Ryokan Yuasa

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.

Crystalline alignment of Tl2Ba2CaCu2Oδ thin films annealed in a sealed capsule

C-axis oriented Tl2Ba2CaCu2Oδ thin films have been prepared on MgO with good reproducibility by annealing them in an alumina capsule sealed with a gold gasket. The crystalline alignment of the thin films depends on the annealing temperature, the Tl content of the pellets used for Tl compensation, and the film thickness. The electric properties of the thin films depend on their alignment. The critical temperature T c of c-axis oriented thin films is above 90 K, whereas that of randomly oriented thin films is below 85 K. The highest T c is 108 K and the highest critical current density J c is 4.5×105 A/cm2.

Step-Edge Josephson Junctions using Tl2Ba2CaCu2Ox Thin Films on MgO Substrates

Tl2Ba2CaCu2Ox(Tl-2212) step-edge Josephson junctions (SEJJs) which are suitable for millimeter wave applicationshave been successfully fabricated on MgO substrates. The Tc, Jc and IcRn products are 100–104K, 1.9–3.4X104A/cm2 and 0.17–0.47mV at 77 K, respectively. Series connected six step-edge Josephson junctions (6-SEJJs) have been also fabricated as millimeter wave broad-band detectors. Their responsivities are two to three orders of magnitude larger than those of single SEJJs. This is due to the improvement in impedance matching which is attributed to the increase in normal resistance (Rn).

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.

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.