Verlag des Forschungszentrums Jülich
JUEL-3563
Ruck, Bernhard
Entwicklung von Multilagenbauelemten für HTSL-RSFQ-Schaltungen und erste Messungen zur Fehlerrate
178 S., 1998
Superconducting digital circuits based on the concept of rapid
single quantum logic (RSFQ) offer high speed operation at very low
power dissipation. The product of switching time and dissipated power
of these circuits is very small compared to semiconductor devices.
However, due the cooling demands of superconducting
circuits, reasonable applications are only those where the higher
performance justifies higher cooling costs.
RSFQ circuits in niobium technology are already well developed. High
temperature superconductors (HTSC) like YBa2Cu3O7-[delta] allow
operation temperatures above the temperature of liquid helium and higher
clock frequencies due to the larger energy gap. Unfortunately, the
technology for HTSC is still immature.
In this work, different aspects of HTSC RSFQ
circuits have been investigated, with the emphasis on multilayer
technology and on measurements of the bit error rate. In addition, a
first step towards a three-dimensional integration of low temperature
RSFQ circuits was developed.
A new device consisting of two vertically stacked separately shunted
Nb/AlO[chi]Al/Nb Josephson junctions was used to test the possibility of
three dimensional integration for RSFQ circuits for the first time. A
T-flipflop was designed its operation as a voltage divider was
verified experimentally.
For HTSC circuits, basic devices like resistors and multilayer
structures, e. g., cross overs, insulating layers and inductances
with groundplane were fabricated, and the electrical properties were
investigated. These devices are indispensable for a future technology
allowing the realization of more complex circuits.
Based on stacked bicrystal junctions in multilayer technology, a
first order delta sigma modulator for analog to digital conversion
was designed and characterized by electrical measurements.
Because of the small switching energies, the stability against
thermal noise is a critical issue for RSFQ circuits, especially at
higher temperatures. In order to estimate the static error rate, the
escape rate of a flux quantum stored in a dc-SQUID was measured with
two different circuits. One circuit was designed for stacked
bicrystal junctions; the second consisted of only one superconducting
layer. Both circuits showed error rates
which are consistent with a model for thermally activated events.
However, the single layer circuit showed error rates indicating
to a two to three times enhanced noise temperature.
In order to measure the dynamic error rate, a Josephson comparator as
basic RSFQ switching device was integrated into a Josephson
transmission line ring oscillator. With this circuit it was possible
to test the Josephson comparator at high speed and to detect
seldom errors at the same time. Depending on the bias conditions, bit
error rates of less than 10-11 at 39 K could be observed.
The measured error rates show that HTSL RSFQ circuits could be useful for
several digital applications and should encourage further efforts to
improve especially the technology and reproducibility of HTSL devices.
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