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LITHOGRAPHY
Drawing the circuit with light
Resist
A light-sensitive polymer layer.
Expose and develop
Optical for most of the chip, electron beam for junctions.
Etch and strip
Cut the metal, then clean thoroughly.
The same steps that make computer chips make qubits.
ETCHING
Wet or dry?
Wet
Etches in every direction and undercuts the mask.
Dry
A directional plasma makes vertical walls.
Over-etch
A shallow trench can lower surface loss.
Dry etching is the standard for fine qubit features.
LIFTOFF
Patterning without etching
Resist first
Patterned with an overhang.
Evaporate
Metal arrives in straight lines.
Dissolve
The unwanted metal lifts away.
Josephson junctions are always made by liftoff.
IMPEDANCE
Design a 50-ohm line
50 Ω
The impedance of every cable in the fridge.
A ratio
Strip width to gap sets the impedance.
Precision
Widths must hold to a fraction of a micrometre.
Mismatched lines reflect signals.
DEFECTS
Fab detective
Shorts and opens
Break circuits.
Residues
Add loss.
Size errors
Shift frequencies.
Each fab mistake leaves its own fingerprint in the measurements.
UNIT 23 STUDY COMPLETE
✏️
Ready for the Fab Challenge?
You've followed a lithography cycle, chosen an etch, learned liftoff, designed a 50-ohm line and solved fab mysteries.