Multilayers, as considered in the following articles, are manmade thin-film materials periodic in one dimension in composition or in composition and structure. This composition/structure variation is generated during synthesis, which is typically accomplished using atom-by-atom technologies. Individual component layers in a multilayer may vary in thickness from one atomic layer (~2 Å) to hundreds of atomic layers (~1,000 Å) of a given material.

An example of these synthetic micro-structures, a lattice image transmission electron micrograph of a cross section of a hundred period titanium (63 Å)/titanium-nickel (40 Å) multilayer microstructure fabricated using magnetron sputtering, is shown on the cover of this issue of the MRS BULLETIN. The titanium-nickel layers are amorphous as a result of the low substrate temperature (<75°C) and the very large atomic quench rates characteristic of vapor deposition (>1012 K/s). The elemental titanium layers are fiber textured with the basal plane of this hexagonal close-pack structure element in the plane of the layers. These (00.1) planes are the ones lattice imaged in this electron micrograph.

The general concept of a multilayer structure, as illustrated above, is now well accepted because the ability to synthesize such materials for scientific study and technological application has been demonstrated at many nationally and internationally based laboratories.

It is well known that thin-film technology relies increasingly on multilayered structures. As dimensions become smaller, the interfacial or contact region assumes a larger and often dominant role in the performance or properties. Many examples come readily to mind. In magnetic hard disks, the active cobaltalloy layer, itself only about 50 nm thick, is grown either on a crystalline chromium thin film or directly onto amorphous nickel-phosphorous, and capped with a protective carbon or chromium-carbon coating (see Figure 1). The recording head “flies” at 90 mph and about 0.1 ü above this combination, which is expected to be mechanically durable and magnetically reliable for thousands of recordings. Atomic-scale multilayers are being investigated to provide the ability to “tune” the magnetic properties of the active recording layer or head materials. Exchange coupled magneto-optical media consisting of a few tens of angstroms of cobalt or nickel layers on amorphous TbFeCo alloys are showing promise for improving magneto-optical coupling while maintaining perpendicular anisotropy. In microelectronic circuits, aluminum or silicide contacts to silicon are essential to any device, and multilevel integration involving a series of metal, alloy, silicon (amorphous, poly- or monocrystalline) and dielectric layers (some of which might be 1-10 nm thick) are increasingly required to achieve large-scale integration. Metal-metalloid (e.g., MoSi, W-C) multilayers are used for x-ray optical elements. Artificially produced metallic superlattices and multilayers are being used to probe the fundamental magnetic, electronic, mechanical, and structural properties of metal-metal interfaces.

The study of Artificially Layered Superconductors (ALS) started more than 20 years ago with the search for unusual and/or high temperature superconductivity in a variety of metal-semiconducting layers. Renewed interest was motivated by the advent of novel preparation techniques that allow control of layer thicknesses close to interatomic distances. In this way layered superconductors can be used as model systems to study a variety of physical phenomena, prepare structures with improved properties and discover novel metastable phases which do not exist in nature. Examples of these studies include: a diversity of dimensional transitions, interaction between superconductivity and magnetism, interaction between superconductivity and electron localization, enhancements of critical fields and critical currents, and the study of incommensurate systems.

Recent developments in high temperature ceramic superconductors further increase the importance of studies of Artificially Layered Superconductors. The newly discovered ceramic superconductors are structurally layered and therefore many of their properties will also be determined by this structure. Because of this, particularly in the search for the mechanism of superconductivity, it is important to understand which properties are a consequence of the layered nature of the material and which are due to the presence of some unusual, yet undetermined physical phenomena.

What makes Artificially Layered Superconductors especially attractive for investigation? The main reason rests on the fact that the characteristic lengths which determine the superconducting properties, i.e., the coherence length and penetration depth, are quite long in conventional low temperature superconductors.

This article considers a particular class of optic microstructure — multilayers for soft x-ray (SXR, 100 eV[124 Å] to 3,000 eV[4.1 Å] and extreme ultraviolet (EUV, 15 eV[800 Å] to 100 eV[124 Å] optics. Multilayers are manmade vapor-deposited depth-periodic layered micro-stractures of high enough quality to be considered synthetic crystals. Layers of two materials, A and B, having significant differences in their scattering powers for x-rays (electron densities) and of uniform thicknesses ta and tb are combined to form a sample of uniform in-depth period d0 ( = ta + tb). These microstructures are of scientific and technological significance since the high angle of incidence SXR and EUV reflectivities of single film reflectors is ~10−4) and since only a limited number of naturally occurring or synthetic large lattice constant materials efficiently diffraction reflect long wavelength radiation (SXR and EUV).

Multilayer microstructure based SXR and EUV optics are direct analogues to standard quarterwave stacks applied at longer wavelengths with the requirement that absorption must be included. They are also analogous to atomic crystalline lattices in that one multilayer period is equivalent to one lattice plane. Thus, the multilayer structure forms a superlattice that diffraction reflects incident radiation. Multilayer diffraction may be modeled using x-ray scattering theory and obeys the refraction-corrected Bragg's law of crystal lattice diffraction. As will be discussed later, many factors determine the character of the multilayer response to an incident spectrum.

The Center for Advanced Materials Processing (CAMP) was established in 1985 by Clarkson University in Potsdam, New York. At that time, nearly half of the research at Clarkson was materials-related but was conducted in seven separate departments of science and engineering. To coordinate and encourage this strong materials program, CAMP was created as an interdisciplinary center dedicated to research on high-technology materials processing.

The current corporate sponsors of CAMP are Corning Incorporated, Eastman Kodak, Xerox Corporation, and IBM. These and over 30 other industrial members support individual research projects. In 1987 the New York State Science and Technology Foundation designated CAMP as the New York State Center for Advanced Materials Processing, entitling CAMP to $1 million per year in operating funds. In addition, CAMP is supported by federal and University sources.

In its role as an education and research initiative, CAMP has three goals:

1. Enhance Clarkson University's expertise and reputation as a center of excellence in materials processing research.

2. Greatly increase the mutually beneficial relationships between industrial organizations and the University; and

3. Strengthen graduate and undergraduate education in materials processing.

Innovative research by Egon Matijević, Distinguished University Professor, has contributed greatly to the development of the fundamental principles for the formation and interactions of colloidal dispersions. Using Matijević's work as a foundation, CAMP has developed four programs in high-technology materials research: electronic materials processing, fine-particle processing, particulate control in process equipment, and polymer processing.