Handbook of Thin-Film Technology

Thin films are generally used to improve the surface properties of solids. Transmission, reflection, absorption, hardness, abrasion resistance, corrosion, permeation and electrical behaviour are only some of the properties of a bulk material surface that can be improved by using a thin film. Nanotechnology also is based on thin film technology.

Vacuum coating processes are characterized by a number of advantages. These include variability of the coating materials, reproducibility of the film properties, and adjustment of the film properties by changing the coating parameters, and the great purity of the coatings. Despite the influence of residual gases in the recipient, and that of the coating material and the condensation rate, the entrapping residual gas molecules into the film can be kept arbitrarily small, if only the residual gas pressure in the recipient is kept accordingly low. Therefore, the conception of the vacuum system during the design and technical execution of vacuum coating plants for Physical Vapor Deposition (PVD) procedures is especially important.

PVD (physical vacuum deposition) methods are the following: [1]: Vacuum evaporation, Ion plating, Cathodic sputtering. These three techniques are also used in reactive processes for coatings with chemical compounds, as well as molecular beam epitaxy [1–5], which is a variant of vacuum evaporation. With ion implantation [6–10], one can change the properties of solid surfaces without coatings. This is not a coating process.

Thin film technologies are generally based on plasma-supported methods (see Fig. 4.1). Plasma means:

- When in a liquid or a gas the number of free charge carriers is so large that charge carriers affect the physical properties of the medium substantially.

- When electromagnetic interactions between the charged particles take place.

- When the number of positive and negative charge carriers is for each unit of volume equally large in each case; the total quantity can be arbitrary.

Consider the following reactions: where U, V, W and Z are molecules. The reactions are denominated elementarily, if they occur in one step. This means all molecules in a), b) must collide at the same time, and/or in c) the decomposition “in a step” must take place. Reaction a) is in all probability not an elementary reaction, since five molecules must collide at the same time. For the reactions b) 2; an elementary reaction is conceivable: likewise for reaction c): On the other hand, it is well known that the following reaction is not elementary: This reaction corresponds to b). From this it follows that on the basis of the stoichiometric equations a)–c) it cannot decided whether a reaction is elementary or not. However, chemical reactions can be divided into different basic steps:At high pressure triple molecular reactions can also occur: In low pressure plasmas the reactions are usually complex. By means of the so-called stoichiometric coefficients the reaction rates can be defined. The reaction rates for reactants (U, V in Eq. (5.1)) are negative, for products positive: here

Cathode sputtering [1, 2] is more frequently used in thin film technology than evaporation. The reasons for this are the following:

- High melting material or dielectrics with a high frequency generator/source can be sputtered from a solid target.

- Sputtering is a ballistic process in which the target remains relatively cold. The composition of the released particle flux corresponds to the stoichiometry of the sputtering target.

- Sputtering with a reactive gas or a mixture of gases generates films of chemical compounds with a defined stoichiometry from elementary target material.

- The sputtering process can be used to deposit large areas with very high lateral homogeneity.

- By ion bombardment, the substrates can be cleaned before coating, and the properties of the sputtered film can be influenced, e.g. adhesion, strength, structure, etc.

Plasma polymerization is a contactless coating method with monomers of polymers in a plasma discharge [1, 2]. The monomers polymerize in the plasma and are deposited on the substrate. Thin polymer coatings have other properties than polymer foils, which are manufactured by conventional methods, e.g. by evaporation of solvents from the polymer solution or by lamination from the polymer melt. A condition for this process is the presence of chain-forming atoms such as carbon, silicon or sulphur in the process gas. Since the monomer molecules are fractured in the plasma to reactive particles, the chemical structure of the outlet gas in the polymer coating remains at most partial, which entails crosslinking and unordered structuring.

In this section we consider the fate of energetic ions (1–100 keV) incident on a solid surface. The 10 ways in which ions can interact with a surface are illustrated in Fig. 8.1. An incoming ion can be backscattered by an atom or group of atoms in the bombarded sample (1). The backscattering process generally results in a deflection of the ion’s incident path to a new trajectory after the encounter and an exchange of energy between the ion and the target atom. The energy exchange can be either elastic or inelastic, depending on the constituent particles and the energy of the ions. The momentum of an ion can be sufficient to dislodge a surface atom from a weakly bound position on the sample lattice and cause its relocation on the surface in a more strongly bound position (2).

In chemical vapor deposition (CVD) the compounds of a vapor phase, often diluted with an inert carrier gas, react at a hot surface to deposit a solid film [1, 2]. The importance of CVD is due to the versatility for depositing a large variety of elements and compounds at relatively low temperatures and at atmospheric pressure. Amorphous, polycrystalline, epitaxial, and uniaxially oriented polycrystalline films can be deposited with a high degree of purity. Aspects of CVD include the chemical reactions involved, the thermodynamics and kinetics of the reactors, and the transport of material and energy to and from the reaction site.

The characterization of surfaces contains the identification of the atoms existing at the surface, their classification in crystalline or amorphous structure, the chemical bonding, and the electronic structure of the surface. By means of this knowledge we can determine further characteristics, such as dynamic, electrical, thermal, and chemical properties, etc. With the analysis methods discussed in this chapter, the atoms and molecules at the surface are identified due to their mass or their characteristic electronic energy states. The method of the diffraction of electrons (low energy electron diffraction LEED, reflection high energy electron diffraction RHEED) uses the periodic arrangement from surface atoms to the determination of the crystalline structure. The investigation of the electron work function also uses a collective characteristic, the potential jump at the surface.

A contactless method to determine the film thickness is the eddy current method, which can be used also for in-situ analysis [1]. An alternating primary magnetic field induces electric currents, so-called eddy currents in an electrical conductor. Their magnetic field is converse to the primary field. Its intensity depends, among other things, on the electrical conductivity of the material, in which the eddy currents are induced. Over the connection of the electrical conductivity of the film with the attenuation of the primary magnetic field the film thickness can be determined.

The thermal conductivity of thin films is important for the nucleation process and the film growth. The heat conductivity λ of a material is defined through: The heat flow \(\mathrm{d}\dot{Q}\) (quantity of heat for each time unit) flows through, under the effect toward the surface-normal existing temperature gradient \((\partial\delta/\partial s)\) the area \(\mathrm{d}A\).

This chapter gives a short introduction to nanomaterials and a description of the synthesis of nanoparticle films. It further describes in detail two nanoparticle film systems, i.e., magnetic nanocomposites in which magnetic nanoparticles are randomly distributed and nanostructured magnetic nanocomposites in which magnetic nanoparticles are embedded in a structured insulating matrix, and nanoparticle ZnO-based films.