Growth of polycrystalline and nanocrystalline diamond films on pure titanium by microwave plasma assisted CVD process

Well-faceted polycrystalline diamond (PCD) films were deposited along with nanocrystalline diamond (NCD) films on the pure titanium substrate by a microwave plasma assisted chemical vapor deposition (MWPCVD) system in the environment of CH₄ and H₂ gases at a moderate temperature. Diamond film deposition on pure titanium and Ti alloys is always extremely hard due to the high diffusion coefficient of carbon in Ti, the big mismatch in their thermal expansion coefficients, the complex nature of the interlayer formed during diamond deposition, and the difficulty of attaining very high nucleation density. A well-faceted PCD film and a smooth NCD film were successfully deposited on pure Ti substrate by using a simple two-step deposition technique. Both films adhered well. Detailed experimental results on the preparation, characterization and successful deposition of the diamond coatings on pure Ti are discussed. Lastly, it is shown that smooth NCD film can be deposited at moderate temperature with sufficient diamond quality for mechanical and tribological applications.     

Structural and optical characterization of thick and thin polycrystalline diamond films deposited by microwave plasma activated CVD

Preliminary results of growth of thin diamond film in a recently installed 3 kW capacity microwave plasma activated CVD (MW-PACVD) system are being reported. The films were deposited on Si (100) substrate at 850°C using methane and hydrogen mixture at 1·5 kW MW power. The grown polycrystalline films were characterized by micro-Raman, transmission electron microscope (TEM), spectrophotometer and atomic force microscope (AFM). The results were compared with that of a thicker diamond film grown elsewhere in a same make MW-PACVD system at relatively higher power densities. The presence of a sharp Raman peak at 1332 cm^− 1 confirmed the growth of diamond, and transmission spectra showed typical diamond film characteristics in both the samples. Typical twin bands and also a quintuplet twinned crystal were observed in TEM, further it was found that the twinned region in thin sample composed of very fine platelet like structure.     

Formation and characterization of diamond crystals synthesized by microwave plasma CVD system

Abstract

Diamond crystals have been successfully synthesized on (100) Si wafer using microwave plasma CVD. The growth was conditioned in a flowing system in which the parameters, such as CH₄/H₂ ratio, pressure, temperature and microwave power were varied. Cubo‐octahedra or tetrakaidecahedra are the equilibrium shape of diamond single crystals obtained under all conditions and are therefore the basic unit for the formation of polycrystalline diamond films, mostly through repetitive twinning and secondary growth of diamond crystals on {100} habit planes of cubo‐octahedra. Both X‐ray diffraction and Raman spectroscopy were used to facilitate the analysis of the diamond crystallinity and purity. These qualities are similar to those of natural diamonds.     

氧碳比对MPCVD法同质外延单晶金刚石的影响

以Ib型(100)取向高温高压(HPHT)单晶金刚石为基底、H2-CH4-CO2混合气为反应气源,利用10kW、2.45GHz不锈钢谐振腔式微波等离子体化学气相沉积(MPCVD)装置进行金刚石同质外延生长。通过光学显微镜表征外延生长金刚石的表面形貌;Raman光谱表征金刚石的结晶质量;螺旋测微仪测厚再计算生长速率,着重探讨工艺因素中氧碳比对同质外延金刚石生长速率、表面形貌、金刚石结晶质量的影响。结果表明随着氧碳比的增加,金刚石生长模式由二维形核模式转变为台阶流模式,结晶质量提高,生长速率变慢;在微波功率7.8kW、CH4浓度(与H2的比例)8%、气压18kPa、基底温度1080℃条件下,氧碳比为0.8时,金刚石结晶质量好且生长速率高(达16μm/h)。反应气源中引入合适比例的CO2是获得高的生长速率同时有效改善同质外延单晶金刚石结晶质量的有效方法。     

Further investigations on the use of polycrystalline CVD diamond as a UV detector

Metal-semiconductor-metal planar structures were fabricated on free-standing diamond films. The devices were found to operate successfully as photodetectors for deep ultraviolet light with their response being dependent on the post-growth treatment. However, evidences of trapping were found both in the spectral photoresponse and temporal response to monochromatic light transients.     

Growth and characterization of diamond films deposited at high-pressure using a low-power microwave plasma reactor

Diamond films were deposited on molybdenum substrates from mixtures of methane diluted in hydrogen using a high-pressure microwave plasma reactor. In this reactor, a compressed waveguide structure was used to increase the electric field strength, and accordingly the reactor was able to operate stably with low gas flow rate and microwave power. The films deposited on 12 mm diameter substrates were characterized by film morphology, Raman spectra, growth rate and crystalline quality. The morphology of diamond films deposited in this reactor depends mainly on the substrate temperature. When the deposition pressure was 48 kPa and microwave power was only 800 W, high quality diamond films could be uniformly deposited with a growth rate around 20 μm/h.     

Correlation between the OES plasma composition and the diamond film properties during microwave PA-CVD with nitrogen addition

The mechanisms of nitrogen incorporation in diamond are still an unsolved riddle. This is mainly due to the complexity of the processes involved as they not only depend on empirical parameters (e.g. vessel pressure, substrate temperature, the gas phase composition, type and concentration of the nitrogen containing compound used), but also on the plasma chemistry and the surface chemical reactions. In this study, small quantities (ppm range) of diatomic nitrogen are added to a conventional hydrogen-methane feed gas mixture in order to investigate the effect of nitrogen incorporation in diamond films prepared by microwave plasma assisted chemical vapour deposition (CVD). Optical emission spectroscopy (OES) is used to survey the plasma composition during deposition. The intensities of the CN, CH and C₂ emitting radicals and the Balmer atomic hydrogen emission lines are correlated to the Raman film quality and to the nitrogen content in the film measured by secondary ion mass spectrometry (SIMS).     

Optical and mechanical properties of diamond like carbon films deposited by microwave ECR plasma CVD

Diamond like carbon (DLC) films were deposited on Si (111) substrates by microwave electron cyclotron resonance (ECR) plasma chemical vapour deposition (CVD) process using plasma of argon and methane gases. During deposition, a d.c. self-bias was applied to the substrates by application of 13·56 MHz rf power. DLC films deposited at three different bias voltages (−60 V, −100 V and −150 V) were characterized by FTIR, Raman spectroscopy and spectroscopic ellipsometry to study the variation in the bonding and optical properties of the deposited coatings with process parameters. The mechanical properties such as hardness and elastic modulus were measured by load depth sensing indentation technique. The DLC film deposited at −100 V bias exhibit high hardness (∼ 19 GPa), high elastic modulus (∼ 160 GPa) and high refractive index (∼ 2·16–2·26) as compared to films deposited at −60 V and −150 V substrate bias. This study clearly shows the significance of substrate bias in controlling the optical and mechanical properties of DLC films.     

Growth, processing and properties of CVD diamond for optical applications

In recent years there has been an increased need for optical materials for use in adverse chemical, thermal, abrasive, and/or radiation environments. Diamond is a natural candidate for many of these applications because of its radiation hardness, superb resistance to chemical attack and abrasive wear, high thermal conductivity, and low absorption coefficient throughout the visible and much of the infrared. The use of synthetic (high pressure-high temperature) and natural diamond in optical components has been limited by the size and shapesof available crystals, and the inability to coat optical elements. The chemical vapor deposition (CVD) of polycrystalline diamond does not suffer the same limitations, and is therefore the focus of an expanding worldwide research effort. CVD diamond is not without its own shortcomings, however, and in this paper a status report is given on the obstacles and current research related to using CVD diamond as an optical material. Natural diamond's relevant physical properties and the optical applications envisioned for CVD diamond are also discussed.     

Influence of bowl shaped substrate holder on growth of polymeric DLC film in a microwave plasma CVD reactor

The properties of diamond like carbon (DLC) films grown in modified microwave plasma CVD reactor is presented in this paper. By using bowl shaped steel substrate holder in a MW plasma CVD reactor (without ECR), films have been grown at relatively high pressure (20 Torr) and at low temperature (without heating). The input microwave power was about 300 W. Earlier, under the same growth conditions, no deposition was achieved when flat molybdenum/steel substrate holders were used. In this study, two different designs of bowl shaped steel substrate holder at different bias have been experimented. Raman spectra confirm the DLC characteristics of the films. FTIR results indicate that the carbon is bonded in the sp ³ form with hydrogen, and this characteristic is more pronounced when smaller holder is used. UV-visible spectra show high visible transmittance (~85%) for films grown in both the holders. The nanoindentation hardness of the films have a wide range, about 4–16 GPa. Field emission scanning electron microscope (FESEM) images reveal that the films have featureless and smooth surface morphology. These films are polymeric in nature with moderately high hardness, which may be useful as anti-scratch and anti-corrosive coatings.     

Growth behavior of CVD diamond films with enhanced electron field emission properties over a wide range of experimental parameters

In this study, diamond films were synthesized on silicon substrates by microwave plasma enhanced chemical vapor deposition (CVD) over a wide range of experimental parameters. The effects of the microwave power, CH₄/H₂ ratio and gas pressure on the morphology, growth rate, composition, and quality of diamond films were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). A rise of microwave power can lead to an increasing pyrolysis of hydrogen and methane, so that the microcrystalline diamond film could be synthesized at low CH₄/H₂ levels. Gas pressure has similar effect in changing the morphology of diamond films, and high gas pressure also results in dramatically increased grain size. However, diamond film is deteriorated at high CH₄/H₂ ratio due to the abundant graphite content including in the films. Under an extreme condition of high microwave power of 10 kW and high CH₄ concentration, a hybrid film composed of diamond/graphite was successfully formed in the absence of N₂ or Ar, which is different from other reports. This composite structure has an excellent measured sheet resistance of 10–100 Ω/Sqr. which allows it to be utilized as field electron emitter. The diamond/graphite hybrid nanostructure displays excellent electron field emission (EFE) properties with a low turn-on field of 2.17 V/μm and β = 3160, therefore it could be a promising alternative in field emission applications.     

Characterization by optical emission spectroscopy of an oxygen plasma used for improving PET wettability

Polymers have excellent bulk physical and chemical properties but usually poor surface properties. For wettability improvement plasma technology is one of the most promising techniques. Several studies about surface modifications of polyethylene terephthalate (PET) exposed to an oxygen plasma have been already carried out. In this work an analysis of the plasma phase by optical emission spectroscopy (OES) has been employed in order to establish a correlation with the surface effects induced by plasma exposition on PET chemical composition and wettability, investigated by X-ray photoelectron spectroscopy (XPS) and water contact angle measurements, respectively. The treatment has been carried out for a time of 60 s at a constant pressure (15 Pa) and at different process powers ranging from 20 to 200 W. As expected, the best performance has been obtained at a power of 200 W due to the larger presence of oxygen radicals (OI) with the assistance of ionic species (OII, O₂⁺) which create dangling bonds on the substrate surface.     

Influence of feedstock and spraying parameters on the depositing efficiency and microhardness of plasma-sprayed zirconia coatings

In this work, nanostructured and conventional zirconia coatings were deposited by atmospheric air plasma spraying (APS) under different spraying conditions using spray-dried nanostructured powders and conventional powders as feedstocks, respectively. The microstructures of the feedstocks were characterized by TEM and SEM. The depositing efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. It is significant from economic view of reducing cost. In addition, the influence of spraying parameters on microstructure and microhardness of zirconia coatings were discussed also. It was found that the microhardness of nanostructured zirconia coatings was larger than that of its counterparts.     

Correlation between structure and optical properties in low emissivity coatings for solar thermal collectors

We have investigated the relation between the structure and morphology of TiN coatings with their optical properties. Samples were deposited by magnetron sputtering and, by changing the deposition parameters, different textures and chemical compositions can be obtained as measured by X-ray diffraction and glow discharge optical emission spectroscopy respectively. The transmittance in the visible range, measured by spectroscopic ellipsometry, and the emittance, derived from reflectance in the far infrared range as measured by Fourier Transform Infrared Spectroscopy have been related to the nitrogen atomic content and the preferred crystalline orientations present in the TiN coatings. The visible transmittance of the coatings was found not to be dependent on the preferential orientation, while the emittance clearly improves with increasing the film thickness and the presence of both (111) and (200) crystal orientations.     

Features of the spectra of optical emission from high-frequency discharge plasma during bismuth ferrite sputtering

he spectra of optical emission from plasma of the RF discharge in oxygen during the sputtering of bismuth ferrite (BiFeO₃) and iron-containing alloy targets have been studied. Two anomalously intense lines of emission from iron atoms at 613.6 and 306.7 nm have been observed during the sputtering of bismuth ferrite, which are much less pronounced in the case of metal targets. The lines of emission from bismuth atoms are completely absent. A mechanism responsible for excitation of the anomalous emission lines during the sputtering of BiFeO₃ is proposed.

     

Characteristics of a well-adherent nanocrystalline diamond thin film grown on titanium in Ar/CH4 microwave CVD plasma

The fabrication of a well-adherent diamond film on titanium and its alloys is always problematical due to the different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density. In this work, well-adherent and smooth nanocrystalline diamond (NCD) thin film is successfully deposited on pure titanium substrate by microwave plasma-assisted chemical vapor deposition (MWPCVD) method in Ar/CH₄ environment. It is found that the average grain size is less than 20 nm with a surface roughness value as low as 12 nm. Morphology, surface roughness, diamond crystal orientation and quality are obtained by characterizing the sample with field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy, respectively. Detailed experimental results and mechanisms for NCD film deposition are discussed.     

Synthesis and purification of diamond films using the microwave plasma of a CO-H2 system

A variety of diamond films were deposited using the microwave plasma of a CO-H₂ system. Qualities of the synthesized films were correlated with the gas phase atomic hydrogen concentration monitored using optical emission spectroscopy. The amorphous components contained in the synthesized films were of a polyacetylene structure, which was possibly formed by the successive polymerization of C₂H₂ in the gas phase.Excess atomic hydrogen allowed highly crystallized diamond films to be deposited at high growth rates which included only a small amount of polyacetylene components. Two possible explanations for these results were proposed: the suppression of polyacetylene formation and the production of appropriate precursor (CH₃) for diamond synthesis under the excess atomic hydrogen condition.Finally, the ratioI _H/I _Ar (whereI is the optical emission intensity) was suggested as a decisive parameter indicating the suitability of the plasma conditions for the growth of pure diamond with good crystallinity.     

等离子体喷涂涂层细观结构与工艺相关性研究

采用改进的颗粒沉积模型和一种新建议的循环算法，利用数值方法模拟了等离子体喷涂中涂层的生长过程及涂层的细观结构．数值模拟，主要包括了陶瓷液滴的高速变形与凝固、涂层材料的堆积、涂层中细观空洞的形成与温度场的迭代计算等过程．研究结果表明，涂层中孔隙率的分布与一些关键工艺参数和基底表面状态等有关，液态陶瓷颗粒的直径和飞行速度的加大会引起涂层内孔隙率的增加，而基体温度和表面粗程度的升高则有利于提高涂层的致密度．本文的研究结果将有助于定量或半定量地优化选取工艺参数以便获得所需的涂层结构和改善涂层的力学性能．     

Implementation of the time-modulated process to produce diamond films using microwave-plasma and hot-filament CVD systems

A newly developed process called time-modulated chemical vapour deposition (TMCVD) was employed to deposit smooth polycrystalline diamond films onto silicon substrates using both microwave plasma CVD (MPCVD) and hot-filament CVD (HFCVD) systems. The distinctive feature of the TMCVD process, which separates it from the conventional diamond CVD process, is that it pulses methane (CH₄) at different flow rates for different time durations into the vacuum reactor during the entire diamond CVD process. Generally, both MPCVD and HFCVD systems produced results that displayed similar trends, except that the growth rate results obtained using the two CVD systems were conflicting. In comparison to the conventional CVD diamond films, the time-modulated films, deposited using both MPCVD and HFCVD techniques, were generally found to be (i) smoother, (ii) consisted of smaller diamond crystallites and (iii) displayed approx. similar film quality. The diamond-carbon phase purity of the as-grown films was assessed using Raman spectroscopy. In addition, the surface roughness, Ra, values of the deposited films were obtained using surface profilometry.     

Correlation between optical path modulations and transmittance spectra of a-Si:H thin films

The optical constants of plasma-enhanced chemical-vapor-deposited amorphous silicon (a-Si:H) thin film upon a transparent substrate are determined within the UV-visible region by measurement of the transmittance spectrum. Apart from thickness irregularities, the effects of vertical film inhomogeneities (refractive-index distribution) on the spectrum are discussed. In this respect, although consideration of any possible variation in thickness of the film within the area illuminated by the probe beam is sufficient for correcting the modulation of the extrema of interference fringes, including in the model the thin transitional regions at substrate-film and film-air interfaces might be an alternative method for understanding the overall optical behavior of the spectrum.