RF characteristics of Nb vertical type microbridges

We have been fabricating Nb microbridges with a novel vertical structure and measuring their basic characteristics. The bridges have a three dimensional structure which connects two Nb planes separated by a thin insulating film (SiO
2
or Si
3
N
4
) through a small pinhole in the insulator. With this structure, self heating effect can be much reduced because of the three dimensional structure and it is easy to have a short bridge with a small cross sectional area. Bridges fabricated have a dimension of 50-300 nm in length, 100-200 nm in pinhole diameter and 150-400 nm in Nb film thickness. We have measured temperature dependence of critical current and rf response under an irradiation at 9.55 GHz. Current steps were observed up to a voltage higher than 1.2mV on 1-V curves for a 50nm long bridge. This indicates that a self heating effect is much reduced by the three dimensional structure.