Colloid aspects of chemical-mechanical planarization.

The essential parts of interconnects for silicon based logic and memory devices consist of metal wiring (e.g. copper), a barrier metal (Ta, TaN), and of insulation (SiO2, low-k polymer). The deposition of the conducting metal cannot be confined to trenches, resulting in additional coverage of Cu and Ta/TaN on the surface of the dielectrics, yielding an electrically conducting continuous but an uneven surface. The surplus metal must be removed until a perfectly flat surface consisting of electrically isolated metal lines is achieved with no imperfections. This task is accomplished by the chemical-mechanical planarization (CMP) process, in which the wafer is polished with a slurry containing abrasives of finely dispersed particles in submicrometer to nanometer size. The slurries also contain dissolved chemicals to modify the surfaces to be planarized. Eventually the final product must be cleared of any adhered particles and debris left after polishing is completed. Obviously the entire process deals with materials and interactions which are the focal subjects of colloid and surface science, such as the natures of abrasive particles and their stability in the slurry, the properties of various surfaces and their modifications, adhesion and detachment of the particles and different methods for the characterization of constituents, as well as elucidation of the relevant interfacial phenomena. This review endeavors to describe the colloid approach to optimize the materials and processes in order to achieve desirable polish rates and final surfaces with no imperfections. Specifically, the effects of the composition, size, shape, and charge of abrasive particles on the polish process and the quality of planarized wafers is described in detail. Furthermore, the interactions of metal surfaces with oxidizing, chelating, and other species which affect the dissolution and surface modification of metal (copper) surfaces are illustrated and related to the planarization process. Finally, using the packed column technique the adhesion phenomena of abrasives on metals and oxides is evaluated on suitable model systems, that contain the same additives in the slurries as in the actual planarization process. A close correlation is established in all cases between the attachment and detachment results with experimentally determined polish rates.

Noise reduction in relaxation kinetic experiments.

The use of high-speed digital data-acquisition devices makes it possible to obtain a large number of points (>4000) for a single experiment during a short time interval. A suitable numerical treatment capable of reducing the periodic and random noise in relaxation-curves and of providing relaxation times and amplitudes of high accuracy is presented.

Gelatin induced reduction of uniform nano-platelets of Ni(OH)2 to Ni metal.

Uniform stable nanosize nickel platelets were prepared by first coating Ni(OH)2 particles of the same shape with gelatin and then heating them at moderate temperatures in nitrogen without addition of any reducing agents. The resulting platelets consist of a mixture of Ni and NiO. Depending on the amount of gelatin coating and temperature, up to 73% of solids were converted to the metal. Subsequent heating of such powders at 190 degrees C in a stream of 5% H2 in N2 produced only Ni platelets with a modal length of approximately 60 nm and several nm thick.

Coating of Nanosize Silver Particles with Silica.

The formation of silica shells on core silver particles by a modified Stöber process was investigated and the coated particles were characterized using UV-visible spectroscopy, transmission electron microscopy, and electrophoresis. The deposition conditions, such as reagent concentrations and reaction time, were optimized in order to obtain uniform surface layers of silica and to avoid excess precipitation of the latter. Copyright 2000 Academic Press.

Preparation of Micrometer Size Budesonide Particles by Precipitation.

Particles of different shape, size and crystallinity of budesonide, a hydrocortisone streroid, were prepared by precipitation. The method is based on taking advantage of the different solubility of this material in certain organic solvents (ethanol, acetone) and water or alcanes. In principle, to solutions of budesonide, water was added until the solids precipitate or the same solute in mixed solvents was carefully separated by evaporation of the more volatile component at room temperature. The morphology, size, and stability of the resulting particles were sensitive to the evaporation technique, stirring, and presence of a stabilizing agent. Copyright 2000 Academic Press.

Preparation of Uniform Needle-Like Aragonite Particles by Homogeneous Precipitation.

Uniform needle-like aragonite particles were obtained by aging solutions of calcium salts in the presence of urea at 90 degrees C. The effects of various experimental conditions, such as the concentration of reactants, temperature, aging time, agitation, mixing procedure, and the addition of divalent cations, surfactants, and polymers, on the precipitation of aragonite were investigated. It was found that the formation of different polymorphs of calcium carbonate was affected mostly by agitation and concentration of reactants, as well as by the mixing procedure. Thus, under otherwise the same experimental conditions, needles of aragonite were formed without stirring or at low-power ultrasound; rhombohedral calcite appeared when aging experiments were carried out under the influence of magnetic stirring, while irregular particles of mixed vaterite and calcite crystal structure were generated at higher CaCl(2) concentrations. Mixing the preheated reactant solutions resulted in unique "flower-type" vaterite particles. Finally, the presence of sulfonate or sulfate ions promoted precipitation of vaterite and calcite. Copyright 1999 Academic Press.

Optical properties of monodispersed hematite hydrosols.

The experimentally determined extinction spectra of colloidal hematite, consisting of spherical particles of 0.10, 0.12, 0.13, 0.15, 0.16, and 0.51 microm in modal diameter, respectively, are compared with the corresponding theoretically calculated spectra using the Lorenz-Mie theory. These spectra are most sensitive to the particle size in the 0.10-0.20-microm range due to the damping effects which broaden the resonance at the expense of its height. For large particles (0.51 microm) the extinction becomes independent of the wavelength because of the broader size distribution of the hematite dispersion. As the small particles aggregate, a shoulder appears at 550 nm corresponding to the onset in fluctuations in the extinction efficiency of various sized particles. This shoulder may suggest the existence of certain surface electromagnetic modes caused by aggregation. A comparison is made in the size-dependent extinction efficiency at various wavelengths between colloidal hematite and gold sols.

Color effects of uniform colloidal particles of different morphologies packed into films.

The experimentally determined chromaticities and reflectance spectra of films consisting of uniform ellipsoidal or spherical colloidal hematite particles are compared with calculated values and are found to be in good agreement. The theoretical treatment of the light-scattering problem involves the Mie theory for the spheres and the T-matrix method for the ellipsoids. The reflectance spectra for the pigment films are calculated through the use of the Kubelka-Munk analysis.