JPH0136274B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a Josephson integrated circuit having a Josephson junction.

(2) Background of the technology Normally, the opposing electrode that forms the Josephson junction uses a different material from the base electrode and the control electrode (for example, the base electrode and the control electrode are made of an alloy of lead, indium, and gold, while the opposing electrode (The electrode is an alloy of lead and bismuth.) For example, when a counter electrode and a control electrode are connected, grain migration may occur or contact resistance may occur due to problems with the material of the counter electrode. To avoid this, the opposite electrode is connected to the base electrode via a Josephson junction, which has a sufficiently wider area than the Josephson junction used as a switch element, and then the base electrode and the control electrode are connected.

(3) Problems with the prior art At this time, in the past, the inductance of the wiring was reduced, and a ground plane for the stripline configuration was also laid at this connection.

For example, if the counter electrode 1 and the base electrode 2 are connected via an in-line Josephson junction as shown in FIG. Therefore, the threshold value of the Josephson junction becomes as shown in FIG. 1b.

The reason will be explained in more detail.

FIG. _1c is a cross-sectional view _of FIG. Currents i ₁ and i ₄ flow on the lower surface of each electrode. At this time, an image current i ₂ of currents i ₁ and i ₄ flows on the surface of the ground plane 3, and an image current i 3 of current i ₄ flows on the upper surface in contact with the Josephson junction of the base electrode ₂ .
flows in the direction of the arrow. This situation is shown in Fig. 1(d) when viewed from the right side of Fig. 1(c).

As is clear from FIG. 1d, the electrode i ₄ flowing on the lower surface of the counter electrode 1 is directed toward the front perpendicular to the paper, and the image current i ₃ flowing on the upper surface of the base electrode 2 is directed toward the back of the paper. The electrode i ₁ flowing on the lower surface of the ground plane 2 flows toward the front, and the image current i ₂ flowing on the surface of the ground plane 3 flows toward the back. The magnetic field produced by each current therefore occurs around each current in the direction indicated by the arrow.

Here, the current flowing above and below Josephson junction 4
Regarding i ₃ and i ₄ , the direction of the magnetic field generated by each current is from the left to the right with respect to the Josephson junction.

That is, if there is a ground plane, the current
Due to i ₃ and i ₄ , the state is the same as if an external magnetic field was applied.

Therefore, the magnitude of the critical current Ic that allows the Josephson junction 4 to remain in a superconducting state is smaller than the critical current value when no magnetic field is applied to the junction.

Furthermore, the maximum critical current is achieved only when a magnetic field is applied from the outside that is sufficient to cancel out the magnetic field generated by the currents i ₃ and i ₄ .

For the above reasons, the Josephson junction 4 has the characteristics shown in FIG. 1b. As is clear from the previous discussion, if there is a ground plane,
Even though no external magnetic field is applied, the maximum critical current determined by the connection area etc. cannot flow.

Therefore, in order for the Josefson junction to flow a large superconducting current without being in a voltage state, the junction area must be increased, which has been an obstacle to higher integration.

(4) Purpose of the Invention The purpose of the present invention is to eliminate the above-mentioned drawbacks and to enable the maximum critical current to flow without increasing the area of the connection portion.

(5) Structure of the Invention The above object is to provide a Josephson integrated circuit having a base electrode and a counter electrode on a ground plane, wherein the ground plane is provided with a window, and the base electrode and the counter electrode are connected on the window. This is achieved by being connected.

(6) Embodiment of the Invention FIG. 2 is a diagram illustrating an embodiment of the present invention, in which FIG. 2a is a plan view and FIG. 2b is an equivalent circuit diagram thereof.

In Figure 2a, 21 and 21' are bias currents
Base electrode through which Ib flows, 22 is load resistance, 23,2
3' is a counter electrode, 24 and 24' are control electrodes to which the input signal S is input, 25 is a base electrode, 26 is a ground plane, and 27 is a window provided in the ground plane. The ground plane 26 is provided over one surface except for a window 27 below the connection between the counter electrode 23 and the base electrode 25.

FIG. 2c is a partial cross-sectional view showing the connecting portion between the base electrode 25 and the upper electrode 23 on the window portion 27 of FIG. 2a.

Josephson junction for the gate in Figure 2a
When J ₁ is in a voltage state and a current flows through the loop consisting of the load resistor 22 and the control electrode 24', the current flows across the window 27 as shown in FIG. 2c.
Flows in the direction of the arrow on the upper and lower surfaces of each electrode. Looking at this from the right, as shown in Figure 2 d, the currents flowing above and below Josephson junction 4 flow toward the same depth in the plane of the paper, so the direction of the magnetic field generated by both currents is They are opposite to each other and cancel each other out.

Therefore, no magnetic field is applied to the junction 4 unless a magnetic field is applied from the outside.

Therefore, the maximum critical current can be obtained when the external magnetic field φ=0, that is, the peak point of the characteristic curve shown in FIG. 1b can be brought to the point φ=0.

Therefore, the maximum critical current can be passed without increasing the area of the connection portion.

FIG. 2b is an equivalent circuit of FIG. 2a, and according to the present invention, a large current can be passed through the loop including the load resistor 22 without increasing the connection area.

(7) Effects of the Invention According to the present invention, the maximum critical current can be passed without increasing the connection area between the base electrode and the counter electrode in the Josephson integrated circuit, thereby increasing the degree of integration of the integrated circuit using the Josephson junction. I can do things.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining a conventional example, and FIG. 2 is a diagram for explaining an embodiment of the present invention. 21, 21',
25: base electrode, 23, 23': counter electrode, 2
6: Ground plane, 27: Window section.

Claims (1)

[Claims] 1. A Josephson integrated circuit having a base electrode and a counter electrode on a ground plane, characterized in that the ground plane is provided with a window, and the base electrode and the counter electrode are connected on the window. Josefson integrated circuit.