Ion implantation is a superb method for modifying surface properties of materials since it offers accurate control of dopant composition and structural modification at any selected temperature. In the field of semiconductor technology there was a time lag of some 20 years from the initial development of ion implantation to its becoming a cornerstone of production technology. A similar delay in the acceptance time occurred for metal surface treatments. For insulating crystals and glasses, the use of ion beams to modify such crucial optical parameters as refractive index, reflectivity, colour centre content, and luminescence has now passed this 20-year apprenticeship, and the subject is expanding to include the valuable application phase. Appreciation of possible uses of ion implantation is gaining momentum, in part as a result of the ease with which one can fabricate optical waveguides and waveguide lasers and tailor electro-optic and non-linear properties of the key materials of modern optics. Our own experience with these ion implanted property changes, and potential applications, encompasses a diversity of examples, from lasers to studies of fundamental imperfections in insulators, to fabrication of car rearview mirrors.

Since Sussex has been among the pioneers in the study of work with optical materials, we have written a text which has perhaps presented a disproportionate number of examples using our own data. They do, however, typify many aspects of the subject. The topics cover the basic ion beam interactions in solids, followed by the optical effects of absorption and luminescence. We have then included a chapter on waveguide theory and analysis in order to lead into the very exciting examples of ion implanted lasers, second harmonic generation and nonlinear waveguide optics.

Luminescence processes

Luminescence transitions may occur within localised defect sites, for example to give the characteristic line emissions of rare earth ions, narrow emission bands of chromium in ruby, or they may produce broad bands from charge transfer between defects. Overall, emission bands may vary greatly in width, but nevertheless the luminescence spectra provide a measure of specific defect types, and even offer some quantitative measure of the changes in defect concentrations. Since the excitation energy for luminescence may be provided by many routes, ion implantation is no exception and it frequently produces strong luminescence from insulating targets. This feature is often used as a means of aligning the ion beam and it is common practice to have defining apertures with silica plates to check the ion beam position visually. Such intense luminescence can reveal a number of features relating to the changing defect state of the target. For example, in many of the materials used to form optical waveguides by ion implantation there is a decrease in luminescence intensity which approximately follows the amorphisation in the crystal. Hence, one has a visual estimate of the progress of the amorphisation. Quantitative recording of the wavelength dependence of the signal, in terms of luminescence efficiency and spectral changes, should provide details on not only the defects pre-existing in the material but also the ion beam induced changes. Consequently, a number of research groups have used the luminescence produced during implantation to follow such modifications.

Lattices modified by the ion beam will show changes in their subsequent luminescence performance and the effects of implantation have been recorded in photoluminescence, laser emission, cathodoluminescence and thermoluminescence.

Analysis methods using absorption, ESR and RBS

Optical methods of studying defects have the advantage that if each defect has characteristic energy levels which lie within the forbidden energy gap, then they show separable optical absorption and luminescence bands. Higher photon energy absorption generally monitors electronic transitions, whereas infra-red absorption records vibrational spectra. Many of the optical transitions which result from the presence of impurities have energies in the visible part of the spectrum and consequently the defects are referred to as colour centres. Examples of colour centres are widespread and include the impurities which give colouration to ruby and sapphire or stained glass. They are the basis for photographic and photochromic materials, and frequently involve a mixture of impurity and intrinsic defects. Whilst analysis of absorption bands may determine defect symmetry and inter-relationships of different colour centres, it is unusual to be able to confirm precise models of defect sites solely from the absorption data. In this respect the processes which involve hyperfine interactions such as Electron Spin Resonance (ESR), Electron Nuclear Double Resonance (ENDOR), Mossbauer spectroscopy or spin precession techniques provide more specific answers if they can be applied. In the ion implantation literature there are frequent presentations of data from Rutherford Back-scattering Spectrometry (RBS) to give the depth distributions of impurities or damage in the target material. In part, RBS appears to be popular for implantation analysis because it requires a high energy ion accelerator, which is normally a feature of an implantation laboratory. The information is useful but, like electron microscopy, it rarely gives precise details of individual defect arrangements.

Over the past decade ion implantation has been demonstrated as a suitable technique for the fabrication of waveguide structures in an ever increasing number of optical materials. The stage has now been reached where actual device-oriented structures are being designed and implemented using this technology. This chapter will deal with the progress which is being made in the development of useful devices, and how ion implantation is being used to achieve these ends.

Many authors have recognised the advantages of waveguide structures for signal processing, for coupling to optical fibres or for frequency conversion. Optical circuitry has been proposed which, at least initially, was purely hypothetical, but has advanced to include ideas of entirely solid state waveguide structures (sometimes termed holosteric systems). Forseeable objectives include multi-frequency, or tunable, compact lasers in which the pump power is provided by a semiconductor diode. This inherently waveguide power source could then be matched into other waveguides for a combination of frequency conversion, SHG, pumping a tunable laser waveguide such as alexandrite or Ti:sapphire, or driving an optical parametric oscillator (OPO). The conversion efficiencies in the various stages are often as high as 30%, hence even in a several-stage process a 100 W semiconductor array could result in a few watts of tunable laser power. Such systems could of course supersede the highly inefficient Ar or Kr gas laser sources for many applications. The concepts are clear, and this final chapter will indicate how close the components are to realisation when using ion implantation fabrication routes.

Characteristics of ion implanted waveguides

A waveguide is characterised by a region of high refractive index bounded by regions of lower index. The confinement of the light, as well as the spatial distribution of optical energy inside the guiding layer depends on the refractive index profile. There are several conventional techniques for fabricating optical waveguides. These techniques, including epitaxial growth, metal diffusion and ion exchange, increase the refractive index of the surface layer for a few microns (Figure 5.1), and this high index layer is surrounded by the low index of air and substrate to form an optical waveguide. Ion implantation, as a surface modification technique, can modify the optical properties of an insulator surface. However, when light ions are used, particularly when dealing with crystals, instead of changing the refractive index of the surface layer, a low index optical barrier is built up at the end of the ions' track due to elastic energy deposition from ions to the lattice. Therefore, the surface layer, ideally the same as the substrate, is surrounded by the low index of air and this optical barrier (Figure 5.1(c)). During the ion implantation, some point defects may be produced in the surface layer due to ionisation and excitation when the ions are travelling fast. These simple defects will change the properties of the material, and induce absorption and scattering loss. In practice, it has been found that post-implant annealing at a moderate temperature can either reduce or aggregate these defects depending on the material in question. In many materials, a low loss optical waveguide (∼0.5dB/cm) can be produced by ion implantation and subsequent annealing.

Development of ion implantation

The control of surface properties is of paramount importance for a wide range of materials applications, and craftsmen and technologists of all scientific disciplines have battled with problems of corrosion, surface hardness, friction and electrical and optical behaviour for many hundreds of years. Even for the simplest of articles, whether they be knives, bottles or non-stick frying pans, the manufacture of materials which have the desired surface properties is often incompatible with bulk performance, and so there is an emphasis on finding ways to modify surface layers. Processes such as thermal quenching prove effective for hardening steel and glass bottles but lack the finesse which is required for more sophisticated technology. Instead, these use more controllable treatments, including the deposition of surface coatings or diffusion of impurities into the surface layer and, of course, ion implantation.

Historically, ion implantation has generally been the last of the treatments to receive widespread acceptance. The reason for this is that, compared with coating and diffusion treatments, it appeared to require more complex and expensive equipment which was not readily available. Figure 1.1 indicates that implantation systems may come in several levels of complexity. There are those similar to sophisticated laboratory research machines, which have ion sources, pre-acceleration, mass analysis followed by additional acceleration and then the target region. Commercial applications with requirements of uniformity and a large sample throughput may result in sample handling and beam sweeping equipment as complex and expensive as the accelerator.

Predictions of range distributions

An essential first step in the consideration of ion implantation effects is to understand how energy is coupled into the target material. We will first present examples of energy transfer and ion range, and then indicate how these features have been calculated. In practice there has been a continuous interaction between the theoretical and experimental assessments of ion ranges. This has resulted in modifications to the theories so that there are now tabulations and computer codes which predict ion ranges in virtually any ion/target combination. These computations are accurate to within 5–15%. Consequently, although it is useful to know the underlying assumptions of the range theories, and hence their limitations, the majority of the profiles for the distributions of implanted ions are calculated from standard computer simulations. Since knowledge of the ion range, damage distribution or surface sputtering involves many factors in addition to the initial ion range, the existing level of accuracy is perfectly acceptable. Indeed, divergence between measured and computed ranges is frequently not a result of a failure of the computation, but, rather, it results from the fact that such computer codes do not allow for subsequent migration and secondary processes. As has already been mentioned briefly in Chapter 1, there are two main processes which slow down the incoming ion. These are electronic excitations and nuclear collisions. The rate of energy transfer for each process is a function of the nuclear charge and mass of the incoming ion (Z1, M1), and the target (Z2, M2), as well as the energy.

Ion implantation may be used to change the optical properties of insulators, either because of the chemical presence of the dopant ions, or more generally because of the radiation damage caused during their implantation. The latter effect produces a significant change in the refractive indices of most materials, and consequently He+ implantation has been used to define optical waveguides in a wide variety of substrates. These include electro-optic, non-linear and laser host materials, with key successes in quartz, LiNbO3, KNbO3, KTiOPO4 (KTP), Bi4Ge3O12 (BGO), garnets such as Y3Al5O12 (YAG), and amorphous glasses such as silica and lead germanate.

Although this technique has wide applicability, the refractive index profiles vary considerably between materials, and even between different indices of the same material. The index change may vary in degree, and even in sign, for both the nuclear collision and the electronic ionisation regions. These effects are discussed in this chapter, together with their applicability in the formation of optical waveguides, and more complex structures. Of particular interest are the three detailed examples of quartz, LiNbO3 and Bi4Ge3O12 since between them they embody most of the features so far observed in ion implanted waveguides in insulating materials. The performance of the implanted waveguides is considered in terms of their thermal stability and their attenuation due to absorption, scattering and tunnelling losses. The He+ guides are first compared with those produced by conventional chemical diffusion methods. At the end of the chapter, waveguides formed by implantation of chemically active components are discussed.

Introduction

Chapter 2 provides a review of basic electromagnetic theory as applied to static magnetic fields. Neither wave motion nor eddy current effects caused by conductive media are considered: it is presumed that all time scales are long compared to these phenomena. Time enters only through the constant head-to-medium relative speed, v, so that all temporal information is transformed immediately into the fundamental spatial recording process by x = vt or dx/dt. The purpose of this chapter is to provide useful relations for the determination of fields, both from integrals over field sources and solutions of differential equations for field potentials. Thus, the framework will be provided for the determination of magnetic fields from magnetized heads (Chapter 3) and media (Chapter 4), and in addition, expressions for Fourier and Hilbert transforms will be presented for utilization in spectral analysis of recording signals and noise. These transforms involve operations on the spatial variable, x, which represent the head-to-medium motion direction. In particular, the Fourier transform will be expressed in terms of the spatial transform variable, k: the wavenumber or inverse wavelength (2π/λ). For direct correspondence with measured frequency the simple transformation f = vκ/2π can be utilized.

In magnetic recording the track width is generally large with respect to dimensions in the nominal recording plane, which includes the head-to-tape motion direction and the direction perpendicular to the medium surface (thickness direction). Therefore, two-dimensional field expressions are useful and will be given explicitly in this chapter. In two dimensions the Fourier transforms acquire a particularly simple form yielding the familiar exponential spacing loss.

Introduction

The fundamental noise in magnetic recording is due to the granularity of the medium. If amplitude as well as phase modulation noise sources are not present, particulate noise remains. This noise can exhibit a different character in tapes than in thin films, because thin films have strong magnetic interactions and are densely packed. Nonetheless, particulate noise is basic to all recording media. The structure of this chapter will be to discuss first granularity noise neglecting particle correlations. The total noise power is simply a sum of the independent noise power from each particle or grain. Next, correlation effects are discussed that can involve spatial as well as magnetic correlations. A general formalism will be given, but only simple examples will be examined. The difficulty is that particulate noise modeling is based on Poisson statistics, which are valid only for point or infinitesimally small particles. The effects of finite particle size for moderately dilute systems, which leads to non-overlap effects, can only be included approximately. Granularity noise in thin films, where the grains are tightly packed, must be analyzed differently. A simple approach will be discussed in Chapter 12. In this chapter signal-tonoise ratios are estimated, and wherever appropriate, comparison will be made with the results for continuum fluctuations calculated in Chapter 10. The chapter begins with a calculation of the replay voltage pulse and spectrum of a single particle following previous analyses (Thurlings, 1980, 1983; Arratia & Bertram, 1984). A simplified model of particle clustering is presented at the end of this chapter. Only stationary correlations are discussed in this chapter. Non-stationary effects are the focus of Chapter 12.

Introduction

This chapter presents the formalism associated with the calculation of playback voltages. Expressions for both real time waveforms, such as isolated pulses, as well as spectra will be derived. The playback process involves low flux levels in the playback head; thus, linear system theory may be utilized to relate a recorded magnetization pattern to the reproduce voltage at the head terminals. The chapter begins with a simple example of the waveform obtained by direct calculation of the playback flux. However, it is generally much more convenient to utilize the formalism of reciprocity. The principle of reciprocity states that the playback flux at any instant is equal to a correlation of the recorded magnetization and the field per unit current of an energized playback head. This principle will be derived and the conditions for its usage discussed in detail. Following that, general playback formulas will be given and specific examples will be discussed for both longitudinal and vertical recording. In this chapter the playback of isolated pulses will be treated. The effects of pulse superposition, ‘linear superposition’, for both the ‘roll-off curve’ as well as linear bit shift will be analyzed in Chapter 6. The discussion will focus on playback by an inductive head. However, since reciprocity may be adapted to magnetoresistive (MR) playback heads, the results presented here apply only with slight modification. Reciprocity as applied to MR heads is discussed in Chapter 7.

In analysis of the recording process the head can be placed either above (e.g. Fig. 5.1) or below (e.g. Fig. 5.3) the medium.

Introduction

In this chapter analysis of the playback process is extended to consider the effects of multiple transitions. First the concept of linear superposition in magnetic recording is introduced. Two examples are discussed in detail: (1) square wave recording of alternating transitions with a fixed bit spacing and (2) dibit recording of a pair of transitions. In the former both the ‘roll-off’ curve of the peak voltage versus density will be derived as well as the spectrum or Fourier transform. The utility of spectral measurements for analysis of the recording process will be emphasized as in Chapter 5. The relation of the square wave spectrum to an ‘effective’ channel transfer function will be given. In this chapter, as in the previous one, a comparison will be made between longitudinal and perpendicular recording. The effects that relate solely to differences in a change in magnetization orientation will be emphasized. In this way the playback effects and record phenomena discussed in Chapter 8 can be distinguished.

Linear superposition

The magnetic recording process is not strictly linear. A linear integral relation does not occur where a change in input amplitude yields a proportional change in output voltage. In fact an impulse does not exist because the ‘input’ to the recording channel consists of step functions of voltage or current that produce transitions of magnetization. However, a restricted linearity applies as long as transitions are not written too closely. A transfer function can be defined whose product with the input spectrum yields the output spectrum.

Introduction

Noise in magnetic recording arises from three predominant sources: the playback amplifier, the playback head, and the recording medium. Amplifier noise depends on current or voltage noise sources. Head noise arises from the loss impedance of the head due to the complex part of the permeability (Figs. 3.2, 3.3). Since the head impedance is matched to the amplifier, inductive head noise results as Johnson noise with the loss impedance as the effective noise resistor (Davenport & Root, 1958). Playback head loss impedance and head noise limited system signal-tonoise ratios have been discussed in detail (Smaller, 1965). In Chapters 10, 11 and 12, analysis of the predominant medium noise mechanisms will be presented. The discussion will focus on calculations of the power spectral density. Measurements of noise spectra can be utilized readily to identify and analyze medium noise sources.

Medium noise arises from fluctuations in the medium magnetization. This noise can be separated into three somewhat distinct sources: amplitude modulation, particulate or granularity noise, and phase or transition noise. An illustration of modulation and transition noise is shown in Figs. 10.1 (a), (b), respectively. In conventional amplitude modulation noise, the fluctuations are proportional to the recorded medium magnetization or flux levels. As the recording density is increased, the noise regions decrease relative to the bit length or transition separation and the noise decreases, as measurements in Fig. 10.2(a) show. Transition noise refers, in general, to fluctuations that are concentrated near the recorded transition centers (Fig. 10.1(b)).

Introduction

This chapter will address noise arising from fluctuations localized at the transition. Transition noise is dominant in metallic thin films where the average magnetization lies in the longitudinal or recording direction (Bertram, et al., 1992). Transition noise can also occur in perpendicular film media. These media are prepared either by sputtering or by plating processes and are extremely uniform so that conventional sources of amplitude modulation noise, as discussed in Chapter 10, are not present. The fundamental feature of thin films that gives rise to transition noise is the almost completely dense packing of these polycrystalline media (Fig. 1.2). With dense packing the medium noise depends strongly on the state of magnetization.

In Figs. 12.1 (a), (b) illustrations of the magnetization configurations for grains with solely longitudinal orientation are shown for a saturated medium (a) and an erased medium (b). For the saturated case the ‘poles’ at the ends on one grain cancel those of each adjacent grain. In this case no noise voltage will occur. In the erased case the average magnetization vanishes. To achieve this state, adjacent grains with opposite magnetization will occur, giving rise to localized ‘poles’ at the grain interface of twice the magnitude of that of individual grains. Even with magnetization correlation, a random distribution of these ‘double’ poles will occur, of both polarities, leading to a replay noise voltage. In general, noise voltage sources will occur for all configurations where the medium is not saturated. The greatest number of noise sources will occur for states where the average magnetization vanishes. Media uniformly magnetized at various states along the major loop will exhibit fluctuations that maximize at the remanent coercive state.

Introduction

In this section fields and Fourier transforms of a variety of inductive head configurations will be presented. The field patterns are of interest for both the recording process and, via reciprocity, the playback waveforms. First, the concept of head efficiency will be introduced. Following that, approximate and exact expressions for the fields in the gap vicinity of a very wide head will be derived and compared. The effect of finite head length as well as finite track width will be discussed in terms of approximate expressions. The results of studies of head field saturation will be included. The chapter will conclude with a discussion of the effect of keepered media on head fields.

The function of a recording head is to transfer efficiently the mmf that results from a current applied to the windings into field at the gap region where the recording medium passes. Two state-of-the-art structures are shown in Figs. 1.4 and 3.1. Figure 3.1 shows a head designed for highdata- rate recording in high-density helical scan tape recorders (Ash, et al., 1990). This structure exhibits the extreme dimensional scaling typical of most head structures: the gap length is sub micron (g ∼ 0.25μm), the gap depth and gap width are between one and two orders of magnitude larger (∼ 10–30μm), and the head major dimensions are in the mm range. The example in Fig. 3.1 is of a structure completely laminated to allow for operation at frequencies ∼150MHz. An inductive head, commonly of this geometry, is either completely ferrite for medium coercivity applications or ‘metal-in-gap’ for use on high coercivity media for video (8mm or DAT) or data recording (Jeffers, 1986; Iizuka, et al., 1988).