We humans have employed and improved materials for millennia, but it required the Industrial Revolution of the last century to birth the systematic, science-based development of materials. During this time, effort expended in understanding the process-microstructure-properties relationships of materials conferred great economic and military advantage upon the successful. The introduction of machine power in this era created great leverage for improvements in the strength, ductility, corrosion resistance, formability, and similar properties of materials. Response to this opportunity led to the emergence of the materials profession. Stimulated by opportunity, materials scientists and engineers of the day met many of the challenges by first understanding and then controlling the composition and microstructure of materials. In the process, they defined the materials-engineering profession and left their names as a part of its vocabulary: Martens(ite), Bain(ite), Austen(ite), Schmid, Bessemer, Charpy, and Jomminy, to name a few. In fact the understanding and control of microstructure is the hallmark of materials science and engineering. Of course the ancient art of finding, mining, concentrating, and refining materials from the earth's crust does not apply to this definition since we wish to focus on the engineering of materials.

Five decades ago, a new chapter in the evolution of this profession began by the invention of the transistor. This invention and the development of integrated circuitry that followed from it spawned a new era of materials achievement, again stimulated by the enormous economic and performance gains available. In this arena however, the object of the game was to completely eliminate microstructure while doing away with impurities, save for a desired few, to levels previously unimagined. Today a material thus prepared is a blank slate upon which we can write the microstructure of an integrated circuit.

It has been estimated that more than 80% of all medication doses are administered as tablets—that is, in unit dosage forms prepared by compacting powders in dies. For drugs with acceptable oral-absorption profiles, the compacted tablet is the first choice as a delivery system in drug development. Modern tablets range in size from 25 milligrams (veterinary implants) to several grams (veterinary “boluses”). The most common tablet shape is the double convex-faced disk. Shapes range from capsule to triangular forms, in many instances with bisects embossed or debossed with identification codes and company logos. Tablets may be either film- or sugar-coated, may be designed for immediate or extended release, or may be prepared as multilayer or laminated forms. In addition to oral administration, tablets may be predissolved or designed to be disintegrated in the mouth, or may be administered by injection or as suppositories. Production rates for over-the-counter products such as acetaminophen tablets can exceed three 1,000-kg batches per week, requiring high-speed multiple-station tablet presses for production. In contrast, implants—which have low-volume market requirements—must be made under aseptic conditions on special singlestation presses.

The microstructure and properties of a ceramic component are largely predetermined by the processes and process controls used to manufacture them. The metric for success in manufacturing is often based on gross density. For example, optimizing pressure-density response, maximizing overall density, and minimizing springback and delaminations in powder pressing all focus on characterization and control of the overall (macroscopic) state of a powder compact. Unfortunately this focus on macroscopic effects has contributed to a general neglect of the compact at the microstructural level. Process-control variables in powder compaction have been defined and discussed by many workers, but their quantitative application to predict and control compaction behavior is limited. Advances in characterization technology and computer modeling now allow us to quantitatively characterize and simulate microstructures more easily. These and other tools can help provide the scientific and technological foundation necessary to predict and control microstructure and microstructural evolution during processing.

The group-III-N materials (GaN and its related alloys, InGaN and AlGaN) are a rapidly emerging family of compound semiconductors that have great potential for a wide range of optoelectronics and electronics applications.

GaN-based blue light-emitting diodes (LEDs) are commercially available and can already be found in full-color, billboard-size displays in major cities in Japan. Achievement of continuous-wave (cw) laser operation at blue wavelengths will lead to the commercialization of these coherent sources probably within the next few years. Like other semiconductor lasers, these devices can be manufactured relatively cheaply (eventually for a few dollars), and they are expected to replace current ($1,000 and up) ultraviolet and blue-laser sources in medical spectroscopy, high-density information storage, and in projection displays.

This article reviews the key role of the crystal structure of GaN grown on sapphire, which is currently the most widely used substrate. We trace the origin of this crystal structure from nucleation and show how it evolves as growth proceeds. We show that the final GaN film is an ordered polycrystal (rather than a single crystal), in which the individual grains are of high crystalline quality but are “tilted” and “twisted” with respect to one another. We conclude by speculating on how this microstructure may explain the unusual optical and electrical properties of GaN.

The key to engineering a material lies in exploiting its beneficial characteristics while minimizing its inherent weaknesses. Whether the weakness is, for example, poor corrosion resistance or low hardness, applying a relatively thin coating of another material that mitigates the shortcoming of the underlying material is a practical solution allowing the composite pieces to be used in demanding environments. This method has been utilized in a wide variety of cases ranging from paint on wooden fences and ceramic thermal barriers on single-crystal superalloy turbine blades to tungsten carbide hard-facing layers on drilling equipment. Some materials may suffer from high cost and therefore are used as a thin layer to impart their desired properties. For instance, gold leaf is applied to buildings for appearance while diamond films are deposited onto normal cutting tools to improve their performance. The specific application typically dictates both the material and the deposition method for the coating. The gold leaf does not need to offer much resistance to abrasion or mechanical stress in order to maintain its beautiful shine far into the future. In contrast the diamond film must be strongly adhered to the underlying cutting tool surface if it is to survive the punishing wear and thermal stresses of machining operations.

Most pairs of chemically different homopolymers of any useful molecular weight are thermodynamically immiscible. This is because any tiny repulsive interaction per monomer, multiplied by the large number of monomers in a typical polymer, overwhelms entropy of mixing (of the order of kBT per molecule where kB is the Boltzmann constant and T the temperature). Nonetheless blends of immiscible pairs of homopolymers are industrially ubiquitous. Two broad categories of such materials are rubber-toughened plastics and stiffened elastomers. Both are fine dispersions of one polymer in another, with the desirable properties of the resulting material (e.g., stiffness-toughness balance) depending crucially on its composite structure and the mechanical contrast between the two components (e.g., difference in moduli).

Typically a fine dispersion, consisting of submicron particles, is necessary for good final properties. Also necessary is a strong interfacial bond between the particles and the matrix so that the resulting material does not fail at these interfaces. A typical volume fraction of the suspended component is perhaps 20–30%. Such polymer blends are prepared by two time-honored techniques (ancient, really, if one considers their antecedents in cookery): stirring and the use of polymeric surfactants—copolymers. The copolymers can be synthesized beforehand and added to a mixer where the components are stirred, but more often they are produced from the homopolymers during stirring by a grafting reaction, which somehow joins the two dissimilar homopolymers to form some sort of block copolymer.

Low-k organic polymers such as polytetrafluoroethylene (PTFE) are promising materials for use as interlayer dielectrics (ILD) because their dielectric constants are generally lower than those of inorganic materials. However poor adhesion with Si substrates, poor thermal stability, and production difficulties have hindered their use in microelectronics.

On the other hand, plasma-enhanced chemical vapor deposition (PECVD) of polymer films (plasma polymerization) has many advantages that help to overcome these problems. Plasma-enhanced chemical vapor deposition uses a glow discharge to create activated species such as radicals and ions from the original monomer, and the polymer films are deposited through various gas-phase and surface reactions of these active species, including ablation of the deposited film. No water is generated during plasma polymerization, and the influence of a solvent can be ignored. Also a layered structure that promotes adhesion can be easily fabricated by changing the source compounds.

Recently the use of fluorinated amorphous carbon thin films (a-C:F) as new low-dielectric-constant interlayer dielectrics has been proposed. These thin films have an amorphous C–C cross-linked structure (including sp3 and sp2 bonded carbon) and have the same C–F bonds found in PTFE. The strong C–F bonds decrease the dielectric constant, and the C–C crosslinked structure maintains the film's thermal stability. The a-C:F film can be deposited from fluorocarbon source materials using PECVD. Typically fluorocarbons such as CF4, C2F6, C4F8, and their hydrogen mixtures are used as source materials. First the a-C:F films for low-k ILD, with a dielectric constant of 2.1, were deposited from CH4 + CF4 mixtures by using parallel-plate PECVD.

The recent developments in III-V-nitride thin-film technology has produced significant advances in high-performance devices operating in the blue and green range of the visible spectrum. These materials are grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. Highly specular surfaces are possible by use of low-temperature buffer layers following the method developed by Akasaki et al. The thin films thus grown have an interesting microstructure, quite different from other known semiconductors. In particular, epilayers with high optoelectronic performance are characterized by high dislocation densities, several orders of magnitude above those found in other optoelectronic semiconductor films. The lattice mismatch between sapphire and GaN is ∼14%, and the thermal-expansion difference is close to 80%. In spite of these large differences, little thermal strain is measurable at room temperature in epilayers grown at temperatures above 1000°C. Epitaxy on other systems, like SiC, with much better similarity in lattice parameter and thermal-expansion characteristics, has failed to produce better performance than films grown on sapphire. The origin of these puzzling properties of nitrides on sapphire rests in its microstructure. This article presents a survey of the microstructure associated with epitaxy of nitrides by MOCVD.

Interest in semiconductor grain boundaries relates to the development of polycrystalline materials for photovoltaics and integrated-circuit interconnects. Although these structures are responsible for deleterious electrical effects, there are few experimental techniques available to study them at the required atomic scale. Therefore models of the physical processes occurring at grain boundaries have necessarily taken a macroscopic approach. Fortunately recent developments have resulted in tools that provide unprecedented glimpses into these interfaces and that will allow us to address anew the connection between grain-boundary structure and properties.

Z-Contrast Imaging

When exploring the unknown, we rely heavily on our eyes (incoherent imaging) to provide a direct image of a new object. In order to explore the unforeseen atomic configurations present at extended defects in materials, it again would be desirable if one could obtain a directly interpretable image of the unfamiliar structures present in the defect cores. Z-contrast electron microscopy provides such a view with both atomic resolution and compositional sensitivity.

This high-resolution imaging technique differs from conventional high-resolution phase-contrast imaging. The phase-contrast technique produces a coherent image, an interference pattern formed by recombining the waves diffracted by the specimen. In the Z-contrast technique, the image is incoherent; it is essentially a map of the scattering power of the specimen. Additionally as was first determined by Lord Rayleigh, the incoherent mode of image formation has double the resolving power of the coherent mode.