Fabrication and performance of NbN thin film planar squids

Planar SQUIDs with a Dayem bridge and two holes have been fabricated by sputter etching NbN thin films deposited epitaxially on MgO substrates and their performance has been tested in a conventional mode of operation at 15 MHz. It has been found that the devices operate with satisfactory reliability and their operating temperature covers over a wide range nearly from the superconducting transition temperature of the basic films e.g.∼15 K down to the pumped liquid helium temperature. The fabrication technique and the result of the performance test are presented, and the bridge characteristics and possible application of the device are discussed from a technical point of view.