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During this step, argon ions are accelerated with an electric field of around 500 V
towards the sample where they impact the surface and remove material through
physical bombardment. Since this process is not based on a chemical reaction, the
resulting etch cannot distinguish between the different materials on the substrate.
Thus, the etch needs to be timed such that it only removes the aluminum’s oxide
layer, but not the trace beneath. But as the mill rate can be closely controlled
with the beam current density, this is not hard to achieve.
5.5.2 Aluminum Deposition
Using the same aluminum sputter technique as described above, one can now
deposit 200 nm of aluminum to form the top wiring layer.
5.5.3 Junction Gap Cut
For reasons that will become clear during the junction definition, the first step
in defining the top wiring traces is to cut holes only in those regions necessary to
prevent a shorting of the qubit junctions (see Figure 5.5). The cutting is performed
with exactly the same lithography – ICP etch – strip process that was used to
define the base wiring traces.
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