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.     

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.     

A chemical adsorption growth model for hot filament chemical vapor deposition diamond

In this paper, a chemical adsorption model for hot filament chemical vapor deposition (HF-CVD) of diamond films has been proposed based on some recent experimental data. The coverages of H and CH₃ precursors on the growing surface have been calculated according to the equilibrium between the adsorption and desorption of the two precursors at a certain substrate temperature T_s. The result shows that the H coverage decreases markedly with increasing T_s when T_s is over a critical temperature T_c. Below the temperature T_c, it comes close to 1. Thus, the quality deterioration of diamond films deposited at rather high substrate temperatures may be attributed to the poor H coverage on the surface. The value of T_c is determined by H atom concentration n_H in the reactor. When n_H is greater than 3.2×10⁻¹¹ mol cm⁻³, T_c is above 1000 K. The CH₃ coverage shows a maximum within the range of the studied T_s. With the typical CH₃ concentrations, the CH₃ coverage reaches the maximum at T_s∼1100 K. A growth rate formula has been developed on the basis of the temperature dependent CH₃ coverage. The formula shows that the growth rate follows the Arrhenius law at relative low T_s, but it rapidly decreases when T_s is rather high, which is in good agreement with the experimental results.     

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.     

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.     

Biased enhanced growth of nanocrystalline diamond films by microwave plasma chemical vapor deposition

Smooth nanocrystalline diamond thin films with rms surface roughness of ∼17 nm were grown on silicon substrates at 600°C using biased enhanced growth (BEG) in microwave plasma chemical vapor deposition (MPCVD). The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the samples with a combination of Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy and Auger electron spectroscopy. The Raman spectra of the films exhibit an intense band near 1150 cm⁻¹ along with graphitic bands. The former Raman band indicates the presence of nanocrystalline diamond. XRD patterns of the films show broad peaks corresponding to inter-planar spacing of (111) and (220) planes of cubic diamond supporting the Raman results. Auger line shapes closely match with the line shape of diamond suggesting high concentration of sp³ carbon on the surfaces of the films. The growth of dominantly sp³ carbon by BEG in the MPCVD system at the conditions used in the present work can be explained by the subsurface implantation mechanism while considering some additional effects from the high concentration of atomic hydrogen in the system.     

Analysis of diamond nucleation on molybdenum by biased hot filament chemical vapor deposition

The nucleation and initial growth of diamond on molybdenum using biased hot filament chemical vapor deposition were investigated by scanning electron microscopy, Raman spectroscopy and adhesion force tests. The studies showed that the negative biased pre-treatment greatly enhanced the nucleation density and adhesion force of diamond films on molybdenum. The experimental evidence was confirmed that there is large stress near the interface between the diamond and the Mo substrate, which were originated from the disordered graphite phases and molybdenum carbide near the interface. This may play an important role during nucleation stage. However, larger stress can cause the degradation of the adhesion force of diamond films on Mo substrate. However, the adhesion force was enhanced with increasing bias voltage. The theoretical relationship between the adhesion force and the bias voltage is given by theoretical calculation.     

Growth of nanocrystalline diamond films by biased enhanced microwave plasma chemical vapor deposition

Nanocrystalline diamond (NCD) films were grown using biased enhanced growth (BEG) in microwave plasma chemical vapor deposition on mirror polished silicon substrates at temperatures in the range from 400 to 700°C. The films were characterized by Raman spectroscopy, X-ray diffraction (XRD), Auger electron spectroscopy and atomic force microscopy (AFM). Hardness of the films was measured by nano-indentor. Apart from graphitic D and G bands in the films, the Raman spectra exhibit NCD features near 1140 cm⁻¹. The relative intensity of the NCD to graphitic G band in the Raman spectra of the films is negligible in the films grown at 400°C. It increases with temperature and attains a maximum at 600°C following a sharp decrease in the films grown at higher temperatures. XRD results also indicate a maximum concentration of NCD in the film grown at 600°C. Average hardness of the films increases with temperature from ∼5 GPa to ∼40 GPa up to 600°C followed by a decrease (∼24 GPa) in the film grown at 700°C. Substrate temperature seems to play a crucial role in the growth of NCD in BEG processes. An increase in growth temperature may be responsible for evolving bonded hydrogen and increasing mobility of carbon atoms. Both factors help in developing NCD in the films grown at 500 and 600°C with a combination of subplantation mechanism, due to biasing, and a high concentration of H atoms in the gas-phase, typical of CVD diamond process. At 700°C the implanted carbon atoms may be migrating back to the surface resulting in domination of surface processes in the growth, which in turn should result in increase in graphitic content of the films at such a high methane concentration and continuous biasing used in the present study.     

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.     

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH₄–H₂–Ar precursor gas mixture with 1% CH₄, at the chamber pressure and the substrate temperature of 7.5 Torr and 840 °C, respectively. In the H₂-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.     

High compressive stress in nanocrystalline diamond films grown by microwave plasma chemical vapor deposition

Hard and smooth nanocrystalline diamond (NCD) thin films were deposited on mirror polished silicon substrates by biased enhanced growth in a microwave plasma chemical vapor deposition system. The films were characterized by Raman spectroscopy, X-ray diffraction and atomic force microscopy. Stress in the films was calculated by measuring the radius of curvature of the films on substrates and hardness was measured using a Nanoindenter. Stress in the films increases, first, with decreasing methane concentration in the gas phase while keeping biasing voltage constant, and second, with increasing biasing voltage while keeping the methane concentration constant. Observation of enormous stress (∼30 GPa) was possible in the films, which is due to strong adhesion between the films and substrates. To the best of our knowledge, this is the maximum value of stress reported so far in any kind of carbon thin films. It was hypothesized that it is mostly hydrogen content of the films in the methane series and graphitic content of the films in voltage series that are responsible in generating compressive stress in the respective films. The hardness follows almost a reverse trend than stress with the two growth parameters and can be well-defined from the relative concentration of NCD to graphitic content of the films, as estimated from Raman spectroscopy.     

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.     

CVD纳米金刚石涂层工具研究进展

概述了CVD纳米金刚石涂层工具的研究开发现状、存在的主要问题,重点介绍了硬质合金基体表面预处理方法及纳米金刚石生长工艺参数对CVD纳米金刚石涂层工具结构和性能的影响,其中,介绍了硬质合金基体表面预处理方法主要有酸液浸蚀去钴、施加中间过渡层、机械或等离子体处理、负偏压等;蚋米金刚石生长工艺参数则主要从碳源浓度、基体温度、...     

Fracture mechanics of microcrystalline/nanocrystalline composited multilayer chemical vapor deposition self-standing diamond films

Though microwave plasma chemical vapor deposition (MPCVD) diamond films exhibit extraordinary strength, the toughness improvement is still a huge challenge. The catastrophic fracture of the diamond films is undesirable in many applications, especially for the application of fabricating cutting tools. In the present study, adopting the pre-notched and unnotched cantilever bending tests, fracture behaviors of monolayer microcrystalline diamond (m-M), monolayer nanocrystalline diamond (m-N), and microcrystalline/nanocrystalline composited multilayer diamond films were observed and compared. Typical fracture mechanics, including Young's modulus (E), fracture strength (${\sigma }_F$), and fracture toughness (K_IC) of different diamond films were investigated. Effects of diamond crystal structure, intrinsic stress, and tensile-side roughness were systematically analyzed. Grain size and graphite phase content dominated the E of self-standing diamond films. Tensive-side surface roughness and intergranular bonding strength affected the ${\sigma }_F$. When the growth side in tension, E of the multilayer film (modulation period Λ = 3 h) was 16.0% lower than m-M and 26.6% higher than m-N, ${\sigma }_F$ of multilayer film was 16.8% higher than m-M and 8.3% lower than m-N. At fixed Λ, doubling the total deposition period, E, ${\sigma }_F$, and K_IC of the multilayer film increased 72.3%, 22.3%, and 2.7%, respectively. MCD/NCD composited multilayer architecture presented significant strengthening and toughening effects on self-standing diamond films.     

Effects of cobalt and cobalt oxide buffer layers on nucleation and growth of hot filament chemical vapor deposition diamond films on silicon (100)

An initial study on the nucleation and growth of diamond, using hot filament chemical vapor deposition (HFCVD) technique, was carried out on Co and CoO thin buffer layers on non-carbon substrates (Si (100)), and the results were compared with conventional scratching method. The substrate temperature during the growth was maintained at 750±50 °C. A mixture of CH₄ and H₂ (1: 100 volume %) was used for deposition. The total pressure during the two hour deposition was 30±2 Torr. X-ray photoelectron spectroscopy (XPS) study showed the diamond nucleation at different time periods on the Co and CoO seed layers. It is observed that Co helps in nucleation of diamond even though it is known to degrade the quality of diamond film on W-C substrate. The reason for improvement in our study is attributed to (i) the low content of Co (~0.01%) compared to W-C substrate (~5–6%), (ii) formation of CoSi₂ phase at elevated temperature, which might work as nucleation sites for diamond. SEM analysis reveals a change in the morphology of diamond film grown on cobalt oxide and a significant reduction in the size of densely packed crystallites. Raman spectroscopic analysis further suggests an improvement in the quality of the film grown on CoO buffer layer.     

Thermal chemical vapor deposition of fluorocarbon polymer thin films in a hot filament reactor

Formation of fluorocarbon polymer films with a linear (CF₂-CF₂)_n molecular structure similar to polytetrafluoroethylene, PTFE is described by a hot filament chemical vapor deposition method. Growth process is analyzed by infrared absorption and C(1s), O(1s) and F(1s) core level electron spectroscopy of films deposited at −5 and +70 °C. Absorption doublet at 1220 and 1160 cm⁻¹ assigned to C-F₂ asymmetric and symmetric stretches, rock at 518 cm⁻¹ and wag at 637 cm⁻¹ indicate formation of linearly organized CF₂ groups with minimum hindrance to molecular vibration modes in CVD grown films. Absorption bands at 1660 and 3389 cm⁻¹ show O and OH groups in the films which diminish on annealing. The C(1s) components, CF₃, CF and C-CF bonding show branching, cross-liking and defects sites which increase as substrate temperature is increased. The O(1s) line analysis shows O₂ in fluorocarbon films is chemically bonded as C-O and F₂CO with relative ratio depending on the film growth temperature. Both O₂ and OH are the result of additional reaction pathways involving the species generated from fragmentation of CF₃C(O)F. Molecular structure of fluorocarbon polymer films involving these species are discussed which are in conformity with the XPS and IR absorption data.     

The effect of d.c. glow discharge on hot filament chemical vapor deposition of diamond

The origin of bias-enhanced nucleation and growth of diamond has been investigated in hot filament chemical vapor deposition with two parallel W electrode wires inserted between filament and two floating Si substrates. An electrical voltage has been applied to the W electrodes to produce a glow discharge, which occurs in bias-enhanced deposition processes, during deposition. Substantial nucleation and growth enhancements occurred on the substrate near the negative glow of the d.c. discharge under the cathode wire. These observations indicate that the enhanced deposition of diamond in both positive and negative bias processes arises from chemically active species in the negative glow region rather than from electron or ion bombardment effects.     

Heteroepitaxial nucleation of diamond on Si(100) via double bias-assisted hot filament chemical vapor deposition

A new process has been developed to obtain high density epitaxial diamond nucleation via a double bias-assisted hot filament chemical vapor deposition (HFCVD). In the process, a negative bias voltage is applied to the Si substrate and a positive bias voltage is applied to a steel grid placed on top of the hot filaments. With this arrangement, a stable plasma can be generated between the grid and the hot filaments. Ions in the plasma are then drawn to the substrate by a negative substrate bias voltage. The impinging rate of these ions can be easily controlled by adjusting the grid current, and the ion energy can be independently controlled by adjusting the substrate bias voltage. Hence, the energy and dosage of ion bombardment onto the Si(100) substrate can be controlled easily and independently. With the controlled ion bombardment, high density and heteroepitaxial nucleation can be achieved routinely. After the nucleation process, highly textured diamond films were grown by either the HFCVD or the microwave plasma chemical vapor deposition process (MPCVD).     

Nitrogen-doped diamond films selected-area deposition by the plasma-enhanced chemical vapor deposition process

The nitrogen-doped diamond films have been successfully synthesized by using urea as the nitrogen source. Selected-area deposition of diamond nuclei was formed by using a SiO₂ layer as the masking material. Diamond pads, around 9 μm in diameter, were obtained when the N-doped diamond films were deposited on these patterned diamond nuclei using the chemical vapor deposition process. An emission current density as high as 200 μA/cm², with a turn-on field of around 8 V/μm, was obtained. However, the diamond emitters broke down easily, which is ascribed to the localized melting of the substrate materials surrounding the diamond pads.