High Reactor Pressure Research Articles

Optimizing utilization efficiencies in electronegative discharges: The importance of the impedance phase angle

We have investigated the operating conditions that result in the greatest utilization efficiencies (UEs) of NF3, CF4, and C2F6 in a capacitively coupled GEC reference cell. We have also independently measured the rf electrical characteristics and optical emission spectra of the plasmas. To avoid inadvertently attributing changes in the UE, discharge impedance, rf currents, or atomic emission intensities to parasitic losses in the matching network or rf delivery system, the rf generator was adjusted to ensure that the same amount of power was dissipated within each discharge. For the NF3 plasmas, argon was used as a diluent and both the NF3 concentration and reactor pressure were varied. For the CF4 and C2F6 based plasmas, the gas compositions were fixed (86 mol % CF4/O2 and 50 mol % C2F6/O2) and the reactor pressure was varied. The greatest NF3 UEs occurred within a narrow range of NF3 partial pressures. The greatest CF4 and C2F6 UEs occurred within a narrow range of reactor pressures. For all mixtures, operating conditions that yielded the highest UEs also yielded the brightest plasmas, the lowest impedance magnitudes, the greatest fraction of current flowing to the grounded electrode, and impedance phase angles within a narrow window centered near φpe=−40°. Within this region, plasma power is most efficiently utilized to dissociate the source gas and excite the atoms that emit light. Collapsed plasmas, observed for high pressure highly electronegative conditions, exhibited very low UEs. At optimal operating conditions the UE of the fluorinated source gases were found to decrease in the order: NF3>C2F6>CF4. The results of this study suggest that the baseline corrected fluorine atom emission intensity (703.7 nm), the magnitude of the discharge impedance, or phase angle of the discharge impedance could be monitored to determine the relative fluorinated source gas UE in an arbitrary plasma reactor as the operating conditions are varied. The concept of an ideal NF3 partial pressure could prove to be a useful strategy to prevent the formation of collapsed plasmas at high reactor pressures while maintaining high NF3UEs.

Thermal Rearrangement of Ethynylarenes to Cyclopentafused Polycyclic Aromatic Hydrocarbons: An Electronic Structure Study

BLYP/6-311G** electronic structure calculations are employed to elucidate the reaction mechanism of the thermal rearrangement of ethynylarenes to cyclopentafused polycyclic aromatic hydrocarbons (CP-PAHs). Out of two potentially possible unimolecular rearrangement pathways, a one-step process involving hydrogen migration immediately followed by ring closure is clearly favored thanks to a significantly lower energy barrier. Contrary to common belief, neither 2-(1-naphthyl)ethylidene carbene nor its annelated analogs are genuine chemical species. Consequently, these carbenes do not constitute intermediates pertinent to this reaction mechanism, which has the predicted activation energy of 55−62 kcal/mol depending on the extent of strain in the parent ethynylarene molecule. The present study conclusively rules out free-radical mechanisms that commence with hydrogen abstraction. However, in the case of strainless substrates and higher reaction pressures, the pathway involving initial hydrogen addition might be...

HREM study of 3C–SiC nanoparticles: influence of growth conditions on crystalline quality

In this contribution, we report on the influence of the growth conditions on the final crystalline quality of 3C–SiC nanoparticles with dimensions around 3–10 nm. The nanoparticles are grown by chemical vapour synthesis in a hot wall reactor using tetramethylsilane [(CH 3) 4Si] as precursor. By altering the decomposition temperature and reactor pressure, the clusters may be amorphous at low temperatures or crystalline at elevated temperatures. Crystalline nanoparticles show a one-dimensional stacking disorder with slip planes lying on different families of {1 1 1} planes when prepared at high reactor pressure. By lowering the pressure, only one-dimensional disordered particles along single (1 1 1) planes are observed.

Effects of hydrogen on carbon incorporation in GaN grown by remote plasma-enhanced metal-organic chemical vapor deposition

The effects of hydrogen on carbon incorporation in GaN layers grown by remote plasma-enhanced metal-organic chemical vapor deposition were investigated. Gas-phase decomposition of TEGa was enhanced at high RF power and reactor pressure conditions. With increase in hydrogen fractions in the downstream plasma, optical emission intensity of CN abruptly decreased. It is suggested that active hydrogen species may scavenge the carbon-containing species in the gas phase, resulting in reduced carbon incorporation in GaN layers measured by 10K photoluminescence and secondary-ion mass spectroscopy.

Surface Chemistry and Extreme-Pressure Lubricant Properties of Dimethyl Disulfide

The growth kinetics of a film formed by the thermal decomposition of dimethyl disulfide on an iron foil are measured using a microbalance where the growth kinetics are parabolic (film thickness X varies with time as X2 ∝ t) at high reaction temperatures and pressures, indicating that it is limited by diffusion through the film. The activation energy for this process is 54.5 ± 0.5 kcal/mol. The growth rate becomes linear as the reaction temperature and/or reactant pressure is lowered, indicating that, under these circumstances, the reaction rate is limited by thermal decomposition of dimethyl disulfide at the growing interface. The activation energy for thermal decomposition at the interface is found to be 37.6 ± 0.7 kcal/mol, and a half-order kinetics pressure dependence for the surface reaction rate is found consistent with a reaction limited by the rate of dimethyl disulfide dissociation. Analysis of the resulting film using Raman and X-ray photoelectron spectroscopies as well as X-ray diffraction reveal the formation of FeS, which may be slightly nonstoichiometric. This film is similar to that formed by methanethiol, suggesting that they may both initially form a surface thiolate species that further reacts to form FeS. The half-order reaction kinetics noted above are consistent with this. Measurement of dimethyl disulfide as an extreme-pressure (antiseizure) additive reveals a plateau at an applied load of ∼4000 N in the seizure load versus additive concentration curve. It has previously been suggested that the plateau corresponds to the load at which the interface reaches the melting point of the solid lubricant layer (in this case proposed to be FeS). Estimation of the interfacial temperature using a method developed previously yields an interfacial temperature of ∼1480 K, in good agreement with the melting point of FeS.

Improved growth and thermal stability of Parylene films

Experimental results for the deposition of low dielectric constant Parylene-N and Parylene-F films are explained by a mechanism based on bulk phase diffusion reaction in the film. The deposition rate of Parylene-F decreases significantly as time increases and ultimately achieves steady state. Very favorable agreement with experiments at a substrate temperature of −10 °C is obtained for parylene-F with a rate constant of kf=5×10−5/s, and a diffusion coefficient Df=1.3×10−8 cm2/s. In contrast, the much higher rate constant at room temperature for Parylene-N films is kf=0.16/s and the predicted deposition rate is constant after a very small period of time (tss<4/kf=25 s). This prediction is confirmed by the experiments. The model predicts correctly that low deposition temperatures improve both the deposition rate and thermal stability of Parylene films. Our experiments confirm this prediction and one obtains both high growth rates and high molecular weight films at lower deposition temperatures. Furthermore, experiments show that the longer chains formed at low temperatures significantly improve the thermal stability of the films. The deposition rate also can be improved by using higher reactor pressures. However, the pressure that can be employed is limited to about 100 mTorr to maintain high quality films. At liquid nitrogen temperature (77 K), the deposition rate is three orders of magnitude larger than the film deposition rate at room temperature. The increased monomer concentration on the substrate due to higher condensation leads to this increased deposition rate.

Inorganic fibres and microfabricated parts by laser assisted chemical vapour deposition (LCVD): Structures and properties

Laser assisted chemical vapour deposition (LCVD) is a new generic process for fabricating advanced inorganic fibres, as well as three-dimensional microsprings and solenoid, directly from the vapour phase. Using high reactor pressures (> 1 bar) and a unique rate control mechanism, chemically pure and structurally uniform boron, carbon, silicon, silicon nitride and carbide, and germanium fibres with small diameters (> 6 μm) and high growth rates (0.3–1.1 mm/s) were obtained. Among others, ultra strong (> 7.5 GPa) boron and stoichiometric silicon carbide fibres were obtained, as well as highly flexible carbon fibres representing a new form of carbon. This paper deals with structure-property relationships relative to market requirements. Growth rates and process economics are comparable to those by which commercial sapphire fibres are obtained from the melt. The process promises to afford new continuous single crystal fibres (including silicon and tantalum carbide) for structural composites uses at temperatures ranging from 1500 to 2200 °C, new sensor fibres (including ZnSe) and fibres for consumable high temperature sensor systems.

Optimization of saturation current density of PECVD SiN coated phosphorus diffused emitters using neural network modeling

Emitter surface passivation by low temperature plasma enhanced chemical vapor deposition (PECVD) silicon nitride is investigated and optimized in this paper. We have found that the saturation current density of a 90±10 μ/sq phosphorus diffused emitter with Ns ≈3 x 1019 and Xj ≈0.3 µm can be lowered by a factor of eight by appropriate PECVD silicon nitride deposition and photoassisted anneal. PECVD silicon nitride deposition alone reduces the emitter saturation density (Joe) by about a factor of two to three, and a subsequent photoanneal at temperatures ≥350°C reduces Joe by another factor of three. In spite of the larger flat band shift for direct PECVD silicon nitride coating, the silicon nitride induced surface passivation is found to be about a factor of two inferior to the thermal oxide plus PECVD silicon nitride passivation due to higher interface state density at the SiN/SiO2 interface compared to SiO2/Si interface. A combination of statistical experimental design and neural network modeling is used to show quantitatively that lower radio frequency power, higher substrate temperature, and higher reactor pressure during the PECVD deposition can reduce the Joe of the silicon nitride coated emitter.

A study of parasitic reactions between NH3 and TMGa or TMAI

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH3 is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH3. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH3.

Gamma-ray (electron) irradiation effects on tensile properties of ferritic alloys

We demonstrate that high-energy electron irradiation can be used to study gamma-ray embrittlement of reactor pressure vessels. Observed changes in tensile properties, in ferritic alloys electron irradiated at low temperature (⩽ 50°C), correlate well with neutron irradiation data when compared simply on a displacement per atom basis. The similarity in embrittlement behavior between gamma-rays (electrons) and neutrons can be explained in terms of a reaction-rate theory model of defect production and clustering. The present results, obtained under well-controlled experimental conditions, provide strong support for a previous analysis implicating gamma-ray damage as the source of the “accelerated” embrittlement of the High Flux Isotope Reactor (HFIR) pressure vessel.

Effect of Reaction Variables on TPA-EG Continuous Esterification with Recycling in PET Production Process

ABSTRACTSimulations of a continuous recycle process were carried out for the direct esterification between terephthalic acid and ethylene glycol in the production of poly(ethylene terephthalate). The influence of reaction pressure, recycle ratio and residence time on the oligomer characteristics was examined and the following results were obtained: (1) The main reactions go on further under higher reaction pressure but the side reactions on diethylene glycol (DEG) proceed to a greater extent than the main reactions. (2) The higher residence time ratio of the first reactor to the total promotes esterification remarkably as the pressure becomes high. (3) As the recycle ratio becomes high, the polymerization degree becomes high, but, at higher degrees of polymerization, the tendency is reversed. (4) The oligomer characteristics are almost the same at the same esterification degree independently of the reaction conditions.

Skeletal Isomerization of n-Butane on Zeolites and Sulfated Zirconium Oxide Promoted by Platinum: Effect of Reaction Pressure

The isomerization of n-butane catalyzed by sulfated zirconium oxide and H-form zeolites with the promotion of platinum has been studied by measurements of the conversion and product distribution at temperatures of 240 ∼ 390°C at atmospheric pressure and at 20.4 atm. The skeletal isomerization proceeds mainly via the bimolecular disproportionation mechanism on Pt/H-zeolite at 1 atm and via the monomolecular carbocation mechanism on Pt/SO2−4–ZrO2 at temperatures of ≤ 300°C at both 1 and 20.4 atm. At 20.4 atm pressure, the monomolecular cracking and isomerization of butane can also occur on Pt/H-zeolite. Hydrogen molecules may be dissociated and converted to hydride and proton ions on Pt/SO2−4–ZrO2, promoting the desorption of surface carbenium ions and the cracking of butane molecules, which have been enhanced under high reaction pressures.

Fischer-Tropsch Synthesis in a Slurry Reactor. Pretreatment Effect Studies

Effects of pretreatment procedures, using H 2 , CO, and syngas (H2/CO = 0.7) as reductants, on the performance (activity, selectivity, and stability with time) of a precipitated iron catalyst (100 Fe/5 Cu/4.2 K/16 SiO 2 on a mass basis) during Fischer-Tropsch (FT) synthesis were studied in a stirred tank slurry reactor. The syngas conversion varied between 70 and 87% during the first 300 h on stream at baseline process conditions (260 °C, 1.48 MPa, 1.5 L(STP)/g-cat/h, H2/ CO = 0.67), and the catalyst deactivation rate was less than 1% per day in all three tests. The catalyst selectivity did not vary markedly with time on stream. The reactor space-time-yield increased by 32-47% in all three tests, upon increasing the reaction pressure and gas space velocity to 2.17 MPa and 2.2 L(STP)/g-cat/h, respectively. The catalyst deactivation rate increased and the water-gas-shift (WGS) activity decreased during testing at the higher reaction pressure. This resulted in a slight decrease of secondary olefin hydrogenation and isomerization reactions in all three tests, and in a shift of hydrocarbon distribution toward higher molecular weight products (CO pretreated catalyst). Total olefin content decreased in the following order with the pretreatment conditions: CO > syngas > H 2 , whereas the olefin isomerization activity was essentially independent of the pretreatment conditions employed. Methane selectivity varied with the pretreatment conditions in the following order: CO < H 2 < syngas.

The effects of oxygen on diamond synthesis by hot-filament chemical vapor deposition

The effects of oxygen addition on the synthesis of diamond are extensively studied by using the hot-filament chemical vapor deposition (HFCVD) method, in which it is simple and easy to control the deposition parameters independently. Diamond films are deposited on silicon wafers under the conditions of substrate temperature 530–950 ‡C; total reaction pressure 700–8000 Pa; and methane concentration 0.4–2.4% in both CH4–H2 and CH4–H2–O2 systems. At deposition conditions of low substrate temperature, high CH4 concentration or high total pressure, soot-like carbon and/or graphite are deposited without oxygen addition. When even a small amount of oxygen (about 0.6%) is added, well-faceted diamond films are observed in scanning electron microscopy micrographs and a sharp diamond peak in the Raman spectra appears. The range of deposition parameters for high-quality diamond syntheses are extended by oxygen addition (low substrate temperature, high methane concentration and high reaction pressure).

Comparison of advanced plasma sources for etching applications. II. Langmuir probe studies of a helicon and a multipole electron cyclotron resonance source

Radial profiles of electron temperature, electron density, and ion current density near the wafer surface are presented for a rf-inductively coupled helicon and a multipole electron cyclotron resonance source as a function of applied source power, applied rf-bias power, and reactor pressure. Both sources show similar trends with respect to changing process conditions, with electron densities in the mid 1011 cm−3 range and electron temperatures between 5.5 and 6.5 eV. An increase in applied source power results in an increase in both electron density and ion current density as expected. An increase in applied rf-bias power results in a 10% change in the measured electron density and ion current density; however, the electron temperature shows a much stronger dependence, indicating that ion flux and ion energy are not completely independent. Radial uniformity is similar in both sources, with the helicon exhibiting better uniformity at the conditions explored. Neither applied source power nor rf-bias power significantly affect radial plasma uniformity above the wafer in both sources, but higher reactor pressure does degrade uniformity in the helicon. We attribute this to the 2:1 geometric expansion between the upper antenna section and the lower plasma confinement section where the measurements are taken. The conditions resulting in optimum radial uniformity of electron temperature, electron density, and ion current density are similar to those for the optimum etching rate uniformity measured in previous studies; yet the uniformity obtained from the Langmuir probe diagnostic is not affected by changing process conditions to the degree that etching uniformity is affected. This can be a result of the difference in chemistry between the two studies since hydrogen bromide was used for etching and argon was used for Langmuir probe measurements. It may also indicate that the correlation between plasma uniformity and etching uniformity is relatively weak or second order compared to other variables affecting uniformity such as rf bias of the wafer.

An evaluation of low temperature radiation embrittlement mechanisms in ferritic alloys

Investigations underway at Oak Ridge National Laboratory (ORNL) into reasons for the accelerated embrittlement of surveillance specimens of ferritic steels irradiated at 50°C at the High Flux Isotope Reactor (HFIR) pressure vessel are described. Originally, the major suspects for the precocious embrittlement were a highly thermalized neutron spectrum, a low displacement rate, and the impurities boron and copper. Each of these possibilities has been eliminated. A dosimetry experiment made at one of the major surveillance sites shows that the spectrum at that site is not thermalized. A new model of matrix hardening due to point defect clusters indicates little effect of displacement rate at low irradiation temperature. Boron levels are measured at 1 wppm or less, which is inadequate for embrittlement. Copper and nickel impurities are shown to promote radiation strengthening at high doses but not at the low doses pertinent to the surveillance data. It is shown that a copper embrittlement scenario has other drawbacks, and it is argued that copper impurity is not responsible for the accelerated embrittlement of the HFIR surveillance specimens. The dosimetry experiment revealed unexpectedly high levels of reaction products in some of the fast flux monitors, which are found to be caused by an exceptionally high ratio of gamma ray flux to fast neutron flux at the pressure vessel. Gamma rays can also induce atomic displacements, leading to the suggestion that the accelerated embrittlement may be provoked by gamma irradiation.

Low pressure growth and nitrogen doping in metalorganic vapor phase epitaxy of ZnSe

Low pressure (0.1–1.0 Torr) metalorganic vapor phase epitaxy (MOVPE) of ZnSe, using tertiarybutylselenol (t-BuSeH) as a selenium source precursor, was studied. Although several growth properties at such a low pressure were different from those at a higher reactor pressure, the quality of the layers grown at 1.0 Torr was as good as that of the layers grown at 300 Torr. Nitrogen doping into ZnSe was carried out at 1.0 Torr using nitrogen plasma excited by 2.45 GHz microwave. Nitrogen concentrations [N] of the doped layers ranged from 2X10 17 to 8X10 18 cm -3, measured by secondary ion mass spectrometry (SIMS), and photoluminescence spectra measured at 4.2 K of such layers were dominated by donor-acceptor pair emissions. However, all the as-doped layers were highly resistive n-type. It was found that hydrogen was incorporated into the doped layers, and its concentration was about 1.7 times more than [N] as established by SIMS measurement. Therefore it is probable that hydrogen passivates nitrogen acceptors in the doped layers. Several doped layers were annealed at 550–700°C in atmospheric N 2 after deposition of SiO 2 cap layers. The carrier types of part of the annealed samples were inverted to p-type, and their net acceptor concentrations were (3–7)X10 15 cm -3.

Amorphous silicon nitride fibers grown from the vapor phase

Using high reactor pressures (&gt;1 bar) and a unique rate control mechanism, three fibers were recently obtained by laser assisted chemical vapor deposition (LCVD) having elemental (i.e., boron, carbon, and silicon) compositions, small diameters (&gt;9 μm), and surprisingly high growth rates (0.3–1.1 mm/s). By reacting silane and ammonia at high pressures (&gt;1 bar) near the focus of a Nd-YAG laser beam, we have now obtained the first LCVD fibers with binary (i.e., silicon-nitrogen and silicon nitride) compositions having small diameters and high growth rates (0.34–0.74 mm/s). These fibers were amorphous.

Inorganic fibers and microstructures directly from the vapor phase

By using high reactor pressures (>1 bar) and a unique rate control mechanism, chemically pure and structurally uniform boron, carbon, silicon, silicon nitride and silicon carbide fibers were obtained by laser-assisted chemical vapor deposition (LCVD) with high growth rates (0·3–1·1 mm/s), high tensile strengths (up to 7·0 GPa) and small diameters (>9 μm). The boron, carbon and silicon nitride fibers were amorphous, the silicon carbide fibers polycrystalline, and the silicon fibers either glassy, polycrystalline or single crystals. The latter were formed by a vapor-liquid-solid (VLS) mechanism. The process speeds, and most likely the first-order process economics, are about the same as those reported for the commercial manufacture of single crystal sapphire fibers (which are, however, made by a different process). A commercial LCVD multifilament process is being developed. LCVD was also used to grow free-standing, three-dimensional microstructures (grids, springs, solenoids) directly from the vapor phase. Related microstructures (microwafers with VLS silicon bristles) are also becoming commercially available.

Novel Carbon Fibers by Laser Assisted Chemical Vapor Deposition (Lcvd)

ABSTRACTChemically pure carbon fibers with small fiber diameters and high growth rates were obtained by laser assisted chemical vapor deposition (LCVD) using high reactor pressures and a unique rate control mechanism. Depending upon growth conditions and gases, these fibers were either flexible (elastic), brittle (thickened) or graphitic (strong). The elastic carbon fibers were uniform and appear to represent a novel form of carbon. The reactants were acetylene, ethylene or methane, and the reaction pressures ranged from 1.9 to 7.5 bar. The highest fiber growth rate was 0.33 mrn/s, and the lowest fiber diameter was 10 μm.