Josephson Junctions are made of weak connections between superconductors through which the Josephson effects are realized. Historically, superconductor insulator-superconductor tunnel junctions have been used to study the Josephson effect, primarily because these are physical situations for which time-averages go to zero except when detailed calculations can be made. However, the Josephson effect is not necessarily a tunneling phenomenon, and the Josephson effect is indeed observed in other types of junctions, such as the superconductor- normal metal-superconductor junction. A particularly useful Josephson junction, the point contact, is formed by bringing a Josephson tunnel sharply pointed superconductor into contact with a blunt superconductor. The critical current of a point contact can be adjusted by changing the pressure of the contact. The low capacitance of the device makes it well suited for high- frequency applications. Thin-film microbridges form another group of Josephson junctions. The simplest microbridge is a short narrow construction (length and width on the order of a few
micrometers or smaller) in a superconducting film known as the Anderson-Dayem bridge. If the microbridge region is also thinner than the rest of the superconducting film, the resulting variable-thickness microbridge has better performance in most device applications. If a narrow strip of superconducting film is over-coated along a few micrometers of its length with a normal metal, superconductivity is weakened beneath the normal metal, and the resulting microbridge is known as a proximity-effect or Notarys-Mercereau microbridge. Among the many other types of Josephson junctions are the superconductor-semiconductor-superconductor and other artificial- barrier tunnel junctions, superconductor-oxide- metal-superconductor junctions, and the so- called SLUG junction, which consists of a drop of lead-tin solder solidified around a niobium wire. HYPRES Inc. has developed and sustains several fabrication processes for superconductor electronics [37]. It currently offers three processes with three different critical current densities
of Nb/AlOx/Nb trilayer: 0.3 µA/µm2, 10 µA/µm2and 45 µA/µm2. The Josephson junctions can
be interconnected into circuit configurations using four superconducting layers (junction base electrode (layer M1), two Nb wiring layers (layers M2 and M3) and superconducting Nb ground plane (layer M0). One normal metal layer is used to provide medium-value resistors, which can be used for shunting Josephson junctions, current distribution and other applications. The sheet resistance of this layer is given in the Table 3.1 for all three processes. Silicon dioxide is deposited to provide insulation between the conducting layers. Anodization of the base electrode of the trilayer provides additional insulation to Josephson junctions. Their standard fabrication process uses 6-inch (150 mm) diameter oxidized Si wafers. In the fabrication process, HYPRES uses only refractory materials, with the exception of a Ti/Pd/Au metallization layer used primarily for contact pads. Niobium is used as the superconducting material due to its comparably high critical temperature, electrical and thermal stability, and ability to be thermally
cycled many times without degradation. Niobium/Aluminum-Oxide/Niobium Josephson tunnel junctions are made by depositing an in-situ trilayer across the entire wafer and subsequently defining junction areas by photolithography and etching. This method yields good uniformity and reproducibility of junction parameters. Table 3.2 lists the physical characteristics of the different types of Josephson junctions for niobium-based superconducting integrated circuits. Some different types of Josephson junctions are illustrated in Figures 3.1.
Table 3.1 HYPRES sheet resistance of the layer for all three processes
Process Jc [µA/µm2] Sheet Resistance at 4.5 Kº [Ω] Material Tc [Kº] Thickness [nm] 0.3 2.0 ±0.20 Ti/AuPd/Ti 0.0 100±10 10 1.0 ±0.15 Mo 0.9 70±10 45 2.1 ±0.30 Mo 0.9 40±6
Table 3.2Josephson junction samples
Name Type Junction Area Cj L C Method
L1 Al/AlOx/Al 60μm2 1 pF 10 nH - Surface mount
Lc1 Al/AlOx/Al 100μm2 4 pF 10 nH 10pF Surface mount
Hypres1 Nb/AlOx/Nb 100μm2 4 pF 10 nH 80pF On-chip
(a)
(b)
(c)
Figure 3.1 Some types of Josephson junctions. (a) Thin-film tunnel junction. (b) Point contact. (c) Thin-film weak link.
The dc current-voltage characteristics of different types of Josephson junctions may differ, but all show a zero-voltage supercurrent, and constant-voltage steps can be induced in the
dc characteristics at voltages given by V=hf/2e when an ac voltage is applied. The dc current-
voltage characteristics for a weak link (microbridge) and a tunnel junction are compared in Figure 3.2.
Figure 3.2 DC current-voltage characteristics for a weak and tunnel junction
Josephson junctions, and instruments incorporating Josephson junctions, are used in applications for metrology at microwave frequencies, frequency metrology, magnetometry detection and amplification of electrical signals. Josephson junction devices also have been used in ultrahigh sensitive detectors, and voltage standards. The Josephson junction device has also been applied as a frequency mixer in heterodyne detection and as a generator of high-frequency electromagnetic radiation, as computer elements (with switching time from a zero-voltage to a
finite-voltage state of order less than 10−9 sec) and in thermometry. Extremely high frequencies
(1010 – 1012 Hz) of the supercurrent provide other important applications, especially in logic
circuits and memory cells. Potentially, these high speed and low power digital circuits can be very useful for signal processors and high-performance computers. Also a Josephson junction is used in a qubit realization block for quantum computers. It was for some of these applications and the fundamental nature of the Josephson effect that the discoverer, Brian Josephson, was honored with the 1973 Nobel Prize in Physics.