, a contribution to the shift independent of ion velocity which at E,, = 2 keV provided about 50% of the observed AICC (this contribution was estimated by extrapolation of the AICC versus E, plot to zero energy). This result is consistent with the existence of the two distinct mechanisms for ion-induced secondary electron emission, viz. potential ejection (via primary ion neutralization) and kinetic ejection (via ion-lattice or

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M.A. Karolewskr,

O*ipolyx

R.G. Cauell / Measurement

of ICC. I

o 0 ICC

In

AICC vs SIMS O-

0 0

_o<-o-o

0”

0

0 o” o

.a0

00

12pA

o” c,

SIMS o-

I

2ocps

/

Fig. 2. Influence of carbonaceous contamination on O,-induced ICC shifts for polyx Cu surfaces. Clean Cu surfaces were exposed to the residual vacuum for varying times prior to oxidation. E,=5keVAr’. 1,<15nAcm-~‘, 0(incident) = normal. Target bias zero.

ion-electron collisions) [15-171. The former mechanism is largely independent of ion velocity and therefore predominates at low energies. Oechsner [18] has derived an empirical relationship describing the variation of potential electron yields (y) in terms of the Fermi energy (b,), primary beam ionization potential ( Ei) and the surface work function (+) for 1.05 keV primary ions incident on metal surfaces. If the weakly chemisorbed oxygen overlayer is considered to alter the surface work function, whilst leaving the Cu Fermi energy essentially unperturbed [lo], then it is possible to predict the ICC shift for 1.05 keV primary ions on O,/Cu using Oechsner’s relation [18]: y = (0.2/&,)(0.8Ei - 2~). The ICC is taken to be a measure of the sum of the primary beam current plus y. Therefore, AICC = -0.2(24$)/&r. An appropriate oxygen-induced work function shift (A4) for polycrystalline Cu is 0.40 f0.27 eV, which is the average for low-index faces of Cu [19]. With these assumptions we obtain a predicted ICC reduction of 1%3% for this system under Ar+ bombardment which essentially agrees with our experimental value at the same energy of 2.0%. It seems, therefore, that work function variations alone might account for both the direction and magnitude of the shift at low energies for O,/Cu. The true situation is probably more complex, however, since the above calculation does not take into account any contributions to y by potential emission arising from adsorbate electron states. The corresponding calculation cannot be made for O/In because the relevant work function

M.A. Karolewski,

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data is unavailable. Moreover, the loss of metallic character on oxidation of In would render the assumption of invariant 8, unreasonable for that system. The kinetic mechanism of electron ejection becomes increasingly important at primary ion velocities above 10’ cm s-i [16] (or about 2 keV for Ar+). This mechanism of ejection might influence the measured AICC through such energy dependent effects as subsurface ion penetration, or induced emission from sputtered or adsorbed species. Such phenomena are second-order effects insofar as they arise as a result of ion-surface collisions, in contrast to potential ejection. They are therefore accompanied by some degree of damage at the bombarded site and do not represent emission from the initial surface structure. This remark applies even under static SIMS conditions [20]. The AICC dependence on primary ion angle of incidence (for 5 keV Ar’) was also investigated. As shallow angles of incidence were approached ( < 30”) a reduction in the AICC was observed. Between 35 o and loo a factor of 6 reduction in AICC was measured (compared to a reduction of - 10% for the total ICC of clean Cu over the same angular range). This result indicates that shallow angles of incidence suppress the secondary yield from the clean Cu surface more readily than from oxidized surface, but the reason for this is not known. We observed that heavily sputtered Cu surfaces (100 PA cm-’ min at 5 keV Art) examined at normal ion incidence showed the opposite effect, i.e., a marked increase in both the clean Cu ICC current and the O,-induced ICC shift. It would seem, therefore, that the AICC behavior at shallow angles of incidence is not simply a topographic effect due to surface roughness, but involves some further undetermined feature of the ion-surface interaction. The AICC for large angles of incidence (35”-90°) was essentially invariant with only a weak angular dependence. The results presented here indicate that ICC measurements may be employed as a useful adjunct to conventional SIMS experiments, particularly in studies of clean metal surfaces whose secondary ion yields are low. In the above discussion we have tacitly ignored the effect of ion emission current on ICC because, in general, absolute yields for secondary ions are much lower than for electrons [15]. Oxidized surfaces of electropositive metals can, however, show secondary ion yields approaching unity [20], and this ionic contribution would have to be taken into account in any quantitative analysis of ICC shifts. The sensitivity of ICC measurements to adsorbed gases is high (< lop2 ML) as is the speed of measurement (<< 1 s). Because of the elimination of quadrupole parameters (including emission angle) ICC shifts may prove to be more reproducible between laboratories than secondary ion intensities. We anticipate that, as with other crystal current techniques, ICC measurements will be sensible to the phases of overlayer development on single crystal surfaces [l-7]. The distinction between the potential and kinetic modes of emission may in future become important in relation to surface structural

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of ICC. I

properties. The use of He+ ions may be preferred in some applications as favouring a high potential yield in combination with low sputtering rates. We shall report investigations in these areas in due course. The subject of surface imaging by means of ICC detection will be dealt with in part II [21]. We thank the Natural Sciences Research Council of Canada, the University of Alberta and The Alberta Oil Sands Technology and Research Authority (AOSTRA) for capital funds for the establishment of the instrumental facility and for operating support.

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

M.G. Barthes-Labrousse and G.E. Rhead, Surface Sci. 116 (1982) 217. C. Argile and G.E. Rhead, J. Phys. C30 (1982) L193. C. Argile and G.E. Rhead, Surface Sci. 135 (1983) 18. SD. Parker and G.E. Rhead, Surface Sci. 167 (1986) 271. C. Argile, M.G. Barthes-Labrousse and G.E. Rhead, Surface Sci. 138 (1984) 181. D. Chadwick, M.A. Karolewski and K. Senkiw, Surface Sci. 175 (1986) L801. M.A. Karolewski and D. Chadwick, Surface Sci. 175 (1986) L806. A. Mueller and A. Benninghoven, Surface Sci. 41 (1974) 493. S. Evans, E.L. Evans, D.E. Parry, M.J. Tricker, M.J. Walters and J.M. Thomas, Faraday Disc. Chem. Sot. 58 (1975) 97. S. Evans, D.E. Parry and J.M. Thomas, Faraday Disc, Chem. Sot. 60 (1975) 102. R.W. Hewitt and N. Winograd, J. Appl. Phys. 51 (1980) 2620. W.F. Tseng, J.W. Mayer, S.U. Campisano, G. Foti and E. Rimini, Appl. Phys. Letters 32 (1978) 824. F.P. Fehlner and N.F. Mott, Oxidation Metals 2 (1970) 59. NJ. Chou, J.M. Eldridge, R. Hammer and D.W. Dong, J. Electron Mater. 2 (1973) 115. G. Carter and J.S. Colligon, Ion Bombardment of Solids (Heinemann, London, 1968). R.A. Baragiola, E.V. Alonso, J. Ferron and A. Oliva-Florio, Surface Sci. 90 (1979) 240. H.D. Hagstrum, Phys. Rev. 96 (1954) 336. H. Oechsner, Phys. Rev. B17 (1978) 1052. T.A. Delchar, Surface Sci. 27 (1971) 11. A. Benninghoven, Surface Sci. 53 (1975) 596. M.A. Karolewski and R.G. Cavell, Surface Sci. 180 (1987) L119.