Preparation and characterization of submicron stacked Josephson junctions on Tl-2212 epitaxial films.

Y.Koval, A.Franz, P.Mueller

Physikalisches Institut III, Universitaet Erlangen-Nuernberg, D-91058 Erlangen, Germany

H.Schneidewind

Institut fuer Physikalische Hochtechnologie, D-07702 Jena, Germany

There are two tasks of great interest which demand a preparation of stacked Josephson junctions of submicron dimensions.
1.	Experimental investigation of charge soliton influence on electron transport and observation of Coulomb blockade effect at elevated temperature.
2.	High frequency application as a mixers and possibly as a powerful radiation sources in the THz range.
Up to the moment stacked junctions were prepared on single crystals. We present a technology in order to realize sub-micron sized mesas on top of Tl2212 epitaxial films on sapphire substrates. A new method for mesa insulation was used. It is based on the properties of the electron resist poly methyl methacrylate (PMMA): 
1)	PMMA can be used both as a positive and as a negative resist just by variation of exposure dose.
2)	PMMA in cross linked state (negative mode) can be effectively used as an insulator [1]. 
Minimum dimensions of mesa areas were - 0.3x0.3 µm^2.
We present transport measurements of these devices. 
It was found that a very high critical current density of 2000-10000 A/cm^2 is typical for junctions of 0.7x0.7 µm^2 . Sub-gap structures were observed similar to the ones reported in [2]. Hysteric IV characteristics can be observed up to 110K which is rather unusual for such kind of junctions. Decrease of junction areas to 0.3x0.3 µm^2 leads to a strong suppression of critical current density which never exceeds 1000 A/cm^2. These results are in agreement with results published in [3].

1.	Y.Koval, A. Wallraff, M. Fistul, N. Thyssen, H.  Kohlstedt, A.V.  Ustinov, IEEE Trans. Appl. Supercond. , 9, 1999, 3957.
2.	K. Schlenga, G. Hechtfischer, R. Kleiner, W. Walkenhorst, P. Müller, H. L. Johnson, M. Veith, W. Brodkorb, and E.Steinbeiß, Phys.Rev. Lett. 76, 4943 (1996)
3.	Yu. I. Latishev, S.-J. Kim, T. Yamashita, JETP Lett. 69, 84 (1999).