Pulsed laser deposition of silicon nitride thin films by laser ablation of a Si target in low pressure ammonia

We report the deposition of Si-N films by multipulse excimer laser ( = 308 nm, _FWHM = 30 ns) ablation of Si wafers placed in a slow flow of NH₃ in the pressure range (1 bar-1 mbar). The films are deposited on to a Si collector placed parallel to the Si target. We succeeded in depositing pure amorphous Si₃N₄ films at a pressure of 1 mbar of NH₃. The deposition rate reached a maximum value of 0.2–0.3 nm per pulse. At lower pressures, the deposited films consist of a fine mixture of three amorphous phases (amorphous stoichiometric silicon nitride, amorphous non-stoichiometric silicon nitride and amorphous silicon). The amorphous silicon is prevalent in films deposited at a pressure of several to several tens of bars. Droplets of polycrystalline -Si are sometimes visible on the film surface. The experimental evidence, is analysed with a view to elucidating the participation in the chemical synthesis of the three main stages of the process: the substance expulsion from the target by laser ablation, the transition through the gas of the expulsed substance and it's final impact on the collector. We conclude that silicon nitride is mostly synthesized during the impact on the collector of the flow of the ablated substance.     

Modeling of the supersonic argon plasma jet at low gas pressure environment

Numerical simulation results are presented concerning the heat transfer and fluid flow within the supersonic argon plasma jet encountered in low pressure (or soft vacuum) plasma spraying (LPPS). The plasma parameters at the inlet section of the plasma jet are taken from our modeling results of the subsonic-to-supersonic d.c. arc plasma torch. The mach number, temperature and static pressure at the center of the plasma jet on the torch exit section are 2.8, 13 200 K and 6000 Pa, respectively, whereas the environment (i.e. vacuum chamber) pressure is 0.1 atm. Those parameters are typical for LPPS. The plasma jet is assumed to be axi-symmetrical and in local thermodynamic equilibrium state. The All-Speed SIMPLE algorithm is coupled with the FAST-2D program to simulate the whole plasma jet containing both the supersonic and subsonic flow regions. Modeling results clearly show that there exist several successive temperature, velocity and static wave crests and troughs. The fluctuation magnitudes of those parameters reduce rapidly in the flow direction, along with the flow transformation from the supersonic flow regime into the subsonic flow regime. The existence of a series of compression and expansion waves in the region near the torch nozzle exit shows clearly the over-expanded characteristics of the supersonic plasma flow.     

气体膜分离技术在低压瓦斯回收中的应用

说明了有机蒸气膜分离的原理、性能、应用．介绍了大庆炼化公司应用有机蒸气膜分离技术回收低压瓦斯中的C3、C4．原料气进行净化后通过膜装置对瓦斯中的C3以上组分富集，然后通过压缩机把渗透气送到ARGG装置回炼，投用后取得了较大经济效益．     

Atmospheric pressure chemical ionization source. 1. Ionization of compounds in the gas phase

A novel chemical ionization source for organic mass spectrometry is introduced. This new source uses a glow discharge in the flowing afterglow mode for the generation of excited species and ions. The direct-current gas discharge is operated in helium at atmospheric pressure; typical operating voltages and currents are around 500 V and 25 mA, respectively. The species generated by this atmospheric pressure glow discharge are mixed with ambient air to generate reagent ions (mostly ionized water clusters and NO+), which are then used for the ionization of gaseous organic compounds. A wide variety of substances, both polar and nonpolar, can be ionized. The resulting mass spectra generally show the parent molecular ion (M+ or MH+) with little or no fragmentation. Proton transfer from ionized water clusters has been identified as the main ionization pathway. However, the presence of radical molecular ions (M+) for some compounds indicates that other ionization mechanisms are also involved. The analytical capabilities of this source were evaluated with a time-of-flight mass spectrometer, and preliminary characterization shows very good stability, linearity, and sensitivity. Limits of detection in the single to tens of femtomole range are reported for selected compounds.     

Absorption of the laser radiation by the laser plasma with gas microjet targets

An upper limit of absorption of the laser radiation in the plasma produced in a gas jet Xe target with the average density of (3–6) × 10¹⁸ cm^–3 and the effective diameter of 0.7 mm is found. It is equal to ≈50% and remains constant under any variation in this range of densities. This result contradicts both theoretical assessments that have predicted virtually complete absorption and results of earlier experiments with the laser spark in an unlimited stationary Xe gas with the same density, where the upper limit of absorption was close to 100%. An analysis shows that nonlinearity of absorption and plasma nonequilibrium lead to the reduction of the absorption coefficient that, along with the limited size of plasma, can explain the experimental results.     

Chemical oxygen-iodine laser: aerooptics and gas dynamics

We have investigated the interaction of regular and irregular structures — systems of shock waves, rarefaction waves, and turbulent layers — in a chemical oxygen-iodine laser.     

A new low pressure plasma nitriding method

A new method of nitriding in low pressure plasmas was developed. The method can be used to combine the benefits of low pressure ion plating with plasma nitriding to produce wear-resistant coatings for various applications since both treatments can be carried out within the same vaccum chamber as consecutive steps. The low pressure plasma nitriding process is quite different from the conventional methods used in industrial applications so far. However, extensive tests confirm that low pressure plasma nitriding compares favourably with other nitriding processes in applications where wear resistance is required. The results are believed to be due to the creation of a thin and homogeneous compound layer during the low pressure treatment.     

Improvement of the characteristics of a plasma torch at low gas flow rate

It is shown that the gas dynamics of a plasma torch and its characteristics depend on the configuration of the discharge chamber. Tapered boring at the cylindrical anode inlet (10° over a length of 11 mm) makes it possible to increase the arc voltage, torch efficiency, and enthalpy of the plasma jet at low gas flow rates. At high gas flow rates, the above parameters are higher in a torch where the anode does not have tapered boring. This is explained by turbulization of the heated gas.Academic Scientific Complex A. V. Luikov Heat and Mass Transfer Institute, Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 3, pp. 338–339, March, 1994.     

Microwave plasma torch operating at a low pressure for material processing

Most operations involving the use of a microwave plasma torch are carried out at atmospheric pressure as an open system. A microwave plasma torch operated in a closed and isolated environment may provide an opportunity for the mass production of chemically active radicals for various chemical and biological processes. This study presents a microwave plasma torch operating at a low pressure in an isolated chamber. A microwave torch operating at a low pressure may be economical in terms of capital costs, maintenance costs, and operational costs compared to other plasma devices used in material processing operations. The properties of the torch plasma at a low pressure were investigated. It is found that the plasma profile at a low pressure is asymmetric with higher density on the incoming side of the microwaves. This behavior of the torch inhibits high-power operation of the microwave plasma torch at a low pressure. However, the asymmetry of the plasma profile disappears under a high gas flow rate. It was also found that a microwave plasma torch used at a low pressure can efficiently produce an abundance of chemical radicals.     

Dry gas seals for low and high pressure

The following article reviews test results accumulated on gas lubricated seals for compressors. The influence of various combinations of materials on the operating behaviour of the seals is also analyzed.     

Ionization of a substance evaporated from a droplet in nonequilibrium plasma

An exact analytical solution is obtained for a linear stationary problem of the ionization of a spherically symmetric flux of a substance expanding in an unbounded nonequilibrium plasma. It is shown that the radial distribution of the degree of ionization in this flux is determined by a single dimensionless scaling parameter.     

A study of the laser plasma in stationary gases at low pressures

Stretched along the laser beam and symmetrical relative to the focus forms of the laser spark induced with the Nd:YAG laser have been observed in stationary gases at low pressures. The spark length rose as the gas pressure increased amounting up to several millimeters. Based on shock waves, conventional mechanism of laser plasma propagation along the beam has been placed in question. Plasma temperature has been deduced from laser energy absorption in the plasma.     

Nonlinear dynamics of a plasma torch generated by a laser pulse of large width

Experimental data on the nonlinear dynamics of a plasma torch generated by a laser pulse of large width acting upon a graphite target are presented and discussed. The mushroom shape of the luminous region and the duration of emission observed in experiment are explained by the development of the Richtmyer-Meshkov instability at the carbon plasma-air interface and by the formation of nanoparticles in the plasma expanding into the buffer gas.     

Role of particles of the target material in the dynamics of an erosive jet formed by a laser beam

Empirical substantiation is offered for a model of the destruction of metal by moderate-intensity (10⁵–10⁸ W/cm²) laser radiation. The model allows for the presence of a finely dispersed liquid-droplet phase of the target material in the erosive jet. This phase is formed by the mechanism of bulk vaporization.A. N. Sevchenko Scientific-Research Institute of Applied Physics Problems, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 62, No. 5, pp. 665–684, May, 1992.     

The growth of diamond in microwave plasma under low pressure

The synthesis of diamond particles in a low pressure plasma has been studied, with emphasis on the investigation of the substrate effect and the plasma conditions. It was found that a special pre-treatment of silicon substrate made it possible to form dense films, and a thickness of about 15m could be reached by 20 h discharge. Unfortunately, however, the prepared films had poor adhesion. Observations by scanning electron microscope (SEM) showed that the poor adhesion was due to the fact that the film consisted of large particles with a diameter of about 10m, and each particle had contacted to the substrate only by a point, not by a face. In addition, the plasma diagnostics of optical and ultraviolet emission spectroscopy (OES, 200–750 nm) revealed that CH and H radicals have come to be criteria for the formation of diamonds, and the ratio of radicals drastically affected the characteristics of the deposits. Nucleation and growth mechanism are also discussed.     

Computational simulation of laser plasma emission with shock-wave-affected density distribution in the gas-jet target

Based on the results of numerical fluid dynamics simulation, an imitation parameter has been constructed which simulates the observed intensity of the laser plasma emission in a short-wave range. Within the computational model frame, a high-temperature perturbation is created in the jet that generates a strong shock wave. The resultant complicated target structure and its evolution lead to nonmonotonic time variations of the simulation parameter. This result agrees well with the experimentally measured behavior of emission from the laser plasma formed on the target perturbed by an additional laser prepulse.