There is a growing demand for nonlinear optical materials for a variety of applications—lasers and coherent sources, electrooptic devices, communication technologies, and optical processors and computers. Nonlinear optics is a vast field requiring materials with diverse performance features. Photorefractive (PR) materials, which experience a change in the refractive index under the effect of inhomogeneous illumination, constitute a relevant branch of the field. They behave as third-order nonlinear materials, which can be considered, in general, as photorefractive. However, the materials more commonly designated as photorefractives involve a charge-transport-induced nonlinearity, and it is these materials which are the object of this issue of the MRS Bulletin.

At variance with conventional (often designated as Kerr) nonlinear materials, photorefractives are sensitive not to the local light intensity but to its spatial variation; i.e., they are nonlocal materials. This feature makes them more complicated to deal with than their conventional counterparts, since a χ(3) susceptibility cannot be properly defined (except as a k-dependent function). On the other hand, this sensitivity gives them some unique and interesting features. In particular, an interference light pattern illuminating the crystal and the generated index grating are phase-shifted, leading to remarkable beam coupling and amplification effects. The coupling gain can be markedly enhanced by applying alternating electric fields or by oscillating the interference fringes with a piezoelectric mirror. Efficient image amplifiers have been made using this effect.

The emergence of the field ion microscope, invented by E.W. Müller in 1951, allowed, for the first time, direct observation of metallic materials in real space on an atomic scale. Field ionization of rare gas atoms near the surface of the material allowed production of an atomic resolution image of the material; field evaporation of surface atoms allowed observation into the depth of the sample.

Field ion microscopy (FIM) has produced numerous impressive results, one of them being three-dimensional reconstruction of defect cascades produced by heavy ion irradiation in metals. FIM techniques have also contributed in a spectacular way to the knowledge of the crystallographic structure of grain boundaries. Surface diffusion as well as surface reconstruction phenomena are applications where, again, FIM gave surprisingly detailed information. This overview of applications of FIM in materials science emphasizes recent work. Detailed information and descriptions of earlier work can be found elsewhere.

During the development of FIM techniques, the basic shortcomings of the instrument for investigating metallic alloys were rapidly recognized. In Cu3Pd alloys, for instance, Cu atoms are darkly imaged and therefore appear as vacancies similar to Co atoms in PtCo and Pt3Co. It is thus impossible to distinguish vacancies from solute atoms. This clearly limited the quantitative use of FIM to study short-range

order in dilute alloys or to investigate long-range order (Figure 1). Similarly, in two-phase materials, precipitates often give rise to quite visible contrast. For instance, in nickel-based alloys, aluminum-enriched precipitates appear in bright contrast. It is therefore possible to find the size and also the number density of particles present in the material. However, FIM does not provide any composition data.

By virtue of their unique structures, fullerenes exhibit novel chemical transformations. Particularly pertinent to this article are the interesting properties exhibited by fullerenes in the solid state. These molecules are spherical or near-spherical in shape. Molecules with high point-group symmetry, which are not bound strongly in the solid state, tend to crystallize into structures with long-range periodicity of the molecular centers of mass, but the molecular orientations are random or even dynamically disordered. When dynamically disordered, the

molecules rotate about some preferred axis. C60 and C70 satisfy the criteria for such orientationally disordered solids and exhibit rich phase behavior in the solid state. Since C60 has high electron affinity, it forms anion salts with alkali and alkaline-earth metals as well as with strong organic donor molecules. With tetrakis dimethylaminoethylene (TDAE), which is a very powerful electron donor, C60 forms a 1:1 solid that is ferromagnetic. C60-TDAE is the molecular organic ferromagnet with the highest Tc (of 16 K) known to date. Some of the alkali and alkaline-earth fullerides, on the other hand, show superconductivity, with transition temperatures going up to 33K. We shall briefly examine some of these solid-state properties.

During the past two decades, mathematical modeling has been gaining acceptance as a legitimate part of materials science and engineering. However, as common to all relatively new disciplines, we still lack a realistic perspective regarding the uses, limitations, and even the optimal methodologies of mathematical modeling techniques.

The term “mathematical modeling” covers a broad range of activities, including molecular dynamics, other atomistic scale systems, continuum fluid and solid mechanics, deformation processing, systems analysis, input-output models, and lifecycle analyses. The common point is that we use algebraic expressions or differential equations to represent physical systems to varying degrees of approximation and then manipulate these equations, using computers, to obtain graphical output.

While it is becoming an accepted fact that some kind of mathematical modeling will be needed to make most research programs complete, there is still considerable ambiguity as to what form this should take and what might be the actual usefulness of such an effort.

Among the more seasoned and successful practitioners of this art, clear guidelines have emerged regarding the uses and limitations of the mathematical modeling approach. We seek to illustrate these uses through the successful modeling examples presented by some leading practitioners. Some general principles may be worth repeating as an introduction to this interesting collection of articles.

Colloidal aggregation is a very old field of research. The basic mechanism that controls the stability of colloidal solutions has been described in the celebrated papers of Derjaguin, Landau, Verwey, and Overbeek (DLVO), and indeed the prevention of undesired aggregation is one of the primary concerns in many industrial processes involving colloids. In the last 10 years a resurgence of interest in aggregation rather than in stability has been spurred by a number of fortuitous coincidences. Both through simulations and experiments, it was realized that colloidal aggregates exhibit a fractal nature. More importantly, it was found that there are two universal modes of aggregation, irrespective of the type of colloidal system. The aggregates grown according to these modes have different fractal dimensions, different reaction kinetics, and different mass distributions. The two modes are called diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA), according to whether the probability that a bond formed as a consequence of an encounter is equal to, or much smaller than one, respectively. The study of these limiting cases has been the object of extensive studies in the late 1980s, and many of the features of these modes are by now well understood.

This article describes some recent results in the area of colloidal aggregation. Although we present mostly static light-scattering data from our laboratory, connections to similar work from other groups are also discussed. For the convenience of the reader, we start by recalling some of the relevant features of both DLCA and RLCA.

This issue of the MRS Bulletin aims to highlight the innovative and exciting materials science research now being done using in situ electron microscopy. Techniques which combine real-time image acquisition with high spatial resolution have contributed to our understanding of a remarkably diverse range of physical phenomena. The articles in this issue present recent advances in materials science which have been made using the techniques of transmission electron microscopy (TEM), including holography, scanning electron microscopy (SEM), low-energy electron microscopy (LEEM), and high-voltage electron microscopy (HVEM).

The idea of carrying out dynamic experiments involving real-time observation of microscopic phenomena has always had an attraction for materials scientists. Ever since the first static images were obtained in the electron microscope, materials scientists have been interested in observing processes in real time: we feel that we obtain a true understanding of a microscopic phenomenon if we can actually watch it taking place. The idea behind “materials science in the electron microscope” is therefore to use the electron microscope—with its unique ability to image subtle changes in a material at or near the atomic level—as a laboratory in which a remarkable variety of experiments can be carried out. In this issue you will read about dynamic experiments in areas such as phase transformations, thin-film growth, and electromigration, which make use of innovative designs for the specimen, the specimen holder, or the microscope itself. These articles speak for themselves in demonstrating the power of real-time analysis in the quantitative exploration of reaction mechanisms.

The first transmission electron microscopes operated at low accelerating voltages, up to about 100 kV. This placed a severe limitation on the thickness of foils that could be examined: Heavy elements, for example, had to be made into foils thinner than 0.1 μm. It was felt that any phenomenon whose “mean free path” was comparable to the foil thickness would be significantly affected by the foil surfaces, and therefore would be unsuitable for study in situ. However, technology quickly generated ever higher accelerating voltages, culminating in the giant 3 MeV electron microscopes. At these voltages, electrons can penetrate materials as thick as 6–9 μm for light elements such as Si and Al, and 1 μm for very heavy ones such as Au and U.

Significant progress has been made in building multilevel copper interconnection systems for advanced microelectronics. In this article, we examine some of the materials science issues underlying this progress, and indicate where significant materials challenges remain. It is probable that several approaches to process integration will be developed for copper interconnections, as has been the case with aluminum systems. The first successful demonstration of a fully integrated 4-level copper/polyimide (Cu/PI) interconnection system has been described by Luther et al. of IBM. A schematic cross section of this interconnection system is shown in Figure 1, indicating multiple layers of BPDA-PDA polyimide (PI 5810), Cu lines and studs, and layers of Ta and Si3N4 which serve as diffusion barriers, adhesion layers, and stopping layers in the patterning and planarization processes. This system demonstrates excellent planarity, as shown in the SEM cross section in Figure 2. The electromigration lifetime of this Cu/PI system is greatly improved relative to state-of-the-art aluminum-based systems, and the dielectric integrity appears adequate. Signal propagation studies also confirm the performance improvements anticipated for copper as a low-resistivity conductor and the use of Cu may allow significant capacitance reduction (≃ 25%) simply by scaling Cu lines to equal the resistance of Al lines. In parallel with efforts to introduce Cu metallization for its low resistivity, extensive efforts are under way to replace SiO2 with lower dielectric constant insulators.

This issue of the MRS Bulletin, on the crystal engineering of high Tc and related oxide films, essentially responds to four questions. The first three are

∎ Why has a high Tc superconducting tunnel junction not yet been fabricated?

∎ Is it possible to fabricate a high Tc tunnel junction and if so, how?

∎ How can thin-film technology contribute to the progress of high Tc superconductor research beyond the fabrication of electronic devices?

Experimental efforts to answer these questions and solve associated problems must focus on atomic-scale control and characterization of surfaces and interfaces, which we designate here as crystal engineering of high Tc and related oxide films. Meanwhile, we have started to realize that the crystal engineering technology of oxides need not be limited to high Tc super-conductor research; it can be extended to electronic materials research, where it may have a somewhat more profound significance. In this light, I would like to pose the fourth question, one that may be especially relevant to this particular topic in the near future:

∎ Will we find or develop an electronic material superior to silicon?

Ever since the first success in superconducting La-Sr-Cu-O film deposition in early 1987, extensive studies have been conducted on the fabrication of high Tc oxide films and devices. Success has been achieved to some extent in relatively simple devices using plain superconducting films or grain boundary and step edge junctions. Nobody, however, has succeeded in fabricating an SIS (superconductor-insulator-superconductor) runnel junction that shows a clear hysteresis in I-V characteristics at a temperature above 77 K. Such a junction is presumably the key to the wide electronic device application of high Tc superconductors, for example, for switching devices expected to work much faster than conventional semiconductor transistors.

One of the many wonders that fullerenes have brought to us during the past few years is the variety of their shapes. When the elusive C60 finally showed up in 1990, the perfect symmetry and astounding beauty of its molecular structure touched the hearts of scientists before they could consider the molecule's vast technical possibilities. Already much has been said about the unique shape of C60 and its potentialities. C70 and higher fullerenes have simultaneously been found in the same soot that produced C60 and were quickly revealed to be shaped like rugby balls or oblong eggs. Hence we were aware that there had to be an extensive series of roundish polyhedral clusters of carbon atoms.

Then, in the following year, multilayered tubular fullerenes (Figures 1a and 1b) were discovered by Iijima and were named buckytubes (see the article by Iijima in this issue). Iijima also observed similarly huge and multilayered carbon balls, before C60 was discovered. Soon after, buckyonions were recognized as an important class of fullerene (Figure 1c, see article by Ugarte in this issue). So, in the early days of fullerene research, we already knew three forms of fullerene: sphere, tube, and particle. At that time, however, nobody anticipated that this was only the beginning of a big show of stunning variations in the shapes of fullerenes. This article introduces current developments in the study of these fullerene styles.

The discovery of superconductivity in yttrium barium cuprate above the boiling point of liquid nitrogen by Wu et al. initiated a race in the search for higher critical temperatures (Tc) which was singular in the history of science. Since the ceramic superconducting pellets could be prepared nearly overnight and superconductivity easily demonstrated with liquid nitrogen and a levitating magnet, a great deal of effort went into garnering funds for a “new technological era.” High Tc superconductivity (HTSC) was expected to have applications in many areas of our life, be it for energy transport and high-power generators, transformers, current limiters, or for ultrafast computers and communication technology, or in medicine for NMR tomography and for SQUIDs in magnetic encephalography. Large-scale uses were also foreseen, such as levitation for trains, magnetohydrodynamic propulsion in ships, and applications for fusion reactors. Thousands of physicists and engineers searched for new materials with high Tc, and explored thin-film preparation by physical vapor deposition. In the early phase only a few scientists recognized that materials engineering problems would greatly hinder the development of applications of the cuprate superconductors. Most early efforts on HTSC went toward physical investigations of samples (crystals, layers, sheets, wires, etc.) which due to their complexity could not be reproducibly prepared, and which were not adequately characterized (see the section on Characterization). The Bi-, Tl-, and Hg-containing cuprates with Tc up to 135 K, for example, were found empirically, not by physical understanding of the HTSC phenomenon. Applications of HTSC have been limited to very specific areas, but a concerted effort should reveal opportunities for the materials research and superconductivity communities. Significant applications could be developed

which in the long run would also benefit fundamental physical investigations of the HTSC phenomenon.

The behavior of fluids confined in porous materials has been of interest to engineers and scientists for many decades. Among the applications driving this research are the use of porous membranes to achieve liquid-liquid separations and to deionize water, the use of porous materials as beds for catalysis, and the need to extract liquids (especially oil and water) from such media. Many of these applications depend on transport, which is governed by flow or diffusion in the imbibed fluids. Both the flow and diffusion of multiphase fluids in porous media, however, strongly depend on the morphology of phase-separated domains, and on the kinetics of domain growth. Thus, it is worthwhile to study the behavior of multiphase fluids in porous media in the absence of flow. Recently, much attention has focused on even simpler systems that still capture these essential features, namely, near-critical binary liquid mixtures and vapor-liquid systems in model porous media, such as Vycor and dilute silica gels. Although near-critical fluids may seem rather artificial as models for multiphase liquids, there are several advantages associated with them. In general, domain morphology and growth kinetics are governed primarily by competition between interfacial tension and the preferential attraction of one phase to the surface of the medium. In near-critical fluids, the relative strength of these two energy scales is sensitive to temperature, and can therefore be altered in a controlled fashion. In addition, the kinetics of domain growth are sensitive to the temperature quench depth, and can be controlled.

“Lignocellulosic materials” refer to the organic matter produced by plants on the Earth in the form of grasses, shrubs, agricultural crops, and trees. Worldwide, this plant growth (also called biomass) serves two sustaining functions for our planet: it degrades carbon dioxide to oxygen and it has been the origin or storage media of all non-atmospheric carbon. Leaving aside the related critical issues of fossil fuel depletion and global warming/cooling, lignocellulosic materials growing on the surface also serve many commercial purposes: food production, fuel sources, structural materials for housing and industry, paper products, and chemicals.

Although lignocellulosic materials represent a sustainable resource base, the expanding use of paper and wood products has caused the developed countries to reevaluate the use and recovery of these natural biopolymerics. This issue of the MRS Bulletin explores some of the intricacies surrounding the recovery and reuse of these biomaterials. The applications for materials research are by no means exhaustively covered in these articles. Instead we illuminate the critical issues facing the paper recycling community and the technologies being applied to this resource to create marketable materials.

Most metals are thermodynamically unstable and tend to change to more stable phases by reaction with their service environment. These oxidation or corrosion processes are usually controlled by kinetic factors, and the rate often increases with increasing temperature. Degradation by these processes is an important, often life-limiting, factor for critical components in thermal-power systems and other high-temperature process plants, and it can have considerable economic and technical consequences. Corrosion can be particularly severe, for example, in

Gas-turbine blades and vanes, Gas coolers in coal-gasification systems, Superheaters and reheaters in coaland oil-fired boilers, In-bed tubes in fluidized-bed coal combustion systems, Components in high-temperature fuel cells, Components in steam-reforming plants, Components in incinerator plants, and Exhaust systems (e.g., automobiles).

Although there is considerable awareness of such problems and much attention is given to the materials requirements for high-temperature systems, failures still occur and can be catastrophic (fire, explosion, burst tube, etc). Plant conditions and control are particularly important since damage can occur due to overheating and overstressing when a material is taken beyond its physical, mechanical, and chemical limits or due to so-called dew-point corrosion when the system is cooled and acidic vapors condense to electrolytically corrode unprotected surfaces.

The oxidation of metals and alloys in oxygen at high temperatures is now reasonably well-understood. Indeed, alloys have been successfully developed to resist environmental attack in such atmospheres. The oxidation resistance of these materials is due to the presence of alloying elements—such as chromium, aluminum, and silicon—which can react with oxygen to form stable, protective, external oxide scales on the component surface, thereby inhibiting further high-temperature degradation.

The Semiconductor Industry Association (SIA) roadmap calls for the incorporation of Cu plugs (vias) integrated with interconnects in 1997. Copper is being evaluated for ULSI metallization because of its lower bulk electrical resistivity and its superior resistance to electromigration and stress voiding as compared to commonly used aluminum and its alloys. One of the major drawbacks of Cu is its fast diffusion in Si and drift in SiO2-based dielectrics, resulting in the deterioration of devices at low temperatures. Hence a diffusion barrier is necessary between Cu and Si or SiO2. Figure 1 is a cross section of

a three metal level interconnect structure using Cu as the conductive material. The interlevel dielectrics (ILD) could be conventional SiO2-based materials or more ideally, materials with low dielectric constants such as polyimide. If conventional SiO2 is used, then Cu plugs and interconnects have to be enclosed in diffusion/drift barriers so that Cu will not move into Si or SiO2 under thermal stress or biased temperature stress (BTS).

This article reviews the published studies on conductive diffusion barriers between thin Cu films and Si substrates. In addition, the work on diffusion and drift of Cu into commonly used inorganic dielectric systems is also summarized. Finally, some concerns involving diffusion/drift barriers between Cu and Si or SiO2 for sub-0.5 μm feature size with high aspect ratios are discussed.

Lithium niobate (LiNbO3) is a paradigmatic photorefractive (PR) material. It provided the first experimental evidence of the PR effect and still maintains a key position in the field. This position is fostered by the fact that large congruent single crystals with good optical quality are routinely grown, enabling technological applications. A remarkable example, a PR narrow-band interference filter has recently been developed and put on the market.

A relevant property of LiNbO3, the high electrooptic figure of merit, n3r, assures efficient PR performance. Another unique feature is the occurrence of a bulk photovoltaic (PV) effect, i.e., the generation of a voltage (in an open circuit) or a current (in a short circuit) as a consequence of homogeneous illumination. The bulk PV effect acts, in a way, like an internal electric field, enhancing the PR effect. On the other hand, thermal fixing or stabilization of PR gratings has been successfully accomplished through an interplay between proton and electron dynamics. LiNbO3 is also the choice substrate for the commercial fabrication of waveguide devices such as modulators, wavelength filters, multiplexers, and demultiplexers. Moreover, lasing action as well as nonlinear effects have been achieved due to the marked inhibition of the PR effect caused by heavy Mg doping. This illustrates the intimate connection between PR behavior and defect structure, i.e., between optics and materials science.

Rustum Roy recently observed that, operating from a finite number of elements, materials science faces an over-supply problem, too many scientists, and too few elements. Like all Malthusian dilemmas, relief is unlikely within the assumptions. Materials scientists, however, enjoy opportunities to develop new materials through morphological engineering of traditional substances. Carbon, after all, provided a century of progress for polymer chemists and a revolution in the manufacturing world. The trend from atomic- to molecular- to chain-level engineering can obviously be extended to mesostructure engineering and beyond. The tailoring of porosity is an example of such an extension.

This issue of the MRS Bulletin examines the concept of engineered porosity from three perspectives. Smith, Hua, and Earl discuss characterization based on classic gas adsorption and more recent NMR and scattering methods. Two articles, one by Harold and Lee, and the other by Fain, look at the pore structure requirements for important applications, reactive separations and gas separation. Finally, the materials science of engineered porosity is discussed in four articles. Even describes emulsion-derived foams in which surface-active species impose micron-scale structure that is retained during polymerization. Shapovalov broadly reviews methods to introduce porosity in metals. Durian discusses recent work on relaxation phenomena in aqueous foams. Finally, I review the factors that control structure in mesoporous aerogels.