Study of metallurgical phenomena

Ion implantation is a powerful tool, useful in the study of alloying phenomena in metals, but the technique has been exploited in that capacity by only a few researchers. The following discussion, taken from the work of S. M. Myers, gives examples of its use for this purpose. Myers (1980) was one of the first to fully utilize ion implantation to study metallurgical phenomena.

The as-implanted ‘surface’ alloy is often metastable on the basis of extended solubilities, as discussed in Chapter 10. Upon heating, the implanted structure returns to an equilibrium situation, and the tracing of this evolution to equilibrium serves to help determine properties such as diffusion rates, solid solubilities, and solute trapping. The study of this transition can be aided by the use of ion beam analysis methods, as well as by conventional electron microscopy, as described below. Myers outlines the evolution of the ion implanted depth distribution and the formalism required to extract the pertinent solid state parameters from the analysis; his approach is paraphrased below.

Diffusion and the composition profile

The quantitative determination of metallurgical properties relies principally on analysis of the time-dependent composition profile obtained during annealing. This analysis involves certain approximations, depending upon the particular experiment, and Myers has outlined the mathematics for certain specific cases, assuming the host to be semi-infinite. The evolution of the implanted distribution during thermal annealing, performed after the implantation has been completed, is of greatest interest.

Introduction

Ion implantation has been investigated with the intention of beneficially modifying surface sensitive properties since the early 1970s. A large share of the early work in this field was performed at Harwell, the UK Atomic Energy Establishment, with an emphasis on (i) tribological properties as modified by nitrogen implantation and (ii) oxidation resistance. Subsequently, several other laboratories worldwide became engaged in ion implantation research, and the range of topics explored expanded to cover other topics and substrates (i.e., ceramics and polymers). Interests started turning to the hybrid technique combining concurrent ion bombardment and physical vapor deposition in the early 1980s, and it continues to the present (1995).

Ion implantation – advantages and limitations of the technique

Ion implantation for the controlled modification of surface sensitive properties has had two principal thrusts: (i) as a metallurgical tool for studying basic mechanisms in areas such as aqueous corrosion, high-temperature oxidation, and metallurgical phenomena (e.g., impurity trapping); and (ii) as a means of beneficially modifying the mechanical or chemical properties of materials. This chapter includes examples of both usages, and will review the present status of some of the most active research fields outside of the semiconductor area. Table 14.1 shows a compilation of material properties influenced by ion implantation.

Some of the advantages and limitations of ion implantation in comparison with other surface treatments, such as coatings, are listed in Table 14.2.

Introduction

Materials under ion irradiation undergo significant atomic rearrangement. The most obvious example of this phenomenon is the atomic intermixing and alloying that can occur at the interface separating two different materials during ion irradiation. This process is known as ion beam mixing. An early observation of the ion mixing phenomenon was made following the irradiation of a Si substrate coated with a thin Pd film. A reaction between Pd and Si was observed when the irradiating Ar ions had sufficient energy to penetrate the Pd/Si interface (van der Weg et al., 1974). This process is schematically displayed in Fig. 11.1 for a layer M on a substrate S for successively higher irradiation doses. Early in the irradiation, when ion tracks are well isolated, each incident ion initiates a collision cascade surrounding the ion track. Atoms within the cascade volume will be mobile and undergo rearrangement for a short period of time, resulting in an intermixed region near the interface. At this stage of the ion mixing process, the interfacial reaction is considered to be composed of many localized volumes of reaction (Fig. 11.1(a)). As the irradiation dose is increased, overlap of localized regions occurs (Fig. 11.1(b)), and for higher doses a continuous reacted layer is formed at the interface (Fig. 11.1(c)).

A major driving force in the development of the ion beam mixing process is its ability to produce ion-modified materials with higher solute concentrations at lower irradiation doses than can be achieved with conventional high-dose implantation techniques.

Introduction

The bombardment of a growing film with energetic particles has been observed to change for the better a number of characteristics and properties, critical to the performance of thin films and coatings, such as adhesion, densification of films grown at low substrate temperatures, modification of residual stresses, control of texture (orientation), modification of grain size and morphology, modification of optical properties, and modification of hardness and ductility.

The process of simultaneous thin-film deposition and directed ion bombardment from an ion source has been labeled by a variety of terms including: ion assisted coating (IAC); ion assisted deposition (IAD); ion vapor deposition (IVD); ion beam enhanced deposition (IBED); dynamic recoil mixing (DRM) at high energies; and ion beam assisted deposition (IBAD). This term, ion beam assisted deposition, or IBAD, will be used here in favor of its growing acceptance by the energetic-particle–solid interaction research community.

The important role of ions in thin-film deposition techniques has long been realized by the coating community. It is difficult, however, in many of the plasma based coating techniques, to separate out the degree to which ion and neutral particle fluxes as well as ion energies affect resultant coating properties. Mattox (1982) showed as early as 1963 that energetic ions within plamsa had an important influence on coating properties in his early development of ion plating. In addition, other plasma-based deposition processes, such as activated reactive evaporation (ARE), developed by R. F. Bunshah and co-workers (Bunshah, 1982), employ ionization to promote film properties.

Gases, liquids and solids

The three most common states, or phases, of matter, gases, liquids and solids are very familiar (Walton, 1976). Phases that are not so well known are plasmas and liquid crystals (although these are both found in electrical and electronic devices in everyday use). All these states are generally distinguished by the degree of translational and orientational order of the constituent molecules. On this basis some phases may be further subdivided. For example, solids, consisting of a rigid arrangement of molecules, can be crystalline or amorphous. In an amorphous solid (a good example is a glass), the molecules are fixed in place, but with no pattern in their arrangement. As shown in figure 1.1, the crystalline solid state is characterized by long-range translational order of the constituent molecules (the molecules are constrained to occupy specific positions in space) and long-range orientational order (the molecules orient themselves with respect to each other). The molecules are, of course, in a constant state of thermal agitation, with a mean translational kinetic energy of 3kT/2 (k is Boltzmann's constant, T is temperature; kT/2 for each component of their velocity). However, this energy is considerably less than that associated with the chemical bonds in the material and the motion does not disrupt the highly ordered molecular arrangement.

Deposition principles

The Langmuir—Blodgett (LB) technique, first introduced by Irving Langmuir and applied extensively by Katharine Blodgett, involves the vertical movement of a solid substrate through the monolayer/air interface (Blodgett, 1934; Blodgett, 1935; Langmuir, 1920). Blodgett's and Langmuir's original papers contain a wealth of useful experimental advice and are still excellent starting points for anyone considering working in the area today (Blodgett, 1935; Blodgett and Langmuir, 1937).

The surface pressure and temperature of the monolayer are first controlled so that the organic film is in a condensed and stable state. For fatty acid type materials, deposition generally proceeds from either the L2′, LS or S phase (with surface pressures in the range 20–40 mN m-1 and temperatures 15–20°C). However, it is also possible to start from one of the other monolayer states. The molecular organization in the resulting LB film will depend on these initial conditions.

Figure 3.1 shows the commonest form of LB film deposition. The substrate is hydrophilic and the first monolayer is transferred, like a carpet, as the substrate is raised through the water. The substrate may therefore be placed in the subphase before the monolayer is spread. Subsequently, a monolayer is deposited on each traversal of the monolayer/air interface.

Refractive index

The refractive index of materials is determined by the interaction of electromagnetic (EM) radiation with the molecules which they comprise (appendix B). This depends not only on the orientation of the electric field vector of the incident EM wave, but also on that of the electric dipoles produced in neighbouring molecules. Figure 7.1 shows an ideal arrangement of molecules in an LB monolayer. The sample coordinate system is (x, y, z) while that of the principal axes of the molecules is (x′, y′, z′). Careful measurements on fatty acid LB layers show that the films possess a biaxial symmetry with three independent permittivity values (Barnes and Sambles, 1987). However, two of the indices are very close in value and LB films are often approximated as uniaxial.

Several approaches can be used to measure the refractive indices of thin organic films (Petty, 1990). In some techniques, the film thickness is also obtained (section 5.8). The more popular methods, based on ellipsometry, surface plasmon resonance and waveguiding are discussed below. Figure 7.2 summarizes the results of such experiments by various workers using cadmium eicosanoate LB films (Swalen et al., 1978). It is evident that the refractive index for the extraordinary ray ne (p-polarization) is greater than that for the ordinary ray n∘ (s-polarization) by 0.04.

Bonding in organics

Carbon has an atomic number of six and a valency of four. Its electron configuration is 1s2, 2s2, 2p2, i.e., the inner s shell is filled and the four electrons available for bonding are distributed two in s orbitals and two in p orbitals. The s orbital is spherically symmetrical, as shown in figure A.1(a), and can form a bond in any direction. In contrast, the p orbitals, figure A.1(b), are directed along mutually orthogonal axes and will tend to form bonds in these directions. When two or more of the valence electrons of carbon are involved in bonding with other atoms, the bonding can be explained by the construction of hybrid orbitals by mathematically combining the 2s and 2p orbitals. In the simplest case, the carbon 2s orbital hybridizes with a single p orbital. Two sp hybrids result by taking the sum and difference of the two orbitals, as shown in figure A.2, and two p orbitals remain. The sp orbitals are constructed from equal amounts of s and p orbitals; they are linear and 180° apart.

Other combinations of orbitals lead to different hybrids. For example, consider three groups bonded to a central carbon atom. From the 2s orbital and two p orbitals (e.g., a px and a py), three equivalent sp2hybrids may be constructed.