EASY PROCESS FOR FABRICATION OF HIGH-RELIABILITY DC-SQUIDS

We have developed an easy process to fabricate high-reliability DC-SQUIDs for the study of biomagnetism on wafers by minimizing the SQUID fabrication process. Al films have been used for the shunt and damping resistors to decrease the number of materials used in the fabrication. The large beta L DC-SQUID device was adopted to increase the reliability of the process because of small number of input coil turns needed. Using this process, we have fabricated devices and tested their reliability by examining the current-voltage and flux-voltage characteristics of the DC-SQUIDs. These characteristics were in close agreement with the calculated values. The critical current and the modulation depth of about 200 DC-SQUIDs across a single wafer agreed within +/-5%. After 30 thermal cycling tests between 4.2 K and room temperature, the changes of the current-voltage and flux-voltage characteristics of the fabricated DC-SQUIDs were less than the-measurement uncertainty of 1 %. The first-order, 7 channel DC-SQUID gradiometer system using these devices is sensitive enough to measure the magnetic field generated from the brain. The sensitivity of the system has remained unchanged after operation of 8 months.