Atsushi Matsushita

Focal Lengths of High-Tc Sintered Superconducting Lenses (Supertrons) for Intense Electron Beams

To determine the optimum dimensions of high-critical-temperature (T c) sintered superconducting lenses (Supertrons), we observed intense electron beams (~340 keV, ~1 kA, ~10 ns) which were focused using axial-length-variable Bi(2223)-based lenses, with a Faraday cup at the exit of the lenses. The lens pipes examined were 13, 28, and 50 mm long and had a wall thickness of 1.5 mm and an inner diameter of 20 mm. Among the lenses, the 50-mm-long lens exhibited the best performance and was found to have the optimum dimensions by comparing the experimental results with electron trajectories simulated for single electrons injected in parallel to the axis of the lenses under a condition of space-charge neutralization. The simulation seems to be a useful method for designing Supertrons.

Deep states in silicon on sapphire by transient-current spectroscopy

It is demonstrated that deep states in silicon on sapphire (SOS) films can be evaluated by transient-current spectroscopy (TCS). In the TCS spectra, a broad peak extending over 100–200 K was observed for the 6000-A-thick n-type SOS film. Assuming the value of capture cross section to be 10−15 cm2 and independent of temperature, the density distribution of deep states was estimated. The density distribution shows a peak of 1.2×1012 cm−2 eV−1 at EC−0.25 eV. Raman backscattering spectroscopy was also performed to evaluate the stress in the silicon film. It was concluded that the defects detected by TCS should be caused by the compressive stress of 6.2×108 Pa in the silicon film.

Self-Magnetic Fields of Electron Beams Confined to Bores of High-Tc Sintered Superconducting Pipes

High-critical-temperature (T c) bismuth-based sintered superconducting pipes (Supertrons) confined the self-magnetic fields of single pulsed intense electron beams (~340 keV, ~1 kA, ~10 ns), ~200 G at 90 K, to their bores. The pipes had inner diameters of 20 and 17 mm and walls 1.5 and 3 mm thick, respectively, with a funnel-type entrance with a total axial length of 55 mm. The thick pipe confined higher fields than did the thin one, but the confined fields for the thick pipe were less than twice those for the thin one. The confinement of the fields and resultant focusing of the electron beams are ascribable to viscous diffusion of the fields through superconducting walls at a velocity of ~104 m/s. The confined fields and diffusion velocities will be used as criteria for applying the superconductor to high-frequency devices.

Silicon fine structure formation on sapphire with focused ion beam

A tetra-methyl-ammonium-hydroxide (TMAH) aqueous solution etches crystalline silicon without removing the damaged silicon and sapphire. Crystalline silicon films with 600 nm thickness on sapphire were irradiated with Si/sup 2+/ focused-ion-beams (FIB), and amorphous fine patterns were formed on the surfaces of the silicon films. Crystalline silicon regions were selectively etched off with the TMAH solution without removing the amorphous patterns acting as the etching mask, and silicon fine structures with the maximum feature size of 800 nm were formed. The feature size depends on the film thickness and can be optimized by the FIB irradiation and etching conditions. The technique is expected to be utilized in the nano-electronic device processing.

Thin CoSi2 Formation on SiO2 with Low-Energy Ion Irradiation

Cobalt silicides were formed by heat treatment and ion irradiation for stacked 7 nm Co and 25 nm Si layers on 20 nm SiO2 films. Irradiation was performed with 25 keV Ar+ ions to a dose of 5 ×1015 cm-2 at sample temperatures between room temperature and 700°C. The phase, atomic concentration profiles, and sheet resistance of the silicide layers were investigated as a function of the processing temperature. X-ray diffraction measurement showed that the phase of CoSi2 was formed by irradiation at temperatures above 300°C, and X-ray photoelectron spectroscopy measurement revealed uniform distributions of Co and Si atoms with the atomic ratio of Co:Si=1:2 for samples irradiated at temperatures above 200°C. Sheet resistance measurement showed that almost complete di-silicidation occurred by irradiation at 700°C. It is concluded that the energy deposited by ions contributes to the migration of the species for silicidation at a lower temperature, and Co/Si mixed layers with an atomic ratio of 1:2 are easily obtained by irradiation of the stacked thin films with low-energy ions. Since the silicide regions formed in the deposited thin films were decomposed during the irradiation at temperatures below 700°C, thermal annealing at 700°C is necessary to obtain completely uniform CoSi2 layers after the irradiation.

Narrow CoSi/sub 2/ line formation on SiO/sub 2/ by focused ion beam

We propose a technique for formation of CoSi/sub 2/ line structures on SiO/sub 2/ films, which utilizes irradiation with 40 keV Si/sup 2+/ FIB to the Co (14 nm)/Si (50 nm)/SiO/sub 2/ (20 nm) stacked layer structures. Ion irradiation was performed at room temperature (RT) in order to prevent the thermal formation of cobalt silicide in regions without irradiation. After ion irradiation, unreacted Co layers remaining on the surfaces were removed by dipping the samples into the solution of HNO/sub 3/:H/sub 2/O/sub 2/=1:3, and unreacted Si layers were removed by H/sub 3/PO/sub 4/ at 160/spl deg/C. The hot H/sub 3/PO/sub 4/ removes the deposited Si without etching the irradiated regions or SiO/sub 2/ films. After the etching, the samples were heat treated at 700 and 900/spl deg/C for 20 min. As a result, cobalt silicide line structures were formed. The resistivity evaluated for samples heat treated at 700 and 900/spl deg/C was in agreement with that of CoSi and CoSi/sub 2/, respectively. CoSi/sub 2/ line structures with less than 200 nm width can be made by the procedure.

Diffusion of Self-Magnetic Fields of Electron Beams through High-Tc Bulk Superconductor Lenses (Supertrons)

When intense electron beams (~340 keV, ~2 kA ~10 ns) were injected, in a vacuum, into a tapered tube (Supertron) made from Bi-based powder-pressed superconductor, the beams were focused to narrow ones with a diameter of a few millimeters. At the same time, the self-magnetic fields of the beams were observed outside the tube using Rogowski coils. The tube had an entrance and exit of 40 and 4 mm diameter, respectively, with an axial length of 55 mm. Contrary to our anticipation that the fields might be fully confined in a bore of the tube, the self-magnetic fields with a duration of ~10 ns diffused through, the wall of the superconducting tube, with an average velocity of ~4 × 10 m/s. Despite these magnetic diffusion, such slow viscous diffusion resulted in the effective focusing of the electron beams of nanosecond order.