Two-step growth of HFCVD diamond films over large areas

This paper reports the results of a two-step hot filament chemical vapor deposition method to improve the quality of diamond films. Diamond films were deposited on a Si(100) substrate having an area of 45 cm² and a thickness of 60 μm, employing a HFCVD system. The first step is the growth of CVD diamond in the HFCVD reactor. In the second step, the samples were treated in a saturated solution of H₂SO₄:CrO₃ and rinsed in a (1:1) solution of H₂O₂:NH₄OH. After this procedure, a second diamond layer was deposited. The diamond films were analyzed by Raman scattering spectroscopy (RSS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The films showed a high degree of purity with a thickness of 60 μm, presenting uniform characteristics over a large area.     

Characteristics of nano-crystalline diamond films prepared in Ar/H2/CH4 microwave plasma

Characteristics of nano-crystalline diamond (NCD) thin films prepared with microwave plasma chemical vapor deposition (CVD) were studied in Ar/H₂/CH₄ gas mixture with a CH₄ gas ratio of 1-10% and H₂ gas ratio of 0-15%. From the Raman measurements, a pair of peaks at 1140 cm^− 1 and 1473 cm^− 1 related to the trans-polyacetylene components peculiar to nano-crystalline diamond films was clearly observed when the H₂ gas ratio of 5% was added in Ar/H₂/CH₄ mixture. With an increase of H₂ gas content up to 15%, their peaks decreased, while a G-peak at roughly 1556 cm^− 1 significantly increased. The degradation of NCD film quality strongly correlates with the decrease of C₂ optical emission intensity with the increase of hydrogen gas contents. From the surface analysis with atomic force microscopy (AFM), it was found that grain sizes of NCD films were typically of 10-100 nm in case of 5% H₂ gas addition.     

Field emission studies of CVD diamond thin films: effect of acid treatment

Field emission from CVD diamond thin films deposited on silicon substrate has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. Field emission studies of as-deposited and acid-treated films were carried out using ‘diode’ configuration in an all metal UHV chamber. Upon acid treatment, the field emission current is found to decrease by two orders of magnitude with increase in the turn-on voltage by 30%. This has been attributed to the removal of sp² content present in the film due to acid etching. Raman spectra of both the as-deposited and acid-treated films exhibit identical spectral features, a well-defined peak at 1333 cm⁻¹ and a broad hump around 1550 cm⁻¹, signatures of diamond (sp³ phase) and graphite (sp² phase), respectively. However upon acid treatment, the ratio (I_d/I_g) is observed to decrease which supports the speculation of removal of sp² content from the film. The surface roughness was studied using atomic force microscopy (AFM). The AFM images indicate increase in the number of protrusions with slight enhancement in overall surface roughness after acid etching. The degradation of field emission current despite an increase in film surface roughness upon acid treatment implies that the sp² content plays significant role in field emission characteristics of CVD diamond films.     

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.     

CVD金刚石膜的场发射机制

利用热灯丝化学气相沉积方法在光滑的钼上沉积了金刚石膜,用扫描电子显微镜和Raman谱对金刚石膜进行了分析。结果表明金刚石膜是由许多金刚石晶粒组成,晶粒间界主要是石墨相,并且在膜内有许多缺陷。金刚石膜的场发射结果表明高浓度CH4形成的金刚石膜场发射阈位电场较低浓度CH4形成的金刚石为低。这意味着杂质（如石墨）和缺陷（悬挂键）极大地影响了膜的场发射性能。根据以上结果,提出了一种CVD金刚石膜的场发射机制即膜内的缺陷增强膜内的电场,石墨增大电子的隧穿系数以增强CVD金刚石膜的场发射。     

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.     

Spectroscopic studies of hydrogen related defects in CVD diamond

Thin diamond films prepared by the hot filament chemical vapour deposition (HFCVD) method at various deposition pressures have been characterized using a variety of spectroscopic techniques. Interpretation of the spectral details have provided useful information about the nature of the films. Deposition pressure appears to affect the quality of the diamond films which is reflected in terms of the position and width of the characteristic Raman peak of diamond. Raman spectra of the films prepared at low deposition pressures showed the presence of a sharp peak at ∼1332 cm⁻¹ characteristic of theT _2g mode of diamond. The study of the effect of deposition pressure on the diamond growth, shows that in the range between 20 torr and 60 torr, there is little effect on the width and the shift of the 1332 cm⁻¹ Raman peak. However, at higher pressures the peak showed a blue shift and was considerably broadened. These studies indicate the development of strain in the lattice due to the introduction of unetched hydride layer, at higher deposition pressures, as well as distortions in the lattice leading to partial lifting of the degeneracy of theT _2g mode. A broad band corresponding to the non-diamond phase (which exists at the grain boundaries, interface or as inclusions inside the grain), which can be attributed to the effect of hydrogen impurity creeping into the lattice at higher deposition pressures is also observed. SEM and XRD patterns have confirmed the dominance of diamond phase in these films.     

热丝CVD大面积金刚石薄膜的生长动力学研究

在传统工业型热丝化学气相沉积（HFCVD）反应腔内，相关工艺参数取模拟计算优化值的条件下，采用XRD，SEM及Raman光谱等分析手段研究了单晶Si（100）上较大面积金刚石薄膜的动力学生长行为，讨论了晶格取向的变化规律。结果表明：优化工艺参数条件下，在模拟计算的衬底温度和气体温度分布均匀的区域内，沉积的金刚石薄膜虽存在一定的内应力，但整体薄膜连续、均匀，几何晶形良好，质量较高，生长速率达1．8μm／h。薄膜生长过程中晶形显露面受衬底温度和活性生长基团浓度的影响较大。     

Application of N- and B-doped CVD diamond layers for cyclic voltammetry measurements

Conductive polycrystalline diamond layers prepared by the CVD process have received attention from electrochemists owing to such superior electrochemical properties as the wide potential window, the very low background current, the stability of chemical and physical properties.In this paper, the cyclic voltammetry application using N- and B-doped diamond electrodes was studied. Diamond layers, doped with boron and nitrogen, were synthesized on a silicon substrate in a hot-filament CVD reactor. The obtained diamond layers were characterized using Raman spectroscopy and scanning electron microscopy (SEM).The electrochemical properties of diamond layers were measured in KCl and NaCl basic solutions to gain knowledge about their potential application as an electrode material.It was found that boron doped diamond electrodes showed potential windows up to about 7 V which were almost twice wider than those observed for conventional Pt electrodes.     

Ion irradiation-induced modifications of diamond nanorods synthesised by microwave plasma chemical vapour deposition

Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma chemical vapour deposition (MPCVD) have been implanted with nitrogen ions. The nanorods were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The DNRs consist of single-crystalline diamond cores of 3–5?nm in diameter and several tens of nanometres in length. For purification from non-diamond contents, hydrogen plasma etching of DNRs was performed. Structural modifications of etched DNRs were studied after irradiating with 50?keV nitrogen ions under the fluence of 5?×?10¹⁴, 1?×?10¹⁵, 5?×?10¹⁵ and 1?×?10¹⁶?ions?cm^?2. Nitrogen-ion implantation changes the carbon–carbon bonding and structural state of the nanocrystalline diamond (NCD). Raman spectroscopy was used to study the structure before and after ion irradiation, indicating the coexistence of diamond and graphite in the samples. The results indicated the increase in graphitic and sp²-related content, at the expense of decrease in diamond crystallinity, for ion implantation dose of 5?×?10^15?cm^?2 and higher. The method proves valuable for the formation of hybrid nanostructures with controlled fractions of sp³–sp² bonding.     

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.     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Cyclic voltammetry response of an undoped CVD diamond electrodes

The polycrystalline undoped diamond layers were deposited on tungsten wire substrates by using hot filament chemical vapor deposition (HFCVD) technique. As a working gas the mixture of methanol in excess of hydrogen was used. The morphologies and quality of as-deposited films were monitored by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy respectively. The electrochemical activity of the obtained diamond layers was monitored by using cyclic voltammetry measurements. Analysis of the ferrocyanide–ferricyanide couple at undoped diamond electrode suggests that electrochemical reaction at diamond electrode has a quasireversibile character. The ratio of the anodic and cathodic peak currents was always close to unity. In this work we showed that the amorphous carbon admixture in the CVD diamond layer has a crucial influence on its electrochemical performance.     

CVD diamond films with hydrophilic micro-patterns for self-organisation of human osteoblasts

Nanocrystalline diamond (NCD) films were prepared by microwave plasma-enhanced chemical vapour deposition (CVD) on Si substrates of different roughness (1 and 500 nm). Diamond nano-crystals are up to 50 nm in size and RMS surface roughness is less than 20 nm. The NCD films were cleaned chemically and terminated by hydrogen using plasma treatment (800 °C, 10 min) to generate a hydrophobic surface. Photolithography mask and oxygen plasma (300 W r.f. power, 3 min) were used to generate O-terminated (hydrophilic) patterns (30-200 μm wide) separated by a H-terminated (hydrophobic) surface. Osteoblast-like human cells were seeded on the patterned flat and rough NCD films in McCoy's 5A medium supplemented with 15% fetal bovine serum (FBS). After two days incubation the cells preferentially adhered on the O-terminated stripes. This phenomenon is not suppressed by the surface roughness and is general for other cell types (fibroblast and cervical carcinoma cells), too. The data are discussed with view to further application of NCD thin films in biotechnology and bio-electronics applications.     

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。     

Temperature enhanced gas sensing properties of diamond films

Nanocrystalline diamond (NCD) film was used as a functional part of gas sensor. The gas sensing properties of H-terminated nanocrystalline diamond films were examined to oxidizing gases (i.e., COCl₂ and humid air). Pronounced increase in the surface conductivity (3 orders of magnitude) was found after sensor exposure to phosgene gas and was explained by the surface transfer doping effect. We also present a possible way how to achieve sensor selectivity, i.e. how to distinguish between phosgene and humid air (the mostly present background gas in a common environment).     

Broad band photoluminescence studies of diamond layers grown by hot-filament CVD

Photoluminescence and Raman spectroscopy were employed to investigate the broad band luminescence in thin diamond films grown on a silicon substrate by the HF CVD technique. The broad band luminescence with a maximum emission at 1.8–2 eV observed for CVD diamonds is characteristic for amorphous carbon with sp²-hybridized carbon bonds. As was shown by the Raman spectroscopy our diamond layer contained certain amounts of amorphous carbon phase and diamond nanocrystals which were the source of an additional energy state within the diamond energy gap. The experimental results precluded the possibility of broad band luminescence being due to the electron–lattice interaction. The amorphous carbon and diamond nanocrystals admixture in polycrystalline diamond layer introduced a defect state in the energy gap not in the form of point defects but rather in the form of a line or extended defects. In consequence these extended defects were responsible for the broad PL spectrum in the CVD diamond films.     

The influence of working gas on CVD diamond quality

The effect of the working gas pressure and its composition on diamond quality and particles size was investigated. The diamond layers were grown in Hot Filament Chemical Vapor Deposition (HF CVD) reactor. A methanol–hydrogen gas mixture was used as the precursor gas. The structure of these films was characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. Typically, the diamond's crystallite size decreased with increasing pressure and increasing with methanol concentration. Additionally the admixture of non-diamond (sp²-hybridized carbon) phase also increased with increasing both of deposition pressure and of methanol concentration. It was observed that the deposition pressure has a weaker influence on diamond quality than the methanol concentration.     

Simplified procedure for patterned growth of nanocrystalline diamond micro-structures

Two technological strategies to generate patterned diamond growth have been tested. The diamond micro-structures (i.e. linear stripes and 5 µm narrow channels) were grown in the thickness of 450 nm on Si/SiO₂ substrates by a microwave plasma chemical vapor deposition process. Strategy 1, employing a metal mask, resulted in unsatisfying patterned diamond growth due to instability of metal mask. Strategy 2 was based on a direct lithographic patterning of the seeding layer and resulted in a strongly selective, homogenous, and compact growth of diamond on the polymer-coated seeding patterns. This is assigned to the high seeding yield. The diamond micro-structures formed in this way exhibit surface conductivity of 10^− 7 (Ω/□)^− 1 as assessed by I–V characteristics. The observed results appear promising for the development of directly grown diamond-based transistors.     

CVD金刚石膜产业化制备技术研究

为了解决化学气相沉积金刚石膜产业化进程中存在的生长速率慢、沉积尺寸小的难题,自行研制了适宜于大尺寸金刚石膜高速生长的电子辅助热灯丝式化学气相沉积(EAHFCVD)装置,通过反应气体中加氧将碳源浓度提高到10%以上,并优化反应压力与直流偏流密度二参数间的匹配,研究了该装置的生产特性,同时利用SEM、XRD和Raman光谱对沉积的金刚石膜进行了分析表征.研究结果表明,应用该装置高质量金刚石膜的沉积尺寸可达100mm以上,生长速率达到约10μm/h的水平,并制备出100mm×1 5mm的完整金刚石自支撑膜片,该技术可满足产业化生产的要求.