Materials have always been interwoven throughout the very fabric of man's history. The present reawakening to the value and importance of materials, however, has become a dominant factor in manufacturing, national security, international competition and trade, consumer products (quality and reliability), and even education. Other renewals of interest have occurred over the centuries, probably beginning with the formation of the first pot from clay. These renewals were associated with discoveries such as copper, iron, and the transistor. However, in the past 40 years the base for renewed interest has broadened.

A true coupling of science and engineering into the field of materials was probably initiated in the 1940s and 1950s. Emphasis at that time was on metals and the “new” semiconductors, with an interest that incubated and grew to where their central position in national economies and man's daily life was recognized. In 1970 the National Academy of Sciences appointed a committee to conduct a comprehensive analysis and assessment of the field of “materials science and engineering.” The COSMAT report which resulted from that study had a dramatic impact on the field and has been a frame of reference for the past 17 years. These years have seen a virtual explosion of ideas, processes, and materials in the field.

Journalists provide an important function in a free society. Through their efforts, the public receives a low cost and accessible continuing education about its world — from who's inciting war, or where interest rates are likely to go in the next three months, to what made the space shuttle explode. In fact, it can be argued that the fast pace of modern society is making each of us increasingly more reliant on the news and knowledge journalists impart. As a member of the news-gathering fraternity, I have been asked to go behind the scenes and describe what it is that journalists, especially science writers, do — including what drives and limits us. There are lessons in this for everyone, not the least of which may be a greater appreciation for the growing symbiosis between the news media and science.

With some 65 U.S. newspapers already carrying regular science sections and a population of probably more than 1,000 accredited science writers, it may come as some surprise to learn that science journalism is largely an invention of this century. In their book Scientists and Journalists: Reporting Science as Neius (Free Press, New York, 1985), Sharon Friedman, Sharon Dunwoody, and Carol Rogers note that prior to the world wars, science achievements were seldom treated as news and “scientists rarely served as sources.”

The science and technology of ultrasmall three-dimensional materials systems has been developing rapidly the last 20 years or so. Catalysts, coatings, composites, as well as electronic device structures—all rely on materials properties on an atomic scale. To develop such new materials and understand the chemical and physical properties that determine their unique behavior, we also require analytical tools with atomic level spatial resolution and at the same time the desired measurement capability. This need, along with extensive scientific interest in the fundamental chemical and physical properties of free surfaces, has led to the continued development of microanalytical chemical analysis techniques over the past 20 years. Most readers will be familiar with many of these techniques with acronyms such as AES, XPS, RBS, SIMS, ESCA, etc. This issue of the MRS BULLETIN will review some recent advances in the development of these techniques as well as introduce new techniques with significant advantages over the older ones.

As you can see from the thickness of this issue, it is difficult to cover the entire field in a finite amount of space. This led us to limit the discussion to those microanalytical tools which can easily be applied to the analysis of buried interface structures such as those found in semiconductor devices.

The following series of articles focuses attention on five important groups of glassy materials: silica, oxynitride, phosphate, chalcogenide, and halide glasses. Each author has made a concerted effort to indicate the nature of the scientific questions and technological applications which make his individual area of materials research both exciting and timely. Nevertheless, these five articles can cover only a few of the identifiable areas of glass science and technology which are active and find new importance today. A broader picture can be obtained by perusal of the general texts listed in References 1-13.

Glasses are usually defined as a sub-class of amorphous materials which exhibit a glass transition and are formable by quenching from the melt. They include numerous oxides, chalcogenides, halides, polymers, organic glasses, and amorphous metals. We have chosen to discuss only the first three of these groupings, which largely comprise the traditional inorganic glasses. The oxide glasses are only partly covered by our first three articles; completeness would require descriptions of activities involving alkalisilicate glasses, alumino-silicates, boro-silicates, boro-phosphates, and more.

Amorphous materials which are not viewed as glasses include tetrahedral amorphous semiconductors like a-Ge and a-Si, either pure or containing alloy levels of H, F, Cl, B, P, or other elements.

Since the pioneering work of Carothers which led to the introduction of strong nylon fibers by DuPont in the 1930s, polymer scientists have pursued the development of high-performance fibers to replace natural or metallic products, both to improve mechanical properties and to reduce weight. That development was relatively slow and evenly paced until DuPont again revolutionized the field with the release of Kevlar™, an aromatic polyamide with unprecedented mechanical properties. Since then, the field has literally boomed with new developments, and now organic fibers are available with properties that compete with the best inorganics and are far superior to metal fibers.

Strong motivation for the invention of new organic fibers comes from the aerospace industry, which seeks fibers to use in reinforced composite structural materials. Composites bring new advances in stiffness (airplane wings can't bend too much!) in weight savings (every kilogram saved in the structure of an airplane saves $120 over its lifetime, and in a spacecraft $10,000), and in radically new ideas, such as radar-invisibility (stealth)5 and mission-adaptive wings (in-flight variable-shape wings). Hence, for a variety of specialty applications, otherwise commercially indefensible materials become viable.

It may be somewhat counter intuitive to materials scientists unfamiliar with polymers to expect polymer mechanical properties to be greater than in the best metals. The origin of high strength and stiffness in a polymer fiber is the covalent bond, especially when aligned in an ordered array of long chain molecules.

Ion beams are used extensively in materials research for processing and synthesis as well as for characterization. In the last few years, enormous advances have been made regarding the use of ion beams for processing or synthesis, and this issue of the MRS BULLETIN will review some of those advances. (The use of ion beams for materials characterization will be the subject of a future issue of the BULLETIN.) The areas covered in this issue are ion implantation, ion beam mixing, ion-assisted deposition, and direct ion beam deposition. For each area, recognized experts in the field prepared overview articles that should be very interesting to those who are not active in the field, and that should be useful to other experts in the field.

The first large-scale use of ion beams for materials modification took place in the semiconductor industry more than 20 years ago when ion implantation began to be used to dope the near-surface region of silicon with Group III or Group V dopants. The use of ion implantation in the semiconductor industry has undergone explosive growth, and today almost all electronic devices are fabricated utilizing at lest one ion implantation step.

In addition to the semiconductor area, research is being carried out using ion implantation in a multitude of other areas which include ceramics, metals and alloys, insulators, etc. The article on “Ion Implantation” by S.T. Picraux and P.S. Peercy provides an excellent overview of current research activities involving ion implantation of a wide spectrum of materials.

This article discusses the underlying principles of Rutherford backscattering spectrometry (RBS). Consideration of the theory of the interaction of high energy ions with solids leads to the conclusion that quantitative elemental analysis of the near-surface composition of solids can be performed by RBS. Examples are given.

Fifty years ago, who could have imagined that silicon dioxide—the material of ordinary beach sand—would become one of the most important materials of present-day optics and electronics? Yet SiO2 is arguably the most crucial material component in current-generation fiber optics and metal-oxide-semiconductor (MOS) device technology. In MOS field-effect transistors (MOSFETs), SiO2 serves not only as the gate insulator, but also as the “field oxide” (which isolates various components of an integrated circuit) and as the packaging material which seals the device from outside contamination. In these roles silica acts as a “perfect dielectric,” being characterized by an essentially infinite resistivity (actually ~1016 Ohm · m at 300 K). The ability to form such a high quality dielectric film with a near-perfect lattice match on single-crystal silicon continues to favor silicon-based MOS technology over technologies founded on electrically superior GaAs.

In the rapidly developing fiber optic arena, fused silica is still “king” due to a combination of properties, including extremely high transparency over a range of usable wavelengths (Figure 1), low material dispersion (~0 at 1.3/üm), high tensile strength (~ 150 kpsi), and high chemical durability. In addition, bulk forms of silica continue to find application in lenses, prisms, windows, and low-coefficient-of-thermal-expansion reflective optics; thin silica films are common components of the highly reflective and anti-reflective surface coatings which are laid down on reflective and transmissive optics, respectively.

We present the possibilities that metallic superlattices offer for the study of materials at length scales ranging from a few to hundreds of angstroms. The materials problems being studied span practically all interesting solid-state phenomena, including superconductivity, magnetism, elastic behavior, development of novel materials, diffusion, ion beam mixing, crystallization, and amorphization. Several applications are also being pursued. We present a few examples of problems that can be studied at different length scales. Emphasis is made that for a proper study of materials properties, extensive structural characterization is imperative.

The importance of crystalline defects in determining the macroscopic properties of metals, ceramics, and semiconductors is well known. Crystalline polymers also exhibit defect structures, but analyzing of defects in polymers is more difficult because the topological connectivity of the covalently bonded polymer chains means that solid polymers typically exhibit larger amounts and types of disorder than crystals of low molar mass molecules. Imaging is also more complex because of the inherent electron beam sensitivity of these typically organic materials. Recent developments in synthesis and processing have made it possible to achieve very highly ordered polymer systems. Defects in these materials will undoubtedly play a critical role in determining mechanical properties, optical behavior, and transport properties such as electrical conductivity.

Direct imaging techniques are particularly important for studying defects because the typically low volume fraction of defects in the crystalline phase makes diffraction experiments difficult if not impossible. Also, in diffraction information about the relative orientation of individual crystallites is lost. It is not surprising, then, that High Resolution Electron Microscopy (HREM) has become extremely important for studying defects in inorganic materials systems. HREM studies of polymers have been limited by the beam sensitivity of organic compounds. Recently, however, high voltage instruments, low dose techniques, and image processing procedures have made it possible to obtain lattice images in systems with beam sensitivities as low as 0.003 C/cm2 (2 e-/A2).

This review discusses the nature of defects in crystalline polymer systems and illustrates how proper application of high resolution imaging techniques promises to answer questions in polymer morphology.

X-ray powder diffraction is one of the most widely used techniques by scientists engaged in the synthesis, analysis, and characterization of solids. It is estimated that there are now about 25,000 users throughout the world, of which about one third are in the United States. Any single-phase polycrystalline material gives an x-ray pattern which can be regarded as a unique “fingerprint,” and modern automated search-and-match techniques used in conjunction with the Powder Diffraction File (maintained by the International Center for Diffraction Data, Swarthmore, PA) allow routine analysis of samples in minutes. From an x-ray pattern of good quality it is possible to determine unit cell parameters with high accuracy and impurity concentrations of 1-5%, so that powder techniques are extremely valuable in phase equilibrium studies and residual stress measurements, for example. In addition, a detailed analysis of line shapes gives information about physical properties such as the size and shape of the individual crystallites, microscopic strain, and stacking disorder.

In the early days of crystallography many simple (and some not-so-simple) structures were solved from x-ray powder diffraction patterns, but the obvious limitations to the number of individual reflection intensities which can be estimated and the increasing sophistication of single-crystal techniques resulted in a decline in the importance of this application in the 1950s and 1960s.

There is considerable excitement and some turmoil in undergraduate and graduate education in the broad field of materials, as well as in its subfields of metallurgy, ceramics, and polymers. While the reasons are many, the underlying driver is the growth in the visibility and diversity of a young field evolving rapidly into a true discipline, much like chemical engineering and biology from their early roots mainly in chemistry.

The University Materials Council (UMC), the group representing accredited materials departments has had a longstanding, obvious interest in education and has been actively involved in assessing current and future directions. More recently, a comprehensive study under the auspices of the National Research Council (the Materials Science and Engineering Study), has been undertaken to develop a unified view of recent progress and new directions in materials science and engineering (MSE), and to assess future opportunities and needs. As part of this ambitious endeavor, a panel on education (Panel 5) was established with the following charges:

• To investigate and document existing human resources in MSE;

• To identify future directions of education in MSE, including education in and out of materials departments;

• To identify needs and opportunities for increasing interdisciplinarity in MSE; and

• To identify needs and opportunities in lifelong education.

Until the 1980s, most people thought of ceramics as artware and artifacts. The recent development of advanced ceramics for high performance thermal insulation (space shuttle tiles), high temperature structures (heat engines), and electronics (superconductors) has dramatically changed perceptions about the utility of ceramics.

High technology ceramics are related to “traditional” ceramics only to the extent that they are inorganic, nonmetallic materials. Traditional ceramics are derived from minerals. For example, dinnerware and bricks consist mostly of clay, while sand is the major ingredient in flat glass and containers. Abundant raw materials, simple processing, adequate performance at low cost, and technological evolution have kept these industries viable for several thousand years.

But, for demanding electronic or structural applications, synthesizing ceramics from minerals is often unacceptable. The chemical variability of mineral deposits, the difficulty of obtaining a homogeneous mixture of powders, and the problems of consolidating the powders into a uniform ceramic with desirable grain morphology, chemistry and grain boundary phases have stimulated the development of chemically derived ceramic precursors.

Polymeric materials are typically considered as insulators, and in fact important applications do rely on their poor conductivity— e.g., electrical cable insulation and charged dielectric films (electrets, electrical analogs of magnets), the latter finding use in microphones. Research in the last decade, however, has lead to the discovery of polymeric materials with extremely high conductivity, approaching that of copper. This brief article will highlight recent work in the synthesis, processing and applications of these novel materials.

Typical polymers, the oxidant (or “dopant”) used to create carriers, and room temperature conductivities are given in Table I. A key feature shared by these materials is delocalized (at least over a few repeat units) π-electron density. Such unsaturated polymers facilitate carrier generation because of the ability for resonance derealization of the resulting radical ions, which can also offer good intramolecular carrier mobility. In addition, the geometry of π-orbitals allows for good orbital overlap and encourages inter molecular carrier transport.

That the polymer chains are considerably shorter than typical sample dimensions indicates that intermolecular transport is dominant, especially in view of the disorder observed in most conducting polymer systems. However, as the number of defects (crosslinks, “twists” which inhibit conjugation) decreases, it might be anticipated that carriers can travel greater distances along a chain before a (presumably) higher activation energy, inter molecular electron transfer becomes necessary, thus affording higher conductivity. Indeed, it has been recently reported that samples of very high quality, oriented polyacetylene, when treated with iodine, exhibit conductivities approaching that of copper at room temperature.

Spectroscopic ellipsometry (SE) is the newest nondestructive and nonperturbing technique for characterizing surfaces, interfaces, and multilayer structures. The technique was originally developed and perfected by Aspnes of Bell Laboratories and a commercial instrument is currently available. After a brief description of the basic principles involved in this technique, one of its many applications — the depth profiling of multilayer structures — is described. Further details about SE and its other applications can be found elsewhere.

The automated spectroscopic ellipsometer that has been built in our laboratory is based on the design of Aspnes and Studna. A schematic diagram of the instrument is shown in Figure 1. It is basically a rotating analyzer ellipsometer operated by an on-line Apple computer, and it has spectroscopic scanning capability. Plane polarized monochromatic light is allowed to be incident on the sample at a chosen angle of incidence. The characteristics of the elliptically polarized reflected light is analyzed by a rotating analyzer. As Budde has shown, Fourier analysis of the detector output in a rotating analyzer ellipsometer yields the desired ellipsometric parameters Δ and ψ characterizing the material under study. Aspnes has pointed out that with proper design, alignment and operation, the precision attained is very high, and accuracy of the data is also as high as that attainable with null ellipsometers. The results of our own studies confirm these conclusions. Such measurements are carried out at a number of discrete wavelengths (˜100) distributed uniformly in the UV-visible-near IR spectral range.

In the last quarter of a century, modification of the near-surface region of materials has become of major technological importance. The principal surface modification technique utilized in integrated circuit technology is ion implantation, a technique which has more recently been applied in the metal-processing industry as well. The very high doses required for applications such as increasing the hardness of steel or forming buried oxide layers in silicon have pushed ion implantation to its limits. Ion beam mixing, the intermixing of surface layers by the penetration of energetic ions through them, was developed to overcome these limits. Additionally, ion beam mixing has been able to produce new phases, amorphous and crystalline, which have technologically and scientifically interesting properties.

Ion beam mixing was studied extensively in silicide forming systems, due partly to applications to electrical contacts for silicon devices. In intermetallic alloy systems, research has concentrated on determining the interplay between the formation of amorphous and crystalline structures and that between equilibrium and metastable phases. Although over 50 alloy systems have been studied, this article will concentrate on the Al-based alloys. These alloys, particularly the near-noble-metal alloys, demonstrate nearly all the features associated with ion-induced phase formation. Further, Al-rich refractory metal alloys form quasicrystalline icosahe-dral alloys. Ion-beam mixing results parallel those of splat-quenching, the technique first used to produce the fivefold symmetric structure.

Research on oxynitride glasses has expanded considerably since the first systematic compositional investigations were begun some ten years ago. This review will briefly introduce that work with an emphasis on more recent developments. Also included is some discussion of the applications of oxynitride glasses in both bulk and thin film form. The literature review is selective, however, and the recently developed phosphorus oxynitride glasses are only briefly mentioned. For more detailed information on earlier work on silicon oxynitride glasses see the reviews by Jack and Loehman. For discussions of phosphorus oxynitride glasses see Marchand, Peng and Day, and Rajaram and Day.

The development of bulk oxynitride glasses has been closely associated with research on Si3N4 and other nitrogen-containing ceramics. The initial discovery that large amounts of nitrogen can be incorporated in silicate glasses originated with analyses of glassy grain boundary phases in Si3N4 that was hot-pressed or sintered with different additives to promote liquid phase formation at the processing temperature. (Some applications of oxynitride glasses take advantage of that close connection to Si3N4. See Applications of Oxynitride Glasses below.) Later, systematic study of the bulk oxynitride glasses showed that nitrogen incorporation increases density, hardness, glass transition temperature and fracture toughness, and, to a certain extent, decreases the thermal expansion coefficient. Nitrogen additions also tend to lower glass solubility in aqueous solutions.