Microwave Chemical Vapor Deposition System Research Articles

Influence of Diamond CVD Growth Conditions and Interlayer Material on Diamond/GaN Interface

In this study we present the diamond deposition on AlGaN/GaN substrates focusing on the quality of the diamond/GaN interface. The growth of diamond films was performed using microwave chemical vapour deposition system in different gas mixtures: standard CH4/H2 (at low and high ratio of CH4 to H2) and addition of CO2 to CH4/H2 gas chemistry. The diamond films were grown directly on GaN films either without or with thin interlayer. As interlayer, 100 nm thick Si3N4 was used. Surprisingly, in the case of standard CH4/H2 gas mixture, no diamond film was observed on the GaN with SiN interlayer, while adding of CO2 resulted in diamond film formation of both samples with and without SiN interlayer. Moreover, adding of CO2 led to higher growth rate. The morphology of diamond films and the quality of the diamond/GaN interface was investigated from the cross-section images by scanning electron microscopy and the chemical character (i.e. sp3 versus sp2 carbon bonds) was measured by Raman spectroscopy.

Diamond growth on copper rods from polymer composite nanofibres

The potential uses of diamond films can be found in a diverse range of industrial applications. However, deposition of diamond films onto some foreign materials is still not a simple task. Here we present the growth of adherent diamond films on copper rods with the focus on substrate pre-treatment by polyvinyl alcohol composite nanofibres. The primary role of the polymer fibres substantially act as a carbon source which enhances the diamond nucleation and accelerates a homogenous CVD growth. Diamond growth was carried out in pulsed linear antenna microwave chemical vapour deposition system, which is characterized by cold plasma due to larger distance of hot plasma region from the substrate, at various gas compositions. The large distance between plasma source and the substrate holder also allows the uniform deposition of diamond on a large number of substrates with complex geometry (3D objects) as well as for the vertically positioned substrates. Moreover, the inhomogeneity in diamond film thickness deposited on vertically positioned substrates was suppressed by using polyvinyl alcohol nanofibre textile. Combination of PVA polymer fibres use together with this unique deposition system leads to a successful overcoating of the copper rods by continuous diamond film without the film cracking or delamination. We propose that the sequence of plasma-chemical reactions enhances the transformation of certain number of carbon atoms into the sp3-bonded form which further are stabilized by atomic hydrogen coming from plasma.

Pulsed plasmas study of linear antennas microwave CVD system for nanocrystalline diamond film growth

Abstract

Growth and characterization of nanodiamond layers prepared using the plasma-enhanced linear antennas microwave CVD system

Industrial applications of plasma-enhanced chemical vapour deposition (CVD) diamond grown on large area substrates, 3D shapes, at low substrate temperatures and on standard engineering substrate materials require novel plasma concepts. Based on the pioneering work of the group at AIST in Japan, the high-density coaxial delivery type of plasmas has been explored (Tsugawa et al 2006 New Diamond Front. Carbon Technol. 16 337–46). However, an important challenge is to obtain commercially interesting growth rates at very low substrate temperatures. In this work we introduce the concept of novel linear antenna sources, designed at Leybold Optics Dresden, using high-frequency pulsed MW discharge with a high plasma density. This type of pulse discharges leads to the preparation of nanocrystalline diamond (NCD) thin films, compared with ultra-NCD thin films prepared in (Tsugawa et al 2006 New Diamond Front. Carbon Technol. 16 337–46). We present optical emission spectroscopy data for the CH4–CO2–H2 gas chemistry and we discuss the basic properties of the NCD films grown.

Nucleation of nanocrystalline diamond on masked/unmasked Si 3N 4 ceramics with different mechanical pretreatments

Results are presented concerning different mechanical pretreatments performed on silicon nitride substrates and their influence on the nucleation and growth of nanocrystalline diamond (NCD). All substrates were equally sintered and finished, but differently pretreated. Then, they were diamond coated in a microwave chemical vapor deposition system (MPCVD) for relatively short periods, using Ar/H 2/CH 4 gas mixtures. The main objective was to identify the best pretreatment among those proposed, while verifying how it correlates with film uniformity and surface roughness after post-growth. The effect of a molybdenum mask during growth is investigated. The top surface analysis revealed major differences in the nucleation morphology of diamond nuclei on the pretreated samples, two different nucleation types having been identified. For all pretreatments, samples exhibited a very smooth and uniform underlayer of very fine grain particles before the formation of larger aggregates, suggesting a bi-phase nucleation mechanism. When no mask is used considerable changes in the nucleation concentration are found, the resulting films showing grain enlargement near the edges, where the morphology assumes microcrystalline nature. This effect is suppressed by the use of a mask that allowed obtaining very uniform smooth films ( R rms∼ 30 nm, thickness ∼ 1.3 μm, MUS pretreatment), indicating a strong edge effect for the unmasked case. This fact can be attributed both to increased local temperature, plasma density and gas turbulence.

Synthesis of unstrained failure-resistant nanocrystalline diamond films

Nanocrystalline diamond films are highly strained materials, in general. This situation precludes from taking advantage of the many superlative properties of diamond that make it suitable for protective and tribological coatings (i.e. chemical inertness, hardness). Unstrained failure-resistant nanocrystalline diamond films were obtained by performing bias-enhanced nucleation in a microwave chemical vapor deposition system followed by nanocrystalline diamond deposition in a hot-filament chemical vapor deposition system. Thermal-shock tests showed that the bias-seeded films return to their initial unstrained state after differentially contracting and expanding, unlike nanocrystalline diamond films deposited on standard abrasion-seeded substrates, which undergo substantial stress changes that indicate the rupture of interfacial bonds. These results are discussed in terms of the nanocomposite interfacial structure induced by bias-seeding that enhances the nanocrystalline diamond film's tolerance to failure.

Deposition of NCD films using hot filament CVD and Ar/CH 4/H 2 gas mixtures

Ar/CH 4/H 2 gas mixtures have been used in an attempt to deposit nanocrystalline (NCD) diamond and ultra-nanocrystalline (UNCD) diamond films using hot filament (HF) chemical vapour deposition (CVD). A detailed composition map has been developed for the type of films deposited in the Ar/CH 4/H 2 system. It was found that the standard gas mixtures of 1%CH 4/Ar (+ 1–2%H 2) that are used successfully to grow UNCD films in microwave plasmas produce only graphitic film growth in a HF system. A 2-dimensional computer model was used to calculate the gas phase composition for these conditions. The non-uniform temperature distribution arising from the hot filament produces a substantial decrease in gas phase H atoms near to the substrate surface, whilst [CH 3] remains almost constant. We find that the [H] : [CH 3] ratio near the surface decreases from ∼5 : 1 for 1%CH 4/H 2 gas mixtures to 1 : 36 for 1%CH 4/Ar mixtures, and that this can explain the decrease in growth rate and the reduction in film quality toward nanocrystalline or graphitic films. Increasing the H 2 content in the gas mixture improves the situation, but NCD growth was confined to a limited composition window at the boundary of the microcrystalline diamond growth region and ‘no growth’ region. A 2D model of a microwave CVD system has also been developed which gives the gas phase composition for the various Ar-rich gas mixtures. We find that due to the higher temperatures within the plasma ball, plus the fact that the gas temperature close to the substrate surface is in excess of 2000 K ensures that the [H] : [CH 3] ratio remains ≫ 1, and thus permits growth of diamond, NCD or UNCD. Furthermore, since the model shows that [CH 3] and [C 2H] are always much greater than [C 2], this suggests that CH 3 and C 2H species may be more important growth precursors than C 2 under typical UNCD deposition conditions.

Heteroepitaxial nucleation of diamond by bias pretreatment in an electron cyclotron resonance microwave chemical vapor deposition system

Bias-enhanced nucleation (BEN) pretreatment was used for the nucleation of diamond particles on β-silicon carbide substrates in an electron cyclotron resonance (ECR) microwave chemical vapor deposition (CVD) system. Consequently, experimental conditions enabled the pretreatment of large-area substrates. After BEN and diamond growth, the density of diamond particles was strongly dependent on the BEN pretreatment parameters, such as the negative bias voltage, the total pressure, the duration of pretreatment and the methane concentration. Moreover, under appropriate BEN conditions, heteroepitaxial nucleation of the diamond particles was obtained and the proportion of particles oriented with the single-crystal substrates was also determined as a function of the BEN conditions. The maximum rate of oriented diamond particles was around 30% and typical particle densities were in the range 5 × 10 7 cm −2 to 5 × 10 8 cm −2. Heteroepitaxial nucleation was also obtained on substrates over a 40 mm length. For pressures increasing from 13 Pa (0.3 Torr) to 270 Pa (2.0 Torr), the maximum rate of oriented particles was obtained with an optimum bias voltage increasing from −30 V to −90 V. Epitaxial nucleation of diamond could then be strongly dependent on the energy distribution of positive ions involved in the BEN process. Although particle densities and rates of oriented particles deteriorated with the decrease in the substrate temperature from 900 °C to 500 °C, temperatures around 400 °C were suitable for obtaining particle densities close to those obtained at high temperatures.

Deposition and characterization of nanocrystalline diamond films

Highly uniform, smooth nanocrystalline diamond films have been fabricated with a magnetoactive microwave chemical vapor deposition system. The top and bottom magnet currents were 145 and 60 A, respectively, while the microwave power and substrate temperature were controlled at 1500 W and 850 °C, respectively during deposition. The total processing pressure was regulated at 40 Pa (300 mTorr) with gas-flow rates of 30 sccm of hydrogen, 2.4 sccm of methane, and 1 sccm of oxygen. Diamond films obtained under these conditions have grain sizes between 0.1 and 0.3 μm, and a mean roughness of 14.95 nm. The growth rate is 0.1 μm/h. Characterization techniques have involved x-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Both x-ray and electron diffraction patterns show no evidence of graphitic phase. Although a high density of twins and stacking faults was revealed by high-resolution electron microscopy, compact diamond grains, and clean intergranular boundaries (no graphitic phase) were observed.

Microscopic investigations of silicon surfaces pretreated for use in diamond deposition

The influence of pretreatment of silicon by diamond powder and cubic coron nitride (cBN) powder, as well as by substrate biasing, was investigated. The uncoated silicon surfaces were investigated by scanning electron microscopy, transmission electron microscopy and electron energy loss spectroscopy. The mechanical pretreatment was carried out in an ultrasonic water bath. Polished silicon wafers were dipped in a beaker containing an ethanol-powder mixture. It has been shown that very small diamond or cBN crystals are pressed into the silicon surface. These crystals are not removable even with thorough post-treatment. The size of the crystals, which were pressed into the silicon surface, is between 2 nm and 50 nm, and the density is in the range 10 9–10 10 crystals cm −2. Increasing pretreatment time causes an increase in crystal number density. All substrates were coated in a microwave chemical vapour deposition system in order to determine the nucleation density. Under the same conditions non-pretreated silicon substrates were coated with the help of substrate biasing. It has been supposed that the nature and the density of residues left on the silicon surface after mechanical pretreatment are important for nucleation. Clusters of diamond microcrystals were formed on the surface of biased pretreated substrates. After deposition a silicon carbide film can be observed in areas not occupied by diamond particles.