**4.2 Pulsed standards for waveform synthesis**

128 Modern Metrology Concerns

In general a problem for the SINIS junctions is due to the high transparency of the two insulating barriers, with the necessity to realize very smooth planarization of the underlying films, since the probability of defects such as pinholes is increased. The fabrication process, requiring that the two barriers and the three thin aluminum layers been highly homogenous, has set a limitation to the fabrication of really high number of junctions, as in

In SNS junctions the damping of the IV characteristic for non-hysteretic behavior is obtained by using a normal metal as barrier. SNS have high values of *Jc,* but typically the metals used have very low resistivities. Nb/PdAu/Nb junctions have been the first developed type of SNS junctions for this application by NIST group, and have produced stable voltage outputs at 1 V as binary arrays for programmable potentiometers, being also used in circuits for pulse driven AC synthesis [14],[15]. They feature values in the range of 100 kA/cm2 and *Vc*  between 5 and 30 V with typical size of few µm2, even if *Jc* values would support also submicron dimensions. The values of *Vc* limit their use to drive frequencies of few GHz and

Impressive results have been obtained with NbN/TiN/NbN junctions, developed by the Tsukuba group, since very large arrays, with more than 300.000 series connected junctions have been succesfully tested in a 8 and 11 bit DAC circuits with quantized steps at 10 V at 10 K [36],[37]. Features of these junctions are Jc about 10 to 10 A/cm and Vc 10-20 V at 10 K with areas of few square micrometer. 20 V output has been also achieved, by series connecting two arrays of different chips, minimizing interconnecting dissipation [9].

Materials have been also proposed as normal metal barrier which are at the metallic insulator transition. In such a way it was possible to tune the barrier resistivity and the characteristic voltage of the junctions [38],[39]. Among the experimental results we mention Nb/TaOx/Nb, NbN/TaN/NbN and Nb/NbxSi/Nb junctions. Nb/TaOx/Nb junctions fabricated at IEN-INRiM, showed resistivities varying different order of magnitudes depending on the bias voltage of the cathode during the sputtering deposition of the TaOx film [40]. The junctions have however a marked aging, featuring a reduction of at values less than 10 A/cm. Also NbN/TaN/NbN junctions studied in [41] have not yet been suitable for large circuits production. These junctions featured very high *Vc*, up to more than 0.5 mV at 4.2 K with *Jc* 104-105 A/cm2, but required a difficult tuning of fabrication parameters and also the temperature dependence of these

In this category, Nb/NbxSi1-x/Nb junctions represent the most successful attempt so far. Although previously studied by Barrera and Beasley in the '80 [42], the most promising results have been achieved in the last years by the NIST researchers, which experimented these junctions in arrays at 1 V level [43]. These authors achieved a transition from a conductive to an highly resistive phase by varying the sputter deposition power of the two elements. Jc varying from 10 to 10 kA/cm2 and Vc from few V to 150 V and more at 4.2 K have been obtained. The stabilization of the barrier stoichiometry, critical aspect of this type of junctions, can be achieved by a thermal annealing after the deposition of the trilayer. Depending on the Nb content and the barrier thickness, also the temperature stability of

these junctions can be optimized, see in a next section [44].

arrays for programmable voltages at 10 V and more [35].

circuits have about 30.000 junctions for 1 V.

parameters was critical.

The series of pulses used for waveform synthesis has a very rich harmonic content and requires proper optimization of the microwave behavior in circuits for rf signal distribution. Most of the techniques adopted for analysis and design of circuits with distributed parameters assumes operation with narrow band signals, thus cannot be directly applied for pulse transmission. The most reliable solution to guarantee pulse operation is to reduce array dimensions allowing lumped analysis of circuit. To that aim array length must be shorter than /8, where is the wavelength of the microwave signal. This constraint sets strong requirements on junctions technology, in particular on junction size.

Therefore the main stream for the realization of these devices is a) submicrometric junction technology development or/and b) vertical multistacking of junctions

In order to realize the array configuration described, materials resulting in improved steepness of the etch profile are preferred, especially when multistacks of junctions are employed.

SINIS junctions, initially developed for programmable voltage standards and previously described, have also been tried for AC synthesis, notwithstanding their limitations [33].

However, as mentioned above, the first, and presently the best results in pulse driven standards had been attained with Nb/PdAu/Nb junctions, which, due to their high current density allow to strongly reduce their dimensions [14].

Nb/HfTi/Nb junctions have been experimented by PTB team with successful results in the realization of large submicrometric circuits for AC synthesis which have recently reached quantized voltages from 2000 series junctions. These junctions feature about 80 kA/cm for HfTi 20 nm thick and of 15 V in junctions 0.2 m0.2 m [33].

Other type of SNS junctions with similar low characteristic voltage, such as Nb/Ti/Nb junctions developed by Jena team and Nb/Al/Nb junctions studied at INRiM have not been resulted in large circuits yet [45]. Ti barrier junctions feature of 104 -105 A/cm2 and from 10 V up to 100 V Vc, while for Al film 100 nm thick, also higher values of and similar or lower values for were obtained. In both case the junction size was in the micrometer and submicrometer range.

Nb/MoSi/Nb junctions also studied and experimented by NIST team, have been used to realize multistacked junctions for lumped arrays for AC synthesis [46]. They feature critical current density between 104 and 105 A/cm2, depending on barrier thickness, while characteristic voltage is typically few tens of V in junctions of few m2.
