H-atom Density Research Articles

Technique for the production, preservation, and transportation of H atoms in metal chambers for processings

Three different coating materials for lowering the H-atom recombination probability on stainless-steel chamber walls were investigated and the results were compared. SiO2 films prepared by natural oxidation of perhydropolysilazane, polytetrafluoroethene (Teflon) films, and H3PO4 coated SiO2 films were used as coating materials. Among them, the SiO2 film was found to be the most useful for this purpose. The densities of H atoms produced by catalytic decomposition of H2 on heated tungsten surfaces were measured by a vacuum-ultraviolet laser absorption technique under various conditions. The H-atom density increased by one order of magnitude with SiO2 and Teflon coating, the former of which is easier to use and more economical. No further increase in H-atom density was observed when the chamber was coated with H3PO4. SiO2 films prepared from perhydropolysilazane were not etched by H atoms. Quadrupole mass spectrometric analysis showed that the production of either SiH4 or H2O is extremely minor. No surface etching was confirmed by x-ray photoelectron spectroscopy (XPS), either. Scanning electron microscopic (SEM) observations showed that the SiO2 films are not porous even after H-atom exposure. It is also suggested that cooling of the chamber walls is important to preserve the H-atom density.

H 2 dilution effect in the Cat-CVD processes of the SiH 4/NH 3 system

Gas-phase diagnostics in the catalytic chemical vapor deposition processes of the SiH 4/NH 3/H 2 system were carried out to examine the effect of H 2 dilution. The decomposition efficiency of NH 3 showed a sharp decrease with the introduction of a small amount of SiH 4, but this decrease was recovered by the addition of H 2 when the NH 3 pressure was low. On the other hand, at higher NH 3 pressures, the decomposition efficiency showed a minor dependence on the H 2 partial pressure. The addition of SiH 4 to the NH 3 system decreases the H-atom density by one order of magnitude, but this decrease is also recovered by H 2 addition. H atoms produced from H 2 must re-activate the catalyzer surfaces poisoned by SiH 4 when the NH 3 pressure is low.

Quantification of Gas-Phase H-Atom Number Density by Tungsten Phosphate Glass

It is shown that H-atom densities in the gas phase can be evaluated by simply measuring the change in optical transmittance of tungsten phosphate glass plates. Tungsten oxide (WO3) doped in phosphate glass plates can be reduced by exposure to H atoms and the degree of reduction can be evaluated from the change in their optical transmittance. The difference in the logarithms of the transmittances before and after the reduction showed a linear dependence on the H-atom density evaluated by a vacuum-ultraviolet laser absorption technique. No change in the transmittance was observed in the absence of H atoms, showing that reduction of WO3 by H2 molecules can be ignored.

Spectroscopic diagnostics and modeling of Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond deposition

In this paper Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% CH4 and H2 percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of 200mbar. Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C2 density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as CH4, C2H2, and C2H6. A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and composition. A relatively high gas temperature ranging between 3000 and 4000K is found for the investigated discharge conditions. The C2 density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between 1013 and 1014cm−3 in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between 1013 and 1015cm−3. The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and C2 population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth.

Effect of laser property fluctuations over multipulses on H atom measurement bytwo-photon absorption laser-induced fluorescence

In recent years the two-photon absorption laser-induced fluorescence technique has widelybeen applied to measure the density of atomic hydrogen. In most studies theinfluence of laser property fluctuations over multipulses in the determination of the Hatom density has not been considered. In this work we derive the effect of suchfluctuations and give a method to experimentally determine the influence of thefluctuations in the laser’s properties over multipulses on the measurement of Hatoms.

Density Distributions of H and H2 in Pulsed Microwavehydrogen Plasmas

Temporal distribution of the H atom and H2 densities in a pulsedmicrowave hydrogen plasmas has been measured simultaneously bytwo-photon absorption laser-induced fluorescence (TALIF). Themeasurement of the H-atom absolute density obtained by NO2titration in a flow tube reactor shows that the density of H2could be determined by the measured effective lifetime of the TALIFsignal via the quenching equation. The H-atom density of about1.5×1015cm-3 in both pulsed and stationary phases doesnot obviously change. It is found that the gas temperature volumeeffect plays an important role in governing the distributions of theH-atom density and it is mole fraction. The calculated gas temperatureis in good consistent with the rotational temperature of H2measured by optical emission spectroscopy in pulsed phase.

New driving parameters for diamond deposition reactors: pulsed mode versus continuous mode

Experimental investigation and modeling of pulsed H2/CH4 plasmas used for diamond deposition are presented. Two plasma configurations are studied : a 2.45 GHz microwave cavity configuration and a 915 MHz surface-wave configuration. Time-resolved measurements of the gas temperature determined from the Doppler broadening of the Balmer ­Ha line, of the H-atom relative density and of the discharge volume (Vpl) are reported. The experimental time-variations of the gas temperature are characterized by a sharp increase at the beginning of the pulse (t 1 ms). The simulations enable us to estimate time-variations of the electron energy distribution function, gas temperature and chemical species densities. The in-pulse steady state temperature obtained from the model is in agreement with the measured one, although a discrepancy is obtained on the shape of the early time-variation. Calculations were carried out in order to study the effects of the in-pulse power, the duty cycle and the off-plasma time on the H-atom and CH3-radical densities. It is seen that, at a constant power density averaged over a period, low duty cycles favor high H-atom and CH3 - radical densities, while too long off-plasma times reduce the H-atom density during the pulse. In addition, the production of H atoms was seen to be governed by thermal dissociation in the 2.45 GHz microwave cavity system, and by electronic impact dissociation in the 915 MHz surface wave system, the latter operating under high gas velocities.

On the thermodynamics of MHD wave-heated solar wind protons

It has been clearly observed by the NASA deep-space probes that the solar wind protons do not adiabatically cool as expected towards larger solar distances, but appear to be heated by non-collisional energy sources. In some papers these heating sources were directly or indirectly ascribed to pick-up ions incorporated as suprathermal ions into the background solar wind. Neutral interstellar H-atoms penetrate into the inner heliosphere and at ionization they are converted into pick-up ions. Here we do not consider how the magnetized solar wind flow incorporates these ions into the plasma bulk when enforcing their co-motion. We simply take the first step of their incorporation for guaranteed, namely the fast redistribution of pick-ups from an initially unstable toroidal to a quasistable bi-spherical distribution. The free energy lost by pick-ups during this redistribution goes into the turbulent MHD waves, and as such cascades down to the proton dissipation scale and finally is absorbed by solar wind protons. Here we investigate the thermodynamics of solar wind protons being heated by absorption of this free energy of pick-ups. In addition we also consider as a relevant and competing proton heat source the heating due to absorption of wave energy of convected MHD turbulences, showing that the latter source always dominates inside some critical solar distance, whereas the first one dominates in the outer heliospheric regions. We then solve the resulting differential equation for the solar wind proton temperature and show in the solutions obtained that a quasipolytropic behaviour of the solar wind protons with a distance-dependent polytropic index is found. The expression for the pressure clearly shows the change from an adiabatic to a quasipolytropic behaviour with a decreasing polytropic index at increasing distances as observed by the VOYAGERs. The quantitative run of the temperature and the polytropic index with solar distance thereby is strongly influenced by the interstellar H-atom density. The (pick-up ion)-induced heating also evidently leads to a wind-asymmetric solar wind temperature distribution with higher temperatures occuring in upwind direction compared to downwind direction.

Heliospheric pick-up ions influencing thermodynamics and dynamics of the distant solar wind

Abstract. Neutral interstellar H-atoms penetrate into the inner heliosphere and upon the event of ionization are converted into pick-up ions (PUIs). The magnetized solar wind flow incorporates these ions into the plasma bulk and enforces their co-motion. By nonlinear interactions with wind-entrained Alfvén waves, these ions are then processed in the comoving velocity space. The complete pick-up process is connected with forces acting back to the original solar wind ion flow, thereby decelerating and heating the solar wind plasma. As we show here, the resulting deceleration cannot be treated as a pure loading effect, but requires adequate consideration of the action of the pressure of PUI-scattered waves operating by the PUI pressure gradient. Hereby, it is important to take into proper account the stochastic acceleration which PUIs suffer from at their convection out of the inner heliosphere by quasi-linear interactions with MHD turbulences. Only then can the presently reported VOYAGER observations of solar wind decelerations and heatings in the outer heliosphere be understood in view of the most likely values of interstellar gas parameters, such as an H-atom density of 0.12 cm-3 . Solar wind protons (SWPs) appear to be globally heated in their motion to larger solar distances. Ascribing the needed heat transfer to the action of suprathermal PUIs, which drive MHD waves that are partly absorbed by SWPs, in order to establish the observed SWP polytropy, we can obtain a quantitative expression for the solar wind proton pressure as a function of solar distance. This expression clearly shows the change from an adiabatic to a quasi-polytropic SWP behaviour with a decreasing polytropic index at increasing distances. This also allows one to calculate the average percentage of initial pick-up energy fed into the thermal proton energy. In a first order evaluation of this expression, we can estimate that about 10% of the initial PUI injection energy is eventually transfered to SWPs independent of the PUI injection rate.

Direct detection of H atoms in the catalytic chemical vapor deposition of the SiH4/H2 system

The absolute densities of H atoms produced in catalytic chemical vapor deposition (Cat-CVD or hot-wire CVD) processes were determined by employing two-photon laser-induced fluorescence and vacuum ultraviolet absorption techniques. The H-atom density in the gas phase increases exponentially with increases in the catalyzer temperature in the presence of pure H2. When the catalyzer temperature was 2200 K, the absolute density in the presence of 5.6 Pa of H2 (150 sccm in flow rate) was as high as 1.5×1014 cm−3 at a point 10 cm from the catalyzer. This density is one or two orders of magnitude higher than those observed in typical plasma-enhanced chemical vapor-deposition processes. The H-atom density decreases sharply with the addition of SiH4. When 0.1 Pa of SiH4 was added, the steady-state density decreased to 7×1012 cm−3. This sharp decrease can primarily be ascribed to the loss processes on chamber walls.

CVD diamond films: from growth to applications

Abstract The present review provides an up-to-date report on the main potential of CVD diamond films for industrial applications as well as on recent basic research which seeks to understand diamond deposition microwave plasma reactors. This review includes firstly an overview of diamond film applications. Elements which explain variations in diamond film characteristics as a function of synthesis conditions are given. Also experimental results are reported which show variations in diamond characteristics (quality, microstructure, growth rate, growth mechanisms) as four plasma variables (pressure, power, percentage of methane, substrate temperature) are systematically changed. In the second part, we discuss the effects of these variables on local parameters such as electron temperature, gas temperature, carbon-containing species and H-atom densities. Finally, based on these results, relationships between key local parameters and diamond characteristics are established and discussed.

Diagnostic of a pulsed CH 4–H 2 plasma to improve microwave plasma assisted chemical vapour deposition process for diamond synthesis

A microwave plasma assisted chemical vapour deposition (MWPACVD) process used for diamond growth was studied under continuous wave (CW) and pulsed mode. Depending on the plasma conditions, it is shown that the discharge exhibits two different regimes. One is characteristic of a resonant cavity and allows a stationary wave to be created in the centre part of the reactor. Thus the discharge can be placed in the middle of the tubular reactor, just above the treated substrate. Within the second regime, the microwave power is absorbed as soon as it enters the reactor, thus implying a displacement of the plasma ball on the side wall of the reactor. These two regimes are explained by considering the wave propagation according to the plasma parameters, especially the electron density and the collisions through the gas pressure. It is pointed out that the pulsed mode allows the input peak power to be higher than in CW operation and consequently to increase the H-atom density in the close vicinity of the substrate. Experiments with specific conditions have shed light on contradictory results dealing with pulsed MWPACVD processes.

Excited species in H 2, N 2, O 2 microwave flowing discharges and post-discharges

The density of O and H atoms have been measured by emission spectroscopy and by catalytic probes in a flowing post-discharge reactor. In the post-discharge reactor at 1 torr and a time of 2–3×10 −2 s after a 2.45-GHz Ar10%O 2 discharge (quartz tube of diameter 6 mm) at 60 W, approximately the same values of 1(±0.3)×10 15 cm −3 oxygen atom densities were obtained by NO titration and by a Ni catalytic probe. In ArH 2 post-discharge, the Ni catalytic probe gave a H-atom density of (1–1.2)×10 15 cm −3 for an Ar4%H 2 gas mixture at 3 torr and a time less than 10 −3 s after a microwave discharge of 50 W.

Global Model of Plasma Chemistry in a High Density Argon/Hydrogen Discharge

A mathematical Model is developed to evaluate the influence of operating parameters on hydrogen dissociation and the nature of the major species present in Ar/H2 plasmas. The Boltzmann equation is solved to obtain the electron energy distributions. Mass and energy conservation equations are then solved simultaneously to calculate the species density and electron temperature as a function of pressure, power, and feed composition. Good agreement is found between the model results and experimental measurements. The primary effect of argon addition to hydrogen plasma is to increase the electron density, which in turn enhances the degree of hydrogen dissociation. It is also found that Ar/H2 plasma retains the basic properties of hydrogen plasma even for feed gases containing more than 70% argon. That is, values of electron temperature and H-atom density are similar, and the dominant ionic species is H3+ ion.

Anomalous atomic hydrogen shock pattern in a supersonic plasma Jet

A two-photon laser-induced fluorescence study on the transport of ground-state atomic hydrogen in a supersonic plasma jet, generated from an Ar-H (2) mixture, reveals an unexpected shock pattern. Whereas both the axial-velocity profile and the temperature profile of hydrogen atoms along the jet centerline can be interpreted in terms of a supersonic expansion of an Ar-H gas mixture, the H-atom density profiles do not satisfy the well established Rankine-Hugoniot relation leading to a nonconservation of the forward flux. The experimental results show that H atoms escape from the supersonic expansion by a diffusion process due to strong density gradients between the core of the jet and its vicinity.

Vacuum ultraviolet absorption spectroscopy employing a microdiacharge hollow-cathode lamp for absolute density measurements of hydrogen atoms in reactive plasmas

We have developed a measurement technique for absolute H-atom densities in process plasmas using vacuum ultraviolet absorption spectroscopy employing a high-pressure microdischarge hollow-cathode lamp (MHCL) as a Lyman α (Lα, 121.6 nm) emission light source. Characterization of the Lα emission-line profile could be simplified by using a high-pressure discharge at about 1 atm. The effect of self-absorption in the MHCL was reduced to an insignificant level by decreasing the H2 partial pressure. The contribution of the collisional broadening to the Lα emission profile was estimated from the saturation characteristics of the absorption intensity when the optical thickness of the plasma was varied. The technique was applied to the measurement of the absolute H-atom density in an inductively coupled H2 plasma.

Nonlinear optical generation in a hydrogen discharge

Sum-frequency generation with electromagnetically induced transparency (EIT) has been examined in a hydrogen discharge. EIT is demonstrated in the $3p\ensuremath{-}1s$ and $4p\ensuremath{-}1s$ transitions at 102.6 and 97.3 nm, respectively, with increased efficiency of generation arising from higher H-atom density, although competing effects due to the presence of ${\mathrm{H}}_{2}$ distort the line shapes of these spectra. Only for shorter-wavelength transitions (such as $7p\ensuremath{-}1s$ and $8p\ensuremath{-}1s)$ that show very large shifts and distortions is there a decrease in intensity in comparison with generated radiation from H atoms external to the discharge.

Non-Maxwellian effect on species and energy transport in moderate pressure H2 plasmas

An investigation of the electron kinetics, and species and energy transport in moderate pressure H2 plasmas is presented. The two-term expansion of the electron Boltzmann equation is solved for determining the electron energy distribution function (EEDF), which is used to estimate the rates of chemical reactions and of energy transfer between electrons and heavy species. These rates are then used in a one-dimensional transport model, which enables estimation of species concentrations and plasma temperatures. Results show that, in the considered plasmas, the electron-heavy species reaction rates depend mainly on the average electron energy. The assumption of a Maxwellian EEDF leads to a strong overestimation of the ionization degree (which is approximately equal to the electron mole fraction) and to the underestimation of H-atom density and average electron energy. The predicted gas and vibration temperature are only slightly affected by such an assumption.

Diagnostic Experiment and Kinetic Model Analysis of Microwave CH4/H2 Plasmas for Deposition of Diamondlike Carbon Films

Medium-pressure CH4/H2 plasmas were generated in a discharge tube on the axis of a cylindrical 2.45-GHz-microwave TM011-mode-resonance cavity for chemical vapor deposition of diamondlike carbon films. Diamondlike carbon synthesis, plasma diagnostic measurements and kinetic model analyses were conducted to examine the correlation between plasma properties and film features. The characteristics of the calculated H-atom density of the excitation quantum level 4 agreed with those of the measured spectral intensity of the Hβ line. Consequently, high density ratios of C atoms to H atoms were required for deposition of diamondlike carbon. On the other hand, the density ratio of CH3 radicals to H atoms was considered to be closely related to the hardness of deposited diamondlike carbons.

Study of the phosphine plasma decomposition and its formation by ablation of red phosphorus in hydrogen plasma

Mass spectrometry and optical emission spectroscopy have been used to study the chemistry of PH3 plasma decomposition as well as its formation by ablation of red phosphorus in hydrogen plasma. It has been shown that PH3 decomposition easily equilibrates at low levels of PH3 depletion (15%–30 %), this depending mainly on the rf power. The ablation of red phosphorus in H2 plasma produces phosphine in significant amount, depending mainly on the total pressure but also on the rf power. It has also been found that H* and PH* emitting species originate not only by the dissociative excitation of H2 and PH3, respectively, but also by the direct excitation of the same species in the ground state. Considerations are developed on how to derive the H-atom and PH radical densities by actinometry, under specific experimental conditions. Besides, the linear dependence of PH3 formation rate, rPH3, on H-atom density, [H], leads to the definition of the kinetic equation rPH3=k[H], and to the hypothesis that the formation of PH radical on the surface or its desorption is the dominant mechanism for PH3 production.