In manufacturing microelectronic and optoelectronic devices, thin solid films of various sorts are routinely deposited and etched using low pressure, weakly ionized plasmas. The term “plasma” in this context implies an ionized gas with nearly equal numbers of positive and negative charges. This definition is not very restrictive, so. there are an enormous number of phenomena that are termed plasmas. For example, very hot, magnetized, fully ionized plasmas exist in stellar environments and thermonuclear fusion experiments. High temperature electric arcs are a form of plasma as well. In contrast, the plasmas used in electronic materials processing are near room temperature and the gas is usually weakly ionized. Indeed, due to the sensitivity of electronic devices to high temperatures, their low operating temperature is one of the major advantages of plasma processes.

Plasma processing is attractive because of two important physiochemical effects: energetic free electrons in the plasma (heated by applied electric fields) dissociate the neutral gas in the plasma to create chemically reactive species; and free positive ions are accelerated by the plasma electric fields to surfaces bounding the plasma. Reactive species created in the plasma diffuse to surfaces and adsorb; wafers to be processed are typically placed on one of these surfaces.

The combination of neutral species adsorption and positive ion bombardment results in surface chemical reaction. If the products of the surface reaction are volatile, they leave the surface and etching results. If the products are involatile, a surface film grows.

This is the second of a short series on the federal budget process. Here, we will focus on how the science budget is set inside the Executive Office of the President. The main players here are the Office of Management and Budget (OMB) and the Office of Science and Technology Policy (OSTP).

The budget process for a given fiscal year starts in the agencies. For example, the fiscal year 1991 budget request to Congress was put together by the agencies during the winter, spring and summer of 1989. In the Department of Energy (DOE) for example, budget requests are obtained from the field offices. Typically, these requests are an exercise in optimism, founded on the time-honored notion that you are certain to get nothing if you don't at least ask.

In any event, DOE headquarters staff, after receiving these requests, pare down the totals in a process not unlike that followed within the OMB. This can resuit in weeping and gnashing of teeth at the field offices even at this early stage. While not generally appreciated, OMB staff can often do much of their budget cutting work before even receiving the formal budget request by a judicious combination of friendly advice, jawboning or more extreme measures which this writer will not go into. (On a more positive note, previous administration guidance—e.g., a planned doubling of the National Science Foundation (NSF) budget in five years, or a commitment to a given schedule for space probes, satellites, etc., in the case of NASA—can largely “hard wire” the budget process before the budget submission to OMB).

From the beginning of the activity in high temperature superconductivity (near the end of 1986), there were efforts to grow single crystals of the compounds. The push for single crystals arose because earlier work on Chevrelphase superconductors (e.g., PbMo6S8) or re-entrant superconductors (ErRh4B4) showed that the properties of complex superconductors were easily masked by impurities, grain boundaries, imperfectly averaged properties, etc.

As early as the so-called “Woodstock of Physics” (the American Physical Society Meeting in New York, March 1987), a single-crystal, x-ray diffraction structure was given for YBa2Cu3O7-x (YBCO-123, with Tc = 93 K), the composition of which had been announced only three weeks earlier. The sample qualified as a single crystal because a single grain had been separated out for the x-ray study, although it was too small for any other measurements.

But, single crystal studies are not infallible, and may not always be the best route. For instance, the aforementioned x-ray work on YBCO gave the wrong structure because the x-ray scattering length from oxygen atoms is too small to yield enough scattering intensity to locate them. It remained for neutron-scattering work with large powdered samples to give the correct orthorhombic structure, having Cu-O chains and warped Cu-O planes (see Figure 1).

“Sand” represents a “state of matter” to which not much attention has been paid. A sand pile can be formed into different shapes; it can exist in many stable states, almost all of which are not the flat lowest energy state. Thus, sand contains memory; one can write letters in sand. If a heap of sand is perturbed, for instance by adding more sand, by tilting the pile, or by shaking it, the system goes from one metastable state to another. In some sense, this happens by a diffusion process, but this process is very different from the process which relaxes a glass of water to equilibrium after shaking. The diffusion process in sand can stop at any of many states, and the process is a threshold process, where nothing happens before the perturbation reaches a minimum magnitude.

The threshold dynamics of sand is a paradigm of many processes in nature. Earthquakes occur only when the stress somewhere on the crust of the Earth exceeds a critical value, and the earthquake takes the crust from one stable state to another. Economie systems are driven by threshold processes: the individual agents change their behavior only when certain factors reach a certain level. Biological species emerge or die when specifie conditions in the ecology are fulfilled. Neurons in a network fire when the input reaches a threshold level, etc.

This article aims to review in rather cursory fashion the ways the concepts of lattice defects have contributed at an early stage to our understanding of solid-state kinetics in metals and insulators.

The importance of point defects in solid-state kinetics was just beginning to be widely recognized in the late 1930s. The crucial experiments of Kirkendall and others were still to come. Many of the leaders in the field of metallurgy believed almost tacitly that diffusion in substitutional alloys occurred by direct interchange or perhaps a ring mechanism.

For the ionic salts, however, basic calculations were further advanced, and it was possible to figure quite confidently the role of Schottky and Frenkel defects in facilitating atom movements. In their seminal paper, Mott and Littleton made specific calculations as to the energies involved in diffusion by the various mechanisms and hence to the relative importance of these mechanisms in the kinetics of these materials. They began by taking over the Born-Mayer short-range formula for ionic repulsion. Next they treated in detail the polarization response of the salt to an extra charge in the lattice, whether interstitial or vacancy. This polarization included the individual polarizabilities of the ions and, for the static case, the ion displacements. Application of this analysis gave good quantitative results for the activation energies to be expected for diffusion and ionic conductivity. For the alkali halides it was made clear that the Schottky defect would dominate and that Frenkel defects would be few.

Imaging is about communicating. Through images, it is possible to convey information about a subject in an exceedingly efficient manner. We are all familiar with the nightly news reports and how a few visual images can rapidly convey a deep impression far beyond that of the accompanying spoken word. The worlds of art and photography are based on images and their power to invoke a sense of beauty and awe in our minds. Both of these aspects are also important for imaging in materials science, although the beauty and awe we wish to convey concerns our understanding of the behavior of materials or the laws of physics and not the aesthetic appeal itself.

An image can convey an insight into a materials problem which is unrivaled in depth and breadth. In many cases, it can reveal aspects of the problem which were completely unforeseen. An image may even reveal the key to the entire problem and so provide a fundamental advance in our understanding, in contrast to the incremental advances which usually result from more or better data. It is this theme which links the articles in this issue of the MRS BULLETIN. Covering semiconductors to cement, the atomic scale to the macroscopic, the theme in common is the power of the image to unravel the story. Often this involves changing our preconceived ideas on the subject (images provide constant proof that “truth is stranger than fiction”), but this is what major advances in research and development are all about.

Polymerization reactions are generally divided into two broad classes: step growth or polycondensation reactions (examples would include the synthesis of polyamides and polyesters), and chain growth processes such as those used to prepare polyethylene or polystyrene. These processes are illustrated schematically in Figure 1.

The statistical nature of step and chain growth polymerization processes ensures that the products of such reactions must be heterogeneous. Conventional polymeric materials thus consist of mixtures of chains, often characterized by relatively broad distributions of chain length or composition. In many materials applications, this kind of molecular heterogeneity is advantageous since it suppresses crystallization and helps to preserve desirable properties such as optical clarity or elasticity. On the other hand, synthetic developments that afford improved control of macromolecular architecture have had profound impact on materials science and technology. As examples, one can cite the discovery of Ziegler-Natta polymerization, now used to prepare billions of pounds per year of crystalline polyolefins, or the development of living anionic polymerization of olefins, which led directly to block copolymers and the commercially important thermoplastic elastomers.

The advent of recombinant DNA methods has provided a basis for developing polymeric materials characterized by essentially absolute uniformity of chain length, sequence, and stereochemistry. This article outlines the principles governing the cloning and expression of artificial genes, and examines the potential role of artificial proteins in polymer materials science.

The word “viscoelastic” is used to describe the mechanical response of materials exhibiting both the springiness associated with elastic solids and viscous flow characteristics associated with fluids. A familiar example of a material called viscoelastic is Silly PuttyTM. If a blob of Silly Putty is rolled into a ball and then dropped onto a hard surface, it will bounce like an elastic ball. If the ball is placed on a hard surface, its own weight will cause it to flow into a puddle. This behavior indicates that time is an intrinsic parameter in discussing viscoelastic response of materials. The elastic response is associated with a contact force of very short duration. The flow into a puddle occurs when forces act for a long period of time.

Viscoelastic response occurs in materials such as soils, concrete, cartilage, biological tissue, and polymers. Soils and cartilage can be thought of as porous solids filled with fluid. Viscous response is due to the flow of the fluid in the pores; elastic response is due to the distortion of the porous solid.

During the 20 years I have been working in the field of metal matrix composites, I have always been drawn to the study of interfaces. Any problems the materials researcher or developer encounters will eventually be tied to some issue involving the interface. The interface controls the in-situ fiber strength and hence the axial strength of the composite. The transverse strength of composites is also controlled directly by the strength of the interface. It follows that in order to optimize a fiber/matrix reinforcement system, one must also optimize the interface. It is no accident, then, that so soon after the two excellent issues on interfaces edited by D. Wolf and S. Yip (MRS Bulletin, September and October 1990), we launch into the subject again. This time, we approach the interface from the point of view of integrating the fields of materials science and engineering, i.e., integrating the structure-processing-properties-materialssynthesis relationships.

The concept of interface design is relatively new. Until recently, we simply controlled damage and limited fiber degradation by controlling processing variables, and contented ourselves with what nature allowed. For the most demanding applications, nature was not permitting reliable structures.

Our emphasis in this issue of the MRS Bulletin is the development of the skills and strategies to design interfaces to a specifie application. In the future, we will be asking materials to perform structurally at temperatures approaching 3000°C in some propulsion applications. The success of such applications will depend on how well we design the interfaces.

This issue of the MRS Bulletin follows up on the November issue's five articles on point defect phenomena in a wide range of materials with five more articles on point defects. The present articles emphasize the behavior of different phenomena in various materials—nonstoichiometric metal oxides, intermetallic compounds, type II superconductors and semiconductors—in terms of fundamental properties of point defects. Again, point defects is the unifying theme but the emphasis shifts to material behavior.

This issue begins with Marshall Stoneham's article on the roles theory plays in predicting and understanding material behavior in terms of point defects in the different classes of materials. The following article by Rüdiger Dieckmann discusses the relationships between point defect concentrations in nonstoichiometric metal oxides and diffusion, i.e., mass transport. Next, Georges Martin and Pascal Bellon review their new approach for analyzing the role played by antisite defects in nonequilibrium phase transitions in intermetallic compounds. Then, Donglu Shi focuses on the effect of point, line, and planar defects on three major properties of type II superconductors—the critical transition temperature, the upper critical magnetic field, and the critical current density. Finally, Lionel Kimerling shows how defect engineering is used to achieve a high degree of complexity in product fabrication and greater sophistication in product performance; he illustrates what he means by defect engineering with examples from basic processes used in electronic materials processing.

The Von Hippel Award is especially significant to me for several reasons. The name Von Hippel is legendary in the halls of Building 13 at the Massachusetts Institute of Technology, where I have worked for the last 12 years. Professor Von Hippel was one of the first to appreciate the interdisciplinary nature of materials research and engineering, and his insulation laboratory at MIT provided an early prototype for the National Science Foundation-supported system of interdisciplinary materials research laboratories in existence today. These laboratories, both at MIT and earlier at Cornell, have helped me in my research in many ways over the years.

Also I have been a recent councillor of the Materials Research Society and have been privileged to witness the phenomenal growth of MRS at first hand. The stature of the Von Hippel Award must be increasing at least in proportion to this growth. I, therefore, feel very honored and grateful indeed! I would also like to add that much of my work has been done in collaboration with valued colleagues and students. They deserve a substantial share of this award.

Over the years, my research has been devoted mainly to the experimental study of crystal defects and their behavior. These defects can be classified according to their dimensionality as shown in the accompanying table, and they are involved in one way, or another, with a host of phenomena in crystalline materials. Since I address a rather diverse audience, it might be appropriate to discuss some of the history of lattice vacancy research and some of the details of my personal contribution to it.

The term “polymeric liquids” is used to describe both solutions of polymers and molten polymers. All polymeric liquids exhibit non-Newtonian flow behavior, including a shear stress-dependent viscosity and elasticity. However, concentrated solutions and melts of polymers whose molecular weights exceed a certain critical value (the “critical molecular weight for entanglement,” Mc) exhibit quite remarkable deviations from classical fluid behavior, especially marked elasticity. Among the remarkable rheological phenomena exhibited by these materials are elastic recoil and flow-induced an-isotropy. Indeed, in certain situations, such materials can exhibit elastic effects that are almost indistinguishable from those exhibited by cross-linked rubbers. This behavior is important, because most commercial “thermoplastics,” such as polyethylene and polystyrene, have high molecular weights (M > Mc) and are processed in the molten state.

A given generic polymer, polyethylene for example, can exhibit a wide range of properties depending on the molecular weight distribution. Another important aspect of molecular structure is branching. For many monomers (the molecular building blocks that make a polymer molecule), two types of polymer structure are possible, linear and branched. For example, ethylene can be polymerized in two ways to form either linear polyethylene or branched polyethylene. Branching enhances the non-Newtonian and elastic aspects of the melt flow behavior. Yet another possible aspect of polymer molecular structure is the presence of a comonomer.

Among the numerous experimental techniques available for comprehensive materials characterization, light optical methods play an outstanding role. Based, in all cases, on the interaction of electromagnetic radiation with matter in various configurations (e.g., for absorption, thermalization, re-emission, elastic and inelastic scattering, or for microscopic imaging experiments), optical techniques provide detailed information about many structural, electronic, and other materials properties. Typically, propagating electromagnetic waves (“normal” photons) are used for these studies, and a broad spectrum of powerful optical instruments is available for investigating all kinds of bulk materials. However, where thinner and thinner samples are of interest—eventually only one monomolecular layer (or even less) thick—some of these standard techniques lack the sensitivity necessary to be useful. On the other hand, there is a growing (nano)tech-nological demand for thin-film preparations especially in the polymeric materials area, e.g., as thin lubricating, isolation, or protective coatings, for integrated or nonlinear optical devices, or to increase the biocompatibility of inorganic materials. This has stimulated the development of novel optical techniques with increased sensitivity, specificity, and spatial resolution. For interfacial properties and thin film samples, the use of a different kind of light, called evanescent waves, has proved in recent years to be particularly helpful because their high surface specificity allows sensitive monitoring of the properties of interfaces and thin layers without the interference of information from the bulk.

During the last decade a powerful new approach for designing semiconductor structures with tailored electronic and optical properties, bandgap engineering, has spawned a new generation of electronic and photonic devices. Central to bandgap engineering is the notion that by spatially varying the composition and the doping of a semiconductor over distances ranging from a few microns down to ~2.5 Å (~1 monolayer), one can tailor the band structure of a material in a nearly arbitrary and continuous way. Thus semiconductor structures with new electronic and optical properties can be custom-designed for specific applications.

The enabling technology which has made bandgap engineering an exciting reality with far reaching implications for science and technology is molecular beam epitaxy (MBE), pioneered by Cho and Arthur in the late 1960s.

In the subsequent decade MBE demonstrated jts ability to create ultra-thin (10–100 Å) layers and atomically abrupt interfaces between two different semiconductors (heterojunctions).

Atomic-resolution techniques for surface characterization are often limited in their ability to probe individual atoms or molecules. In the last few years, scanning tunneling microscopy (STM) has emerged as a powerful probe for studying the atomic-scale structure of surfaces. One of the greatest features of STM is that it is applicable to a wide variety of problems, restricted only by the condition that the material to be studied must be a conductor. Although STM has been applied to a wide variety of materials science problems, the greatest concentration of STM work has been in the study of semiconductor surfaces. This results not only because semiconductor surfaces exhibit complicated and interesting reconstructions of the atoms in the outermost surface layers, but also because many important properties of semiconductor surfaces and interfaces are dominated by defects and other surface imperfections with atomic dimensions.

The goal of this issue of the MRS BULLETIN, with its focus on the physics of complex materials, is to point out some of the fascinating features, both fundamental and applied, of complex materials: liquid crystals and polymers. Over the past 20 years, we have witnessed impressive advances in the understanding of liquid crystals and polymers on all fronts—physics, chemistry, materials research, and applications.

Physicists are interested in the fundamentals of a phenomenon. Our assumption is that once we understand how the pieces of a System work, the understanding of how the whole System works immediately follows. However, those of us who have been involved in materials physics research quickly learn that complexity generates rules of its own on scales much larger than the microscopic scale of the molecules involved. Some-times these rules are beautifully simple and elegantly described, but most often they are not. The following articles high-light some important current research in the domain of complex materials, particularly for liquid crystals and polymers.

Contributing to this special issue are: Pierre-Gilles de Gennes; J. William Doane; Wolfgang Meier and Heino Finkelmann; Paul Keyes; Patrick Oswald, John Bechhoefer and Francisco Melo; and Walter Zimmerman. They give us their current thinking on polymers in shear, novel electro-mechanical effects observed in polymeric liquid crystals, and how liquid crystals in a solid polymer matrix make useful high-speed color displays.