Showing papers in "Plasma Chemistry and Plasma Processing in 1982"

Plasma-assisted etching

Abstract: The mechanistic and parametric complexity of a plasma etching environment often causes confusion and delays in the development of a suitable plasma etching process. This paper is an attempt to alleviate this problem by discussing some of the important physical and chemical phenomena and, where possible, relating these phenomena to apparatus selection and operation.

The use of electrostatic probes for plasma diagnostics—A review

Abstract: The use of electrostatic, or Langmuir, probes for plasma diagnostics is reviewed. The emphasis is on experimental implementation and current techniques, and particular attention is paid to sources of error in theoretical interpretation as well as to experimental problems that can occur in complex, reactive plasmas.

Heat transfer to a single particle exposed to a thermal plasma

Abstract: This paper is concerned with an analytical study of the heat and mass transfer process of a single particle exposed to a thermal plasma, with emphasis on the effects which evaporation imposes on heat transfer from the plasma to the particle. The results refer mainly to an atmospheric-pressure argon plasma and, for comparison purposes, an argon-hydrogen mixture and a nitrogen plasma are also considered in a temperature range from 3000 to 16,000 K. Interactions with water droplets, alumina, tungsten, and graphite particles are considered in a range of small Reynolds numbers typical for plasma processing of fine powders. Comparisons between exact solutions of the governing equations and approximate solutions indicate the parameter range for which approximate solutions are valid. The time required for complete evaporation of a given particle can be determined from calculated values of the vaporization constant. This constant is mainly determined by the boiling (or sublimation) temperature of the particles and the density of the condensed phase. Evaporation severely reduces heat transfer to a particle and, in general, this effect is more pronounced for materials with low latent heat of evaporation.

Diagnostics and decomposition mechanism in radio-frequency discharges of fluorocarbons utilized for plasma etching or polymerization

Abstract: Optical emission (180–800 nm) and mass spectroscopy have been used to study the CF4, CF4+O2, C2F6, C2F6+H2, CF3Cl, and C2F4 decomposition in radio-frequency discharges. The analysis of the stable and unstable discharge products has allowed the suggestion of decomposition channels for the various gases and to classify the fluorinated gases according to their predominant etching or polymerizing characteristics on the basis of the active species present in the plasma. A new broad emission continuum centered at λ=290 nm (FWHM=66 nm) has also been identified and it has been tentatively assigned to CF+2.

Temperature and velocity fields in a gas stream exiting a plasma torch. A mathematical model and its experimental verification

Abstract: A mathematical model was developed to predict the velocity and temperature fields in a free plasma jet issuing from a D.C. plasma torch. It was assumed that the temperature and velocity at the torch nozzle were specified and the turbulent Navier-Stokes equations were solved in conjunction with a two-equation model of turbulence and the energy transport equation. The model was formulated in terms of the two-dimensional elliptic equations to facilitate its future extension to nonparabolic problems. The predictions of the model were compared with experimental measurements obtained from laser Doppler and spectroscopic techniques. Good overall agreement was found between the theoretical predictions and the experimental measurements for two sets of initial conditions corresponding to nitrogen/hydrogen and argon/hydrogen plasmas. Radiation was found to have a small effect on the overall heat transfer process, and it is suggested that the assumption of an optically thin radiation loss per unit volume is sufficiently accurate for most engineering applications. The significance of this work lies in the fact that, for the first time, it is possible to test the assumptions of the current model against a reliable set of experimental measurements.

Unsteady heating and radiation effects of small particles in a thermal plasma

Abstract: Based on exact solutions for the heat flux to a particle exposed to a thermal plasma given in a previous paper, initial unsteady heating (including heating of the solid phase, melting of the solid phase, heating of the liquid phase, and evaporation) and radiation effects are considered. Closed-form solutions can be obtained for particles with infinite thermal conductivities. The results show that the time periods required for the various steps are all proportional to the square of the particle radius, suggesting that reduced time periods which are independent of the particle radius are appropriate bases for comparison. Results are presented for three materials (alumina, tungsten, and graphite) and three types of plasmas (argon, argon-hydrogen mixture, and nitrogen). It is shown that evaporation (or sublimation) is by the slowest step among all processes in a plasma reactor if complete evaporation (or sublimation) of the particles is desired. Studies of the temperature history of particles with finite thermal conductivities show that temperature gradients within the particles depend on the ratio of the particles' thermal resistance to that of the plasma. In spite of the difference in initial heating, the analytical expressions based on infinite thermal conductivities predict the correct total time spent for both heating and evaporation even for low-conductivity materials such as alumina. The effect of radiation losses from a particle during heating becomes important for large particles, for high-boiling-point materials, and for low enthalpy differences between the plasma and the particle surface.

Ion and radical reactions in the silane glow discharge deposition of a-Si:H films

Abstract: A mass spectrometric analysis of the positive ions and neutral products in a silane glow discharge has been performed. The active species, created by dissociation, disproportionation, and ion-molecule reactions are mainly SiH2, SiH3, and H. A calculation of the distribution of the SiH + ions shows that the silane concentration monitors the abundance of SiH 3 + . The diffusional transport of radicals toward the discharge-tube walls can explain the observed deposition rates. The study of SiH4-SiD4 and SiH4-D2 plasmas emphasizes several reactions which modify the free-radical populations depending on the discharge conditions: disproportionation, termination, recombination, and abstraction. Heterogeneous reactions have also been observed: etching of the film by H atoms and direct incorporation of hydrogen in the growing film. A general scheme for the plasma deposition mechanism is proposed.

Influence of velocity and surface temperature of alumina particles on the properties of plasma sprayed coatings

Abstract: In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.

Electron-impact-induced anisotropic etching of silicon by hydrogen

Abstract: The etch rate of silicon in a hydrogen low-pressure discharge plasma can be strongly enhanced by electron bombardment, reaching presently up to ∼1000 A/min. The etch rate increases linearly with increasing electron current density and hydrogen pressure (range ∼0.05–0.7 mbar) and decreases with increasing temperature, yielding an activation energy of −4.2 kcal/mole in a temperature range of ∼80 to 300°C. The etching remains anisotropic within the whole pressure range studied.

Plasma etch chemistry of aluminum and aluminum alloy films

Abstract: The chemistry occurring in glow discharges used to etch aluminum and aluminum alloy films is examined and is related to recurring problms such as initiation and reproducibility of etching, polymer or residue formation, photoresist degradation, aluminum corrosion, and safety aspects. The relative effects of different etch gases on these problems is discussed in light of aluminum surface chemistry and gas-phase plasma chemistry.

Kinetic study of the heterogeneous Si/H system under low-pressure plasma conditions by means of mass spectrometry

Abstract: The kinetics of the silicon/hydrogen low-pressure discharge system have been measured using a flow technique and mass spectrometry. Results show that at long residence times the system operates under a partial chemical equilibrium even though it is not at thermodynamic equilibrium. The present work indicates that the decisive parameter controlling the structural properties of the deposit (i.e., the formation of either amorphous or microcrystalline silicon) is the departure of the system from the partial chemical equilibrium.

Operating characteristics and energy distribution in transferred plasma arc systems

Abstract: A specially designed plasma chamber was constructed to study the operating characteristics of a dc plasma-transferred arc of argon, struck between a fluid convective cathode and a water-cooled anode. The arc voltage increased markedly with arc length and with an increase in the inlet velocity of the argon flow past the cathode tip, and much less with an increase in current. Radiation from the plasma column to the chamber walls and transfer of energy to the anode were the two principal modes of transfer of the arc energy. The former was dominant in the case of long arcs and at high inlet argon velocities. At the anode, the major contribution was from electron transfer, which occurred on a very small area of the anode (∼5 mm in diameter). Convective heat transfer from the plasma was somewhat less. In all cases, the arc energy contributions to cathode cooling and to the exit gas enthalpy were small. From total heat flux and radiative heat transfer measurements, it was estimated that the plasma temperature just above the anode was in the range 10,000–12,000 K. Preliminary experiments with an anode consisting of molten copper showed that the arc root was no longer fixed but moved around continuously. The arc was othwewise quite stable, and its operating characteristics differed little from those reported for solid anodes, in spite of the greater extent of metal vaporization.

The quenching of vibrationally excited N 2 + ions by various neutrals

Abstract: Using a selected ion flow tube, the quenching of the vibrational excitation of N 2 + (X, v≠0) by Ne, N2, O2, NO, and CO2 was investigated, and the following thermal quenching rate coefficients, kq, were obtained respectively (all in units of 10−10 cm3 sec−1): 0.045, 5, 1.2, ≤0.3, ≤1. For the charge transfer of N 2 + with O2, NO, and CO2, the respective rate coefficients (in units of 10−10 cm3 sec−1) 0.5, 3, and 7 were obtained independently of whether N 2 + (X) was vibrationally excited or not.

Kinetic modelling of the reduction of SiCl4 in an arc heater for the production of silicon

Abstract: The conversion of SiCl4 into Si has been achieved from reduction by a hydrogen plasma produced in an arc heater. As the results (conversion yield about 60%) are far from chemical equilibrium predictions, a kinetic model is proposed using the few kinetic data available in the literature and a temperature history of the reactants deduced from measurements of the temperatures and velocities of the flowing chemical mixture.

Neutral and ion chemistry in a C2H4-SiH4 discharge

Abstract: This paper reports on a mass spectrometric study of the neutral and ionic species in a low-pressure rf discharge sustained in a C2H4-SiH4 mixture diluted in helium. It is shown that C2H4 is readily decomposed into C2H 2 * and C2H3. The formation of secondary products such as C4H2, C4H4, and C4H6 is observed and confirms the presence of C2H2 in the discharge. Methylsilane (CH3SiH3) and ethylsilane (C2H5SiH3) are also synthesized in this discharge. It is also observed that the major ions C2H 4 + , C3H 5 + , SiH 3 + , Si2H 4 + , SiCH 3 + , SiC2H 3 + , and SiC2H 7 + are not representative of the direct ionization of neutral species. Their formation is thus interpreted on the basis of ion-molecule reactions.

Effect of heating on dissociation of water vapor in high-frequency plasmas and formation of hydrogen peroxide in a cold trap downstream of the plasma

Abstract: At low flow rates (0.7–2.8 mmol hr−1) and long residence times (2.3–8.5 s) nearly 60% of the input water vapor was decomposed by a 13.56-MHz rf discharge. Downstream of the discharge a trap cooled by liquid nitrogen collected nearly constant yields of H2O2. The decomposition is representable by the equation 2H2O=H2O2+H2. The overall rate of decomposition was found to depend on the absorbed power density. Heating the rf plasma and its spatial afterglow from 25 to 600°C did not significantly change the percent decomposition of H2O and the formation of H2O2. Above 600°C, however, a continuous decrease in H2O2 yield was observed with increasing temperature, and this was associated with the increasing formation of H2O from the dissociated products such as highly excited OH radicals which otherwise produce the precursors of H2O2. The same heating effects were observed in the case of the spatial afterglow of a 2.45-GHz microwave cavity discharge in water vapor under essentially similar conditions. It appears that at the high temperatures the reaction OH+OH→H2O+O is favored over the reaction O+OH→O2+H. This limits the formation of O2 and consequently decreases the H2O2 yield.

Quenching of air plasma effluents

Abstract: The concentration of nitric oxide in the effluent of an air plasma (2350–2600 K) was studied as a function of the rate of quenching. Rapid quenching leads to concentrations higher than those predicted from stagnant equilibrium conditions. This observation may be attributed to a lack of chemical equilibrium under the conditions investigated.

Effect of frequency on the deposition of microcrystalline silicon from tetrachlorosilane in low-pressure r.f. plasmas

Abstract: The efficiency of reduction of silicon tetrachloride and the rate of deposition of Si in a low-pressure r.f. plasma was investigated at two frequencies (0.4 and 27 MHz) as a function of position with regard to the rf coil, pressure, and time of deposition. At 27 MHz the decomposition efficiency of silicon tetrachloride and the deposition rate of Si are about three times higher than at 0.4 MHz.

Kinetics and mechanism of the decomposition of CS2, OCS, and SCl2 in a radiofrequency pulse discharge

Abstract: The identification of transient species in r.f. discharges and measurement of rate coefficients for their reactions contributes to the understanding of the complex mechanisms in r.f. plasma chemistry. Using kinetic spectroscopy in conjunction with a short-duration r.f. pulse to investigate the decomposition of CS2, OCS, and SCl2 at low pressure, it has been shown that the predominant primary dissociation steps are CS2→CS+S, OCS→CO+S(1D), and SCl2→S+Cl+Cl. With OCS the most important subsequent steps involved the formation and removal of S2: S(1D)+OCS→CO+S2(a1Δ), S2(a1Δ)+M→S2(X3Σ)+M, (13), and 2S2+M→S4+M(15). Taking the previously published value of k12, computer simulation gave the rate coefficient values k13=6.4±2.4×108 mol−1 dm3 s−1 and k15=1.8±0.5×1013 mol−2 dm6 s−1 at 295±3 K.

Formation of niobium boride from NB2O5 and boron with plasma arc

Abstract: When a mixture of Nb2O5 and boron is heated to about 3000°C and subsequently annealed at about 2000°C in an argon plasma arc, NbB2 is obtained The products are found to be superconductors, theTcof which is higher than 4 K Some properties of the products, such as Vickers hardness and density, are comparable with those of NbB2 prepared from niobium and boron In the reaction forming NbB2, a white powder is also obtained The powder is identified as H3BO3 and is formed by the reaction between BO and water in the atmosphere Therefore, the reaction of NbB2 formation can be written as Nb2O5+9B=2NbB2+5BO

Experiment planning, mathematical modelling, and nonlinear optimization of the ion-nitriding process in a glow-discharge plasma

Abstract: A new, efficient method for investigating and optimizing the ion-nitriding process in a glow-discharge plasma is proposed and worked out in detail for the mass kinetics of Rp-3 steel. This method, which is based on the concepts of experiment planning, mathematical modelling, and nonlinear optimization, is quite general and potentially applicable to a wide class of technological and other processes depending on several parameters.

Reaction of hydrogen with aromatic compounds in a 13.6-MHz discharge

Abstract: A series of aromatic compounds C6H5X (X=CH3, Cl, NO2, NH2, OCH3, CO2CH2CH3, COCH3, CN) were reacted with hydrogen in a 13.6-MHz inductively coupled glow discharge. The flow rates of aromatic and hydrogen were typically 0.5 mmol/min and 18 mmol/min, respectively. The applied power was varied from 50–200 W and the total pressure was varied from 2–14 torr. The products were collected and analyzed by gas chromatography. Three types of reactions were observed: (1) addition of hydrogen to the aromatic, (2) replacement of the group X by hydrogen, and (3) reactions characteristic of aromatic in the absence of hydrogen. The toluene reaction was studied most carefully. Methylcyclohexenes and benzene were the major products identified. The benzene was optimized by increasing the power and decreasing the pressure of either hydrogen or toluene. Reaction of toluene-d8 with hydrogen revealed that hydrogens were sequentially exchanged for deuteria on toluene and each of the products. A new apparatus is described which allows flow rates and pressure to be preselected and controlled and which allows a series of product samples to be collected without quenching the plasma.