Interaction of slow multicharged ions with solid surfaces

A. Arnau a, F. Aumayr b, P.M. Echenique a, M. Grether c, W. Heiland a, J. Limburg e, R. Morgenstern e, P. Roncin f, S. Schippers e, R. Schuch g,

N. Stolterfohff, P. Varga b, T.J.M. Zouros h, HP. Winter b

a Departamento de Fisica de Materiales, Universidad del Pais Vasco, Apartado 1072, San Sebastian 20080, Spain

b lnstitutfiir Allgemeine Physik, Technische Universitdt Wien, Wiedner Hauptstrafle 8-10/E134, A-1040 Wien, Austria

c Hahn-Meitner-Institut Berlin, D- 14109, Berlin, Germany d FB Physik, Universitdt Osnabriick, D-49069, Osnabriick, Germany

e Kernfysisch Versneller Instituut, NL-9747 AA Groningen, The Netherlands f Lab. des Collisions Atomiques et MolOculaires, UniversitO Paris Sud, F-91405 Orsay, France

g Atomic Physics Department, Stockholm University, S-10405 Stockholm, Sweden h Institute of Electronic Structure and Laser, 711 10 Heraklion/Crete, Greece

ELSEVIER

Amsterdam-Lausanne-New York-Oxford-Shannon-Tokyo

114 A. Arnau et al./SmJ}lce Science Reports 27 (1997) 113 239

Contents

1. Introduction 1.1. The role of slow multicharged ions in atomic- and surface-collision physics 1.2. Brief history of slow MCI surface collision studies

1.2.1. One-electron transitions 1.2.2. Two-electron transitions 1.2.3. Radiative transitions

1.3. Simple scenario for phenomena occurring in slow MCI surface collisions 1.4. Applications and relations to other fields 1.5. Scope of the EU HCM Network ISHCISS

2. Experimental methods 2.1. Introduction 2.2. Target preparation 2.3. Multicharged ion-sources and -optics

2.3.1. General remarks on production and transport of slow MCI beams 2.3.2. Characterisation of ion beam quality 2.3.3. Modern high-performance multicharged ion sources

2.3.3.1. E C R I S - electron cyclotron resonance ion source 2.3.3.2. EBIS - electron beam ion source 2.3.3.3. EBIT electron beam ion trap

2.4. Detection of slow and fast electrons 2.4.1. Total yield and number statistics of emitted slow electrons 2.4.2. Measurement of ejected electron energy distributions

2.5. Detection and spectroscopy of X-rays 2.5.1. Semiconductor detectors 2.5.2. Crystal spectrometers 2.5.3. Calorimetric X-ray detectors

2.6. lon scattering and recoils 2.7. MCl-induced sputtering and secondary ion emission

2.7.1. Secondary ion emission 2.7.2. Total sputter yield of thin films measured by the quartz micro-balance

2.8. Coincidence techniques 3. Theoretical methods

3.1. Introduction 3.2. Screening in the bulk

3.2.1. Introduction 3.2.2. Linear response theory 3.2.3. Non-linear screening, scattering approach, density functional

3.3. Screening at surfaces 3.4. Neutralisation and de-excitation dynamics

3.4.1. The classical over-the-barrier model 3.4.2. Charge exchange between inner-shell orbitals

3.4.2.1. Molecular orbital diagrams 3,4.2.2. Charge exchange cross-sections

3.4.3. De-excitation of the hollow atom below the surface 3.4.3.1. Cascades in the bulk 3.4.3.2. Example for cascade model calculations

118 118 119 119 121 122 124 126 126 127 127 127 130 130 130 132 132 133 134 134 134 137 139 140 140 141 141 143 143 144 145 147 147 147 147 148 149 156 163 163 165 165 167 169 169 172

A. Arnau et al.,Su~;lilce Science Reports 27 ( 1997 ) 113 239 115

3.4.4. Auger electron spectroscopy 3.4.4.1. He 2 ÷ projectiles: decay of doubly excited He states 3.4.4.2. 0 7 ~ projectiles: decay of 1s21231 s configurations 3.4.4.3. Configurations with more than two L-electrons

3.4.5. Radiative processes 4. Discussion of selected results

4.1. MCI impact on metal surfaces 4.1.1. Hollow atom formation and image charge acceleration 4.1.2. Low energy electron yields and number statistics of electron emission 4.1.3. K LL-Auger electron emission: Above- or below-surface?

4.1.3.1. Reflected projectiles 4.1.3.2. Penetrating projectiles

4.1.4. X-ray emission from hollow atoms 4.1.5. Time scales for projectile inner-shell filling 4.1.6. Emission of target Auger electrons 4.1.7. Charge state and energy loss of scattered projectiles

4.2. MC1 impact on semiconductor surfaces 4.3. MCI impact on insulator surfaces

4.3.1. Low energy electron yields and electron number statistics 4.3.2. Auger electron spectra 4.3.3. Ion scattering from LiF surfaces 4.3.4. MCl-induced emission of atoms and secondary ions from LiF

4.3.4.1. Secondary ion emission from LiF 4.3.4.2. Potential sputtering of lithium fluoride by slow muhicharged ions

4.3.5. First results obtained by coincidence techniques 5. Summary and outlook

Acknowledgements Appendix: Lists of symbols and abbreviations References

74 76 77 8(1 81 82 82 83 85 89 90 93

197 201 205 207 21)9 212 2t3 214 217 219 219

226 228

230 2311 232

ELSEVIER Surface Science Reports 27 (1997) l l3 239

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surface science reports

Interaction of slow multicharged ions with solid surfaces

A. Arnau a, F. Aumayr b, P.M. Echenique a, M. Grether c, W. Heiland d, J. Limburg e, R. Morgenstern e, P. Roncin f, S. Schippers ¢, R. Schuch g,

N. Stolterfoht ¢, P. Varga b T.J.M. Zouros h, HP. Winter b.,

a Departamento de Fisica de Materiales, Universidad del Pals Vasco, Apartado 1072, San Sebastian 20080. Spain b lnstituth'ir A/Igemeine Physik, Technische Universitfit Wien, Wiedner Hauptstra/3e 8- I0/E134. A-I040 Wien, Austria

Hahn-Meitner-Institut Berlin, D-14109, Berlin, Germany d FB Physik, Universitfit Osnabrfick, D-49069, Osnabrfick, Germany

e Kernfvsisch Versne/ler Instituut, NL-9747 AA Groningen, The Netherlands f Lab. des Collisions Atomiques et Mol~cu/aires, Universitk Paris Sud, F-91405 Orsay, France

g Atomic Physics Department, Stockholm University, S-10405 Stockholm, Sweden h Institute of Electronic Structure and Laser. 711 10 Herak/ion/Crete, Greece

Manuscript received in final form 10 January 1997

Abstract

The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on "Interaction of Slow Highly Charged Ions with Solid Surfaces", which has been carried out during the last three years.

After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of "hollow atoms" formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understand- ing of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.

Keywords: Multicharged ions; Hollow atoms; Ion surface collisions

* Corresponding author. Tel.: +43 l 58801 5710; Fax: +43 1 5864203; e-mail: winter(a iap.tuwien.ac.at. 1 Permanent address: Institut fiir Kernphysik, Universifiit Giessen. Germany.

0167-5729/97,/$32.00 ,~) 1997 Elsevier Science B.V. All rights reserved Pll SO 167-5729197)00002-2

118 A. A rnau et al./SutJace Science Reports 27 ( 1997 ) 113 239

1. Introduction

1.1. The role of slow multicharged ions in atomic- and surface-collision physics

In this survey we will deal with the interaction of slow multicharged ions (MCI) with solid surfaces and the adjacent subsurface layers of metals, semiconductors and insulators. In a global way we will regard all processes initiated by surface impact of relatively slow MCI, with "slow" referring to impact velocities below 1 a.u. or 2.18 x 106m/s, which corresponds to projectile energies below 25 keV per atomic mass unit (ainu). At such low MCI impact velocity the electronic transitions between projectile and target surface are generally much faster than significant changes of the projectile distance from the surface. In fact, most experimental and theoretical work covered by this survey involves impact velocities of only a few l0 s m/s or less. Processes of our present interest cover a large variety of often intimately inter-related atomic reactions, e.g. electron exchange between solid and approaching projectile, inelastic scattering of projectile, electronic excitation in the projectile and/or the solid, ejection of electrons, photons, neutral and ionised target particles, and eventual slowing down and stopping of the projectile in the solid.

If an MCI approaches a solid surface with sufficiently low impact velocity, any so-called "kinetic influences" (i.e. effects caused by the kinetic projectile energy) are less important than the influence of the potential projectile energy, which will be deposited during and upon surface impact. This potential energy is equal to the total ionisation energy which had to be spent for generating the original MCI from its neutral atomic ground state. Due to primarily technical reasons, earlier collision studies involving MCI have mainly been conducted at relatively higher kinetic energies and low ion charge states where possible potential effects are superimposed and usually dominated by kinetic effects. With the now available MCI sources it is possible to produce slow MCI beams in such a way that in surface collisions the potential ion energy greatly exceeds its kinetic energy. As will be explained later on, the related "potential effects" depend critically on the respective target surface conditions which determine the nature of electronic transitions between projectile- and surface- states. Therefore, careful investigations of these potential effects call for well-defined projectile- and target-surface conditions in ultrahigh vacuum (UHV) environment, as well as for application of suitable methods for surface analysis, -definition and -preparation which are, however, common in surface physics. Up till now the interest on MCI surface collisions for surface physics itself remained limited because of the rather complicated nature of processes involved, although their close relations with ion-induced desorption, secondary ion emission and low-energy ion reflection from surfaces are well known, the latter phenomena being of considerable importance for surface-physics and -analysis.

On the other hand, for impact of slow MCI on metal surfaces the appearance of a new kind of short-lived (transient) atomic state, the so-called "hollow atom" has been identified. The name "hollow atom" refers to projectiles with empty inner shells but filled outer shells such that transient neutral species are created. Such hollow atoms are initially created by multiple-electron capture above the surface, then collapse at and below the surface under the influence of dynamic screening and eventually become fully de-excited inside the solid.

This novel hollow atom state has raised considerable interest in the physics community [1-3] and henceforth established a new field of research in atomic collision physics.

In Section 1.2 we conduct a short historical tour into studies for impact of slow MCI on surfaces. Recently, these studies have been greatly promoted by the availability of new kinds of sources for

A. Armm et al./SuiJ~u'e Scielwe Reports 27 ( •997 ) 1 I3 239 119

MCI, which will be briefly reviewed in Section 2.3. Nowadays, such high-performance MCI sources permit unique investigations on atomic structure, spectroscopy and collisions [-2 I.

Section 1.3 describes a simple qualitative scenario for some of the so far explored phenomena occurring during slow MCI surface interaction. This scenario will later on be extended and further detailed in Sections 3 and 4. Section 1.4 summarises possible applications of the processes discussed here, and their relations to other fields of research and technology. Finally, Section 1.5 introduces the subsequent Sections of this review, which are dedicated to pertinent experimental techniques (Section 2), essential theoretical aspects (Section 3) and the discussion of recent results obtained both within the EU HCM Network "ISHCISS ~' and further groups active in this field [Section 4). "ISHCISS" designates a "Human Capital and Mobility Network "~ within the 3rd Framework of the European Union dedicated to the "Interaction of Slow Highly Charged Ions with Solid Surfaces", within which the authors of this review and their affiliates in the period 1993 1996 have investigated in a co-operative manner various aspects of the processes to be discussed in this paper.

1.2. Brief history ¢~/s/ow M C I surface collision studies

Particular interest in slow MCI collisions with solid surfaces resulted from the need to understand ion-induced ~'secondary electron emission" at ion collectors, in pursuit of precision measurements of small ion currents in mass spectrometry. Pioneering work of Hagstrum at Bell Laboratories in the US started in the early 1950s and provided a foundation of ISHCISS studies both from the experimental [4,5] and theoretical [6] points of view. It soon became clear that the total electron yields and ejected electron energy distributions for ion-induced ~'potential emission" (PE) are due to fast electronic transitions between the impinging ion and the metal surface. PE results from ion-neutralisation and de-excitation in front of and at the surface, for which essentially no kinetic projectile energy is needed (no impact velocity threshold). Therefore, PE is the principal source of ion-induced electron emission as long as the ion kinetic energy remains less important than the ion potential energy. Consequently, for MCI-induced PE the most significant role is played by the ion charge. However, for excited atoms and ions in low charge states, ion species as well as surface conditions are also relevant. Based on early studies of resonant and Auger electron transitions between singly charged ions and metal surfaces [ 7 - i I I , Hagstrum adopted the following four electronic transitions as important for PE.

1.2.1. One-electron transitions Resonant transitions into excited projectile states (cf. Fig. l(a)), if possible, dominate the

particle-solid interaction because of the large spatial extension of excited state wave functions in comparison with the respective ground state wave function. These resonant transitions act as non electron-emitting precursors for subsequent electron-emitting two-electron transitions (see below).

Resonant neutralisation (RN) (cf. Fig. l(a)) involves transfer of an electron from the surface valence band into the incident ion. It can take place as soon as unoccupied electronic states in the projectile become energetically degenerate with occupied surface valence band states. In a potential curve diagram such RN transitions occur at crossings between the respective initial (ion plus surface) and final states (excited neutralised particle plus surface).

120 A. Arnau et al./Surface Science Reports 27 (1997) 113- 239

V=O

E F m

RI

R=O Z q"

a

wo_ j, A N

R=O Z q*

h

V=O

% E F

R=O Z q+ R

V=O

El; AI

R=O Z q`

V=0

% E F - -

"44

I I

R=O q*

x /x , , ' , , / x .~

R D

Fig. 1. Electron energy diagrams showing resonant electronic transitions (a) and the Auger neutralisation process (b) for ions in front of a metal surface (lie,: electron binding energy, R: particle surface distance). The shaded region indicates the occupied part of the conduction band (W~: work function, Ev: Fermi energy). Electron energy diagrams showing Auger de-excitation (c} and autoionisation processes (d) for ions in front of a metal surface. For other details see (a) and (b). (e) Electron energy diagram showing radiative de-excitation for ions in front of a metal surface. For other details see (a) and (b}.

A. Arnau et al./SutJ'ace Science Repor ts 27 ( 1997 ) 113 ~ 239 121

Resonant ionisation (RI) (cf. Fig. l(a)) as the inverse process to RN takes place if the binding energy of occupied ex-'~ited projectile states becomes smaller than the surface work function W,l ~, and if empty levels in ~the conduction band are available.

1.2.2. Two-electron transitions Auger neutralisation (AN) (cf. Fig. l(b)) ejects an electron from the surface valence band if the

involved neutralisation energy is at least twice the work function W,~. Two electrons of the surface conduction band are involved, one neutralising the ion and the other, by gaining sufficient energy via electron-electron interaction, being ejected with a kinetic energy E e ~< W'i - 2 W,. W'i as the effective recombination energy of the neutralised particle (neutralisation energy) decreases with the AN process occurring closer to the surface because of the increasing level shift with decreasing distance R. The energy distribution of electrons ejected due to AN corresponds to a self-convolution of the electronic surface density-of-states (S-DOS).

Auger de-excitation (AD): In the absence of empty resonant levels in the valence band (i.e. if the binding energy W'~- Wex of the excited electron in the projectile is larger than the surface work function W,), an excited electron (cf. fig. l(c)) will interact with an electron of the surface valence band and thus gets ejected, while the surface electron is captured into a lower projectile state.

The energy of such emitted electrons is limited at Wex- W~, with We~ (the excitation energy) remaining nearly independent of the particle-surface distance R, since respective initial and final charge states are similar and thus subject to comparable level shifts. The energy distribution of electrons emitted due to such AD processes directly reflects the electronic S-DOS. Hagstrum treated the above four electronic transitions between a metal surface and an incident ion within an adiabatic model (no coupling between electronic and nuclear motion). Due to the interaction of the active electron with the negative image charge of the positively charged projectile core, which is stronger than the electronic self-image interaction, the projectile energy levels involved are slightly shifted upwards, corresponding to lower binding energies than for the same particle far from the surface. The resulting transition rates for electron transfer from the surface valence band into the projectile decrease roughly exponentially with increasing projectile-surface distance, because of their depend- ence on the overlap of atomic wave functions of the surface atoms and projectile states. All more recent quantum-mechanical calculations of such transition rates support the applicability of this conclusion (see e.g. [12] and references therein). Consequently, highest transition probabilities are expected from surface states with the lowest binding energy (i.e. those situated near the Fermi edge). Within this adiabatic picture, Hagstrum assumed transition probabilities independent of the ion impact energy and found that the most probable distances for these transitions are of the order of several ~.ngstr6ms. Apart from these four transitions, three further ones are of relevance for MCI-surface interaction.

Quasi-resonant neutralisation (QRN) (cf. Fig. lta)) is a near-resonant transition from tightly bound localised target (core)to projectile states. It can only occur in a sufficiently close collision when overlapping of the involved inner electronic orbitals becomes appreciable. QRN was found responsible for oscillatory ion survival probabilities in ion-surface scattering (ISS) [ 13,14], and is of interest for the spectra of fast Auger electrons in recent pertinent work (cf. Section 4).

Autoionisation (A1) is the intra-projectile AD of transiently formed doubly [15,16] or multiply excited [173 particles, where one or more electrons are ejected into vacuum with other electron(s) in the projectile being demoted to lower states (cf. Fig. l(d)). AI is important in MCI-induced electron

122 A. Arnau et al./Surface Science Reports 27 (1997) 113-239

emission from surfaces, following the formation of multiply excited projectile states by multiple RN (cf. Section 3).

1.2.3. Radiative transitions De-excitation of a singly charged ion near a surface via photon emission (cf. Fig.l(e)) is highly

improbable as compared with the above "radiationless" transitions AN and AD, since the radiative transition rates involved of typically 10Ss 1 are about 106 times smaller than those for the "radiationless" Auger processes. However, since the latter do not essentially depend on the ion charge, for higher charged projectiles radiative de-excitation can become competitive, since the related transition rates (or, in other words, the respective fluorescence yields) increase approximately with the fourth power of the effective ion charge [18].

Apart from the briefly described foundations by Hagstrum and his co-workers, further important contributions to both theoretical and experimental aspects of MCl-induced PE have been made by the group of Arifov in Tashkent/Uzhbekistan [19,20].

In particular, they have pointed out the role of multiple RN followed by AI (see above) [17]. Further relevant work conducted during the 1950s and early 1960s has been reviewed by Kaminsky [21], and Carter and Colligon [22].

At higher impact velocity, PE processes will be superimposed and eventually dominated by kinetic electron emission (KE). Transfer of the kinetic projectile energy onto the electrons and atomic cores in the solid or in adsorbed layers at the solid surface can lead, via different mechanisms, to the ejection of electrons. For reviews on KE see Hasselkamp [23] (primarily experimental aspects) and R6sler and Brauer [24] (theory). For KE, kinetic energy and mass of the projectile are of foremost importance, but other projectile properties as its charge and electronic configuration, and also the surface species and conditions are of relevance.

The ejected electron energy distribution due to KE usually peaks at a few eV and decreases rapidly toward higher electron energy. If the impact velocity involved is not sufficiently low to exclude all KE related effects (i.e. below the respective "KE threshold" which, however, is not really a sharply defined limit), neither the electron yields nor the ejected electron energy distributions give clear information on the relative importance of PE and KE.

As already.mentioned, meaningful investigations of PE pose rather stringent experimental demands on UHV conditions and target surface preparation. In fact, Hagstrum has even tried to utilise the very surface sensitivity of PE for surface analysis (ion neutralisation spectroscopy - INS; [25,26]). However, this INS found no wider application and has eventually been superseded by the closely related metastable atom de-excitation spectroscopy [27,28].

Not many further efforts have been devoted to PE and related phenomena before a decisive boost to this field was given by the novel powerful sources for slow MC1 (see Section 2.3). These devices have greatly expanded the experimental opportunities for studying "potential effects" even at relatively high impact energy where still inclined or grazing ion incidence can be used (see Section 2). Under such considerably more convenient conditions, PE related phenomena have received attention in a much broader parameter space regarding both MCI charge and impact energy. Here we refer to work by Delaunay et al. [29,30] on PE yields for impact of up to Ar ~e+ on clean polycrystalline W, and by de Zwart [31] on ejected electron energy distributions involving Ar q+ (q < 12) impact on clean monocrystalline W. For q _> 8 impact the latter author found a distinct feature near 200 eV in the corresponding ejected electron energy distribution, which he ascribed to

A. Arnau et al./Surface Science Reports 27 (1997) 113 239 123

inner-shell (LMM, LMN, etc.) transitions in the projectile (see [29,32]). This important finding could explain irregularities found by Delaunay et al. for the PE yield dependence on MCI charge state q. At about the same time Zehner et al. [33] also demonstrated projectile-KLL-Auger electron emission in grazing incidence of N q + and O q + on clean monocrystalline gold. With these first measurements of the high-energy part of ejected electron energy distributions, an important new dimension for studying the potential effects in MCI surface collisions has been added (for further developments, in particular by the groups in Groningen (Morgenstern et al.), Berlin (Stolterfoht et al.) and Oak Ridge (Meyer et al.), cf. Section 4). Although principally similar electronic transitions had already been exposed by Hagstrum and Becker [15,16] for impact of metastable He + and fully stripped He 2. on various target surfaces, this was followed up only much later by ejected electron energy distributions measured and simulated for impact of other singly and doubly charged ions on well-defined monocrystalline surfaces (e.g. [14,34,35]). In somewhat faster MCI surface collisions projectile inner shell vacancies can also be transferred into inner shells of target atoms, thus causing target- characteristic Auger electron emission [33,36]. Indirect evidence for such processes has been provided by the dependence of secondary ion yields on projectile ion charge state for impact of Ar q + on polycrystalline Si [37].

In addition to total electron yields and ejected electron energy distributions, for studies on MCI- induced PE also the number statistics of ejected slow electrons are of interest. Obtainable by means of relative measurements, these statistics not only provide as their mean value the respective absolute total electron yield ([38]; for further details cf. Section 2.4.1 t, but also permit distinction between PE and KE, as long as the projectile velocity does not reach too far beyond the KE threshold [39]. Moreover, the number statistics of ejected electrons furnish important details on the hollow atom formation in MCI surface collisions (cf. Section 4).

Apart from studying slow and fast electron emission processes, one should keep in mind that the potential energy deposited during particle-surface collisions may be disposed of by the emission of photons. "Bombardment-induced light emission" has been investigated mainly in conjunction with sputtered excited target particles and secondary ion emission (for a recent review see Yu [40]). As already mentioned, for impact of excited atoms and singly charged ions such processes remain insignificant, since de-excitation via Auger transitions is usually much more probable (see above).

This situation changes quite drastically for recombination of inner-shell vacancies in high-Z hollow atoms, where emission of soft X-rays competes with the above discussed fast Auger electron ejection. This has first been shown by Donets [41,42] and later investigated in more detail by Briand et al. E43] and Andr~i et al. [44] (for further details cf. Section 4).

Another more recent branch of ISHCISS related research comprises the reflection, charge state distribution and inelastic energy loss of MCI scattered from solid surfaces. The charge state distribution of non-neutralised projectiles after their reflection from a clean W surface first indicated the survival of projectile inner-shell vacancies during the collision [45]. More recently, final charge state fractions of slow MCI channelled along a Au single crystal surface have been investigated e.g. by Folkerts et al. [46] (for details cf. Section 4). These fractions are almost independent of the incident projectile charge, proving that projectile charge equilibration is achieved near the surface already within the very short time of about 30fs.

Scattering of MCI under grazing incidence on carefully prepared clean flat single crystal surfaces yields rather precise information on the respective projectile image-charge attraction [47]. Another recent line of research comprises MCI-induced-sputtering and -secondary ion emission from

124 A. Arnau et al./Sur[ace Science Reports 27 (1997) 113 239

insulator surfaces. No influence of the projectile ion charge on the neutral sputtering yield is found for metallic [48] or semiconducting target surfaces [37]. This is expected as long as sputtering of neutrals [49] and ions [50] is caused by the kinetic projectile energy. For MCI impact on insulator surfaces an effect called "Coulomb explosion" has been predicted by Bitensky et al. [51] and Bitensky and Parilis [52]. It should occur due to rapid extraction of many electrons into approaching MCI, leading to highly localised surface charge-up with subsequent ejection of target ion cores. Evidence for "Coulomb explosion" has primarily been searched for with alkali-halide targets, where "electronic sputtering" and ion-induced desorption due to impact of singly charged ions are well-established phenomena [53,54]. The "Coulomb explosion" process has been experi- mentally inferred from secondary ion yields (Radzhabov et al. [55] for alkali halides and Si; Schneider et al. [56] for SiO2) and micro-crater sizes increasing with projectile charge (Radzhabov and Rakhimov [57] for alkali halides; Schneider et al. [56] for mica). However, recent absolute measurements of the neutral sputtering yields for MC! impact on polycrystalline LiF by Neidhart et al. [58] showed no impact energy threshold, well in contrast to the common kinetic sputtering (for further details see Section 4). This "potential sputtering" process is thus not related to "Coulomb explosion" and it should be noted that absolute MCI-induced secondary ion emission yields for LiF [59] are typically by a factor of hundred smaller than the related neutral sputtering yields.

1.3. Simple scenario for phenomena occurrinq in slow MCI-sur face collisions

Current views on the interaction of slow MCI with a metal surface evolved during the last decade from numerous experimental and theoretical studies of the respective total electron yields, slow electron number statistics and fast Auger electron energy distributions, scattered projectile charge- states and -energies, and soft X-ray emission (further details to be given in Section 4). An appropriate scenario (cf. Fig. 2) can be divided into four stages [60,61], covering the projectile's approach toward

Fig. 2. Hollow atom development during slow MCI impact on a metal surface (cf. text; after Ref. [61]).

A. Arnau et al./SurJace Science Reports 27 (1997) 113 239 125

the surface (A), its close contact with the surface (B, C), and finally its subsequent penetration into the target bulk (D) or back scattering into vacuum. After earlier attempts by Delaunay et al. [30], Appell [62,63], Varga [64], Snowdon [65], Andr~i [66,67] and Bardsley and Penetrante [68], the above-surface stage (A) of hollow atom formation has been theoretically described in great detail by Burgd6rfer et al. [69 71] within the so-called "classical over-the-barrier" (COB) model (see also Section 3.5). Above the metal surface the MCI induces a collective response of free metal electrons which at sufficiently large distance can be described by the classical image potential. The latter accelerates the MCI towards the surface and thus sets a lower limit for the effective projectile impact velocity, with a correspondingly upper limit for the MCI above-surface interaction time.

The image interaction shifts up and Stark-mixes the projectile electron states and enhances the height of the potential barrier between the MCI and the surface. Within a critical distance which in first approximation depends on the metal work function and the MC1 charge state, electrons from the solid can be captured into highly excited projectile states (resonant neutralisation). This RN should go on until complete neutralisation of the MC1 is achieved, corresponding to formation of a "'primary hollow atom" (Fig. 2, stage B [60,61]). Although this hollow atom is subject to rapid A1, it will be kept dynamically neutral by ongoing RN. Simply speaking, with further approach the levels of this hollow atom will be shifted up because of image interaction and screening of the projectile charge by already captured electrons. Both AI and shifting give rise to slow electron emission from the target as well as the projectile, and increasingly tighter bound projectile states will become populated as they fall into resonance with occupied electron states in the surface.

On the other hand, electrons in shifted states may be recaptured into the metal via resonant ionisation. This interplay of different electronic transitions will continue until close contact with the surface, with both electron emission and gradual shifting of electrons bound to the projectile into lower n-shells (Fig. 2, stage C), and involved projectile states becoming filled and again depopulated within a few femtoseconds, thus being of a highly transient nature. At close contact with the surface, screening by surface electrons will become dominant (leading to the "peel ofF' of remaining outer shell electrons) and relaxation of a then formed "secondary hollow atom" will proceed below the surface via three competing processes (Fig. 2, stage D). At low impact velocity, AN can populate the still present projectile inner-shell vacancies, whereas at higher impact velocity they will also be directly filled in close collisions with target atoms via quasi-molecular transitions (further details on these processes, which have only recently been explained in a satisfactory way, are given in Sections 3.4 and 4). The resulting subsurface AN accounts for the bulk of observed fast projectile Auger electrons. In competition with Auger electron emission, in MCI with higher Z the transient inner-shell vacancies may also become de-excited by characteristic projectile X-ray emission. Auger electron as well as soft X-ray emission take place mainly below the surface, where projectile shielding by free metal electrons strongly influences all respective electronic transitions (see Fig. 2, stage D). Many-electron processes like multiple-electron capture (as shown for binary collisions by Barat and Roncin [72]) may play an increasing role for higher initial charge of the projectile. In addition to the fast Auger electrons a considerably larger number of slow electrons is emitted mainly before and at close surface contact (see Fig. 2, stages B, C, which also are relevant for elastic and inelastic MCI scattering, MCI-induced sputtering and secondary ion emission).

126 A. Arnau et al./SurJuce Science Reports 27 (1997) 113 239

1.4. Applications and relations to other fields

MCI can carry with them a rather large amount of potential energy (e.g. 250 keV for Ne-like ThS°+). Transfer of such a large energy onto a very small surface area of about 100/~ 2 within the rather short time of < 100 fs corresponds to the immense power flux o f> 1014 W/cm z, which can give rise to various non-linear processes whose consequences are largely unexplored. Many electrons are extracted from the surface to be captured by the MCI or emitted into vacuum, thus producing strange "hollow" atoms whose outer Rydberg orbitals are filled while the inner shells stay largely empty. In short, the high potential energy of MCI is released via emission of electrons, photons, atoms and ions. Slow highly charged ions can therefore serve as local probes for gleaning information about the many-body dynamics in exotic atoms and the non-linear properties of solid surfaces. Since MCI deposit their potential energy in a rather localised region, the corresponding high-energy concentration may also possibly lead to characteristic surface modifications.

The above described processes are of interest, e.g. in the following fields:

• Astrophysics and cosmology (MCI-induced desorption from interplanetary and interstellar clusters and grains).

• Atomic physics of multiply excited species (hollow atom spectroscopy). • Information technology (high-density storage media based on MCI-induced micro-structures). • Plasma-chemistry and -technology. • Semiconductor industry (novel cleaning procedures, e.g. preferential removal of insulating layers). • Thermonuclear fusion reactor development (plasma wall interaction, forced radiation cooling in

the plasma edge region).

1.5. Scope of the EU HCM Network ISHCISS

Considering the steadily increasing scientific interest in MCI surface interactions and also the continuously improving conditions for experimental studies involving slow MCI beams in conjunc- tion with atomically clean solid surfaces in UHV (see Sections 2.2 and 2.3), about five years ago a subgroup of the present authors has agreed to pool their different resources into a common effort within a "Human Capital and Mobility (HCM)" Network of the European Union. This HCM Network was named "Interaction of Slow Highly Charged Ions with Solid Surfaces (ISHCISS)". It was submitted for funding to the EU in May 1992 by the coordinator N. Stolterfoht (HMI Berlin) and has eventually been granted for a period of three years, starting by October 1993.

Within this Network it was planned to study a number of model systems, involving certain MCI species (primarily N u +) and solid surfaces (Au, Si, LiF). The latter were primarily chosen because of their wider application in former similar studies, following their comparably easy experimental handling. For the experimental techniques to be used within the Network, spectroscopy of fast Auger electrons, measurement of slow electron number statistics, and ion-scattering and -energy loss studies were most important. However, other techniques have been used as well, either separately (e.g. investigation of MCI-induced sputtering with the quartz micro-balance) or in conjunction with the above methods. A number of these "central" techniques have also been applied in mutual coincidence, which greatly enhances the potential of all of them. Furthermore, most earlier experiments on the present subject have suffered from insufficient theoretical support. This

A. Arnau et al./SuJJ~we Science Reports 27 (1997) 113 239 127

unfortunate situation can be understood from the great complexity of the MCI surface system, which puts it beyond the primary interest of atomic collision- and surface-physics. However, several of the experimentally oriented groups within the present Network had already fruitful cooperations with the San Sebastian group, which then agreed to provide the principal theoretical support (see Section 3) for investigations carried out within the Network.

In Section 4 we present and discuss a number of results obtained within the ISHCISS Network, which have considerably improved our understanding of the subject and also put our results into perspective with relevant work from other groups.

2. Experimental methods

2.1. Introduction

In this section the basic experimental requirements and presently used experimental schemes in the field of slow (MCI) surface collisions will be discussed. Some of the problems encountered are quite common in surface physics experiments, i.e. the preparation and the probing of the actual status of a surface with respect to cleanliness and structure. Other aspects like the sources and optics used to produce beams of MCI are rather important for the field naturally. These developments are fairly recent especially for the low ion energy regime. Fast MCI can be produced using high energy accelerators and "stripper foils". For the detection and the analysis of "MCI-induced electrons", X-rays and scattered or sputtered ions the experiments started with conventional means. Special developments will be a major part of this section. Two examples are the instrument for the measurement of the total yield and the number statistics of emitted slow electrons [73] and the improvement of the quartz micro-balance method for the measurement of sputter yields [58]. Another special development is the the experimental set-up for coincidence experiments E743.

2.2. Tar qet preparation

A basic requirement for reproducible experiments and results to be comparable to theory are well- defined surface samples. It is useful to distinguish polycrystalline material and single crystals. In case of polycrystalline materials the sample cleanliness is of major concern, however structural problems cannot be neglected completely. Especially at low ion energies the possible preferential orientation of the grains of a polycrystal may cause channelling effects leading to structural effects in particle penetration, scattering and sputtering. In the experiments discussed in the present paper polycrystal- line samples are used for the electron number statistics (Au) (Sections 2.4.1 and 4) and for the sputtering experiments (Au, Si, S i O 2 and LiF) (Sections 2.1.2 and 4). So far, the set-up for the electron emission statistics does not allow in situ control of single crystal surfaces. Targets have to be prepared and then transported into the electron detection set-up. Since this can also be done by usual manipulation there is no principal problem. Details for target preparation have been discussed previously [-75]. For the sputtering experiment evaporated layers have been used as described below. Only in case of S i O 2 the actual quartz of the micro-balance can be used which is single crystalline. For the use of single crystals in the sputtering system a combination with a MBE (molecular beam

128 A. Arnau et al./Surface Science Reports 27 (1997) 113 239

1400

1200 ¢/)

,4,,,,a

= 1000 o

800 >,

600 ¢.)

400

200

' ' I ' ' ' ' t ' ' ' ' I '

2keY He + -~ Pt(l 10) + K

2keV He + --~ Pt(l 10) clean

K

#

0 500 1000 1500 2000

Energy / eV

Fig. 3. IS-spectrum of a clean Pt( 1 l 0) surface and a K contaminated surface after heating to ca. 1000 K [76].

epitaxy) system could be a possibility. In the Auger electron emission- and ion-scattering experi- ments metal single crystal surfaces have been the majority of the targets. In general the target preparation followed conventional recipes [75]. The metal single crystals are polished and oriented before entering the vacuum chamber. Stress free mounting on the usual UHV goniometers is afforded by cutting a groove into the side of the sample and holding it with clamps there. The target preparation consists of sputtering and annealing cycles. The target cleanliness is in most cases measured by Auger electron spectroscopy (AES) or ion scattering spectrometry (ISS) (Fig. 3; [76]). In experimental systems for the analysis of electron energy spectra from MCI-sol id interaction an electron gun is all what is needed to measure Auger electron spectra. The combination of the ion sputter gun with an electrostatic energy analyser (here a 90 ° spherical analyser) provides the means for ISS [77]. The annealing temperatures can be kept below 0.5T m where T m is the bulk melting temperature. High temperatures may c a t ~ severe segregation of impurities. In cases of obnoxious impurities gas-surface reactions, i.e. O to remove C, or H to remove O, may be necessary. The structural quality of the surface is controlled in many cases by low energy electron diffraction LEED. In all experiments discussed here only a visual inspection of the beam spots is being used. The surface structure is also controlled by surface channelling effects [77]. For low index directions of a single crystal surface characteristically shaped spatial ion distributions are observed [46,78], whereas for high index or "random" directions the angular peak width of reflected ions is a good indicator for the "flatness" of a surface [79].

For semiconductor surfaces the sputtering and annealing procedure is less effective or simply destructive. Due to the covalent bonding annealing of a sputtered semiconductor surface is poor. Semiconductor surfaces are prepared by following chemical recipes in case of Si and a heating process in vacuum [75,80]. In some cases, i.e. I I I -V semiconductors, cleaning in vacuum is

A. Arnau et al./Surface Science Reports 27 (1997) 113 239 129

a possibility. In case of oxides the extreme sensitivity to ion bombardment causes problems. Besides the structural damage by preferential sputtering, i.e. the depletion of O in the near surface region, the defects created cause severe changes of surface properties. In some cases thin oxide layers can be grown in situ on the proper metal substrate [81,82]. This procedure has the advantage of rather straightforward reproducibility. Furthermore the bulk conductivity inhibits charging effects. So far samples of this type have been used in ISS experiments only, but not for using MCI.

A severe difficulty in ion beam experiments with insulator surfaces is the generation of electrostatic charges at the surface. Both the impact of positively charged ions and the subsequent electron emission will form a positively charged surface layer. This will not only change the impact energy and beam geometry, but also the energy distribution of the emitted charged particles. Since the energy of charged secondary particles is usually very low (the maximum of the energy distribution is around a few eV) even an electrostatic charge-up of a fraction of one volt can influence the measured total yield.

Several methods can be used to overcome the charge build-up at the surface:

(1) Flooding the target with carriers of appropriate polarity (in our case electrons). (2) Deposition of the insulating target material as thin (j2m) or ultra thin films (nm) on metal

substrates, to reduce the resistance of the surface layer. (3) Heating of the sample up to temperatures where it becomes a good ionic conductor (e.g.

alkali-halide targets).

Typical experimental set-ups for MCI experiments (Fig. 4) using single crystals within the HCM network are described in [83,84]. The essential parts of these systems are the electrostatic energy

ion beam from

ECR-source

Faraday~ ,ter cup ~

pumps ~ "

Fig. 4. Typical experimental set-up for the study of electron emission from MCI-solid interaction. The system includes an upper preparation stage with an ion sputter gun (not shown), an electrostatic analyser (ESA) and a LEED system. The lower stage for electron spectroscopy is shielded by la-metal [83].

130 A. Arnau et al./Sur[ace Science Reports 27 (1997) 113 239

analyser and the magnetic shielding. Electrostatic analysers are described in detail in Section 2.4.2 and in text books [85,86]. The magnetic shielding is necessary for the measurements of low energy electrons. Since the spectroscopy is double differential, i.e. energy and angle, particle counting using channeltrons is necessary. For the detection of ions time-of-flight (TOF) methods are also applicable using a pulsed primary beam [86].

2.3. Multichar,qed ion-sources and -optics

2.3.1. General remarks on production and transport of slow M C I beams The ideal MCI source delivers bright beams of Z q+ ions with atomic Z and charge state q from

virtually any chemical species and up to the fully stripped ("naked") state q = Z. In reality, MCI beams may be produced in many different ways (for a general overview on MCI source-principles and -technology see [87]), of which the successive electron impact ionisation is the most effective one. MCI production by single-electron impact on neutral particles involves reaction cross-sections which dramatically decrease with increasing q, whereas cross-sections for subsequent removal of single electrons from increasingly higher charged ions decrease much slower with q. For utilising this successive ionisation mode efficiently, the ions need to be kept within an environment assuring the necessary ionisation times r~,q, which depend on the relevant ionisation- and recombination-volume processes and on the respective ion confinement conditions. However, intense ion beams which are characterised by their emittance and brightness (see below) can only be efficiently produced and transported toward an experimental set-up if the ion beam has a sufficiently small energy spread or, in more relaxed terminology, a low temperature. Therefore, the art of MCIS-construction and -operation consists in a subtle balancing between the rather contradictory requirements of efficient MCI production and keeping the resulting ions as cold as possible.

2.3.2. Characterisation of ion beam quality

The concept of ion beam emittance derives from Liouville's theorem, which states that the density of non-interacting particles subject to conservative forces in the six-dimensional phase space stays constant along a given particle trajectory. If we regard an ion beam drifting along the z-direction and neglect the ion beam space charge (see below), we may substitute [88] the six-dimensional phase space {x, y, z; Px, P~,, P:} with its two-dimensional sub spaces .{x, Px} and [y, Pyl. By further assuming that p.,., p~,

A. Arnau et al./Sur/'ace Science Reports 27 (1997) 113 239 131

30

20

1 0

- 1 0 -

-20

l i l l l [ l i l l l i i l [

l l t i l L t I i l l [ i t l

- 2 1 - 1 8 - 1 5 - 1 2 - 9 -6 -3 0 3 6 9 12 15 18 21

x [ m m ]

Fig. 5. Emittance plots with contours for different total ion beam fractions (solid curve 95%, dotted curve 85"/0, dashed curve 60%) for a 40 keV Ar s+ beam [89] extracted from a 5 GHz ECRIS [91].

Fig. 5 shows a typical emittance plot with contours for different fractions of the total ion beam intensity [89]. Since such emittance figures have an elliptical shape, it is customary to give the size of the respective emittances in units of ~ mm mrad.

Based on the concept of emittance, the most relevant figure of merit of an ion beam is its ion current density in phase space (so-called brightness or normalised brightness, respectively):

B - I/~:x.ey, B, ~ I/g,,n,x'~n,~.. (3)

Customarily, brightness is given in units of A/(zt mm mrad) 2. We can thus simply state for high quality ion beams that the lower their emittance the higher their brightness.

If ion beams involve non-negligible space charge due to mutual repulsion of the ions by their Coulomb interaction, the conditions given above for the validity of Liouville's theorem break down. However, since c.w.- or slowly pulsed positive-ion beams drifting in good technical vacua (typically above 10 -7 mbar) remain space charge compensated by those electrons which they themselves produce in collisions with background gas molecules, the concepts of emittance and brightness may still be adhered to. Space charge influence along acceleration- or deceleration- regions as well as imperfect ion-optical systems enhance the ion beam emittance and give rise to characteristic distortions from the ideal elliptical shape, which will cause a considerably larger "effective emit- tance" (i.e. the elliptical envelope of a real emittance figure, cf. Fig. 5). Such distortions can affect the ion beam transport in ion-optical systems or experimental environments, the ion-optical properties of which can be characterised by their "acceptance" according to the same criteria as for the ion beam emittance.

132 A. Arnau et al./Sur/~tce Science Reports 27 (1997) 113 239

Electrical coils T---

RF inj,ection I I I

Gas inlet

S / / i

/

/ Multipole (permanent magnets - Ioffe bars)

~ ~ Ion Beam

Fig. 6. Main components of an ECRIS for MCI production (cf. text}.

2.3.3. Modern hiyh-performance multicharged ion sources

2.3.3.1. E C R I S - e l e c t r o n cyclotron resonance ion source. An ECRIS capable of efficient MCI production consists of the main components shown in Fig.6. A discharge chamber filled with the working gas (pressure of typically < 10 -5 mbar; for ion production from non-volatile substances see below) is immersed in a "min-B" magnetic field geometry (i.e. the magnetic field strength increases from the plasma centre outward) which provides the necessary ion confinement. Such magnetic field configurations can be generated by sets of solenoids (which for larger set-ups may be superconduct- ing) combined with multipolar magnetic fields produced by high current-carrying conductors ("Joffe bars") or, for more compact constructions, permanent magnets (Sin Co, Fe Nd B). Microwaves with a frequency of presently up to 28 GHz are fed into the discharge chamber, to produce a plasma wherein the electrons are preferentially heated by electron cyclotron resonance (ECR) when drifting through the so-called ECR zone (cf. Fig. 6) where the electron cyclotron frequency vec = eB/2rcm e matches the applied microwave frequency (e and m e are electron-charge and -mass, respectively, and B is the local magnetic field). In this way the electrons can reach rather high energies (up to hundreds of keV) and thus ionise efficiently even highly charged ions, which themselves stay rather cold. This provides effective step-by-step electron impact ionisation conditions up to high q values and permits extraction of high quality MCI beams.

Different techniques have been developed for producing MCI from solid compounds, e.g. sputtering or melting from suitable electrodes situated next to the ECR zone, or vaporisation from a crucible inside the discharge chamber. Stable operation of ECRIS calls for preferential replenish- ment of the plasma electrons, which in early ECR types has been achieved by a first discharge stage

A. Arnau et al./Sur[ace Science Reports 27 (I997) 113 239 133

featuring higher gas pressure. More recently, these extra electrons are either delivered by hot filaments or ion-induced electron emission from oxide-coated inner walls, or negatively biased probes with respect to the ECR plasma. Since their first practical realisation in the late 1960s by Geller and co-workers at C.E.N. Grenoble/France (see references in Melin and Girard [-90]), the ECRIS has undergone a rapid development toward an already high degree of maturity. Modern ECRIS are typically operated with ECR frequencies of 10--15 GHz at a power level of several 100 W and can produce e.g. 4 ~tA c.w. beams ofAr ~'+ or 12 laA of U 33+ [90]. The kinetic energy spread of the extracted ions is typically between 5 and 10eV times charge state q, Emittance measurements for 5 x q keV N 4+ or Ar ~+ ions extracted from a single-stage 5 GHz ECRIS [91] delivered values for c,9 s,,

134 A. Arnau et al./Sur[ace Science Reports 27 (1997) 113 239

variable potentials is focused by a strong magnetic field of typically 1 T, which is being delivered by a long (usually super conducting) solenoid. This electron beam, by virtue of its strong negative space charge, provides excellent confinement for positive ions as long as suitable positive potential wells are applied on both ends (cf. Fig. 7). This confinement can last for a few milliseconds up to many seconds, until the full space charge compensation of the electron beam is reached. Ions brought into the electron beam during the confinement time, either by gas injection or from suitable external beam sources, will be ionised in a step-by-step manner. The lower the surrounding background gas pressure, the more efficiently this ionisation proceeds. The background pressure can be kept below 10- 10 mbar by cryogenic pumping on the inner walls of the super conducting solenoid. As a special feature, at any time during ion confinement only narrow groups of MCI charge states are present.

With EBIS used in short-pulsed fashion, desired MCI are extracted by rapidly lowering one axial potential barrier (extraction times typically 5- 50 Ias). In this mode an EBIS is also excellently suited as MCI injector for heavy ion synchrotrons. On the other hand, it may be run c.w. in the so-called "leaky mode", where it delivers less highly charged ions than in the (short) pulsed mode.

Existing EBIS are capable of producing up to fully stripped Xe 54 + ions (about 104 per pulse), or fully stripped Ar 18 + (about 108 per second [94]). However, there is still considerable potential for further development, which requires further systematic studies.

In particular, emittances of MCI beams from EBIS [95] should be measured more systematically. According to Becker [96], typical EBIS emittances of < l0 ~mm mrad can be expected, which values would be considerably smaller than for ECRIS (see above). Technical details on the construction and operation of cryogenic EBIS can be found in St6ckli [97].

2.3.3.3. E B I T - electron beam ion trap. EB|S and EB|T feature as common working principle the step-by-step ionisation of ions trapped in the space charge of dense electron beams. However, an EBIT (first constructed by Marrs et al. [98]) involves a much shorter electron beam than an EBIS (only a few cm), which is both easier to realise and probably more stable, thus giving rise to a comparably more efficient ion confinement. Secondly, the EBIT has originally not been conceived as an MCI source, but as a device for studying radiation frorn the trapped MCI.

The presently most advanced EBIT version (Lawrence Livermore National Laboratory "super- EBIT") applies a 200 keV electron beam, which permits studies on electron impact-excitation and -ionisation for up to H-like U 91 + [2]. MCI can be extracted along the electron beam direction for forming a free MCI beam, as was first demonstrated by Schneider et al. [99]. In this case similar ion charge spectra and MCI yields can be obtained as with EBIS (see above). For production of MCI from non-gaseous compounds, the respective singly charged ion species can be injected in a similar way as for EBIS, e.g. from a metal vapour vacuum arc source ("MEVVA" [87]). In summary, EBIT are quite compact devices (comparable in size, technology and costs to a medium-performance electron microscope), which can produce up to fully stripped ions of virtually all elements.

2.4. Detection of slow and fast electrons

2.4.1. Total yield and number statistics of emitted slow electrons The total electron yield 7 (mean number of electrons emitted per single projectile impact) can

usually be determined by measuring fluxes of both the incoming projectiles Ip and emitted electrons Ie = 7" lp/q, respectively (so-called current measurement) [100]. With charged projectiles this can be

A. Arnau et al./Surthce Science Reports 27 ( 1997 ) 113 239 135

accomplished in a straightforward manner by measuring target currents with and without permit- ting electrons to leave the target, which can be secured by appropriate target biasing with respect to its environment. Whereas the large majority of electrons (typically 95-99%) are emitted with kinetic energies of less than 50 eV, highly charged ions carrying inner-shell vacancies will also give rise to Auger electron emission with energies of hundreds of eV, which cannot be suppressed by common target biasing. Precautions have also to be taken against possible disturbances from charged particle reflection, secondary ion emission and, especially, spurious electron production due to impact of reflected or scattered projectiles or electrons, all these effects possibly causing additional electron emission from the target region. In general, for such measurements projectile currents of typically above 1 nA are necessary. Very highly charged ions (q > 20) are usually not available in such large quantities. Therefore a technique for determination of the total electron yield 7 has been introduced that measures the electron emission statistics (ES), i.e. the probabilities W, for ejection of 1,2 . . . . . n electrons per incident projectile, where

/

W, = 1. (4) n 0

The relation between 7 and W, is then given by /

Z nw.. (5) n - I

A set-up for measuring such ES of slow MCI as devised by TU Wien [38,73,101-104] is shown in Fig. 8. In this set-up the incoming MCI can be accelerated or decelerated by a four-cylinder lens assembly to any desired nominal impact energy, before hitting the target surface under normal incidence. In some of these experiments, the impact energy was actually limited only by the projectile image charge interaction with the surface [103]. Practically all ( > 97% [105]) electron ejected from the target with energies smaller than 60eV into the full 2n solid angle are deflected by a highly transparent (96%) conical electrode and then, after extraction from the target region, accelerated and focused onto a surface barrier detector connected to U e >_ + 20 kV with respect to the target. The resulting ejected electron trajectories have been indicated in Fig. 8. All electron emission events induced by one impinging projectile particle will be finished within less than 10- ~ s, which is much

electron detector target ion beam

Fig. 8. Set-up for measuring the number statistics of ion-induced electron emission (from [ 102], cf. text).

136 A. Arnau et al./Surface Science Reports 27 (1997) 113-239

z_

m

e -

0 . 0 1 ~ , ~ ~ , i ' ' i ' ' ' - ~ ~ , ~ - ~ - A r 9* --~ A u 0.01 ~ e ~ 100eV

0 5 10 15 20 25

n

! A r 9+ --> A u [] exp, data (7 = 11.2) Binomial distribution (N = 32, p = 0.35; 0.15~ 1 0 0 e V ~ y = N p = 11.2)

i / ~ _ 1 B ~ ......... .oisaon distribution

t i ' l l l . l (7= 11.2,

°°f lllll 0.05 ~ /" ",..

o.oo f~_ . . . . . . . . . . . . 0 5 10 15 20 2 5

Fig. 9. (a) Measured pulse height spectrum for impact of 100 eV Ar 9 + (normal incidence) on clean polycrystalline gold. Experimental data have been fitted by solid line for deconvolution into the individual probabilities W, (data from [73]). (b) Best fit of the resulting emission probabilities W,, ( = electron number statistics) with bionomial and Poisson distribution, respectively.

shorter than the resolution time of common detection electronics (> 10-6S). Thus, n electrons emitted due to impact of one projectile are registered like one electron of hUe keV rather than a number n of U e keV-electrons. Consequently, the area below the nth peak of the resulting electron "energy" spectra (cf. Fig 9) is directly related to the probability W, for emission of n electrons. The probability W o that no electron is emitted cannot be determined directly, but may practically be neglected for yields 7 > 3. Electrons which are reflected from the detector surface (fraction ~ 15% [106]) will deposit only part of their kinetic energy in the detector and thus add to the simple Gaussian peak shapes (FWHM ~ 6 keV) on the low-energy side of each peak a broader structure of however well-interpretable shape [101]. These structures become dominant for peaks correspond-

A. Arnau et al./Surface Science Reports 27 (1997) 113 239 137

ing to higher electron number n. As described in more detail in [38,101], least-squares fits of linear combinations of such peak shapes to the measured pulse height spectra deliver the actual emission probability distributions W,, from which the total electron yields can be directly obtained via Eq. (5). The ES technique requires ion fluxes at the target surface of less than 104 projectiles/s and is therefore ideally suited for comparably small MCI beams from EBIS and EBIT (cf. same section). Addition- ally, charging-up of insulator surfaces under HCI bombardment will completely be avoided [107,108]. Furthermore, since MCI-induced potential electron emission depends strongly on the MCI charge state, respective ES spectra can be utilised to distinguish between different MCI species with equal or nearly equal charge-to-mass ratios present in "mixed" ion beams [109].

Apart from giving access to precise total electron yields, the ES method delivers the emission (number) statistics itself. This statistical distribution can provide unique information on the respective interaction processes, namely the total number of electrons involved in a particular emission process and the mean "single emission probability" for these electrons, since only a fraction of all involved electrons can actually escape into vacuum [110,111].

2.4.2. Measurement of ejected electron energy distributions For the energy analysis of electrons various types of electrostatic spectrometers are used.

Generally, the theory of the electrostatic spectrometers is relatively complex so that only general aspects can be discussed here. For more details the reader is referred to the articles by Ewald and Liebl [112,113], Rudd [114], Dahl [115] and Wannberg et al. [116]. Each spectrometer consists essentially of two plates producing a well-defined electric field. The notations of the spectrometers refer primarily to the shape of these plates. There are the parallel plate spectrometer [117,118], the cylindrical mirror spectrometer [119], the cylindrical plate spectrometer [120] and the spherical plate spectrometer [ 121].

The following discussion deals with general aspects of such spectrometers. Electrons from the source enter the electric field through the entrance slit. After appropriate deflection the electrons leave the field by passing the exit slit. The slit denoted "object" defines the beam of electrons before passing the entrance slit. Another defining slit denoted "image" is located at the image of the object. The object and image may coincide with the entrance and exit slit, respectively.

For electrons originating from a point source the object slit determines the polar and azimuthal acceptance angle. An important quantity of the spectrometer is the solid angle defined as the product of the polar and azimuthal acceptance angles. Similarly, a finite source produces the angular cone limited by the trajectories passing through the centre of the image slit. Hence, the maximum divergence of the electron beam is essentially the sum of these angles. Another important quantity of the spectrometer is the (relative) energy resolution AE/E defined as the accepted energy interval AE divided by the electron energy E. Also, the quality of a spectrometer is determined by its focusing power. An electron spectrometer can focus in different orders [112,113].

Various aspects have to be considered when a choice is made between different types of spectrometers. Favourable features of the spectrometers are: high resolution, efficiency and simplic- ity of design. High resolution is required when individual Auger lines are to be measured. Natural line width and separation of adjacent lines are of the order of 1 eV. Accordingly, resolutions AE/E of 10 .-4 10 3 are required using e.g. 1000eV electrons. Such resolutions may in principle be realised by reduction of the spectrometer slits or by deceleration of the electrons. The latter method takes advantage of the fact that AE/E is usually constant so that the reduction of E decreases

138 A. Arnau et al./Surlace Science Reports 27 (1997) 113 239

AE. The deceleration method is advantageous, since the loss in the spectrometer efficiency is relatively low. Furthermore, the deceleration method allows for varying AE during the measurements.

A measure of the efficiency is the luminosity of the spectrometer which is proportional to the product of the solid angle and the accepted area of the source. Both the solid angle and the accepted source area are determined by the focusing power of the spectrometer. In ion-surface collision experiments, the source area is the product of the length and the diameter of the beam seen by the spectrometer. The beam current usually increases when the beam diameter increases. Hence, the luminosity of the spectrometer is determined by this length and the current. This shows that for a given resolution the spectrometer with large luminosity is favoured, as a high count rate is desirable.

The simplicity of the spectrometer design is related to disturbance effects involved in the electron measurements. Significant experimental problems are produced by stray electrons reaching the detector and by spurious electric and magnetic field. For high resolution measurements it is required to reduce the earth magnetic field by a factor of about 100. This can be made by a g-metal vessel placed inside the scattering chambers. Similarly, spurious electric fields must be avoided. They are generally prodced by electrons collected at insulating surfaces (e.g. contaminated by finger prints). Such fields are critical near the target region as they may produce energy shifts of the electrons of some eV.

The experiments of angular-resolved electron spectroscopy at solid surfaces are generally performed in UHV chambers (Fig. 4). The vacuum chamber may have two sections, the upper one being a "service section" and the lower a g-metal shielded "spectroscopy section". In connection with the electrostatic analyser (ESA), the ion cleaning gun affords surface chemical analysis by ISS. The target surface structure can be judged from LEED patterns using e.g. a "reverse view" LEED system.

In the "spectroscopy section" a tandem parallel-plate electron-energy analyser [122] is mounted on a rotable feed through such that angular-resolved energy spectra can be measured in an angular range from 0 to 180 °. The energy resolution of the spectrometer as used here is AE/E = 2.5%. The energy resolution has been checked using an electron gun and measuring the elastic scattering peak as a function of the primary electron energy. The spectroscopy section is screened against magnetic fields by an inner [J-metal chamber. The field inside the section is of the order of 10raG.

Fig. 10(a) shows a low resolution electron spectrum obtained for Ne 9+ incident on a solid surface (we remark that various spectra obtained with higher resolution will be presented in Section 4). The spectrum shows two pronounced peaks due to the emission of LMM- and KLL-Auger electrons superimposed on a background of continuous electrons. A kinetic electron background was estimated as shown in the upper part of the figure. The curve is chosen so that it follows the continuum background in the energy region from 0 to --~ 400eV. At higher energies the curve is arbitrarily extrapolated. It is noted that this extrapolation is uncritical as the background is negligible in the region of the Ne KLL-Auger electrons. Only the LM M- and KLL-Auger lines of Ne become visible after background subtraction, as shown in Fig. 10(b).

The measured spectra at this stage contain contributions from the electron transport from the emitting atom through the solid and across the surface barrier [123-126].

In Fig. 10(c) we show an example for the contribution of the electron transport phenomena to the Ne KLL spectrum. For the evaluation Tougaard's model [127-] was adapted to the present situation,

A. Arnau et al./'Surface Science Reports 27 (I 997 ) 113 239 139

10 6

c- 5 10

"~ 104 LU x 10 3

Z Q ~'o ~ 102

101

J

g "-- 6 0 1 - -

x 4

Z ~

0 0

v

' ~ ' . . . ' ' I Ne q+ ~ Sol id ung~nal / . . . . . . . .

NeS+ - _____._~_ ---. ._ -

Substracted and Normalized

200 400 600 800 1000 1200 Energy (eV)

[Ne 9 ~ . Pt(110 measured spectrum

~ ~ t t e r i n g contribution

!il

700 800 900 1000 Lab. Electron Energy (eV)

Fig. 10. (a) Typical electron spectrum obtained with Ne 9+ incidenl on an AI surface. The background mainly due to kinetic electron emission is arbitrarily extrapolated toward higher energies. (b) Spectrum of (a) after background subtraction, showing at 175 eV the LMM- and at 850 eV the KLL-Auger peaks. (c) Example for the electron scattering contribution to the KLL-Auger peak for Ne 9+ impinging on Pt. The shaded area shows the "primary spectrum" [83].

i.e. the electron emitters have a depth distribution which is estimated from the lifetime of the K-shell hole in the moving Ne atoms.

2.5. Detection and spectroscopy of X-rays

The detection of low-energy photons, soft and hard X-rays gives direct information on radiative stabilisation during neutralisation of highly charged ions interacting with the surface and bulk of

140 A. Arnau et al./Sur['ace Science Reports 27 (1997) 113 239

a solid. X-rays have the advantage that there is no energy shift when passing through material, since photons can be absorbed but their spectral distribution is practically not distorted. Background of continuous radiation is virtually non-existent. The emission lines can be measured at high resolution with still good detection efficiency. This allows one to reveal clearly the atomic shells involved and even the population of spectator electrons [128]. A main disadvantage is a generally low and strongly population dependent fluorescence yield and an applicability to inner-shell processes only. This technique has therefore a predominantly complementary character compared to the other detection techniques discussed in this article.

X-ray detection instruments are divided with respect to the detection method and characterised by their spectral resolution, detection efficiency, spectral range, and time resolution. For the studies of MCI-surface interactions the spectral range, resolution, and detection efficiency are most important, the latter one because of the very often low intensity with which MCI with inner-shell vacancies can be obtained. Solid state photon detectors with sufficiently high detection efficiency and energy resolution are often implemented which have a time resolution that even allows coincidence measurements of decay cascades. The range of energy for which X-ray measurements for MC| hitting surfaces were performed, lies typically between 1 and 20 keV. We will only describe here detector types which were used in the studies to be discussed later on (cf. Section 4.1.4).

2.5.1. Semiconductor detectors Diodes of Si or Ge material for X-ray detection are commercially available. The Si(Li) consists of

a Li drifted p-doped silicon wafer which is contacted by few tens of nm gold, In an intrinsic germanium detector an ultra pure Ge wafer is contacted via ion implantation. The diodes have diameters of typically 6-25 ram, which can give a rather large solid angle of a few percent of 4~ at a distance of a few mm from the source point. Both detector types are operated at the temperature of liquid N z. This requires usually a vacuum tight window (mostly of few ~tm Be or polypropylene) which sets the lower limit of the spectral range, given by the absorption of the window and contact layer, at a few keV.

The depletion zone is built up by a potential of around 1 kV. X-rays being absorbed in this zone give rise to electron-hole pairs and lead subsequently to a voltage drop in a resistor which is amplified. The ratio of signal to thermal noise in this resistor and the statistics of the number of electron-hole pairs determine the resolution. At a signal strength far above the noise level the resolution is proportional to xfN and has typical values between 120 and 220eV at the energy of the Mn K~ line, obtained from beta decay of 55Fe, i.e. at K~ = 5.89keV. In the high energy range the detection efficiency is determined by photo absorption in the depletion zone of the diode. It declines above typically 20keV for a Si(Li) and at 80keV for a Ge(I). The striking advantage of semiconductor detectors of covering a large spectral range with high detection efficiency and solid angle was used in the first experiments on MCI-induced X-ray emission [42].

2,5.2. Crystal spectrometers The crystal spectrometer is based on the dispersion property of Bragg reflection by the crystal

planes. Differently bent and plane crystals, source- and detector-geometries determine the design of various types of X-ray spectrometers. In the experiments described here for MCI-surface studies a flat crystal geometry and a linear position resolution gas proportional counter with backgammon

A. Arnau et al./Sur[ace Science Reports 27 (I997) 113 239 141

anode was used [43]. The resolution which can be reached with such spectrometers is in the order of 10 3 and below. A compromise between resolution and detection efficiency has to be found. This is dictated by the system under study. For the case of At 17+, where most of the high resolution X-ray measurements were done, the Lyman alpha lines are at around 3.5 keV. The L-shell satellites are separated by around 20eV, i.e. they can be resolved for a AE/E < 4 x 10-7. Using a flat crystal of graphite with mosaic structure at a resolution of 5 eV, the detection efficiency of such a spectrometer is of the order of 10 -6 of 4~z.

2.5.3. Calorimetric X-ray detectors Calorimetric X-ray detectors provide high-resolution spectra without the bandwidth and sensitiv-

ity limitations of a crystal spectrometer. In a micro-calorimeter X-ray photons are absorbed and thermalised in a detector which is weakly coupled thermally to a cold bath at typically 50 inK. The resulting rise in the absorber temperature is measured with a thermal sensor. In order to make the temperature rise measurably large the material must possess a very small heat capacity. With proper choice of materials, the resolution of a calorimeter at 100 mK could be in principle 1 eV, independent of the X-ray energy. The absorber consists, in practice, of a high-Z superconducting material with low heat capacity, such as Sn. The thermistor must be made as small as possible, in order to keep its heat capacity low. With the system used in an experiment of highly charged Ar ions hitting a Be surface, a spectral resolution of 20eV at a quantum efficiency of 90-100% in a range of about 1 7 keV was reached [129].

2.6. Ion scattering and recoils

The first MCl-surface interaction experiments regarded the study of multiply charged Cu '~+ recoil ions produced by 60 and 90 keV Ar + bombardment [l 30]. In the same experiments the production of multiply charged projectile ions Ar r+ was found. The highest charge states were given by q = p = 4. The vacuum system was pumped to 10 -6 Torr by a baffled mercury diffusion pump. The targets were Cu single crystals with low index (1 0 0), (1 1 0) and (1 1 1) orientations, respectively. The scattered and recoiled ions were analysed using an electrostatic analyser. The next surface scattering experiments involving MCI came 16 years later [45], the difference being that then the primary ions were higher charged. Again an electrostatic energy analyser was used to study the yield of the primary keV Ar q+ ions (q = 1 to 9) after striking a W surface. Here no creation of higher charge state ions was observed, nor were any highly charged recoils reported. The main effects were the observation of strong neutralisation and evidence for the survival of inner-shell vacancies through the surface interaction.

These results have been confirmed in more recent studies using the electrostatic analyser also used for electron detection. For the measurement of scattered ions, i.e. the charge and energy distribution, a new electrostatic analyser has been developed (Fig. 11, [ 131]). It is a two-plate analyser with a flat and a parabolic electrode. The properties of this analyser are calculated numerically. The spec- trometer constant k~ is evaluated thus giving the relations U = ksE. U is the voltage applied across the analyser and E is the ion energy. Fig. 12 shows the angular intensity profile of 90keV N 7+ scattering off Au(1 1 1) at a glancing angle of incidence of 1.5 °. The MARLOWE data are calculated for a perfect, unreconstructed Au(1 1 1) surface. Thermal vibrations are included. Image force effects were included using a sheath potential. The remaining shift between the calculated and measured

142 A. Arnau et al./Surface Science Reports 27 (1997) 113-239

upper plate

/ 1111111 / 1111111 II

I

collirn~Jto~ ',uwer plate chunr~eitr,)r~

Fig. l 1. Ion charge state- and energy analyser for investigation of MCI surface scattering [77].

90 keY N 7+ -> Au(111), ,7=1.5 ° 1 1 0 •

0 , 8 "

E ,.-. 0,6- o i - v

-~ 0,4. ¢-

I- 0,2,

0~0 j ÷

///:/." '".......... :".. --+-• Gaus MARl

'""""O + - - - < + .... +

• Experiment (N 1+) Gaussian (Fit) M A R L O W E

+o+__. I I I !

0 2 4 6 8 10

scattering angle e [°]

Fig. 12. Angular distribution of N + ions scattered at grazing incidence (~ = 1.5 ) from Au(l 1 1), in comparison with calculated data (MARLOWE code). The primary beam is 90 keV N v +. The laboratory scattering angle is 0 = 3.0 _+ 0.1 ~ The experimental data are fitted with a Gaussian [77], the MARLOWE data with a line to guide the eye. The deviation of the experimental data from the calculation at larger scattering angles is due to surface imperfection. The difference between the centroids of the two distributions by 0.2" could be due to image force effects.

distributions of 0.2 ° may reflect the inaccuracy of the sheath potential and also of the ZBL-potential used [77]. The intensity of the outgoing ions decreases towards higher charge states by about a factor 10 with each charge state. There is no evidence in these data as in others [46,132] measured

A. Arnau et al./SmJ~we Science Reports 27 (1997) 113 239 143

under grazing incidence which confirm the earlier results indicating ionisation to higher charge states [130]. Angular distribution can also be measured by simply using a movable diaphragm and a channeltron for particle counting [47,133]. This type of data gives detailed insight into the image charge acting on MCI's scattering from surfaces and also yields the distance above the surface at which electron capture starts to occur.

When combining a movable slit, deflection plates and a position sensitive detector [46] the charge state distribution and the angular distribution of the scattered ions can be measured. One advantage of this scheme is that the neutrals can also be measured. At several keV kinetic energy the neutrals have in fact the highest yield (measurements with O +'j, q = 3 to 8, and 3.75 keV/amu and Ar ~1+ at 2keV/amu). These results are also obtained at grazing incidence, i. e. under surface channelling condition. An interesting aspect is the observation of negative ions, e.g. O , which is a clear indication for complete neutralisation of the incident MCI [46,132]. Further channelling results are presented in Section 4. When using a large scattering angle and aligning the incident beam parallel to a low index direction the interaction potential of the MCI and the target atoms can be derived by measuring the backscattered ion intensity versus the angle of incidence [-77~134], with first results for MC1 having been reported recently [1353.

2.7. MCl-induced sputtering and secondary ion emission

For sputtering of insulators with MCI an experimentally determined increase in secondary ion emission with the primary ion charge state [55] has been explained by the Coulomb explosion model [51]. In more recent investigations, secondary ion emission induced by MCI as well as the total sputter yield for different target materials like metals (Au), insulators (LiF, NaC1, MgO, SiO 2) and semiconductors (Si, GaAs) [48,58,59] have been studied. The experimental set-up has been described in detail in [54,59]. The beam was well defined in mass, charge state, geometry (FWHM 0.5 mm at 500eV) and kinetic energy (FWHM 2 3eV at 500eV). It was scanned over the target area to get a homogeneous irradiation. The kinetic energy of the primary ions was varied between 5 and 500 eV to keep kinetic effects as low as possible, for finding out the role of potential energy in sputtering. The MCI current density was below 10 nA/cm 2. The residual gas pressure in the target chamber was always in the low 10 ~0 mbar range.

2.7.1. Secondary ion emission The determination of secondary ion emission yields was performed simply by a quadrupole mass

spectrometer (VG, SXP 800) which could be used for positive and negative ions. With this arrangement relative ion yields can be determined (counts per primary ions) for a fixed geometry. To determine the absolute secondary ion yields an estimate of acceptance geometry, based on the quadrupole solid angle of acceptance and assuming a cosine angular distribution of secondary ions has been performed. From the accuracies of primary and secondary ion current measurements, and by taking into account possible different ejection characteristics, a conservative estimate leads to total errors of the absolute secondary ion yields of _< 50%.

In order to study the influence of electrostatic charges on the emission of secondary ions from LiF, we have used all three methods for avoiding surface charge-up as described in Section 2.2. In particular, measurements on insulators, for which the target temperature had to be kept low, have been performed by flooding the target with low energy electrons.

144 A. Arnau et al./Sur['ace Science Reports 27 (1997) 113 239

Two different LiF targets have been used, a LiF(1 00) single crystal and thin films of LiF. The targets could be heated and bombarded with low energy electrons (electron flood gun). At room temperature, the LiF(100) crystal became charged-up to such an extent that it acted as an electrostatic mirror for the ion beam. By increasing the temperature, charging-up started to decrease. At a temperature of more than 350°C no more change in the yield of the emitted positively and negatively charged particles could be observed. All data taken for LiF(1 00) have therefore been measured at a temperature of 400°C. At this temperature LiF is already a good ionic conductor and a stoichiometric surface is restored because radiation induced lattice defects will be annealed.

Heating experiments have also been performed with thin LiF film targets. The emission of Li +, F + and F- was measured at different target temperatures (20°C and 400°C) with or without electron flooding. The yields of above species might be influenced by surface charging-up as well as by changes in surface composition, the latter resulting from different sample treatments. Great care is needed to avoid errors due to such influences.

2.7.2. Total sputter yield of thin films measured by the quartz micro-balance Whereas quartz crystals are widely used for determination of the area mass and hence the

thickness of deposited material, the rate for material removal has mainly been studied with other techniques such as the conventional micro-balance and catcher foils analysed by Rutherford back scattering. This is not astonishing, since the use of quartz crystals for sputter yield measurements encounters severe problems. The rates of material removal and hence the frequency changes are rather low compared to most deposition applications, requiring high frequency stability of the crystal and of the oscillator circuit as well as high accuracy and resolution of the frequency measurement. Furthermore, a substantial amount of energy is deposited by the primary particles on the sputtered surface, causing problems due to thermal drift. In many deposition applications, the energy deposition per incident atom is only a few eV (sublimation energy plus heat radiation from the evaporation source), while in our case the energy deposited per sputtered atom is rather