Introduction

Ion irradiation of crystalline metallic alloys causes structural changes. Crystalline phase can become amorphous or can change to a different crystalline structure. The transformation can be to metastable or equilibrium phases. The driving force for an irradiation induced transformation is provided by the energy to the lattice during the penetration and subsequent stopping of an energetic ion (see Chapter 7).

To understand irradiation induced phase transformations on a thermodynamic level, let us consider an A–B binary phase diagram and the associated free energy of three of the components (Fig. 12.1). We choose A and B to be fcc metals, AB3 to be a line compound with many atoms per unit cell, AB to be a simple CsCl structure with an extended phase field (the CsCl structure has two atoms per unit cell) that exists over a 10 to 20% range in composition, and the region α to be an fcc solid solution. A free energy curve with small curvature (wide phase field) indicates that the irradiation induced deviations from equilibrium do not result in large increases in the compound's free energy. Large curvature (narrow phase field) indicates just the opposite; small increases in the equilibrium defect concentration or changes in composition produce large increases in the free energy. As a result, compounds with large phase fields can tolerate a higher degree of irradiation damage and should be more irradiation stable relative to compounds with narrow phase fields (Brimhall et al., 1983; Hung et al., 1983).

Introduction

Ion beam processing of materials results from the introduction of atoms into the surface layer of a solid substrate by bombardment of the solid with ions in the electron-volt to mega-electron-volt energy range. The solid-state aspects are particularly broad because of the range of physical properties that are sensitive to the presence of a trace amount of foreign atoms. Mechanical, electrical, optical, magnetic, and super-conducting properties are all affected and, indeed, may even be dominated by the presence of such foreign atoms. The use of energeticions affords the possibility of introducing a wide range of atomic species, independent of thermodynamic factors, thus making it possible to obtain impurity concentrations and distributions of particular interest; in many cases, these distributions would not be otherwise attainable.

Recent interest in ion beam processing has focused on the studies of ion implantation, ion beam mixing, ion induced phase transformations, and ion beam deposition. These interests have been stimulated by the possibilities of synthesizing novel materials with potential applications in the semiconductor, tribological, corrosion, and optical fields.

Ion beam processing provides an alternative and non-equilibrium method of introducing dopant atoms into the lattice. In typical applications, a beam of dopant ions is accelerated through a potential of 10–100 kV. The implantation system shown in Fig. 1.1 illustrates the basic elements required in this technique: ion source, acceleration column, mass separator, and target chamber.

Introduction

The ion beam systems currently employed for surface modification studies involving either direct ion beam implantation or ion beam assisted deposition have evolved from distinctly different predecessors, including isotope separators and exploratory space propulsion devices, respectively. The features of directed beam ion implanters will be discussed first, followed by features of plasma source ion implantation (PSII), and finally the lower-energy broad-beam ion sources commonly used for IBAD studies.

The earliest ion implanters evolved from the isotope separators of the 1940s and later. Ion implanters are frequently classed according to their ion current capabilities, ranging from low currents (i.e., microampères) to high currents (one to several milliampères). The specific design criteria have been mainly driven by the particular fluence (dose) and depth profile requirements for semiconductor device fabrication. The history of ion implanter evolution and development is in itself an interesting study of technology transfer. It is covered comprehensively in a series of Conference Proceedings (see the Suggested reading section) that parallel the developments in Si device technology that has demonstrated such explosive growth since the early 1970s (Rose, 1985). Although this area of accelerator application is not the focus of this book, many ion implanters in research and development usage today, for general materials science studies, are either converted semiconductor ion implanters or are based on their design. Therefore, the basic design and system features of these systems will be briefly treated.

Introduction

The previous chapters have used analytical approaches to ion–solid interactions: ion ranges and radiation damage. Here, we discuss the use of computer simulations to describe the slowing down and scattering of energetic ions in solids. Two different types of computer simulations will be examined: Monte Carlo (MC) and molecular dynamics (MD). The Monte Carlo method relies on a binary collision model, and molecular dynamics solves the many-body problem of Newtonian mechanics for many interacting particles. Eckstein (1991) provides a review of computer simulation of ion–solid interactions.

The defects generated in ion–solid interactions influence the kinetic processes that occur both inside and outside the cascade volume. At times long after the cascade lifetime (t > 10−11 s), the remaining vacancy–interstitial pairs can contribute to atomic diffusion processes. This process, commonly called radiation enhanced diffusion (RED), can be described by rate equations and an analytical approach. Within the cascade itself, under conditions of high defect densities, local energy depositions exceed 1 eV/atom, and local kinetic processes can be described on the basis of a liquid-like diffusion formalism.

Monte Carlo simulations

The Monte Carlo methods, applied to ion–solid interactions, have a number of distinct advantages over analytical calculations based on transport theory. The MC approach allows for a more rigorous treatment of elastic scattering and of the determination of angular and energy distributions. As the name MC suggests, the results require averaging over many simulated particle trajectories.

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.