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Figure 6.1: 4-Wire Measurement – a) On-chip trace layout: Each side of the
junction to be measured is connected to two pads, one of which is used for a
current bias and the other for a voltage measurement. b) Electrical diagram: A
current source drives a known current I through the series resistor of interest
R J . This causes a voltage V = I R J to develop across the resistor, which can be
measured at the indicated terminals. If the volt-meter is assumed to have infinite
internal resistance, the wiring resistances R L1 through R L4 do not influence the
result.
6.2.2 Adiabatic Demagnetization Refrigerator
Unfortunately, the other circuit elements, like the inductor, capacitor, and
squid, can only be tested cold, i.e. at temperatures where the aluminum traces
become superconducting (< 1 K). Since the final cool-down in the dilution refrigerator
(see Section 6.3.1) is rather time consuming and expensive, it makes sense
to screen the candidate dies further in an “Adiabatic Demagnetization Refrigerator”
(ADR). This refrigerator can cool samples to around 100 mK and stays cold
for a few hours at a time. The cooling happens in two stages: The first stage, a
closed cycle pulse tube cooler, works very much like a conventional refrigerator,
except that it circulates helium gas rather than tetrafluoroethane (the environmentally
conscious replacement for Freon).
This cooler brings the refrigerator
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