Interfaces between metal oxides have not been studied as extensively as metal or semiconductor interfaces. Even in magnetic metallic systems, new phenomena are still being discovered, such as giant magnetoresistance, which has been attributed to an interface phenomenon. As general interest in metal oxides increases, researchers are studying a large variety of heterostructures consisting of superconducting oxides, conducting oxides, ferroelectric oxides, magnetic oxides, and optical oxides. As the complexity and level of integration increase, scientists need a better understanding of the interfaces in metal oxide systems. For example, interface issues in grain boundaries in high Tc oxides and in Josephson junctions of the superconductor/normal material/superconductor (SNS) type have been identified as important technological barriers. In heterostructures based on ferroelectric materials, fatigue problems are believed to be associated with the interface between the conducting electrodes and the ferroelectric metal oxides, and with grain boundaries in the ferroelectric materials.

In this article we will focus on the interface issues related to YBCO superconductors. We will describe interface phenomena observed in several systems, such as YBCO/metal contacts, YBCO/YBCO grain boundaries, and YBCO/epitaxial metal oxides. From their seemingly universal behavior, we will try to identify the origin of the interface phenomena and will describe some recent efforts to control such phenomena.

Large amounts of waste paper are generated every day and interest in its reuse has increased steadily due to environmental concerns and improved economics. In 1990 in the United States, for example, 28.9 million tons of recovered papers were collected for recycling. By 1995, that amount is expected to increase to almost 40 million tons with a collection rate of almost 40%. To facilitate the use of secondary fibers, sticky contaminants, or “stickies,” must be controlled.

Classification and Properties of Stickies

Recovered paper bales usually contain extraneous materials such as sand, glass, staples, nails, inks, coatings, plastic, styrofoam, wax, EVA (ethylene vinyl acetate), and SBR (styrene butadiene rubber). A particularly troublesome contaminant in recovered paper is “stickies” which, in their original state, were used as paper adhesives. Inks and coatings can also be a source of stickies. Inks are primarily composed of a pigment, liquid vehicle, binder, and modifier. Coating formulations contain several ingredients that can be classified as pigments, binders, and additives, such as plasticizers, thickeners, dispersants, dyes, preservatives, and defoamers. Note that both inks and coatings mixtures contain binders which contribute to the stickies problem. Common binders used in inks are hydrocarbon resins and rosin esters while those used in coating formulations include starch, soya protein, acrylics, and polyvinyl acetate. An overview of stickies was presented by Moreland.

Stickies can be classified into three categories: hot melts, pressure-sensitive adhesives, and latexes. Although wax can be a contaminant, it is not included separately because it is similar to and an important ingredient of most hot melts.Contaminants such as plastics and styrofoam are also excluded from this discussion because they are not stickies.

Electron microscopy has played an undeniably important role in the development and understanding of new materials. One particular method, plan-view transmission electron microscopy (TEM), has been used to reveal many aspects of materials and to help answer questions that are not accessible by other methods. In planview TEM, materials are studied either by the image contrast caused by diffraction or by direct diffraction analysis. The studies are often at a somewhat lower resolution than is possible with some other electron-based microscopies or, in the case of diffraction, must be interpreted in terms of an average effect. Nevertheless, plan-view TEM is very useful because properties at these intermediate length scales typically determine important materials characteristics. It is plan-view microscopy that gives us the big picture of what is happening in a material.

Until recently, TEM studies of materials have entailed comparing samples taken at various stages of processing and trying to observe and understand the mechanisms that determine the material behavior. With the advent of controllable specimen environments, materials processing has been taken into the TEM, allowing direct observation of the microstructural development. Here we will focus particularly on surface and near-surface processes, such as oxidation.

A large class of materials of great technological importance and scientific interest are highly disordered on length scales comparable to, or larger than, the wavelength of light. As a consequence of this disorder, these materials are often composed of a large number of regions with different indices of refraction, separated by interfaces that are abrupt on the scale of the wavelength of light. Each of these interfaces refracts or scatters light, making these materials quite opaque optically. Even in the absence of absorption, the strong scattering precludes the direct transmission of light rays through the material. This greatly complicates the study of these materials.

Examples of these disordered materials abound. Many are complex fluids; colloidal suspensions and other dispersions such as foams and emulsions often consist of particles or droplets whose diameters are comparable to the wavelength of light. Unless they are very dilute, these dispersions are typically highly opaque. Other materials consist of solid grains of similar sizes without the surrounding fluid. For example, most ceramics are composed of oxide grains sintered together; the voids between the grains lead to the large number of interfaces, and hence to the strong scattering and opaque appearance commonly associated with these materials. Similarly, many porous materials also possess a high density of voids that scatter light; filters, oxide catalyst supports, and even rocks are all optically opaque because of the strong scattering. Sand and other unconsolidated granular materials are also opaque in part because of the large number of interfaces that scatter light.

Cheese is a popular food among people of all ages and its popularity stems from its nutritional value and convenience. Cheese comes in many varieties, shapes, and sizes. It makes an ideal snack food. The recent expansion of ethnic foods that include cheese as an ingredient is somewhat responsible for the increased use of cheese. Per capita consumption of cheese has roughly doubled over the past 20 years. The annual U.S. per capita consumption increased from 12.4 to 24.7 lbs. Rapidly expanding restaurant pizza sales and changes in lifestyles that emphasize convenience food are also major factors affecting cheese trends. The increasing popularity of pizza has caused consumption of mozzarella cheese to more than double in the past decade alone. Cheese sales in 1993 reached about $17.2 billion. Concerns over fat intake and its affect on health has resulted in a growing production of the “light” varieties, which represent about 38% of all new cheese products introduced in the past two years.

The origin of cheese as a staple in the human diet is lost in history. By some accounts, cheese has been around for 6,000 to 7,000 years. It was standard fare for the Egyptians of about 2000 B.C. and many types were made by the Romans. Cheese making skills were brought by settlers who migrated due to famine, conflicts, and invasions. The Roman invasion of Britain brought a new art of cheesemaking to that country, although cheese may have been made there previously.

Depending on their radioactive content and abundance of long-lived radionuclides, radioactive wastes are often described as low, intermediate or high-level. Cements play a major role in the engineered structures, existing and planned, of most national programs for low- and intermediate-level (ILW) radioactive wastes. Final disposal of ILW is usually by burial at considerable depth (>250 meters), e.g., in planned repositories in clay at Mol (Belgium), in salt at Gorleben (Germany), and in volcanic tuffs at Sellafield (United Kingdom). A sample disposal concept is shown in Figure 1. Shallow land burial is also employed, mostly for low-level wastes (LLW) and often in concrete-lined vaults, e.g., at Drigg (UK) and Center de la Manche (France). Cements are likely to be used in a waste repository as structural elements in the engineered structure and for encapsulation of the actual waste itself (see Figure 1). In the UK, cements are also the favored material for infilling of the vault space (backfilling), and sealing.

LLW and ILW are characterized by considerable heterogeneity, and comprise liquids, solids, floes, sludges, exchange resins, and contaminated laboratory equipment. Cements are capable of converting most of these waste streams into solid and stable monoliths that can be further encapsulated in a steel or concrete container. Such a product is ideal for interim storage, transportation, and final emplacement in a repository. The technology of cementation is well-established, and suited to automation and remote handling (thereby reducing the radiation dose to workers).

Catalytic inorganic membranes are among the most challenging and intriguing porous materials. Consisting of a thin film of mesoporous or microporous inorganic material deposited on a macroporous material, catalytic membranes are multifunctional materials that must be engineered for both chemical and physical properties. New approaches to carrying out chemical reactions are possible by tailoring the membrane catalytic activity and selectivity, permselectivity, and other thin film properties. Readers are referred to several recent reviews of inorganic membranes, in particular, Zaspalis and Burggraaf, Armor, Gellings and Bouwmeister, Hsieh, Stoukides, and Tsotsis et al.

Inorganic membranes are most conveniently classified according to pore size (see introductory article). Of particular importance is the ratio of the pore size to the molecular mean free path (MFP). Decreasing pore dimensions lead to increased selectivity with corresponding loss of permeability. Macroporous membranes have a pore size much larger than the MFP, leading to molecular (bulk) diffusion or viscous flow. Knudsen diffusion dominates in the mesoporous regime, where the pore size is comparable to the MFP. In addition, surface diffusion of the molecules along the pore walls may contribute, leading to an enhanced flux of the adsorbed species along the walls. The microporous regime is encountered when the pore size is comparable to the molecules. This regime makes possible much higher permselectivities, which depend on both molecular size and specific interactions with the solid. Finally, in dense membranes, molecular transport occurs through a solution-diffusion mechanism, which also involves specific interactions between the solute and membrane.

Chemical plating techniques have been used in silicon processing for many years for junction delineation and ohmic contact formation. In recent years, interest in this area has been renewed because of the potential use of electroless copper deposition for ultra-large-scale integration (ULSI) metallization and for the formation of thin metal etch masks for deep-ultraviolet lithography. Good deposition selectivity, low operating temperature, high copper purity, good filling characteristics, and planar topography have been among the many advantageous attributes reported from early investigations.

In the June 1993 issue of the MRS Bulletin on Copper Metallization, Cho et al. gave an exposition on the use of electroless copper for VLSI. More comprehensive reviews of the electrochemical fundamentals can be found in References 8,9, and 10. This article summarizes the current understanding of various chemical and material aspects of this deposition method in an attempt to give an overview of the film growth characteristics for thin film applications. Because of length limitations, only selected topics are included. The emphasis is on the initiation conditions and the resultant microstructure and properties obtained. We also discuss special considerations for fine pattern formation.

The information presented here applies primarily to electroless copper deposition on metals and metal silicides since these are the typical substrate surfaces for metallization in contact vias of ULSI circuits. Strictly speaking, metallization of upper-level interconnects occurs mostly on a dielectric base. However, since copper systems usually require a diffusion barrier to shield the copper from diffusing into the silicon, we can treat the deposition process startingfrom this layer onward.

Silicon-based ceramics are leading candidate materials for high-temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica scale (SiO2). However, durability in high-temperature environments containing molten salts, water vapor, or a reducing atmosphere can limit their applications. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species, such as Si(OH)4(g) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing (CO2, H2O) and reducing (CO, H2) gases form SiO2 and then reduce it to SiO(g). In situations with extremely low partial pressures of oxidant, direct formation of SiO(g) occurs. All these reactions can potentially limit the formation of a protective silica scale and thus lead to an accelerated or a catastrophic degradation.

One approach overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor, and/or reducing atmospheres. Refractory oxides such as mullite (3Al2O3 · 2SiO2), yttria-stabilized zirconia (ZrO2-Y2O3), or alumina (Al2O3) are promising candidate coating materials because of their excellent environmental stability in these severe conditions. Refractory oxide coatings can also serve as thermal barrier coatings because of their low thermal conductivity. Key requirements for an adherent and durable barrier coating include coefficient of thermal expansion (CTE) match and chemical compatibility with the substrate. Mullite in general meets all the requirements and thus appears most promising.

As the degree of large-scale integration (LSI) increases, the area of a single transistor will diminish and the density of transistors will increase. Accordingly, technology for high-density wiring to interconnect the huge number of transistors is needed because transistors cannot perform any useful functions without interconnects and electrodes. For advanced microprocessor chips with a sub-half-micron design rule and at least four interconnect layers, the minimum width of the interconnects becomes less than 0.35 μm.

Aluminum or an aluminum alloy is now generally used as the interconnect material in LSI circuits because the physical and chemical properties of aluminum are compatible with current LSI processing: Aluminum forms a thin protective oxide film that withstands various thermal processes; it has relatively low electrical resistivity and halide compounds with a relatively high vapor pressure which are suitable for reactive ion etching (RIE), and it is an inexpensive material. The reliability of aluminum interconnects, however, is a major concern for maintaining the total reliability of advanced LSI. Because of its relatively low melting point, aluminum as an interconnect material is susceptible to stress- and electromigration, which leads to open failure of the interconnect. It is well-known that these failure modes are accelerated by decreasing the width and thickness of the interconnects. Hence, use of aluminum interconnects may be limited for future sub-half-micron LSIs.

Ice cream and frozen desserts are popular throughout the world, but the United States is the largest consumer of ice cream. In the United States it is estimated that 46.8 pints per capita of ice cream and frozen desserts were consumed in 1992.

Physically, ice cream is a foam in which the continuous phase is a partially frozen emulsion. Foams are also important for foods such as whipped toppings, souffle, mousse, meringue, and angel food cake. The structure of the foam affects the appearance, texture, consistency, lightness, and mouthfeel of these foods. A foam is mostly air and is characterized by high viscosity, low density, high surface area, and high surface energy. Food foams such as ice cream have an air content of 40-50% by volume. The continuous phase is a concentrated, unfrozen aqueous solution containing soluble milk salts, lactose, and added sugars. The dispersed solids (i.e., proteins, salts, stabilizer, and fat) are in the colloidal state and in the form of an emulsion. The ice crystals exist in the coarsely dispersed phase, occupying a major portion of the space between air cells.

Some industrial processes require the separation of gas or vapor mixtures. Methods for separating the mixtures vary from separation by diffusion to separation by distillation. Many of the methods, such as distillation, are energy intensive. Membranes can reduce the energy required to produce a desired separation. Because of their corrosion resistance and high temperature applications, engineered inorganic membranes can significantly increase the efficiency of many of these processes. The magnitude of the separation factor, available operating conditions, enrichment, yield, and cost of the membranes play a large role in determining whether membranes can be more economical than other methods of separation. These factors have to be evaluated on a case-by-case basis.

Martin Marietta Energy Systems' Office of Technology Transfer conducted a preliminary market survey with the assistance of the University of Tennessee and commercial marketing experts in inorganic membranes. The survey assumed that membranes could be made with permeabilities a factor of 3 larger and with cost per unit area a factor of 3 smaller than is currently available. The results indicated that active implementation of such technology could expect to achieve the following results:

• $2 billion dollar per year sales market,

• $16.6 billion increase in the national GDP,

• $2 billion improvement in the balance of trade, and

• 6 quads per year decrease in energy use.

Environmental degradation poses a serious limitation on the utility of engineering materials such as metals, plastics, glasses, and structural ceramics. However, at least water would seem to be a harmless environment for ceramics, which are considered to be the most corrosion-resistant of all materials. When we drink water from a glass or a ceramic cup, we never think about an aqueous alteration of these materials. We cannot imagine that our ceramic mug may be dissolved in water or fail due to stress corrosion during breakfast. Even hot water which we use to make coffee or tea does not seem to be a hazardous environment for our china and earthenware. Why then are we going to discuss the effect of water on structural ceramics? The answer is simple: Under conditions of normal use of structural ceramics, which include high temperatures, high pressures, and mechanical stresses (Figure 1), even the most stable and corrosion-resistant materials may interact with water and water may become a hostile environment. It can be adsorbed on the surface of ceramics and act as a solvent for oxide ceramics or as an oxidant for non-oxide ceramics (Figure 2). However, water can cause a degradation of glasses and oxide ceramics even at room temperature if mechanical stresses are simultaneously applied (so-called stress corrosion). Water or humid air can decrease significantly the mechanical properties of certain zirconia ceramics at only slightly elevated temperatures (~200°C).

For surface science, the 1980s were the decade in which the microscopes arrived. The scanning tunneling microscope (STM) was invented in 1982. Ultrahigh vacuum transmission electron microscopy (UHVTEM) played a key role in resolving the structure of the elusive Si(111)-7 × 7 surface. Scanning electron microscopy (SEM) as well as reflection electron microscopy (REM) were applied to the study of growth and islanding. And low-energy electron microscopy (LEEM), invented some 20 years earlier, made its appearance with the work of Telieps and Bauer.

LEEM and TEM have many things in common. Unlike STM and SEM, they are direct imaging techniques, using magnifying lenses. Both use an aperture to select a particular diffracted beam, which determines the nature of the contrast. If the direct beam is selected (no parallel momentum transfer), a bright field image is formed, and contrast arises primarily from differences in the scattering factor. A dark field image is formed with any other beam in the diffraction pattern, allowing contrast due to differences in symmetry. In LEEM, phase contrast is the third important mechanism by which surface and interface features such as atomic steps and dislocations may be imaged. One major difference between TEM and LEEM is the electron energy: 100 keV and above in TEM, 100 eV and below in LEEM. In LEEM, the imaging electrons are reflected from the sample surface, unlike TEM where the electrons zip right through the sample, encountering top surface, bulk, and bottom surface. STM and TEM are capable of ~2 Å resolution, while LEEM and SEM can observe surface features (including atomic steps) with -100 Å resolution.

The Center for Electron Microscopy of Materials Science (EMAT) at the University of Antwerp, Belgium, started in 1967 under the initiative of S. Amelinckx. While most materials science laboratories choose a particular research theme (e.g., superconducting materials) and study it through several available techniques, the EMAT group at Antwerp has taken a different approach. From the beginning, EMAT has concentrated on a single technique: transmission electron microscopy. Since the laboratory, as well as the university, is very small, we felt it would be better to be excellent in a single technique than to have limited proficiency in several techniques. Until recently, EMAT has been able to maintain this strategy, although the group has grown considerably; at present it consists of four academic staff members, four postdocs, and 12 PhD students, supported by a technical and administrative staff of eight people.

Instrumentation at EMAT

The range of electron microscopes and related instruments in the EMAT laboratory is quite extensive and includes several instruments dedicated to specific techniques. The instruments include:

∎ A 1,250 kV high-voltage electron microscope, used mainly for studying processed semiconductor devices and for in situ radiation experiments.

∎ A 400 kV and a 200 kV high-resolution electron microscope with resolving powers of 1.7 Å and 2.5 Å, respectively. Both instruments are equipped with a TV imaging system and a video camera.

∎ A 200 kV and a 100 kV analytical electron microscope with large tilting possibilities, cooling and heating facilities, and capability for energy dispersive x-ray analysis for measuring the local composition.