THE FOUNDATION

Silicon or sapphire?

Silicon High-resistivity and compatible with standard fabs.

Sapphire A perfect insulator with very low loss.

Both Polished single-crystal wafers.

Much of a qubit's field runs through the chip it sits on.

LOSS TANGENT

The substrate sets a lifetime limit

tan δ The fraction of energy lost each cycle.

Q = 1/(p tan δ) p is the share of the field in the substrate.

Today Good substrates allow lifetimes beyond a millisecond.

The bulk substrate is rarely the main loss now; surfaces are.

SURFACE PREP

Native oxide 1 to 2 nm of lossy SiO₂.

HF dip Strips the oxide and leaves hydrogen on the surface.

Speed Into vacuum before the oxide regrows.

The qubit's first interface is made in its first minutes.

CLEANLINESS

Residues Resist and organic films add loss.

Purity Fewer impurities mean fewer lossy charges.

Discipline Gloves, cleanrooms and clean tools.

Most qubit loss hides at surfaces and interfaces.

SCALE

Research labs Usually 100 mm wafers.

Industrial fabs 200 and 300 mm wafers with automated tools.

Dicing Chips are sawn apart at the end.

Bigger wafers mean more chips and more consistent processing.

UNIT 21 STUDY COMPLETE

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You've chosen a substrate, worked out its lifetime limit, cleaned its surface and laid out chips on a wafer.