The physical properties of superlattices have been the subject of considerable interest because a wide range of phenomena associated with very thin films, interfaces, and coupling effects can be studied. Recent areas of activity in metallic superlattices include antiferromagnetic coupling of ferromagnetic layers across nonmagnetic spacer layers, giant magnetoresistance, magnetic surface anisotropy, low-dimensional superconductivity, and anomalous mechanical properties. All of these phenomena are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. Therefore, a detailed understanding of the properties of superlattices requires a nondestructive, quantitative determination of the superlattice structure.

Because superlattices are not in thermodynamic equilibrium, their structure is sensitive to preparation methods and growth conditions. A dramatic example of superlattice structural dependence on growth conditions is shown in Figure 1, for sputtered Nb/Si superlattices. Increasing the Ar pressure during sputtering decreases the kinetic energy of the deposited atoms, thereby changing their surface mobility, and thus altering growth dynamics. Figure 1 shows the low-angle x-ray diffraction and cross-sectional transmission electron microscopy (TEM) images of [Nb(35 Å)/Si(25 Å)]40, superlattices sputtered in, respectively, 3 and 15 mTorr of Ar. The TEM image of the 3 mTorr superlattice clearly shows the smooth and continuous layering across the entire cross section of the image (≈5 μm). This is characteristic of sputtered metal/semiconductor superlattices used for x-ray optics.

Phenomenal rate accelerations, exacting selectivities, and mild reaction conditions characterize biocatalysis and have generated considerable interest in nonbiological applications of enzymes. As a result of greater availability and purity, these molecules are now being used increasingly in organic synthesis to prepare complex natural products and novel materials. They are also being exploited industrially in the production of food additives, pharmaceuticals, and fine chemicals.

Unfortunately, for many commercial applications, natural enzymes may be unsuitable. They may be unstable or difficult to isolate; they may function poorly at the temperatures, pH's, and substrate concentrations needed for reaction; or they may lack appropriate specificity. For many chemical reactions, a natural enzyme may not even exist. For example, natural biocatalysts for the synthetically valuable Diels-Alder cycloaddition have yet to be discovered.

In the mid 1980s, reports of exciting progress from GaAs integrated circuit (IC) performance from R&D laboratories world-wide portrayed a rosy future for GaAs. Now, in the early 1990s, true to their reputation, GaAs ICs are still largely the stuff of the future. In fact, deployment of GaAs ICs in real systems has been disappointingly slow. In 1985, the commercial GaAs IC market was forecast to reach $800 million by 1990. The actual figure was only $142 million. To put this number in perspective, it represents less than 0.4% of the total Si IC merchant market.

In a recent survey of the GaAs industry, Kato explored the causes for GaAs troubles, with startling findings. The issue certainly does not seem to be a performance one because GaAs ICs are sufficiently ahead of alternative technologies. Material quality is not a problem either. Extremely high-quality 3 in. and 4 in. GaAs wafers are now on the market at reasonable prices. On the other hand, several serious deficiencies center around IC manufacturing. The price of the final GaAs ICs is perceived as not competitive with alternative technologies. This is rooted in the low yields and poor repeatability of the manufacturing lines. A great contribution to cost is time-consuming functionality testing, particularly for analog products. For MMICs (monolithic microwave integrated circuits) in particular, final testing can easily become the bottleneck of the entire fabrication process. There is also much uncertainty about reliability. This might explain to a large extent the low customer confidence in the technology. Kato reports that fundamental technical problems in making GaAs ICs are still believed to remain.

Since their discovery in 1987, significant progress has been made in the fabrication of high-quality, high-temperature superconducting (HTS) thin films. Films with reproducible properties can be routinely deposited on single crystal substrates by several well-established processing techniques. Single crystal substrates, however, are not suitable for many applications because of their cost, limitations in size and shape, and lack of flexibility. Hence, a great deal of effort has been directed at the fabrication of thin films on polycrystalline rather than single crystal substrates. For example, metallic substrates are expected to be useful for the fabrication of HTS conductors for such applications as generators, motors, and superconducting magnetic energy storage (SMES) devices. For polycrystalline thin-film applications, lattice matching for epitaxial growth of thin films is no longer possible. Microstructures of these films are generally more complex than those of single crystal films, primarily because of grain boundaries. As a result, the microstructure of polycrystalline films must be carefully controlled to ensure that the critical current density is high enough for practical applications.

Happily, progress in this respect has been substantial. There have been laboratory demonstrations of techniques for controlled processing of high-quality HTS thin films on polycrystalline substrates. Even though the technology development in this area is still in its infancy, many successful processing approaches have been developed to set the stage for the eventual use of HTS thin films in power device applications.

This article describes the current status of implantation in silicon for ion doses up to but not including the onset of the amorphous state. Physical phenomena, applications to VLSI, and equipment issues are discussed and linked from the viewpoint of vertically integrated technologies (see Figure 1). In such a view, engineering applications are the customer of research on phenomena, and the equipment industry is the customer of both the research and applications community. The article highlights areas in phenomena, applications, and equipment requiring particular attention.

Studies in physical phenomena associated with ion implantation are motivated by the need for very shallow junctions. The shallowness is limited by ion channeling effects in crystalline silicon (which directly affect the as-implanted impurity profile) and by diffusion (which occurs during implant activation and annealing of damage). The inventory of ion-damage induced defects includes point defects, cluster defects, and dislocations arising from the combination of damage and annealing. A recent survey by Schreutelkamp et al. of low-dose implantation describes advances in characterization of channeling effects, and the formation of dislocation damage which, in turn, impacts thermally processed profiles. These effects need to be understood in terms of modern MOS (metal-oxide-semiconductor) processes.

A 1989 report by the National Research Council (NRC) entitled, Materials Science and Engineering in the 1990s: Maintaining Competitiveness in the Age of Materials, strongly emphasizes that the United States as a nation has devoted insufficient resources to materials synthesis and processing, yet it fails to suggest why. If we are to correct such an imbalance, it is essential that we understand the cause so we can effect a solution.

In my opinion, the cause of our failure to place sufficient emphasis on synthesis and processing is more fundamental than the study of materials science and engineering alone; it is symptomatic of the way we teach all science and engineering. We have failed to instill balanced attitudes of inquiry within our students. They have emulated our academic approach to defining and solving problems, an approach that works very well in academia but unfortunately is not always effective in the world of business and manufacturing. As a result, to be successful in the commercial sector, the young engineer or scientist must “unlearn” much of what has been taught in the universities.

Advances in the methods of deposition and characterization of crystalline films of submicron thickness have been dramatic over the past two decades. A principal motivation for development of this technology has been the potential for use of thin film semiconductor materials in electronic and optoelectronic devices. The main function of these devices is to control transport of electrons in a way which permits high spatial density, and in which the carriers are highly mobile, that is, they show fast response with little power consumption. Spatial control of mobile electrons can be facilitated by combining materials, forming a material heterostructure, with one or more of these materials being a thin film. Carrier confinement is enforced by a difference in energy band structure across the interface acting as a barrier. The exploitation of this physical effect in device design is called bandgap engineering. A great deal of attention has been focused on film/substrate systems involving the III-V compounds (InGaAs/GaAs, for example), as well as on II-VI compounds for optical applications (ZnSe/GaAs, for example). Current efforts are also directed toward SiGe/Si and GaAs/Si systems to exploit well-developed silicon device technology.

During the greater part of the 15th century, the Burgundian princes created a stable, unified center for industry and the flourishing of the arts in the Netherlands. Philip the Good became one of the most powerful and wealthy princes of the House of Burgundy in the period. Under his rule, the Netherlands became an important center for commerce. The port of Bruges, and later Antwerp, offered easy access to the important trade routes. The German merchants of the Hansa towns of Bremen, Danzig, Lübeck, and Hamburg and ships from England and the Baltic regions brought wares to be bought and sold in Flemish towns. The routes along the Atlantic and Mediterranean provided direct lines of communication between Italian merchants from Venice, Genoa, Florence, and Bruges.

The Netherlands soon became a center of a large part of the business activity in Europe and its prosperity grew. The concentration of trade, the presence of numerous banks, and the commission they charged contributed to the wealth of its bourgeois merchants and financiers. They soon became as rich and sometimes richer than the Burgundian princes. Thus they had the means to become important patrons of the arts so as to display their wealth. The acquisition of rare and exotic goods became an essential part of a society where exhibiting one's wealth was admired.

Flemish artists' corporations were well organized, not unlike modern businesses. They were well-known locally and abroad and had significant influence on the art of the period. Works of art were created in workshops where a long apprenticeship afforded the artists guidance and expert training in their craft. High standards which contributed to the good reputation of the art of Flanders, were maintained by setting the quality of the materials and establishing the techniques used. The painters' guild controlled the production of paintings and took measures to control the supply of materials to keep down prices and to control competition. Also, contracts between artist and patron would sometimes stipulate the type of materials to be used.

Materials science has come of age. The need now exists to establish programs in education and research in materials science at colleges and universities throughout the world. It is not realistic to expect that every university will receive funds to found entirely new programs. However, the infrastructure for materials science already exists at most universities that have offered traditional studies in the natural sciences. It is the purpose of this article to provide ideas and suggestions for initiating new academic programs in materials science by using existing resources. The major prerequisite for success is a faculty and staff who are willing and able to adapt to a rapidly advancing scientific environment and who have the individual initiative to seize and profit from the many new opportunities in materials science. This article reports an experiment in progress at Bowling Green State University, Ohio which uses the strengths of the Departments of Biology, Chemistry, Geology, Physics and Technology to develop a new program in materials science. I hope it will serve as a model for other universities.

All living cells, and many of the structures within these cells (mitochondria, nuclei, chloroplasts) are surrounded by biological membranes which serve to separate the cell contents from the surrounding environment. The biological membrane is an extraordinary material. It controls the highly selective transport of molecules into and out of the cell. It senses the environment outside the cell and transmits information about it to the intracellular machinery. It reports information about the cell to the outside world—its identity and its state of function. It transports electrons, converts sunlight to chemical and electrical energy, pumps small molecules against a concentration gradient, and uses that gradient as a source of energy. The membrane is a generally robust structure, and one that can be modified in a controlled manner, making it adaptable for use in nonbiological applications. It has served as a model for sensors and detectors, for surface modification agents, for drug delivery systems, and for information storage and delivery, as well as other optoelectronic functions.

Narrow, passivated metal lines are generally used as interconnects in VLSI microcircuits at the chip level. In most metals, high electric current densities lead to a mass flow of constituent atoms accompanying the current of electrons. Electromigration (EM) has long been considered an important reliability concern in the semiconductor industry because the current-induced atomic fluxes can give rise to void formation and open circuits, or hillock formation and short circuits between nearby interconnects. The problem is exacerbated because of the continued trend of increasing the density of the devices on the chip. This means that the line widths of the interconnects have been reduced and are now in the submicron range; correspondingly, the current densities have increased and may be as high as 106 A/cm2. Recently, thermal-stress-induced damage in metallizations has also been recognized as an important reliability concern, perhaps of the same gravity as EM. Thermal stresses in the metallizations are caused by the different thermal expansion coefficients of the metal and the substrate. Stress-induced void and hillock formation are the main causes of in terconnect failures before service. More recently, concern has been growing that thermal stresses or thermal-stress-induced voids may enhance the subsequent electromigration damage during the service life of the microchips.

For simplicity, this article addresses the case of pure aluminum metallizations on oxidized silicon substrates. However, much of what is said applies to other metal-rigid substrate systems as well, most notably to various aluminum and copper-based metallizations on ceramic substrates. The present treatment emphasizes void formation and growth in the metallizations during nd after cooldown from elevated temperatures, or those due to electromigration in service or testing conditions. Many of the mechanisms we explain are also applicable to hillock formation under compressive stresses, whether due to EM or thermal cycles during manufacturing.

Experiments in ion implantation were first performed almost 40 years ago by nuclear physicists. More recently, ion implanters have become permanent fixtures in integrated circuit processing lines. Manufacture of the more complex integrated circuits may involve as many as 10 different ion implantation steps. Implantation is used primarily at f luences of 1012–1015 ions/cm2 to tailor the electrical properties of a semiconductor substrate, but causing only a small perturbation in the composition of the target (see the article by Seidel and Larson in this issue of the MRS Bulletin). Applications of implantation had been limited by the small beam currents that were available, but recently a new generation of high-current implanters has been developed. This high-current capability allows implanting concentrations up to three orders of magnitude higher than those required for doping—enough to create a compound.

High-temperature superconductors are brittle oxide ceramics, yet they have been made into wire that has been wrapped into solenoids and used in demonstration magnets and motors. Fabricating wires from these ceramics is an extremely challenging materials science process that requires a precisely engineered microstructure with the correct chemical, mechanical, and electromagnetic properties if these wires are to transport large current densities (Jc) in high magnetic fields. Heine et al. first demonstrated that wires of these materials could carry high Jc in very high magnetic fields. At 4.2 K, the oxide superconducting wires can carry higher Jc at higher magnetic fields than conventional Nb-Ti or Nb3Sn wires (Figure 1), and as shown in the companion article in this issue by Kato et al. they can also have high Jc at 77 K.

Of the three major families of high-temperature superconductors, YBa2Cu3O7-x, Bi-Sr-Ca-Cu-O (BSCCO), and Tl-Ba-Ca-Cu-O, the best wires to date have been made in the BSCCO system. At present, all YBa2Cu3O7-x wires are weak linked and have only small Jc in magnetic fields. In the Tl-based system, the superconducting properties are potentially very interesting, but the toxicity of Tl and the system's complex processing have limited conductor development. For the Bi-based system, the basic processing steps are becoming known, the grains are well connected, and the weak link problem can be controlled. This permits applications in the temperature range 4–77 K, depending on the field and current density requirements of the particular use.

The successful use of pulsed laser deposition (PLD) to fabricate thin film superconductors has generated interest in using the technique to deposit thin films of other materials. The compositional fidelity between laser target and deposited film and the ability to deposit films in reactive gas environments make the PLD process particularly well suited to the deposition of complex multicomponent materials. Cheung and Sankur recently provided an excellent review of the PLD field, including a table of over 100 elements, inorganic and organic compounds, and superlattices that have been laser evaporated. Over 75 of these materials were deposited as thin films.

The goal of this article is to provide an introduction to some of the newer applications of PLD for thin film fabrication. Four classes of materials are highlighted: ferroelectrics, bioceramics, ferrites, and tribological materials. Ferroelectric materials are structurally related to the high-temperature superconducting oxides and therefore are a direct extension of the recent superconducting oxide work. Bioceramics are dissimilar in structure and application to both ferroelectrics and superconducting oxides, but they are complex multicomponent oxides and, therefore, benefit from the use of PLD. Ferrites, also complex, multicomponent oxides, represent another exciting, but only lightly explored opportunity for PLD. In contrast, tribological materials are typically neither complex nor multicomponent. Nevertheless, interesting structures and properties have been produced by PLD. A few of the more important ones will be discussed. These different types of materials demonstrate the diversity of capabilities offered by PLD.

For efficient and economic production of discrete parts with any material, it is always desirable to obtain the final product using one process alone, without secondary operations. This is called “net-shape” manufacturing. Injection molding is perhaps the best net-shape manufacturing process for materials that can be handled by this method. For polymers and some of their composites, injection molding can mass produce high-precision parts with complex geometry at very low cost. In engineering applications, some injection-molded plastic parts of intricate shape with tight tolerance can be snap-fitted together for easy assembly, while others can achieve extremely high quality to meet the optical functional requirements for such parts as optical lenses and information-storage disks.

Since the oil crisis in the early 1970s, the use of polymers as an engineering material has been rapidly increasing to improve energy efficiency for both the processes themselves and the final products. Plastics still constitute one of the world's fastest growing industries. Polymeric materials and their composites offer various desirable properties including high strength-to-weight ratio, corrosion resistance, clarity, and others. Among all the polymer-processing methods, injection molding accounts for approximately one-third by weight of all the polymeric materials processed.

In 1925, P. Auger first observed the so-called Auger electrons in a Wilson cloud chamber. He explained this occurrence as being due to a radiationless transition in atoms excited by a primary x-ray photon source. In 1953, Lander first pointed out that Auger electrons arising from solid samples can be detected in the energy distribution curve of secondary electrons from surfaces subjected to electron bombardment. Moreover, low-energy Auger electrons (∼1 keV kinetic energy) can escape from only the first several atomic layers of a surface since they are strongly absorbed by even a monolayer of atoms. Thus Auger electron spectroscopy (AES) possesses high surface sensitivity. This is one characteristic that makes AES very useful for the study of thin films. For such applications, an important development in AES occurred when Harris showed that the sensitivity of the detection of Auger electrons can be improved by differentiating the electron energy distribution curve with respect to the energy. Furthermore, Weber and Johnson demonstrated that, provided the Auger line profile does not change, the peak-to-peak height in the differentiated energy distribution curves is proportional to the Auger current in the peak. Therefore, in addition to its surface sensitivity, AES also can be used for quantitative studies of thin films.

Like AES, x-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that uses the energy distribution of electrons ejected from a thin film for quantitative analysis. However, in many ways the information provided by AES and XPS is complementary.

Glasses are a minor fraction of Earth materials. However, all igneous processes within the Earth involve a melt phase (magma), and our understanding of the physical and chemical features of these processes is contingent upon an adequate understanding of the melt phase itself. In particular, we wish to relate the behavior of such melts to their composition and structure in the range of physical and chemical conditions in which they occur. The physical conditions of many of these processes are particularly difficult to reproduce in the laboratory, and the ability to predict behavior under such conditions is the goal. From a geological viewpoint, the primary interest is in silicate melts because of the composition of the Earth's lithosphere and mantle. There are significant experimental difficulties in looking at the behavior of silicate melts, so much work has focused on glasses as melt analogues since spectroscopic data on silicate glasses and melts (Figure 1) are similar.

Silicate glasses are also of considerable industrial interest, and early work in the geological field borrowed heavily from materials science. However, the thrust to understand natural magmas led geologically based work into more complex chemical systems (e.g., hydrous alkali-alkaline-earth-aluminosilicates) and their behavior at high pressures.

The structure of glasses is a complex issue, and many different experimental techniques are needed to resolve the details of their structure. Here I will focus on the following techniques: IR (infrared), Raman, Mössbauer, EXAFS (extended x-ray absorption fine structure) and XANES (x-ray absorption near-edge spectroscopy) spectroscopies.

An obvious parallel of structure and function exists between a rhinoceros and a tank, and between a beetle shell and the skin of an aircraft. We can also draw comparisons at the microstructural level between these biological and synthetic materials. Significant differences also exist, however, and the rigid biological materials such as bone and shell have much to teach us. In particular, they are distinctly composite structures. Although they bear loads in much the same way as synthetic composites or ceramics, they have far more complex architectures.

The goal in considering the group of mineralized biological materials as described, for example, in the article by Fink et al. in this issue, and in devising modifications of them, which is the focus of this article and of Mann's, is to learn to devise arrangements of synthetic materials that work more efficiently than the homogeneous substances of simple composites that we use now. In addition to designing better microstructural arrangements we may also learn, again by analogy to the biological materials, how best to process these structures and how to recycle them after use.

Biological structural materials are optimized for their high strength- or stiffness-to-weight ratio. Achieving this in synthetic materials for nonbiological application, for example in cars and airplanes, would be of obvious value. Our own interest here has focused on cuticle and bone as models for our synthetic work. Another property of biomineralized materials, for example biological ceramics, is their increased toughness. In this case we will discuss tooth enamel mimicking.

Pollution prevention and the preservation of environmental quality are issues that can no longer be ignored. For years, environmental organizations have struggled, somewhat unsuccessfully, with U.S. industry and congressional legislators, focusing on local, national, and worldwide issues relating to environmental quality. Meanwhile, global pollutants such as acid rain (from sulfur oxide emissions), carbon dioxide, volatile organics (such as chlorofluorocarbons (CFCs) and benzene), metals (such as chromium, cadmium, mercury, and lead), dioxins, and pesticides continue to be introduced into the environment. Many consider global environmental effects potentially serious due to “partitioning,” a phenomenon where pollutants migrate to the media where they are most stable. The result is contamination in the air, water, and soil, creating a high potential for exposure to pollutants. Consider, for example, the depletion of the earth's stratospheric ozone layer, promoted by the annual emission of various CFCs equivalent to the emission of over 100 million pounds of equivalent CFC-11 worldwide (CFC-11 has an ozone depleting potential of 1.0 with other CFCs having ozone depleting potentials between zero and one). Consider also the heavily debated global warming/greenhouse effect, caused to a large degree by CO2 emissions from spent fossil fuels, one-fourth of which are consumed in the U.S. Many examples can be cited regarding the presence of toxic substances in remote lakes and streams throughout the world. Now, as the ill effects of our past and present actions are becoming visible and finally better understood, the pressure on U.S. manufacturers is increasing to take responsibility for products and processes contributing to the world's environmental degradation. This pressure will continue to rise until real solutions to these problems are developed and implemented.