Accurate measurement of small currents using a CCC with DC SQUID readout

The continuous miniaturisation in the semiconductor industry increases the importance of accurate measurement and control of very small currents. in t he field of precision electrical measurements, cryogenic current comparator s (CCCs) are used as extremely sensitive and accurate instruments for scali ng currents. In an international co-operation project, a special CCC system is being developed, optimised for the measurement of extremely small curre nts to less than 1 pA (10(-12) A). DC superconducting quantum interference device (SQUID) readout is used for monitoring the ampere-turn unbalance of the CCC. In this paper, we report on the accurate determination of the CCC and SQUID input coil inductances. Matching of these inductances is required for obtaining ultimate current resolution with the CCC. We find very good agreement between measurements of the CCC inductance at room temperature an d at 4.2 K, and results from numerical calculations. The measured values fo r the input and mutual inductance of the SQUID sensor are in good agreement with the design values when the effect of the slit in the SQUID washer is correctly taken into account. Final current resolution of our measurement s ystem is expected to be better than 1 x 10(-15) A/root Hz for a CCC with 20 ,000 primary windings. (C) 2000 Elsevier Science S.A. All rights reserved.