Thomas Guillemet

Influence of WC-Co Substrate Pretreatment on Diamond Film Deposition by Laser-Assisted Combustion Synthesis

The quality of diamond films deposited on cemented tungsten carbide substrates (WC-Co) is limited by the presence of the cobalt binder. The cobalt in the WC-Co substrates enhances the formation of nondiamond carbon on the substrate surface, resulting in a poor film adhesion and a low diamond quality. In this study, we investigated pretreatments of WC-Co substrates in three different approaches, namely, chemical etching, laser etching, and laser etching followed by acid treatment. The laser produces a periodic surface pattern, thus increasing the roughness and releasing the stress at the interfaces between the substrate and the grown diamond film. Effects of these pretreatments have been analyzed in terms of microstructure and cobalt content. Raman spectroscopy was conducted to characterize both the diamond quality and compressive residual stress in the films.

Stress and phase purity analyses of diamond films deposited through laser-assisted combustion synthesis

Diamond films were deposited on silicon and tungsten carbide substrates in open air through laser-assisted combustion synthesis. Laser-induced resonant excitation of ethylene molecules was achieved in the combustion process to promote diamond growth rate. In addition to microstructure study by scanning electron microscopy, Raman spectroscopy was used to analyze the phase purity and residual stress of the diamond films. High-purity diamond films were obtained through laser-assisted combustion synthesis. The levels of residual stress were in agreement with corresponding thermal expansion coefficients of diamond, silicon, and tungsten carbide. Diamond-film purity increases while residual stress decreases with an increasing film thickness. Diamond films deposited on silicon substrates exhibit higher purity and lower residual stress than those deposited on tungsten carbide substrates.

Control of crystallographic orientation in diamond synthesis through laser resonant vibrational excitation of precursor molecules

Crystallographic orientations determine the optical, electrical, mechanical, and thermal properties of crystals. Control of crystallographic orientations has been studied by changing the growth parameters, including temperature, pressure, proportion of precursors, and surface conditions. However, molecular dynamic mechanisms underlying these controls remain largely unknown. Here we achieved control of crystallographic orientations in diamond growth through a joint experimental and theoretical study of laser resonant vibrational excitation of precursor molecules (ethylene). Resonant vibrational excitation of the ethylene molecules using a wavelength-tunable CO2 laser steers the chemical reactions and promotes proportion of intermediate oxide species, which results in preferential growth of {100}-oriented diamond films and diamond single crystals in open air. Quantum molecular dynamic simulations and calculations of chemisorption energies of radicals detected from our mass-spectroscopy experiment provide an in-depth understanding of molecular reaction mechanisms in the steering of chemical reactions and control of crystallographic orientations. This finding opens up a new avenue for controlled chemical vapor deposition of crystals through resonant vibrational excitations to steer surface chemistry.

Solid-state graphene formation via a nickel carbide intermediate phase

Direct formation of graphene with a controlled number of graphitic layers on dielectric surfaces is highly desired for practical applications but still challenging. Distinguished from the conventional chemical vapor deposition methods, a solid-state rapid thermal processing (RTP) method can achieve high-quality graphene formation on dielectric surfaces without transfer. However, little research is available to elucidate the graphene growth mechanism in the RTP method (heating rate ∼15 °C s−1). Here we show a solid-state transformation mechanism in which a metastable nickel carbide (Ni3C) intermediate phase plays a critical role in transforming amorphous carbon to two dimensional crystalline graphene and contributing to the autonomous Ni evaporation in the RTP process. The formation, migration and decomposition of Ni3C are confirmed to be responsible for graphene formation and Ni evaporation. The Ni3C-assisted graphene formation mechanism expands the understanding of Ni-catalyzed graphene formation and provides insightful guidance for controlled growth of graphene through the solid-state transformation process.

Thermal characterization of diamond films through modulated photothermal radiometry.

Diamond (Dia) films are promising heat-dissipative materials for electronic packages because they combine high thermal conductivity with high electrical resistivity. However, precise knowledge of the thermal properties of the diamond films is crucial to their potential application as passive thermal management substrates in electronics. In this study, modulated photothermal radiometry in a front-face configuration was employed to thermally characterize polycrystalline diamond films deposited onto silicon (Si) substrates through laser-assisted combustion synthesis. The intrinsic thermal conductivity of diamond films and the thermal boundary resistance at the interface between the diamond film and the Si substrate were investigated. The results enlighten the correlation between the deposition process, film purity, film transverse thermal conductivity, and interface thermal resistance.

Laser-assisted synthesis of diamond crystals in open air through vibrational excitation of precursor molecules

Fast growth of diamond crystals in open air was achieved by laser-assisted combustion synthesis through vibrational excitation of precursor molecules. A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 μm) was used for the vibrational excitation in synthesis of diamond crystals. A pre-mixed C2H4/C2H2/O2 gas mixture was used as precursors. Through resonant excitation of the CH2-wagging mode of ethylene (C2H4) molecules using the CO2 laser tuned at 10.532 Μm, high-quality diamond crystals were grown on silicon substrates with a high growth rate of ~139 μm/hr. Diamond crystals with a length up to 5 mm and a diameter of 1 mm were grown in 36 hours. Sharp Raman peaks at 1332 cm-1 with full width at half maximum (FWHM) values around 4.5 cm-1 and distinct X-ray diffraction spectra demonstrated the high quality of the diamond crystals. The effects of the resonant excitation of precursor molecules by the CO2 laser were investigated using optical emission spectroscopy.

Laser-power-resolved excitations of ethylene molecules in laser-assisted synthesis of diamond films

Laser-power-resolved excitations of precursor molecules in laser-assisted synthesis of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 µm to match a vibration mode of a precursor molecule, ethylene (C2H4). The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5×103 and 1.0×104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5×103∼6.7×103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3×103∼6.7×103 W/cm2. The effects of the resonant laser energy coupling were investigated using optical emission spectroscopy.Laser-power-resolved excitations of precursor molecules in laser-assisted synthesis of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 µm to match a vibration mode of a precursor molecule, ethylene (C2H4). The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5×103 and 1.0×104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5×103∼6.7×103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3×103∼6.7×103 W/cm2. The effects of the resonant laser energy coupling were investigat...

Role of metal/matrix interfaces in the thermal management of metal-carbon composites

The increase in both power and packing densities in power electronic devices has led to an increase in the market demand for effective heat-dissipating materials, with high thermal conductivity and thermal expansion coefficient compatible with chip materials still ensuring the reliability of the power modules. In this context, metal matrix composites: carbon fibers, carbon nano fibers and diamond reinforced copper matrix composites among them are considered very promising as a next generation of thermal management materials in power electronic packages. These composites exhibit enhanced thermal properties compared to pure copper combined with lower density. This article presents the fabrication techniques of copper/carbon composite films by powder metallurgy and tape casting and hot-pressing; these films promise to be efficient heat-dissipation layers for power electronic modules. The thermal analyses clearly indicate that interfacial treatments are required in these composites to achieve high thermo-mechanical properties. Interfaces (through novel chemical and processing methods), when selected carefully and processed properly will form the right chemical/mechanical link between copper and carbon enhancing all the desired thermal properties while minimizing the deleterious effect. In this paper, we outline a variety of methods that are system specific that achieve these goals.The increase in both power and packing densities in power electronic devices has led to an increase in the market demand for effective heat-dissipating materials, with high thermal conductivity and thermal expansion coefficient compatible with chip materials still ensuring the reliability of the power modules. In this context, metal matrix composites: carbon fibers, carbon nano fibers and diamond reinforced copper matrix composites among them are considered very promising as a next generation of thermal management materials in power electronic packages. These composites exhibit enhanced thermal properties compared to pure copper combined with lower density. This article presents the fabrication techniques of copper/carbon composite films by powder metallurgy and tape casting and hot-pressing; these films promise to be efficient heat-dissipation layers for power electronic modules. The thermal analyses clearly indicate that interfacial treatments are required in these composites to achieve high thermo-mech...

Evaluation of thermal resistance at the silicon/diamond interface through infrared photothermal radiometry

Besides knowledge of thermal conductivities, information about the interfacial thermal resistances existing in layered systems such as power electronic packages is of primary importance. Indeed, thermal boundary resistances have a critical influence on the heat transfer process occurring between the layers. In this study, modulated infrared photothermal radiometry was employed to measure the thermal response of diamond films deposited on silicon substrates through laser-assisted combustion synthesis. The thermal resistance normal to the diamond/silicon interface was then estimated from the measurement of the phase and the amplitude of the thermal response. Preliminary results show that the layered diamond/Si system exhibits an interfacial thermal resistance of about 4×10-8 K.W-1. The technique developed in this study enables a precise evaluation of the thermal resistance at the diamond/silicon interface and is promising for various thermal management applications of diamond thin-films in optics, electronics, or mechanics.Besides knowledge of thermal conductivities, information about the interfacial thermal resistances existing in layered systems such as power electronic packages is of primary importance. Indeed, thermal boundary resistances have a critical influence on the heat transfer process occurring between the layers. In this study, modulated infrared photothermal radiometry was employed to measure the thermal response of diamond films deposited on silicon substrates through laser-assisted combustion synthesis. The thermal resistance normal to the diamond/silicon interface was then estimated from the measurement of the phase and the amplitude of the thermal response. Preliminary results show that the layered diamond/Si system exhibits an interfacial thermal resistance of about 4×10-8 K.W-1. The technique developed in this study enables a precise evaluation of the thermal resistance at the diamond/silicon interface and is promising for various thermal management applications of diamond thin-films in optics, electroni...

Laser-induced resonant vibrational excitations of precursor molecules in multi-energy processing for diamond synthesis

A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 f.m) was applied in a multi-energy process for growing diamond crystals in open air. A pre-mixed C2H4/C2H2/O2 gas was used as precursors for the diamond growth. Laser energy was coupled into the reactions through resonantly exciting the CHB2B-wagging mode of ethylene (C2H4) molecules by tuning the laser wavelength to 10.532 µm. Diamond growth rate and diamond quality were both promoted by the laser-induced resonant excitations. High-quality diamond crystals were grown on silicon substrates with a high growth rate of ∼139 µm/hr. Diamond crystals up to 5 mm in height and 1 mm in diameter were grown in open air in 36 hours. Sharp Raman peak at 1332 cm-1 with a full width at half maximum value around 4.5cm-1 and distinct X-ray diffraction spectra indicate the high quality of the diamond crystals.A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 f.m) was applied in a multi-energy process for growing diamond crystals in open air. A pre-mixed C2H4/C2H2/O2 gas was used as precursors for the diamond growth. Laser energy was coupled into the reactions through resonantly exciting the CHB2B-wagging mode of ethylene (C2H4) molecules by tuning the laser wavelength to 10.532 µm. Diamond growth rate and diamond quality were both promoted by the laser-induced resonant excitations. High-quality diamond crystals were grown on silicon substrates with a high growth rate of ∼139 µm/hr. Diamond crystals up to 5 mm in height and 1 mm in diameter were grown in open air in 36 hours. Sharp Raman peak at 1332 cm-1 with a full width at half maximum value around 4.5cm-1 and distinct X-ray diffraction spectra indicate the high quality of the diamond crystals.