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5.6 Junction Layers
5.6.1 Oxidation / Deposition
The next step is to form the qubit junction. The junction consists of a thin
layer of aluminum oxide sandwiched between two aluminum electrodes. The thickness
of the oxide needs to be very well controlled, since it will determine the junction’s
critical current. Thus, the uncontrolled native oxide layer that formed on
the top wiring layer during exposure to air needs to be removed with the argon
mill step described in Section 5.5.1. After, a controlled amount of oxygen is bled
into the chamber of the sputter system to oxidize the exposed clean aluminum
to the desired depth. Immediately after, the entire wafer is covered with another
150 nm of sputtered aluminum to form the junction’s counter-electrode.
5.6.2 Junction Definition via Argon-Chlorine Etch
Since the critical current of the junctions not only depends on the thickness
of their oxide, but also on their area, a range of critical currents can be created
across the wafer by offsetting the position of the features slightly for the different
rows on the wafer during the photo-lithography. This time, the etching of the
junctions is not done via the usual BCl 3 /Cl 2 etch, since the oxide layer on the top
wiring is too thin to allow for a selective ICP etch to stop on it. The usual etch
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