Electric VLSI Design System User's Manual

Chapter 7: Technologies

7-6: Special Technologies
7-6-5: The Josephson Technology

At very cold temperatures, just a few degrees above absolute zero, some metals loose all electrical resistance and become superconductive. The non-linear element for superconductive digital cirucits is the Josephson Junction or JJ. JJs are usually made using the "tri-layer" process developed by Bell Labs which has layers Niobium, Aluminum, and Aluminum Oxide. A JJ is superconductive until the current through it exceeds a "critical current" proportional to its area. JJs are usually circular because a circle changes area least when slightly over or under etched.

Electric's Josephson technology specifies tha layers required to make JJs. It conforms to the MIT Lincoln Laboratory μA/μm² Superconductor Electronics Fabrication process SFQ5ee_1.4. The Josephson technology also specifies some "inductor" devices that allow Elecric to estimate the inductance of superconducting wires.
Everyone knows that current seeks the path of least resistance, but where does currrent go if there is no resistance? Current is lazy. It follows the path that requires least energy, dissipated in resistance, or stored in inductance.
Here is the layout for a 212 microAmp JJ and its parallel damping resistor. The damping resistor is chosen to provide critical damping of the capacitance inherent in two very close layers of superconductor. Larger JJs require damping resistors of lower resistance. Both the JJ and its damping resistor lie beween Metal-5 (red) and Metal-6 (blue) in the MIT Lincoln lab process available in Electric.
Figure 7.32

JJs can be used to build logic circuits, such as this nine-bit counting shift register. The picture includes three I/O converters at the upper left near the counter's most significant bit, and three more I/O converters at the lower right near the least significant bit.

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