Cryogenic wafer prober for Josephson devices

A wafer probing system has been built for the testing of Josephson junction devices at helium temperature. A mechanism moves a probe card from one position to another on a two inch wafer while immersed in liquid helium. The mechanism is actuated by shafts which connect to stepper motors positioned above the helium dewar. A positioning accuracy of ± 50 μ m at the probe tips is achieved. The replaceable probe card is all ceramic and carries 120 rigidly mounted palladium-alloy needles, arranged in signal-ground pairs and positioned in an array which matches the pad design of the particular device under test. Controlled impedance transmission lines are maintained all the way to the wafer's surface. A computer interface is included so that probing of a whole wafer can be conducted under software control. The system is intended for routine testing of Josephson devices in wafer form as well as for testing very large numbers of individual junctions.