Investigation of Low-Frequency Excess Flux Noise in DC SQUIDs at mK Temperatures

The excess low-frequency flux noise in dc superconducting quantum interference devices (SQUIDs) operated at ultra-low temperatures was studied. A large number of single SQUIDs as well as SQUID arrays from 16 wafers fabricated over a period of six years were characterized at 4.2 K and <;320 mK. Considering the large spread in the low-frequency noise at 4.2 K, there was no observable dependence of the low-frequency energy resolution ε_1/f on the SQUID design or fabrication parameters. In contrast, below 4.2 K the low-frequency noise changed moderately or increased strongly depending on whether the bottom Nb or the insulation layer were fabricated in our newer sputter system instead of the older one. The corresponding excess noise levels ε_1/f at <;320 mK and f=10 Hz are typically 40 h and 300 h, respectively (h is Planck´s constant). The excess noise scales as ε_1/f ∝ f^-α with α typically around 0.6 for good devices. For devices with strong low-frequency excess noise, α increases up to about 0.9. The best energy resolution ε achieved so far with a 50 pH test SQUID operated at 16 mK is 0.62 h at 100 kHz, increasing to 1.64 h at 1 kHz and 14 h at 10 Hz.