Surface Analysis Methods in Materials Science

Awareness of the important role played by surfaces in technology has existed for some time, although it is only in the past three decades that we have been able to establish an improved understanding of their properties. In everyday life our perceptions of solid materials, and in particular their surfaces, are strongly distorted by the limitations of visible light. These wavelengths are a thousand times larger than dimensions of the surface region in which well understood bulk properties of materials break down, making way for the transitional interface with another phase, which may be gaseous, liquid or solid. Such are the alteration of bulk properties, structural and compositional, that it is not unreasonable to consider surfaces as an additional phase of matter [2]. Whilst this may serve as a useful general concept for surface scientists, in the various fields of technological endeavour what is thought of as a surface varies enormously, particularly in depth characterisation. Accepting the simplest definition of the surface, as the boundary defined by the outermost atomic layer separating the bulk solid from an adjacent phase, is thus inadequate in the area of practical surface technology. A more meaningful approach is to consider a selvedge layer of variable depth. In fact, the different depth regimes of the surface are defined by that depth which actually plays the definitive role in the technological application (see Table 1.1)..

Modern surface analytical methods over the last three decades have been dominated by those requiring ultrahigh vacuum (UHV) chambers in which to carry out the analyses. This is not a universal requirement for surface analysis and several of the techniques such as Fourier transform infrared (FTIR), scanning tunnelling microscopy (STM) and ellipsometry do not have a mandatory vacuum requirement. Vacuum is of course required by those techniques utilizing beams of particles and higher energy radiation so that the beams may be generated and travel undisturbed until intercepting the surface. The requirement for UHV or vacua of ≦ 10^-10mbar (10^-8Pa) is fundamental to surface analysis when those beams are employed. This arises due to the flux of residual gas molecules striking the surface i.e. the number of molecules per unit area per unit time, which is responsible for the pressure that those gas molecules exert upon the surface. By knowing the pressure the flux can be evaluated. From the kinetic theory of gases [1] the molecular flux Zis given by the Herz-Knudsen equation: where N/V is the number of molecules per unit volume and c is the average speed of the molecules.

Electron microscopy in its various forms has developed over the past fifty years into one of the major techniques of materials science. Surface analytical techniques are more recent additions to the materials scientists’ range of experimental methods for learning about materials properties. Microscopy and spectroscopy are complementary, and the use of one alone can result in an inadequate characterization of a material. In this chapter we will consider the various modes of electron microscopy and attempt to demonstrate the critical importance of applying electron optical imaging together with surface analytical techniques in studies of surfaces.

The understanding and modification of surface properties of materials often requires a detailed knowledge of the spatial distribution of specific elements in both the surface plane and as a function of depth normal to the surface. This chapter will concentrate on ways of analyzing depth distributions of elements, up to a few microns deep, using ion beam sputter profiling. Sputter profiling uses the combination of a surface sensitive analytical technique, such as SIMS, LEIS, AES, XPS or SNMS [1], together with the continuous exposure of a new surface by ion beam sputtering. The sputtering ion beam typically is Ar⁺, (math) or Cs⁺ with energy in the range 0.25–20keV, although other inert gas (Kr⁺ or Xe⁺), liquid metal (Ga⁺ or In⁺) or, more recently, cluster (math) ion sources are also used. Ar⁺ is most commonly used with AES and XPS since it does not form compounds with target constituents and so does not significantly alter bulk atomic concentrations. (math) and Cs⁺ are favoured for SIMS since they increase secondary ion yields and reduce matrix effects. However, this is at the price of reduced sputter rates and profile distortion though compound formation and segregation. Liquid metal sources are normally used for imaging. Profiling rates of up to 2 μm/h can be achieved although 0.1 μm/h is more typical.

Secondary ion mass spectroscopy (SIMS) is an ion beam analysis technique useful for characterising the top few micrometres of samples. Primary ions of energy 0.5–20 keV, commonly O ₂ ⁺ , Cs⁺, Ar⁺ but also ions such as Ga⁺, Xe⁺, O^-, C _{m m} ⁺ and SF ₆ ⁺ are used to erode the sample surface and the secondary elemental or cluster ions formed from the target atoms by the impact are extracted from the surface by an electric field and then energy and mass analyzed. The ions are then detected by a Faraday cup or electron multiplier and the resulting secondary ion distribution displayed as a function of mass, surface location or depth into the sample (Fig. 5.1).

Auger electron spectroscopy (AES) is one of the most commonly used surface analytical techniques available to the materials scientist. It has the ability to measure the chemical composition of the first few monolayers of a given surface with a sensitivity of the order of 0.1 atomic % and a spatial resolution of the order of 10nm [1]. Its ease of interpretation means that AES is often the analysis technique of choice and has been used to study a wide range of different materials.

The detection and energy analysis of photoelectrons produced by radiation whose energy exceeds their binding energies is the subject of an extensively-used technique known as Photoelectron (PE) Spectroscopy. This technique can be conveniently divided into two broad areas, the first employing ultraviolet radiation, hence called Ultraviolet Photoelectron Spectroscopy (UPS), and the second using X-rays, termed X-ray Photoelectron Spectroscopy (XPS). The latter spectroscopy is the subject of this present chapter, while UPS is discussed in Chap. 14.

There are many forms of vibrational spectroscopies applied to surfaces but the most accessible techniques are Fourier transform infrared spectroscopy and Raman spectroscopy. They will be discussed in this chapter. Other vibrational spectroscopies (e.g. PAS, EELS) are listed in the Appendix and Bibliography.

The RBS (Rutherford Backscattering Spectrometry) and NRA (Nuclear Reaction Analysis) are a subset of what is generally known as ion beam analysis (IBA) methods, performed with energetic (typically a few hundred keV to a few MeV) ion beam from accelerators. The energies involved are such that the interaction between the projectile and the target is insensitive to molecular or atomic shell effects, and the methods are thus thus not suitable for measurements of chemical effects. The methods are non-destructive, and provide the elemental composition and/or structure, namely the depth profiles from the surface region spanning the first few hundred layers of atoms to a few microns depth. Other main advantages are the rapidity of analysis (few minutes), and the direct and simple way the information can be obtained from the data. The methods are amenable to simple calibration procedures to facilitate quantitative, standardless analysis. RBS is based on elastic Coulomb scattering between the projectile and the target nuclei, and is usually applied to obtain data for most if not all elements present in the specimen. In contrast, NRA is based on nuclear reactions which are element specific. The most commonly used beam in RBS is ⁴He (alpha particles) with 1–4 MeV energies. Protons are also used for RBS, typically with energies between 100 keV and 2 MeV, but the beam is more suited for NRA applications.

The STM (see list of acronyms at end of book) was invented by Binnig et al. in 1982 [1]. The two main protagonists, G. Binnig and H. Röhrer, were subsequently awarded the Nobel Prize for physics. Thus began the age of SPM. Much of the early development and excitement generated by the unequivocal demonstration of spatial resolution on the scale of the single atom and of local spectroscopies have been described in the literature [2–4]. The rationale for including a chapter on SPM in a book on surface analysis can be inferred from Fig. 10.1 (adapted from Röhrer [5]). The impact of SPM in the broad field of surface and interface science and technology can be illustrated by its prominence at a conference in Birmingham in September 1998, which brought together a representative cross-section of the international surface science community through the 14th International Vacuum Congress, 10th International Conference on Solid Surfaces, 5th International Conference on Nanometer-scale Science and Technology and 10th International Coference on Quantitative Surface Analysis. Of some 1350 invited and contributed papers approximately 20% were based substantially on SPM techniques and methodologies, while the corresponding indices for the ‘traditional’ techniques and ‘other’ were 34% and 46%, respectively.

Low energy ion scattering (LEIS) is the study of the structure and composition of a surface by the detection of low energy (100 eV-10 keV) ions (and atoms) elastically scattered off the surface. This technique is a subset of ion scattering spectrometry which involves the use of incident ions with energies ranging from 100 eV to over 1 MeV. The range of measurements possible over such a large range of energies extends from purely atomic layer resolution at the low energy end to analysis to depths of the order of microns at the high energy end. Some of the high energy effects (> 250 keV) are covered in Chap. 9, while the intermediate energy range (medium energy ion scattering) has been successfully developed as a near surface structure probe mentioned briefly in Chap. 1. The use of low energy ions to measure the surface structure of solids was established by Smith [1] in 1968. In that study the basic elements of LEIS were established and these have been built on over the past 20 years to develop into a powerful surface atomic layer structure and composition probe [2–6]. It has been successfully applied to a wide range of practical surface problems which include the surface composition analysis of:

Reflection high energy electron diffraction (RHEED) was first used in the study of a cleaved calcite crystal by Nishikawa and Kikuchi in 1928 [1]. They observed diffraction spots, and lines attributed to diffuse scattering. Other early workers included Germer (1936) [2] who took diffraction patterns from galena and Miyake (1936) [3] who examined oxide surfaces. Uyeda et al. [4] used RHEED with metal films in 1940 and with adsorbed organic molecules in 1950. Commercial RHEED equipment was developed through the 1950s but this operated at 10^-4 to 10^-6 Torr and hence only on dirty surfaces. However its application was as an alternative to X-ray diffraction: RHEED’s forward scattering nature and comparatively high scattering cross section (10⁸:1) made it competitive. As ultra high vacuum equipment became common in the 1960s, systems were equipped with RHEED guns, but good LEED was then possible, and LEED largely displaced RHEED as a diffraction technique for clean single crystal surfaces. The main reasons for this neglect was that RHEED gives quantitative results only on extremely flat surfaces which were not easily prepared and offered no theoretical advantages over LEED in the central surface science question of the surface atomic structure. Also the LEED apparatus was easily constructed and much cheaper than the traditional magnetically focussed RHEED guns. A good review of RHEED and LEED before 1970 is given by Bauer [5].

One of the most powerful techniques available for surface structural analysis is low energy electron diffraction (LEED). It is widely used in materials science research to study surface structure and bonding and the effects of structure on surface processes. However because it usually requires single crystals and ultrahigh vacuum conditions it has limited value for applied surface analysis, which is often concerned with polycrystallme or amorphous materials. LEED has many similarities to X-ray and neutron diffraction but it is preferred for surface studies because of the short mean free path of low energy electrons in solids.

Compared to X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS) is not generally considered to be an analytic technique for the surface characterisation of materials. It is, however, an extremely surface sensitive technique where even a monolayer coverage of an adsorbate or contaminant is sufficient to grossly alter the signal from a given surface. As we shall see, its main strength lies in its unique ability to explore the electronic structure in the conduction/valence band region of a wide variety of solids. As a technique, it can readily be added to other surface science instrumentation and is indeed often offered as an option by manufacturers of XPS/AES equipment. This chapter is consequently included as a brief introduction to the capabilities of UPS, to alert practitioners of other surface science techniques to the information contained in UPS spectra.

X-ray Absorption Fine Structure (XAFS) spectroscopy is an important technique for determining the local structure around an absorbing element in a sample. It is normally divided into two techniques: Extended XAFS (EXAFS) describing the fine structure more than about 50 eV above an absorption edge, and near edge structure or XANES. In an X-ray absorption spectrum, a series of oscillations in the measured absorption coefficient can be observed on the high-energy side of an absorption edge. These oscillations are due to the fact that the final state wavefunction of the emitted photoelectron consists of an outgoing part and a part that is scattered from neighbouring atoms. The EXAFS oscillations, as they are known, correspond to an interferogram of the spatial distribution of nearby atoms and can be analysed to provide structural information about the local environment of the absorbing atom such as bond length, the number and type of neighbouring atoms, bond angles and a measure of order/disorder and/or chemical lability. EXAFS typically gives very accurate interatomic distances, in particular for nearest neighbours, of the order of ±0.02 Å or better. Achieving precision in the determination of coordination number is more difficult and errors as large as 20% in the fitted result are common. Careful choice of suitable standard compounds with known structure followed by EXAFS analysis, however, can greatly improve this determination.

In materials science and technology applied to minerals, ceramics and glasses, surface analysis is used in three principal modes: problem solving in quality control for existing processes and materials; materials characterisation after adsorption, surface coating, reaction or modification; and development of new materials or processes. We can illustrate each of these modes with a few examples. In problem solving, difficulties with control of contamination, coating or adsorption chemistry, adherence (e.g. delamination), discolouration and changes in surface reactivity are common. Characterisation includes the rapidly expanding industry of surface engineering of ceramic and glass layers for corrosion and wear resistance, alteration of surfaces for composite (e.g. polymer) compatibility and mineral surface weathering. The last area encompasses long-term projects in processes as diverse as minerals separation to bioceramic design for materials as diverse as clays, nuclear waste solids and superconductors.

Solid catalysts are the basis of many important industrial processes. Reaction of gases or liquids to form particular products occurs at specific sites on the catalyst surface. The structure and composition of the catalyst surface is critical in determining the reactivity and selectivity of a catalyst. The techniques of surface analysis provide the means of characterizing a catalyst in terms of the actual composition and structure of the surface rather than by its bulk properties. The objective of such studies is to provide a scientific basis for improving catalyst formulations and understanding the processes of activation and deactivation which the catalyst undergoes. Supported catalysts, the type most widely used in industry, consist of an active component dispersed on the internal surface of a porous inorganic oxide. High area solids and light loadings very highly dispersed are often employed to maximize the catalytic activity of expensive components. Metals and oxides may be formed on a support by decomposing or reducing a salt which has been introduced by solution impregnation. For studies of model catalysts under UHV conditions metals can be deposited in situ on to oxide surfaces usually formed on metal substrates. The preparation, pre-treatment and actual catalytic reaction conditions may result in reaction between the components including the support. It is the purpose of the surface analysis to reveal these processes.

Semiconductor devices form the basis of nearly all applications in electronics and optoelectronics. Silicon integrated circuits (ICs) containing a range of advanced logic devices are dominant in the electronics industry [1]. Other commercial electronic devices based on substrates of III-V compound semiconductors include high electron mobility transistors, heterojunction bi-polar transistors and integrated circuits. Examples of commercially available optoelectronic devices in III-V semiconductors include light emitting diodes and multi-quantum-well lasers. For military and industrial markets, arrays of infrared diodes and detectors are fabricated using Hg_1-xCd_xTe or equivalent bandgap II–VI semiconductors. Recent developments in semiconductor devices include the field of silicon nanoelectronics in which Si/SiGe/SiO₂ structures are used to fabricate single electron metal-oxide-semiconductor transistors with nano-scale gate lengths. The performance of all of these devices is critically dependent on the integrity of the fabrication process in terms of their dimensional accuracy, the control of interfacial reactions and high purity of the component materials. While electrical measurements on the devices can provide indirect data on these material parameters, their characterisation also requires direct information from the range of surface analytical techniques.

Metals, in general, critically depend on their surface oxide scales for environmental stability, particularly in aggressive oxidising atmospheres at high temperatures. The protective capabilities of oxides are dependent on many physical and chemical properties, as well as on their mechanical adherence to the metal surface. In summary, an “ideal” protective oxide would be:

Metal manufacturing and metal goods figure prominently in the industrial applications of surface analysis [1]. This is not surprising when one considers the enormous surface areas being produced each year, for example, in the sheet metal industry. Such sheet metal products undergo complex multistage industrial processing much of which interacts with the particular surface exposed during each process; and the performance and appearance of these products are judged, at least in part, by surface features, such as surface flatness, corrosion resistance, color, gloss, etc.

The increasing importance of thin films for new technologies has encouraged fundamental and applied research on their physical and chemical structures and on the interfaces made with them [1]. Their physical structures (e.g. morphology, topography, crystallite properties, extent and type of defects) are explored by diffraction and microscopic techniques including X-ray and electron diffraction, scanning tunnelling microscopy and ultrasonic microscopy. Their chemical structures (e.g. element type, concentration and spatial distribution) are explored by microanalytical techniques such as Fourier transform infra-red spectroscopy, secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ion scattering spectroscopy (LEIS, HEIS), as well as dispersive X-ray analysis and electron energy loss spectrometry with scanning and transmission electron microscopy. As an ultimate objective, researchers desire a three-dimensional elemental map on an atomic scale for the thin film and its interfaces. Some progress towards that aim has been made, but achievement is some time away.

Adsorption at a solid surface is the initial step in many heterogeneous processes. The reactivity of solids, corrosion, inhibition, catalytic reaction and some methods of separation depend on adsorption. The process may involve specific chemical interaction with surface sites and, in many cases, results in dissociation of the adsorbate molecule. The formation of the initial monolayer needs to be understood in detail in order to explain the behaviour of adsorbent materials in contact with gas or solution.

The properties and composition of polymer surfaces play an important role in a number of modern applications of polymeric materials, such as wetting, printing, adhesive bonding, membranes, and biomedical devices [1]. Interfacial interactions govern, for instance, the adsorption and denaturation of proteins on the surface of a biomedical implant or a membrane; adverse interactions give rise to biocompatibility problems and membrane fouling [2]. The surface composition of polymeric materials and the resultant interfacial forces must be carefully optimized for such applications; surface analytical methods occupy a key role in such optimizations and in general in the research and development of novel polymeric materials designed to possess specific surface properties [1—4]. Many of the analytical methods detailed elsewhere in this book are well suited to such work since their probe depths are of a similar magnitude to the thickness of the surface layers that interact with the “environment” (e.g. water, ink, adhesives, and biological fluids)

Glow Discharge Optical Emission Spectrometry (GD-OES) is a well established technique for surface and interface characterisation [1]. It was first introduced by Grimm in Germany in 1967 [2]. In the beginning it was only used for bulk analysis of solid conducting samples, but quickly the field of application was widened to include qualitative depth profiles of surfaces and coatings. French, German and Japanese teams of industrial researchers were very active in this field in the 1970s and 1980s, leading to the first methods for quantitative depth profiling [3]. The next big step forward was the development of the radio frequency (rf) version of GD-OES by Richard Passetemps at Renault in France and Ken Marcus at Clemson University in USA, allowing the analysis of insulating as well as conductive solid materials [4–6]. In recent years the technique has also found application in the semiconductor manufacturing process.