A Multi-Aperture Cryogenic Storage Cell

Abstract

Two types of superconducting memory cells are now being considered: the continuous-film memory and cryotron-switched storage loops. The former suffer from low cell operating range compared to cell-to-cell variation. The cryotron cells, although having a wider operating range, must be switched very rapidly to have a detectable output signal. A recent development in cryogenic sensing called ``Current Stretch'' has allowed more freedom in the design of superconducting memory cells. With current stretching the information is transformed to a persistent input current in a cryotron amplifier. The signal produced when the cell is switched is then proportional to the magnetic flux change rather than the rate of change of flux. This feature eliminates the usual memory requirements of fast-access current risetime, pickup noise cancellation, fast cell switching, and the need for a separate sense line with cryotron-accessed memories. A new class of cell which takes advantage of this technique will be described. It consists of a cryotron shunted across a loop, one side of which has a lower inductance and critical current than the other. If the cryotron is activated while current is injected into the cell, a persistent current is created in the loop. Cell-flux changes are detected through one of the cryotron access trees using current stretching. The operating tolerance of these cells is only limited by their geometry. They have no metal-to-metal contacts, use only three metal layers including the ground plane, need no sense line, no noise cancellation, and can be switched arbitrarily slowly. Cells can be fabricated as small as 10 mil2 and operating ranges as large as 40% have been observed. The cell speed is on the order of 50 nsec with a single crossover access cryotron.