8.2.1 Vacuum-and plasma-based techniques: basics of evaporation and sputtering

Evaporation is a very well-known technique for making thin films. It is in constant use in research laboratories all over the world and has been so for 60 years or more. It is widespread also industrially today. This technique entails that the raw material of the film is heated in vacuum so that a vapour comprising atoms or molecules transfers material to the substrate at a sufficient rate (Holland, 1956; Glang, 1970). The energy of the impinging species is typically a fraction of an electron volt. The heating can be produced by drawing current through a resistive coil or boat, often of tungsten, in contact with the substance to be evaporated or by thermionic emission from a wire and focusing of the electron beam onto the substance to be evaporated from a water-cooled ‘electron gun’. The latter technique is referred to as electron-beam, or e-beam, evaporation.

Sputter deposition is very generally employed to make uniform coatings on glass, polymers, metals, etc. In essence, a plasma is set up in a low pressure of inert and/or reactive gases, and energetic ions in the plasma dislodge material from a solid plate or cylinder of the raw material of the film (known as the ‘target’) and deposit these atoms as a uniform film on an adjacent surface (called the ‘substrate’). The technology is discussed in detail in books by Chapman (1980), Cuomo et al. (1989), Konuma (1992), Wasa and Hayakawa (1992) and Depla and Mahieu (2008). The sputter plasma can be inert, typically consisting of argon ions, in which case the target and the thin film have the same composition. Alternatively the plasma can be reactive and contain for example oxygen so that an oxide film can be formed by sputtering from a metallic target; an additional admixture of water vapour can yield an entire cocktail of metastable and highly reactive species in the plasma (Liu et al., 2011). Analogously, nitrides can be made by sputtering in the presence of nitrogen, etc. The great versatility of the reactive sputtering technique should be obvious.

The plasma is normally confined to the target area by magnets placed behind the target, and one then refers to the deposition technique as ‘magnetron sputtering’. Rotating targets can be used for maximum utilization of the deposition material. The deposition species typically have energies of some electron volts, i.e., the energies are higher than in the case of evaporation and are large enough to remove contaminants from the substrate. This self-cleaning feature is conducive to a good adherence between substrate and film, which is an advantage for sputtering when compared with evaporation as a thin film technology.

Evaporation and sputtering are often referred to jointly as ‘physical vapour deposition’ or PVD. What do thin films made by these techniques look like at the nano level? We consider the growth of metal films on dielectric substrates, such as glass or polymer. Figure 8.1 illustrates a series of scanning electron micrographs taken on gold films with the shown thicknesses and deposited by sputtering onto glass at room temperature (Lansåker et al., 2009). The initial deposition is seen to yield tiny metallic nuclei at certain sites on the substrate, and continued deposition makes these nuclei grow, which is expected to occur via diffusion of atoms or molecules over the substrate surface as well as by direct impingement of atoms or molecules. The discrete metal ‘islands’ that are then formed have shapes that somewhat resemble ellipsoids. Continued deposition makes some of the ‘islands’ touch and rearrange into larger and more irregular objects; this is conventionally referred to as ‘coalescence growth’. The growing film then passes through what can be called ‘large-scale coalescence’, meaning that a contiguous and meandering metallic network of macroscopic extent is formed. Only then can metallic conduction be detected along the film. The evolution of electromagnetic properties in a growing metal film is a topic of continued interest and is highly dependent on the prevailing nanostructures (Earp and Smith, 2011).

The development of the structure depends critically on deposition and post-treatment parameters, to an extent that might seem surprising. Thus, for example, depositing a gold film at room temperature and then heating it to a certain temperature is not equivalent to direct deposition onto a substrate at the same elevated temperature, as shown in a sequel to the work from which Fig. 8.1 was reproduced (Lansåker et al., 2012). Another example of this sensitivity to the film preparation conditions is shown in Fig. 8.2 for the case of sputter deposited films consisting of In₂O₃:Sn (Betz et al., 2006), which is a transparent and electrically conducting material of very large importance in energy technology and for transparent electronics of different kinds. The top panel illustrates a scanning electron micrograph for a film deposited onto a substrate at room temperature and then annealing post-treated at 200 °C and the bottom panel pertains to a film that was sputtered onto a substrate at 200 °C. Clearly the two films display striking differences.

A film that is thicker than those in Fig. 8.1 develops a characteristic nanostructure also over its cross section. Figure 8.3 is a recent extension (Anders, 2010) of a well-known structure zone diagram, known as a ‘Thornton diagram’ (Thornton, 1977), and illustrates what happens. The figure applies to sputtering and shows that the film typically exhibits a columnar structure oriented perpendicular to the substrate, and that this structure depends critically on the deposition parameters, especially on the energy of the sputtered species (in its turn related to the pressure in the sputter plasma, typically comprised of argon) and the substrate temperature. The structure of an evaporated film is found in the limit of a small argon pressure; it was described already in the 1960s (Movchan and Demchishin, 1969) and the modelling was subsequently refined (Barna and Adamik, 1998; Hultman and Sundgren, 2001).

For many thin film applications, there is a requirement for high durability, which means that compact films are wanted, and historically the sputter-based technology was developed to prepare films that were more durable than those made by evaporation. Parameters leading to films belonging to ‘zone T’ in the ‘Thornton diagram’ are then preferred. For other applications, however, it is desirable to make films with a carefully chosen nanoporosity, and then one should make use of low substrate temperatures and high plasma gas pressures in order to reach ‘zone 1’. Films of the latter kind are required for a number of eco-efficient technologies such as for energy efficient and comfort enhancing electrochromic ‘switchable glazings’ in buildings, for gas sensors devised for air quality assessment, and for photocatalytic surfaces capable of cleaning air and water (Granqvist, 1995; Smith and Granqvist, 2010); electrochromics is discussed further in Chapter 11.

Multilayer films are readily made by sequential evaporation or sputtering from more than one source and a practical deposition unit can incorporate a large number of sources which the substrate is transported past in a more or less continuous process which we return to below for the case of sputtering. Composite films can be prepared by deposition from one source of a compound material (as long as decomposition of this material does not take place) or from simultaneous deposition from two or more sources. Mixed metal-dielectric films can be deposited reactively, for example in the presence of a small amount of oxygen, so that the deposit comprises a random mixture of metallic and oxidized parts (known as a ‘cermet’ film). The underlying processes for reactive sputtering can be modelled very accurately (Berg and Nyberg, 2005).

Chemical post-treatment of thin films can modify the properties and lead to novel properties. Figure 8.4 shows one example where a sputter deposited film of ZnO:Al – a transparent conductor – is roughened by etching in dilute hydrochloric acid (Ruske et al., 2007). The treated film exhibits significant light scattering. Chemical etching of some alloy films can yield highly nanoporous conducting layers (Maaroof et al., 2005; Cortie et al., 2006). An example is shown in Fig. 8.5, which reports on a sputter deposited AuAl₂ film after etching so that nothing but the gold remains.

8.2.2 Vacuum-and plasma-based techniques: effects of glancing angle incidence and substrate rotation

For the deposition techniques discussed above, it was tacitly implied that the incidence of the deposition species is more or less perpendicular to the substrate. If this is not the case, it is possible to build up coatings with inclined nanostructures and, if a rotation of the substrate is invoked as well, one can arrive at an entire zoo of nanostructures. The pertinent deposition techniques are sometimes referred to as ‘glancing angle deposition’ or ‘GLAD’.

Figure 8.6 shows the build-up of a film with an inclined columnar structure (Brett, 1989). It is based on calculations from a model wherein ‘atoms’ are depicted as two-dimensional hard discs which travel with a well-defined direction but otherwise randomly towards a ‘substrate’. The ‘atoms’ stick wherever they hit the substrate or an earlier deposited ‘atom’. Columns are then formed for the simple reason that a randomly formed protrusion tends to shade whatever is behind it from further deposition. ‘Self-shadowing’ is a term that is used to describe this. The character of the columns as well as the density of the nanostructure depends on the energy given to the ‘atoms’, i.e., to the mobility they attain. Figure 8.6 describes the structures that form when ‘atoms’ are injected at an off-normal angle of 50° and arriving from the upper right. The column orientation typically does not coincide with the direction of the incident species. Films of this type are sometimes referred to as ‘sculptured thin films’ and are theoretically well understood (Lakhtakia and Messier, 2004; Wakefield and Sit, 2011). Figure 8.7(a) shows a scanning electron micrograph of a film made by GLAD (Steele and Brett, 2007). The resemblance to the simulated structure is striking. Films of this type are known to exhibit angular-selective optical properties (Mbise et al., 1997). By changing the direction of the incident species, it is possible to create a zig-zag pattern, as seen in Fig. 8.7(b).

Still larger possibilities to produce nanostructured coatings are obtained if the substrate is rotated while the incident deposition species have an oblique angle of incidence. Figure 8.7(b) and (c) shows two striking examples using slow substrate rotation. With more rapid rotation, it is possible to obtain ‘penniform’ structures, as illustrated in Fig. 8.8 for the case of a titanium dioxide-based structure made by sputter deposition under conditions so that ‘zone 1’ films are to be expected (Rodríguez et al., 2000).

8.2.3 Non-vacuum-and non-plasma-based techniques

There are many thin film technologies that do not require low pressures. For example, coatings can be prepared by dipping a substrate in a solution containing the species to be incorporated in the film, withdrawing at a controlled rate, and heating to remove volatile components in the solution; an alternative technique suitable for coating small objects is to apply the solution in the form of drops, then spin the substrate at a controlled speed in order to make an even layer, and finally heat treat. In either case the deposition can be repeated over and over in order to make a thicker film. As an alternative to dipping and spinning, the chemical solution can be applied by spraying. Film deposition processes of dipping, spinning and spraying are often referred to jointly as ‘Sol-gel deposition’ (Klein, 1994; Frenzer and Maier, 2006). Figure 8.9 illustrates a cross section of a multi-layer coating made by dip coating (Boström et al., 2011). It has two layers of nickel particles in alumina, with different compositions, and a top layer of silica. The coating is backed by metallic aluminium. This coating is an example of a solar-absorbing surface that avoids thermal re-emission and is thus of the kind mentioned as the second example in Section 8.1.

Chemical vapor deposition (CVD) uses heat to decompose a vapour of a ‘precursor’ substance in order to make a thin film of a desired composition (Morosanu, 1990; Pierson, 1999). This deposition technique can be made more efficient by combining it with plasma treatment in what is denoted ‘plasma enhanced CVD’ or ‘PECVD’. A variety of the CVD technique is referred to as ‘spray pyrolysis’; a fluid containing the precursor is then sprayed onto a hot substrate.

Electrochemical techniques embrace cathodic electroplating from a chemical solution (Lowenheim, 1978) and anodic conversion of a metallic surface to form a porous oxide. Anodization is most common in the case of aluminium (Wernick et al., 2001), but other metals – such as titanium (Diamanti and Pedeferri, 2007) and tantalum (El-Sayed and Birss, 2011) – can be used too. Numerous alternative techniques exist as well. The anodization of aluminium can be carried out following several different strategies. Thus ‘mild’ anodization can lead to a self-ordered pore structure at the nanoscale, but this technique is slow and confined to a limited set of process parameters; ‘hard’ anodization, on the other hand, is a fast and industrially viable process leading to thick layers with a disordered pore arrangement. The latter technique is applied routinely for aluminium surfaces exposed to air. It was recently realized that a combination of ‘mild’ and ‘hard’ anodization in what is known as ‘pulse’ anodization can yield particularly interesting nano-features, such as those depicted in Fig. 8.10 (Lee et al., 2008), which shows alternate layers with well developed nanostructures.

8.2.4 Nano-particle-based coatings

Vacuum-based coating methods, as discussed extensively above, had an important historical role for making films based on nanoparticles. In fact, vacuum-based techniques provided some of the first insights into approaches to prepare nanoparticles under controlled conditions, specifically in the manufacture of ‘gold blacks’ for darkening of thermocouples by gold nanoparticles prepared by evaporation at pressures high enough to yield nanoparticle nucleation and growth of fractal aggregates in the gas phase (Harris et al., 1948; Granqvist and Hunderi, 1977; Sotelo et al., 2002).

A major step forward for the latter technique was taken when it was realized that the mean particle diameter and the size distribution could be understood and accurately modelled provided that the vapour source was equipped with accurate temperature control (Granqvist and Buhrman, 1976). This led to the ‘advanced gas deposition’ (AGD) technique, which is now used to mass-produce nanoparticles that can be collected for later use or for coating directly onto substrates. The technique and its implementation are described in detail in a book by Hayashi et al. (1997).

Figure 8.11 illustrates an AGD unit; as shown, it is arranged for tungsten oxide nanoparticle production (Reyes et al., 2004) but the technique can be used reactively or non-reactively to make nanoparticles of a large variety of pure metals, oxides, nitrides, etc. Evaporation takes place in the lower chamber into a laminar gas flow surrounding the vapour source. The vaporized species are then cooled via collisions with gas molecules so that they form tiny nuclei that subsequently grow in the gas flow. A thin transfer pipe collects nanoparticles in a region at a controlled distance from the vapour source and transports them in a gas stream that ends in the upper deposition chamber, which is maintained at good vacuum. A separate evacuation pipe removes nanoparticles outside the growth region. The nanoparticles are then deposited via a nozzle in order to gain momentum onto a substrate that can be moved so that they form a uniform film comprised of nanoparticles. The technique can be implemented with multiple vapour sources and transfer pipes in order to prepare materials consisting of mixed or layered nanoparticles. Figure 8.12 shows some typical nanoparticle deposits for the case of as-deposited tungsten oxide and WO₃:Pd sintered at 600 °C (Hoel et al., 2005). The particle size distribution is seen to be narrow. Evidently the sintering has caused significant grain growth.

A distinctive advantage of the AGD technique is that it separates nanoparticle formation and growth from thin film deposition. This feature makes it possible to fine-tune particle interaction within the film, at least to some extent, which is beneficial for devices that require well-controlled electrical contact between adjacent nanoparticles such as conductometric gas sensors for determining and surveying air quality. Gas phase synthesis of nanoparticles also can use high-temperature processes, since particles can form in a flame or plasma. The reader is referred to the book by Granqvist et al. (2004) for detailed coverage of this subject.

Chemical approaches are now widely used to grow and precipitate metallic, inorganic and semiconducting nanoparticles from solution and have been refined so as to limit size ranges, create elongated particles as well as spheres, and to overcoat nanoparticles or microparticles with nanoshells in ‘core-shell’ structures which enable new or improved functionality. These aspects have been discussed in detail in the literature (Cushing et al., 2004). Thick layers incorporating nanoparticles can effectively function as thin ones provided that metal flakes are included and serve as ‘artificial substrates’ (Kuni et al., 2009). Layers or coatings containing previously prepared nanoparticles are usually made by first dispersing them in a paint binder, a polymer coating solution, or a monomer solution prior to polymerization. The latter process can be used to produce master batches of concentrated nanoparticles in resin, which can subsequently be mixed with clear resin to prepare thin polymer foils doped with nanoparticles by extrusion. These foils can then be stuck onto surfaces or positioned between clear sheets. Nanoparticle-doped polymer sheets and other shapes of plastic can also be made by injection moulding and extrusion from suitable resins. If dilute coatings are needed, it is important to make sure that the particles are dispersed, which usually requires a surfactant (i.e., a ‘soap’ type molecule) on their surface to ensure that they do not stick together.

Carbon-based nanomaterials deserve particular attention and have undergone phenomenal development during recent years. Figure 8.13 illustrates how different nanostructures can be created from a two-dimensional arrangement of carbon atoms to yield C₆₀ units (known as ‘buckminster-fullerene molecules’ or ‘buckyballs’), long nanotubes with metallic and semiconducting properties, and graphene (two-dimensional graphite-like sheaths) (Geim and Novoselov, 2007).

Long nanoparticles can be prepared not only from carbon, as indicated in Fig. 8.13, but from many materials. Figure 8.14 illustrates an example of Ag nanowires that can be made from potentially inexpensive reduction of liquid silver nitrate to make electrically conducting and transparent coatings with some degree of optical scattering (‘haze’) (J.-Y. Lee et al., 2008; Hu et al., 2010).

The final example of nanoparticle production for coatings is one where work has only recently begun and for which the potential is great though not yet easy to assess: microbiological preparation. Plate II (between pages 162 and 163) illustrates the growth of silver nanoparticles inside bacteria of Pseudomonas stutzeri (Klaus et al., 1999; Klaus-Joerger et al., 2001). It is remarkable that particles of this kind can be single-crystalline. They can be used, for example, to make selectively solar absorbing coatings (Joerger et al., 2000). In fact, there seems to be a vast number of organisms that can serve as eco-efficient ‘nanofactories’ and produce inorganic nanoparticles either intra-or extracellularly, including magnetotactic bacteria, diatoms, fungi and others (Mandal et al., 2006; Olenin and Lisichkin, 2011; Rai and Duran, 2011).

The marriage of biotechnology and materials science to make new eco-efficient constructions – as exemplified above – is a particularly exciting line of development, and recent work has demonstrated how viruses can be harnessed for making nanobatteries (Lee et al., 2009) and nanostructured solar cells (Dang et al., 2011).

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