Ion Beam Analysis : a Century of Exploiting the Electronic and Nuclear Structure of the Atom for Materials Characterisation

Subsequently published by: Reviews of Accelerator Science and Technology Vol. 4 (2011) 41-82 (World Scientific Publishing Company) C. Jeynes, University of Surrey Ion Beam Centre, 22 nd September 2011 ION BEAM ANALYSIS : A CENTURY OF EXPLOITING THE ELECTRONIC AND NUCLEAR STRUCTURE OF THE ATOM FOR MATERIALS CHARACTERISATION CHRIS JEYNES University of Surrey Ion Beam Centre Guildford GU2 7XH, England [email protected] ROGER P. WEBB University of Surrey Ion Beam Centre [email protected] ANNIKA LOHSTROH Department of Physics, University of Surrey [email protected] Analysis using MeV ion beams is a thin film characterisation technique invented some 50 years ago which has recently had the benefit of a number of important advances. The review will cover damage profiling in crystals including studies of defects in semiconductors, surface studies, and depth profiling with sputtering. But it will concentrate on thin film depth profiling using Rutherford backscattering, particle in- duced X-ray emission and related techniques in the deliberately synergistic way that has only recently be- come possible. In this review of these new developments, we will show how this integrated approach, which we might call " Total IBA ", has given the technique great analytical power. Keywords : RBS, EBS, PIXE, ERD, NRA, MEIS, LEIS, SIMS, IBIC. 1. Introduction gies, including the semiconductor, sensor, mag- netics, and coatings industries (including both Ion beam analysis is a very diverse group of char- tribology and optics), among others. It is also acterisation techniques which have been applied to valuable in many other disparate applications such every class of material where the interest is in the as cultural heritage, environmental monitoring and surface or near-surface region up to a fraction of a forensics. We will be describing examples in many mm in thickness. Such a field is far too broad to be of these areas in our review of MeV-IBA. reasonably covered by such a review as this; we Historically, IBA labs have tended to split into will concentrate on thin film elemental depth (at least) two "traditions": on the one hand nuclear profiling methods using ions with energies of order methods (RBS, ERD, NRA), and on the other 1 MeV/nucleon. We will emphasise complemen- atomic methods (PIXE). We will outline various tary techniques, including some other closely reasons for this, but will show that recent advances I related IBA methods. have facilitated the integration of these two tradi- Thin film elemental depth profiling is of critical tions giving us what is effectively a new and much importance to a wide variety of modern technolo- more powerful technique. Curiously, this review seems to touch on all the th I see the end of the chapter for expansion and explanation of main breakthroughs in 20 century physics, and all acronyms. Appendix A is a glossary of IBA techniques, and IBA techniques hinge on spectroscopy. So perhaps Appendix B is a glossary of related techniques. we should start by acknowledging our debt to Isaac 1 2 Jeynes, Webb & Lohstroh Newton, who in 1671 was the first to use the word ants)IV , the scanning probe microscopies (AFM "spectrum " with the modern connotation of quan- and variants including the new optical near-field tum phænomenon observed with a dispersive methods), X-ray techniques like XRF and XRD mechanism [1] II . Previously (and subsequently) (also with many variants) and optical methods like "spectrum" had the connotation of "spectre" or ellipsometry, Raman, FTIR and other spectro- "apparition". But philosophers see reality. scopies. Elemental depth profiling can be done destructively using sputtering techniques with 1.1. Scope of chapter SIMS (or, frequently, AES). If destructive techniques are considered then bulk methods like Although concentrating on MeV IBA depth profil- ICP-MS and AMS should be mentioned, and of ing, we will mention methods of depth profiling course there are a wide variety of wet chemical crystallographic defects using channelling and analytical methods. XRF and XRD are frequently MEIS; characterising defects in semiconductors, applied to "bulk" as well as thin film samples, and including their spacial distribution, by IBIC; and other comparable fluorescence techniques are depth profiling using sputtering by SIMS (includ- cathodoluminescence or photoluminescence. ing MeV-SIMS), although we will not discuss Molecular imaging can already be done in air by beam damage extensively. We will also mention MALDI, DESI and DART. ion beam methods sensitive to the true surface Where does IBA fit in this kaleidoscope of tech- (MEIS, LEIS) since they also use RBS. niques? IBA typically uses an accelerator which We will not specifically review microbeam ap- needs a hall of at least 200m 2, a footprint well over plications, taking microbeam technology for an order of magnitude larger than any of the other granted throughout the text. But it will be clear techniques mentioned – it is necessarily a technique that we think that 3D spacial resolution is central to with high running costs. What can it do which the general usefulness of IBA. cannot be done reasonably easily by other techniques? If a materials research organisation (for 1.2. Complementary techniques example, a University) were to set up a central Throughout, we will also mention complementary analytical laboratory to service the needs of all its techniques wherever appropriate. Materials analy- research groups and other collaborators, would sis must be a strongly interdisciplinary field, and IBA be one of the techniques considered "essen- the characterisation problems of modern materials tial"? almost invariably require the use of a variety of We believe that modern integrated IBA meth- techniques for their solution. Any discussion of a ods are exceptionally powerful for a wide range of technique without the context of complementary materials problems, and we will show a number of techniques is likely to be strongly misleading. significant examples that exemplify this. In all these fields the analyst has various stan- 1.3. Overview of chapter dard tools: the electron microscopies and spectro- scopies III (SEM, TEM, XPS, AES and their vari- In §§2, 4 we will summarise the nuclear and atomic IBA techniques we will be concentrating on, and II Using Newton's spelling of "phænomenon" which transliterates in §6 we will address their integration. §3 will the Greek φαινοµενον . In his use of "spectrum" Newton had in mind the Latin etymology of specere , to see. III "spectrometry" or "spectroscopy"? A spectrum is the object lines has shifted to different frequencies. However, the boundary that results from some dispersive process: we do spectrometry between spectroscopy (as in XPS) and spectrometry (as in RBS) is where we measure the spectrum and spectroscopy where we look ill-defined, and it is mostly a conventional distinction. at the spectrum. The Hubble red-shift for example is a spectro- IV see Appendix B for a glossary expanding and explaining scopic effect since the recognisable pattern of atomic absorption acronyms for complementary characterisation techniques. Ion Beam Analysis 3 briefly describe the other IBA techniques, for a tors were available, with the first paper by Georges more complete overview of the field. In §5 we will Amsel on Si diode detectors in 1960 [19] and Turke- discuss the important issue of computer codes used vich's immediate proposal for the Surveyor Moon in IBA, together with a further discussion of the mission in 1961 [20] with the report in 1967 [21]. high accuracy available with these methods. Explicit depth profiles were not published until 1970 A general introduction to the field must mention [22]. the three IBA Handbooks published over the last 35 years. The 1977 "green book" [2] includes a sec- 2.1. Energy loss tion on PIXE which was dropped from the more It was obvious to all the early workers that the extensive "black book" from 1995 [3]. PIXE is energy loss of scattered particles represented depth restored in the recent 2-volume Handbook [4]. in the samples, and the famous Bragg rule (1905) There have been an almost unbroken series of [23] obtained the compound stopping power for a biennial IBA Conferences, the first of which was fast particle from a linear combination of elemental held in 1973 [5]. Currently the most recent pub- stopping powers. lished Proceedings are from the Cambridge confer- To interpret IBA spectra in general it is essen- ence in September 2009 [6]: the Proceedings are tial to have energy loss ("stopping power") data for not yet available from the Brazil conference of the whole periodic table and all the ion beams of April 2011. There is also a triennial PIXE confer- interest. This is a massive task both of measure- ence series, the latest of which was in Surrey in ment and of evaluation against a theoretical model. 2010 [7]. Other useful recent reviews include The measurements are difficult to make and the Giuntini (2011) on the use of external beams [8]. model enables both a valid comparison between different sets and also extrapolation to materials or 2. Nuclear IBA depth profiling methods beams for which measurements are not available. Large angle ion scattering was first observed in Happily this has been done, with comprehensive Geiger & Marsden's experiments in 1909 [9], which stopping power databases now available from Jim were interpreted by Ernest Rutherford in 1911 to Ziegler's SRIM website [24] [25] [26]. Helmut demonstrate the existence of the positively charged Paul has also recently reviewed this field with atomic nucleus [10]. The transition from RBS to references to other compilations (H.Paul [27], EBS as the Coulomb barrier is exceeded was first MSTAR, ICRU…) [28]. demonstrated by Chadwick & Bieler in 1921 (for The Bragg rule is an approximation that clearly alphas on H [11]).

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