Fabrication and performance of NbN thin film planar SQUIDs

Planar SQUIDs, with a Dayem bridge and two holes, were fabricated by sputter etching NbN thin films deposited epitaxially on MgO substrates, and their performance was tested in a conventional mode of operation at 15 MHz. The devices operate with satisfactory reliability, and their operating temperature covers a wide range, nearly from the superconducting transition temperature of the basic films (e.g. approximately 15 K) down to the pumped liquid helium temperature. The fabrication technique and the results of the performance test are presented, and the bridge characteristics and possible applications of the device are discussed