A multiloop SQUID and a SQUID array with 1-µm and submicrometer input coils

We have constructed a niobium multiloop SQUID current sensor and a 60-SQUID series array as a mix-and-match fabrication experiment utilizing our in-house projection stepper. The multiloop SQUID includes an input coil with a linewidth of 0.8 μm and a pitch of 1.6 μm, resulting in the mutual inductance M ^-1 = 1.8 μA/Φ ₀ . The SQUID array is built on the same chip as the second-stage amplifier. The array and the multiloop showed white flux noise of 0.2 and 0.8 μΦ ₀ Hz ^-1/2 at 4.2 K, respectively. Owing to the well-balanced gradiometric narrow-line construction, the two-SQUID system can be cooled down in the Earth's magnetic field and can be operated without superconducting or mu-metal shields. Excess low frequency noise was observed, whose possible origin in Josephson dynamics has been briefly studied with numerical simulation.