Filament metal contamination and Raman spectra of hot filament chemical vapor deposited diamond films

Chemical vapor deposited diamond films grown in a hot filament reactor using three filament metals (tungsten, tantalum, and rhenium) have been analyzed for their metal impurity content. This is the first report wherein all three common CVD filament metals have been examined and a single technique used for diamond film analysis. Tungsten carbide filaments yielded the lowest impurity level (few ppm by mass), whereas rhenium yielded the highest (parts per thousand). The effects of filament temperature and addition of ammonia or oxygen to the reactant gas mixture were examined. A correlation was observed between the metal content of the product films and their quality, as judged using Raman spectroscopy. Films with the highest metal content yielded Raman spectra showing the lowest fluorescence background, the smallest sp² carbon contribution, and the narrowest 1332 cm⁻¹ diamond line.     

Raman scattering characterization of CVD graphite films

Raman spectroscopic study has been performed for thin graphite films grown on nickel substrates by chemical vapor deposition from a mixture of hydrogen and methane activated by a direct current discharge. Depending on the growth conditions, the CVD films are composed of graphene layers parallel to the substrate surface or of plate-like crystallites with the predominant orientation of their graphene layers perpendicular to the substrate surface. A comparison of the Raman spectra for the CVD films and for the highly oriented pyrolytic graphite has been performed. The mechanisms governing the Raman scattering process in the films are discussed. An important role of a double resonance mechanism in the Raman spectra of these graphite-based materials has been revealed. The Raman band positions and intensities and their dependence on excitation wavelength confirm a high degree of the structural order in the CVD graphite films.     

785 nm Raman spectroscopy of CVD diamond films

Raman spectroscopy is a powerful technique often used to study CVD diamond films, however, very little work has been reported for the Raman study of CVD diamond films using near-infrared (785 nm) excitation. Here, we report that when using 785 nm excitation with 1 µm spot size, the Raman spectra from thin polycrystalline diamond films exhibit a multitude of peaks (over 30) ranging from 400–3000 cm^− 1. These features are too sharp to be photoluminescence, and are a function of film thickness. For films > 30 µm thick, freestanding films, and for films grown in diamond substrates the Raman peaks disappear. This suggests that the laser is probing the vibrations of molecular units at the grain boundaries of the disordered crystallites present at the interface between the diamond and substrate.     

Control of abnormal grain inclusions in the nanocrystalline diamond film deposited by hot filament CVD

Formation of abnormal grain inclusions in nanocrystalline diamond films deposited by hot filament CVD (HFCVD) was investigated. The phenomenon was attributed to two different origins: an intrinsic and an extrinsic one. The inclusions due to the intrinsic origin could be either avoided or weakened by controlling chamber pressure, CH₄/N₂ concentrations in H₂, and by positive substrate bias. The extrinsic origin for the abnormal grains was found to be the contamination from the alumina insulation tubes for the thermocouple placed near the substrate, which were degraded by the extended exposure to the high temperature and strongly reducing atmosphere.     

Deep ultra-violet Raman imaging of CVD boron-doped and non-doped diamond films

Deep UV (244 nm) Raman images were recorded on CVD diamond films. The different images recorded on boron-doped polycrystalline film reveal large differences on the boron level within individual crystallites. Nevertheless, inspite of an enhanced intensity of the diamond line, the UV Raman spectra are less informative than the visible Raman spectra due to the weak response of the boron-related peaks. The images recorded on an un-doped homoepitaxial diamond film reveal a defective domain exhibiting in the same areas the sp³ CH_x stretching vibration component and the first and second-order optical phonons of the graphite-like phonon bands of sp² carbons embedded in a compressive zone of polycrystalline diamond.     

Doping of vanadium to nanocrystalline diamond films by hot filament chemical vapor deposition

Doping an impure element with a larger atomic volume into crystalline structure of buck crystals is normally blocked because the rigid crystalline structure could not tolerate a larger distortion. However, this difficulty may be weakened for nanocrystalline structures. Diamonds, as well as many semiconductors, have a difficulty in effective doping. Theoretical calculations carried out by DFT indicate that vanadium (V) is a dopant element for the n-type diamond semiconductor, and their several donor state levels are distributed between the conduction band and middle bandgap position in the V-doped band structure of diamond. Experimental investigation of doping vanadium into nanocrystalline diamond films (NDFs) was first attempted by hot filament chemical vapor deposition technique. Acetone/H₂ gas mixtures and vanadium oxytripropoxide (VO(OCH₂CH₂CH₃)₃) solutions of acetone with V and C elemental ratios of 1:5,000, 1:2,000, and 1:1,000 were used as carbon and vanadium sources, respectively. The resistivity of the V-doped NDFs decreased two orders with the increasing V/C ratios.     

Synthesis of nanocrystalline diamond films by DC plasma-assisted argon-rich hot filament chemical vapor deposition

Continuous nanocrystalline diamond (NCD) films were grown in an argon-rich gas atmosphere with relatively high growth rates by sustaining a low power (5 W) DC plasma in a hot filament chemical vapor deposition system (HFCVD). The parameter window for the synthesis of NCD films was studied as a function of argon, methane and hydrogen concentrations, as well as substrate temperature and DC bias. The results are consistent with reports indicating that the DC plasma induces re-nucleation by ion bombardment during the initial growth step and helps to maintain the atomic H and hydrocarbon species near the growing surface. It was found that DC plasma-assisted HFCVD enables high NCD growth rates and expands the parameter window, rendering it unnecessary to heat the filament above 2800 K.     

High rate of diamond deposition through graphite etching in a hot filament CVD reactor

Although a hot filament diamond CVD reactor has many advantages over other processes, the relatively low growth rate of the films has been a crucial drawback. We developed a new hot filament process that uses no hydrocarbon gas for diamond deposition. The graphite plate was placed below the silicon substrate and only hydrogen was supplied during the process. We could achieve the growth rate of 9 μm/h, which is approximately 9 times higher than that of conventional hot filament CVD using a gas mixture of methane and hydrogen. In spite of the high growth rate, the quality of diamond films was not degraded. Besides, the diamond films consisted of small crystallites with a smooth surface while the conventional diamond films of the same thickness tend to have a columnar structure with a rough surface.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

Diamond films with preferred <110> texture by hot filament CVD at low pressure

The effects of gas pressure on the textured growth of diamond films were investigated in a hot filament chemical vapor deposition (HFCVD) system. Diamond thin film with the growth rate of 1.3 μm/h and with high <110> texture was obtained at 5 Torr when lowering the gas pressure from 40 Torr to 1 Torr. The formation of high density nanocrystalline diamond nuclei elongated along the <110> direction in the nucleation stage and its consequent growth at lower pressure were considered to be responsible for the formation of <110> textured diamond thin film.     

Hot filament CVD diamond coating of TiC sliders

Hot filament chemical vapour deposition (HF CVD) process with low filament temperature (∼ 1650 °C) was utilized for the diamond coating of TiC samples. Porous substrates were fabricated by pulse discharge sintering (PDS) to create more nucleation sites. Nucleation density and morphology of deposited diamond films were studied using scanning electron microscopy (SEM). It was found that the highest growth rate occurs at substrate temperature of 980 °C. Evaluation of the residual stress in deposited films was carried out by Raman spectroscopy. Ball on disk tests were performed with steel as a counterface material. After polishing diamond films demonstrated good sliding performance: friction coefficient of 0.08 and wear rate of 10^− 17 m³/N m.     

Enhanced nucleation of diamond on polycrystalline Ni by d.c. glow discharge in hot filament CVD

If a negative glow discharge is produced around a cathode wire above a polycrystalline Ni substrate, the diamond nucleation rate is greatly enhanced in the hot filament chemical deposition process. The diamond deposition depends on the surface orientation of the Ni grains, and occurs preferentially on faceted surfaces with hill-and-valley structures. The deposited diamond particles show a tendency to orient in one direction.     

机械法抛光加工金刚石膜研究

在平面研磨机上使用金刚石抛光盘对金刚石膜进行了抛光实验。通过观察金刚石膜机械抛光过程中表面形貌的变化，并结合拉曼光谱分析，提出了金刚石抛光盘抛光金刚石膜的抛光机理主要是微切削和压力破碎作用，同时还可能有少量金刚石转变为非晶碳的作用。通过单因素实验研究，发现金刚石盘的粒度对抛光金刚石膜材料去除率的影响最为显著。转速越高，表面粗糙度越小，压力和粒度存在最优值，超过该值后，表面粗糙度并不会随之降低。     

Graphitized filament plasma enhanced CVD deposition of nanocrystalline diamond

A novel approach to the deposition of polycrystalline diamond is presented. The technique is based on the hot filament chemical vapour deposition technique (HFCVD). While it is similar to a high plasma power “bias enhanced growth” HFCVD, it relies on a graphite filament rather than on a metal one. It was found that with an appropriate choice of the growth parameters, 4–9% CH₄ in H₂, filament temperature > 2200 °C, 25 mBar gas pressure, plasma power > 500 W, a long filament lifetime can be achieved, when a simultaneous deposition of graphitic carbon on the hot graphite filament and of nanocrystalline diamond on a substrate facing the filament assembly is realized. In this paper the growth of nanocrystalline diamond films and their characterization (SEM, XRD, AFM) are presented. While the technique is promising for low cost, large area deposition of nanocrystalline diamond films, also the growth of microcrystalline diamond has been observed.     

Electronic properties of CVD polycrystalline diamond films

Electronic properties of polycrystalline diamond films have been analysed using steady-state photoconductivity under UV (λ=190 nm) and red (λ=633 nm) illumination. We look at the effect of several post-deposition treatments: annealing in vacuum at 400°C, successive annealing under air and methane or the reverse, and X-ray exposure. These are shown to affect the photoresponses in different manners. Globally, the mobility–lifetime products of samples are found in the range 10⁻⁷–10⁻⁶ cm² V⁻¹ under UV illumination, and ratios of photoresponses in UV and red are in the range 10³–10⁵. First results obtained using the modulated photocurrent technique are also presented.     

砷化镓上生长金刚石薄膜的研究

以CH4和H2为气源,用微波辅助等离子体装置,在10.0 mm×7.0 mm的砷化镓基底上沉积了CVD金刚石薄膜,用扫描电子显微镜观察沉积效果,拉曼光谱表征沉积质量,分析薄膜附着力与砷化镓材料性能的关系。结果表明,当基体温度为600℃,气压为5 kPa,甲烷浓度为2.0%时,在砷化镓片表面上沉积出了CVD金刚石薄膜,晶粒尺寸均匀,晶形完整、规则,晶界非常清晰。     

High pressure annealing of CVD diamond films

CVD diamond films were annealed from 600 to 1900 °C at 7.7 GPa in a toroidal high pressure (HP) apparatus, always inside the diamond-phase stability region. The annealed films were analyzed by Raman and infrared (IR) spectroscopy and the results showed that the diamond grains remained stable while the non-diamond carbon phases and impurities, responsible for the intricate film structure, changed after processing. For the HP annealing from 600 to 1300 °C, there were no major changes in the Raman spectra of the film, however, the film became easily broken and the IR spectra indicated a high reactivity of carbon with chemical elements from the environment. After annealing at 1500 °C and 7.7 GPa, the formation of diamond-like (DLC) and graphitic structures in-between the diamond grains were observed, while the reaction with the environment elements decreased. For higher temperatures, the DLC and graphitic structures persisted up to 1700 °C and the film incorporated OH in large amounts. The results showed that the non-diamond carbon species are susceptible to the HP annealing, and structural modifications in between the diamond grains are significant for temperatures above 1300 °C at 7.7 GPa.     

Nucleation of diamond over nanotube coated Si substrate using hot filament chemical vapor deposition (CVD) system

We studied the use of carbon nanotubes as a seeding layer for the nucleation of diamond on Si (100) substrate by using a hot filament chemical vapor deposition (HFCVD) system. Prior to deposition, substrates were seeded with multi-wall carbon nanotube (MWCNT) powder which was prepared separately. MWCNTs were used as nucleation precursors. The diamond grains grew essentially over the nanotubes with a higher growth density in comparison with the un-seeded substrates. The scanning electron microscopy (SEM) image of surface morphology shows crystallites of cauliflower shaped grains. The micro Raman spectroscopic results show a sharp peak at 1,332 cm^-1 corresponding to diamond phase. X-ray photoelectron spectroscopic study show the presence of carbon (C_1s) phase. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Characterization of boron-doped micro- and nanocrystalline diamond films deposited by wafer-scale hot filament chemical vapor deposition for MEMS applications

Micro- and nanocrystalline diamond (MCD and NCD) films are deposited on 4-inch silicon substrates by a large-area multi-wafer-scale hot filament chemical vapor deposition (HFCVD) system. The films are in-situ doped by boron. The chemical and crystalline structures are studied by electron probe microanalysis (EPMA), Raman spectroscopy and X-ray diffraction (XRD). The microcrystalline films have a preferred (111) texture, while the nanocrystalline films exhibit (220) texture. Strain gauges and cantilever beam arrays are micro-fabricated by surface micro-machining techniques to characterize the residual strain and strain gradient of the diamond films. Both micro- and nanocrystalline films have small compressive strains of − 0.052% and − 0.040% respectively, with the strain gradient of about 10^− 5 μm^− 1. These values are low enough to enable the realization of many MEMS devices.     

Low temperature growth of highly transparent nanocrystalline diamond films on quartz glass by hot filament chemical vapor deposition

Highly transparent and hard nanocrystalline diamond (NCD) films were prepared on quartz glass by hot filament chemical vapor deposition (HFCVD). The effects of total gas pressure, substrate's temperature, and concentration of CH₄ on the grain size, surface's roughness and hardness, growth rate, as well as the optical properties of NCD films were investigated. The results indicated that with a low total gas pressure and high CH₄ concentration, high frequency of secondary nucleation can be obtained. In addition, low substrate temperature can increase the rate of the hydrogen atom etched sp² graphite carbon in the film, yielding a smooth surface of NCD films and very high sp³ content. Under optimized conditions, the hardness can be enhanced up to 65 Gpa, with 80% maximum transmittance in the visible light region. The aforementioned reaction platform outcomes a 1.2 μm thickness of NCD coating with a low root-mean-square (r.m.s.) surface roughness around 12–13 nm and a high growth rate around 1 μm/h. The influences of the total gas pressure, substrate's temperature, and CH₄ concentration for growing NCD films were also discussed in this paper.