Superconducting MgB2 thin films grown by a two-step process for tunnel junctions

We present two novel preparation methods making use of the two-step process for superconducting MgB₂ thin films. We call them the cap and melt (CM) and the metal-plate cap and melt (MCM) methods. The MCM method, which is an improved CM method, enables us to raise more Mg pressure locally in the sample than the CM method. Using the MCM method, the MgB₂ thin films with T_c>30 K were obtained. We have also succeeded in fabricating MgB₂ mesa structures of the size of 15 μm×15 μm-75 μm×75 μm useful for tunnel-junction fabrication. For mesa-structure fabrication, a lift-off procedure was employed in the MCM method to avoid the slow milling rate of MgB₂ material. The MgB₂ mesas showed almost the same critical temperature as that of the single films, as measured from the observation of Meissner state of the mesa by a scanning superconducting quantum interference device microscopy.