Ion-induced Auger emission and sputtering processes in solids

Nuclear Instruments and Methods in Physics Research B67 (1992) 610-615 North-Holland

Nuclear Instruments & Methods in Physics Research Section B

Ion-induced Auger emission and sputtering processes in solids E.A. Maydell Department of Metallurgy and Engineering Materials, University of Strathclyde, Glasgow G I IXN, Scotland, UK D.J. Fabian Department of Physics and Applied Physics, Unirersity of Strathclyde, Glasgow GI IXN, Scotland, UK

Extensive measurements have been made of the electron emission flom aluminium induced by bombardment with Cs + ions. The equipment used is a quadrupole-based SIMS instrument, especially fitted with an electron-energy analyser for observing electron emission from the point of ion-beam incidence. A strong variation of ion-induced Auger spectra with angle of ion-beam incidence is observed and measurements of Auger emission from sputtered aluminium and caesium-aluminium species are reported in detail. Correlation of depth profiles for an aluminium-on-silicon structure, measured by SIMS and by "dynamic" ion-induced Auger emission, provide evidence for a proposed contribution to the atomiclike peaks of the Auger spectra from sputtered caesium-aluminium molecular species.

1. Introduction Electron emission produced by irradiation of solids with heavy ions of energies ranging from ~ 10 eV to some hundreds of keV, has been studied for more than two decades. Most investigators however have used noble gas ions; for example, even a recent comprehensive review by Hofer [1], covering ion-induced kinetic emission of electrons from solids (in many ways similar to secondary electron emission - except for the excitation step) is limited to noble gas ions. It has been widely observed [2-5] that for light metals (Be, Mg, AI and Si) the ion-induced Auger spectra can be attributed to a combination of a broad energy band corresponding to electronic emission arising from the bulk material or sample surface, similar to that caused by electron excitation, plus a series of sharp atomiclike peaks arising from Auger deeay of free atoms outside the solid. A strong dependence of ion-induced Auger emission on angle of beam incidence has been observed, and has been interpreted in terms of both a litetime effect of the excited states [6] and the angular dependence of sputtering rate [7]. Apart from inner-shell ionization and Auger decay processes studied using high-energy heavy ions, mostly bombarding free atoms [8-10], virtually no reports exist - until recent studies in our laboratories [11,12] - of studies of Auger emissign from solids using incident ions othelr than those of noble gases. We have made detailed investigations of the Auger emission from aluminium and magnesium induced by Cs + ions and have shown that correlations of Auger yields with

secondary-ion yields can be valuable in the interpretation of depth profiles observed in dynamic SIMS measurements. In this paper we present results for the Cs +-induced Auger emission from aluminium and report a remarkably strong dependence of the spectra on angle of incidence of the ion beam. We also report a strong correlation of the observed secondary-ion depth profiles with those obtained from measurements of ion-induced Auger spectra monitored "dynamically" at increasing depth for an aluminium-on-silicon structure, and we find evidence to postulate an additional contribution to the Auger emission from excited aluminium atoms formed on dissociation of transient CsAI + molecular-ion species.

2. I n s t r u m e n t a t i o n

Measurements were made using a specially designed V G SIMS-Auger instrument described elsewhere [13], equipped with an electron energy analyzer that can monitor electrons emitted at the same point on ,,he sample surface from which secondary ions are detected. The experimental arrangement is shown schematically in fig. 1. The sample is mounted on a precision manipulator which permits it to be rotated about an axis normal to the incident Cs + ion beam. The electron spectrometer, a VG CLAM-100, is mounted at an angle of 36 ° vertically below the ion beam. Secondary ions are extracted in a direction that makes an angle of 45 ° with the incident ion beam, in

0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

E.A. Maydell, D.J. Fabian /lon-inducedAES in solids

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4. R e s u l t s a n d d i s c u s s i o n

..... x x iL e-

45* / . y - ~ , ~ ,

In fig. 2 we show AI LMM ion-induced Auger N ( E ) spectra measured for pure aluminium with the Cs + beam incident at angles to the surface normal of 0° (i.e. sample facing the ion beam), 45 ° (sample facing the secondary-ion extraction column) and - 4 5 ° (sample facing away from the secondary-ion extraction electrode; i.e. 9° beyond the position in which it faces the

30

(a)"

I

k 25

2o

l/

Cs*/*z'5°

A

CS+/O°

Fig. 1. Geometrical arrangement for ion-induced Auger electron emission and SIMS measurements, i - ion beam, a CLAM electron-energy analyser, e - - electron beam, b secondary ion extraction column, ~ - angle of ion-beam incidence to sample surface normal ~.

Ib)

tB L

the vertical plane defined by the latter and the electron acceptance lens. In SIMS mode the instrument normally operates with the primary ion beam impinging the sample at 45 ° to surface normal. A 10 keV electron gun, providing a submicrometer beam spot, is mounted in the same horizontal plane as the incident ion column and axis of rotation of the sample, making an angle of 45 ° to the ion beam in a horizontal plane.

k

l

i

I

!

|

!

k so

I

Cs+/1+50

'°I

3. M e a s u r e m e n t s

The ion-induced Auger spectra were measured using a 10 keV, 15 hA, Cs ÷ beam. The ion beam irradiates a sample area which is square ( ~ 350 I~m X 350 v,m) for incidence angles of +45 ° and -45°; for other angles of incidence the area becomes rectangular. The collection of electrons from the emitting area was found to be 100%. Electron-induced "reference" Auger spectra were measured using a 3 keV, 50 nA electron beam impinging the sample at 45 °. Dynamic ion-induced Auger depth profiling of an aluminium-on-silicon structure (1000 A aluminium evaporated onto a single-crystal silicon substrate) was performed by etching the sample with the Cs + beam and measuring sequentially the ion-induced Auger spectra until the desired depth was reached. Dynamic SIMS depth profiles of the same structure were measured using a 10 keV, 50 nA Cs + beam raster-scanned over a sample surface area of ~ 440 I~m × 440 p,m. The secondary ions were detected normal to the sample surface (with the Cs + beam incident at 45* to the surface) using zero ion-extraction field.

k

:'ot

)/

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/ (d) -

~40 k

'°'

120

C o iO0 u n 80

t

s 60

20

30

40

50

60

70

80

90

i00

Kinetic Energy / eV Fig. 2. Auger electron spectra measured for pure aluminium. (a)-(c) Cs +-induced, with ion-beam incidence angles of 45°, 0° and -45 ° (to the sample normal), (d) electron-induced (3 keV, 50 nA electron beam) at 45° angle of incidence. IX. SECONDARY EMISSION

E.A. Maydell, DJ. Fabian / Ion-induced AES in solids

612

Table 1 Electronic transition assignments and energies (theoretical and measured) for principal peaks observed in Cs+-induced (present work) and Ar÷-induced (recent work [14] and Whaley and Thomas [3]) Auger spectra for aluminium Feature

AI 0 AI 1 AI II Al lll Al IV

Transition

Energy [eV]

2p53s23p z ~ 2p63s2(IS) 2p53s23p 2 --* 2p~'3s3p(3P) 2p53sZ3pz --* 2p"3pZ(3P) 2pS3s23p --* 2p63s(2S) 2pS3sZ3p ~ 2p63p(2p) 3p43s23p 2 -~ 2pS3s3p(3P)

Theor. [3]

Exp. ~ (Cs +, - 45°)

Exp. [14]" (Ar +, - 45°)

Exp. [3] b (Ar +, - 60°)

66.37 61.74 54.71 55.7 49.0 75.3

65.53 60.1 53.9 46.3 74.5 71.6

66.48 63.0 56.0 49.1 74.8 72.5

67.8 62.2 56.2 c 55.7 e 49.0 76.0

a Energies corrected for an electron spectrometer 5 eV offset. b Angle ( - 60P) derived from experimental details of ref. [3]. c Whaley and Thomas [3] were unable to resolve these as separate peaks, but they appear to give two values of energy.

electron acceptance lens). For comparison, the electron-induced LVV Auger N(E) spectrum measured for a l u m i n i u m , with t h e electron b e a m incident at 45 °, is also s h o w a (fig. 2d). T h e s p e c t r u m m e a s u r e d with t h e Cs + b e a m at - 4 5 ° (fig. 2¢) is typical o f ion-induced A u g e r electron spectra. It ca)nsists o f a broad b a c k g r o u n d , corresponding to t h e electron-induced L V V a l u m i n i u m A u g e r electron emission (fig. 2d) from t h e solid, plus s h a r p atomiclike A u g e r peaks. T h e s e p e a k s are attributed to A u g e r transitions b e t w e e n known energy levels in free

a l u m i n i u m a t o m s or ions outside t h e solid; their electronic interpretation is s u m m a r i s e d in table 1. W e note that t h e energy s p e c t r u m in t h e region of feature AI 0 shows m o r e t h a n o n e peak for ion b e a m incidence angles o f 0 ° a n d - 4 5 °. W e observe that the ion-induced A u g e r s p e e t r u m m e a s u r e d with t h e Cs ÷ b e a m incident at + 4 5 °, for which the detection angle b e c o m e s 81 ° to t h e surface normal, h a s - as expected - a greatly r e d u c e d backg r o u n d (the intensity of A u g e r emission detected from t h e bulk material b e c o m i n g relatively m u c h less at t h e

Detection angle of Auger electrons -40 !

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60 i

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80 i

I

i

Auger electron yield

-80

-60

-40

-20

0

20

40

60

(])

Angle of ion beam incidence Fig. 3. Angullar dependence of principal peak intensities (measured as differentiated peak-to-peak heights x linewidth, normalized to ! at +45 ° incidence) in Cs+-induced Auger spectrum of aluminium.

l:..A. Maydell, D.J. Fabian / Ion-induced AES in solids oblique angle due to increased escape length needed); while the atomiclike peaks are unaffected by the detection geometry, supporting the interpretation that these arise from sputtered species - from which the Auger emission is isotropic. The energies measured for individual peaks are shifted by ~ 1-2 cV compared to the same peaks measured with Ar+-ion excitation, Maydell [14]. The peaks AI 0, and AI I - I V correlate with those observed, and the assignments given, by Whaley and Thomas [3] for Ar+-induced spectra from aluminium. The feature Al IV, although observed clearly in our work [14] using Ar + bombardment, is, however, weak in Cs+-induced spectra from aluminium. This peak is not further considered in the present paper. The angle of incidence of the Cs + beam to the sample normal was varied in our measurements from + 60 ° to - 85 °. At the same time the angle of detection of electrons (fixed, geometrically, at 36° to the ion beam) varies from 95 ° to - 5 0 °. The angular dependence of the individual Auger peaks - "quantified" as peak-to-peak height in the differentiated ( d N ( E ) / d E ) spectra x linewidth and normalized to 1 at beam incidence +45 ° - is shown in fig. 3. The experimental results for all atomiclike features, for the two angles of incidence 0 ° and + 45 °, are summarised in table 2. The intensity ratio of the Ai I peak measured at an angle of i o n - b e a m incidence of 0 ° to that measured at 45 ° is 1/4.3, and the intensity ratio for the Al II peak measured at these two angles is 1/4.8; while the ratios of intensities for respectively the AI 0 peak and the Al III peak, measured at these angles, are 1/1.4 and 1/2.5. It should be noted that the ratio of sputtering coefficients for aluminium bombarded by Cs + ions at these two angles is only ~ 1/1.4 [16]; i.e. the variation of the sputtering rate alone cannot account for the observed variation of the three Ai I, Al II and Al III peaks with angle of incidence. However (see table 2), the variation of intensity of the AI 0 peak with angle of ion-beam incidence does appear to correspond to the expected variation of sputter rate. The attenuation of ion-induced Auger signals (features AI I and Al II), for the Cs + beam incident at 45 °

613

and at 0 °, is much too large to be explained in terms of the dependence of the sputtering efficiency on angle of incidence at the sample surface. However, as noted, the feature AI 0 shows an attenuation typical of ion sputtering (i.e. 1 / c o s - i 45 ° = 1/ 1.41 ). The emission giving rise to peaks I and II requires of course, the sputtering of excited atoms or ions; the angular dependence of this particular sputtering may be quite different from the total sputter rate variation. Evidence for this being so can be found in the unexpectedly strong dependence of photon emission from sputtered excited atoms of light metals on angle of incidence [17]. The feature A~ III, attenuated by a factor of ~ 2.5, appears to behave as cos -2s qb, which was found to describe the angular dependence bf bulk Auger electronic emission in Ar+-induced total cross section measurements by Hou et al. [7]. Thus the A! I and Al II peaks must contain an additional contribution of excited neutral aluminium atoms and excited aluminima ions, respectively, formed from sources that dominate at oblique angles of ion-beam incidence. We propose that the explanation lies in the formation of the molecular-ion species CsAl +, which we find to be abundant in the secondary-ion spectrum generated by Cs÷-ion bombardment of the aluminium target. A typical Cs + dynamic SIMS depth profile measured for the aluminium-on-silicon structure is shown in fig. 4. All secondary-ion signals appear to reflect the analysed structure, except for the AICs + secondary ion. The intensity of this secondary ion shows a 2.5-fold increase at the A i / S i interface, followed by the expected decline in the silicon suhstrate. Enhancement of sputtered-ion signals at Si and other interfaces i~ a "matrix ionisation" effect, frequently observed in depth profiling of layered structures, while not always easily understood. In our work its correlation with a parallel enhancement in the Cs + ion-induced Auger atomiclike peaks is important in our conclusions regarding the origin of the latter peaks. Fig. 5 shows the depth profiles obtained by monitoring the various features of the Cs+-induced Auger spectra, measured by "dynamic" ion-induced AES

Table 2 Intensities of Cs+-induced aluminium LMM atomiclike Auger peaks expressed as dN(E)/d E peak-to-peak height x peak width. The feature AlIV, although included in table 1, was too weak to be quantified Angle cp 0° 45° Ratio(45°/0 °)

Intensity [cts eV s- ~] of feature AI 0

AII

AI ll

A111i

2366 3380

4432 19296

468 2270

740 1836

1.4

4.3

4.8

2.5

cos qb

cos-2,s

1.0 0.7171

1.0 0.4204

1.41

2.38

IX. SECONDARY EMISSION

E.A. Maydell, D.J. Fabian / Ion-induced AES in solids

614

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~ 1.6 .¢

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l-iching Time (see) Fig. 4. SIMS depth profiles measured for an aluminium-onsilicon structure (1000 ,g, evaporated AI on single-crystal Si substrate).

(Auger spectra measured sequentially while sputteretching with Cs ÷ ions) for the same aluminium-onsilicon structure. Atomiclike peaks in the Si LMM Auger spectra are identified in the same scheme as used for aluminium. All features in the Si Auger LMM depth profiles follow the silicon-related signals in the SIMS depth profiles. However, in the A! Auger LMM profiles, the A I I feature shows a sixfold increase in intensity at the AI/Si interface and the AI II feature a twofold in-

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Fig. 5. Dynamic ion-induced Auger depth profiles measured for the same aluminium-on-silicon structure as for SIMS measurements of fig. 4. Note appearance of Si LMM Auger peaks from substrate.

crease, over their values in the early stages of etching. A "normal" electron-induced AES depth profile, measured using Cs + ion beam etching while monitoring the electron-induced AI LMM and Si LMM Auger emission, shows the entirely expected uniform intensity for the AI and Si signals measured in the aluminium layer and the silicon substrate respectively - with expected sharp changes at the AI/Si interface. Thus we observe a remarkable correlation between the abundance of CsAI ÷ ions, sputtered from the material by Cs ÷ ion bombardment, and the intensity of the principal AI Auger atomiclike A I I and AI II peaks. The A! 0 peak, on the other hand, appears not to correlate as clearly with the CsAI + abundance; as noted, its variation with ion-beam angle of incidence also appears to indicate its origin to be directly sputtered ionised AI atoms. Dissociation of CsAI ÷ molecular ions, or dissociation and ionisation of CsAI neutral molecular species, can - we propose - provide a further source of the excited aluminium atoms and ions that give rise to the Auger electron emission peaks AI I and AI II. It is well recognised [15,3] that the formation of transient CsAI "collision molecules" is a prerequisite to promotion of electrons during the collision event by crossover transitions between the collision partners; the promoted electron remaining with the new partner when the projectile and target atoms separate. With Ar + and AI, or Cs + and AI, this promotion leaves a 2p core hole in the AI atom or AI ÷ ion, resulting in the observed Auger decay, peaks A I I and Ai II, following separation. Our observations, of the abundance of "long-lived" CsAI + species, and of the correspondence of peaks AI I and AI II in dynamic ion-induced depth profiles with the CsAI ÷ (SIMS depth profile) secondary-ion abundance, provide strong evidence not only for the validity of the ion-induced Auger emission mechanism but also for the molecular-ion species itself being a direct additional source of the excited atoms or ions needed to undergo the measured Auger decay. This contribution could explain the observed intensities of the Cs÷-in duced A I I and AI II peaks - which, at 45 ° incidence, show 60-70% increases over that expected from sputtering of neutral or ionised atomic species, The AI III peak appears to follow more the abundance of AI, ions (in particular AI~-) in the secondary-ion mass spectra; while the AI 0 peak can be correlated with sputtered AI atoms. Andreadis et al. [6] considered the effect of lifetime of a 2p excited state, and concluded that this would need to be 50 fs to explain the observed angular dependence of the Ar+-induced Auger emission for aluminium. The life of the 2p excited state in AI r, calculated by McGuire [15] is 134 fs. In our work a much stronger angular dependence of the Cs+-induced Auger emission is observed than for the Ar+-induced

E.A. Maydell, D.J. Fabian / Ion.induced AES in solids

Auger emission [14]; i.e. this would require an even shorter life for a 2p vacancy for the lifetime effect to provide an explanation. Shorter lifetimes would also result in broader atomiclike peaks in the Cs+-induced spectra, which is not observed. We discount the lifetime effect as an explanation of the angular dependence of the Cs+-induced Auger spectra from aluminium.

5. Conclusions Sputtering of aluminium by Cs+-ion bombardment produces ejected excited atoms and ions which contribute to the Cs+-induced Auger electron emission from the solid. The variation in intensity of the principal atomiclike Auger peaks with angle of ion beam incidence on the sample is only partly explained by variation of sputter rate with incident angle; a contribution from Auger decay of dissociation fragments of the sputtered CsAi + molecular-ion species is found to be important and possibly to dominate at oblique angles of incidence.

Acknowledgements The authors thank the SERC for funds in support of this work, Dr. Hassan Bolouri, University of Strathclyde, for technical assistance, and Dr. Roger Webb, University of Surrey, for kindly running computer simulations of sputtering.

615

References [1] W.O. Hofer, Scanning Microsc. Suppl. 4 (1990) 265. [2] C. Benazeth, N. Benazeth and L. Viel, Surf. Sci. 78 (1978) 625. [3] R. Whaley and E.W. Thomas, J. Appl. Phys. 56 (1984) 1505. [4] J.F. Hennequin, R-L. Inglebert and P. Viaris de Lesegno, Surf. Sci. 140 (1984) 197. [5] L. De Ferrariis, O. Grizzi, G.E. Zampieri, E.V. AIonso and R.A. Baragiola, Surf. Sci. Lett. 167 (1986) L175. [6] T.D. Andreadis, J. Fine and J.A.D. Matthew, J. Vac. Sci. Technol. AI (1983) 1159. [7] M. Hou, C. Benazeth, N. Benazeth and C. Mayoral, Nucl. Instr. and Meth B13 (1986) 645. [8] P. Dahl, M. Rodbro, B. Fastrup and M.E. Rudd, J. Phys. B9 (1976) 1567. [9] P. Dahl, M. Rodbro, G. Hermann, B. Fastrup and M.E. Rudd, ibid., p. 1581. [10] See: D.J. Fabian, H. Kleinpoppen and L.M. Watson (eds.), Inner-Shell and X-ray Physics of Atoms and Solids (Plenum, New York and London, 1981). [11] R.H. Milne, E.A. Maydell and D.J. Fabian, Appl. Phys. A52 (1991) 197. [12] R.H. Milne and D.J. Fabian, Appl. Phys. A53 (1992) 574. [13] E.A. Maydell, D.J. Fabian and H. Bolouri, submitted to J. Phys. E Meas. Sci. Technol. (1992). [14] E.A. Maydel, to be submitted to Surf. Sci. [15] E.J. McGuire, Phys. Rev. A3 (1971) 587. [16] R.P. Webb, SUSPRE, private communication (1991). [17] M. Szymonski, A. Poradzisz and L. Gabla, Surf. Sci. 112 (1981) 254.

IX. SECONDARY EMISSION