Toivo T. Kodas

Aerosol flow reactor production of fine Y1Ba2Cu3O7 powder: Fabrication of superconducting ceramics

An aerosol flow reactor operating at 900–1000 °C is used to prepare high‐purity Y1Ba2Cu3O7 powders with a uniform chemical composition and a submicron to micron average particle size by thermally decomposing aerosol droplets of a solution consisting of the nitrate salts of Y, Ba, and Cu in a 1:2:3 ratio. The powders were at least 99% reacted based on thermogravimetric analysis, and the x‐ray diffraction pattern is essentially that of Y1Ba2Cu3O7. Magnetic susceptibility measurements showed the powders to be superconducting with a transition at 90 K even for average reactor residence times as short as 20 s. Sintering cold‐pressed pellets between 900 and 1000 °C provides dense, fine grained (average size on the order of 1 μm) superconducting ceramics with sharp 90 K transitions. The grain size and shape of a final sintered part could be varied depending on powder production, processing, and sintering conditions.

Kinetics of laser‐induced chemical vapor deposition of gold

Rates of photothermal gold deposition onto alumina substrates heated by a focused argon‐ion laser beam were measured by determining the time required for deposits to grow through the focal spot of a HeNe laser probe beam directed parallel to the substrate. Deposition rates from 0.25 to 6 μm/s were measured for deposits with heights ranging from 5 to 100 μm. Rates of gold deposition using dimethyl gold hexafluoroacetylacetonate as a precursor depend linearly on the gold precursor partial pressure and for a wide variety of conditions do not depend on the laser power or focal spot diameter. A theory is presented to describe reactant mass transfer‐controlled deposition with and without buffer gas. Comparison of measured with calculated growth rates showed that gold deposition rates using dimethyl gold hexafluoroacetylacetonate as a precursor were transport limited for the conditions studied. Theory and experiment also showed that the deposition rate decreases inversely with increasing buffer gas pressure abov...

Surface temperature rise in multilayer solids induced by a focused laser beam

Surface temperatures of a laser‐heated structure consisting of intersecting gold and nickel lines on a Si substrate with a SiO2 surface layer were determined experimentally and theoretically. Temperatures were measured with micron‐sized thin‐film thermocouples formed from intersecting gold and nickel lines. Temperature profiles were calculated using a finite difference method which took into account the optical and physical properties of the thermocouple metal lines, the silicon dioxide layer, and the silicon substrate. For a 0.2‐μm SiO2 layer, maximum surface temperatures were much higher with the metal lines than without because of the lower reflectivity of the gold relative to the silicon and the limited ability of the thin metal lines to conduct heat away from the irradiated region. Calculated maximum temperatures on the metal lines depended strongly on the thicknesses of the insulating SiO2 layer and the metal lines. Application of these results to the dynamics of laser‐induced chemical vapor deposit...

Generation of thick Ba2YCu3O7 films by aerosol deposition

Thick superconducting films were fabricated by producing high‐purity Ba2YCu3O7 particles by aerosol decomposition in a gaseous flow system, depositing the particles directly from the gas phase onto surfaces by thermophoresis, and then sintering and annealing the deposited particulate films in an oxygen flow. Particulate films with thicknesses of 1 mm were deposited on the inside surfaces of copper tubes and sintered to provide uniform adherent coatings with sharp superconducting transitions above 91 K. High‐purity powders based on the Bi‐Sr‐Ca‐Cu‐O and Tl‐Ca‐Ba‐Cu‐O systems were also produced and sintered to form bulk ceramics with transitions at 80 and 110 K, respectively, suggesting that the process is general and can be used for a variety of materials. Advantages of the process include the ease of obtaining the correct oxygen content and the ability to fabricate thick films of fine grained material while minimizing exposure to carbon and other contaminants.

Gold crystal growth by photothermal laser-induced chemical vapor deposition

Abstract Gold crystals were grown on alumina substrates using photothermal laser-induced chemical vapor deposition with dimethyl gold hexafluoroacetylacetonate as the gold precursor. The overall shape of the deposits and the size of the crystallites in the deposit depended strongly on the laser power, buffer gas partial pressure and buffer gas type. Addition of oxygen or air resulted in a large increase in the size of the crystallites in the deposit, while addition of inert buffer gas resulted in the formation of dendritic structures.

Generation of thick Ba/sub 2/YCu/sub 3/O/sub 7/ films by aerosol deposition

Thick superconducting films were fabricated by producing high‐purity Ba2YCu3O7 particles by aerosol decomposition in a gaseous flow system, depositing the particles directly from the gas phase onto surfaces by thermophoresis, and then sintering and annealing the deposited particulate films in an oxygen flow. Particulate films with thicknesses of 1 mm were deposited on the inside surfaces of copper tubes and sintered to provide uniform adherent coatings with sharp superconducting transitions above 91 K. High‐purity powders based on the Bi‐Sr‐Ca‐Cu‐O and Tl‐Ca‐Ba‐Cu‐O systems were also produced and sintered to form bulk ceramics with transitions at 80 and 110 K, respectively, suggesting that the process is general and can be used for a variety of materials. Advantages of the process include the ease of obtaining the correct oxygen content and the ability to fabricate thick films of fine grained material while minimizing exposure to carbon and other contaminants.

A diffusive transport relaxation technique for studying laser-induced chemical vapor deposition reactions at high pressures

A diffusive transport relaxation technique has been developed to study the chemical processes occurring during laser‐induced chemical vapor deposition. The technique is based on modulation of the laser light intensity which results in a modulation of the surface reaction rate. This perturbation of the reaction rate produces transients in the gas‐phase reactant and product concentrations for the case of gas‐phase transport‐limited deposition. Product species can be differentiated from species formed from the reactant during ionization and fragmentation due to the phase‐sensitive nature of the detection scheme. The system response is examined theoretically by describing the response of the surface reaction rate to the modulated laser light and then examining the response of the gas‐phase reactant and product concentrations to the modulated surface reaction. The technique was used to identify the gaseous products formed during laser‐induced deposition of gold using dimethyl gold hexafluoroacetylacetonate.

Real time optical profilometry as a probe of rates of laser induced chemical vapor deposition

An optical method for the measurement of rates of laser induced chemical vapor deposition processes is described. Vertical growth rates are obtained from measurements of the power attenuation by a growing deposit of a probe beam of known diameter directed parallel to the substrate and intersecting the axis of the deposit. A theoretical analysis is presented that relates the growth rate of deposits with various geometries to the attenuation of the probe beam as a function of time and the probe laser beam diameter. The method can be used to obtain growth rates for deposits as small as a few μm in height, as long as the surface of the deposit is smooth relative to the diameter of the probe beam. The method is used to determine rates of photothermal gold deposition as a function of gold precursor partial pressure. Measured deposition rates ranged from 0.2 to 5.5 μm/s for dimethyl gold hexafluoroacetylacetonate partial pressures from 0.03 to 0.62 Torr.An optical method for the measurement of rates of laser induced chemical vapor deposition processes is described. Vertical growth rates are obtained from measurements of the power attenuation by a growing deposit of a probe beam of known diameter directed parallel to the substrate and intersecting the axis of the deposit. A theoretical analysis is presented that relates the growth rate of deposits with various geometries to the attenuation of the probe beam as a function of time and the probe laser beam diameter. The method can be used to obtain growth rates for deposits as small as a few μm in height, as long as the surface of the deposit is smooth relative to the diameter of the probe beam. The method is used to determine rates of photothermal gold deposition as a function of gold precursor partial pressure. Measured deposition rates ranged from 0.2 to 5.5 μm/s for dimethyl gold hexafluoroacetylacetonate partial pressures from 0.03 to 0.62 Torr.

Modulated laser beam relaxation spectrometry of laser‐induced chemical vapor deposition

The surface chemical reactions occurring during the laser‐assisted deposition of gold from dimethyl gold hexafluoroacetylacetonate were studied using a photothermal modulation technique. A modulated argon ion laser was used to heat an area roughly 1 mm in diameter on an alumina substrate. Products of the photothermal reaction along with reactants were sampled through a 150×150 μm orifice located at the center of the deposit and were then introduced directly into a quadrupole mass filter. An inert buffer gas was required to confine changes in the reactant and product concentrations to the region surrounding the sampling orifice. Since the reactant concentration at the surface decreased when the laser was turned on while the product concentration at the surface increased when the laser was turned on, modulating the surface reaction rate by modulating the argon ion laser beam and employing phase‐sensitive detection allowed the identification of reaction products. The behavior of the system was described qual...

Mechanisms of Laser-Induced Deposition from the Gas Phase

Laser-induced deposition from the gas phase has been extensively studied in the last several years. Because a high degree of spatial localization can be achieved via deposition employing a tightly focused laser beam, a number of applications for laser deposition have been found in the microelectronics field. For example, highly localized deposition induced by a scanning, focused cw laser beam has been successfully used for direct writing of interconnection lines in integrated circuits (1–4). Related applications have been found in integrated circuit modification and customization (5), direct-writing of waveguides (6), and localized deposition for repair of clear defects in lithographic masks (7–9). A mask repair system based on laser deposition of a metal repair patch is now commercially available (10)