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Tunable Diode Laser Diagnostic Studies of H 2 -Ar-O 2 Microwave Plasmas Containing Methane or Methanol
Abstract
Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and ten stable molecules in H2-Ar-O2microwave plasmas containing up to 7.2% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbons varied between 30 and 90% and the methyl radical concentration was found to be in the range 1010–1012molecules cm-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C2H2, C2H4, and C2H6, produced in the plasma decayed exponentially when increasing amounts of O2were added at fixed methane or methanol partial pressures. In addition to detecting the hydrocarbon species, the major products CO, CO2, and H2O were also monitored. For the first time, formaldehyde, formic acid, and methane were detected in methanol microwave plasmas, formaldehyde was detected in methane microwave plasmas. Chemical modeling with 57 reactions was used to successfully predict the concentrations in methane plasmas in the absence of oxygen and the trends for the major chemical product species as oxygen was added.
... This situation is typical of the discharge conditions employed in this thesis. Investigations of this complex plasma chemistry are being conducted currently but only few results have been published so far (e.g. in [30,97]). ...
... [73,94,104,108]), due to an increased technological demand as well as the availability of the necessary computing power. Nevertheless there exist only few models that describe the chemistry of hydrocarbon plasmas, and oxygen admixtures have been taken into account only very recently by Fan [29] and Röpcke et al. [97]. ...
... A rather recent model that has been developed since 1998 is used by Röpcke et al. [80,97] employing a sophisticated time-dependent Boltzmann solver with a multi-term approximation for the electron kinetics. Only cross sections of methane and hydrogen are employed. ...
... Basically this diagnostic method is not restricted to a specific type of plasma. It was used to determine neutral gas temperatures [1] and to investigate dissociation processes [2][3][4][5]. Due to their small laser line width (about 10 -4 cm -1 ) the lead-salt diode lasers used in the mid infrared region are well suited for high resolution spectroscopy purposes, e.g. of low molecular weight free radicals and molecular ions [6][7][8][9]. One of the most successful applications of TDLAS is for studying the decomposition of hydrocarbons in a variety of PECVD processes. ...
... One of the most successful applications of TDLAS is for studying the decomposition of hydrocarbons in a variety of PECVD processes. Recently, systematic investigations of plasma chemistry and kinetics in plasmas containing hydrocarbons have been published [2,10]. Outside of plasma diagnostics this technique has been used successfully in the field of atmospheric trace gas monitoring and for exhaust gas monitoring of on-road vehicles, e.g. ...
... The experimental set-up of the used TDL arrangement and the microwave reactor is shown in figure 1. Experimental details of this set-up can be found elsewhere. [2,13] The infrared diode laser beam entered the plasma chamber via a KBr window and passes twice through the plasma region. A monochromator in front of the HgCdTe detector was used as a mode filter. ...
In this contribution tunable diode laser absorption spectroscopic (TDLAS) studies of hydrocarbon-containing H2-Ar-N2-discharges in a planar microwave reactor (f= 2.45 GHz, P=1.5 kW) under static conditions are presented. Measurements were performed using various ratios of H2 to N2 and small admixtures of some percents of methane or methanol at a discharge pressure of 1.5 mbar. TDLAS was used to determine the degree of dissociation of the precursor hydrocarbons methane or methanol as well as the concentrations of the produced molecules like acetylene, ethylene, ethane, hydro cyanic acid, ammonia and the methyl radical. The degree of dissociation of methane was found to be between 70 to 95 %, showing a considerable increase at small nitrogen contents. Methanol dissociated almost completely (about 97 %) for all H2-N2-admixtures.
... TDLAS is increasingly being used in the spectral region between 3 and 20 µm for measuring the concentrations of free radicals, transient molecules and stable products in their electronic ground states. TDLAS can also be used to measure neutral gas temperatures [2] and to investigate dissociation processes of molecular low temperature plasmas [3][4][5][6]. The main applications of TDLAS until now have been for investigating molecules and radicals in fluorocarbon etching plasmas [2,5,7], in plasmas containing hydrocarbons [6, 8-14, 60,63] and nitrogen, hydrogen and oxygen [58,59,61,62]. ...
... A mirror spacing of 1.5 m and 40 passes gave an effective absorption length of 60 m. Details on the experimental setup, data acquisition and data processing can be found elsewhere [6,17,18,21]. The pressure was kept constant at 1.5 mbar during the experiments. ...
... The behaviour of the carbon containing molecules mentioned above can be understood qualitatively in terms of a model which was developed for a H 2 /CH 4 /O 2 plasma [6,23]. Although the bath gas was changed (Ar instead of H 2 ) the major reactions for the conversion of CH 4 (eq. ...
Within the last decade mid infrared absorption spectroscopy between 3 and 20 μm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has lead to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Information about gas temperature and population densities can also be derived from IRLAS measurements. A variety of free radicals and molecular ions have been detected, especially using TDLs. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes. The aim of the present article is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals, and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid infrared.
... Tunable diode laser absorption spectroscopy (TDLAS) in the mid-infrared spectral region between 3 and 20 µm is a non-invasive technique for measuring number densities of stable molecules and radicals. TDLAS can also be used to determine neutral gas temperatures [1] and to investigate dissociation processes [2][3][4][5]. Due to their small laser line width (about 10 −4 cm −1 ) the lead-salt diode lasers used in the mid-infrared region are well suited for high resolution spectroscopy purposes, e.g. of low molecular weight free radicals and molecular ions [6][7][8][9]. ...
... One of the most successful applications of TDLAS is for studying the decomposition of hydrocarbons in a variety of PECVD processes. Recently, systematic investigations of plasma chemistry and kinetics in plasmas containing hydrocarbons have been published [2,10]. Outside of plasma diagnostics this technique has been used successfully in the field of atmospheric trace gas monitoring and for exhaust gas monitoring of on-road vehicles, e.g. ...
... The temperature of the lasers could be controlled at milli-Kelvin precision in the range between 25 and 80 K. The infrared laser beam [28] Used for measurements CO (2) 2073.4892 6.640 × 10 −22 [28] Used for identification CO (3) 2073.5252 ...
The time and spatial dependence of the chemical conversion of CO2 to CO were studied in a closed glow discharge reactor (p = 50 Pa, I = 2-30 mA) consisting of a small plasma zone and an extended stationary afterglow. Tunable infrared diode laser absorption spectroscopy has been applied to determine the absolute ground state concentrations of CO and CO2. After a certain discharge time an equilibrium of the concentrations of both species could be observed. The spatial dependence of the equilibrium CO concentration in the afterglow was found to be varying less than 10%. The feed gas was converted to CO more predominantly between 43% and 60% with increasing discharge current, forming so-called quasi-equilibrium states of the stable reaction products. The formation time of the stable gas composition also decreased with the current. For currents higher than 10 mA the conversion rate of CO2 to CO was estimated to be 1.2×1013 molecules J-1. Based on the experimental results, a plasma chemical modelling has been established.
... The diagnostic allows monitoring of densities without disturbing the system making its application indispensable in many areas of fundamental and applied research [6]. For example, recently tunable diode laser absorption spectroscopy (TDLAS) measurements were performed in plasmas at very different conditions [7][8][9][10]. The narrow linewidth of the laser provides high resolution measurements and makes an in-depth analysis of the line profiles possible. ...
Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.
... The results originally presented a schematic pathway for direct methane decomposition in an audio-frequency plasma. Ropcke et al. [27] developed a chemical model with 57 reactions to generally predict the species concentrations and major reaction scheme in a H 2 -O 2 -CH 4 microwave plasma. The later modelling works by Nair, Tomohiro Nozaki [28,29], Yiguang Ju and Wenting Sun [3,23], Young-Hoon Song [30] and others [31][32][33][34][35][36][37], presented more detailed reaction schemes for direct methane or methane oxidative conversion in non-thermal as well as thermal plasmas, and obtained good agreement with the experimental values for conversion and some key species concentration. ...
The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH4 mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H2, C2 and C3 hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C2 and C3 formation dramatically decreased, and syngas (H2 and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH4 mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH3, CH2, CH, H, O and O (¹D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.
... SNR issues can be improved by using multi-pass cells, often at the expense of spatial resolution [27][28][29][30]. As alternatives, cavity ring-down spectroscopy (CRDS) [14,[31][32][33][34][35][36] and multi-pass tunable diode laser absorption spectroscopy (TDLAS) absorption methods [37][38][39][40][41][42][43][44][45][46] have been developed to obtain high SNR when concentrations are very small. ...
Accurate measurement techniques for in situ determination of soot are necessary to understand and mon- itor the process of soot particle production. One of these techniques is line-of-sight extinction, which is a fast, low- cost and quantitative method to investigate the soot volume fraction in flames. However, the extinction-based technique suffers from relatively high measurement uncertainty due to low signal-to-noise ratio, as the single-pass attenuation of the laser beam intensity is often insufficient. Multi-pass techniques can increase the sensitivity, but may suffer from low spatial resolution. To overcome this problem, we have developed a high spatial resolution laser cavity extinction technique to measure the soot volume fraction from low- soot-producing flames. A laser beam cavity is realised by placing two partially reflective concave mirrors on either side of the laminar diffusion flame under investigation. This configuration makes the beam convergent inside the cavity, allowing a spatial resolution within 200 µm, whilst increasing the absorption by an order of magnitude. Three different hydrocarbon fuels are tested: methane, propane and ethylene. The measurements of soot distribution across the flame show good agreement with results using laser- induced incandescence (LII) in the range from around 20 ppb to 15 ppm.
... Tuneable infrared diode laser absorption spectroscopy (TDLAS) in the mid infrared spectral region between 3 and 20 µm, using lead salt lasers, is a well known non-invasive technique for the detection and the measurement of stable and radical molecular species not only in the gas phase but in gas discharges as well [1,2]. It can also be used to determine neutral gas temperatures and to investigate dissociation processes in a wide range of molecular discharge plasmas. ...
Quantum cascade lasers absorption spectroscopy in the mid IR can measure absolute concentrations of plasma or gas species for a large variety of applications. Recently, a compact quantum cascade laser measurement and control system (Q-MACS) has been developed for time-resolved plasma diagnostics, process control and trace gas monitoring. The Q-MAC System contains a tuneable quantum cascade laser which can be directed through a plasma or into a multi path cell for exhaust or trace gas detection.
... Based on the TDLAS method, online measurements with temporal resolutions down to the µs range can be performed [28]. Such time-resolved observations of concentration changes of molecular species, e.g. after abrupt changes in experimental conditions, can provide valuable information about kinetics in the plasma and can be used to estimate gas-phase reaction constants or surface reaction probabilities [29][30][31][32]. ...
Low-pressure pulsed dc H2?N2 plasmas with admixtures of CH4 or CO2 used for conventional (CPN) and active screen plasma nitriding (ASPN) were studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were performed in two modes: (i) the plasma at an internal model probe, driven by a bias voltage operated only, representing a CPN approach, and (ii) the screen plasma operated only, which corresponds to an ASPN experiment. Combining in situ tunable diode laser absorption spectroscopy with ex situ Fourier transform infrared spectroscopy the evolution of the concentrations of the methyl radical and of eight stable molecules, C2H2, CH4, C2H4, CO, CO2, NH3, HCN and H2O was monitored. The degree of dissociation of the carbon-containing precursor molecules varied between 40% and 98%. The methyl radical concentration was found to be in the range 1011?1012?molecules?cm?3. By analysing the development of molecular concentrations with changes in gas mixtures and plasma power values, it was found that (i) C2H2, HCN and NH3 were the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF (CH4)???(0.5?4)?×?1016?molecules?J?1, RF (CO2)???(0.5?3.4)?×?1016?molecules?J?1) and the respective conversion efficiencies to the product molecules (RC (product) ??1013?1015?molecules?J?1) were determined for different gas mixtures and plasma power values. With the help of OES the rotational temperature of the screen plasma could be determined, which increased with power from 600 to 850?K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the
-(0?0)-band, increased strongly in relation to the intensity of the neutral component, represented by the N2-(0?2)-band.
... The TDLAS can also be successfully used to measure neutral gas temperature 30 and to investigate dissociation processes in low-temperature molecular plasmas. [31][32][33] In this work we have used a compact transportable measurement equipment named "Quantum Cascade Laser Measurement and Control System" (Q-MACS) developed by Neoplas Control GmbH. This system provides an effective and reliable real-time plasma diagnostics and process control. ...
Quantum cascade laser absorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticle nucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generating plasmas.
... In plasmas the measurement of absolute molecular species number densities can be done using absorption spectroscopic methods in the mid infrared spectral region. Tuneable infrared diode laser absorption spectroscopy (TDLAS) in the spectral region between 3 and 20 µm is a well known noninvasive technique for the detection and the measurement of concentrations of stable and transient molecular species not only in the gas phase but in gas discharges as well [1,2]. It can also be used to determine neutral gas temperatures and to investigate dissociation processes in a wide range of molecular discharge plasmas. ...
Quantum cascade lasers (QCL) offer attractive options for the application of mid-infrared absorption spectroscopy for industrial process monitoring and control. In particular the in-situ measurement of reactive plasma compounds can give new insight into the reaction kinetics of industrial plasma processes. For these purposes the compact quantum cascade laser measurement and control system (Q-MACS) has been combined with an infrared-fibre to allow a flexible and completely dust-sealed incoupling of the infrared radiation into the reactor chamber of an industrial plasma surface modification system. The system has been completed with a fast and compact detector and a data acquisition system to monitor concentrations of species of interest in the plasma process online. First tests proofed the potential of this approach.
... The mole fraction of water was typically in the order of 1 %. Further details on the experimental setup, data acquisition and data processing can be found elsewhere [28][29][30][31] . It can be seen in figure 4 that the ammonia concentration is quite sensitive to even small amounts of oxygen in the plasma, and as its concentration decreases the NO and OH concentrations increase. ...
Mid infrared (MIR) absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).
In situ measurements are reported giving insight into the plasma chemical conversion of the precursor BCl3 in industrial applications of boriding plasmas. For the online monitoring of its ground state concentration, quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range was applied in a plasma assisted chemical vapor deposition (PACVD) reactor. A compact quantum cascade laser measurement and control system (Q-MACS) was developed to allow a flexible and completely dust-sealed optical coupling to the reactor chamber of an industrial plasma surface modification system. The process under the study was a pulsed DC plasma with periodically injected BCl3 at 200 Pa. A synchronization of the Q-MACS with the process control unit enabled an insight into individual process cycles with a sensitivity of 10⁻⁶ cm⁻¹Hz-1/2. Different fragmentation rates of the precursor were found during an individual process cycle. The detected BCl3 concentrations were in the order of 10¹⁴ moleculescm⁻³. The reported results of in situ monitoring with QCLAS demonstrate the potential for effective optimization procedures in industrial PACVD processes.
The recent development and commercial availability of quantum cascade lasers (QCL) offers an attractive option for mid-infrared absorption spectroscopy. The positive features of QCL systems have opened up new fields of application in research and industry, including studies of gases in atmospheric, environmental and plasma chemistry but also for in-situ control of industrial plasma processes. A compact quantum cascade laser measurement and control system (Q-MACS) based on infrared absorption spectroscopy has been developed for time-resolved plasma diagnostics, process control and trace gas monitoring. Rapid scan software with real-time line shape fitting provides a time resolution up to 1 us to study kinetic processes of infrared active compounds in plasmas or gases. The capabilities of the Q-MAC system have been demonstrated in basic research and in industrial applications for trace gas detection and in plasmas.
The answer to the question "how can we deposit diamond of very high purity/high quality at ultrahigh growth rate by chemical vapor deposition" can be expressed in very simple words: just produce as much as possible hydrogen atoms in a very clean system, and prevent their loss until they get to the growing surface! To that, we will add "manage" carefully hydrocarbon injection into the reactor.This chapter is dedicated to the understanding of how a 2.45GHz microwave (MW) cavity-based diamond deposition reactor working under moderate pressure and high power density in H2-CH4 mixtures operates. The focus is to identify how and where the key species for growing diamond, H atoms and CH3 radicals, are formed and lost into the plasma and at the plasma/surface interface until they reach the diamond surface where they are consumed by surface reactions, including diamond growth. The study is based on plasma modeling and spectroscopic analysis means. It is shown that one of the best ways to increase these species densities at the surface and grow ultrafast diamond is to couple as much as possible MW power, increasing simultaneously the pressure and the power in order to raise the gas temperature and to reduce the boundary layer thicknesses on the diamond surface. However, limitations appear at high power density in terms of thermal management, filamentation of the discharge, and methane addition due to soot formation that prohibits long-time deposition. Other effects are studied such as using pulsed plasmas, adding argon, changing the cavity, and/or its excitation frequency. Comparison is made with the results obtained in fully dissociated plasma flowing at high speed.
InfraRed Tunable Diode Laser Absorption Spectroscopy technique has been implemented in a H2/CH4 Micro-Wave (MW frequency f = 2.45 GHz) plasma reactor dedicated to diamond deposition under high pressure and high power conditions. Parametric studies such as a function of MW power, pressure, and admixtures of methane have been carried out on a wide range of experimental conditions: the pressure up to 270 mbar and the MW power up to 4 kW. These conditions allow high purity Chemical Vapor Deposition diamond deposition at high growth rates. Line integrated absorption measurements have been performed in order to monitor hydrocarbon species, i.e., CH3, CH4, C2H2, C2H4, and C2H6. The densities of the stable detected species were found to vary in the range of 1012–1017 molecules cm−3, while the methyl radical CH3 (precursor of diamond growth under these conditions) measured into the plasma bulk was found up to 1014 molecules cm−3. The experimental densities have been compared to those provided by 1D-radial thermochemical model for low power and low pressure conditions (up to 100 mbar/2 kW). These densities have been axially integrated. Experimental measurements under high pressure and power conditions confirm a strong increase of the degree of dissociation of the precursor, CH4, associated to an increase of the C2H2 density, the most abundant reaction product in the plasma.
Titanium dioxide (TiO2) films are produced by inductively coupled plasma-assisted (ICP) CVD at various H2 flow rates. Anatase and rutile TiO2 films are obtained without any external heating. The surface morphologies, structures, and deposition rates of the TiO2 films are strongly affected by the H2 flow rate. By plasma diagnostics, using a single Langmuir probe (SLP) and optical emission spectroscopy (OES), it is found that the amount of H2 plays a dominant role in determining the deposition rate of the TiO2 films, while the substrate temperature or plasma density do not appreciably affect the deposition rate.
The effect of catalyst deactivation in the plasma catalysis of methanol decomposition is investigated. A dielectric barrier discharge is generated in the Cu/ZnO/γ-Al2O3 catalyst-packed bed reactor. Methanol conversion and generation of CO/CO2 and HC species using the fresh and deactivated catalyst are compared. Catalyst deactivation usually causes a reduction in the synergetic effect of the plasma and catalyst, mainly as a result of changes in the relative generation of primary radicals by the plasma and subsequent interactions between the catalyst-adsorbed and plasma-generated species. The results show that deactivation and changes in catalyst characteristics, as well as active control of the plasma generation technique, can be the controlling parameters in the kinetics of plasma–catalyst reactions.
This paper reviews recent progress in the development of time-resolved diagnostics to probe high-density pulsed plasma sources. We focus on time-resolved measurements of radicals' densities in the afterglow of pulsed discharges to provide useful information on production and loss mechanisms of free radicals. We show that broad-band absorption spectroscopy in the ultraviolet and vacuum ultraviolet spectral domain and threshold ionization modulated beam mass spectrometry are powerful techniques for the determination of the time variation of the radicals' densities in pulsed plasmas. The combination of these complementary techniques allows detection of most of the reactive species present in industrial etching plasmas, giving insights into the physico-chemistry reactions involving these species. As an example, we discuss briefly the radicals' kinetics in the afterglow of a SiCl4/Cl2/Ar discharge.
Infrared diode laser spectroscopy has been used as a diagnostic probe to measure the concentrations of the methyl radical and stable products in an ac methane/hydrogen/oxygen (CH4−H2−O2) plasma. Among the products detected were all of the stable C-2 hydrocarbons and oxygen-containing species including methanol, formaldehyde, formic acid, carbon monoxide, and carbon dioxide. A simple one-dimensional chemical modeling program has been written to calculate and compare the model concentrations of all the detected species with their observed concentrations. Good agreement between these values has been obtained which enables some insights to be gained into the gas-phase mechanism in mixed methane plasmas.
A reactiv molecular discharge with methane and oxygen as feed gases is investigated by laser absorption spectroscopy and a numerical model. Some aspects of the plasma chemistry, specially that of the methyl radical, are discussed and a few remarks are made on the temporal volution of methan and carbon monoxide.
In Ar and He radio-frequency (RF) plasmas with admixtures of C2H2 and CH4 the hydrocarbon chemistry has been studied in relation to dust particle formation by means of infrared tunable diode laser absorption spectroscopy (TDLAS) combined with Fourier transform infrared (FTIR) spectroscopy. The experiments were performed in a RF capacitively coupled parallel plate reactor at a frequency of f = 13.56 MHz, a pressure of p = 0.1 mbar and a flow rate of Φ = 8 sccm of Ar or He with admixtures of 0.5 sccm C2H2 or 1 sccm CH4. The power was P = 15 W. Using TDLAS, the temporal evolution of the concentrations of the methyl radical and of four stable molecules, C2H2, CH4, C2H4 and CO, was monitored in the plasma. Simultaneously, the growth process of the dust particles was analysed by FTIR spectroscopy. The degree of dissociation of the acetylene precursor was found to be nearly constant in the range of 96% under stabilized conditions for both the Ar and He plasmas. In contrast, the degree of dissociation of the methane precursor varied between 45% and 90% depending (i) on the appearance of dust particles in the reactor volume and (ii) on the Ar or He plasma conditions. The methyl radical concentration was found to be in the range of 1011 molecules cm−3. The concentrations of all hydrocarbon species were strongly correlated with the dynamic of the dust formation. Fragmentation efficiencies of acetylene (RF (C2 H2) = 3.2 × 1016 molecules J−1) and of methane (RF (CH4) = (0.16–2.5) × 1016 molecules J−1) and conversion efficiencies to the produced hydrocarbons (RC = (0.23–8.5) × 1014 molecules J−1) could be estimated in dependence on the discharge conditions in the RF plasma.
In situ measurements are reported giving insight into the plasma chemical conversion of the precursor BCl3 in industrial applications of boriding plasmas. For the online monitoring of its ground state concentration, quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range was applied in a plasma assisted chemical vapor deposition (PACVD) reactor. A compact quantum cascade laser measurement and control system (Q-MACS) was developed to allow a flexible and completely dust-sealed optical coupling to the reactor chamber of an industrial plasma surface modification system. The process under the study was a pulsed DC plasma with periodically injected BCl3 at 200 Pa. A synchronization of the Q-MACS with the process control unit enabled an insight into individual process cycles with a sensitivity of 10-6 cm-1\cdotHz-1/2. Different fragmentation rates of the precursor were found during an individual process cycle. The detected BCl3 concentrations were in the order of 1014 molecules\cdotcm-3. The reported results of in situ monitoring with QCLAS demonstrate the potential for effective optimization procedures in industrial PACVD processes.
Polarization-spectroscopy (PS) line shapes and signal intensities are measured in well-characterized hydrogen-air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) pump beam intensities in the counterpropagating pump-probe geometry. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The PS signal intensities are used to measure relative OH number densities in a series of near-adiabatic flames at equivalence ratios (Ï) ranging from 0.5 to 1.5. The use of saturating pump intensities minimizes the effect of pump beam absorption, providing more accurate number density measurements. When calibrated to the calculated OH concentration in the Ï=0.6 flame, the saturated PS number density measurements probing the Pâ(2) transition are in excellent agreement with OH absorption measurements, equilibrium calculations of OH number density, and previous saturated degenerate four-wave mixing OH number density measurements. (c) 2000 Optical Society of America.
Broad band UV-visible absorption spectroscopy is widely used to measure the concentration of radicals in reactive plasmas. We extended the applicability of this technique to the VUV (115 nm to 200 nm), the spectral range in which the electronic transitions from the ground state to the Rydberg or pre-dissociated states of many closed-shell molecules are located. This gives access to the absolute densities of species which do not, or weakly absorb in the UV-visible range. The technique is demonstrated by measuring the densities of HBr and Br2 molecules in HBr high-density ICP plasmas.
A compact and transportable two laser beam infrared (TOBI) system based on infrared absorption spectroscopy has been developed for time-resolved plasma diagnostics. The TOBI system contains two independent tunable diode lasers which can be directed through a plasma or into a multipass cell for exhaust gas detection. Rapid scan software with real-time line shape fitting provides a time resolution up to 10 μs to study chemical kinetic processes of infrared active compounds in plasmas. The capabilities of the TOBI system have been demonstrated in plasmas of pulsed H2–N2 surface wave and in pulsed air–CH4 dc discharges. © 2003 American Institute of Physics.
A compact and transportable infrared multicomponent acquisition (IRMA) system based on infrared absorption spectroscopy has been developed for plasma diagnostics and control. The IRMA system contains four independent tunable diode lasers which can be temporally multiplexed and directed into plasma reactors or into a multipass cell for exhaust gas detection. Rapid scan software with real-time line shape analysis provides simultaneous measurements of the absolute concentrations of several molecular species. © 2000 American Institute of Physics.
The signal generation process in the wavelength modulation absorption spectrometry (WMAS) technique, mainly applied to diode lasers, is scrutinized in detail. The basic foundations of the technique, including its partly unique nomenclature and its relation to conventional absorption spectrometry, are reviewed. Most of the description of the signal generation process is made in terms of a newly developed formalism based on Fourier series. It is shown that the nth harmonic output of the lock-in amplifier in a WMAS instrument is given by the nth Fourier coefficient of the detector signal. This implies that many of the intricate characteristics of the WMAS technique can be derived from various properties of Fourier analysis. Analytical expressions for an arbitrary harmonics of the WMAS signals from Lorentzian, Gaussian and Voigt absorption profiles are given. It is shown how an associated laser-power modulation affects the WMAS signals and how multiple reflections, so-called etalons, give rise to background signals that often limits the applicability of the technique. It is furthermore shown that the traditional description of the WMAS technique, applicable only when small frequency-modulation amplitudes are used and often referred to as derivative spectroscopy, is a subset of the new formalism. Additional features covered are: WMAS signals from multiline transitions; the temperature dependence of the signal; WMAS under optically thick conditions (including the concept of an extended dynamic range); the advantages of multi-harmonic detection; WMAS background signals from frequency-doubled diode laser light; and double modulation techniques. It is also demonstrated that the new formalism can be used to predict the shift of zero crossings of odd harmonics, which is a feature often used for frequency-locking of lasers. Although some of these topics have been discussed previously in the literature, this work presents much new information. It also constitutes the first review of the WMAS technique based upon the new Fourier series-based formalism. (C) 2001 Elsevier Science B.V. All rights reserved.
Cited By (since 1996): 4, Export Date: 13 April 2010, Source: Scopus, CODEN: IJHMA, doi: 10.1016/S0017-9310(01)00008-4, Language of Original Document: English, Correspondence Address: Goldstein, R.J.; Department of Mechanical Engineering, Institute of Technology, University of Minnesota-Twin Cities, 111 Church Street S.E., Minneapolis, MN 55455-0111, United States; email: rjg@me.umn.edu, References: Das, A.K., Sadhal, S.S., Thermal constriction resistance between two solids for random distribution of contacts (1999) Heat and Mass Transfer, 35 (2), p. 101;
Infra-red tuneable diode laser spectroscopy (IR TDLAS) has been used to detect and quantify the methyl radical and three stable carbon-containing species (CH4, C2H2 and C2H6) in a moderate pressure microwave (f = 2.45 GHz) bell-jar reactor used for diamond films deposition. A wide range of experimental conditions was investigated, with typical pressure/power required to perform diamond deposition, i.e. pressure from 2500 to 12 000 Pa and power from 600 W to 2 kW, which means gas temperatures ranging from 2200 to 3200 K, when the power density increases from 9 to 30 W cm−3. Since TDLAS is a line of sight averaged technique, the analysis of the experimental data required the use of a one-dimensional non-equilibrium transport model that provides species density and gas temperature variations along the optical beam. This model describes the plasma in terms of 28 species/131 reactions reactive flow. The thermal non-equilibrium is described by distinguishing a first energy mode for the electron and a second one for the heavy species. Parametric studies as a function of power density and methane percentage in the gas mixture are presented. The good agreement obtained between measurement and one-dimensional radial calculations allows a validation of the thermo-chemical model, which can be used as a tool to enlighten the chemistry in the spatially non-uniform H2/CH4 microwave discharge used for diamond deposition. This is especially of interest for high power density discharge conditions that remain poorly understood.
Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH4, CH3OH, C2H2, C2H4, C2H6, NH3, HCN, CH2O and C2N2, in H2–Ar–N2 microwave plasmas containing up to 7% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20% and 97%. The methyl radical concentration was found to be in the range 1012–1013 molecules cm−3. By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol (RF(CH4) = (2–7) × 1015 molecules J−1, RF(CH3OH) = (6–9) × 1015 molecules J−1) and the respective conversion rates to methane, hydrogen cyanide and ammonia (RCmax(CH4) = 1.2 × 1015 molecules J−1, RCmax(HCN) = 1.3 × 1015 molecules J−1, RCmax(NH3) = 1 × 1014 molecules J−1) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H2–N2–Ar–CH4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed.
In semiconductor processes, reactive plasma is the most important technology for etching, deposition and surface modification of thin films. Radicals have played key roles in plasma processing. In order to realize the high performance of semiconductor processing, important molecular radicals have been measured and their behaviour has been clarified using laser spectroscopic methods such as infrared diode laser absorption spectroscopy, laser induced fluorescence spectroscopy, cavity ring down spectroscopy and recently atomic radicals have also been measured using compact vacuum ultraviolet absorption spectroscopy. Quantitative understanding of kinetics of radicals in plasma will be necessary for nano-scaled semiconductor processing. Their progress is reviewed and the future prospects are presented.
Magnetron plasma processes involving metal atoms and clusters are reviewed. The formation of metal atoms near the cathode and their nucleation in a buffer gas flow are discussed. The flow of a buffer gas with metal clusters through a magnetron chamber disturbs the equilibrium between the buffer gas flow and clusters near the exit orifice and is accompanied by cluster attachment to the chamber walls. Cluster charging far
off the cathode, the disturbance of equilibrium between the buffer gas flow and cluster drift, and the attachment of charged clusters to the chamber walls — the factors determining the output parameters of the cluster beam escaping the magnetron
chamber — are analyzed. Cluster deposition on a solid surface and on dusty plasma particles is considered.
Boron trichloride (BCl3) is used as a source gas in various industrial plasma applications. The online monitoring of its ground state concentration in the plasma process reactor is vital for improved insight into the plasma chemistry and to increase productivity, reliability and reproducibility of the process. Infrared absorption spectroscopy was applied for in situ measurements of absolute densities of BCl3 and HCl in planar microwave plasma sources as well as in an industrial pulsed dc discharge reactor. In this paper procedures for the determination of HITRAN format line strength data files for BCl3 are described together with results of concentration measurements. The results show the influence of the discharge parameters on the dissociation of BCl3 and its conversion to HCl. Observations using tuneable diode lasers are compared with measurements using quantum cascade lasers and the applicability of this diagnostic technique for industrial plasma process monitoring is shown.
Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four stable molecules, CH4, C2H2, C2H4 and C2H6, in a H2 surface wave discharge (f = 2.45 GHz and power density ≈10-50 W cm-3) containing up to 10% methane under different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr). The degree of dissociation of the methane precursor varied between 20% and 85% and the methyl radical concentration was found to be in the range of 1012 molecules cm-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C2H2, C2H4, C2H6, produced in the plasma, increased with an increasing amount of added CH4 as well as with increasing pressure. For the first time, fragmentation rates of methane (RF(CH4) = 1×1015-2.5×1016 molecules J-1) and conversion rates to the measured C-2 hydrocarbons (RC(C2Hy): 5×1013-3×1015 molecules J-1) could be estimated with dependence on the flow and pressure in a surface wave discharge. The influence of diffusion and convection on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered by a simple model.
The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH4, N2 and O2, is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H2O, N2 and H2. The molecules CH4 and O2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH3, NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows Φ(O2)/Φ(CH4) > 0.5.
Three plasma diagnostic methods, tunable infrared diode laser absorption spectroscopy, optical emission spectroscopy and microwave interferometry have been used to monitor concentrations of transient and stable molecules, CH3, CH4, C2H2, C2H6, and of electrons in capacitively coupled CH4-H2-Ar radiofrequency (RF) plasmas (fRF = 13.56 MHz, p = 100 Pa, total = 66 sccm) for various discharge power values (P = 10-100 W) and gas mixtures. The degree of dissociation of the methane precursor varied between 3% and 60%. The methyl radical concentration was found to be in the order of 1012 molecules cm-3 and the electron concentration in the order of 1010 cm-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons, C2H2 and C2H6, produced in the plasma, increased with discharge power. The fragmentation rates of the methane precursor and conversion rates to the measured C-2 hydrocarbons were estimated in dependence on discharge power. Radial distributions of the electron and methyl radical concentrations, and of the gas temperature were measured for the first time simultaneously in the plasma region between the discharge electrodes. The measurements allow us to draw qualitative conclusions on the main chemical processes and the plasma chemical reaction paths.
Tuneable infrared diode laser absorption spectroscopy at 16.5 µm and broadband ultraviolet absorption spectroscopy at 216 nm have both been used to measure the ground state concentrations of the methyl radical in two different types of non-equilibrium microwave plasmas (f = 2.45 GHz): (i) H2–Ar plasmas of a planar reactor with small admixtures of methane or methanol, at a pressure of 1.5 mbar, and (ii) H2–CH4 plasmas of a bell jar reactor, at pressures of 25 and 32 mbar under flowing conditions. For the first time, two different optical techniques have been directly compared to verify the available data about absorption cross sections and line strengths of the methyl radical. It was found that application of the CH3 absorption cross section of the transition at 216 nm, reported by Davidson et al (1995 J. Quant. Spectrosc. Radiat. Transfer 53 581) and of the line strength of the Q(8,8) line of the ν2 fundamental band near 16.44 µm, given by Wormhoudt et al (1989 Chem. Phys. Lett. 156 47), leads to satisfactory agreement.
Time resolved tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four related stable molecules, CH4, C2H2, C2H4 and C2H6, in H2 surface wave plasmas (f = 2.45 GHz, power density ≈10-50 W cm⁻³) containing 10% methane under static conditions at different pressures (p = 0.1-4 Torr). For the first time, the time dependence of the conversion of methane to the methyl radical and three stable C-2 hydrocarbons was studied in a fixed discharge volume nearly up to a stationary state. The degree of dissociation of the methane precursor was found to increase by up to 96% in the stationary state, and the methyl radical concentration was measured to be in the range of 10¹²-10¹³ molecules cm⁻³. The concentrations of both C2H2 and C2H4 produced in the plasma showed a maximum at a distinct time before decreasing. In contrast, the C2H6 concentration was observed to increase with time to a nearly constant value between 6×10¹² and 2×10¹⁴ molecules cm⁻³ varying with pressure.
Based on time resolved concentrations, conversion rates to the measured C-2 hydrocarbons (RC(C2Hy) = 10¹¹-10¹³ molecules J⁻¹) could be estimated in dependence on pressure in a surface wave discharge. The influence of diffusion on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered. A qualitative model has been developed in order to describe the chemical processes and to identify the main plasma chemical reaction paths.
In this paper we present primary hexamethyldisiloxane (HMDSO) molecule dissociation paths in a remote argon-fed expanding thermal plasma, where the HMDSO dissociation is initiated by charge exchange reactions with argon ions followed by dissociative recombination reactions with electrons. Investigation of the argon/HMDSO plasma chemistry by means of cavity ring down spectroscopy and threshold ionization mass spectrometry has allowed the detection and identification of radical species, such as CHx, SiCxHy and SiCHxO, and oligomerization products. The charge exchange reaction rate constant between argon ions and HMDSO molecules has been found to be equal to (4 ± 2) × 10−16 m3 s−1. This reaction has a dissociative character, i.e. the dissociation occurring at the Si–C and Si–O bonds accompanied by the abstraction of radicals, e.g. CH3 and OSi(CH3)3. Under the experimental conditions investigated, the ions produced via charge exchange reactions lead to negligible ion-induced oligomerization routes. Instead, they undergo recombination reactions with electrons, leading to Si–O bond dissociation and further abstraction of methyl radicals.
The densities and temperatures of methane, oxygen, the methyl radical, carbon monoxide and carbon dioxide have been measured using absorption spectroscopy in a capacitively coupled radio frequency (rf) (13.56 MHz) discharge with varied mixtures of methane and molecular oxygen as feed gases under flowing conditions (flow rate ≤10 sccm, total pressure 100 Pa, input power ≤80 W). Whereas neither translational nor rovibrational temperatures rise much above room temperature, up to 90% of the feed gas species are dissociated, and the methyl radical attains densities of the order of 1017-1018 m-3. The observed depletion of the feed gases and the production of methyl as a function of the applied rf power are described consistently by means of analytical expressions derived from simple rate equations of the volume chemistry. Some experimental findings, however, cannot be explained by volume reactions only, indicating that surface processes probably play an important role in the overall plasma chemistry.
In reactive plasma processing, species produced in the plasma reach the surface of a substrate and cause etching, deposition and surface modification through surface reactions. These reactions are characterized by the densities and energies of species incident on the surfaces. In order to realize nano-scale plasma processing, important species for plasma processing have been identified and characterized, and their behavior, not only in the gas phase, but also on the surface, have been clarified and controlled. One of the most critical parameters for insights into surface reaction kinetics of radicals is sticking and surface loss probability. On the basis of radical densities measured by various methods, the sticking and surface reaction loss probabilities have been compiled, and they enable the quantitative understanding of the kinetics of radicals on the surface in the plasma. In this article, the sticking and surface reaction loss probabilities measured thus far are reviewed focusing on fluorocarbon gas, silane gas and methane gas based plasma processes. The establishment of a smart plasma process and the development of an autonomous production device with control of radicals on the basis of insights into the surface reactions for nano-scale plasma processing are presented.
Within the last decade mid-infrared absorption spectroscopy over a region from 3 to 17µm and based on tuneable lead salt diode lasers, often called tuneable diode laser absorption spectroscopy or TDLAS, has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry in molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has led to further applications of TDLAS because most of these compounds and their decomposition products are infrared active. TDLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetic phenomena. Information about gas temperature and population densities can also be derived from TDLAS measurements. A variety of free radicals and molecular ions have been detected by TDLAS. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes. The aim of the present paper is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid-infrared.
Recent studies of phenomena in molecular non-equilibrium microwave plasmas, which have been performed within the framework of the DFG-Sonderforschungsbereich 198, project A12, are presented in this review article. Special attention is devoted to the properties and kinetics of transient molecular species. The experimental methods used were optical emission and absorption spectroscopy. The main objectives of this contribution were threefold: (i) the determination of the degree of dissociation of hydrogen in plasmas, (ii) the quantitative detection of the methyl radical, and (iii) plasma process monitoring using quantum cascade lasers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Within the last decade mid-infrared absorption spectroscopy between 3 and 20 μm, known as infrared laser absorption spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has lead to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes.
The aim of the present article is twofold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas using TDLs and (ii) to report on selected new applications of QCLs in the mid-infrared.
The destruction of methane by a nonthermal plasma in atmospheric pressure gas streams of nitrogen with variable amounts of added oxygen has been investigated. The identities and concentrations of the endproducts are determined by online FTIR spectroscopy and the plasma chemistry is interpreted using kinetic modelling. For a deposited energy of 118kJ m–3, the destruction is 12% in nitrogen decreasing monotonically to 5% in air. The major endproducts are HCN and NH3 in nitrogen and CO, CO2, N2O, NO and NO2 for gas streams containing oxygen. The chemistry in pure nitrogen is predominantly due to reactions of electronicallyexcited nitrogen atoms, N(2D). The addition of oxygen converts the excited state nitrogen into nitrogen oxides reducing the methane destruction which then arises by O and OH reactions yielding CO and, to a lesser extent, CO2. The modelling correctly predicts the magnitude of the methane destruction as a function of added oxygen and the concentrations of the endproducts for processing in both nitrogen and air.
Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and three stable molecules,
CH4, C2H2 and C2H6, in radio frequency plasmas (f=13.56 MHz) containing hexamethyldisiloxane (HMDSO). The methyl radical concentration and the
concentration of the stable hydrocarbons, produced in the plasma, have been measured in pure HMDSO discharges and with admixtures
of Ar, while discharge power (P=20–200 W), total gas pressure (p=0.08–0.6 mbar), gas mixture and total gas flow rate (Φ=1–10
sccm) were varied. The methyl radical concentration was found to be in the range of 1013 molecules cm-3, while methane and ethane are the dominant hydrocarbons with concentrations of 1014–1015 mol cm-3. Conversion rates to the measured stable hydrocarbons (RC(CxHy): 2×1012–2×1016 molecules J-1 s-1) could be estimated in dependence on power, flow, mixture and pressure. Under the used experimental conditions a maximum
deposition rate of polymer layers of about 400 nm min-1 has been found.
Radio frequency plasma-plasma chemistry-hexamethyldisiloxane-laser diagnostics-TDLAS-CH3
-CH4
-C2H6
Within the last decade, mid-infrared absorption spectroscopy between 3 and 20μm – known as infrared laser absorption spectroscopy (IRLAS) and based on tunable semiconductor lasers, namely lead salt diode lasers, often called tunable diode lasers (TDLs), and quantum cascade lasers (QCLs) – has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, and organosilicon compounds has led to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Information about gas temperature and population densities can also be derived from IRLAS measurements. A variety of free radicals and molecular ions have been detected, especially using TDLs. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes.
Radio frequency (rf) generated methane plasmas are commonly employed in the deposition of hydrogenated amorphous carbon (a- C:H ) thin films. However, very little is known about the rf discharge chemistry and how it relates to the deposition process. Consequently, we have characterized a low-pressure methane plasma and compared the results with those obtained theoretically by considering the steady-state kinetics of the chemical processes present in a low-pressure plasma reactor, in order to elucidate the dominant reaction channels responsible for the generation of the active precursors required for film growth. Mass spectrometry measurements of the gas phase indicated little variation in the plasma chemistry with increasing electron temperature. This was later attributed to the partial saturation of the electron-impact dissociation and ionization rate constants at electron temperatures in excess of ∼4 eV. The ion densities in the plasma were also found to be strongly dependent upon the parent neutral concentration in the gas phase, indicating that direct electron-impact reactions exerted greater influence on the plasma chemistry than secondary ion–neutral reactions. © 2003 American Institute of Physics.
Diamond films were successfully synthesized in both parallel-plate radio frequency (rf: 13.56 MHz) CH4 and CH3OH plasmas with injection of H and OH radicals generated in the remote microwave (2.45 GHz) H2/H2O plasma. Effects of H, OH, and CH3 radicals on the diamond film formation in the rf plasma reactor were investigated by the formation of diamond films employing radical injection technique and the measurement of density in the plasma. Under the condition of diamond film formation, CH3 density was measured by infrared diode laser absorption spectroscopy (IRLAS). The kinetics of CH3 in rf CH4 and CH3OH plasmas with injection of H and OH radicals were evaluated from the results of optical emission spectroscopy and lifetime of CH3 radicals estimated by IRLAS.
The influence of N 2 additions (5%–40% N 2 /CH 4 ) on various features of hot‐filament chemical vapor deposition (CVD) of diamond layers is presented and discussed using experimental results and calculations of the thermodynamic equilibrium. Small N 2 additions (5%–10% N 2 /CH 4 ) improved the diamond phase purity but led surprisingly either to an increase or decrease of the growth rate depending upon the filament temperature. The effects are attributed to a reduction of carbon supersaturation due to an abstraction of adsorbed hydrogen atoms caused by CN and HCN. Higher N 2 additions (20%–40% N 2 /CH 4 ) revealed a deterioration of the diamond phase purity and reversal growth rates. These results are probably caused by a beginning reconstruction of the diamond surface which is originated from enhanced abstraction of adsorbed hydrogen and the inefficiency of CN or nitrogen species to stabilize the diamond surface structure. © 1996 American Institute of Physics.
For the first time semiempirical data on the radiative transition probabilities for rovibronic transitions between 3p Σ, Π; 3d Π−,Δ− and 2p Σ, Π, states and radiative lifetimes of upper rovibronic levels of the hydrogen molecule were analysed in detail and presented in a table form useful for applications. The intensities of e3Σu+, d3Πu± → a3ϵg+,i3Πg−, j3Δg− → c3Πu and I1Πg−,J1Δg− → C1Πu band systems have been measured in d.c. capillary-arc and microwave discharges under pressures of 1–600 Pa and a power input of 1–500 W. It was observed, that recalculated for the ground X1Σg+ vibronic state, values of rotational temperature, derived from the populations of e3Σu+, d3Πu+, d3Πu−, j3Δg− and J1Δg− states, coincide within 10–20% and may be used for plasma diagnostics. In contrast, the rotational population density distributions in I1Πg− and i3Πg− states show abnormal behaviour corresponding to effective temperatures, which are about two times higher. The effect needs further investigations in crossed beams and gas-beam experiments.
A new plasma chemical vapor deposition (P-CVD) system was developed for
synthesis of diamond. This system consisted of a parallel-plate radio
frequency (RF) (13.56 MHz) plasma reactor, with a radical source using a
microwave (2.45 GHz) discharge plasma and substrate heating using a
cw-CO2 laser. In this system, hydrogen (H) radicals were
generated in the microwave H2 plasma and preferentially
injected near the substrate in the parallel-plate RF magnetron methanol
( CH3OH) plasma region. By scanning electron microscope (SEM)
and X-ray diffraction (XRD) analyses, it was found that diamond was
successfully synthesized using this system. The effects of H radical on
the diamond formation were also investigated from the results of optical
emission measurements in the RF plasma region, thin-film deposition and
etching of the nondiamond phases by varying amounts of H radical
injection.
The CH3 radical density and the deposition rate of
carbonthin film were measured under the same conditions in RF-discharge
CH4 and CH4/rare gas plasmas. The behavior of the
CH3radical density showed a similar tendency as the
deposition rateof carbon thin film as a function of power and
CH4 pressurein CH4 plasma. In CH4/Xe
plasma, where a selective formation mechanismincreases the
CH3 radical density with increasing Xe dilution whereas other
CH x radicals are expected to decrease, the carbon deposition
rateincreased with increasing Xe dilution. These results strongly
suggestthat the CH3 radical is the dominant precursor in the
film formation.The increase of film formation rate in CH4/Xe
plasma was slower thanthat of the CH3 radical density with
increasing Xe dilution. This couldbe attributed to the sputtering of the
film by heavy Xe ions.In CH4/He plasma, where the effect of
sputtering is expected to be small,the film deposition rate and the
CH3 density varied in a much moresimilar manner.
A direct measurement of the transition dipole moment, μ3, of the degenerate v3 in-plane asymmetric C–H stretching vibration of the methyl radical has been made. The measurements were carried out in a flow reactor using laser-photolysis transient infrared absorption spectroscopy. Cyano (CN) radicals (and Cl atoms) were produced by laser photolysis of BrCN (or ClCN) at 193 nm and reacted with methane to give both CH3 and HCN (and HCl). The intensities of 18 rotational lines of the v3 fundamental band were measured relative to the R(8) line of the C–H stretching vibration (v3) of HCN(001←0). The best estimate of the transition dipole moment of the CH3 (00110←0) transition was provided by the measured line intensity for the CH3 (00110←0)rR(3,3) transition and was determined to be μ3=0.0327±0.0021 D.
Gaseous methyl radical was produced through flash photolysis of methyl iodide and of dimethyl mercury and a portion of its infrared spectrum was recorded (450–740 cm−1) using a rapid scan infrared spectrometer with scan rates up to 1 cm−1∕μsec. Two absorptions were observed for CH3, at 607.0 and 603.3 cm−1, two for CH2D, at 561.2 and 558.0 cm−1, and one for CD3 at 460.6 cm−1, with a second possible feature at 457 cm−1. These frequencies are close to the corresponding matrix frequencies, which permits these bands to be assigned to ν2, the out-of-plane (umbrella) mode of CH3. The possibility that the doublets are due to inversion doubling of a pyramidal molecule is considered but evidence is presented to show that the lower frequency component is due, instead, to vibrationally excited methyl radical. The out-of-plane vibrational potential function is calculated assuming CH3 is planar. If the harmonic oscillator approximation is used, k2=0.174 mdyn∕Å but the CD3 frequency shows that the quartic contribution is not negligible. If a quartic term is included, the force constants obtained are k2=0.137 mdyn∕Å and a=0.098 mdyn∕Å3.
CH3 radical densities in electron cyclotron resonance (ECR) discharge methanol ( CH3OH), methanol diluted with hydrogen gas ( CH3OH/H2) and methane ( CH4) plasmas were measured for the first time using infrared diode laser absorption spectroscopy (IRLAS). CH3 radical densities in the CH3OH and CH3OH/H2 plasmas were estimated to be of the order of 1011 cm-3 under the conditions of total pressure of 1.3 Pa and microwave power of 50-800 W.
On the other hand, CH3 radical density in the CH4 plasma was estimated to be less than 1010 cm-3. Moreover, the production and loss processes of CH3 radical in the CH3OH plasma were discussed on the basis of the results of emission intensity of Ar*, the absorption ratio of CH3OH molecule and the decay curve analysis of CH3 radical after termination of the discharge.
The total dissociation cross section for methane is reported for electron energies between 10 and 500 eV. A maximum cross section of 4×10-16 cm2 occurs at 85 eV. Dissociation into ionic and neutral fragments have about equal probability for energies between 50 and 500 eV. At lower energies most of the fragments are uncharged, ground state, or long lived excited state molecules. The experimental technique is discussed in detail and criteria established for determining gases for which similar methods can be applied.
Shock tube experiments which incorporate improved selection and handling of chemical precursors provide a corrected determination of 216.615 nm absorption coefficient for methyl radicals at high temperatures (1311–2506 K). The revised expression is approximately one-half that given by us previously.
Large-area microwave plasmas generated by special planar microwave plasma sources were tested for their capability to enhance surface cleaning with oxygen plasmas. With a laboratory experimental reactor equipped with only one plasma source and an industrial prototype device equipped with an array of four sources, the influence of parameter variation on organic surface layer removal was investigated and compared with a numerical model. Aluminium foil pieces coated with thick hydrocarbon lacquer layers were used as test objects. Processing conditions combining low pressure and low gas flow rate were identified as the optimum for an advantageous combination of the high reactivity of microwave plasmas with the large plasma area provided by an array of planar plasma sources. With correspondence between experiments and numerical modelling saturation of removal rates at about 100 μm h−1 was observed.
Understanding of specific plasmachemical reactions occurring in plasma sources with the addition of complicated molecules requires knowledge of the particular plasma conditions. The subject of the present paper is a comparison of the spectrometrically measured relative concentration distributions and energies of species in low-pressure argon discharges containing an organosilicon compound, hexamethyldisiloxane (HMDSO), which has a large molecular size. The optical diagnostics were performed with three different plasma sources: a special planar microwave plasma source (v= 2.45 GHz), an r.f. planar reactor (v= 13.56 MHz) and a capacitively coupled r.f. model discharge tube (v= 460 kHz). The energy distribution functions of each plasma are not the same, but their general forms are similar to a Maxwell–Boltzmann distribution. The results of comprehensive, spatially resolved measurements of the relative concentrations of atoms and radicals (H, Si, CH and C2), the neutral species gas temperature, rotational temperature and optical excitation temperature are reported. The use of the same gas mixture in three plasma sources, each of distinct construction, excited by different frequencies, once again clearly indicates that the comparison and interpretation of optical diagnostic results has to be done very carefully, taking into consideration the discharge conditions. Results obtained by actinometry show considerably different particle density gradients in the plasmas. Gradients of the excitation temperatures (Texc= 0.45–0.7 eV) are less, but the neutral gas temperatures also exhibit large spatial gradients (Tg= ambient temperature, about 2000 K). This clearly indicates the absolute necessity of spatially resolved optical emission measurements for the purpose of comparisons between different plasma sources.
The infrared band strength of the ν2 band of the methyl radical has been measured using tunable diode laser absorption by the Q8 (8) line at 608.3 cm−1. Experiments were performed in a discharge-flow apparatus, using the homogeneous recombination decay to quantify CH3 concentrations. The measured line strength at 300 K is (3.2±1.0) × 10−19 cm−1 (molecule/cm2)−1, resulting in a band strength of (2.5±0.8) × 10−17 cm−1 (molecule/cm2)−1. This agrees with an earlier value to within the combined error limits.
Both the CH3 radical density and carbon thin-film formation were investigated in an RF-discharge CH4/H2 plasma. In this plasma, although CH3 radical density was almost constant, the deposition rate decreased markedly with increasing H2 partial pressure. These results suggested that the surface loss of the radicals was decreased due to the change in surface composition of the film, and also the film etching was enhanced with increasing H2 partial pressure. Therefore, the deposition rate of the carbon thin film decreased.
The deposition of hydrogenated amorphous silicon and diamond can be achieved using plasma-enhanced chemical vapor deposition. In order to optimize the deposition process and thus the property profiles of the materials obtained it is important that the gas-phase and surface reactions are understood. The modeling methods and the diagnostic techniques used to follow the processes involved are reviewed and the plasma chemistry of diamond and amorphous silicon deposition compared.
The spectra of many diatomic free radicals like those of CH, OH, NH, C 2 , CN have been known and identified as such almost since the beginning of molecular spectroscopy. They occur readily in emission in a variety of flames (including the ordinary Bunsen burner) and in electric discharges. It was through the study of these spectra that our knowledge of the structure of diatomic molecules was developed and in particular that the different coupling conditions between the electron spin and the other angular momenta were recognized. Most of the resonance transitions of the diatomic free radicals lie in the visible or near ultra-violet region where a detailed high-resolution study is easily possible. It is for this reason also that these spectra play a most prominent part in the spectra of astrophysical sources: comets, low-temperature stars and the interstellar medium. The spectra of free polyatomic radicals have become recognized and analyzed only in the last ten or fifteen years. The reason for this delay is the much greater complexity of the spectra of polyatomic as compared to the spectra of diatomic molecules combined with the greater difficulty of exciting these spectra in emission. Ordinary polyatomic molecules can easily be studied in infra-red absorption or by means of the Raman effect. The infra-red and Raman spectra are simpler than the electronic spectra and their interpretation has been established for a long time; but up to now no infra-red (or Raman) spectrum of a free polyatomic radical has been obtained. One is entirely dependent on electronic spectra for the study of the structure of polyatomic free radicals, and these electronic spectra exhibit many complexities.
Pressure broadening coefficients for an infrared transition of the methyl radical have been measured for the first time. CH3radicals, generated by pyrolyzing di-tert-butyl peroxide in a flow of either N2or Ar, were probed using a tunable diode laser and a multipass absorption cell. The Lorentz half-width of theQ(6,6) line of the ν2band of CH3at 607.024 cm−1was measured as a function of pressure at 295 K. The broadening coefficients (HWHM) areb(Ar) = 0.0310 ± 0.0012 cm−1atm−1andb(N2) = 0.0390 ± 0.0020 cm−1atm−1. These coefficients are lower than those for CH4–Ar, N2broadening. This may be due to a lower polarizability or smaller effective hard collision diameter for CH3relative to CH4.
By using a tunable diode laser, the decay of the absorption intensity with time, after the discharge generating the radical was switched off, was measured for a line [Q4(4)] of the CH3 ν2 fundamental band. When combined with a literature value of the CH3 recombination rate constant, the present result gave an estimate for the steady-state concentration of the CH3 radical. The steady-state absorbance of the Q4(4) line was also measured, and the two results were used to calculate the transition moment of the CH3v2=1←0 band to be 0.280±0.049 D.
Experimental results concerning basic heat transport processes to and from substrates in diamond-forming low-pressure microwave discharges are presented. Special properties of the used planar microwave plasma source allow to distinguish clearly between different transport processes. As a rule, at pressures of few kPa substrate heating is dominated by surface recombination of atomic hydrogen. Substrate cooling is dominated by thermal conductivity of neutral gas. Detailed neutral gas temperature measurements demonstrated the completely non-isothermal character of the plasma with gas temperatures down to ambient temperature. Therefore, a basic problem of achieving very low deposition temperatures is to get detailed information about the concentration of atomic hydrogen that is absolutely necessary for high-quality diamond growth and its effect on substrate temperature.
We have measured relative excitation functions for the production of electronically excited fragments by electron impact dissociation of methane, ethylene and methanol from 0 to 2000 eV. At low impact energies, 10–50 eV, the measured appearance potentials are correlated to specific dissociation limits and, when possible, to specific excited states of the parent molecules. It is found that below ∼25 eV superexcited states play the dominant role in producing these fragments. Fano plots have been constructed to determine the types of molecular excitation involved. In general, most of these excited fragments are produced through optically forbidden transitions.
A single-stage time-of-flight mass spectrometer used in conjunction with resonance enhanced multiphoton ionization has been employed to study the dynamics of surface photodissociation processes as well as methyl radicals produced from a continuous source. By utilizing ion rather than neutral flight times, species that have an impressed velocity along the detection axis can be readily distinguished from species that exhibit an isotropic velocity distribution. This allows for experimental discrimination between photofragments produced from adsorbate photolysis and those produced as a result of probe laser photolysis of gas-phase species photodesorbed from the surface. For species generated in continuous sources, such as methyl radicals produced from azomethane pyrolysis, the same approach permits an unambiguous determination of the total-energy content, despite the presence of additional radicals within the ionizing volume that have scattered from the chamber walls. © 1997 American Institute of Physics.
The v2 = 1←0, 2←1, and 3←2 bands of the methyl radical were observed in the gas phase by infrared tunable diode laser spectroscopy at 606.4531, 681.6369, and 731.0757 cm−1. The observed vibration–rotation spectra were analyzed to derive precise values for the rotational constants, centrifugal distortion constants, and spin–rotation interaction constants. The absence of N = odd, K = 0 levels in the v2 = even states and of N = even, K = 0 levels in the v2 = odd states was confirmed, indicating that each vibrational state is nondegenerate. The observed band origins, when analyzed, led to a potential function for the out‐of‐plane bending vibration that has no potential hump at the planar configuration. The large negative anharmonicity of this potential was ascribed to a vibronic interaction with excited electronic states. The methyl radical was generated by a 60 Hz discharge in di‐tert‐butyl peroxide and was found to have a lifetime of about 1.4 msec; its concentration in the absorption cell was estimated to be about 1013 molecules/cm3, provided that a recombination reaction is the only route for eliminating methyl radicals.
The ν3 fundamental band of the CH3 radical has been detected in absorption with a tunable difference frequency laser. Zeeman modulation is found to be inefficient, because the spin‐rotation interaction is small. The molecular constants in the ν3 vibrational state have been determined with the molecular constants in the ground state fixed at the values determined from the analysis of the ν2 band. The main parameters thus obtained are the band origin ν0=3160.8212(12), B3=9.471 10(14), C3=4.701 67(15), (Cζ)3=0.345 88 (22), and q3=0.006 42(25), all in cm−1 with one standard deviation in parentheses. The transition frequencies and the molecular constants of the ν3 band may be useful in a search for interstellar CH3.
The dynamics for the H+CH4→H2+CH3 reaction has been studied using reduced dimensionality quantum‐mechanical theory. The system is treated as a linear four‐atom chemical reaction, reducing the system to a three‐dimensional scattering problem. The vibrational modes of ν1 and ν4 of CH4, the stretching vibration of H2, and the umbrella ν2 mode of CH3 are taken into consideration in the reaction dynamics based on the vibrational analysis along the reaction path. The semiempirical potential energy surface which has recently been developed by Jordan and Gilbert [J. Chem. Phys. 102, 5669 (1995)] is employed. Rotationally averaged cross sections and thermal rate constants are calculated using an energy‐shifting approximation in order to take into account the effect of all the degrees of freedom. It is shown that excitation of the ν1 mode of CH4 significantly enhances the reactivity, indicating that there is a strong coupling between the ν1 mode of CH4 and the reaction coordinate. The vibrational state distributions for the products H2 and CH3 have also been studied. In the energy range considered here, the population of vibrationally excited H2 is found to be very small, while the umbrella ν2 mode of CH3 is found to be excited. © 1996 American Institute of Physics.
The behavior of the CH3 radical density in a parallel-plate
RF CH4 plasma diluted with rare gases (He, Ne, Ar, Kr, and
Xe) was investigated systematically using infrared diode laser
absorption spectroscopy. The CH3 radical density increased in
CH4/Xe and CH4/Kr plasmas with increasing rare gas
dilution. The Xe* atom densities in the lowest metastable
state 3P2 and the resonant state
3P1 were measured in CH4/Xe plasma
through absorption spectroscopy using a Xe hollow cathode lamp in order
to clarify the role of Xe* (3P2 and
3P1) atoms. It was shown that the increase in the
CH3 radical density in CH4/Xe plasma was mainly
caused by the collision of Xe* atoms with CH4
molecules.
The ν2 fundamental band of H2CO has been studied using a combination of sub-Doppler laser Stark spectroscopy and Doppler-limited Fourier transform spectroscopy. A combined analysis of the Stark and Fourier infrared data together with previous microwave data on the ν2 = 1 state yielded improved molecular parameters for formaldehyde, including the excited state dipole moment. A small perturbation was noted at K′a = 7 which may be ascribed to a ΔKa = 2 interaction with the v3 = 1 state. Precise treatments of ν2 with K′a > 6 will thus require a combined analysis taking into account Coriolis interactions among ν4, ν6, ν3, and ν2.
The elementary mechanisms are described which determine the plasma and surface processes during the plasma-enhanced chemical vapour deposition of hydrogenated carbon films from methane. Corresponding model calculations are reviewed and critically discussed in comparison to experimental results. A realistic modeling requires the simultaneous and self-consistent treatment of plasma and surface effects. Several experimental data sets on plasma parameters and the growth and the composition of the films have been reproduced successfully. However, a broader experimental data base is needed for more critical tests of the models. The reliability of the modeling, in particular of the surface effects, is still limited due to the poor availability of elementary data.
The chemistry of hydrogen-rich hydrocarbon-hydrogen mixtures is of primary interest for the understanding of the low-pressure synthesis of diamond. We per formed experiments under well-defined conditions like temperature, pressure, initial gas composition, etc. The gas composition at the end of a flow reactor was analyzed by a calibrated mass spectrometer and compared to results obtained from the Chemkin computer code. Residence thne in the reactor as well as other process parameters were similar to those of diamond-growing PA CVD processes performed earlier with the same experimental set-rip. Modeling and experiment under isothermal conditions show quantitative agreement. We realized time-resolved mass .spectrometry by means of a helium-flushed gas sampling probe. There is evidence that the commonly used reaction kinetic data for the dissociation C2H6 (+ M) 2CH,(+M) gives 2 too small C2H4 concentrations for hydrogen-rich conditions. This could be attributed to the poorly known third-body efficiencies of the H2 molecules compared to Ar or C2H6 from which kinetic data are commonly derived.
The plasma plume of a hydrogen plasma jet used for diamond synthesis is analyzed by a Pitot tube and by mass spectrometry.
In the investigated pressure range of 2–10 mbar, supersonic gas velocities with Mach numbers of up to 2 were observed, which
decreased with increasing pressure and increasing distance from the nozzle. The injection of the carbon-containing species
either at the exit of the jet nozzle or simply into the background gas of the reaction chamber confirmed the importance of
recirculation of background gas into the plasma plume. In the case of background injection the rise of the total carbon content
in the plume with increasing distance from the nozzle is much slower than in the case of nozzle injection. The results of
a numerical model of the hydrocarbon gas-phase reactions in the jet are presented. The model considers the entrainment of
background gas into the plasma plume. Two domains along the jet axis can be distinguished. The first one in the vicinity of
the nozzle is dominated by methyl radicals, the second one by atomic carbon. Increase of the hydrogen dissociation level results
in the broadening of the atomic carbon domain and the rise of C2 far from the nozzle. Background injection of CH4 leads to lower total carbon content in the plume but has little effect on the species distribution along the jet axis.
Tunable diode laser absorption spectroscopy has been used to measure the concentrations of reactive species in an rf (20 kHz) chemical‐vapor‐deposition reactor as a function of methane flow rate, pressure, current, and added inert gas. The IR measurements have been supplemented by mass spectrometry and optical emission spectroscopy. The reactive hydrocarbon species detected by IR were CH 4 , C 2 H 2 , C 2 H 4 , and C 2 H 6 , and the free radical CH 3 . No significant spatial variation in methyl radical concentration was measured between the parallel‐plate electrodes under our conditions. The growth rate of amorphous carbon films on the lower (ground) electrode was found to vary linearly with the concentration of CH 3 measured close to this electrode, and these results are compatible with a CH 3 sticking coefficient of ≤0.02. One‐dimensional modeling calculations using the facsimile program were carried out to simulate the variation of the concentration of the principle species with current (electron number density). Satisfactory agreement with the measurements of CH 4 , CH 3 , and C 2 H 6 concentrations was obtained when ionization and ion molecule reactions, as well as reactions of neutral species, were included in the chemical model.
The gas‐phase composition at the surface of a growing diamond film was measured as a function of the initial methane (CH 4 ) fraction and, for a 2% methane fraction, as a function of added oxygen (O 2 ). The results were modeled with a one‐dimensional reactor flow code that includes diffusion and detailed chemical kinetics. We found that most of the ethylene (C 2 H 4 ) and ethane (C 2 H 6 ) that was detected was actually not present in the growth chamber but was instead formed in the probe by recombination of methyl radicals (CH 3 ) that were present in the gas phase. Thus, C 2 H 4 and C 2 H 6 acted as surrogates for CH 3 in our system, and measurement of those two stable species allowed us to estimate the mole fraction of the CH 3 radical. We then took advantage of the fact that CH 3 , CH 4 , H 2 , and H were in partial equilibrium in the diamond growth chamber in order to estimate the concentration of H. A comparison between the mole fractions of CH 3 and H, as determined from our experiments, and the mole fractions calculated from the model shows very good agreement.
Experimental measurements and theoretical modeling of methane deposition plasmas have led to the identification of the most likely homogeneous and heterogeneous reaction paths leading to the deposition of amorphous carbon thin films. Experimental measurements of the voltage, current waveforms, mass flow rates, and pressure are used as inputs to the model. The magnitude and flow‐rate dependence of the discharge luminosity, film deposition rates, and downstream mass spectra are compared with the model predictions and used to identify the dominant reaction paths. The model uses Monte Carlo simulation of the electron kinetics to predict the electron impact dissociation and ionization rates. These rates provide input for a plug flow chemical kinetics model.
The gas kinetics of a 30 mTorr radio‐frequency methane glow discharge are studied as a function of methane depletion including conditions suitable for hard carbon thin‐film deposition. Mass spectrometry is used to measure the partial pressures of the species C 2 H 6 , C 2 H 4 , C 2 H 2 , C 3 H 8 , C 3 H 6 , and C 3 H 4 . Net film growth was calculated using mass balance and corroborated by direct measurements of deposition rate. Using a combination of static and flowing discharge measurements, the net yields of C 2 H 6 , C 2 H 4 , and C 2 H 2 are described using a simple analytic model. C 2 H 6 is modeled as a production from CH 3 recombination, and the production of C 2 H 4 is modeled as reaction of CH with CH 4 where the CH can be produced both by direct electron collisional dissociation of CH 4 as well as reaction of CH 2 with H. C 2 H 2 production is modeled as arising principally from C 2 H 4 depletion. The principal dissociation mechanism of these molecules appears to be electron collisional dissociation. The CH 3 radical densities deduced from this analysis are in good agreement with threshold ionization radical measurements reported in the literature. In addition, the methane electron collisional dissociative branching is inferred to be approximately 68% CH 3 and 32% CH 2 +CH. The results of this analysis when compared to the observed film yield imply that the CH n radicals do not make a significant direct contribution to amorphous carbon film growth. Rather, the film appears to result from the depletion products of discharge‐produced gas molecules such as C 2 H 6 , C 2 H 4 , and C 2-
H 2 . © 1996 American Institute of Physics.
Infrared tunable diode laser absorption studies of radicals and stable molecules formed in radio frequency plasmas are being carried out in a laboratory reactor which allows a long absorption path. In this paper we describe studies of CH4 RF plasmas. We report absolute concentration measurements as functions of total pressure and RF power for CH3 and C2H2 in CH4 plasmas, as well as measurements of the CH4 rotational temperature and dissociation fraction.
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@inproceedings{OPL:8124782,author = {Wormhoudt,J.},title = {Radical and Molecular Product Concentration Measurements in CH4 RF Plasmas by Infrared Tunable Diode Laser Absorption},booktitle = {Symposium I – Characterization of Plasma-Enhanced CVD Processes},series = {MRS Proceedings},volume = {165},year = {1989},doi = {10.1557/PROC-165-35},URL = {http://journals.cambridge.org/article_S194642740040727X},}
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Radical and Molecular Product Concentration Measurements in CH4 RF Plasmas by Infrared Tunable Diode Laser Absorption
J. Wormhoudt (1989).
MRS Proceedings , Volume 165 , 1989, 35
http://journals.cambridge.org/action/displayAbstract?aid=8124782
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Radical and Molecular Product Concentration Measurements in CH4 RF Plasmas by Infrared Tunable Diode Laser Absorption
J. Wormhoudt 1989
MRS Proceedings,
,Volume165,
1989,
35
http://journals.cambridge.org/abstract_S194642740040727X
J. Wormhoudt
(1989).
Radical and Molecular Product Concentration Measurements in CH4 RF Plasmas by Infrared Tunable Diode Laser Absorption.
MRS Proceedings,
165,
35
doi:10.1557/PROC-165-35.
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We present preliminary results from our methanol–water plasma system using a radio frequency (rf) discharge reactor without a substrate. Second-harmonic, tunable diode laser absorption spectroscopy (TDLAS) was used to detect the number densities of the dilute molecular species, CH <sub> 3 </sub> and C <sub> 2 </sub> H <sub> 2 </sub> . TDLAS was also used to measure the dissociation fractions of CH <sub> 3 </sub> OH and H <sub> 2 </sub> O . The plasma pressure was 1.0 Torr, and the molecular density fraction, [CH <sub> 3 </sub> OH]/[CH <sub> 3 </sub> OH+H <sub> 2 </sub> O ] , was 0.6 to give the atomic ratio of C, H, and O most favorable for diamond film growth. We found that about 73% of the water was dissociated with 1500 W of rf power while more than 98% of the methanol was dissociated with only 500 W of power. The CH <sub> 3 </sub> and C <sub> 2 </sub> H <sub> 2 </sub> concentrations gradually increased with rf power up to 900 W. The ranges of the number densities were 0.56×10<sup>12</sup>/ cm <sup> 3 </sup>–4.34×10<sup>12</sup>/ cm <sup>3</sup> for CH <sub> 3 </sub> and 0.50×10<sup>13</sup>/ cm <sup> 3 </sup>–3.18×10<sup>13</sup>/ cm <sup>3</sup> for C <sub> 2 </sub> H <sub> 2 </sub> . The number density of CH</roma-
-
n><sub> 3 </sub> showed an abrupt decrease at 1000 W, while a sudden increase of C <sub> 2 </sub> H <sub> 2 </sub> occurred at the same power. These sudden changes are due to a transition of the plasma from a low-density state plasma to a high-density, more localized state, caused by a change in the plasma coupling mechanism. This transition also occurs in the dissociation of water. © 1997 American Vacuum Society.
Present diamond deposition reactors have been very successful in furthering our understanding of how diamond grows at low pressures. However, their primary disadvantage is that changes in reactor conditions (e.g., filament temperature or microwave power, substrate temperature, and CH 4 :H 2 concentration ratio in methane‐based reactors) change all chemical species and their relationships. Changes in diamond growth created by independent changes to a single species are not directly accessible. For example, it is known that making atomic hydrogen is required for the growth of diamond films. However, changes to film growth rate have not been tied quantitatively to changes in the amount of atomic hydrogen available. A high‐yield atomic hydrogen source was fabricated and pretested in a diamond growth setup. A previously developed atomic hydrogen sensor was used to measure atomic hydrogen density output as a function of both power and pressure. Diamond was then grown as a function of atomic hydrogen density, and growth rate was shown to be linear with density with a lower bound relationship of 0.27 μm/h/10<sup>16</sup> cm<sup>-3</sup>. Since the magnitude and trend of the data appear to be inconsistent with other reported results, they are analyzed with respect to a simplified growth model. Based on reasonable assumptions, we conclude that the data are consistent with growth rate being independent of surface atomic hydrogen concentration and linear with methyl radical concentration. Because sufficient methyl radical density is produced through the interaction of atomic hydrogen with methane with correspondingly negligible reduction in total atomic hydrogen density, it is inferred that growth rate linearity with atomic hydrogen enters only through the production of methyl radical. © 1996 American Vacuum Society
Tunable infrared diode laser spectroscopy has been applied to the study of CH 4 plasmas generated in a deposition reactor run at 20 kHz. Absolute number densities of CH 4 , C 2 H 4 , and CH 3 have been measured as functions of pressure and current. A qualitative explanation of the results is presented.
Infrared diode laser absorption spectroscopy is employed as an in situ method to examine gas phase species present during filament‐assisted deposition of diamond films. From a reactant mixture of 0.5% methane in hydrogen, methyl radical (CH 3 ), acetylene (C 2 H 2 ), and ethylene (C 2 H 4 ) are detected above the growing surface, while ethane (C 2 H 6 ), various C 3 hydrocarbons, and methylene (CH 2 ) radicals are below our sensitivity levels. The growth of polycrystalline diamond films on Si wafers and polycrystalline Ni is confirmed with x‐ray and Raman scattering, scanning electron microscopy, and Auger electron spectroscopy.
A sensitive and quantitative absorption diagnostic has been developed for measurement or C2H4 in shock tube kinetics experiments. This diagnostic has been calibrated from 300 to 2571 K at 1.2 atm in shock tube experiments, resulting in an absorption coefficient at 174.4 nm of kabs = 3.11 × 105/T[K]-51.0 cm-1 atm-1. The absorption coefficient for C2H2 at this wavelenght was also measured over the same temperature and pressure range. Kinetics experiments were performed to measure the rate of ethylene decomposition at reflected shock conditions of 1600–2300 K at 1.2 atm, with mixtures of 189–444 ppm C2H4 in argon. Sensitivity analysis based on a comprehensive hydrocarbon mechanism confirmed that the rate of ethylene decomposition was significantly affected by the rate of only one reaction: C2H4 + M → C2H2 + H2 + M. The rate coefficient for this reaction near the low-pressure limit was determined to be k = 9.17 × 1016 exp(- 37780/T) cm3 · mol-1 · s-1 + 20% over the entire temperature range, in good agreement with past measurements.
Experiments to show the influence of the relative atomic hydrogen content on diamond deposition in hydrogen plasmas with small admixtures of alcohols are reported. By using a planar microwave plasma source and placing the substrates outside the active plasma excitation region, we could ensure that the substrate heating was dominated by the surface recombination of atomic hydrogen. Therefore, the concentration of atomic hydrogen could be assumed to be proportional to the substrate temperature. By varying the position of the substrate, deposition experiments with a variation in concentration of 60% were possible. Growth conditions yielding well isolated crystals were used. A dependence of the medium crystal diameter on position, similar to the spatial dependence of the atomic hydrogen concentration was observed. This indicated a power law dependence of the linear growth rate on the concentration of atomic hydrogen. An analysis of the crystallite size revealed an asymmetric size distribution. In contrast to the crystal diameters, the nucleation densities were independent of the atomic hydrogen concentration.
The influence of a constant concentration of atomic hydrogen on the steady state gas-phase composition (quasi-equilibrium) and its development in hydrocarbon-hydrogen mixtures was studied by means of computational modeling. In the thermodynamic equilibrium, methane is virtually the only stable gas species at temperatures below 1000 °C, whereas acetylene prevails at higher temperatures. The steady state of the gas phase in the presence of atomic hydrogen is also divided between these two stability regimes. The transition temperature between them, however, is considerably lowered even by small H concentrations. Concentrations of other species, especially CH3, are increased by several orders of magnitude over their equilibrium concentrations, having peak values in the vicinity of this transition temperature. Cyclic reaction sequences, consisting of H-consuming reactions, result in a catalytic effect on atomic hydrogen recombination. Generally, atomic hydrogen accelerates the development of a steady state, but not necessarily with a time constant decreasing with increasing temperature.
We describe in this paper the modifications, improvements, and enhancements to the HITRAN molecular absorption database that have occurred in the two editions of 1991 and 1992. The current database includes line parameters for 31 species and their isotopomers that are significant for terrestrial atmospheric studies. This line-by-line portion of HITRAN presently contains about 709,000 transitions between 0 and 23,000 cm-1 and contains three molecules not present in earlier versions: COF2, SF6, and H2S. The HITRAN compilation has substantially more information on chlorofluorocarbons and other molecular species that exhibit dense spectra which are not amenable to line-by-line representation. The user access of the database has been advanced, and new media forms are now available for use on personal computers.
An IBM PC AT/XT compatible version of the GEISA database has been recently created with the purpose of extending its use to potential non mainframe users and because of the now generalized employment of personal computing facilities. Thanks to a collaboration between the Laboratoire de Météorologie Dynamique du CNRS, in France, and the Laboratory of Theoretical Spectroscopy of Institute of Atmospheric Optics, in Russia, an enhanced and extended management system of GEISA has been built up. This paper provides an overview of the new GEISA-PC system, and details the updates of the database contents since its previous edition.
A time-resolved technique has been developed for the detection of CH3 radicals in the gas phase by diode laser absorption at 608 cm−1 on a rotation-vibration transition of the v2 mode of CH3. The usefulness of the technique as a probe of the reaction kinetics is demonstrated by the measurement of the recombination rate of CH3 radicals and the third-body reaction rate of CH3 with O2 at room temperature.
The paper presents a review of recent results of emission (UV/visible) and absorption (visible and IR) spectroscopical investigations in molecular tube and planar microwave discharges. As a new approach by emission spectroscopy seven band systems of the hydrogen molecule have been analysed to investigate H2 radiative characteristics and to determine the gas temperature completed by Doppler broadening measurements of Hα, Dα and H2 spectral lines. H2 dissociation continuum was used for an estimation of the radiative dissociation rate/degree. By absorption spectroscopy the populations of 4s ArI levels and concentrations of hydrocarbons (e.g. CH3, C2H2, CH4) were determined. In particular, investigations were focused on spatial distributions of plasma parameters. La publication présente un aperçu de nouveaux résultats de mesures de spectroscopie démission (UV/visible) et d'absorption (visible et Infrarouge) dans des décharges microondes moléculaires dans des tubes et en structure plane. La nouveauté consiste en l'analyse de l'émission de système à sept bandes de la molécule d'hydrogène. On étudie ainsi les caractéristiques radiatives de H2 et on détermine la température du gaz par des mesures d'élargissement Doppler des lignes spectrales de Hα, Dα et H2. Le continuum de dissociation de H2 est utilisé pour estimer le degré de dissociation radiative. Les populations des niveaux 4s de l'Ar I et les concentrations des hydrocarbures (par exemple CH3, C2H2, CH4) sont déterminés par spectroscopie d'absorption. En particulier on s'est intéressé à la distribution spatiale des paramètres du plasma.
The paper discusses basic concepts of slotted waveguide microwave plasma excitation which is technically very reasonable for the generation of long extended and large area plasmas. Results of different experiments aimed at the details of the spatial plasma structure are reported for an improved version of slotted waveguide plasma excitation which allows maximum suppression of plasma homogeneity limiting effects. They demonstrate that some residual influences of the local discreteness of coupling have to be expected in any case and that distributions of applied fields and local fields in the plasma can differ strongly. Ce papier discute des concepts de base de l'excitation d'un plasma à partir d'un guide d'ondes à fentes qui, techniquement, peut s'avérer très utile pour l'obtention de plasmas très longs et de grande surface. Les résultats des différentes expériences visant aux détails de la structure qui sont présentés sont ceux d'une version améliorée qui permet de réduire aux mieux les effets limitant l'homogénéité du plasma. Ces résultats démontrent l'influence résiduelle des couplages locaux discrets sur les écarts forts qu'on peut attendre entre distributions de champs appliqués et de champs locaux.
A chemical kinetic model is developed that includes detailed descriptions of both gas-phase and surface processes occurring in gas-activated deposition of diamond films on diamond (111) surface. The model was tested by simulating diamond film deposition in a hot-filament reactor using methane-hydrogen, methane-argon, and methane-oxygen-hydrogen gas mixtures. The gas-phase part of the model includes transport phenomena and predicts correctly the measured concentrations of major gaseous species. The surface part of the model reproduces the general experimental trends–the effects of temperature, pressure, initial methane concentration, and the addition of oxygen–for the growth rate and film quality. Analysis of the computational results revealed the factors controlling the growth phenomena. Among several reaction pathways describing deposition of diamond initiated by different gaseous species, including CH3 and several C2Hx species, the H-abstraction–C2H2-addition mechanism appears to dominate. The key role of hydrogen and oxygen is identified to be the suppression of the formation of aromatic species in the gas phase, which prevents their condensation on the deposition surface. Activation of the growing surface by H atoms and gasification of sp2 carbon by OH radicals are other important factors. The developed model does not support the theory of preferential etching by H atoms advanced to explain the kinetic competition between diamond and nondiamond phases. Instead, it establishes the critical role of aromatics condensation and interconversion of sp2 and sp3 carbon phases mediated by hydrogen atoms.