Optical emission spectroscopy of electron-cyclotron-resonance-heated helium mirror plasmas

In this experiment emission spectroscopy in the 3000–5000 Å range has been utilized to determine the electron temperature (15–60 eV) and ion density (2–5 x 10¹¹ cm^–3) of helium plasmas produced by the Michigan mirror machine⁽¹⁾ (MIMI). The plasma is generated and heated by whistler-mode electron-cyclotron resonance (ECR) waves at 7.43 GHz with 400–900 W power in 80-ms-long pulses. Gas fueling is provided at the midplane region by a leak valve with a range in pressure of 3 x 10 to 2 x 10⁴ Torr. Emission line intensities are interpreted using a model of the important collisional and radiative processes occurring in the plasma. The model examines secondary processes such as radiation trapping, excitation transfer between levels of the carne principle quantum number, and excitation front metastable states for plasmas in the parameter range of MIMI (n _c = 1–6 x 10¹¹ cm^–3). Front the analysis of line intensity ratios for neutral helium, the electron temperature is measured and its dependence upon the gas pressure and microwave power is determined. These temperatures agree with those obtained by Langmuir probe measurements. Art analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement of the ion density which is in good agreement with electron density measurements made by a microwave interferometer.     

Attachment and ionization in a self-consistent treatment of the electron kinetics of a collision-dominated rf plasma

This paper deals with the self-consistent determination of the rf field amplitude for sustaining the steady-state collision-dominated weakley ionized plasmas in the bulk of the rf discharge and of the time-resolved behavior of the isotropic part of the distribution function as well as of relevant macroscopic quantities in plasmas whose particle loss is dominantly determined by electron attachment. The strict timeresolved treatment is based on the nonstationary Boltzmann equation of the electrons and its numerical solution including, apart from electron number conservative collision processes, the electron attachment and ionization. The investigations are related to an rf plasma in a model gas and in SF₆ and are performed for reduced rf field frequencies around 10 MHz Torr^–1 which are of particular interest from the point of application of rf discharges for plasma processing. The numerical results show that a large field amplitude of around 160 V cm^–1 Torr^–1 is necessary to maintain the discharge and that the isotropic distribution, the relevant collision frequencies for attachment and ionization, and the electron density undergo a large modulation during a period of the rf field.     

Characterization of laser induced breakdown plasmas used for measurements of arsenic,antimony and selenium hydrides

Studies have been performed to characterize laser induced breakdown spectroscopy (LIBS) plasmas formed in Ar/H₂ gas mixtures that are used for hydride generation (HG) LIBS measurements of arsenic (As), antimony (Sb) and selenium (Se) hydrides. The plasma electron density and plasma excitation temperature have been determined through hydrogen, argon and arsenic emission measurements. The electron density ranges from 4.5 × 10¹⁷ to 8.3 × 10¹⁵ cm^?3 over time delays of 0.2 to 15 μs. The plasma temperatures range from 8800 to 7700 K for Ar and from 8800 to 6500 K for As in the HG LIBS plasmas. Evaluation of the plasma properties leads to the conclusion that partial local thermodynamic equilibrium conditions are present in the HG LIBS plasmas. Comparison measurements in LIBS plasmas formed in Ar gas only indicate that the temperatures are similar in both plasmas. However it is also observed that the electron density is higher in the Ar only plasmas and that the emission intensities of Ar are higher and decay more slowly in the Ar only plasmas. These differences are attributed to the presence of H₂ which has a higher thermal conductivity and provides additional dissociation, excitation and ionization processes in the HG LIBS plasma environment. Based on the observed results, it is anticipated that changes to the HG conditions that change the amount of H₂ in the plasma will have a significant effect on analyte emission in the HG LIBS plasmas that is independent of changes in the HG efficiency. The HG LIBS plasmas have been evaluated for measurements of elements hydrides using a constant set of HG LIBS plasma conditions. Linear responses are observed and limits of detection of 0.7, 0.2 and 0.6 mg/L are reported for As, Sb and Se, respectively.     

Use of CF3,Br/Al,discharges for reactive ion etching of III-V semiconductors

The reactive ion etching of GaAs, InP, InGaAs, and InAlAs in CF₃Br/Ar discharges was investigated as a function of both plasma power density (0.56-1.3 W - cm^–2) and total pressure (10-40 mTorr) The etch rate of GaAs in 19CF₃Br:1Ar discharges at 10 m Torr increases linearly with power density, from 600 Å min^–1 at 0.56 W · cm^–2, to 1550 Å · min at 1.3 W · cm^–2. The in-based materials show linear increases in etch rates only for power densities above – 1.0 W · cm^–2. These etch rates are comparable to those obtained with CCI₂F₂:O₂ mixtures under the same conditions. Smooth surface morphologies and vertical sidewalls are obtained over a wide range of plasma parameters. Reductions in the near-surface carrier concentration in n-type GaAs are evident for etching with power densities of >0.8 W cm^–2, due to the introduction of deep level trapping centers. At 1.3 W· cm^–2, the Schottky barrier height of TiPtAu contacts on GaAs is reduced from 0.74 to 0.53 eV as a result of this damage, and the photoluminescent intensity from the material is degraded. Alter RIE, we detect the presence of both F and Br on the surface of all of the semiconductors. This contamination is worse than with CCl₂F₂-based mixtures. High-power etching with CF₃Br/Ar together with Al-containing electrodes can lead to the presence of a substantial layer of aluminum oxide on the samples if the moisture content in the reactor is appreciable.     

Conformational Stability from Variable-Temperature Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r o Structural Parameters of Chlorocyclopentane

The infrared spectra (3500–400 cm^–1) of krypton solutions of chlorocyclopentane, c-C₅H₉Cl, at variable temperatures (–101 to –150°C) have been recorded and the fundamental vibrations of the axial conformer and several of those for the equatorial form have been assigned. Utilizing two pairs of fundamentals for the two conformers in the krypton solution, an enthalpy difference of 145±15 cm^–1 (1.73±0.18 kJ-mol^–1) has been obtained with the axial conformer the more stable form. It is estimated that there is 67±2% of the axial conformer present at ambient temperature. Convincing spectroscopic evidence shows that a significant percentage of the chlorocyclopentane molecules are undergoing pseudorotation at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation and these quantities have been compared to the experimental values when appropriate. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2 calculations as well as by density functional theory (DFT) by the B3LYP method with several different basis sets. The adjusted r ₀ structural parameters have been obtained for both conformers by combining the ab initio data with the previously reported microwave rotational constants. These new values of the structural parameters for both conformers are compared to those previously reported from electron diffraction and microwave studies. These results are compared to the corresponding quantities of some similar molecules.     

The 3d←3p endothermic collisional population transfer of Li in He and Ar at 298 K

The 3p state of Li was excited in He and Ar buffer gases at room temperature (298 K) and the time profiles of sensitized fluorescence from the 3s and 3d states were measured. The 3d←3p endothermic population transfer rates determined from the pressure dependence of the time profiles were 6.5×10⁻¹¹ cm³ s⁻¹ for He and less than 10⁻³ cm³ s⁻¹ for Ar. The origin of this large difference between He and Ar is discussed in terms of non-adiabatic transitions between the 3p and 3d molecular states of the Li–He and Li–Ar molecules.     

Microwave and RF surface wave sustained discharges as plasma sources for plasma chemistry and plasma processing

The surface wave produced plasma belongs to a class of RF and microwave induced plasmas. It results from the propagation of an electromagnetic wave which uses the plasma column it sustains and the plasma tube as its sole propagating media. This type of plasma offers several advantages compared to the positive column plasma of dc discharges or to other RF and microwave produced plasmas. Surface wave plasmas require no internal electrodes, and they can be applied over an extremely broad range of wave frequencies (27 MHz to 10 GHz demonstrated) and gas pressures (about 10^–4 Torr to a few times the atmospheric pressures). Using the surface wave plasma technique, a large variety of plasma column diameters have been created (0.5–150 mm demonstrated) and no limitation on plasma column length (column up to 6 m long demonstrated) has been found. The surface wave produced plasma is used in elemental analysis and to sustain emission in lasing media. This article is intended as a guide for potential users of surface wave plasmas in the field of plasma processing and plasma chemistry.     

Thomson scattering on argon surfatron plasmas at intermediate pressures: Axial profiles of the electron temperature and electron density

The axial profiles of the electron density n_e and electron temperature T_e of argon surfatron plasmas in the pressure range of 6–20 mbar and microwave power between 32 and 82 W have been determined using Thomson Scattering of laser irradiation at 532 nm. For the electron density and temperature we found values in the ranges 5 × 10¹⁸ < n_e < 8 × 10¹⁹ m^− 3 and 1.1 < T_e < 2.0 eV. Due to several improvements of the setup we could reduce the errors of n_e and T_e down to 8% and 3%, respectively. It is found that n_e decreases in the direction of the wave propagation with a slope that is nearly constant. The slope depends on the pressure but not on the power. Just as predicted by theories we see that increasing the power leads to longer plasma columns. However, the plasmas are shorter than what is predicted by theories based on the assumption that for the plasma-wave interaction electron–atom collisions are of minor importance (the so-called collisionless regime). The plasma vanishes long before the critical value of the electron density is reached. In contrast to what is predicted by the positive column model it is found that T_e does not stay constant along the column, but monotonically increases with the distance from the microwave launcher. Increases of more than 50% over 30 cm were found.     

Raman and Infrared Spectra, Ab Initio Calculations, Normal Coordinate Analysis, and Conformational Stability of 2-Methoxypropene

The Raman (3500–40 cm^–1) and infrared (3500–70 cm^–1) spectra of gaseous and solid 2-methoxypropene, CH₃O(CH₃)C=CH₂, and the isotopomers, CD₃O(CH₃)C=CH₂ and CH₃O(CD₃)C=CD₂ have been recorded. In addition, the Raman spectra of the liquids have been recorded with qualitative depolarization measurements. All of these data indicate that only one conformer is present in the fluid phases at ambient temperature and this form is the cis conformer, which remains in the solid. Assignments are provided for the fundamentals of all three isotopomers for the cis conformer with C_s symmetry. The far-infrared spectra of all three isotopic species have been recorded at a resolution of 0.1 cm^–1 in the gas and 1.0 cm^–1 in the solid. The parameters of the potential function governing the asymmetric torsion are determined to be V₃ = 1485 ± 9 cm^–1 and V₆ = –55 ± 4 cm^–1 for the d₀ compound, where only two terms were determined, since a second conformer was not evident. The barrier to internal rotation for the methyl group attached to the oxygen atom is 1370 ± 8 cm^–1 and the C—CH₃ barrier is 772 ± 5 cm^–1. Ab initio calculations with full electron correlation have been carried out by the perturbation method to second order to obtain the equilibrium structural parameters, harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, depolarization values, and conformational stability. The predicted values have been compared to the experimental values where appropriate.     

Reactivity of tri-n-bytylphosphate towards solvated electrons

Decomposition yields of tri-n-butylphosphate in methanol solutions saturated with Ar or N₂O were determined. On the basis of difference between yields in Ar and N₂O saturated solutions the rate constant k_{/e s} ^– _+TBP/=5.0×10⁶ dm³.mol^–1.s^–1 was calculated.     

Hybrid electron cyclotron resonance-radio-frequency plasma etching of III–V semiconductors in Cl2-based discharges. Part II: InP and related compounds

Electron Cyclotron Resonance (ECR) discharges of CCl₂F₂ or PCl₃ have been used to etch InP, InAs, InSb, InGaAs and AlInAs. The etch rates of these materials increase linearly with additional RF power level applied to the cathode and are in the range 50–180 Å · min^–1 for 50 W (DC bias 308 V), 10 mTorr, 38 CCl₂F₂/2 O₂ plasmas. The etch rates fall rapidly with increasing pressure or increasing O₂-to-CCl₂F₂ ratio. Polymeric surface residues up to 40 Å thick are found on all of these semiconductors when using Freon-based gas mixtures. Etching at practical rates is possible with only 100 V self-bias when using PCl₃ discharges, and the addition of microwave excitation under these conditions enhances the etch rates by factors of 2–9. At higher self-biases (300 V) etch rates of 3500–8000 Å · min^–1 are possible with PCl₃ although the surface morphologies are significantly rougher and the etching less anisotropic than with CCl₂F₂-based mixtures.     

Tunable Diode Laser Absorption Studies of Hydrocarbons in RF Plasmas Containing Hexamethyldisiloxane

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 (=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(C_xH_y): 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.     

Solubility of nonpolar gases in tetrahydropyran at 0 to 30°C and 101.33 kPa partial pressure of gas

Solubility measurements of several nonpolar gases (He, Ne, Ar, Kr, Xe, H₂, D₂, N₂, CH₄, C₂H₄, C₂H₆, CF₄, and SF₆) in tetrahydropyran at the temperature range 0 to 30°C and 101.33 kPa partial pressure of gas are reported. Thermodynamic functions for the solution process (Gibbs energy, enthalpy, and entropies) at 25°C are evaluated from the experimental values of the solubility of gases as mole fraction and their variation with the temperature. Lennard-Jones 6–12 pair potential parameters for tetrahydropyran are estimated by using the scale particle theory (SPT); and experimental solubilities are compared with the calculated values using this model. Experimental solubilities of gases in tetrahydropyran and intermolecular potential parameters are compared with those obtained for the same gases in other cycloethers.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.     

Comparison of the Active Species in the RF and Microwave Flowing Discharges of N<Subscript>2</Subscript> and Ar–20 %N<Subscript>2</Subscript>

We report a detailed comparison between RF and microwave (HF) plasmas of N₂ and Ar–20 %N₂ as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10^?2 s) of the active species in the N₂ RF plasma [compared to that (10^?3 s) of HF], providing a condition for an efficient vibrational excitation of N₂(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N₂(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N₂(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γ_v destruction probability of N₂(X, v > 3–13) on the tube wall (2–3 × 10^?3 for both cases) obtained from a comparison between the density of N₂(X, v > 3–9) in the plasmas to that of the N₂(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N₂(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N₂ ⁺ ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N₂ ⁺ ions are very sensitive to oxygen impurity .     

CCl<Subscript>4</Subscript> Decomposition in RF Thermal Plasma in Inert and Oxidative Environments

The decomposition of carbon tetrachloride was investigated in an RF inductively coupled thermal plasma reactor in inert CCl₄–Ar and in oxidative CCl₄–O₂–Ar systems, respectively. The exhaust gases were analyzed by gas chromatography-mass spectrometry. The kinetics of CCl₄ decomposition at the experimental conditions was modeled in the temperature range of 300–7,000 K. The simulations predicted 67.0 and 97.9% net conversions of CCl₄ for CCl₄–Ar and for CCl₄–O₂–Ar, respectively. These values are close to the experimentally determined values of 60.6 and 92.5%. We concluded that in RF thermal plasma much less CCl₄ reconstructed in oxidative environment than in an oxygen-free mixture.     

Densities,Apparent Molar Volumes and Viscosities of Concentrated Aqueous NaNO<Subscript>3</Subscript> Solutions at Temperatures from 298 to 607 K and at Pressures up to 30 MPa

Densities of four (2.124, 2.953, 5.015 and 6.271 mol-kg⁻¹) and viscosities of eight (0.265, 0.503, 0.665, 1.412, 2.106, 2.977, 5.015 and 6.271 mol-kg⁻¹) NaNO₃(aq) solutions have been measured with a constant-volume piezometer immersed in a precision liquid thermostat and using capillary flow techniques, respectively. Measurements were made at pressures up to 30 MPa. The temperature range was 298–607 K for the density measurements and 298–576 K for the viscosity measurements. The total uncertainty of density, viscosity, pressure, temperature and composition measurements were estimated to be less than 0.06%, 1.6%, 0.05%, 15 mK and 0.02%, respectively. The temperature, pressure and concentration dependence of density and viscosity of NaNO₃(aq) solutions were studied. The measured values of density and viscosity of NaNO₃(aq) were compared with data and correlations reported in the literature. Apparent molar volumes were derived using the measured density values. The viscosity data have been interpreted in terms of the extended Jones–Dole equation for strong electrolytes. The values of the viscosity A-, B-, D- and F-coefficients of the extended Jones–Dole equation for the relative viscosity (η/η₀) of NaNO₃(aq) solutions were evaluated as a function of temperature. The derived values of the viscosity A- and B-coefficients were compared with the results predicted by Falkenhagen–Dole theory of electrolyte solutions and calculated with the ionic B-coefficient data.     

Gas-phase reactions of CF3 and CF2 with atomic and molecular fluorine: Their significance in plasma etching

Reaction rate coefficients have been measured at 295 K for both CF₃ and CF₂ with atomic and molecular fluorine. The reaction between CF₃ and F was studied over a gas number density range of (2.4–23)×10¹⁶ cm^–3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10^–11 cm³ s^–1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF₂+F increased from (0.4–2.3)×10^–12 cm³ s^–1 as the helium gas number density increased from (3.4–28.4)×10¹⁶ cm^–3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.Both these association reactions are examined from the standpoint of the Gorin model for association of radicals and use is made of unimolecular dissociation theory to examine the expected dependence on gas number density. The calculations reveal that CF₃+F can be explained satisfactorily in these terms but CF₂+F is not well described by the simple Gorin model for association.CF₃ was found to react with molecular fluorine with a rate coefficient of (7±2)×10^–14 cm³ s^–1 whereas only an upper limit of 2×10^–15 cm³ s^–1 could be placed on the rate coefficient for the reaction between CF₂ and F₂. The values obtained for this set of reactions mean that the reaction between CF₃ and F will play an important role in plasmas containing CF₄. The high rate coefficient will mean that, under certain conditions, this particular reaction will control the amount of CF₄ consumed. On the other hand, the much lower rate coefficient for reactions between CF₂ and F means that CF₂ will attain much higher concentrations than CF₃ in plasmas where these combination reactions are dominant.     

Conformational stability,barriers to internal rotation,structural parameters,ab initio calculations,and vibrational assignment of cyclopropanecarboxaldehyde

The asymmetric torsional potential function, conformational energy difference, vibrational frequencies, and structural parameters of Cyclopropane-carboxaldehyde have been obtained from ab initio calculations at the 3–21G and/or 6-31G^* baiss set levels. These results have allowed for a reinterpretation or clarification of some of the corresponding results obtained from experiment. The conformations that have the oxygen atom oriented cis and trans to the three-membered ring are observed and calculated to be the most stable and high energy forms in the gaseous phase, respectively. From the ab initio calculations using the 6–31 G^* basis set, the energy difference between the two conformers is 114 cm^–1. For the liquid, the trans conformer is more stable and is the only rotamer present in the annealed solid. Based on a combination of results obtained from ab initio calculations, microwave spectroscopy, and the electron diffraction technique,r _o structural parameters have been obtained for both conformations.