Metallic Surfaces and Thin Films

Clearly, the most prominent imperfection in a crystalline solid is its surface, since it represents a cutoff of the lattice periodicity. The surface can be defined as constituting one atomic-molecular layer. This definition is sometimes not particularly useful, however. lu certaiu cases the system or property of iuterest requires that additioual layers be cousidered as the surface.  

Fundamentals of Electrochemical Deposition, Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

At this point it is natural to wonder how the type of information discussed above is obtained. It is possible to group the analysis techniques for the characterization of metal surfaces and/or thin films into three major categories  

As representative techniques of the second group, we discuss two methods x-ray photoelectron spectroscopy (XPS), sometimes referred to as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). The main principle of the first method (XPS) is the excitation of electrons in an atom or molecule by x-rays. The resulting electrons carry energy away according to the formula 

The other method. Auger electron spectroscopy, is considered appropriate for studying the chemical makeup (composition) of surfaces, with a sensitivity down to 1% of a single atomic layer (monolayer). It is also easier to perform than many other methods of surface studies of the present group. It is based on the principle that if an 

A large number of chemical reactions are made possible by surface atoms of heterogeneous catalysts. Those are in small particle form. This includes catalysts to produce high-octane fuels, for instance. 

When adsorption takes place on an ordered metal-crystal surface, the adsorbed material forms ordered surface structures. The root cause of this is in mutual atomic interactions, which may be categorized into adsorbate-adsorbate and adsorbate-substrate interactions. In case of chemisorption, the former is considerably the weaker of the two. The possible long-range ordering of the overlayer formed is dominated by adsorbate-adsorbate interaction, however. Ordering of the adsorbed material is also dependent on the degree of surface coverage. Thus, for instance, at 

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Reconstructed clean metal surfaces may thus be interesting templates. However, their given structure does not provide the experimentalist with a control on the nature and physical dimensions of the nanostructure. This implies that these surfaces can be suitable candidates for the investigation of the basic properties of templates on an atomic scale but are less interesting in terms of appHcations. To overcome this limitation systems are needed for which the pattern formation by self-organization can to some extent be controlled by the experimentaUst. This leads us to two promising systems stepped metal surfaces and thin films on metal surfaces. 

In recent years, metal nanoparticles and thin films supported on oxides have become fundamental components of many devices as their small dimensions present structures with new chemical and physical properties, often enhancing the reactivity of these surfaces relative to their bulk counterparts. Numerous theoretical and experimental studies show that the metal particle size and shape as well as direct adsorbate interactions with the oxide support can each play a key role in enhancing the reactivity of these surfaces. Further investigations imply that the support material may... 

However, continuing development of spectroscopic sampling techniques in recent years has increasingly led to the use of non-metallic surfaces as thin-film substrates, and the structure of the adsorbed film on these surfaces is now being elucidated via spectroscopic techniques. Included in this group is the class of... 

Nanoindentation techniques were used to determine the hardness of Cu, Ta W metal discs and thin films on silicon substrates as a function of load or indentation depth. Cu films exposed to oxidizing solutions containing H2O2 exhibited a higher hardness at the surface while no such change was observed for W exposed to ferric nitrate. The implication of these measurements and their relationship to chemical-mechanical polishing rates are discussed. 

As we mentioned, oxide surfaces are important in the field of nanocatalysis by supported metals. In practical applications, the support has the crucial role of stabilizing small metallic particles, which act as the actual catalysts in a chemical process. Once the oxide surface is sufficiently well characterized, one can deposit small metal clusters and study their reactivity as a function of the support, of the metal, of the size of the cluster, etc. In this way, complex catalytic processes can be divided into a series of substeps, which allow a more detailed microscopic characterization. Despite the fact that only recently well-defined metal clusters have been deposited under controlled conditions on oxide surfaces and thin films, great advances have been obtained in the understanding of the mechanisms of adhesion and growth of the metal particles to the oxide surface. In this process, the role of theory is quite substantial. 

Effects of intense ion beams and plasma flows on metal and semiconductor surfaces are the most prospective directions of recent studies in materials science. Formation of superfine metal coatings and thin films on the treated surface was observed during metal injection into the plasma. The structure and composition of the deposited coating determine its unique physical and chemical properties [1]. 

While silica is probably the most frequently encountered oxide surface, other materials particularly alumina, titania and zirconia also have considerable use and spectroscopic characterization is beneficial. One study mentioned above explored the potential for modifying alumina, zirconia, titania and thoria surfaces in a marmer similar to silica [6]. While bonding of the moiety is usually the method of choice, in some cases adsorption is sufficient. For example, polyacrylates adsorbed on alumina are a useful dispersent in the production of certain ceramic products. The successful adsorption of these compounds on aluminia has been monitored by FTIR using the carbonyl stretching frequency for the acrylate species which appears between 1602-1606 cm [15]. Polybutadiene which has been adsorbed on alumina for use as a chromatographic phase can be detected by FTIR [16]. A similar adsorption process has also been tested on zirconia [17,18]. It has also been shown that FTIR can be used to detect Langmuir-Blodgett layers on the metal surfaces of thin-film devices [19]. 

The physics of surface plasmons propagating along a metal/dielectric interface has been studied intensively, and their fundamental properties have been found to be in good agreement with theoretical concepts based upon the plasma formulation of Maxwell s theory of electromagnetism. The phenomenon has been utilized extensively by physical scientists in studies of the properties of surfaces and thin films. Current interest in the properties of thin surface coatings stems partly from increased applications to thin film devices and, in particular, to recent developments in biosensor devices. This article focuses on the characterization of the surface plasmon resonance phenomenon, with emphasis on the conditions of optical excitation of plasmon resonance and the theoretical analysis of different types of surface resonances. 

For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most usehil techniques for studying the optical proponents of the bulk (semiconductors or metals) and surface (semiconductors) of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors. Modulation Spectroscopy is gainii in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 

The 10 volumes in the Series on characterization of particular materials classes include volumes on silicon processir, metals and alloys, catalytic materials, integrated circuit packaging, etc. Characterization is approached from the materials user s point of view. Thus, in general, the format is based on properties, processing steps, materials classification, etc., rather than on a technique. The emphasis of all volumes is on surfaces, interfaces, and thin films, but the emphasis varies depending on the relative importance of these areas for the materials class concerned. Appendixes in each volume reproduce the relevant one-page summaries from the Encyclopedia and provide longer summaries for any techniques referred to that are not covered in the Encyclopedia. 

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

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