Some readers may wonder about the reasons for having two issues focus on fine particles. Hopefully, this introduction and the articles to follow will offer the necessary justification.

It is fully expected by everybody, professional or laie, that the properties of matter will change if its chemical composition is altered. It is less frequently recognized that many characteristics of materials can be dramatically affected by varying dimensions, yet countless examples in everyday life clearly demonstrate such phenomena. Consider water in droplet form. Driving in rain creates little problem, providing the car has working windshield wipers. In contrast fog, which consists of droplets of much smaller size, represents one of the major hazards in transportation. Obviously, the optical properties of the same matter have greatly altered with diminution. In another example, it is well known that finely dispersed carbon is an excellent adsorbent, a property used in gas masks and in many industries as a purifying agent. Large-sized lumps of such carbon would be useless in these applications.

The effect of particle size on properties of materials can be readily understood from a simple consideration. The total energy of any System, (Etot), consists of two contributions, the internai energy, Ei, and the surface energy, Ei.

Recently the science and technology of fine particles have been greatly advanced to meet the urgent demands of modern industries for specific and sophisticated functions for various materials, including catalysts, sensors, electromagnetic devices, and photosensitive materials. Monodispersed colloidal Systems are invaluable for this purpose because the entire system's uniform physicochemical properties directly reflect the properties of each constituent particle. Procedures for modifying monodispersed particles have progressed remarkably in recent years so that specific characteristics can be achieved. This article focuses on the backgrounds of science and technology for controlling the properties of inorganic monodispersed particles and on new developments in this field.

Colloidal particles are normally formed through a sequential process of nucleation and growth of the nuclei. To obtain a monodisperse System, the two stages must be strictly separated and nucleation avoided during the growth period. Since the steady concentration of monomers in the growth stage is determined by the balance between the rates of generation of monomers and their consumption by particle growth, the generation or introduction rate of monomers must be controlled so that it is low enough to keep the balanced monomer concentration below the critical supersaturation after the nucleation period. Typically, the initial concentrations of metal sait, pH, and temperature are adjusted to meet this requirement for the formation of monodispersed metal hydrous oxide particles by forced hydrolysis of metal ions.

An artificial separation between nucleation and growth processes may be achieved by “seeding,” in which foreign particles are introduced into the solution of monomers below the critical supersaturation. One may also lower the pH in hydrolysis of metal ions, dilute with solvent, add chelating agents, or suddenly change the temperature just after limited nucleation. All these procedures could cause the monomer concentration above the critical supersaturation to plunge to a level below it.

One of the newer tendencies in materials science has been to tailor-make classical products (long associated with old applications) with controlled properties for special uses, especially in high technology. Preparing dispersed systems in which all particles have nearly uniform size (monodisperse solids) is a typical example. This goal can be achieved in some cases through cleverly controlled particle growth from a liquid medium. Examples of such preparations include gold colloids prepared by Zsigmondy and later by Turkevich et al., sulfur sols obtained by LaMer, metal oxides and hydrous oxides prepared by Matijević et al., silica, etc. These dispersions have been used either to check theories of colloid science, or to a lesser extent, for industrial purposes. In the case of fine metal particles, a uniform size distribution associated with a low degree of agglomeration, and sometimes the spherical shape, appear as particularly convenient characteristics for certain applications. The production of conductive inks or pastes for electronic materials and for the preparation of conductive paints are particularly good examples.

In so-called thick film technology, conductive inks and pastes are screen printed on a ceramic substrate in order to form, after firing, a conductive film with a thickness less than 10 μm. This technique is, for instance, used to form the network in hybrid integrated circuits or the internal electrodes of multilayer ceramic capacitors.

Metallic powders in thick film compositions are usually precious metals (Au, Ag, Pt, Pd), their mixtures, or alloys. Cheaper metals such as copper or nickel are tested and may be potential substitutes for precious metals in different specific applications. Powders for thick film composition are mainly obtained through chemical precipitation from aqueous or organic solutions, which yield high purity powders. Modification of precipitation parameters (such as the nature and the concentration of the starting metallic compound and of the reducing agent, reaction temperature, viscosity of the medium) and the addition of additives and surfactants, can often be used to control particle size and agglomeration.

Over the past few years, we have developed a new process for preparing finely divided metal powders of easily reducible metals (such as precious metals and copper) or less reducible metals (such as cobalt, nickel, cadmium, or lead) by precipitation in liquid polyols. This reaction will be used as an example in order to discuss the mechanism of formation of uniform micrometer and submicrometer size metal particles by precipitation reactions.

Magnetic particles are important not only in the technology, but also in the function of some biosystems. In addition, they are of great scientific interest in developing a better understanding of magnetic phenomena. Ever since magnetic recording media were first prepared, extensive efforts have been made to produce improved magnetic dispersions. The particle s for magnetic media must be of single domain, high saturation magnetization, and proper coercive force. However, the magnetic interactions between such particles are very strong, and stable dispersions are difficult to obtain. Originally, their use was limited to audio tapes, but presently they are employed in a variety of applications. Thus, small particles of different magnetic properties are constituents of magnetic fluids.

In 1975, magnetic particles were identified in the bodies of some bacteria, which can navigate along geomagnetic fields. It is also believed that certain animais have the ability to detect a magnetic field due to the presence of magnetic particles in their cells.

Techniques are being developed to introduce new functions to materials by incorporating magnetic particles. For example, magnetic particles that are encapsulated by nonmagnetic compounds (e.g. polymers) or particles are used for binding and separating enzymes from reactant mixtures. Efforts are under way to use magnetic particles in drug delivery Systems and for diagnosis.

Small inorganic particles with a narrow size distribution have been prepared from aqueous and organic solutions under varied temperatures, pressures, and concentrations. This article reviews the preparation of such particles from microemulsions and liquid crystals and shows how these amphiphilic association structures can have a pronounced influence on the size, shape, and even chemical composition of the precipitated particles.

Surfactants associate in the presence of water to form micelles and lyotropic mesophases. This is a consequence of the molecular geometrical packing requirements, superposed on the dissolution behavior of these substances. Associated structures that have shapes with the lowest free energy are preferentially formed. A balance is attained between the interactions of the hydrophobic portions of the surfactant, reducing the unfavorable contact with aqueous environs and the interactions between the headgroups. The composition regions of these different phases have been widely studied. Micelles exist as normal and reversed structures (Figure 1), while the liquid crystalline phases show a variety of structures which have been studied in great detail.

Figure 1 gives a simplified example of the structural conditions in different parts of a phase diagram of the three components water/surfactant/amphiphilic substance (e.g., medium chain length alcohol). The interaction between water and the surfactant is sufficiently pronounced to permit the formation of reversed micelles. The association structures of different regions shown in Figure 1 include normal (L1) and reversed micelles (L2). The hexagonal phase (E) consists of a hexagonal array of cylinders with the hydrocarbon chains situated in the center of the cylinder, while the reverse hexagonal phase (F) is built of cylinders with aqueous centers. The lamellar phase (Lα) is made up of infinite sheets of bimolecularly arranged surfactant molecules separated by aqueous layers. The physical properties and reasons for the formation of such lyotropic phases have been discussed extensively.

A novel excimer laser process has been developed for generating cylindrical via holes with an aspect ratio of about one. The fabrication process is being successfully run on a production line for a highly miniaturized printed circuit board used for the multichip module in the new Siemens 7500 H 90 mainframe computer. The process is outstanding in terms of reliability and reproducibility. To the best of our knowledge, this is the first that that excimer lasers have been put into large-scale use in an industrial environment.

Since signal delay times for chips have decreased much more rapidly than delay times for packaging, the computing speed of high-speed computers is restricted by the packaging techniques used. Therefore, further development of packaging technology became a prime objective for those developing high-performance computers. Packaging delay times had to be reduced drastically to keep up with increasingly shorter chip delay times. This, in effect, meant that a greater packaging density had to be implemented.

A novel planar packaging technique has lead to considerable progress in solving this problem. This technique has been described in detail elsewhere. A key component in this technology is a multichip module, which can take in each of 16 areas, either an LSI module with 320 leads or 9 MSI modules with 52 leads as “bare” ICs. This means that a micro-wiring printed circuit board of this kind can accomodate between 16 (LSI) and 144 (MSI) chips. This article describes how these printed circuit boards are manufactured.

As the specifications (Table I) show, blind vias 80 μm in diameter at a pitch of 0.5 mm have to be made in a 16-layer printed circuit board. It is intended that these blind vias will provide the through-contact for neighboring layers. The excimer laser plays a major role in this process.