Methane-hydrogen Plasma Research Articles

Certification of diamond HPHT-plates for their use as substrates for CVD-synthesis

Incoming control of substates and gases is very important part of technological chain. The properties of diamond layer on substrates with different misorientation angles may differ considerably, therefore, determining the deviation of diamond growth plane (100) from the direction [100] is important. Also, in order to select diamond substrates that are most suitable for synthesis of pure quality crystal, it is required to determine the content of impurities in them and the roughness of their surface. In this research, HPHT-plates of one lot were examined using Fourier transform infrared spectrometry, high-resolution diffractometry and scanning probe microscopy in order to classify them according to the studied parameters. Further the classified diamond substrates were used to synthesize diamond layers. Ardis 300 was used for deposition of monocrystalline diamond; synthesis was carried out from a methane-hydrogen plasma with 3% methane at a temperature of 1100 °C, a pressure of 26.7 kPa and a power of 3800 W. The studies have shown that certification of diamond HPHT-plates allows to select substrates for the synthesis of pure quality monocrystalline diamonds.

Single crystal diamond growth by MPCVD at subatmospheric pressures

Microwave plasma assisted chemical vapor deposition (MPCVD) is the established technique to produce high quality single crystal diamond (SCD). While typical pressures for SCD growth regimes in methane-hydrogen plasma are currently within 100–300 Torr, a transition to much high pressures promises enhanced growth rates. Here, we report on successful SCD synthesis by MPCVD in CH4-H2 gas mixtures at pressures up to 600 Tорр. A strong change of the plasma shape and volume (the latter shrinks by 10 times at fixed MW power) with pressure rise from 100 to 600 Torr was observed, still keeping the plasma stable. The record high absorbed MW power density of ≈1800 W/cm3 was achieved at 600 Torr. Optical emission spectroscopy (OES) was used for the plasma analysis via monitoring emission intensities of radicals Hα, C2 and CH. The gas temperature Tg determined from analysis of rotational fine structure of OES Swan transitions of dimer C2 (516 nm) turned out to be essentially constant ∼ 3100 ± 150 K over the pressure range explored. The diamond growth rate is found to increase by an order of magnitude with pressure to achieve 57 μm/h at 500 Torr at relatively low (4%) CH4 concentration, as measured in situ using low-coherence interferometry, but declined at further pressure increase. The produced films were characterized with SEM, XRD, Raman and photoluminescence spectroscopy, and a high/moderate quality of the obtained material was confirmed.

Specific surface defects arising from diamond CVD-synthesis from methane-hydrogen plasma

Diamond is unique material that have long been known to humanity. It has extra high hardness, thermal conductivity, as well as unique electrophysical characteristics that can be managed by changing its impurity composition. Therefore, this material has gained the potential of widespread use in engineering, which is currently complicated by the high cost of diamonds, difficulties of its synthesis, mechanical and physical processing. A promising direction for obtaining high-quality poly- and single crystals and, accordingly, expanding the areas of diamond use is the method of its deposition from the gas phase (CVD). The main problem that researchers face during deposition the diamond is the formation of surface defects in the growth zone. In this work, we discuss defects observed on single-crystal diamond CVD-films of 300-500 μm thick obtained from methane-hydrogen plasma with 3% methane. The surface was studied by scanning electron microscopy. As a result of the work, we revealed that during the diamond films CVD-growth mainly three types of defects are formed: etch pits, isolated diamond particles (“straying” diamonds) and coarse-grained diamond formed along the diamond plates periphery. Methods for eliminating these defects are proposed.

Express-analysis of nitrogen content in CVD-diamonds by FTIR-spectrometry

Natural and synthetic diamonds most often contain nitrogen impurities, which can form tens of impurity and impurity-vacancy centers in the crystal structure of diamond. At the same time, crystals properties of and possibility of their use in specific application strongly depend on the diamonds impurity composition. In this work, we investigated by the method of Fourier-transform infrared spectrometry (FTIR) the impurity composition of HPHT-diamond substrates and changes in it after the deposition of diamond layers by the CVD-method. We used Ardis 300 for deposition of diamond layers from methane-hydrogen plasma with 3% methane at a temperature of 1100 °C, a pressure of 26.7 kPa and a power of 3800 W on diamond HPHT-substrates containing from 70 to 200 ppm of nitrogen. The proposed technique makes possible to study the integral content of impurities in diamond, which will make it possible to quickly evaluate the result of the diamond layers synthesis and conclude about the possibility of their application in various fields of science and technology, as well as use in jewelry industry.

Polycrystalline diamond CVD-synthesis on modified silicon substrates from methane-hydrogen plasma

The widespread use of diamond in electronics, photonics and other fields of science and technology is currently constrained by the possibility of obtaining high-quality material – polycrystalline diamond CVD-films on silicon, molybdenum, copper and other materials, as well as a small deposition area. Substrates special pretreatment will allow solving both problems. This paper presents the results of the synthesis of several types of polycrystalline diamond film (light-gray, gray and dark-gray) on a modified silicon substrate. We discuss the technical aspects of modifying silicon substrate surface and the synthesis parameters of each type of film. The results obtained in this work allow in the future significantly increase the geometric dimensions of deposited diamond films without reducing their quality, which will make it possible to use them as optical windows.

Equilibrium calculations for plasmas of volatile halides of III, IV and VI group elements mixed with H2 and H2 + CX4 (X = H, Cl, F) relevant to PECVD of isotopic materials

The composition of hydrogen and hydrogen-methane plasmas containing ~ 10% of BX3, SiX4, GeX4 (X = F, Cl), SF6, MoF6 and WF6 is calculated for the temperature range ~ 300–4000 K using the equilibrium chemical model. The calculations provide valuable information about thermodynamic parameters (pressure, temperature) needed for condensation of pure elements (in H2 plasma) and their carbides (in H2 + CH4 plasma) and about intermediate reaction products. Using volatile fluorides for plasma chemical deposition alleviates obtaining monoisotopic elements and their isotopic compounds because fluorine is monoisotopic. PECVD is promising method for one-step conversion of fluorides to elemental isotopes and their carbides. For fluorides, further insight is needed into properties of plasmas supported by different types of discharges.

Self-consistent simulation studies on effect of methane concentration on microwave assisted H2-CH4 plasma at low pressure

Self-consistent simulations of low-pressure hydrogen-methane plasmas in a microwave-resonating chamber have been conducted. The dynamics of the plasma are dictated by the dominant ion species in the reactor, which is affected by the methane chemistry. Even small concentrations of methane have substantial effects on the plasma, and self-consistent simulations are necessary to capture the wave plasma interaction. In particular, the input power at which the transition between two different coupling regimes, namely a stable single plasma ball regime and a two ball unstable and undesirable plasma regime, takes place depending on the methane composition. The first experimental results show good agreement with the simulations.

Effect of methane on stability of plasma in a MW-assisted hydrogen-methane plasma

Effect of methane on stability of plasma in a MW-assisted hydrogen-methane plasma

Experimental study of plasma decay in pulsed microwave discharges of H2, CH4 and their mixtures

The results of the experimental investigations of the plasma decay in X-band pulsed microwave discharges produced by surface waves in hydrogen and hydrogen–methane mixtures under pressures of 0.5–5 Torr are presented. The electron density in the decaying plasma was measured by the resonator method using a high-Q quasioptical resonator. These investigations demonstrated a significant influence of ion conversion processes on the composition of ions in a hydrogen–methane plasma. Such conversion processes result in a change in ionization-recombination balance, which, in turn, influences the parameters of the discharge plasma. Ion recharging processes quickly convert the ‘primary’ ions in hydrogen into ions. It was found that the addition of even a few percent of CH4 to hydrogen, after a sequence of ion-molecular reactions, leads to the formation of complex ions Cx. Recombination coefficients and the dominant type of ions in hydrogen and hydrogen–methane plasma are reported.

Gas mixture influence on the reactive ion etching of InSb in an inductively coupled methane-hydrogen plasma

In this paper, inductively coupled plasma etching of InSb material has been investigated using methane–hydrogen chemistry. Plasma conditions were first studied in terms of bias autopolarization, partial methane quantity in a CH4/H2 mixture and chamber pressure. The surface morphology of the etched samples was analyzed using an atomic force microscope, scanning electron microscope and x-ray photoelectron spectrometry (XPS) measurements. The results highlight the difficulties in removing etching products related to In, and the surface roughness is mainly correlated with the methane ratio in the mixture. The best surface stoichiometry, with a surface roughness of 7 nm and an etch rate of 110 nm min−1, was obtained with the addition of argon. To evaluate the feasibility of high performance infrared photodiodes, InSb monopixels were fabricated by dry etching, electrically characterized under illumination and compared with devices obtained by wet etching.

Correlation between optical emission spectra and the process parameters of a 915 MHz microwave plasma CVD reactor used for depositing polycrystalline diamond coatings

In this paper, the hydrogen and hydrogen-methane mixed plasma have been generated inside a 33 cm diameter quartz bell jar with a low power (9 KW) and lower frequency 915 MHz microwave plasma chemical vapor deposition system. The reactor is being used for growing polycrystalline diamond (PCD) over large area (100 mm). The generated plasma is diagnosed by in situ optical emission spectroscopy method with wave length ranging from 200 to 900 nm. The effects of microwave power, chamber pressure and gas concentration on plasma characteristics have been studied in this work. Within the optical range, Balmer H α , H β , C2swan band and CH lines have been detected at the wavelengths of 655.95, 485.7, 515.82 and 430.17 nm, respectively. It has been observed that for hydrogen plasma, the amount of transition from hydrogen atom inner shell 3 to 2 (H α ) is almost constant with increasing microwave (MW) power (from 2000 to 2800 W) and pressure (from 15 to 30 Torr) initially, after that it increases with further increase of MW power and pressure, whereas, the transition from 4 to 2 (H β ) is slowly increased with increasing MW power and pressure. For hydrogen-methane plasma, intensities of C2 swan band, i.e., the transitions from D3π g to A3π μ energy levels, are also increased with the increasing microwave power and reactor pressure. It has been observed that the radicals present in the plasma are affected by variation of different reactor parameters like pressure, MW power, CH4 concentration, etc.

A reduced model for nanoparticle coating in non-equilibrium plasma

In this Letter, a reduced model is developed based on the full model presented earlier [Yarin et al., J. Appl. Phys. 99 (6) (2006) 064310] for the deposition of amorphous hydrogenated carbon onto particles in a methane–hydrogen plasma. The reduced model is developed based on the assumption that, under certain conditions, chemistry may be decoupled from transport. The results from the reduced model are compared to the results from the full model for particle charge and growth rate of the deposited layer. It is shown that the two models are in good agreement for submicron particles that are of interest in nanoparticle coating in low-pressure plasma reactors. The reduced model is computationally far less expensive as compared to the full model and can be implemented for simulation of a large number of nanoparticles in plasma reactors.

Effect of nitrogen and oxygen addition on morphology and texture of diamond films (from polycrystalline to nanocrystalline)

We investigate the effect of simultaneous nitrogen and oxygen addition into conventional methane–hydrogen plasma on morphology and texture of diamond films produced in a high power high pressure 5 kW microwave plasma chemical vapor deposition (MPCVD) reactor. Diverse diamond films ranging from large-grained polycrystalline to nanocrystalline can be achieved by simply changing the amount of nitrogen and oxygen addition, while keeping other parameters the same. This opens a new operating window for tailoring the growth of diamond for different applications. Our study shows that: (1) a small amount of oxygen addition leads to a {111}facet dominated large-grained polycrystalline diamond film of mixed <211> and <110> texture; (2) a small amount of nitrogen addition favours the formation of a <110> textured nanocrystalline diamond film; whereas (3) the coupled effect of simultaneous nitrogen and oxygen addition is in between them. The versatility of nitrogen and oxygen addition on tailoring diamond growth ranging from polycrystalline to nanocrystalline diamonds with different microstructures, morphologies and textures is clearly demonstrated.

Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10 11/cm 2 on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.

Particle growth in hydrogen–methane plasmas

Particle growth and the behavior of particle clouds in hydrogen–methane capacitively coupled rf plasmas are investigated. At room temperature, most for different wall temperatures and gas compositions of these particles are due to flakes of layers delaminated from the electrode surfaces. Heating of the electrodes up to 800 K and dilution by hydrogen (up to H 2:CH 4 = 20:1) suppresses the production of the particles from the electrode surfaces. The electron temperature in the particle levitation region is controlled by introducing an additional electrode made from a grid (= gridded electrode) in between the levitation electrode and the driven electrode. If we introduce diamond seed particles (∼ 2.8 μm in diameter) into the plasma with the gridded electrode in place, we observe nucleation of new grains (∼ 100 nm) on the surfaces of the diamond particles. On the other hand, without the gridded electrode, we do not observe nucleation but growth of amorphous carbon films on them.

Development of Laser Photodetachment Technique Using Heated Probe to Eliminate the Effect of Probe Surface Ablation Phenomena

The characteristics of the phenomena caused by laser irradiation to an electrostatic probe in plasmas are studied to avoid the disturbance of the laser photodetachment signals for negative ion density measurement. In helium–hydrogen and hydrogen–methane plasmas, a probe surface ablation phenomenon was observed as an anomalous excess electron current in response to the laser irradiation to the electrostatic probe, while the phenomenon was not observed in pure hydrogen plasmas. Contaminations of the probe surface appear to be the mechanism causing the ablation phenomena. In order to clean the probe surface, a filament-type heated probe, which is the same type of conventional emissive probe, is applied to the laser photodetachment technique. When the surface is cleaned by heating the probe, the ablation phenomenon disappears, and the negative ion density can be evaluated at a sufficiently high laser pulse energy to saturate the photodetachment rate of negative ions. The method developed in this paper is useful for the measurement of negative ion density in plasmas where the probe surface is easily contaminated.

Field emission characteristics of diamond edge-shaped emitters fabricated using nitrogen–methane plasma

We have previously reported electron field emission characteristics of edge-shaped diamond micropatterned field emission cathodes fabricated from methane–hydrogen plasma deposition having a turn-on field of 6 V/μm. Recent work has shown that nitrogen-incorporated diamond films are highly conductive and exhibit low turn-on field. In this paper, we report the improved field emission characteristics of edge-shaped diamond micropatterned devices fabricated using nitrogen–methane plasma. The edge arrays were fabricated via a mold transfer technique on a silicon substrate utilizing silicon micropatterning and etching, followed by the plasma chemical vapor deposition of diamond. Arrays of micropatterned edge-shaped diamond emitters were formed and tested for field emission. The deposited diamond film was characterized by energy dispersion spectroscopy (EDS), Raman spectroscopy and SEM. Edge emitter diodes fabricated from the nitrogen–methane plasma displayed a low turn-on field of 2 V/μm and a high emission current of 5.3 mA at 8 V/μm.

Diamond nucleation in low electron temperature CH 4/H 2 plasma

Nucleation of diamond crystal is observed on a surface of fine diamond particles of micron meters attached to a nickel plate placed in methane–hydrogen plasma. The electron temperature T e in the plasma is quite important for the nucleation. When T e is reduced to ∼0.4 eV, we observed an appearance of nanometer diamond nucleation on the surface of the fine diamond particles. The size and the number of nucleation cite increase with the deposition time with average growth rate of ∼10 nm/h.

Diamond heteroepitaxy: pattern formation and mechanisms

Pattern formation that occurs during the initial condensation of diamond can provide insights into the physical mechanisms responsible for crystal growth. From a practical perspective, it can be exploited to develop reliable heteroepitaxial growth processes. Using (001) Ir on sapphire (α-Al 2O 3) or strontium titanate (SrTiO 3) as substrate, and a methane–hydrogen plasma in the presence of d.c. bias, we reproducibly generate dense arrays, 10 12 cm −2, of orientationally-ordered (001) diamond nanocrystallites. They appear on the Ir substrate after the rapid quench brought about by abrupt bias termination. The crystallite sizes are highly monodisperse, with typical lateral size below 10 nm. Under the appropriate conditions, the nascent diamond crystals self-organize into a lattice with six-fold coordination and correlation lengths of several lattice parameters. The regularity of the patterns raises the possibility of a collective instability of the excited carbon condensate. Since the crystallite density is so high, subsequent growth leads to rapid coalescence within a few minutes, resulting in a smooth, continuous film that covers the substrate. When the growth step is extended in time, thick single crystal plates have been grown over areas approaching 1 cm 2. Particular emphasis is placed on the time evolution of emergent patterns during the bias stage, early coarsening of diamond grains, and the textures during extended growth. These results are placed in the context of dynamical systems driven far from equilibrium.

High quality diamond formation by electron temperature control in methane–hydrogen plasma

We report a new method for the nucleation and growth of diamonds by employing an electron-temperature control technique in CH4/H2 radio frequency glow discharge plasma under a low gas pressure of 100 mTorr. The electron temperature in the plasma is controlled under constant gas pressure in a range from 0.5 to 2.5 eV continuously by changing the open area of the slits situated around a grid that is kept at the floating potential. It is observed that the film quality is changed in accordance with the variation of electron temperature, and we can produce high quality diamond in a low electron temperature plasma, even though usually only graphite film is deposited, unless the electron temperature is controlled.