JPS603797B2 - Josephson tunnel junction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Josephson tunnel junction device, particularly a Josephson tunnel junction device suitable for use as a millimeter-wave heterodyne mixer in a radio telescope or as a video detector.

A Josephson tunnel junction device consisting of an insulating layer or a tunnel barrier layer sandwiched between a pair of superconductor electrode layers has a superconductor-insulating layer-superconductor (SuperConductor-1ns peakator-SuperCond structure) structure, abbreviated as SIS structure. This is called the SIS type Josephson element in this book, but this SI
Recently, S-type Josephson devices have been able to exhibit outstanding performance as the above-mentioned millimeter wave heterodyne mixers and video detectors, because even though they are passive devices, power gain can be detected through quantum effects. With this hope in mind, research has been carried out on SIS structures and electrode materials for this type of analog use.

The present invention is an extension of this series of research processes, and aims to provide this type of analog SIS type Josephson device with superior electrical and physical properties. In order to explain the types of electrical characteristics, first, Figure 1 shows the current-voltage characteristics (1-
V curve).

As is well known, these SIS type Josephson elements are
Both electrode layers sandwiching an insulating layer operate in an extremely low temperature environment where superconducting transition occurs, but when the device current 1 reaches a certain critical current value lc.
Until the value exceeds 0, the device current becomes a so-called Josephson tunnel current, and therefore, a state in which no voltage is generated continues at both ends of the junction (between both electrode layers) of the SIS structure.

This state is said to be that the junction is in a zero voltage state. When the element current reaches the critical current lc, the arrow . As shown by T, the junction transitions to a point P where a predetermined voltage value (gap voltage) V6 determined by the electrode layer material is generated at both ends of the junction, and the junction enters a so-called voltage state.

Once the junction transitions to the voltage state in this way,
This voltage state is maintained until the element current 1 is returned to zero, and when the element current 1 becomes zero, it returns to the drop voltage state again.

Therefore, an analyzer that has such a hysteresis curve and is the object of the present invention.

The areas that require electrical attention are inherently different for digital applications and for digital applications. In digital applications, emphasis is placed on the transition from one voltage state to another.

For example, whether the critical current lc does not vary from element to element and satisfies design specifications. On the other hand, when used as an analog mixer or detector, the curve region in the voltage state, that is, the almost vertical curve that includes point P and allows the current value 1 to be varied while maintaining the gap voltage Vg. A portion Ca, a curved portion Cb roughly along the voltage axis V that rapidly moves toward the origin 0 without being able to maintain the gap voltage Vg, and a knee that forms the joint between both curved portions.
Attention is focused on a portion Ck called (knee).

Here, required when used in an electromagnetic wave receiving system,
Let us enumerate the electrical characteristics that would be desirable to improve.

(i) Knee characteristic>Knee characteristic refers to the quasi in the 1-V curve that allows the current value 1 to be varied while maintaining the gap voltage Vg, as schematically shown by angle 8 in Fig. 1. Particle current rise portion Ca and gap voltage V
The angle 8 formed with the part Cb that returns to the zero voltage state from g is 9
It is said to be based on how obtuse the angle is to 00, or how close it is to 900.The closer it is to 900, the rounder the knee is.The closer it is to 900, the sharper the knee. etc.

In conclusion, the sharper the knee, the better.From another perspective, if the curve part Cb is parallel to the voltage axis V, and the slope of the rising slope in the current axis direction is 4. be done.

This is because the raised portion of this portion Cb is equivalently S
This is because it indicates the leakage current of the IS junction, and in the case of a mixer or the like, it is natural that the conversion efficiency increases as the leakage current decreases. In the end, a curve close to the target curve Cai, Ck;, Cbi shown by the imaginary line in FIG. The rise of the current is steep, and the gap voltage V
The closer a curve is found to a curve in which the curve portion Cbi from g to zero voltage state is approximately on the voltage axis, the more desirable the strong nonlinearity is and the higher the conversion efficiency can be obtained.

However, even if we use angular expressions such as "the knee is round,""the knee is sharp," or "the knee is good" to describe the knee characteristics, as mentioned above, the measurement system cannot quantify it at an angle of 8. In practice, the ratio Ra/Rb of the resistance value Ra at a voltage value △V (generally 0.5 V) higher than the gap voltage Vg and the resistance value Rb at a voltage value △V lower is used. is searched and normalized to determine the knee characteristics.

In other words, the smaller the ratio Ra/Rb, the better the knee characteristics, and the ratio Ra'/Rb' in the target curve described above.
becomes extremely small.

As described above, obtaining good knee characteristics is one of the conditions for obtaining strong nonlinearity, and is an important factor in improving conversion efficiency.

(ii) <Gap Voltage Vg> The gap voltage Vg is determined by the material of the electrode layer used for the SIS junction, and it is said that the higher the voltage Vg, the better.

This is because this gap voltage Vg corresponds to the energy band gap 2△/q (q: electron charge amount) of the electrode layer material, so 2△/q = Na skin x h (
h: Planck's constant), the higher the value, the higher the maximum operating frequency.
This is because it is possible to increase the ax and expand the receivable range. For example, it is desirable for a millimeter wave band heterodyne mixer to cover the millimeter wave band 30 to 3000 generations with sufficient margin. Even if it is not used as a mixer, being able to increase the maximum receivable frequency means that it has the ability to receive a wide spectrum distribution, which will lead to an expanded range of applications in the future. (
iii) <Current density at gap voltage Vg> The higher the current density (A/ground) at gap voltage Vg, the better.

For example, if the maximum current at the gap voltage Vg in FIG. becomes smaller. Furthermore, the range of energy incident on the junction is expanded, which in turn leads to the expansion of the dynamic machine. Specifically, it is desirable that the current is from several hundred A/ground to more than 1000 A'. The above are the main electrical requirements for analog SIS elements such as mixers, but as mentioned earlier, they are essentially different from those for digital systems.

Also, although it does not appear on the 1-V curve, in digital applications,
Attention is also paid to the dielectric constant of the barrier layer and the insulating film between the two electrode layers, and if it is too large, a reduction in operating speed becomes a problem, but in applications such as the present invention, its absolute value is not a problem.
However, there are some requirements that are common to both systems.

These are exclusively physical properties, in particular the so-called thermal cycling properties or temperature cycling properties. In other words, the device must not be damaged even if it is cycled many times between room temperature and operating temperature (generally liquid helium temperature 4.5 or lower in a reduced pressure atmosphere). ) is an important factor that must be satisfied from a practical point of view.

Taking all of the above into account, the SIS type Josephson element, which is originally suitable for uses such as mixers, has been researched and developed as a device exclusively for mixers due to the differences and special characteristics of various electrical characteristics that should be noted. It should be.

However, it has traditionally been meaningless if the physical strength of the object cannot be achieved, so regardless of whether it is for analog or digital systems, a certain level of physical strength is required for the time being. (i) and (ii) for use in mixers, etc. mentioned above from among those with physical strength and temperature cycle characteristics.
Comparative research has been conducted to see if there is a product that can satisfy the specifications of (iii) to some extent. One of the things that was discovered during this research process was that although this was originally developed for digital systems, it was possible to use P as the lower electrode layer.
A b/ln (t% of 1)/Au (4M%) alloy is used and oxidized to form an oxide film as a tunnel barrier layer. %) There is an SIS type element made of an alloy.

This element has good temperature cycle characteristics and is excellent in that it can be tested for more than 500 thermal cycles, but the gap voltage Vg is about 2.3 mV, which is an unsatisfactory value, and the Au
Therefore, there is a drawback that leakage current is large and good nonlinearity cannot be obtained.

On the other hand, Pb containing no Au in the lower electrode layer
/ln has been developed for mixers, but this also has a gap voltage of about 2.8V, which is not high, and the nonlinearity or knee characteristics are also lower than the resistance value Ra mentioned above.
/Rb, 4. For foxes, the value is 0.15, which is not too small.

Incidentally, when calculating the maximum operating frequency of the element consisting of this Pb/ln lower electrode layer and Pb/Bi upper electrode layer, it is approximately 55 Hz from the above formula Vg white 2 △/q = Pu skin h, The value is not that high. Furthermore, the leakage current is reduced due to the absence of Au, but it is not extremely small. As another conventional element, there is also an element in which both the upper and lower electrode layers are made of Pb/Bi (2% by weight).

29 weight percent of bismuth Bi is a uniquely determined value for forming a good alloy. In this element, the gap voltage Vg is approximately 3.4 V, and therefore the maximum operating frequency has a fairly wide range of ``'81'', but it has been found that Bi acts in the direction of lowering the physical strength. In fact, this device cannot withstand even several temperature cycles.

Temperature cycling is required at least ten times even in the current transition period for commercialization of mixers. In addition to this, Nb has higher physical strength compared to lead alloys.
Some materials are also used, such as Nb/Miyako Yamashita electrode layer monoxide film, Pb/Bi upper electrode layer, and Nb lower electrode layer.
SIS structures such as a frequently oxidized Katsuichi Pb upper electrode layer have also been proposed, but both have the disadvantage that they have a large current density and cannot be used for digital applications, mixers, etc.

If anything, Nb-based materials are unsuitable for use in mixers, and their high physical strength has a conspicuous drawback in that they adversely affect microfabrication workability. The present invention
As mentioned above, in the complexly intertwined situation where each element causes various effects, the desirable electrical characteristics already mentioned for millimeter wave heterodyne mixers and video detectors... (ii), (iii' The purpose of this invention is to provide an element or its electrode layer composition that can improve all of the above and satisfy the physical strength of G as the fourth target specification. However, none of the above four characteristics were improved, even if only slightly, and at least one of them ended up getting worse.

To put it simply, the SIS type element of the present invention has at least a lower electrode layer having a composition of Pb/Bi/1.

Although indium ln will be described later, the weight percent of bismuth Bi is 2% by weight in order to form a good alloy with lead Pb, as explained above. Hereinafter, a manufacturing example of an SIS type Josephson element according to an embodiment of the present invention will be described as a second example.
This will be explained based on the diagram.

First, as shown in FIG. 2A, a metal mask (not shown) for the required lower electrode layer is applied on a suitable substrate 1 such as a quartz substrate, and a vacuum atmosphere is applied within the sputter deposition layer. , a lower electrode base material layer 2' made of Pb/Bi is formed at room temperature.

The weight percentage of Bi in the Pb/Bi alloy is
%. Further, the layer thickness is 2000Δ. here,
The lower electrode "base material" layer 2' was selected because ln has not yet been introduced at this point, and it is a layer from which the Pb/Bi/ln lower electrode layer referred to in the present invention will be formed in the future. However, the planar shape itself is the same as that of the required lower electrode layer.

Next, as shown in FIG. 2B', the above Pb/Bi layer 2
On top of ', an ln layer 3' from loo to 200 is formed in the same sputter deposition apparatus without applying a vacuum.

When this layer 3' is exposed to an atmosphere of dry oxygen 02 at a substrate temperature of room temperature for several hours as shown in FIG. 2C, an indium oxide layer as the oxide film 3 is obtained. At the same time, it is thought that ln diffuses into the base layer 2' during this oxidation step, and as a result, the required Pb/Bi/ln lower electrode layer 2 is formed. More practically, it can be considered that the area plane relatively close to the surface of the Pb/Bi layer 2' constitutes the Pb/Bi/IJ member 2. Thereafter, after applying a mask for the upper electrode, an upper electrode layer 4 made of Pb/Bi (29%) with a thickness of about 2000 Å was formed, and an SIS junction 5 was formed as shown in FIG. 2D. .

The Josephine device 10 of the present invention has the above-mentioned features in the SIS junction 5, especially Pb/Bi/ln of at least the lower electrode layer.
However, in more practical applications, for example, as a separately excited mixer element for millimeter-wave heterodyne, at least one of the upper and lower electrode layers 2 and 4 has a
As shown in the plan view of the figure, a filter section 7 consisting of capacitance and inductance may be provided in the open circuit section 8 extending from the above-mentioned joint section 5 to the output terminal surface section 6 to a desired value by adjusting the geometry.

In this way, only the intermediate frequency can be extracted between the output terminals 6, 6, and the element 10' of FIG. 3 becomes a single mixer cell.

However, in the case shown, the filter section 7 is connected to both electrode layers 2 and 4.
Although it is provided in the electric circuit portion 8 of , it is also possible to provide only one of them. The cross section of the joint 5 along the Rollo line in FIG. 3 corresponds to the cross-sectional structure shown in FIG. 2D, and in the plan view shown in FIG. This is a knife edge-shaped overlapping portion, and the overlapping area, that is, the joint area is 3 to 12 mm in the example manufactured by the present applicant.

Also, although not shown, the substrate 1 and the output cross-section portions 6, 6
A chrome contact surface layer may be provided between them. This element 1
The mixer cells 0 to 10 can be used by being fitted inside a conventional millimeter wave waveguide 11, as shown in FIG.

The waveguide 11 is usually equipped with a short plunger 12 and a tuning stub 13 using a known structure.
In addition, only the essential parts of the device 10 are shown with the substrate omitted, but in such a configuration, the millimeter wave input angular frequency signal i (generally no i
='s-'s. can be obtained. Now, referring to the characteristics of the joint portion 5 of the device 10 of the present invention according to the above manufacturing example, the following favorable results are reached. In the above production example, when the indium layer is oxidized for 10 minutes in an environment of about 2 mom, a current density of about 1000 A/N can be obtained, and even if various conditions are explored, the current density is at least 250 A/N, which is a sufficient value, but it cannot reach the maximum. It is possible to obtain a completely satisfactory range of up to 1600 A/ground.

Figure 5 shows a device with a sufficient but relatively low current density among devices of the present invention (with a junction area of 6; each layer thickness is as described above) between liquid helium temperature and room temperature. The fact that this data was obtained after cycling back and forth 10 times confirms that the temperature cycle characteristics have not been impaired in any way, but further examination of the electrical characteristics reveals the following.

First, the relative relationship regarding the operating temperature of the device of the present invention itself is shown by the solid line 4. It is obvious that the characteristics under the operating temperature of ice shown by the imaginary line are better than the curve at the time of arc, and the 1-V curve at the group is excellent.

In general, the lower the operating temperature, the better the characteristics. For comparison with the conventional example mentioned at the beginning, let's take a concrete numerical example.The gap voltage in this example is about 3.4V.
This is an extremely good value.

As mentioned above, in the related example, the Pb/B: lower electrode layer element had a value of this level, but the temperature cycle characteristics were so bad that they were not a problem. On the other hand, compared to the gap voltage of the conventional element whose temperature cycle characteristics are satisfactory to some extent, the element of the present invention has an improvement rate of about 15 to 30%. When determining the maximum operating frequency Mx based on this gap voltage, it is approximately 81 from the above equation, which is a completely satisfactory value.

Next, regarding knee characteristics or nonlinearity, as mentioned above, assuming that the deviation voltage △V from the gap voltage Vg is 0.5 V, Vg (3.4 wV) and △V (0.5 mV) ) The ratio of the resistance value (DC resistance) Ra and Rb found at
When calculating Rb, 4. Gabut Ra/Rb (4. Ga) = 0
．． Shows an excellent value of 06, and on ice Ra/Rb (pine) = 0
．． 013, which is an extremely excellent value.

Note that the equivalent normal conductive resistance of the element of this example is 1400, which provides sufficient impedance matching when used as a mixer.
As mentioned above, the data shown in Figure 5 has already undergone ten temperature cycles, but when the 11V curve of the above-mentioned element is examined using the number of cycles as a parameter, it becomes as shown in Figure 6.

This measurement was carried out with consideration given to the situation as close as possible to the situation when this element is assembled as a component in an actual device as shown in FIG.

The sample holder was filled with helium gas and the device was sealed. Helium gas was filled in to ensure thermal coupling between the device and liquid helium when the holder was immersed in liquid helium. Before starting the measurement,
It was stored for 6 days at a temperature of -8°C in a sealed warehouse. In FIG. 6, the circled numbers are the number of cycles, and the curve shown by the circle is the 1-V curve after the cycle rotation, and 11 cycles were performed over two weeks. During the process, the helium gas is pumped out to expose the element to the atmospheric environment, or it is exposed directly to liquid helium. Then,
According to the data shown in FIG. 6, the resistance value decreases during the first few cycles, then increases, and then decreases again from the 10th to the 11th cycle.

In this way, strictly speaking, the 11V curve is not completely stable, but its changes are very slight and are sufficiently small compared to other conventional examples.

FIG. 7 is a 1-V curve of the device shown in FIG. 5 of another example device according to the present invention, but in this device, the operating temperature is 2. In fox, the current density is 1000A/ground with a junction area of 10A,
Gap voltage approximately 3.45 skin V, knee characteristics Ra/Rb (2
．． pine) = 0.043 and equivalent resistance of 220, which are also sufficient values.

By the way, in the lower electrode layer 2 in the present invention, l
The question is what range should the n content be in order to achieve the basic effect of the present invention, which is not to significantly impair any of the electrical properties including the temperature cycle properties. According to the research and experiments of the present inventors, it can be said that the range is approximately 1 to 1 t% of the skin.

If there is too much ln, Pb
This is because the -ln relationship becomes stronger and the drawbacks of the Pb/ln-based conventional example described above begin to appear.

Further, in the above embodiment, the upper electrode layer is made of Pb/Bi and does not contain ln, but if it is based on a Pb/Bi alloy, it may contain ln as well as other elements.

This is because, while the lower electrode layer must withstand various subsequent elemental steps in manufacturing the device, the upper electrode layer, which is formed almost last, does not need to do so.

Conversely, if the lower electrode layer is affected by any adverse effects during the device manufacturing process, problems will arise in the temperature cycle after the device is completed. This is obvious even when looking at the development process of conventional examples. In any case, according to the present invention, none of the electrical and physical characteristics required for millimeter-wave heterodyne mixers, video detectors, etc. are pseudo-viral, and a balance is achieved. The fact that the SIS type Josephson element can be provided is due to the fact that
If desired, it is possible to obtain an element with extremely high improvement rate of characteristics, and it exhibits an extremely remarkable effect that has not been achieved in the past.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram using current-voltage curves to explain the required characteristics of the SIS type Josephson tunnel junction element, Figure 2 is an explanatory diagram of the manufacturing process of the device of the present invention, and Figure 3 is for mixers. FIG. 4 is a plan view of a device according to an embodiment of the present invention, and FIG. 4 is a perspective view of an example of use of the device shown in FIG.
FIG. 5 is an explanatory diagram of the electrical characteristics of the device of the first embodiment, FIG. 6 is an explanatory diagram of the physical characteristics that are also converted into electrical characteristics, and FIG. 7 is the same as FIG. 5 of the device of the second embodiment. FIG. In the figure, 2 is a lower electrode layer, 3 is an insulating layer or oxide film, 4 is an upper electrode layer, 5 is a Josephson tunnel junction, 10
is the element as a whole. Figure 1 Figure 3 Figure 2A Figure 2B Figure 2C Figure 2D Figure 4 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] 1 In a Josephson tunnel junction device in which an insulating film is sandwiched between a pair of upper and lower superconductor electrode layers, the upper polarity is P.
A Josephson tunnel junction device characterized in that the lower electrode layer is made of an alloy material of P_b and B_i and I_n is included in the alloy material of P_b and B_i.