Substrate crystal recovery for homoepitaxial diamond synthesis

Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.     

Partially stabilized zirconia substrate for chemical vapor deposition of free-standing diamond films

In this work we investigated the use of partially stabilized zirconia (PSZ) as the substrate for deposition of CVD diamond films. The polycrystalline PSZ substrates were sintered at high temperatures and the results showed that this material has unique properties which are very appropriated for the growth of free-standing diamond films. The diamond nucleation density on PSZ is high, even without seeding, and the CVD diamond film was totally released from the substrate after the deposition process, without cracking. Micro-Raman analysis revealed that the free-standing diamond film had a good crystallinity on both surfaces with practically no stress in the structure. The same PSZ substrate can be reutilized for the deposition of a large number of diamond films. The average growth rate is about 5–6 μm/h in a microwave plasma reactor at 2.5 kW. The deposition process causes the reduction of ZrO₂, producing ZrC. The high mobility of oxygen in the zirconia matrix at high temperature would probably help to etch the interface region between the substrate surface and the diamond film, decreasing the adhesion strength and eliminating some defects in the film structure related to non-diamond carbon phases.     

Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Comparative study of homoepitaxial single crystal diamond growth at continuous and pulsed mode of MPACVD reactor operation

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in pulsed regime of a 2.45 GHz MPACVD reactor operation at pulse repetition rates of 150 and 250 Hz was investigated. The high quality CVD diamond layers were deposited in the H₂-CH₄ gas mixture containing 4% and 8% of methane, gas pressures of 250 and 260 Torr and substrate temperature of 900 °C without any nitrogen addition. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. At pulse repetition rate 150 Hz the high quality single crystal diamond was grown with growth rate of 22 μm/h. The comparison of the single crystal diamond growth rates in CW and pulsed wave regimes of MPACVD reactor operation at microwave power density 200 W/cm³ was made. It was found that at equal power density, the growth rate in pulsed wave regime was higher than in CW regime. Differences in single crystal diamond growth for two sets of experiments (with continuous and pulsed wave regimes) were explained.     

Growth strategy for controlling dislocation densities and crystal morphologies of single crystal diamond by using pyramidal-shape substrates

The growth of millimetre-thick diamond single crystals by plasma assisted CVD is complicated by the formation of unepitaxial defects, particularly at the edges of the crystal. These defects tend to encroach on the top surface hence limiting the maximum thickness to typically a few hundreds of micrometres. Dislocations are another type of defects that are also particularly formed at the edges of the crystal. They thread through the diamond film, strongly affecting its characteristics. The growth on pyramidal-shape substrates having different angles and orientations was carried out in an attempt to solve those issues. It was found that the pyramidal-shape tends to disappear after a certain thickness is grown. The inclined faces of the pyramid not only helped in preserving the crystal morphology over a large thickness but also deviated dislocations towards the edges of the crystal, hence limiting their occurrence at the surface. Using this strategy, millimetre-thick diamond single crystals presenting a reduced dislocation density were successfully grown.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

High quality,large surface area,homoepitaxial MPACVD diamond growth

The use of CVD diamond in electronics has very stringent requirements. For a CVD diamond industry to become viable it is mandatory to obtain very large growth rates (> 5 µm/h), all the while maintaining extremely high purity, a crystalline defect density as low as possible, and large usable surface areas. At the same time, one must keep the stress level within the growing crystal below acceptable limits to avoid crack formation and preserve the crystal structural integrity. These imperatives imply to work to improve both the plasma deposition process and the CVD diamond crystal growth. In this paper, we propose a three-pronged approach: (i) We use detailed plasma models to establish the influence of process parameters (in particular deposition pressure) on plasma chemistry in order to optimize film growth rate and diamond quality; (ii) We emphasize the need for careful substrate pre-treatment and selection (including choosing a single-sector face) to minimize defects in the growing films; (iii) We employ a 3D geometrical model to predict the crystal shape under given growth conditions, and exploit this knowledge to devise a growth strategy maximizing the usable film surface area while minimizing stresses inside the films.     

The effect of substrate temperature and growth rate on the doping efficiency of single crystal boron doped diamond

The substrate growth temperature dependence of the plasma gas-phase to solid-phase doping efficiency in single crystal, boron doped diamond (BDD) deposition is investigated. Single crystal diamond (SCD) is grown by microwave plasma assisted chemical vapor deposition (MPACVD) on high pressure, high temperature (HPHT) type Ib substrates. Samples are grown at substrate temperatures of 850–950 °C for each of five doping concentration levels, to determine the effect of the growth temperature on the doping efficiency and defect morphology. The substrate temperature during growth is shown to have a significant effect on the grown sample defect morphology, and a temperature dependence of the doping efficiency is also shown. The effect of the growth rate on the doping efficiency is discussed, and the ratio of the boron concentration in the gas phase to the flux of carbon incorporated into the solid diamond phase is shown to be a more predictive measure of the resulting boron concentration than the gas phase boron to carbon ratio that is more commonly reported.     

Antibacterial properties of polycrystalline diamond films

Electronic and mechanical properties, and their biocompatibility, make diamond-based materials promising biomedical applications. The cost required to produce high quality single crystalline diamond films is still a hurdle to prevent them from commercial applications, but the emergence of polycrystalline diamond (PCD) films grown by chemical vapour deposition (CVD) method has provided an affordable strategy. PCD films grown on silicon wafer have been used throughout and were fully characterised by SEM, XPS, Raman spectroscopy and FTIR. The samples contain nearly pure carbon, with impurities originated from the CVD growth and the silicon etching process. Raman spectroscopy revealed it contained tetrahedral amorphous carbon with small tensile stress. The sp² carbon content, comprised between 16.1 and 18.8%, is attributed to the diamond grain boundaries and iron-catalysed graphitisation. Antibacterial properties of PCD films were performed with two model bacteria, i.e. Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) using direct contact and shaking flask methods. The samples showed strong bacteriostatic properties against S. aureus and E. coli with the direct contact method and no influence on planktonic bacterial growth. These results suggest that the bacteriostatic mechanism of PCD films is linked to their surface functional groups (carbon radicals and –NH₂ and –COOH groups) and that no diffusible molecules or components were involved.     

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.     

The influence of recess depth and crystallographic orientation of seed sides on homoepitaxial growth of CVD single crystal diamonds

Microwave plasma chemical vapor deposition (MPCVD) has gained increasing attention as a feasible and effective way to produce large, high-quality, single-crystal diamonds. However, the growth of polycrystalline diamond on the periphery of the seed crystal and the cracking generated by the internal stress during the growing process lead to significant decline of the quality and integrity of the CVD diamond, thus increasing the difficulty of synthesizing large diamond layers. Although optimized growth parameters and refined substrate holders have been employed by some researchers to improve the periphery quality of CVD diamond layers, more research needs to be done in this area. In this study, we used a specially designed substrate holder with a circular recess, in which the seed crystal was placed. By designing substrate holders with different recess depths and a seed crystal with different side-surface crystallographic orientations, we aimed to determine the influence of the recess depths and the crystallographic orientation of seed sides on the growth quality according to polarizing microscope, laser Raman spectroscopy, UV fluorescence imaging, and photoluminescence (PL) mapping measurements. The results demonstrate that as the recess depth increases, the amount of polycrystalline diamonds and the internal stress on the periphery are controlled effectively. The crystalline quality is improved, and the growth rate is decreased. In addition, compared to the periphery with {100} seed sides, the periphery with {110} seed sides displays better crystalline quality, lower internal stress, and fewer polycrystalline diamonds after growth, which is probably due to the intrinsic nature of the growth steps propagating on the {100} diamond surface and the effect of nitrogen atoms on the growing process in the diamond lattice.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Control of surface and bulk crystalline quality in single crystal diamond grown by chemical vapour deposition

In order to improve the performance of existing technologies based on single crystal diamond grown by chemical vapour deposition (CVD), and to open up new technologies in fields such as quantum computing or solid state and semiconductor disc lasers, control over surface and bulk crystalline quality is of great importance. Inductively coupled plasma (ICP) etching using an Ar/Cl gas mixture is demonstrated to remove sub-surface damage of mechanically processed surfaces, whilst maintaining macroscopic planarity and low roughness on a microscopic scale. Dislocations in high quality single crystal CVD diamond are shown to be reduced by using substrates with a combination of low surface damage and low densities of extended defects. Substrates engineered such that only a minority of defects intersect the epitaxial surface are also shown to lead to a reduction in dislocation density. Anisotropy in the birefringence of single crystal CVD diamond due to the preferential direction of dislocation propagation is reported. Ultra low birefringence plates (< 10^? 5) are now available for intra-cavity heat spreaders in solid state disc lasers, and the application is no longer limited by depolarisation losses. Birefringence of less than 5 × 10^? 7 along a direction perpendicular to the CVD growth direction has been demonstrated in exceptionally high quality samples.     

Experimental study of nucleation and quality of CVD diamond adopting two-step deposition approach using MPECVD

Effects of the deposition conditions on quality and nucleation density of CVD diamond were investigated using a microwave plasma enhanced chemical vapor deposition (MPECVD) method with methane-hydrogen gas mixtures. Diamond films were deposited at pressures of 665–4000 Pa, temperatures of 660–950 °C, and methane concentrations of 0.5–5 vol.%. Deposited diamond films were characterized by scanning electron microscopy, field emission scanning electron microscopy, micro-Raman spectroscopy, and X-ray diffraction. Diamond quality and nucleation density significantly affected by the deposition pressure, substrate temperature, and methane concentration. The findings of this work were discussed in terms of the effects of deposition conditions on the plasma composition. A two-step deposition approach was applied to improve nucleation density and quality of CVD diamond films. Polycrystalline diamond films were grown using the two-step deposition process changing a combination of parameters in the two steps. Growth and quality of the deposited diamond films were improved altering the deposition pressure and substrate temperature in the two steps.     

Ultraviolet-irradiated precision polishing of diamond and its related materials

In this study, a novel ultraprecision polishing process for single-crystal diamond substrates was developed utilizing ultraviolet (UV) irradiation. This polishing is basically a mechanochemical polishing (MCP) process combined with a UV-induced photochemical reaction. Carbon atoms on the topmost surface of diamond are oxidized by active species such as hydroxyl radicals (OH radicals) and oxygen radicals at localized high temperature and finally removed as CO and CO₂. This polishing process was applied to diamond substrates, chemical vapor deposition (CVD) diamond-coated films and polycrystalline diamond (PCD). The results showed that the surface roughness of the entire substrate reached 0.2 nmRa within 1 – 3 h at a comparatively high removal rate. The characteristics and removal mechanism of UV polishing were also discussed.     

Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

Crystallographic anisotropy of growth and etch rates of CVD diamond

The investigation of orientation dependent crystal growth and etch processes can provide deep insights into the underlying mechanisms and thus helps to validate theoretical models. Here, we report on homoepitaxial diamond growth and oxygen etch experiments on polished, polycrystalline CVD diamond wafers by use of electron backscatter diffraction (EBSD) and white-light interferometry (WLI). Atomic force microscopy (AFM) was applied to provide additional atomic scale surface morphology information. The main advantage of using polycrystalline diamond substrates with almost random grain orientation is that it allows determining the orientation dependent growth (etch) rate for different orientations within one experiment. Specifically, we studied the effect of methane concentration on the diamond growth rate, using a microwave plasma CVD process. At 1% methane concentration a maximum of the growth rate near <100> and a minimum near <111> is detected. Increasing the methane concentration up to 5% shifts the maximum towards <110> while the minimum stays at <111>. Etch rate measurements in a microwave powered oxygen plasma reveal a pronounced maximum at <111>. We also made a first attempt to interpret our experimental data in terms of local micro-faceting of high-indexed planes.     

A study of the crystalline growth of highly boron-doped CVD diamond: preparation of graded-morphology diamond thin films

An investigation was made of graded-morphology diamond thin films deposited on Si substrates by use of the microwave plasma chemical vapor deposition (CVD) technique. The preparation of graded diamond thin films not only offers new perspectives into functional materials, but it also gives clues to the growth mechanism of CVD diamond. Although it is clear that the substrate temperature and the deposition time affect the grain size in particular, the difference depending on the boron concentration in the growing process was studied in the present work. As a result, in highly boron-doped (10⁴ ppm) diamond films, the Raman peak which was ascribed to the boron-doping was not observed at the very initial growth stage. However, the crystal growth process was almost irrelevant for the boron-doping quantity. Furthermore, we showed the morphology dependence of electrochemical properties as one example of the excellent functions of the graded-morphology highly boron-doped diamond film.     

Deposition of diamond films on sapphire: studies of interfacial properties and patterning techniques

Polycrystalline diamond films were grown on single crystal sapphire substrates using hot filament chemical vapour deposition (CVD). Problems with poor adhesion, stress and film cracking became severe for deposited areas greater than about (100 μm)². Scanning electron microscopy analysis showed the films to be failing both at the interface and in the diamond layer itself. Transmission electron microscopy cross-sections of the interface showed that the interface was clean and free from non-diamond carbon impurities. Spallation problems in the diamond film could be reduced by introducing a barrier layer of epitaxial silicon grown on the sapphire prior to the diamond CVD step. Patterned silicon-on-sapphire wafers were then used as substrates for CVD of diamond in order to define features of linewidth more than 10 μm in the diamond films. Two methods were used: selective nucleation and lift off.