E. Blank

Characterization of ballas diamond depositions

Abstract Unfaceted, polycrystalline spherically grown diamond deposits having a radial structure have been observed since the early days of low pressure CVD diamond synthesis. Because the structure is quite similar to natural ballas stones, unfaceted CVD diamond is called ballas. So far, the general trend in diamond deposition has focused on well-faceted diamond layers, so CVD ballas deposits have not been systematically investigated. Low pressure growth of ballas always occur under conditions that are “non-optimal”, i.e. at least one parameter exceeds the range for a diamond growth leading to well-faceted diamond crystals. CVD ballas can consist of more than 99% of pure diamond; its microstructure reveals high amounts of micro-twins. Several morphological ballas structures have been observed by varying the deposition conditions, i.e. ballases having faceted areas, flat ballases, ballases with graphitic inclusions etc. Various deposits were characterized by Raman spectroscopy and impurities were measured by S IMS . Low pressure ballas diamond layers have a hardness quite similar to pure diamond. Of particular interest is the fact that cleavage and crack propagation along crystallographic planes can — due to the presence of micro-twins — be expected to be much lower in ballas than in single-crystalline diamonds. Thus, ballas structures are of particular interest for wear applications. Ballas type diamonds containing fine graphite particles could also be of interest for flat panel displays, as the graphite permits high electron emissions.

Analysis of coating fracture and substrate plasticity induced by spherical indentors: diamond and diamond-like carbon layers on steel substrates

Hard coatings as diamond or diamond-like carbon (DLC) layers are widely used as protective coatings on metal substrates, such as steel or hard metal. Failure mechanisms of the substrate/coating composite are studied in this paper through a parametric elastic-plastic finite element analysis, for the common load case of the indentation of spherical bodies into a layered surface considering a wide range of coating thicknesses. For the case of typical DLC layers on tool steel, the first damage of the layer/substrate composite is found to occur by substrate plasticity at or below the interface for most geometries. The fracture mode of the coating changes with varying ratio of layer thickness to indentor radius. If this ratio decreases from large to small values, radial and/or circumferential cracks are initiated first at the surface at the edge of the contact, then preferentially cracking occurrs at the interface on the symmetry axis or at the surface near to the contact edge, and finally again at the interface below the contact area, but without preference for the symmetry axis. Indentation experiments with DLC films on tool steel validate the fracture mechanisms deduced from calculations. For typical DLC layers on tool steel and diamond layers on tool steel, the critical forces for the onset of plastic deformation in the substrate and coating fracture are calculated by means of finite element analysis and analytical approximations of the contact problem. The results can be subsumed in normalised failure maps, from which the optimal coating thickness for the special load case of a spherical indentor can be estimated. Approximate analytical solutions to the results from finite element calculations are derived from simple mechanical analogues. They give more insight into the role of materials and geometry parameters and allow the extrapolation of the results to similar substrate coating systems. For instance, for coating thicknesses equal to or larger than the indentor radius, the load necessary to induce plasticity is shown to vary linearly with the substrate yield strength and the square of the layer thickness. Similarly, the load to initiate coating fracture varies with the square of the layer thickness and was linear with the coating fracture strength if the coating thickness is in the order of the indentor radius.

Residual stress in diamond films: origins and modelling

Abstract The evolution of stress in diamond layers is investigated both experimentally and theoretically in order to develop a comprehensive view of the formation of residual stress. A compressive stress maximum associated with grain coalescence, a decreasing stress with increasing layer thickness and strong stress inhomogeneities at the level of the grain size are observed by in situ macro- and ex situ micro-Raman spectroscopy in diamond films grown on silicon (001) substrates. For most diamond deposits on silicon, neglecting wafer bending for the calculation of thermal stress turns out to be an inappropriate approximation, but even the exact modelling of the thermal stress by means of plate theory and finite element calculations only explains a minor part of the observed stress. Detailed finite element calculations reveal that the average thermal stress, and the thermal stress distribution, are largely modified by temperature gradients during deposition, and by film morphology. Tensile stresses can form due to temperature gradients and surface roughness relaxes an essential part of the thermal stress. The expected average stresses are calculated for common cases. Stress measurements using micro-Raman spectroscopy confirm these predictions obtained from modelling.The microstructure, in particular coherency strains, surface energy effects and disclinations, can contribute substantially to the observed compressive stress maximum at small layer thickness. During grain coalescence, the formation of disclinations can be energetically more favourable than small angle grain boundaries. The related stress fields are estimated to be of the order of several GPa. The formation of large local compressive stresses during grain coalescence is confirmed by micro-Raman spectroscopy. At small layer thicknesses, the evolution of stress is dominated by the microstructure and morphology, whereas at higher thicknesses the thermal stress, including bending effects and temperature inhomogeneities during deposition, is more important.

Optical emission diagnostics and film growth during microwave-plasma-assisted diamond CVD

High-resolution spectroscopy of atomic and molecular lines in a hydrogen-methane-argon plasma was used for the investigation of the gas temperature and the atomic hydrogen concentration as a function of process parameters (power, pressure, and methane concentration) during microwave-plasma-assisted CVD of oriented diamond films on silicon(100) substrates. Translational temperatures were derived from the Doppler broadening of H2 lines and the Hα line. Rotational temperatures of H2 as calculated from the Q-branch of the Fulcher-α system yield much lower temperatures which can be explained by theoretical considerations. Information about the changes in hydrogen concentration is obtained by actinometry, i.e. by relating the intensity of the Balmer lines to an argon line. The results are relatively independent of the choice of the Balmer and actinometer lines, except when varying the methane concentration. The diamond films show distinct changes in morphology with increasing microwave power or methane concentration. From the growth rate at constant methane content, limits for the activation energy of diamond growth have been derived by using a simple growth law which takes into account the influence of substrate temperature and gas phase activation. Microstructure formation at high temperatures is controlled by the growth competition between {100} and {111} facets, while twinning dominates in the low-temperature range.

The effect of nitrogen on low temperature growth of diamond films

The intentional addition of small amounts of nitrogen to different C/H/O gas systems in microwave plasma-assisted deposition of diamond films at low substrate temperatures has been studied. The effect on growth is qualitatively different for gas mixtures with or without oxygen. Adding nitrogen to C/H mixtures results in a significant change of film morphology, growth rate, defect formation and incorporation of hydrogen. The film quality seriously deteriorates with increasing nitrogen concentration in the gas phase. The influence of nitrogen on gas phase processes has been monitored by optical emission spectroscopy. There is evidence that nitrogen affects growth primarily by surface related mechanisms. By contrast, its effect on growth from CO-rich C/H/O systems is much less pronounced. These films show a constant quality and a lower defect content. The interaction of nitrogen and oxygen in low temperature growth of diamond films has been thoroughly examined for gas mixtures containing comparatively low oxygen fractions. The presence of oxygen effectively counteracts the deleterious effect of nitrogen on the formation of defects. Elastic recoil detection has shown, however, that the incorporation of nitrogen into the film always increases when its gas phase concentration is raised, no matter which gas system is chosen.

Microstructure evolution and non-diamond carbon incorporation in CVD diamond thin films grown at low substrate temperatures

Abstract We investigated the development of the microstructure and the incorporation of non-diamond carbon close to the low temperature border of the CVD diamond domain. Thin diamond films were deposited at low substrate temperatures (560°C–275°C) by microwave plasma-assisted CVD on silicon, varying only the substrate temperature. At elevated temperatures (560°C–430°C) the film mainly consists of nearly defect free near 112 oriented grains with smooth 111 facets, exhibiting steps and risers at the surface. Decreasing the substrate temperature an apparently sharp transition occurs, below which the film quality undergoes a rapid deterioration as evidenced by Raman spectroscopy, while crystalline faceted grains with a size of several microns and a growth texture of 〈100〉 remain. However, X-ray diffraction reveals a strongly decreasing crystal size (from about 1 μm to 10 nm) which can be attributed to an increased twin density within the macroscopic grains. High resolution transmission electron microscopy reveals that these twins consist of small twin lamellae with a spacing of only several atomic planes. Transmission electron microscopy of near surface areas evidences re-entrant corners at the grain surfaces formed by twin lamellae and the presence of steps and risers. Non-diamond carbon was detected in the form of amorphous inclusions at incoherent twin boundaries and probably at higher order twin boundaries. The observations will be discussed by means of two different competing nucleation mechanisms: above the low temperature limit the grains grow by lateral ledge motion and preferential nucleation at re-entrant corners. Approaching the low temperature limit, two-dimensional nucleation at growth facets becomes an alternate nucleation mechanism, which introduces a high density of microtwins. If two-dimensional nuclei grow together, non-diamond carbon is incorporated during growth at this interface.

Evolution of the density of graphite-like defects during CVD diamond growth

Abstract In order to quantitatively measure the quality of CVD diamond by Raman spectroscopy the spectra of diamond films were decomposed into Raman peaks and luminescence bands. The evolution of the diamond and “G-band” Raman peaks were studied as a function of film thickness for two different growth morphologies. A normalization procedure was used to eliminate the effects on signal intensities of varying probe size, light absorption and scattering. Electron spin resonance was applied to scale normalized peak intensities in terms of defect densities. The grain structure of the thin films strongly affects the normalized G-band intensity. It is demonstrated that graphite-like defects are concentrated in grain boundaries.

Defects in chemically vapour-deposited diamond films studied by electron spin resonance and Raman spectroscopy

Abstract Diamond films were deposited by microwave-assisted chemical vapour deposition on silicon substrates using methane as a hydrocarbon precursor. The films were investigated by scanning electron spectroscopy, Raman spectroscopy, electron spin resonance (ESR) and X-ray diffraction. With increasing methane concentration, the diamond crystallites underwent morphological changes which were accompanied by important changes in the Raman diamond line and the ESR signal. Increasing width and decreasing intensity of the Raman diamond line were observed to correlate with increasing spin densities and decreasing grain size.

IMPURITY AND DEFECT INCORPORATION IN DIAMOND FILMS DEPOSITED AT LOW SUBSTRATE TEMPERATURES

Abstract The quality of CVD diamond films degrades severely with decreasing substrate temperatures. In this report, the impurity and defect incorporation in diamond films deposited from a carbon-hydrogen-oxygen gas system at substrate temperatures between 560 and 345 C has been investigated using elastic recoil detection (ERD), FTIR and micro-Raman spectroscopy. In approaching the low temperature limit which coincides with the formation of cauliflower morphologies, the hydrogen incorporation rises steeply. Hydrogen contents beyond 1 at.% have been measured, roughly 20 times higher than in the upper temperature range. By contrast, there was a much smaller rate of rise in the concentration of nitrogen and oxygen, despite a marked change in the microstructure of the deposited films. At the lowest substrate temperatures, the absolute hydrogen content measured by ERD increases more steeply than those measured by FTIR spectroscopy, which refers to C-H stretch vibrations only. There is evidence that hydrogen is incorporated also in the bulk rather than being concentrated at grain boundaries as at higher temperatures. This conclusion is supported by micro-Raman spectroscopy exhibiting significant peak broadening in the low temperature region.

A new interpretation of bulge test measurements using numerical simulation

Analysis of the deflection of a circular membrane under differential pressure (bulge test) is a well-known method of determining the elastic properties of thin films. However, analytical models always suffer from simplifying hypotheses. In this study we present a new approach, based on numerical modeling, to interpret pressure-deflection curves. By adjusting Young’s modulus and Poisson’s ratio in the simulation program, it is possible to reproduce the experimental curves faithfully. The method was successfully tested with two different materials (silicon and aluminium) with known elastic properties and was then used to determine biaxial Young’s moduli of CVD diamond thin films for three different microstructures. The values of E varied from 565 to 620 GPa (assuming a Poisson ratio of 0.1). Grain boundaries are thought to be responsible for the relatively low values of Young’s moduli. Uncertainties in E are relatively large (lo%-15%) because the method is highly sensitive to experimental parameters such as thickness or membrane diameter and to the initial residual stress state which is known only approximately.