Ultra low noise all niobium DC-SQUIDs

The noise and signal properties of SQUIDs with amorphous silicon barriers and Al₂O₃ barriers are studied. The barrier material is found to be of great importance for the value of the 1/f noise component. The best results were obtained for SQUIDs with Al₂O₃ barriers and a 1/f noise level at 1 Hz of about 1×10^-6Φ/√Hz was found. After integration of coupling coils onto the SQUIDs, a signal limitation and a dramatic increase of the noise were found. Implementation of a damping circuitry over the coupling coil results in optimized signals (Δ V(Φ₀/2)≃I_cR) and a white noise level comparable to the white noise level without a coupling coil. The 1/f noise component for SQUIDs with a damped coupling coil is higher than for 1/f noise component of SQUIDs without a coupling coil. For SQUIDs with Al₂O₃ barriers, the 1/f noise level keeps below 3×10^-6 Φ₀/√Hz at 1 Hz. For SQUIDs with an amorphous silicon barrier the 1/f noise component changes per cooling cycle in an irregular way. The stability for thermal cycling and room-temperature storage is very good for all the devices