We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by e-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in situ fabrication of junctions and capacitors without the use of the well-known suspended bridge (Dolan 1977 Appl. Phys. Lett. 31 337-9). The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 10(-2) µm(2) to more than 10(4) µm(2). We have fabricated Al/AlO(x)/Al Josephson junctions, phase qubit and capacitors using a 100 kV e-beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20 kV e-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillation decays and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this bridge-free technique.

We have performed a detailed study of the time stability and reproducibility of sub-micron Al/AlOx/Al tunnel junctions, fabricated using standard double angle shadow evaporations. We have found that by aggressively cleaning the substrate before the evaporations, thus preventing any contamination of the junction, we obtained perfectly stable oxide barriers. We also present measurements on large ensembles of junctions which prove the reproducibility of the fabrication process. The measured tunnel resistance variance in large ensembles of identically fabricated junctions is in the range of only a few percents. Finally, we have studied the effect of different thermal treatments on the junction barrier. This is especially important for multiple step fabrication processes which imply annealing the junction.Sub-micron sized metal/AlO x /metal tunnel junctions are used in a wide range of applications in many fields, from superconducting and single charge electronics to calorimetry, nanomagnetism and spintronics. However, the time instability of the AlO x barrier is a frequently reported drawback. Systematic measurements of junction aging showed that the resistance of the tunnel barrier could double its value within a period of days 1 . Similarly, a reduction of the capacitance C of the junctions was also observed 2 . The junction aging is usually associated with two types of phenomena. Either (i) the diffusion of oxygen atoms from the oxide barrier to the electrodes 3 or (ii) the change of the chemical composition of the barrier, by absorption or desorption of atoms or molecules other than oxygen 4 . It has been shown that mechanism (i) plays a secondary role in the aging of the barrier, only accounting for the slow drift of the junction parameters at long time scales. Moreover, this slow diffusion of oxygen atoms can be suppressed by surface nitridation of the electrodes 5 . Mechanism (ii) is believed to play the dominant role in the aging of the junction barrier. Vacuum anneals at temperatures between 200 o C and 450 o C will accelerate the relaxation processes in the AlO x barrier and the resulting junctions show improved characteristics 1,6 .The origin of the junction contamination, which leads to the chemical relaxation in mechanism (ii), has been up to now subject of speculation. Using a standard bilayer double angle evaporation technique 7 , Koppinen et al. 1 observed a decrease in the junction aging when the substrate was cleaned with an oxygen plasma just before the electrodes evaporation. This observation suggests that the aging of the barrier is linked to the existence of resist residues in the vicinity of the junction during the deposition. Indeed, by using fabrication methods which avoid the direct contact between the photoresist and the insulating layer, it is possible to obtain perfectly stable junctions 8,9 . In the following we will show that by aggressively cleaning the substrate before the Al deposition, we could obtain perfectly stable Al/AlO x /Al junctions, using the standard bilayer double angle ev...

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