Ar Beam Research Articles

Systematic studies of scintillation detector with timing resolution of 10 ps for heavy ion beam

We have tested Time-Of-Flight detectors consisting of several combinations of scintillators and photomultiplier tubes for the identification of produced heavy ion beam. Timing resolution of 6.8 ps (RMS) is achieved with energy deposited of 150– 160 MeV in the plastic scintillator, where 40 Ar beam is used as a primary beam at the total energies of 3.8 GeV . Our systematic studies suggest the importance of maximizing the number of photoelectrons, where the timing resolution seems to be scaled by the pulse height according to the characteristic function of the photomultiplier tube. In addition, scintillator with faster timing property seems to be also essential for better timing resolution.

Fabrication of single electron transistors in multi-wall carbon nanotubes using Ar beam irradiation

The Ar beam with an acceleration voltage of 300 V has been irradiated to multi-wall carbon nanotubes (MWNTs) through a narrow opening of the resist, in order to form a tunnel barrier. The tunnel resistance could be set by appropriate dose, in the range greater than h/ e 2, and single electron transistors (SETs) have been fabricated with two tunnel barriers in series. Electrical transport measurements have been carried out at 2.6 K. Most of the samples have shown periodic Coulomb diamonds, suggesting the formation of a single quantum dot with a maximum Coulomb gap ranging from 5 to 26 meV. In a few samples, there was still a finite Coulomb gap at the gate voltages where two adjacent diamonds meet, suggesting an unexpected multi-barrier formation. The present technique may be useful for the fabrication of more complicated quantum dot-based nanodevices in carbon nanotubes.

Probing a cold surface with slow heavy-atom scattering: Experimental results and theoretical calculations

Slow heavy atoms scattering from cold surfaces excite many phonons, but still have a finite elastic scattering fraction. We have measured the coherent elastic intensity of low-energy Ne, Ar, and Kr beams scattered from a Cu(111) surface, and compared the results to accurate semiclassical calculations. Earlier experiments have been extended to a range of incident beam energies, while earlier calculations have been improved by using state-of-the-art atom-surface potentials and surface phonon densities of states. All inputs to the calculations are taken from the literature, so they involve no adjustable parameters. We compare both the zero-temperature limit and the thermal attenuation of the elastic scattering at elevated temperatures. Our comparisons confirm that the probability for elastic scattering is (i) independent of particle mass m at low surface temperatures and (ii) depends on $\sqrt{m}$ at elevated temperatures. Corresponding experimental observations for the comparatively light particles ${\mathrm{H}}_{2},$ ${\mathrm{D}}_{2},$ and He show clear deviations from the semiclassical predictions. Our analysis shows that this regime provides new and complementary information about both surface phonons and atom-surface potentials.

Stopping energetic beams in gas

The stopping of Ar beams at energies up to 100 MeV/nucleon in a combination of Al energy degrader and He gas volume was investigated. The range straggling was not largely enhanced by the energy degrader. A Monte Carlo simulation was applied in order to calculate the spatial distribution of stopped ions in the gas.

Capture and fusion dynamics in heavy-ion collisions

The dependence of the difference between the processes of capture and complete fusion is studied as a function of the entrance channel in the framework of the combined dynamical and statistical model. The excitation functions for the 126 Ba compound nucleus formed in the 19 F + 107 Ag and 28 Si + 98 Mo reactions were calculated to compare the E min and E max values of the beam energy window for the fusion cross section. The importance of the competition between quasi-fission and complete fusion in interpreting the experimental data obtained for the 40 Ar + nat Ag reaction is discussed. The increase in the difference between the experimental evaporation residue production and the extracted fusion cross section at increasing 40 Ar beam energy is explained as an increasing effect of the quasi-fission process.

Experimental determination of neutron spectra produced by bombarding thick targets: Deuterons (100 MeV/u) on 9Be, deuterons (100 MeV/u) on 238U and 36Ar (95 MeV/u) on 12C

Experimental results concerning neutron distributions from thick Be and U targets bombarded by deuterons at 200 MeV incident energies and from a thick C target bombarded by 36Ar at 95 MeV/u are reported. The measurements were performed with the activation technique using an improved set of detectors. A comparison was carried out with theoretical results in the framework of the Serber model for deuteron beams and using a schematic fireball model for Ar beams.

Characteristics of ion beams from a Penning source for focused ion beam applications

Focused ion beam characteristics, measured by using PMMA resist exposures, show significant improvement of the previously reported results for hydrogen ion beams. A beam current density of >50 mA cm−2 at 6–7 keV has been obtained over a spot size of 2.3 μm, estimated from the full width at half maximum of the intensity distribution. A single Einzel lens with magnification of about 0.1 was used. A target current density of >1 A/cm2 can be achieved with this beam for a lens system with overall magnification of ∼0.01. In experiments with Ar, both the discharge current and extracted beam current increased by more than a factor of 2 when the discharge cell length was increased by about 25% with respect to the nominal cell length for operation with H. Ar ion beams were extracted with an angular beam intensity of >5 mA sr−1, brightness (90% beam) of ∼103 A sr−1 cm−2 at 10 keV, and energy spread of ∼4.5 eV. Greatly improved beam characteristics, with brightness >104 A sr−1 cm−2, are expected by optimization of the source for Ar beams.

Transmission Electron Microscope Observations of Si/Si Interface Bonded at Room Temperature by Ar Beam Surface Activation

Silicon wafers are strongly bonded at room temperature by Ar beam surface etching in ultrahigh vacuum. The bonding interface is investigated using a transmission electron microscope (TEM). Residual strain originating from surface roughness exists in the interface region. It is observed only in specimens bonded at room temperature, because it relaxes even by low-temperature annealing at 400°C. An amorphous-like intermediate layer is observed in high-resolution images of the interface. The layer is quite unstable and different from the oxide layer which is often observed at the Si/Si interface prepared by the conventional hydrophilic wafer bonding method. Both annealing and electron beam irradiation during TEM observation cause recrystallization of the layer. The layer contains Ar, but the O concentration is less than the detection limit of analytical TEM. The layer thickness changes according to the kinetic energy of Ar beam. These points demonstrate that Ar atom implantation during the surface etching introduces surface damage, forming the amorphous-like layer.

Evidence for particle stability of [formula omitted]F and particle instability of [formula omitted]N and [formula omitted

The neutron drip line up to fluorine has been investigated by the projectile fragmentation of a 94.1 A MeV 40 Ar beam at the fragment separator RIPS at RIKEN. A new neutron-rich isotope, 31 F, has been observed for the first time while clear evidence for the particle instability of 24,25 N, 27,28 O and 30 F has been obtained. The sudden change in stability from oxygen to fluorine may demonstrate the onset of deformation for the neutron-rich fluorine isotopes.

Effect of a metal–dielectric structure introduced in the plasma chamber of the Frankfurt 14 GHz electron cyclotron resonance ion source

A new approach of the possibility to significantly increase the high charge state ion beams delivered by electron cyclotron resonance (ECR) ion sources by using metal–dielectric (MD) structures characterized by high secondary electron emission properties is presented. The intensities of argon ion beams extracted from the 14 GHz electron cyclotron resonance ion source of the Institut für Kernphysik (IKF) der Johann Wolfgang Goethe-Universität in Frankfurt/Main were measured when a 26 mm diam disk of a specially treated MD structure (Al–Al2O3) was introduced axially close to the ECR plasma. The Ar beam intensities and charge-state distributions obtained with this disk are compared to measurements with disks of iron and pure aluminum at the same position relative to the plasma. All measurements were performed with the disk at the plasma chamber potential. The results with the MD structure show a net shift of the beam intensity towards higher charge states as compared with the other disk materials. Enhancement factors of the beam current of up to 10 (for Ar12+) when using a MD disk compared to the output when using an aluminum disk and up to 40 (for Ar11+) when using an iron disk were measured.

Si/Si Interface Bonded at Room Temperature by Ar Beam Surface Activation

Si wafers are successfully bonded at room temperature by Ar beam surface etching in ultra high vacuum. The bonding interface was investigated by transmission electron microscope (TEM). Residual strain originated from surface roughness is observed in the interface region. The residual strain is relaxed by low temperature annealing. An amorphous-like intermediate layer exists at the interface. The layer is different from the oxide layer which often observed at Si/Si interface prepared by the conventional wafer bonding. It is supposed that damages by Ar atom impact formed this layer, because thickness of the layer changes according to kinetic energy of Ar beam. This layer is quite instable. Low temperature annealing or electron beam irradiation during TEM observation cause recrystallization of the layer. Therefore, the residual stress and the intermediate layer are observed only in the specimens which is not subjected any heat treatments.

Low-temperature direct bonding of silicon and silicon dioxide by the surface activation method

We have investigated low-temperature bonding of Si and SiO 2 by the surface activation method in vacuum. In the method, Ar beam etching is used to create a clean surface which has strong bonding ability. The specimens are bonded in the vacuum without exposing them to the atmosphere. The strength of Si/Si bonding prepared at room temperature by the method is equivalent to the bulk strength. SiO 2/SiO 2 bonding by the method is twice as strong as conventional bonding before annealing. In addition, the bonding prepared by Ar beam is stronger than that prepared by reactive molecule beam etching such as H 2O and NH 3. The influence of surface oxidation was examined by exposing an etched Si surface to residual gas in the vacuum chamber. Adsorption of reactive molecules such as H 2O on the etched surface causes reduction of bonding strength, whereas Ar gas does not affect the bonding. These results mean that a clean surface etched by Ar beam has strong bonding ability even at room temperature.

Erosion and surface morphology of graphite materials under high flux beam irradiation

Surface morphology and its relation to the erosion mechanism of graphite materials (pyrolytic graphite (PG) and isotropic graphite) irradiated by high flux beams were studied. The irradiation was made by a 5 keV Ar beam with a maximum flux of 1.3 × 10 21 Ar/m 2 s with irradiation angles and fluences, and sample temperatures as experimental parameters. In the case of PG, the surface morphology was different between irradiation at RT (physical sputtering) and at elevated temperatures (1980 K, radiation enhanced sublimation (RES)). In addition, irradiation at oblique incidence (60°) tended to flatten the surface at RT, while at 1980 K the surface roughness developed to some extent. No clear flux dependence of the surface morphology was observed at 1980 K as long as the mean eroded depth was similar, though the RES yield was flux-dependent (decrease with flux). In the case of isotropic graphite, the distinctive surface morphology observed for PG was not found.

Detailed study of radiation enhanced sublimation of graphite under high flux beam irradiation

The dependence of the erosion yield on angle of incidence and irradiation flux for graphite materials at elevated temperature (1980 K) and at room temperature was studied in high flux regime close to edge plasma condition. Irradiation was made with 5 keV Ar beam with the highest flux of 10 21 m −2 s −1. It is found that similar reduction of the yield at 1980 K with flux was observed for isotropic graphite (IG-430 and ISO-630) and pyrolytic graphite at normal incidence and for IG-430 at 60° incidence. The yield at RT (physical sputtering) increased with angle of incidence in the same manner for isotropic and pyrolytic graphite. The irradiated surface of pyrolytic graphite at RT shows dense spinous processes, which could relate to the result. The RES yield at 1980 K, defined as total yield at 1980 K minus physical sputtering yield at RT, is almost constant with angle of incidence in low flux but shows some decrease in high flux. The RES yield calculated based on an interstitial diffusion model shows very weak angular dependence.

Effect of Surface Roughness on Room-Temperature Wafer Bonding by Ar Beam Surface Activation

Using Ar beam etching in vacuum, strong bonding of Si wafers is attained at room temperature. With appropriate etching time, the bonding occurs spontaneously without any load to force two wafers together. However, surface roughness of the wafers increases during Ar beam etching. Because surface roughness has a strong influence on wafer bonding, long etching time degrades the bonding strength. Using atomic force microscope, we measured surface roughness enhancement caused by Ar beam etching, and investigated the relationship between surface roughness and bonding properties such as strength and interfacial voids. The results agree well with theoretical predictions using elastic theory and energy gain by bond formation. A guideline for successful room-temperature bonding is proposed from these results.

Collision-induced activation of the β-hydride elimination reaction of isobutyl iodide dissociatively chemisorbed on Al(111)

The collision-induced activation of the endothermic surface reaction of isobutyl iodide chemisorbed on an Al(111) surface is demonstrated using inert-gas, hyperthermal atomic beams. The collision-induced reaction (CIR) is highly selective towards promoting the β-hydride elimination pathway of the chemisorbed isobutyl fragments. The cross section for the collision-induced reaction was measured over a wide range of energies (14–92 kcal/mol) at normal incidence for Ar, Kr, and Xe atom beams. The CIR cross section exhibits scaling as a function of the normal kinetic energy of the incident atoms. The threshold energy for the β-hydride elimination reaction calculated from the experimental results using a classical energy transfer model is ∼1.1 eV (∼25 kcal/mol). This value is in excellent agreement with that obtained from an analysis of the thermally activated kinetics of the reaction. The measured cross section shows a complex dependence on both the incident energy of the colliding atom and the thermal energy provided by the surface where the two energy modes are interchangeable. The dynamics are explained on the basis of an impulsive, bimolecular collision event where the β-hydride elimination proceeds via a possible tunneling mechanism. The threshold energy calculated in this manner is an upper limit given that it is derived from an analysis which ignores excitations of the internal modes of the chemisorbed alkyl groups.

Ionization Efficiency from Matrix Surface Bombarded with Xe,Ar, and He Beams.

The ionization efficiency under fast atom bombardment conditions with liquid matrix have been studied. The dependence of fast atoms, Xe, Ar, and He, for the ionization efficiency from the surface of matrix solution bombarded with 6 keV atom beams has been rationalized from the standpoint of elastic/inelastic collisions between a fast atom and a matrix molecule in the surface layer. A relationship between ionization efficiency and atomic mass, which was formulated by a collision theory, predicted a tendency that the efficiency approaches asymptotically to a finite value with increasing atomic mass. The curves for the ionization efficiency versus atomic mass, obtained by using a few compounds, were in agreement with the theoretical prediction in the tendency.

Glancing-angle ion bombardment for modification and monitoring of semiconductor surfaces

Using glancing-angle ion bombardment for surface modification rather than conventional near-normal incidence ions has the advantages of reducing damage and implantation projected ranges, reducing channeling, reducing sputtering, and preferentially removing surface asperities leading to flat surfaces. The effects of bombardment conditions on the surface morphology and perfection of GaAs (001), InP (001), and Si (001) surfaces are reported. Air-exposed surfaces were cleaned and smoothened to near atomic flatness without damage under optimal conditions. Sputtering yield, measured using film thicknesses and changes in reflection high-energy electron diffraction oscillations, decreased with decreasing incidence angle. The low sputtering yield and minimal damage make a glancing-angle geometry ideal for real-time characterization by ion scattering spectroscopy. Surface composition measurements on single monolayers of InAs on GaAs showed that the glancing-angle Ar beam did not measurably change the In coverage over relatively long times. A new ion beam monitoring technique was also developed that utilizes the advantages of glancing-angle ions. Specular scattering of 3 keV He ions was observed for incidence angles of 2–6° from GaAs (001). Oscillation in the specularly scattered ion current during GaAs growth were observed with periods corresponding to monolayer growth times. The oscillations allow a simple quantitative interpretation based on scattering by adatoms and step edges.

Erosion of Pyrolytic Graphite and Ti-doped Graphite due to High Flux Irradiation

The erosion of pyrolytic graphite and titanium doped graphite RG-Ti above 1,780K was investigated by 5keV Ar beam irradiation with the flux from 4x1019 to 1x1021 m−2·s−1. The total erosion yields were significantly reduced with the flux. This reduction would be attributed to the reduction of RES (radiation enhanced sublimation) yield, which was observed in the case of isotropic graphite with the flux dependence of RES yield of φ−0.26 (φ: flux) obtained in our previous work. The yield of pyrolytic graphite was roughly 30% higher than that of isotropic graphite below the flux of 1020 m−2·s−1 whereas each yield approached to very close value at the highest flux of 1x1021 m−2·s−1. This result indicated that the effect of graphite structure on the RES yield, which was apparent in the low flux region, would disappear in the high flux region probably due to the disordering of crystal structure.In the case of irradiation to RG-Ti at 1,780K, the surface undulations evolved with a mean height of about 3μm at 1.2×1020 m−2·s−1, while at higher flux of 8.0×1020 m−2·s−1 they were unrecognizable. These phenomena can be explained by the reduction of RES of graphite parts excluding Tic grains.

Active Control of the Relative Population of 3P0 Fine-State in the Glow Discharge of a Supersonic Ar Beam

Abstract An active control for changing the relative population of the 3P0 fine-state of Ar* in a supersonic beam was investigated in a glow discharge by adding a small amount of foreign gases to pure Ar gas. The Ar* mean velocity was determined to be constant at major positions of the time profile, except for a rise and the decay of the beam for all gas mixtures, and also pure Ar. The relative population of 3P0 decreased with the discharge potential for the addition of ethylene, cyclohexane, and benzene. However, the addition of N2 and CH4 and pure Ar, did not show such a 3P0 decrease. These dissimilar behaviors of the foreign mixed gases could be interpreted based on a difference in the ionization potential of the molecules. A useful application of this population control is discussed.