摘要:

Laser‐induced fluorescence of CF2is used to monitor heterogeneous processes in ≊300 mTorr CF4/O2plasmas. CF2is rapidly removed at fluorinated copper and silver surfaces in 13.56‐MHz rf discharges as judged by a distinct dip in its spatial distribution. These metals, when employed as etch masks, are known to accelerate plasma etching of silicon, and the present results suggest catalytic dehalogenation of CF2is involved in this process. In contrast, aluminum and silicon dioxide exhibit negligible reactivity with CF2, which suggests that aluminum masks will not appreciably accelerate silicon etching and that ground state CF2does not efficiently etch silicon dioxide. Measurement of CF2decay in a pulsed discharge coupled with direct laser sputtering of metal into the gas phase indicates the interaction between CF2and the active metals is purely heterogeneous. Aluminum does, however, exhibit homogeneous reactivity with CF2. Redistribution of active metal by plasma sputtering readily occurs; silicon etch rates may also be enhanced by the metal’s presence on the silicon surface. Polymers contribute CF2to the plasma as they etch. The observation of an induction period suggests fluorination of the polymer surface is the first step in its degradation. Polymeric etch masks can therefore depress the silicon etch rate by removal of F atoms, the primary etchants.

ISSN:0021-8979    

DOI:10.1063/1.346551

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Axial magnetic fields earlier observed in laser‐produced plasma experiments can be generated in the case of nonlinear interactions of lasers with under dense plasmas. It has been shown that for high‐frequency laser (viz. Nd‐glass laser having the wavelength (&lgr;)=1.06 &mgr;m and the power flux (P)=1016W/cm2) the background dc magnetic field would be important only when electron cyclotron frequency is comparable to the laser frequency.

ISSN:0021-8979    

DOI:10.1063/1.346552

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The excitation of resonance and excimer radiation in mixtures of mercury and the rare gases has been investigated. The gas mixtures were excited in a dielectric barrier discharge. The emission features of such discharges which are mostly in the UV and VUV range were studied in detail. Whereas resonance radiation dominates in mixtures of mercury with He, Ne, and Ar, efficient excimer formation takes place in mixtures of mercury with Kr and Xe. All the emission features of the HgXe excimer which are expected between and around the resonance lines of mercury were found and measured. A theoretical model of HgXe excimer formation is presented. This model takes both the discharge formation and the charged particle kinetics as well as the excimer and excited species formation into account. Experimentally determined Boltzmann temperatures show a decline with mercury density as is predicted by the theory for the average electron temperature. Furthermore it is, e.g., possible to calculate the efficiency of the generation of the various species observed. Due to the geometrical flexibility and high obtainable power levels, dielectric barrier discharges are very well suited for the generation of UV radiation. Apart from well‐established techniques like UV lithography, disinfection, or UV curing, also new techniques in photochemistry such as, e.g., photochemical vapor deposition, profit from these new sources.

ISSN:0021-8979    

DOI:10.1063/1.346553

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The dynamics of relativistic electron beam and the oscillations of the virtual cathode in the vircator (virtual cathode oscillator) have been investigated by electrostatic particle‐in‐cell computer simulations. A specially developed PIC (particle‐in‐cell) computer code is employed in this study. In the diode region, cathode to anode, the solid electron beam is modeled by one‐dimensional sheets for simplicity. In the drift tube region the potential at the center line is obtained by solving the Poisson’s equation analytically in the radial direction and via PIC simulation in the axial direction. The electron beam in the drift tube region is modeled as a solid and uniform beam of constant radius equal to the cathode radius. The simulation results show that the potential minimum in the drift tube region oscillates at broad‐band frequencies. The injected electron beam in the diode region is found to be modulated by the bunched reflex electrons. The oscillation frequency of the potential minimum in the drift tube region is found to have strong dependence on the transit time of the electrons between the real cathode and the virtual cathode, and it scales inversely proportional to the cathode to anode distance. The effect of applied voltage, injected current, cathode‐to‐anode distance, and beam radius on the oscillation frequency has also been studied. The results are compared with predictions of theoretical and empirical formulas. The potential minimum is found to oscillate at a single constant frequency when the reflected electrons are not allowed to enter the diode region.

ISSN:0021-8979    

DOI:10.1063/1.346554

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

During laser welding, the plasma plume affects the amount of energy reaching the weld surface and the composition and properties of the welds. Light emissions during welding were recorded by emission spectroscopy to understand the energy absorption and the nature of the plasma formed during welding of various grades of steels. The flow of gases and the concentrations of the various metal vapors were computed by solving the Navier Stokes equation and the equations of conservation of various species. The variables studied were shielding gas composition and flow rate and the base metal composition. Until now, self‐absorption of emissions arising from species present at high concentrations within the plasma has kept researchers limited to either analyzing ideal situations that are unrelated to the welding process or not accounting for the attenuation of the emissions. It is demonstrated that during welding, the peaks in the emission spectra that are affected by the self‐absorption process can be eliminated on the basis of the initial and the terminal energy levels for electronic transitions. By selectively eliminating the affected transitions and by using the numerically computed local concentrations of metal vapors, the absorption of the laser beam energy by the plasma can be accurately determined.

ISSN:0021-8979    

DOI:10.1063/1.346555

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A plasma–microwave system was proposed by one of the authors for the production of microwave radiation. The device exploits the electromagnetic instabilities of nonequilibrium plasmas. During single‐mode operation of the plasma maser, the resonant particles become trapped and cease to radiate before a substantial fraction of the energy available could be extracted from the plasma in the form of radiation. (This is not true for multimode operation. During multimode operation, the total energy available is distributed over a large number of resonant lines and the amplitudes of the various frequency components of the waves are not at all large enough to cause particle trapping and subsequently saturation. Thus, the linear theory remains valid until the anisotropy disappears due to radiation.) One should, therefore, consider the interaction of the bulk plasma with the nonresonant monochromatic electromagnetic wave in order to estimate the total energy transferred from the plasma to the wave. In the framework of the linear treatment, due to the resonant nature of the instability considered in previous work by one of the authors, the transfer of energy from the nonresonant plasma to the wave is zero. Consequently, the efficiency of the device during single‐mode operation can only be evaluated after considering the nonlinear evolution of the plasma–microwave system. In this paper we describe in some details the nonlinear evolution of the plasma–wave system using a single particle analysis. The current densityJis calculated using the second moment of the particle distribution function (evaluated using Liouville’s theorem of conservation of phase space density along particle orbits), and the energy balance equation is solved numerically.

ISSN:0021-8979    

DOI:10.1063/1.346556

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The properties of vacuum gap closure initiated by a plasma cathode are presented. The plasma cathode consisted of approximately 60 surface flashover sites distributed over a 10‐cm2area. Vacuum gap dimensions were 1×7.5 cm in diameter. Faraday cup measurements indicated an ion density greater than 1012cm−3, which was controllable by the amplitude of the initiating high‐voltage pulse. Although the field‐free expansion velocity of the plasma was measured to be 7 cm/&mgr;s, plasma‐cathode‐initiated closure of the vacuum gap indicated closure speeds on the order of 0.5 cm/&mgr;s. Also, increased injected ion density and increased anode‐cathode potential resulted in increased closure velocity.

ISSN:0021-8979    

DOI:10.1063/1.346557

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The size and surface effects on the nematic‐isotropic phase transition of a liquid crystal are analyzed using a functional form of the surface energy proportional to the square of the scalar order parameter. The evaluated temperature shift agrees with the published experimental observations. This form seems confirmed by experimental data of the anchoring energy versus temperature, determined by means of the Freedericksz technique. The obtained formula for the temperature shift is similar to the well‐known Thomson formula, usually considered in the classical phase transitions.

ISSN:0021-8979    

DOI:10.1063/1.346558

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Hard amorphous films ofa‐C:D were deposited by a rf glow discharge in CD4. The implantation of 40‐keV3He+ions in these films was studied as a function of the helium fluence at temperatures between 100 K and room temperature. The trapped amounts of helium and the deuterium losses in the layer were measuredinsituusing nuclear reaction analysis with 0.5‐MeV D+and 1‐MeV3He+beams, respectively, and subsequentex‐situelastic‐recoil detection analysis. A transient helium retention appears in the carbonized layers, occurring only at temperatures below 200 K. Above a critical fluence which depends on temperature and ion flux, outdiffusion of He is observed which is ascribed to the formation of diffusion channels by radiation damage. The deuterium depletion induced by helium bombardment below 200 K sets on in correlation with the helium outdiffusion, but is otherwise independent of the helium trapping.

ISSN:0021-8979    

DOI:10.1063/1.346559

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In a strained layer grown epitaxially on a substrate, the motion of a dislocation on any particular glide plane in the layer can be influenced by the presence of dislocations on other glide planes. The focus here is on the glide of a dislocation extending from the free surface of the layer to the layer‐substrate interface, the so‐called threading dislocation. A general definition of driving force for glide of a threading dislocation in a nonuniform stress field is adopted to calculate the driving force on a threading dislocation due to an encounter with an interface misfit dislocation on an intersecting glide plane. The result is examined in detail for the case of cubic materials, taking into account different combinations of Burgers vectors. The analysis makes it clear that the misfit dislocation forces the threading dislocation to glide through a channel of width less than the full layer thickness. A blocking criterion is proposed, based on the presumption that blocking will occur if the channel width is less than the critical thickness for the local reduced strain. The results indicate that this effect can be significant in blocking the glide of a threading dislocation, depending on the mismatch strain magnitude and the layer thickness.

ISSN:0021-8979    

DOI:10.1063/1.346560

出版商:AIP    

年代:1990

数据来源: AIP