Sub-micrometer and especially nano-meter sized structures made from thin superconducting films are crucial for development of the modern generation of ultra-sensitive detectors and mixers for a wide spectrum of electro-magnetic radiation [1, 2]. We present results on systematic study of the critical current in superconducting bridges from 50 nm up to 10 micrometers in width. The bridges as well as meander line structures were made from Nb and NbN films thinner than 20 nm and patterned by means of electron-beam lithography and ion milling technique. We have observed three areas in the dependence of the critical current density on the bridge width. For bridges wider than 1 - 2 micrometers the critical current density is almost constant and has a larger value for thicker bridges. The critical current in this range of bridge width is due to de-pinning of magnetic vortices. Narrowing of bridges down to a few hundreds nanometer leads to an increase of the critical current density up to a value, which is about two times smaller than the theoretical value of the de-pairing current. Further reduction of the width results in a drastic decrease of supercurrent density. We consider the bridges as a superconducting core surrounded by a non-superconducting shell [3]. The proximity effect between this normal shell and the central superconducting part of the bridge leads to an effective suppression of superconductivity and as result to a reduction of supercurrent density in nanometer wide bridges. The reasons for formation of the normal shell will be discussed.

[1] A. Semenov et al., Eur. Phys. J. AP 21, 171-178, 2003.
[2] J.J.A. Baselmans et al., Appl. Phys. Lett. 84, 1958-1960, 2004.
[3] K.S. Ilin et al., Proceedings of the 6th European Conference on Applied Superconductivity, 14-18 September 2003, Sorrento Napoli - Italy.

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1. Fluctuations and Dark Count Rates in Superconducting NbN Single-Photon Detectors