**4.4 Application to RSFQ**

The application of superconductive technology to RSFQ requires a high level of reliability, suitable for the development and testing of very complex circuits, with thousands of junctions implementing many different functional blocks. The best established technology, well mastered by many foundries, is based on SIS junctions, usually made of a Nb/AlOx/Nb, with an externally shunt resistor [49]. The minimum junction area that can be fabricated with this technology is 12 um2, and the whole process involves as many as 12 mask steps. Impressive results have been obtained with the standard Nb/AlOx/Nb technology: in [50] the operation of a RSFQ quantum DAC with 6000 Josephson junctions, subdivided in several functional blocks, is reported. SINIS junctions have proven to be suitable for RSFQ, and some fundamental circuits based on this technology were successfully operated [51]. More recently, owing to the wide range of tunability of their electrical parameters, co-sputtered Niobium-silicide barrier junctions appear interesting for a wide range of applications in digital electronics and well suited to RSFQ [52]. Even more complex circuits, comprehending RSFQ electronics as a pattern generator of pulses, followed by semiconductor amplifier and a Josephson junction series array as a quantizer are studied, and realization of preliminary building blocks is in progress.

Fig. 6. Example of fabrication process sequence of a Josephson junctions array. The process is started with the trilayer deposition, by sputtering, followed by the area patterning, by photolithographic process and reactive ion etching, and the insulation of the electrodes by liquid anodization. The deposition of contact layers concludes the process.

However, in principle, no constraint on *Vc* is needed, while the need of a sufficient, but not

The application of superconductive technology to RSFQ requires a high level of reliability, suitable for the development and testing of very complex circuits, with thousands of junctions implementing many different functional blocks. The best established technology, well mastered by many foundries, is based on SIS junctions, usually made of a Nb/AlOx/Nb, with an externally shunt resistor [49]. The minimum junction area that can be fabricated with this technology is 12 um2, and the whole process involves as many as 12 mask steps. Impressive results have been obtained with the standard Nb/AlOx/Nb technology: in [50] the operation of a RSFQ quantum DAC with 6000 Josephson junctions, subdivided in several functional blocks, is reported. SINIS junctions have proven to be suitable for RSFQ, and some fundamental circuits based on this technology were successfully operated [51]. More recently, owing to the wide range of tunability of their electrical parameters, co-sputtered Niobium-silicide barrier junctions appear interesting for a wide range of applications in digital electronics and well suited to RSFQ [52]. Even more complex circuits, comprehending RSFQ electronics as a pattern generator of pulses, followed by semiconductor amplifier and a Josephson junction series array as a quantizer

exceedingly high overlapping of the two steps is the only fundamental aspect.

are studied, and realization of preliminary building blocks is in progress.

Fig. 6. Example of fabrication process sequence of a Josephson junctions array. The process is started with the trilayer deposition, by sputtering, followed by the area patterning, by photolithographic process and reactive ion etching, and the insulation of the electrodes by

liquid anodization. The deposition of contact layers concludes the process.

**4.4 Application to RSFQ** 

## **4.5 An example of a multifunctional junction: Nb/Al-AlOx/Nb overdamped SNIS junctions**

SNIS Josephson devices of the Nb/Al-AlOx/Nb type do belong to the family of internally shunted junctions, exhibiting important similarities, but demonstrating additional advantages comparing with other types of self-shunted junctions, namely an extended range of electrical parameters and an advanced temperature stability of transport characteristics.

The main difference respect to the structure of the basic Nb/AlOx/Nb SIS junctions is the thickness of the aluminum film which is increased at tens of nm, up to about 100 nm, and the augmented transparency of the AlOx [53],[54].

An essential feature of SNIS is that, at 4.2 K a transition from the hysteretic to the nonhysteretic state can be induced, when the aluminum thickness is in the above mentioned range, by changing the AlOx exposure dose. Conversely, once chosen Al thickness and AlOx exposure values, this transition is observed as function of the junction temperature in measurements below 4.2 K (see Figure 7) [63].

Fig. 7. Transition from non-hysteretic to hysteretic behavior for a SNIS junction, obtained by changing the operating temperature.

Critical current densities *Jc* of the SNIS junctions are sufficiently high (also 2 mA/m2, along with characteristic voltage values *V*c up to several hundred microvolts at 4.2 K. In particular, junctions with Al-layer thickness ranged from 40 to 120 nm and exposure doses of AlOx between 160 and 250 Pas showed *Jc* from 0.01 to 1 mA/m2 and Vc from 0.1 to 0.7 mV at 4.2 K. The dependence of *Jc* on *E*ox within the indicated interval of *E*ox is almost linear in the log-log scale, as for other types of Josephson devices with oxide barriers, while the characteristic voltages are more dependent on *d*Al.

The specific normal conductance of these devices ranges from 0.3 108 Ohm-1cm-2 to 0.7 108 Ohm-1cm-2 and, hence, is of the same order of magnitude as that in high-currentdensity single-barrier Josephson junctions and double-barrier SINIS heterostructures [54].

As indicated in the literature, the main explanation of the self-shunting phenomenon in high specific-conductance devices both SIS and in SINIS structures, consists in the enhanced subgap conductivity. The same result has been verified in SNIS junctions where the subgap resistance *R*sg at 4.2 K and very low voltages is of the order of 0.3 Ohm which is higher but comparable with the normal-state junction resistance *RN*.

The main contribution to this subgap enhancement may be related to Andreev reflection processes in the interlayer. Really, since the Al film thickness in SNIS junctions is comparable with the electron mean free path, the transparency of the weak link between the Nb electrodes is dominated by that of the insulating layer. Its average resistance nearly coincides with that in high-specific-conductance Nb-AlOx-Nb junctions where appearance of multiple Andreev reflections was revealed by analyzing comparatively high subgap 'leakage' currents in Nb-AlOx-Nb junctions. Hence, the explanation of the phenomenon suggested for these structures and based on the existence of a universal distribution of transparencies in dirty disordered tunnel barriers [55],[56] in the interlayer. Really, since the Al film thickness in SNIS junctions is comparable with the electron mean free path, the transparency of the weak link between the Nb electrodes is dominated by that of the insulating layer, Its average resistance nearly coincides with that in high-specific-conductance Nb-AlOx-Nb junctions where appearance of multiple Andreev reflections was revealed by analyzing comparatively high subgap 'leakage' currents in Nb-AlOx-Nb junctions . Hence, the explanation of the phenomenon suggested for these structures and based on the existence of a universal distribution of transparencies in dirty disordered tunnel barriers is appropriate to these devices as well. They do not arise from trivial pinholes because after suppressing the Josephson effect, usually no leakage supercurrents were observed.
