K. Tankala

Role of heat transfer and fluid flow in the chemical vapor deposition of diamond

The role of fluid flow and heat transfer in determining the quality of the diamond films and the rate of their deposition in a hot‐filament chemical vapor deposition (HFCVD) reactor was investigated both experimentally and theoretically. The equations of conservation of mass, momentum, and enthalpy were solved numerically to calculate the temperature and fluid flow fields. Experiments were conducted with various flow configurations, and the deposition rates and the spatial variations of film thickness were examined in each case. The films were characterized by Raman spectroscopy, x‐ray, and scanning electron microscopy. The influences of free and forced convection, and diffusion due to concentration and temperature gradients (Soret effect) were examined. Comparison of the computed results with the experimental data revealed the importance of thermal diffusion in the HFCVD of diamond.

Modeling of the Role of Atomic Hydrogen in Heat Transfer During Hot Filament Assisted Deposition of Diamond

The temperature and atomic hydrogen concentration profiles in a hot filament type diamond deposition reactor were determined experimentally and theoretically to demonstrate that the reaction of atomic hydrogen on the substrate surface plays an important role in the heating of the substrate. For a given filament temperature, the substrate temperature in helium was significantly lower than that in either pure hydrogen or 1% methane‐hydrogen atmospheres. The presence of small amounts of methane in hydrogen did not have any significant effect in influencing the shape of the atomic hydrogen concentration profile. In the space between the filament and the substrate, the concentration field is established mainly due to the diffusive mixing of the atomic hydrogen with the molecular hydrogen and other species in the gas phase. Homogeneous chemical reactions in the gas phase do not significantly affect the atomic hydrogen concentration distribution in this region.

Hydrogen assisted heat transfer during diamond growth using carbon and tantalum filaments

Much of the previous work on the role of atomic hydrogen in diamond growth has been focused on its formation on various refractory metal filaments, its reaction in the gas phase and its role in the growth mechanism. In contrast, the effect of atomic hydrogen recombination on substrate heating is addressed in this letter. Experiments were conducted in vacuum, helium, and hydrogen environments. Tantalum and carbon filaments were used to vary atomic hydrogen generation rates. Furthermore, methane was added in some experiments to determine its effect on hydrogen assisted ‘‘chemical’’ heating of the substrate. The results indicate that when substantial amounts of atomic hydrogen are generated at the filament, reactions of atomic hydrogen at the diamond growth surface have a pronounced effect on the substrate temperature. Use of carbon filaments lead to significantly diminished atomic hydrogen generation rates and much lower substrate temperatures. Additions of small amounts of methane to hydrogen also resulted...

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

Abstract In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs. The kinetics of formation of Al2O3 in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol−1. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.

Optical emissions during plasma assisted chemical vapor deposition of diamond-like carbon films

Abstract The deposition of diamond-like carbon (DLC) films onto silicon wafers and polyethyleneterephthalate (PET) from methanehydrogen gas mixtures by plasma assisted chemical vapor deposition was investigated by optical emission spectroscopy. Film growth rates, crack formation, and average electron energy in the plasma were analyzed for various deposition conditions. The cracks in the DLC films deposited onto PET could be removed by increasing the hydrogen content in the gas mixture. No adjustment of ion energy, substrate temperature control, or addition of inert gas was necessary to avoid crack formation. In the commonly used range of bias voltage above 500 V, the intensity of the Hα line (656.3 nm) correlated well with the film deposition rate. The hydrogen peak intensity can be used for on-line, non-contact, instantaneous monitoring of the deposition rate.

Transport phenomena in the scale-up of hot filament-assisted chemical vapor deposition of diamond

Abstract Heat transfer and fluid flow in a diamond deposition reactor were examined to identify parameters important to reactor design and scale-up. Physical and mathematical modeling of hot filament-assisted diamond deposition reactors indicated that both ordinary and thermal diffusion are equally important in the transport of nutrient species to the substrate surface. The atomic hydrogen concentration field in the reactor is established mainly by diffusive mixing of the atomic hydrogen with molecular hydrogen and other species in the gas phase. Homogeneous chemical reactions in the gas phase do not significantly affect the atomic hydrogen concentration profiles. In hot filament reactors, in addition to convection, conduction and radiation, filament-to-substrate heat transfer takes place by dissociation of molecular hydrogen at or near the filament and recombination of atomic hydrogen at the substrate surface Computer simulation of the heat transfer and fluid flow in a typical hot filament reactor for diamond deposition indicated that system geometry, filament temperature and pressure are important parameters in the design of reactors for coating large areas. The investigation clearly indicates considerable promise of science-based design and scale-up of hot filament reactors for diamond deposition.

Modeling of substrate surface temperature distribution during hot-filament assisted diamond deposition

Abstract The importance of substrate temperature in determining the quality, uniformity and growth rate of diamond films is now well recognized. In the hot-filament assisted chemical vapor deposition of diamond, the mechanism of heat transfer is unique. In addition to conduction, convection and radiation, filament-to-substrate heat transfer takes place by dissociation of molecular hydrogen at or near the filament and recombination of atomic hydrogen at the substrate surface. In this paper, the role of atomic hydrogen recombination in heat transfer is examined. Furthermore, the effects of system geometry and process variables on the substrate temperature distribution are analyzed. The results indicate that atomic hydrogen recombination at the substrate plays a significant role in substrate heating. In hot-filament assisted diamond deposition, system geometry, filament temperature and pressure are the most important factors in determining the substrate temperature distribution.