**4. Current junction technologies and fabrication issues**

The fabrication of arrays for AC generation and waveform synthesis is a difficult, challenging task, and many different technologies have been proposed and tested, yet the choice of material and fabrication techniques still represents an open question. Independent of the technique adopted for signal generation, a relevant problem is posed by the high number of junctions needed to reach practical voltages, since junctions for AC typically operate on the first step and the drive frequency is limited for technical and economical reasons [25]. A high noise immunity, low power dissipation, reduced dimensions are also essential properties in metrological applications and in view of spreading quantum standards to a wider range of users.

The chip dimensions are set by the area and the number of junctions, both essential parameters for technology, because a reduced area along with a high number of junctions make it difficult to guarantee the uniformity of their electrical properties, which is essential to observe decent collective steps.

These requirements can be translated into well-defined specifications on junction parameters.

First, the critical current *Ic*, which sets an upper limit on the amplitude of the quantized steps, should be large enough for steps with suitable width and noise immunity, yet a too high value increases dissipation in junctions. The area of the junction must be small, to reduce array dimensions, but avoiding the excessive difficulties in fabrication imposed by deep submicron patterning, today still hard to achieve for high integration superconductive circuits. It follows that an optimal range exist for critical current density. Presently, values ranging from few kA to tens of thousands kA/cm2 appear to be the best choice.

The characteristic voltage, *Vc* determines the microwave optimal drive frequency *fd*, from the relation *Ω = fd* / *fc*, where *fc* is *Vc × (2e/h)* and thus the step voltage (i.e. the voltage resolution of the array) and the number of junctions needed to achieve the maximum requested voltage output. To obtain the maximum step amplitude, *Ω* must be ≈ 1 [26]. In order to use commercial microwave instrumentation and reduce as much as possible the number of junctions, achieving the maximum possible voltage output, drive frequencies close to 70 Ghz are used, and *Vc* around 150 µV are needed. Of course even larger values, which on the other hand are absolutely advantageous for speed applications, can be used. But in voltage standard application this causes a sensible reduction of the step width.

Moreover, the characteristic voltage defines also the highest speed of RSFQ circuits, since this is proportional to τ*car=* 1/*fc*. In this case, the highest *Vc,* the higher the speed.

On the contrary, where we focus on the best possible voltage resolution of the standard, junctions with reduced Vc should be used.

In this chapter we discuss extensively niobium and niobium nitride based junctions, considering high *Tc* junctions in the section devoted to the use at temperatures above 4.2 K. A challenging problem to be solved in the next future for voltage metrology and superconductive electronics applications, is operation in cryocoolers at a temperature greater than 4.2 K. Indeed, in order to make the Josephson quantum standards available to a widespread market, rather than limited only to the National Laboratories, as well as affordable for private companies needing an accurate voltage reference, refrigeration systems cheaper and more compact than those nowadays used for niobium junctions are required. Moreover, as it will be discussed in a next section, the use of a cryocooler would allow a reduction of the measuring leads between the Josephson device and the measuring system, reducing the indetermination associated to the transition between voltage levels.

Considering the electrode configuration, non-hysteretic IV characteristic can be obtained by three main classes of junctions:


126 Modern Metrology Concerns

of the technique adopted for signal generation, a relevant problem is posed by the high number of junctions needed to reach practical voltages, since junctions for AC typically operate on the first step and the drive frequency is limited for technical and economical reasons [25]. A high noise immunity, low power dissipation, reduced dimensions are also essential properties in metrological applications and in view of spreading quantum

The chip dimensions are set by the area and the number of junctions, both essential parameters for technology, because a reduced area along with a high number of junctions make it difficult to guarantee the uniformity of their electrical properties, which is essential

These requirements can be translated into well-defined specifications on junction

First, the critical current *Ic*, which sets an upper limit on the amplitude of the quantized steps, should be large enough for steps with suitable width and noise immunity, yet a too high value increases dissipation in junctions. The area of the junction must be small, to reduce array dimensions, but avoiding the excessive difficulties in fabrication imposed by deep submicron patterning, today still hard to achieve for high integration superconductive circuits. It follows that an optimal range exist for critical current density. Presently, values

The characteristic voltage, *Vc* determines the microwave optimal drive frequency *fd*, from the relation *Ω = fd* / *fc*, where *fc* is *Vc × (2e/h)* and thus the step voltage (i.e. the voltage resolution of the array) and the number of junctions needed to achieve the maximum requested voltage output. To obtain the maximum step amplitude, *Ω* must be ≈ 1 [26]. In order to use commercial microwave instrumentation and reduce as much as possible the number of junctions, achieving the maximum possible voltage output, drive frequencies close to 70 Ghz are used, and *Vc* around 150 µV are needed. Of course even larger values, which on the other hand are absolutely advantageous for speed applications, can be used. But in voltage

Moreover, the characteristic voltage defines also the highest speed of RSFQ circuits, since

On the contrary, where we focus on the best possible voltage resolution of the standard,

In this chapter we discuss extensively niobium and niobium nitride based junctions, considering high *Tc* junctions in the section devoted to the use at temperatures above 4.2 K. A challenging problem to be solved in the next future for voltage metrology and superconductive electronics applications, is operation in cryocoolers at a temperature greater than 4.2 K. Indeed, in order to make the Josephson quantum standards available to a widespread market, rather than limited only to the National Laboratories, as well as affordable for private companies needing an accurate voltage reference, refrigeration systems cheaper and more compact than those nowadays used for niobium junctions are required. Moreover, as it will be discussed in a next section, the use of a cryocooler would allow a reduction of the measuring leads between the Josephson device and the measuring system, reducing the indetermination associated to the transition between voltage levels.

this is proportional to τ*car=* 1/*fc*. In this case, the highest *Vc,* the higher the speed.

ranging from few kA to tens of thousands kA/cm2 appear to be the best choice.

standard application this causes a sensible reduction of the step width.

junctions with reduced Vc should be used.

standards to a wider range of users.

to observe decent collective steps.

parameters.


The first class, directly derived from the most developed and optimized process of superconductive electronics, namely Nb/AlOx/Nb junctions and which is still the predominant technology for RSFQ circuits, suffers from the disadvantage of a configuration requiring an external resistor or a more complex circuitry. This, with the severe limitation to *Ic* from chaotic instabilities [27] limits the use of these junctions in voltage standard circuits. In spite of this a programmable array based on shunted Nb based SIS, where a reactive shunting was realized for blocks of several junctions, proved suitable operation at 1 V level [28].

In any case we will not consider junctions of this type in the following, where a detailed analysis limited to the last two classes will be carried out.
