Tsutom Yotsuya

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern

An antireflection surface with sub-wavelength structure has been successfully fabricated on a fused silica substrate. The fabricated antireflection structured surface consists of a microcone array of fused silica with a period shorter than the wavelengths of visible light. The microcone array is made by a reactive ion etching method using fluorocarbon plasma. A microdisk array of chromium thin film, formed by an electron-beam lithography and lift-off process, is used as the etching mask. Since an electric field induced near the substrate was focused on the edges of the metal disks, these disks gradually shrank. Consequently, a conical shape was formed. The fabricated cone array has a period of 250 nm and a height of 750 nm. Measured reflectivity of the antireflection structured surface is less than 0.5% in the wavelength range of 400–800 nm for normal incidence.

Polarization-multiplexed diffractive optical elements fabricated by subwavelength structures.

Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated.

Influence of Oxygen Flow Ratio on Properties of Zn2SnO4 Thin Films Deposited by RF Magnetron Sputtering

Zinc stannate (Zn2SnO4) thin films were deposited by RF magnetron sputtering on silica substrates at various [O2/(Ar+O2)] flow ratios. The influences of the [O2/(Ar+O2)] flow ratio on the crystalline structure, and the optical and electrical properties have been investigated. No sharp X-ray diffraction (XRD) peaks were observed in as-deposited thin films. After postdeposition annealing in air at 750°C, the thin films showed a preferred orientation of (111). The thin films exhibited a high transmittance in the visible spectrum irrespective of the [O2/(Ar+O2)] flow ratio or postdeposition annealing. The optical band gap was estimated to be 4.1 eV by analyzing the optical spectra of thin films annealed at 750°C. The composition ratio of Zn/Sn for thin films deposited in an Ar/O2 mixture was 2.0 and their electrical resistivity was on the order of 105 Ωcm. In contrast, the composition ratio of Zn/Sn for a thin film deposited in pure Ar was 1.5 and an electrical resistivity of 4.1 ×10-2 Ωcm was observed.

Fabrication of multilevel phase computer-generated hologram elements based on effective medium theory

A conventional method to synthesize diffractive optical elements and computer-generated holograms (CGHs) with high diffraction efficiency relies on an increase of phase levels. To fabricate such a device, one should perform electron-beam (e-beam) lithography with multiple-dose exposures or multiple-step photolithography. Here we describe a one-step method, which is based on the effective medium theory, for the fabrication of a multilevel phase CGH. The phase modulations required in cells of a CGH are constructed by means of dividing these cells into fine (subwavelength) structures. The surface features of these fine structures control their corresponding indices, and their values can be calculated according to the effective medium theory. By proper selection of the fine structures, based on the requirements of the phase modulation of the cells, a CGH with multilevel phases is synthesized when a binary structure is relieved on the dielectric material. Then the CGH can be fabricated by direct e-beam lithography or one-step photolithography through an amplitude mask followed by an ion-etching treatment. The experimental results showed that the reconstructed wave field is in good agreement with that simulated by a computer, indicating the effectiveness of the proposed method.

Effects of Nitrogen Pressure and RF Power on the Properties of Reactive Magnetron Sputtered Zr-N Films and an Application to a Thermistor

Thin Zr-N films have been prepared by reactive RF magnetron sputtering. The nitrogen partial pressure and the RF power are chosen as sputtering parameters. The relationships between film properties, such as composition, structure, optical and electrical properties, and these sputtering parameters is systematically investigated. The relationships between the nitrogen pressure and the RF power for each property of Zr-N films in the sputtering process are also discussed. It is found that the film properties can be widely changed by controlling RF power as well as nitrogen pressure. A semiconductorlike Zr-N film is obtained with sputtering in the high-nitrogen-pressure region. The conductance of the semiconductorlike Zr-N films changes with temperature, conforming to a function of exp (E/kT) in the temperature range from 200°C to 550^°C, whichisapplicabletoathermistor.

Optimization of diffraction grating profiles in fabrication by electron-beam lithography

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 microm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.

Planar reflection grating lens for compact spectroscopic imaging system

A compact spectroscopic imaging device consisting of a planar reflection grating lens, a probe fiber array, and a two-dimensional image sensor was proposed and discussed. Reflected or luminescent lights from a subject are coupled to the probe fibers, guided to fiber output ends, radiated into the air, diffracted by the grating lens with wavelength-dependent angle, and focused onto lines on the image sensor. Two-dimensional intensity distribution on the image sensor can give one-dimensional spectrum distribution along a specified direction. A grating lens was designed with a fiber array and a CCD image sensor for 100-nm wavelength range and 10-mm fiber array width. A spectral resolution of 5 nm and a spatial resolution of 0.25 mm were experimentally confirmed.

Epitaxial growth of CuScO2 thin films on sapphire a-plane substrates by pulsed laser deposition

An epitaxial film of CuScO2, a transparent oxide semiconductor with a delafossite structure, was grown on an α−Al2O3(112¯0) substrate by a pulsed laser deposition method using a single-phase Cu2Sc2Oδ target. A two-dimensional x-ray reciprocal space mapping measurement revealed that the film was single phase with a rhombohedral crystal structure. The film showed six-fold rotational symmetry in the basal plane, indicating that the film had a twinned domain structure. The epitaxial growth of CuScO2[3R](0001) thin films on α−Al2O3(112¯0) substrates is caused by the uniaxial locked epitaxy mechanism along the ⟨1¯21¯0⟩ direction of the film, and the orientation relationships of the film with respect to the substrate were CuScO2[3R](0001)∕∕α−Al2O3(112¯0) and CuScO2[3R][1¯21¯0]∕∕α−Al2O3[88¯01]. The optical transmittance of the film was larger than 65% in the visible/near-infrared regions, while the energy gap for direct allowed transition was estimated as 3.7 eV. The resistivity of the film, 9.3×106Ωcm at room te...

Low-temperature thermometer using sputtered ZrNx thin film

Abstract A thin film thermometer using sputtered zirconium nitride has been investigated as a low-temperature thermometer. This film deposited on a sapphire substrate can be used to measure a wide temperature range from 1 to 300 K with reasonable sensitivity within a fast response time. The thermometer sensitivity can be altered by changing fabrication conditions. Moreover, the thermometer is almost insensitive to magnetic fields. The temperature error in a magnetic field of 6 T is less than 10 mK at 4.2 K.

Fabrication of Bi(Pb)SrCaCuO Thin Film of High-Tc Phase

Lead-added Bi-Sr-Ca-Cu-O (BSCCO) thin film of high-Tc phase has been successfully prepared by means of rf-magnetron sputtering and the post annealing process. The content of lead in the BSCCO thin film was decreased during an ordinary annealing process. In order to minimize the decrease of lead, the BSCCO thin film was placed on a bulk ceramics whose composition was similar to that of the film, and annealed for 50 hours at 855°C in air. As the result, the content of lead in the thin film was almost unchanged during the annealing process. The intensity ratio of the X-ray diffraction lines due to the (002) of high and low-Tc phases was about 4/1. The film showed a superconducting transition at 110 K; a long tail of some residual resistance remained down to 90 K.