The term “solar energy” refers to a wide variety of techniques for using the energy available as sunlight. Well-known examples are active and passive thermal solar energy and photovoltaic solar energy but, strictly speaking, hydropower, wind energy, and biomass are also forms of solar energy. Today, only hydropower is used in significant quantities, covering approximately 6% of the world's energy demand. Traditional use of biomass, mainly in developing countries, accounts for more than 10% of the total energy consumption, but is sometimes left out of statistics because it falls outside the category of organized and commercial use.

The global potential for solar energy is huge, since the amount of energy that reaches the earth's surface every year exceeds the total energy consumption by roughly a factor of 10,000. There are, however, various barriers to the large-scale use of solar energy technologies. Most technologies have in common that the power density of the generator is low; in other words, one needs large areas to generate significant amounts of energy. This is especially true for biomass, with typical conversion efficiencies (solar energy to chemical energy) of 1% or less. Further, many solar energy technologies have proved technically feasible, but have yet to be proved economically feasible. Last, but not least, the large-scale use of solar energy requires substantial modification of our global energy supply system, which is based largely on fossil fuels.

The use of microwave energy to process a wide variety of ceramic, polymeric and composite materials offers many new and exciting opportunities. Although microwave processing is well established in the food, rubber, and textile and wood products industries, it is a relatively new tool for processing inorganic and other organic materials, chemicals, and certain minerals. Much of its potential has yet to be exploited and carried through to industrial and commercial applications.

Microwave processing is fundamentally different from other heating sources such as conventional ovens and furnaces, lasers, plasmas, and electron beams, where heat is applied externally to the surface of the material, and where time is required for the heat to penetrate the cooler, interior regions via conduction or radiation.

Microwave irradiation penetrates and simultaneously heats the bulk of the material. This feature, along with other microwave characteristics, offers new and unique opportunities to process materials with greater flexibility, to improve quality and product properties, to create or synthesize entirely new types of materials that cannot be produced by alternative methods, and to do all this with greater speed and energy savings.

In recent years, the materials community has become increasingly aware of the potential for microwave processing, and there is now an active R&D effort worldwide to explore, develop, and exploit this potential. In 1988 the Materials Research Society held the first international symposium on microwave processing of materials to review and publish some of these new developments. Every year since then, MRS and the American Ceramic Society have alternated sponsoring symposia on this topic.

Friction, lubrication, wear, and adhesion are all phenomena familiar from everyday experience. We experience friction when we go for a walk on ordinary ground—we do not expect to fall flat on our faces, as we might when walking on ice. Wear is particularly well known because it leads to catastrophic failure and represents one of the most costly problems facing industry today. These phenomena are part of the field of tribology—the science of interacting surfaces in relative motion. A macroscopic understanding of tribology is crucial for the design and engineering of mechanical components. As dimensional tolerances of these components approach the nanometer scale and novel new materials are used in their fabrication, macroscopic understanding must give way to a microscopic atomic-scale understanding. This fundamental microscopic understanding and its practical application remain limited.

With the advent of surface proximity probes, however, those limitations are rapidly disappearing. Researchers are now able to test and develop a microscopic understanding of such tribological phenomena as boundary-layer lubrication and asperity-surface interactions—ideas that underlie our current macroscopic understanding. This emerging field is known as nanotribology or molecular tribology, the focus of this issue of the MRS Bulletin. The new experimental probes include the surface force apparatus for measuring interfacial forces as a function of surface separation and, more recently, rheological properties of molecularly thin layers; a quartz-crystal microbalance for measuring the atomic-scale friction of adsorbed monolayer films; and the atomic force microscope (AFM) for measuring nanomechanical properties of surfaces, atomicscale friction of clean surfaces, and frictional properties of surfaces coated with molecular films.

The semiconductor silicon is the dominant material in microelectronics and is one of the best-studied materials known to humanity. Its inability to emit light efficiently is therefore well documented. Nevertheless, a “holy grail” of semiconductor materials research has for decades been the realization of an efficient Si light-emitting diode (LED). Such a device would enable optoelectronic circuitry to be based entirely on silicon and would revolutionize VLSI technology since the other required Si-based devices (detectors, waveguides, modulators, etc.) have already been demonstrated. Although this holy grail has proved elusive, the 1990s have heralded greatly renewed interest and optimism in the development of such devices for both the visible and near-infrared spectral ranges. Dramatic progress is at last being made. This review focuses, in a somewhat chronological manner, on the progress of specific approaches to realizing crystalline structures of high radiative efficiency, and the materials constraints involved.

In the second of this two-part series for the MRS Bulletin, we present the most commonly used tools for the quantitative determination of structure and chemistry in thin films. The physics of thin films is at a stage during which one encounters phenomena that have a fragile dependence on the structure and chemical properties of the interface; the field, therefore, is at a stage during which quantitiative structural and chemical determination at the atomic level is a must. It is the purpose of the December and January issues of the Bulletin to provide a start for researchers interested in the field. Of course, even during the writing of these articles new techniques are being invented and it is therefore impossible to provide a comprehensive review of all techniques being used these days.

Multilevel metallization of very large-scale integrated (VLSI) circuits has become an area of intense research interest as devices are scaled down in order to increase circuit density. As device dimensions approach the submicron regime, reliability becomes more of an issue. Metallization generally requires good conductivity, electromigration resistance, controllable contact performance, corrosion resistance, adherence, thermal stability, bondability, ability to be patterned into a desirable geometry, and economic feasibility.

Aluminum and its alloys have been commonly used as the main metallization materials because they meet most of the metallization requirements for microelectronic devices. Aluminum, however, suffers from major limitations, such as elec-tromigration and stress-voiding induced open-circuit failure. For the development of ultralarge-scale integration (ULSI) for fast-switching-speed devices, the electrical resistivities of aluminum and its alloys are not low enough. As the minimum geometry is scaled down to one-quarter micron, aluminum and its alloys potentially will be replaced by other materials such as Cu, Au, or superconductors for on-chip interconnection.