Shuji Fujiwara

Superconductivity in BiSrCaCuO Superlattices: Two-Dimensional Properties of CuO Planes

We report resistive measurements on epitaxial superlattices consisting of alternating Bi2Sr2CaCu2O8 (2212) and Bi2Sr2CuO6 (2201) layers grown by atomic-layer-by-layer molecular beam epitaxy (MBE) using oxygen radical beams. The influence of interplane coupling between Cu-O planes was studied by varying the superlattice sequence of different c-axis half-unit cells. A single-half-unit-cell-thick 2212 layer was found to be superconducting in a 2201 matrix, independent of the conductivity and the thickness of neighboring 2201 layers. The fact that all of our superlattices showed superconductivity essentially equal to that of a 2212 single-phase film suggests that this cuprate is a two-dimensional superconductor due to its Cu-O double planes.

Ion bombardment enhanced etching for Bi-Ca-Sr-Cu-O high-Tc superconducting thin films

Ion bombardment enhanced etching (IBEE) has been first applied to Bi–Ca–Sr–Cu–O (BCSCO) high-Tc superconducting thin films. It was found out that the damaged region formed by the ion irradiation with a 200 keV Si++ focused ion beam was selectively dissolved in 1 normal KOH solution. The IBEE process resolves 0.1 µm patterns in the BCSCO films. Other strong alkaline solutions such as NaOH solution were also effective for the selective etching of the BCSCO films, but acid solutions did not give an enhanced etch rate at the irradiated region. The KOH treatment gave no effect on Tc of the BCSCO film. Bridge structures with the width of 1.5 µm were successfully fabricated with little degradation in Tc.

Long-Range Josephson Coupling in BiSrCaCuO-Based Coplanar Junctions

We present measurements of the characteristics of superconducting weak links prepared in epitaxial bilayer structures of Bi2Sr2CuO6 (BSCO)/Bi2Sr2CaCu2O8 (BSCCO) by focused ion beam lithography. The bilayer films with each layer thickness of 150 nm have been grown in situ by atomic-layer- by-layer molecular beam epitaxy and then patterned by implanting Si+ ions to destroy the superconductivity of the BSCCO top layer. We find S-N-S-like I-V characteristics for junctions in which the separation between the two superconducting BSCCO banks is about 300–400 nm. Shapiro steps in the nonhysteretic I-V curves appear in response to microwave radiation. We further find that the critical currents I c for all the weak links fit well with the form given by the Ginzburg-Landau theory, (1-T/T c’)3/2, where T c’ is estimated to be 26–33 K. We conclude that our junctions are not proximity-effect-type but microbridge-type S-S’-S weak links. This characteristics can be interpreted as the formation of filamentary superconducting paths in BSCO.

Ion Bombardment Enhanced Etching for YBaCuO Superconducting Thin Films

Ion bombardment enhanced etching using Br2 solution, previously applied to BiSrCaCuO films, has also been successfully applied to YBaCuO high-Tc superconducting thin films. It was found that the etch rate of the damaged region which was formed by the ion irradiation with a 200 keV Si++ focused ion beam was enhanced with the Br2 treatment. Bridge structures with a width of 0.3 µm were successfully fabricated with little degradation in Tc and Jc. The 0.3-µm bridge had a Jc value of about 106 A/cm2 at 15 K.

Pattern replication accuracy in 1:1 synchrotron radiation lithography

Abstract The replicated pattern distortion for feature sizes down to 0.15 μm, characterized by pattern linearity, pattern density dependence, and corner rounding, is investigated in synchrotron radiation (SR) lithography. In the investigation, a highly accurate X-ray mask delineated by a focused ion beam, a chemically amplified resist, and a Fresnel diffraction model simulator are used. At a 15 μm mask-to-wafer gap, no pattern proximity effect is observed. The corner rounding radius due to Fresnel diffraction is approximately 0.04 μm.

Pattern replication accuracy in 1 : 1 SR lithography

Abstract The replicated pattern distortion for feature sizes down to 0.15 μm, characterized by pattern linearity, pattern density dependence, and corner rounding, is investigated in synchrotron radiation (SR) lithography, using a highly accurate x-ray mask delineated by a focused ion beam. At a 15 μm mask-to-wafer gap, no proximity effect is observed. The corner rounding due to Fresnel diffraction is approximately 0.04 μm.

Exposure and resist-process condition dependence of replicated-pattern accuracy in SR lithography

Replicated pattern accuracy was investigated in synchrotron radiation (SR) lithography, using a chemically amplified (CA) negative-type resist and a highly accurate x-ray mask delineated by focused ion beams (FIB). The size of the hole resist pattern which is arranged in a lattice is affected by the proximity gap and the pattern pitch, but that of the dot resist pattern is hardly dependent on these variables. The dot resist pattern, which was replicated with the 0.25- micrometers -square hole mask pattern, became circular with a proximity gap of over 20 micrometers . In order to obtain highly accurate pattern shape as well as pattern size, the proximity gap should be less than 20 micrometers . The resist pattern size and shape depend on the exposure dose. This was confirmed through the simulation of Fresnel diffraction. The resist pattern was influenced considerably by the post-exposure baking (PEB) conditions, temperature and time. The PEB condition dependence of a replicated pattern was investigated. It is effective to extend the PEB time to improve the sensitivity without deforming the pattern shape.

Fabrication of x-ray masks with 0.15-um level two-dimensional patterns by using highly accurate FIB lithography

A highly accurate focused ion beam (FIB) lithography and its application to the x-ray mask fabrication are discussed. The pattern delineation accuracy in FIB lithography was investigated by drawing various two-dimensional (2-D) test patterns. We could obtain 0.15 micrometers feature resist patterns on the heavy-metal layers of the x-ray mask substrate. FIB lithography suffers little proximity effect and thus various 2-D test patterns were obtained with small distortion. The FIB drawn patterns were precisely transferred into the W absorber layer by the time modulated etching technique. X-ray masks used for the evaluation of the pattern replication accuracy in synchrotron radiation lithography were successfully fabricated.

BiSrCaCuO/BiSrCuO Superlattices: A Prove of Cu-O Plane Superconductivity

The parallel and perpendicular superconducting transport properties of epitaxial superlattices that consist of alternating Bi2Sr2CaCu2O8 (BSCCO) and Bi2Sr2CuO6 (BSCO) layers are reported. The interlayer coupling of Cu-O planes within a cell and in adjacent cells is studied by varying the superlattice sequence of different c-axis half-unit cells. All of our superlattices show superconductivity in the plane of the layer, including superlattices containing isolated Cu-O double planes of BSCCO in a semiconducting BSCO matrix. The overall shapes of superconducting transitions are found to be quite similar independent of the separation of the BSCCO layer. In the perpendicular direction, the transition width increases with the number of BSCO half-unit cells, but a residual resistivity appears for five half-unit cells. We conclude that superconductivity is essentially independent of the BSCCO and BSCO half-unit-cell sequence, and proximity-effect coupling along the c-axis direction cannot occur through the BSCO layer more than two unit cells (49 A) thick.

Focused Ion Beam Processes for Bi-Ca-Sr-Cu-O Superconducting Thin Films

Focused ion beam (FIB) processes have been used for the patterning of Bi-Ca- Sr-Cu-O high-Tc superconducting thin films. Maskless sputter etching was carried out using Bi+ FIB to minimize the effect of impurity. V-shaped fine grooves with the 0.2 μm width near the surface of the film and the depth of 2 μm were obtained by the FIB processes. Bridge structures with the width of 0.7 μm were fabricated with little degradation in Tc. Planer S-N-S Josephson junctions were made by depositing Au into the grooves. Superconducting current flowing across the junctions was clearly observed.