DC SQUIDs with planar input coils

We describe the key parts of our recent work to develop a planar thin-film DC SQUID with a closely-coupled spiral input coil. Our aim has been to make a device that is superior to present RF SQUID sensors in terms of sensitivity and long-term reliability. To be compatible with an RF SQUID the inductance of the input coils must be relatively large, typically 2 μH, and the input noise current in the white noise region should be below 10pA Hz^-1/2. A low level of 1/f noise is also necessary for many applications and should be achieved without the use of complex noise-cancelling circuitry. Our devices meet these criteria. We include a description of work on window and edge junction fabrication using ion beam cleaning, thermal oxidation and RF plasma processing.