Beam Gas Research Articles

In-Situ Monitoring Of Etch By-Products During Reactive Ion Beam Etching Of Gaas In Chlorine/Argon

AbstractMass spectrometry of the plasma effluent during Reactive Ion Beam Etching (RIBE) of GaAs using an Inductively Coupled Plasma (ICP) source and a Cl2/Ar gas chemistry shows that AsCl3, AsCl2 and AsCl are all detected as etch products for As, while GaCl2 is the main signal detected for the Ga products. The variation in selective ion currents for the various etch products has been examined as a function of chuck temperature (30–100°C), percentage Cl2 in the gas flow, beam current (60–180 mA) and beam voltage (200–800 V). The results are consistent with AsCl3 and GaCl3 being the main etch product species under our conditions, with fragmentation being responsible for the observed mass spectra.

A high charge state heavy ion beam source for heavy ion fusion

A high current, low emittance, high charge state heavy ion beam source is being developed. This is designed to deliver a heavy ion fusion (HIF) driver accelerator scale beam. Using a high charge state beam in a driver accelerator for HIF may increase the acceleration efficiency, leading to a reduction in the driver accelerator size and cost. The proposed source system, which consists of a gas beam electron stripper followed by a high charge state beam separator, can be added to existing single charge state, low emittance, high brightness ion sources and injectors. We shall report on the source physics design using 3D beam simulations and experimental feasibility study results using a neutral gas stripper and a beam separator at the exit of the LBL 2 MV injector.

Self-guiding and stability of intense optical beams in gases undergoing ionization.

The propagation of intense optical beams in gases undergoing ionization is analyzed. Two types of optical beam modes are considered: a fundamental Gaussian and a higher-order radially polarized beam. The propagation dynamics include the effects of diffraction, nonlinear self-focusing, and ionization. For sufficiently intense optical beams the neutral gas undergoes ionization, generating a plasma which tends to defocus the beam. An envelope equation governing the spot size for both types of beams is derived, analyzed, and solved numerically. Self-guided solutions, which result from a balancing of diffraction, plasma defocusing, and nonlinear self-focusing, are analyzed for both types of beams. These equilibrium solutions are found to be unstable due to an ionization-modulation instability for which asymptotic growth rates are obtained. A self-guided inverse Cherenkov accelerator based on the higher-order radially polarized mode is proposed and analyzed. In addition, the depletion of the optical field due to collision and ionization losses is analyzed and the attenuation length is derived.

The nonequilibrium beam plasma

The kinetics and the heating of nonequilibrium plasma, formed as a result of injection of intensive relativistic (or subrelativistic) electron or proton beams in the atomic-molecular gas (nitrogen or air with water vapor) at different pressures (from 0.1–10 atm) have been investigated numerically by using the mathematical model “SKIF” which was developed by the author 15 years ago.More than 200 plasma-chemical reactions and elementary processes are taken into consideration, including the action of “non-Maxwell” electrons of the cascade, formed in the ionization of the molecules by the beam particles. For the description of the deviation of energy distribution of such electrons from the equilibrium distribution, a simplified analytical model was used, with the help of which one can calculate the energy spectrum of the cascade electrons with satisfactory precision. This essentially reduces the calculating expenditure.

Gas source molecular beam epitaxy of β-SiC on Si substrates

The growth of β-SiC films on Si(100) substrates using C 2H 2 gas and Si solid sources in a molecular beam epitaxy system has been investigated. Different C 2H 2 and Si fluxes as well as different substrate temperatures have been used. The growth was performed at two steps: the initial optimal carbonisation step followed by the MBE growth with simultaneous supply of Si molecular and C 2H 2 gas beams. The films were analysed using reflected high-energy electron diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy. Thin (< 0.1 μm) single crystalline SiC was grown at 980°C while 850°C was sufficient for the carbonisation of the Si surface. Films thicker than 0.1 μm are partially polycrystalline.

Design of a Closed Loop Computer Controlled System for Selective Area Laser Deposition. I. Laser Systems, Gasflow, and Substrate Temperature Control

Abstract A closed loop computer controlled system to implement selective area laser deposition (SALD) is described. The SALD process uses a laser beam that is moved relative to a substrate to control the region over which chemical vapor deposition occurs. Deposit shape control is achieved by controlling the laser beam path and power. The system combines a remote microwave plasma source with scanning laser induced chemical vapor deposition. Major design goals included computer control of process variables including reactor pressure, source gas flow rates, laser power and beam position, applied microwave power and tuning, and substrate bias temperature. Scanned shape data files are derived from industry standard “.STL” tessellated surface meshes. Control strategies for these process variables are described.

Exploring disordered quantum wires with non-equilibrium acoustic phonons

We examine the results of a recent experiment in which non-equilibrium acoustic phonons were used to probe an electron gas confined to a split-gate GaAs/AlGaAs quantum wire. The results showed a resistance decrease following the onset of the interaction between the confined electron gas and incident phonon beam and also peaks in resistance decrease versus gate voltage which approximately coincided with the conductance step edges. We obtain qualitative agreement with experiment using a model which assumes electron heating due to phonon absorption, together with a wire resistance temperature dependence arising from either (i) impurity scattering in the presence of electron - electron interactions, or (ii) weak localization with phase relaxation due to electron - electron scattering. Magnetoconductance measurements will be needed in order to determine which of these two gives the dominant temperature dependence. A third possible mechanism, the energy dependence of Coulomb scattering from remote donor impurities, gives rise to a resistance increase and is thus ruled out. Taking into account phonon focusing, we estimate the electron temperature increase for the heater geometry and heater temperature of the experiment and find that it is much smaller than the experimental estimate. Several possible explanations are given for this discrepancy.

Beam induced deposition of an ultraviolet transparent silicon oxide film by focused gallium ion beam

We have deposited a silicon oxide (SiOx) film with a high optical transmittance in the DUV region by a focused ion beam induced deposition technique using a gallium ion beam and a mixture of oxygen and TMCTS(1,3,5,7-tetramethylcyclotetrasiloxane) as a source gas. The optical transmittance of a 0.3 μm thick film is higher than 90% at the wavelength of 250 nm. The transmittance of the deposited SiOx film depends on both the source gas and ion beam irradiation conditions. A scaling to explain the transmittance along with the ion beam conditions is proposed.

Energy loss, electron screening and the astrophysical 3He(d, p) 4He cross section

We reanalyze the low-energy 3He(d, p) 4He cross section measurements of Engstler et al. using recently measured energy loss data for proton and deuteron beams in a helium gas. Although the new 3He(d, p) 4He astrophysical S-factors are significantly lower than those reported by Engstler et al., they clearly show the presence of electron screening effects. From the new astrophysical S-factors we find an electron screening energy in agreement with the adiabatic limit.

Controlling the resistive hose instability in relativistic electron beams

The resistive hose instability has long been recognized as the major impediment to the propagation of intense, relativistic electrons beams in dense gas. However, hose is a convective instability, and therefore its growth is limited by the length of the beam pulse, the local growth rate, and the speed at which the instability convects through the pulse. The convective speed and the growth rate depend on the beam and plasma parameters, and these vary strongly from beam head to tail. In this paper, hose theory is reformulated to incorporate these variations, and the reformulated model is then used to compute the maximum hose growth possible in a given beam pulse. In air, the model predicts that hose grows by many orders of magnitude when the beam current is less than 10 kA or has a rise time more than a few nanoseconds long. But the growth is predicted to be less than a factor of 20 if the current is 50 kA or more, the rise time is subnanosecond, and the beam radius is properly tapered from head to tail. The model is supported by extensive numerical simulations and is in general agreement with available experimental data. Many of the issues discussed here may have application to other instabilities as well.

Vibrational spectroscopy of small (HF)n clusters (n=4–8) in size-selected molecular beams

Molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small hydrogen fluoride clusters [(HF)n, n=4–8] in the spectral region between 3100 and 3700 cm−1. As tunable infrared radiation source we used an IR-seeded optical parametric oscillator (OPO) with a LiNbO3 crystal as nonlinear medium. Size-specific information has been obtained by scattering the cluster beam from a secondary rare gas beam. In this way all spectral features are assigned to specific cluster sizes. While the qualitative agreement with earlier experiments is good, the new assignments differ by one or more monomer units. In general, it is found that the absorption bands must be assigned to larger clusters. For some cluster sizes a doublet structure is observed. This observation is discussed in terms of combination bands involving intra- and intermolecular modes; but the possibility of isomeric structures is considered, too.

Transport and centering of high current electron beams in neutral gas filled cells

Conducting tubes filled with neutral gas at pressures between 0.001 and 0.1 Torr can be used to transport, to center, and to reduce the transverse oscillations of high current (≳10 kA) electron beams. Electron impact ionization of the gas leads to partial neutralization of the beam space charge allowing self-focused beam transport and phase-mix damping of injected beam oscillations. In addition, the presence of conducting walls helps center the beam in the transport tube. High current beams, transported through a 1.3 m long tube, were centered to within one-tenth of the beam radius and input transverse oscillations were damped to submillimeter values without significant current loss or emittance growth. Beam transport properties are examined as a function of injected current, gas pressure, and cell geometry. Experimental results are compared with a theoretical model.

Stimulated Raman scattering with a Gaussian pump beam in H2 gas.

Raman scattering of the Q(1) line in ${\mathrm{H}}_{2}$ gas was investigated from the spontaneous to the stimulated scattering region using a frequency-doubled Q-switched Nd:YAG laser (where YAG denotes yttrium aluminum garnet). We measured the Raman Stokes energy as a function of the laser power and the radial distribution of the Stokes intensity for the unfocused and the focused Gaussian laser beam. There is good agreement between the experiments and the results of a nonorthogonal mode theory in the region of stimulated Raman scattering. For cell lengths short compared to the Rayleigh range of the laser beam or for Raman cells far from the beam waist, stimulated Raman scattering is described in a good approximation by an adapted plane-wave theory. In the region of spontaneous Raman scattering, the measured dependence on cell length, solid angle, and focusing conditions is not in accordance with the nonorthogonal mode theory, but agrees with the standard theory of spontaneous Raman scattering.

Metastable liquid phase separation in tungsten inert gas and electron beam copper/stainless-steel welds

The microstructure of Tungsten Inert Gas (TIG) and electron beam copper/stainless-steel welds were investigated using scanning electron microscopy. The relatively high cooling rates entailed in the welding result in high bulk supercooling, causing two microstructural effects: (i) melt separation into two liquids, iron-rich L1, and copper-poor L2, while each solidifies by a path dictated by the stable phase boundary; (ii) enhanced solute trapping of Cu in the α-Fe phase, and Fe in the ɛ-Cu phase.

On the prospects of using runaway electron beams generated in an open discharge for the pumping of metal-vapor lasers

The excitation of gas lasers with the help of electron beams (e-beams) is, at the present time, one of the most promising methods of pumping. In most of the conventional methods, an electron beam is produced as a result of a collisionless acceleration of electrons due to the low gas density in a diode of an accelerator. Because of this, an accelerative gap should be hermetically isolated from a laser cell, which makes the technique of e-beam injection into the active region quite complicated. A radical solution to the problem would be the production of an e-beam in the gas whose density in the accelerative gap corresponds to the working pressure of the laser. This would allow one to place an accelerator in one chamber with the laser. The generation of a beam in a high-density gas would become possible when one provides the conditions for an effective transition of electrons into the runaway regime. Such a motion of electrons can be achieved in a strong electric field as a result of the decreasing cross section of the electron-atom interaction as sufficiently high energy. The runaway effect is manifested in the case where the electrons moving under the action ofmore » a strong external field obtain kinetic energy comparable with the potential difference. The kinetic energy is many times greater than that spent in one collision. Despite the considerable number of collisions, the electrons are transmitted into the steady acceleration regime, and their motion becomes almost directed, thus forming a beam. The present paper is devoted to the development of a physical-mathematical model of an open discharge, the calculation of its characteristics, and the estimation of the potentialities of e-beam pumping of metal-vapor lasers.« less

IR spectroscopy of size-selected clusters at 3 μm wavelength

An IR injection-seeded optical parametric oscillator (OPO) with narrow bandwidth ( Г ⩽0.25 cm −1 ) and high energy E ⩾ 4 mJ/pulse) output between 2.5 and 4 μm has been developed for cluster spectroscopy in molecular beams. The oscillator contains a LiNbO 3 crystal and is pumped by the 1.064 μm fundamental of a Q-switched Nd:YAG laser. The IR seed radiation is obtained by difference frequency mixing the output of a tunable dye laser with the second harmonic of the same Nd:YAG laser in a LiIO 3 crystal. After characterization, the OPO has been used to measure absorption spectra of small hydrogen-bonded clusters (HF) n and (H 2 O) n where n ranges from 2 to 8. The various spectral features could be assigned to specific cluster sized by dispersing the cluster beam with a secondary rare gas beam and by measuring the laser-induced depletion as a function of the scattering angle.

Investigation of Charged Particle Beam Stability: Numerical Method and Physical Results

Experiments on generation and transport of high current electron beams in gases and plasmas excite interest in studying their stability, which is reduced to solving a spectral boundary value problem for an ordinary second-order differential equation for some equilibrium beam configuration. The numerical technique which best satisfies the objectives of studying the beam stability and specific mathematical features of the correspondent spectral problem is the method of parameter evolution (MPE). In this paper a general scheme of the MPE is given, the evolution operator is derived for problems with smooth coefficients and the numerical algorithm is discussed. The problem of the bifurcations arising in the space of physical parameters is considered. An algorithm for predicting the point of bifurcation of a finite order and two algorithms for evolution across such a point are proposed. The following configurations of charged particle beams are studied on stability: a nonvortex tube electron beam in a longitudinal homogeneous magnetic field and radial electric field; and an isorotational vortex charged particle beam without the drift approximation.

Crossed beam rovibrational energy transfer from S1 glyoxal. III. Quantitative H2 and He cross sections for (0, K′=0) and (72, K′=0) and comparison with theory

Results of crossed-molecular beam inelastic scattering experiments from two levels of 1Au (S1) trans-glyoxal in collisions with H2 (Ec.m.=80 meV, 650 cm−1) and He (Ec.m.=95 meV, 770 cm−1) are reported. Relative inelastic scattering cross sections with quantitative error bars are obtained. S1←S0 laser excitation was used to prepare either the 00 level or the 72 level (εvib=466 cm−1) with the high rotational selection of K′=0 and J′=0–10. The final high levels populated by rotationally and rovibrationally inelastic scattering were monitored by dispersed fluorescence with K′ state resolution. Fluorescence from only those molecules involved in inelastic scattering was obtained from the difference signal of spectra with the target gas beam (H2 or He) ON and OFF. Those dispersed fluorescence spectra were analyzed with a computer fitting procedure to extract 52 relative state-to-state cross sections for scattering from the (00, K′=0) level and 84 for the (72, K′=0) level. The cross sections have been compared quantitatively with the results of the three-dimensional quantal scattering calculations of Clary, Dateo, Kroes, and Rettschnick. The agreement between the experimental and theoretical cross sections is nearly quantitative for both target gases and both initial states. Disagreements between experiment and theory occur only in the details of K′ distributions within the rovibrational channels. The vibrationally inelastic scattering is extremely selective among the many accessible channels. The cross sections for Δυ7=±1 changes in the lowest frequency mode ν7′ = 233 cm−1, a CHO–CHO torsion, exceed those involving the other 11 modes by at least an order of magnitude. As judged by the relative magnitudes of rovibrational and rotational cross sections, rovibrational scattering is surprisingly efficient. In fact, when comparing cross sections for transferring energy by (T→R) vs (T→R,V) with similar ΔE, rovibrational cross sections are the greater in numerous examples.

The ‘‘TITAN’’ ion source

TITAN is a new type of ion source capable of generating high current, wide aperture beams of gas and metal ions from a broad range of elements: Mg, Al, Ti, Cr, Fe, Co, Ni, Sm, Zn, W, Pb, Ta, Re, Y, C, He, N, Ar, and Xe. A specific feature of the TITAN ion source is the use of two kinds of arc discharges, each with cold cathodes, to produce plasma for ion beam extraction. Metal ions are generated by means of the vacuum arc in the metal vapor formed in cathode spots. Gas ions, on the other hand, are provided by a low-pressure constricted arc discharge. In a pulsed mode of operation the extraction voltage of the source ranges from 10 to 100 kV. The pulsed beam current for gas and metal ions is on the order of 1 A at pulse repetition rates up to 50 pulses per second and pulse duration of ∼400 μs. For dc operation and at an extraction voltage up to 10 kV, the ion current is as high as hundreds of milliamperes. This work outlines briefly the ion source, its design, and certain physical peculiarities observed when a high current ion beam is generated and transported.

Isotopic selectivity in the IR photosensitized dissociation of molecules. I. Generality of the effect

The dissociation rate of a molecule X, present under several isotopic varieties, X a, X b, … induced by a photosensitizer A, revealed in previous works a strong isotopic selectivity α D X a X b . This effect is investigated again. The couple A X = SF 6 SF 5Cl , with the isotopic species 32S, 33S, 34SF 5Cl, is studied systematically versus fluence, pressure, presence of an inert gas, pulse duration, and laser beam diameter. The maximum isotopic selectivity is α D X 32 33 = 1.62 and α D X 32 34 = 2.53. It is shown that the effect is very general and can be obtained with the following other couples A X : SF 6 CF 3I , SF 6 CF 3Br , BCl 3 SF 5Cl , SiF 4 SF 6 . In contrast, no measurable effect has been observed with SF 6 UF 6 The preferentially dissociated isotopic species X a can be predicted from the relative positions of IR active modes in A, X a, X b. The possible interpretations are systematically reviewed and agruments converge towards an isotopic selectivity α D X a X b of the dissociation, induced by an isotopic selectivity α T X a X b in the collisional transfer rates of the vibrational energy from A to X a and A to X b, in quasi-equilibrium conditions. Ambiguities or weaknesses of the interpretation are recorded in view of being removed in the following papers of this series.