Free-standing diamond wafers deposited by multi-cathode,direct-current,plasma-assisted chemical vapor deposition

Free-standing diamond wafers, 100 mm in diameter, have been deposited by the multi-cathode (seven-cathode) direct-current (DC) plasma-assisted chemical vapor deposition (PACVD) method. The input power was 17.5 kW and the pressure was 100 torr. The methane concentration in hydrogen was between 3.5% and 8% at a constant flow rate of 150 sccm. Intrinsic tensile stress was controlled by introducing thermal compressive stress with step-down control of the deposition temperature during diamond deposition. A higher growth rate of 10 μm h⁻¹ was obtained by raising the methane concentration to 8%, and the deposited diamond wafer showed good thermal conductivity of 12–14 W cm⁻¹ K⁻¹. Crack-free, homogeneous and flat diamond wafers with 100 mm diameter were obtainable.     

Adhesion improvement of diamond coatings on cemented carbide with high cobalt content using PVD interlayer

At present, diamond coating is usually deposited on cemented carbide (WC-Co) tool with low Co content (Co ≤ 6 wt.%). It is more difficult to deposit diamond coating on WC-Co with high Co content because of the strong catalytic effect of Co. However, WC-Co tools with high Co content (Co ≥ 6 wt.%) are more widely used in difficult-to-cut materials machining because of their higher strength and better ductility. In this paper, the research was carried out on the adhesion performance of diamond coating on WC-Co (Co 10 wt.%). The deposition of diamond coating was conducted in hot filament chemical vapor deposition (HFCVD) system with the presence of the strong carbon-forming metallic interlayer (Nb, Cr or Ta), which was prepared using physical vapor deposition (PVD) on WC-Co substrate after chemical etching through a two-step process (Murakami solution and Caro's acid), which is a general way to treat the WC-Co substrate before growth of diamond coating. The results showed that the diamond films grown on the above treated WC-Co substrate have higher nucleation density, purity and adhesion strength than those on WC-Co substrates pretreated only using PVD interlayer or chemical etching. The PVD interlayer restrains the diffusion of Co as a result of high substrate temperature during the diamond film deposition, and consequently prevents the formation of the loosened layer induced by the removal of Co binder phase in the WC-Co substrate. The results also indicated that Nb interlayer leads to the most adhesion improvement of diamond films on the WC-Co inserts among the Nb, Ta and Cr interlayers.     

Extreme insulating ultrathin diamond films for SOD applications: From coalescence modelling to synthesis

The synthesis of diamond films with extreme insulating properties is of great interest for most diamond film applications in nanoelectronics. SOD (Silicon-On-Diamond) is a promising alternative to standard SOI (Silicon-On-Insulator) because of the high heat-spreading capability of diamond material. Current Fully Depleted MOS processing technologies require a thickness of the dielectric buried layer of 150 nm. Synthesis of polycrystalline diamond films is already well documented. Nonetheless, the difficulties here are to keep their high thermal conductivity and their high electrical resistivity in spite of the reduction of the diamond layer thickness. This study aims at the fine control of both the nucleation density and the growth process to enable the fabrication of optimized fully covered diamond films as thin as possible.A mathematical model describing the coalescence was used to determine the surface coverage of the diamond film according to the linear growth of the diamond nanocrystals for different nucleation densities. The model gives information on the nucleation density needed to obtain a covering diamond film within ultrathin diamond layer thickness. To corroborate the coalescence model, diamond layers with different surface coverages were characterized. Our work led to ultrathin diamond layers (thickness below 140 nm) exhibiting electrical resistivities above 2 × 10¹³ Ω cm.     

Diamond coating on hard metals by traveling methods in a plasma-assisted chemical vapor deposition method above the liquid

In this work, two approaches were developed to extend the coating area of diamond by continuous deposition in a plasma-assisted chemical vapor deposition (CVD) method above the liquid. The techniques were based on the methods previously developed by our research group and the characteristic was to use dc (direct current) plasma generated between the liquid surface and the metal electrode. In the first approach, a tungsten rod was rotated in a chamber at reduced pressure so that a diamond film was formed as a ‘belt’ in 6 mm width around the side of the rod. The deposited diamond was polycrystalline with a grain size of 1–3 μm. The film thickness increased almost linearly with deposition time, whereas the grain size was almost constant against the deposition time. The second approach was for a plate substrate. A tungsten plate was hung with an iron wire and the plasma was horizontally generated between the liquid and plate surfaces. When the W plate was vertically slid down slowly, a diamond film was continuously deposited on the surface. The deposited film was covered with a soot-like carbon layer on the top and the post-treatment with H₂O/N₂ gas at 600 °C was effective in removing it. The continuous deposition successfully demonstrated the expansion of the deposition area with the novel plasma CVD method above the liquid.     

Fretting wear and electrochemical corrosion of well-adhered CVD diamond films deposited on steel substrates with a WC–Co interlayer

Diamond films have been grown on carbon steel substrates by hot-filament chemical vapour deposition methods. A Co-containing tungsten-carbide (WC–Co) coating prepared by high velocity oxy-fuel spraying was used as an intermediate layer on the steel substrates to minimize the early formation of graphite (and thus growth of low quality diamond films) and to enhance the diamond film adhesion. The effects of the WC–Co interlayer on nucleation, quality, adhesion, tribological behaviour and electrochemical corrosion of the diamond film were investigated. The diamond films exhibit excellent adhesion under Rockwell indentation testing (1500 N load) and when subjected to high-speed, high-load, long-time reciprocating dry sliding ball-on-flat wear tests against a Si₃N₄ counterface in ambient air (500 rpm, 200 N, 300,000 cycles). A WC–Co interlayer with appropriate chemical pretreatment is shown to play an important role in improving the nucleation, quality and adhesion of the diamond film, relative to that shown by substrates without such pretreatment.     

Electrochemical synthesis and electronic properties of polypyrrole on intrinsic diamond

Thin film of polypyrrole is electrochemically deposited on hydrogen terminated surface of intrinsic diamond exhibiting surface conductivity (5 × 10^? 5 S/□). Based on a high bonding strength (40 nN) estimated by AFM-scratching experiments and local changes of surface work function detected by Kelvin force microscopy we suggest that the polypyrrole is covalently attached to the diamond surface via carbon–carbon bond while replacing the hydrogen termination. Electronic measurements show loss of surface conductivity of diamond after the polypyrrole deposition. These results are discussed in terms of electronic and electrochemical properties of polypyrrole–diamond system also in the perspective of its potential device applications.     

A graphite nanoplatelet/epoxy composite with high dielectric constant and high thermal conductivity

A simple strategy for the preparation of composites with high dielectric constant and thermal conductivity was developed through a typical interface design. Graphite nanoplatelets (GNPs) with a thickness of 20–50 nm are fabricated and homogeneously dispersed in the epoxy matrix. A high dielectric constant of more than 230 and a high thermal conductivity of 0.54 W/mK (a 157% increase over that of pure epoxy) could be obtained for the composites with a lower filler content of 1.892 vol.%. The dielectric constant still remains at more than 100 even in the frequency range of 10⁵–10⁶ Hz. When loaded at 2.703 vol.%, GNP/epoxy composites have a dielectric constant higher than 140 in the frequency range of 10²–10⁴ Hz and a high thermal conductivity of 0.72 W/mK, which is a 240% increase over that of pure epoxy. The high dielectric constant and low loss tangent are observed in the composite with the GNPs content of 0.949 vol.% around 10⁴ Hz. It is believed that high aspect ratio of GNPs and oxygen functional groups on their basal planes are critical issues of the constitution of a special interface region between the GNPs and epoxy matrix and the high performance of the composites.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Thermal conductivity of individual carbon nanofibers

In the present paper, we present results of thermal conductivity measurements in commercially-available, chemical vapor deposition grown, heat-treated and non-heat-treated individual carbon nanofibers (CNFs). The thermal conductivity measurements are made using the T-type probe experimental configuration using a Wollaston wire probe inside a high resolution scanning electron microscope. The results show a significant increase in the thermal conductivity of CNFs that are annealed at 2800 °C for 20 h when compared with the non-heat-treated CNF samples. When adjusted for thermal contact resistance, the highest measured thermal conductivity is 449 ± 39 W/m-K. The average thermal conductivity of the heat-treated samples is 163 W/m-K, while the average thermal conductivity of the non-heat-treated samples is 4.6 W/m-K. The results demonstrate the importance of the quality of the CNFs, in particular their heat treatment (high temperature annealing), in controlling their thermal conductivity for thermal management applications.     

Electron beam physical vapour deposition and mechanical properties of c-ZrO2-ZTA-coatings on alloy 617 substrates

Multilayered zirconia toughened alumina (ZTA) and c-zirconia coatings were prepared using electron beam physical vapour deposition (EB-PVD). Characterizations of the morphology and chemical composition of the deposited coatings were performed using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). Scratch resistance, nano-indentation and bending strength were used for the evaluation of the mechanical properties. X-ray diffraction of the top ceramic TBC surface showed that it consists entirely of cubic ZrO₂ phase. The energy-dispersive X-ray spectroscopy analysis (EDS) showed that α-Al₂O₃ is the only oxide phase present at the interface, while SEM indicated the presence of columnar c-ZrO₂ as the only phase of the top coat. Delamination over a large region was observed in the case of double layer (ZTA) coating. In contrast, the multilayered (ZTA1 + ZTA2 + c-Z) coating showed neither delamination nor cracking. The hardness and scratch measurements showed that the top coat c-ZrO₂ layer is harder than the ZTA layers. The thermal conductivity of the multilayer coatings was estimated using the theoretical density and thermal conductivity values of zirconia toughened alumina (ZTA) and cubic-zirconia (c-ZrO₂) together with their experimentally measured data.     

Enhanced thermal conductivity and stability of diamond/aluminum composite by introduction of carbide interface layer

W-coated diamond/aluminum composites were manufactured by a pressure infiltration method. A continuous and homogeneous carbide coating was formed on the surface of diamond particles, and the selective bonding between the aluminum matrix and different diamond faces was no longer observed. The obtained composites exhibited thermal conductivity as high as 476 W⋅m^− 1⋅ K^− 1. It was attributed to the carbide layer which increased the amount of reactive interfacial bonding and improved the mean interfacial thermal conductance. In addition, compared with the W coated diamond/aluminum composites, the thermal conductivities of uncoated ones were seriously declined by immersing the composites in moisture circumstances. SEM and XRD results indicated that the stabilities of composites thermal behaviors were closely related to the interface of composites. The W carbide layer in the interface region played a critical role in improving the stability of the composite exposure to moist environments.     

Growth of diamond films with high surface smoothness

Diamond films with highly smooth backside surface have been deposited by positively biasing the substrate during diamond growth in a hot-filament chemical vapor deposition (HFCVD) system. By bonding the diamond film on the glass and wet etching to remove silicon, the highly smooth diamond surface can be exposed and used directly for the fabrication of diamond devices.Silicon substrate was first treated by diamond powder of 625 nm in an ultrasonic bath. By positively biasing the substrate, electron bombardment during diamond growth increases the nucleation density from 10⁸ ∼ 10⁹ cm^− 2 to 4 × 10¹¹ cm^− 2. The surface smoothness on the backside of diamond film has thus been improved significantly, inducing root-mean-square roughness of 5 nm. Owing to the extremely high surface smoothness and the high crystalline quality on the backside of diamond film and the high diamond growth rate, the backside surface of the diamond film grown under electron bombardment is particularly suitable for device fabrication.     

Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method

Alumina has high heat resistance and corrosion resistance compared to other ceramics such as silica or mullite. However, for its application to refractory bricks, its high thermal conductivity must be reduced. To reduce this thermal conductivity by increasing the porosity, a GS (gelation of slurry) method that can produce high porosity solid foam was applied here to produce the alumina refractory brick. This method was successfully applied to produce alumina foam with high porosity and thermal conductivity of the foam is evaluated. At room temperature, the thermal conductivity was about 0.12 W/mK when the foam density was 0.1 g/cm³. At elevated temperature above 783 K, thermal conductivity of the foam was strongly affected by heat radiation and increased with increasing temperature, in contrast to the thermal conductivity of alumina itself, which decreased with increasing temperature. The alumina foams developed here achieved sufficient thermal insulating properties for use in refractory bricks.     

High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD

Free-standing diamond films with 1.68 mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness thermal conductivity (κ_⊥) together with the in-plane (κ_//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ_⊥ and κ_// of free-standing diamond films are up to 1916 and 1739 Wm^− 1 K^− 1 at room temperature, respectively, showing small anisotropy (9%), and following the relationship κ ~ T^− n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500 K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed.     

Effect of total reaction pressure on electrical properties of boron doped homoepitaxial (100) diamond films formed by microwave plasma-assisted chemical vapor deposition using trimethylboron

Boron was doped into diamond films which were synthesized homoepitaxially on polished (100) diamond substrates by means of microwave plasma-assisted chemical vapor deposition (MPCVD) using trimethylboron as the dopant at a constant substrate temperature of 1073 K. The morphologies and electrical properties of the synthesized diamond films were dependent on the total reaction pressure. A maximum Hall mobility, 760 cm² V⁻¹ s⁻¹, was obtained for the film synthesized at 10.7 kPa. The values of Hall mobility were comparable with those obtained for B₂H₆-doped films at corresponding hole concentrations.     

Fatigue strength of diamond coating-substrate interface assessed by inclined impact tests at ambient and elevated temperatures

The effect of two different treatments of cemented carbide substrates, prior to the deposition of a nanocrystalline diamond (NCD) coating, on the film interface fatigue strength was investigated at ambient and elevated temperatures. The first substrate treatment of the cemented carbide substrate was a selective chemical Co-etching and the second one the deposition of a Cr-adhesive layer. Inclined impact tests at 25 °C and 300 °C were performed on the NCD coated specimens. The related imprints were evaluated by confocal microscopy measurements and EDX micro-analyses. The thermal residual stresses developed in the film structure at various temperatures were estimated by Finite Element Method (FEM) calculations. A fatigue damage in the NCD coating interface region was induced by the repetitive impacts. After this damage, the compressive residual stresses in the NCD film are released leading to its lifting from the substrate (bulge formation) and subsequent coating failure. The NCD film-substrate interface fatigue behavior is significantly affected by the test temperature. Based on the attained results at diverse substrate treatments, Woehler-like diagrams were developed for monitoring the fatigue failure of NCD coating interface area at 25 °C and 300 °C. The interfacial fatigue strength worsens as the impact test temperature grows in both examined substrate treatment cases. Moreover, Co-etched substrates compared to coated ones by an adhesive Cr-interlayer possess higher interfacial strength at ambient and elevated temperatures. These phenomena were investigated and related explanations are described in the paper.     

Growth of Al2O3–PTFE composite film at room temperature by aerosol deposition method

To fabricate a ceramic-based substrate for 3-dimensional integration modules with a thick film coating process at room temperature, aerosol deposition method was employed. Al₂O₃ was chosen as a main coating material for the requirements of low permittivity and dielectric loss. Especially to give a functionality of plasticity, composite film with polytetrafluoroethylene (PTFE) was also studied. The effects of PTFE, which was incorporated in the film, were investigated by the microstructural characterization. It was confirmed that Al₂O₃–PTFE film with the grain size of 100–200 nm were grown at room temperature using Al₂O₃–0.5 wt% PTFE mixture powders. Dielectric constant and dielectric loss of Al₂O₃–PTFE film were 4.5 and 0.005 at 1 MHz, respectively.     

High thermal conductive diamond/Ag–Ti composites fabricated by low-cost cold pressing and vacuum liquid sintering techniques

Diamond/Ag–Ti composites were fabricated by a low-cost liquid sintering technique. The Ti addition can effectively improve wetting and promote penetration in composite pores during liquid sintering. The interface structure of the diamond/Ag–Ti composite was identified as Ag/TiC/Ag–Ti/diamond. A high thermal conductivity of 719 W/mK was obtained for the 50 vol.% diamond/Ag-1 at.% Ti composite. Using a bimodal mixture (60 vol.% 150 μm + 10 vol.% 50 μm diamond/Ag-2 at.% Ti composite), a low coefficient of thermal expansion of 6.3 × 10^− 6/K still with high thermal conductivity of 687 W/mK was achieved. These composites have potential applications for thermal management of high integration electronic devices.     

Mechanism of diamond nucleation enhancement by electron emission via hot filament chemical vapor deposition

The mechanism of diamond nucleation enhancement by electron emission in the hot filament chemical vapor deposition process has been investigated by scanning electron microscopy, Raman spectroscopy and infrared (IR) absorption spectroscopy. The maximum value of the nucleation density was found to be 10¹¹ cm⁻² with a −300 V and 250 mA bias. The electron emission from the diamond coating on the electrode excites a plasma, and greatly increases the chemical species, as we have seen by in situ IR absorption. The experimental studies showed that the diamond and chemical species were transported and scattered from the diamond coating on the electrode and through the plasma towards the substrate surface, where they caused enhanced nucleation.     

Analysis of diamond film creation by pulsed optical lattices

The ability of a chirped pulsed optical lattice to create diamond films using a molecular beam of fullerene molecules is numerically investigated. Two cases, high and low density, are considered. In both cases, the molecular beam was found to impact the substrate at velocities between 10 and 14 km/s. The proposed scenario for the diamond coating stems from the generation of high velocity beams of fullerene particles, bombardment of the substrate surface by these beams, successive dissipation of kinetic energy at the surface and drastic increase of pressure and temperature in the interaction region and finally, formation of diamond crystal structure from deposited fullerenes. A possible setup for the film deposition is proposed. It is shown that that such a system could possibly achieve diamond film growth rates in excess of 1.4 mm/s.