Pre-nucleation techniques for enhancing nucleation density and adhesion of low temperature deposited ultra-nanocrystalline diamond

Effect of pre-nucleation techniques on enhancing nucleation density and the adhesion of ultra-nanocrystalline diamond (UNCD) deposited on the Si substrates at low temperature were investigated. Four different pre-nucleation techniques were used for depositing UNCD films: (i) bias-enhanced nucleation (BEN); (ii) pre-carburized and then ultrasonicated with diamond powder solution (PC-U); (iii) ultrasonicated with diamond and Ti mixed powder solution (U-m); (iv) ultrasonicated with diamond powder solution (U). The nucleation density is lowest for UNCD/U-substrate films ( 10⁸ grains/cm²), which results in roughest surface and poorest film-to-substrate adhesion. The UNCD/PC-U-substrate films show largest nucleation density ( 1 × 10¹¹ grains/cm²) and most smooth surface (8.81 nm-rms), whereas the UNCD/BEN-substrate films exhibit the strongest adhesion to the Si substrates (critical loads =  67 mN). Such a phenomenon can be ascribed to the high kinetic energy of the carbon species, which easily form covalent bonding, Si–C, and bond strongly to both the Si and diamond.     

Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Highly oriented diamond films have been deposited on silicon substrate by the MPCVD technique (microwave plasma assisted chemical vapour deposition) using an ultra short bias enhanced nucleation process (so called USBEN). We focus our attention on two points: the homogeneity of the deposit in order to perform a precise characterisation whatever surface location (on 1×1 cm² of single silicon substrate); and the simplification of the successive steps usually performed in the BEN process. This is carried out by optimising the microwave cavity and the d.c. discharge extension and by keeping the pretreatments just necessary to obtain high nucleation density with an acceptable epitaxial ratio and a good homogeneity. This leads to a drastic reduction of the bias time of only 30 s for low bias voltage. As we obtain a highly oriented diamond film with a short bias pretreatment without preliminary carburation step, we discuss the substrate transformation under a weak bombardment duration of ions having a quite low energy. We think that the bias step probably consists to a slight modification of the substrate surface.     

Thermoluminescence properties of nitrogen containing chemical vapour deposited diamond films

The behaviour of centres due to nitrogen impurities introduced during the growth process into the chemical vapour deposited (CVD) diamond lattice is reported for the first time by thermoluminescence (TL) studies between 300 and 670 K after 200–400 nm ultraviolet (deuterium lamp) illumination at 300 K (RT) in air. TL curves exhibit some glow peaks at 490, 520 and 620 K, characterized by activation energies of approximately 1.2, 1.4 and 1.9 eV, respectively. Spectral analysis of these peaks which reveals some differences due to nitrogen content in the films shows well defined emission bands and interesting features leading to a better knowledge of the broad red photoluminescence (PL) band observed in our films. Nitrogen addition during the growth of the CVD material leads to the quenching of both of the 1.68 eV line related to Si centre and the broad green band centred at 2.25 eV which were observed for the high quality film.     

Microstructure of c-BN thin films deposited on diamond films

Diamond films were used as substrates for cubic boron nitride (c-BN) thin film deposition. The c-BN films were deposited by ion beam assisted deposition (IBAD) using a mixture of nitrogen and argon ions on diamond films. The diamond films exhibiting different values of surface roughness ranging from 16 to 200 nm (in R_rms) were deposited on Si substrates by plasma enhanced chemical vapor deposition. The microstructure of these c-BN films has been studied using in situ reflexion electron energy loss spectroscopy analyses at different primary energy values, Fourier transform infrared spectroscopy and high resolution transmission microscopy. The fraction of cubic phase in the c-BN films was depending on the roughness of the diamond surface. It was optimized in the case of the smooth surface presenting no particular geometrical effect for the incoming energetic nitrogen and argon ions during the deposition. The films showed a nanocrystalline cubic structure with columnar grains while the near surface region was sp² bonded. The films exhibit the commonly observed layered structure of c-BN films, that is, a well textured c-BN volume lying on a h-BN basal layer with the (00.2) planes perpendicular to the substrate. The formation mechanism of c-BN films by IBAD, still involving a h-BN basal sublayer, does not depend on the substrate nature.     

Influence of nucleation density on film quality,growth rate and morphology of thick CVD diamond films

The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH₄/H₂/O₂ plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm⁻¹, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (10⁴–10⁵ cm⁻²) and high nucleation densities (>10¹⁰ cm⁻²), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>10¹⁰ cm⁻²), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×10⁴–10⁵ cm⁻²) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.     

Grain size dependent physical and chemical properties of thick CVD diamond films for high energy density physics experiments

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH₄ content of the CH₄/H₂ feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm^− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH₄. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.     

Efficient CVD diamond film/alumina composite substrate for high density electronic packaging application

Due to its extreme hardness, chemical and mechanical stability, large band gap, low dielectric constant and highest thermal conductivity, diamond film is expected to be an excellent electronic packaging material for high frequency and high power devices. Under an alcohol concentration of 0.8% and a substrate temperature of 850 °C, high quality diamond films deposited on alumina are obtained by hot filament chemical vapor deposition (HFCVD) method using the optimum parameters determined by an infrared spectroscopic ellipsometer. Prior to the deposition of diamond film, carbon ions are implanted into alumina wafers to release the residual stress between interfaces. The measurement results indicate that dielectric properties and the thermal conductivity of diamond film/alumina composites are improved further with the increase of diamond coating. When the thickness of diamond coating is up to 100 μm, dielectric constant and dielectric loss of diamond film/alumina composite are 6.5 and 1.1 × 10^− 3, respectively. However, a thermal conductivity of 3.98 W/cm·K is obtained.     

Al2O3微粉及Alphabond加入量对无水泥刚玉质浇注料性能的影响

为了改善刚玉质浇注料的热态强度和抗侵蚀性,采用水合氧化铝Alphabond作为结合剂,分别调整Al2O3微粉及Alphabond加入量,研究了Al2O3微粉及Alphabond加入量对无水泥刚玉质浇注料性能的影响。结果表明:Al2O3微粉加入量对浇注料的成型性能和烧后强度影响较大,其最佳加入量在Alphabond含量(质量分数,下同)为3%时为11%左右;增加Alphabond加入量(在0~4%范围内)可大幅度提高浇注料的烘干强度,但1400℃及1600℃烧后常温抗折强度和1400℃高温抗折强度明显降低。     

Microstructural characterization of diamond films deposited on c-BN crystals

The morphology and structure of diamond films, deposited on cubic boron nitride (c-BN) crystals by microwave-plasma-enhanced chemical vapor deposition, is studied by high-resolution scanning electron microscopy and micro-Raman spectroscopy. The c-BN crystals, with sizes of 200 to 350 μm and grown by a high-temperature/high-pressure technique, were embedded in a copper holder, and used as substrates in deposition runs of 15 min to 5 h. The nucleation centers for diamond appear as well-shaped cuboctahedral crystallites, having diameters of approximately 100 nm. With increasing deposition time the diamond crystallites grew larger, forming islands on the c-BN faces. In some cases, epitaxial growth was observed on the (111) c-BN faces where coalesced particles gave rise to very smooth regions. A number of diamond crystals with peculiar shapes are observed, such as a pseudo five-fold symmetry due to multiple twinning. Moreover, both randomly distributed carbon tubes, about 100 nm in diameter and 1 μm in length, and spherically shaped features are observed in samples prepared under the typical conditions of diamond deposition, this effect being ascribed to the influence of plasma-sputtered copper contamination. Quite unusual diamond crystals with a deep, pyramidal-shaped hole in the middle grew on the copper substrate between the c-BN crystals.     

Hydrogen plasma etching of diamond films deposited on graphite

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH₄/15%H₂/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp³/sp² ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.     

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.     

The preparation of high quality oriented diamond thin films via low temperature and hydrogen ion etched nucleation

A three-step process was used to prepare high quality [001]-oriented diamond films. First, diamond crystallites were nucleated for 20 min on Si(001) at a temperature around 740 °C by bias-enhance method, during this step the portion of [001]-oriented diamond nuclei was increased in comparison with the nuclei deposited by a two-step method. Then hydrogen ion etching was performed for 30 min by setting an electric potential of −140 V. After the etching step most of the crystallites were [001]-oriented and twinned crystallites disappeared. Finally, diamond thin films were deposited under conventional conditions for [001]-textured growth. SEM was used to analyse the morphology of diamond crystallites and films. The results indicate that large area, uniform and [001]-oriented diamond thin films can be prepared by three-step growth. The films show good Raman characteristics and higher thermal conductivity than those deposited by a two-step process.     

Effect of ion beam nitriding on diamond nucleation and growth onto steel substrates

Ion beam nitriding was successfully employed to overcome the difficulty of diamond growth on ferrous base substrates. Commercial steels were pretreated by an ion beam method in an ambient environment of nitrogen gas, diamond was then deposited by hot filament chemical vapor deposition (CVD). The deposited films were characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Continuous diamond films with a sharp characteristic Raman peak of 1337.7 cm⁻¹ were grown and adhered well on the nitrided region of the steel substrates. On the other hand, a mixture of diamond crystallites, amorphous carbon and graphitic carbon was loosely deposited on the unnitrided region. A thin layer of iron and chromium nitrides, formed on the steel surface by ion beam nitriding, enabled the subsequent nucleation and growth of high-quality CVD diamond.     

Effect of bias enhanced nucleation on the nucleation density of diamond in microwave plasma CVD

The variation of diamond nucleation density as a function of the conditions of bias enhanced nucleation (BEN) were studied. The nucleation density increased with microwave power, but decreased with the substrate temperature. The nucleation density also increased with bias voltage above 60 V, and had a maximum around 100 V. The crystal growth of diamond took place when either the bias voltage was high or the deposition time was long. The shift of C_1s energy measured by X-ray photoelectron spectroscopy indicated that the ratio of carbon sp³ bonds in the amorphous carbon and/or SiC phases formed before the nucleation of diamond, increased around the bias voltage of 100 V, which seemed to be the reason for enhancement of diamond nucleation by bias voltage. A simple computer simulation was performed in order to understand the effect of BEN conditions on the nucleation of diamond. The simulation reproduced the experimentally observed changes of nucleation density and particle size.     

Fretting wear of thin diamond films deposited on steel substrates

The dominant wear mechanism of thin diamond films deposited onto steel substrates and the effect of film thickness on their lifetime under fretting conditions were studied by analyzing the running-in and the main period of the coatings wear life. Steel plate and steel ball specimens for the present study were both coated with diamond by chemical vapor deposition (CVD). The wear tracks resulting from the fretting tests were investigated by various surface analysis methods. The results showed that the dominant wear mechanism of the diamond coatings, when both surfaces are coated, is an abrasive form of fretting wear. Under these conditions, the lifetime of the diamond films increased with increasing film thickness. It was found that the wear rate during the main period is independent on the initial thickness of the diamond film and therefore its life depends on the residual thickness at the end of the running-in period.     

Microwave dielectric properties of bismuth zinc niobate thin films deposited on alumina by pulsed laser deposition

Bi₂Zn_2/3Nb_4/3O₇ thin films were prepared on Al₂O₃ substrates by pulsed laser deposition. The phase compositions and microstructures were characterized by X-ray diffraction and atomic force microscopy. The as-deposited films were all amorphous in nature. All films were crystallized after the post annealing at the temperature range of 700–900 °C for 30 min in air. The texture characteristics change with annealing temperature. A split post dielectric resonator method was used to measure the microwave dielectric performance at the resonant frequencies of 10, 15 and 19 GHz. For the films annealed at 900 °C, the preferential orientation is similar to the monoclinic BZN bulk. The microwave dielectric constants at 10, 15 and 19 GHz are 69.4, 58.9 and 47.9, respectively, which are closer to these of the monoclinic BZN bulk.     

Antibacterial properties of polycrystalline diamond films

Electronic and mechanical properties, and their biocompatibility, make diamond-based materials promising biomedical applications. The cost required to produce high quality single crystalline diamond films is still a hurdle to prevent them from commercial applications, but the emergence of polycrystalline diamond (PCD) films grown by chemical vapour deposition (CVD) method has provided an affordable strategy. PCD films grown on silicon wafer have been used throughout and were fully characterised by SEM, XPS, Raman spectroscopy and FTIR. The samples contain nearly pure carbon, with impurities originated from the CVD growth and the silicon etching process. Raman spectroscopy revealed it contained tetrahedral amorphous carbon with small tensile stress. The sp² carbon content, comprised between 16.1 and 18.8%, is attributed to the diamond grain boundaries and iron-catalysed graphitisation. Antibacterial properties of PCD films were performed with two model bacteria, i.e. Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) using direct contact and shaking flask methods. The samples showed strong bacteriostatic properties against S. aureus and E. coli with the direct contact method and no influence on planktonic bacterial growth. These results suggest that the bacteriostatic mechanism of PCD films is linked to their surface functional groups (carbon radicals and –NH₂ and –COOH groups) and that no diffusible molecules or components were involved.     

Comparison of the adhesion of diamond films deposited on different materials

Indentation tests combined with acoustic emission spectra were used to compare the adhesion of diamond films deposited on various substrates, including Ti, Cr, Si and Ti coated Cu. We show that indentation in the diamond coatings may cause the following failure modes: (a) the substrate cracking; (b) the film cracking and localised detachment; and (c) the film delamination and the delamination propagation. Acoustic emission during indentation loading provided essential information in predicting what mode of failure occurs. Combined with the acoustic emission spectra, the indentation tests are reliable in comparing the adhesion of diamond films deposited on the same or similar substrate materials. However, the comparison of the film adhesion on very different substrates, like Cu and Ti, is not so straightforward. Acoustic emission spectra also revealed that indentation caused substrate cracking prior to the failure of the film/substrate interface for diamond coatings on Si. In this case, the indentation tests are not valid to compare the coating adhesion.     

Adhesion of gold and copper thin films deposited on alumina and magnesium oxide

Scratch and pull-off methods for adhesion measurement were applied to various film/ substrate systems. The film crystallographic structure was investigated by transmission electron microscopy (TEM) in order to correlate it to the adhesion strength. We also correlate the film and substrate electronic structures (in particular the electronegativity) to the adhesion strength. The results of adhesion measurement are also discussed with respect to the substrate temperature. The main results are that the adhesion strength is greater when the film is deposited on magnesium oxide than on alumina substrates, and for the copper than for the gold deposit. Also, the adhesion strength increases with the substrate temperature.