Low-Energy Ion Irradiation of Materials

Collisions between, on the one hand, the incident ion and the target atoms and, on the other hand, between target atoms themselves are the fundamental processes of ion–solid interaction. In a collision with low and medium ion energies, the interaction between two particles can be described by a screened Coulomb potential. In detail, various screened potentials and their screening function are presented in a summarized form. Interactions between particles in the very low or hyperthermal energy range can, on the one hand, be represented approximately by a Born–Mayer potential and, on the other hand, it is to be taken into account that the attractive part of the interaction potential cannot be ignored any more. In the following, the classical description of the collision processes in both the laboratory system and the center-of-mass system is briefly presented, the collision parameters are introduced, and the scattering angles for the colliding and collided particles are formulated. Subsequently, the total scattering cross section, a quantity indicating the probability of the interaction between ions and target atoms, and the differential scattering cross-section, indicating the number of ions scattered into a differential solid angle at given polar and azimuthal angles, are introduced. The method proposed by Lindhard et al. to represent the differential cross-section as a function of a single parameter for the screened Coulomb interaction is also presented.

Energetic ions are mainly subject to two loss mechanisms during interaction with target atoms, the nuclear energy loss and the electronic energy loss. The dominating interaction processes of the movement of low-energy particles through matter are elastic collisions with target atoms. In this chapter, the different approaches to determine the nuclear energy loss per unit length, also referred to as nuclear stopping power, are presented in detail. The electronic energy loss, which is less significant for low-energy ions, is briefly described. Based on the knowledge of nuclear and electronic energy loss, the range of incidence particles can be approximately determined as a function of acceleration energy. It can be shown that by use of the projected range and its standard deviation, three-dimensional concentration distribution of the implanted ion species below the surface can be determined. Since often the experimentally determined concentration profiles deviate considerably from a simple expected Gaussian distribution, higher order moments of the Gaussian distribution must be included in the calculation of the concentration distribution. Computer simulations can be used to calculate not only the concentration distribution, but also the trajectories of the particles, the number of reflected ions and sputtered atoms, the distribution of vacancies and interstitials, etc. In this chapter, the two most common computer simulation codes for the analysis of particle-solid interactions, molecular dynamical code and the Monte Carlo code, are briefly presented.

Bombardment of surfaces with accelerated ions leads to the formation of defects, where lattice atoms can be displaced after collision with the incident ion or a higher order recoil atom if the transferred energy is higher than the displacement threshold energy. The Kinchin-Pease model, modified Kinchin-Pease model, and the Norgett–Robinson–Torrens model are presented and the calculation of the defect generation rate is demonstrated. Of particular importance is the determination of the number of displacements per atom (dpa), since this number is the benchmark for quantifying radiation-induced changes of material properties after ion bombardment. Assuming that elastic collisions are dominant, the spatial distribution of the deposited energy is discussed. If the assumption of subsequent binary collisions is not valid, the formation of high energy density cascades can be expected. Both the displacement spike and the thermal spike are introduced and the mean features of these cascades are discussed. Then, the behaviour of irradiation-induced point defects as a function of irradiation time (fluence) and the temperature during ion bombardment is described and the main analytical relationships are given. As the concentration of ion beam-induced defects increases, the probability of formation of an amorphous layer also increases. The formation of an amorphous phase for a given material depends on the ion species, ion energy, fluence, ion current density, and irradiation temperature. The various models proposed in the literature for the ion beam-induced amorphization process are briefly summarized.

Sputtering techniques are of great importance for both academic research and commercial applications. This chapter summarizes the fundamentals of the sputtering process, especially after low-energy ion bombardment. The theoretical description of the total sputtering yield, and the energy and angular distribution of the sputtered atoms is based on the assumption of a linear cascade regime. The description of the sputtering process in the linear regime according to Sigmund for amorphous materials by means of the Boltzmann transport equation is briefly presented, the energy dependence of the sputtering yield is demonstrated, and the influence of critical parameters, such as surface binding energy, threshold energy and material correction factor is discussed. The dependence of the sputtering yield on the ion incidence angle and also the energy and angular distributions of the sputtered particles are considered in detail. The particular case of preferential sputtering in low-energy ion bombardment of binary materials is addressed, considering both the mass effect and the bonding effect. An important aspect of this chapter is the measurement of the sputtering yield and energy and angular distributions of the sputtered particles. Finally, the reflection of low-energy ions from surfaces is briefly presented.

The sputtering of the surface by low-energy ion irradiation presented in the last chapter leads directly to roughening of the surface. The behavior can be described by relations of dynamic scaling theory and stochastic growth equations. The surface after ion irradiation with ion energies above the displacement energy is initially characterized by the formation of vacancies, interstitial sites, and adatoms, some of which coalesce into clusters. The kinetics of growth of surface defects in the early stages of ion irradiation is influenced primarily by two factors, the ion energy and the temperature during bombardment. In the specific case of a high nuclear energy loss, carters are created. As ion irradiation progresses, increasingly extended defects form on the surface, whereby a distinction must be made between intra- and Inter-crystalline surface defects. The evolution of individual defects such as cones, pyramids, etch pits, facets, etc. is discussed in detail as a function of the irradiation parameters. The theoretical description of ion beam-induced surface evolution is based on the assumption that the sputtering yield is exclusively a function of the local surface curvature and higher spatial derivatives of the local surface height. This assumption allows the surface evolution to be considered spatially and temporally as a moving wavefront and to apply geometrical methods developed in optics. Finally, secondary ion beam-induced mechanisms, such as grazing incidence ion reflection, re-deposition, shadowing, surface diffusion, non-uniform bombardment, viscous flow, and swelling are presented, which can have an additional significant effect on the topography evolution.

The technologies, ion beam figuring (IBF) and ion beam-induced smoothing (IBS), are used to precisely remove imperfections or correct surface shape in a predetermined and controlled manner. The IBF method uses the computer-aided codes to realize the desired surface topography, where a spatially and temporally stable ion beam is passed vertically over the surface at a fixed distance in a high-vacuum environment. The main input variables of this approach, the ion beam removal function, the surface error function and the dwell time procedure, are presented. The various algorithms (Fourier transform algorithm, iterative dwell algorithm, matrix based algorithm, Bayesian algorithm) for determining the dwell time of the ion beam over each object point to be machined and the effect of temperature during the figuring procedure are discussed. The application of IBF technology for the correction of shape errors on surfaces to achieve depth accuracies in the nanometer and sub-nanometer range over the entire spectrum of the spatial surface wavelength is demonstrated with selected examples. Ion beam-induced smoothing (IBS) focuses on feature processing (spatial wavelength < a few microns and height amplitudes on the order of nanometers) with the aim of producing ultra-smooth surfaces. In addition to direct smoothing by low-energy ions, the technologies of smoothing with a planarization layer and smoothing by means of ions incident at a very oblique angle have also become established. Atomistic surface relaxation processes such as ion beam enhanced viscous flow, thermally activated surface diffusion, effective ion-induced diffusion, and ballistic mass redistribution can contribute to the smoothing process.

An attractive ion beam method is the possibility of spontaneous formation of ordered surface patterns in the form of nanodots/nanoholes or sinusoidal modulations of the surface (ripples) in the nanometer range. This method, based on self-organization, is characterized by the interplay of two low-energy processes induced by ion beams. The ion bombardment roughens the surface, while relaxation processes such as surface diffusion or/and beam-induced viscous flow smooth the surface. In this chapter, the formation of nanoripples with and without metallic contaminants is presented and the dependence of ripple formation on temperature, ion incidence angle, ion energy and co-deposited metal concentration is discussed. Bradley and Harper have proposed a continuum theory to describe the topography evolution and pattern formation. This theory is based on curvature-dependent sputtering, which is proportional to the locally deposited energy. In the following, it will be shown that, on the one hand, this theoretical concept can be extended by introducing nonlinear terms and, on the other hand, that the formation of surface patterns can be also explained by a directional redistribution of mass. Finally, the great application potential of this technology for effective, low-cost and scalable patterning of large areas of all materials is demonstrated.

Direct deposition of low-energy ionized atoms or molecules (IBD) onto a substrate has key advantages in terms of controlling layer properties. In general, this technique is based either on the deceleration of high-energy, mass-separated ion beams or the generation of mass-separated, low-energy ion beams. The growth of the layers is primarily determined by the balance between two energy-dependent effects, deposition (sticking of deposited atoms or molecules) and sputtering. Processes such as sputtering, ion reflection, and athermal generation of defects define an energy window for successful deposition of the layers. The application of molecular ions for layer deposition is characterized by the fact that these ions are fragmented above a threshold energy. The potential of IBD with hyperthermal particles can be advantageously used in the synthesis of both stable and metastable phases in the deposited layers. As examples, the deposition of carbon and diamond-like carbon films, epitaxial silicon and germanium films, metal films and also compound films can be mentioned. High-quality thin organic films can be deposited by a specific variant of the IBD technique, the electrospray deposition. This technique, based on soft landing, allows the non-destructive deposition of large organic molecules on the surface of a substrate, when the energy of these molecules is lower than the activation energy for collision induced dissociation. This technique is presented and the deposition of selected organic layers is demonstrated. Finally, a method for cleaning surfaces by low energy ion bombardment is reviewed. It is characterized by the removal of surface contamination, adsorbates or compound layers on the surface without significantly damaging the underlying structure.

In this chapter, the ion beam-assisted deposition (IBAD) method is presented, which is characterized by the deposition of material in high vacuum and the simultaneous bombardment of the surface with hyperthermal or low-energy ions. The individual sub-processes of the layer deposition, such as the generation of the atoms and ions, the transport of the particles through the vacuum and the processes on the surface during the deposition of the atoms under low-energy ion beam irradiation are discussed. The layer growth under ion irradiation is described. In detail, the significance of the ion energy, the temperature and the ion-to-atom arrival ratio for the layer growth is presented. Individual aspects such as epitaxial growth, evolution of topography, grain size, texture, biaxial orientation, mechanical layer stresses, and densification as a result of ion beam-assisted deposition are considered. The explanations and models for these aspects known in the literature are presented and critically discussed. A decisive advantage of IBAD over conventional deposition methods is the controlled fabrication of thin compound films, in particular nitride and oxide films. Examples of the formation of various nitride and oxide layers are presented.

The method of ion beam sputtering under glancing angle conditions in combination with an additional rotation of the sample holder allows the growth of almost arbitrarily designed nano- and microstructures of all material classes on surfaces. The self-shadowing and the surface diffusion essentially govern the structure evolution. It is demonstrated that by varying the particle incidence angle, the temperature, azimuthal rotation frequency, and the beam divergence of the sputtered particles, a wide variety of nanostructure morphologies (e.g., slanted and vertical columns, screws, spirals, or zigzag columns) can be generated. Ballistic simulations are preferably used to simulate the growth of these structures. It can be shown that two basic alternatives of ballistic simulations, off-lattice simulations and on-lattice simulations, are available to successfully model growth. A remarkable result of all experimental investigations and computer simulations is that the column tilt angle is always smaller than the incidence angle. Various explanations are known to explain this fact. These models will be presented and it will be shown that especially the competition model is able to describe a relation between the tilt angle and the angle of incidence for the complete range of material incidence angles. For various applications, patterning of the substrate prior to growth is required to fabricate arrays for highly regular nanostructures. This fabrication is demonstrated and the application of these structures for the realization of biosensors and magnetic nanotubes is shown.