The Wiring Sky

10 atomic recall cards. Export to Anki and let spaced repetition do its slow work.

In Anki: File → Import, choose this TSV, set field separator to Tab, deck = Sand to Silicon · Ch 10, note type = Basic.

Front	Back
What does 'BEOL' stand for?	Back-end-of-line — the wiring layers above the transistors.
How many metal layers does a Rubin-class GPU's BEOL contain?	Fifteen or more.
How long would all the copper in a single GPU be if laid end to end?	Tens of kilometers.
What metal did copper replace for chip wiring, and roughly when?	Aluminum, replaced by IBM's copper interconnect process in the late 1990s.
Why does copper conduct better than aluminum?	Copper's resistivity is about 40% lower, and it resists electromigration better under high current density.
Why can't copper be plasma-etched like aluminum?	Copper does not form volatile etch byproducts under standard plasma chemistries — it cannot be cleanly removed that way.
What barrier metal is deposited before the copper seed layer, and what does it prevent?	Tantalum nitride (TaN) — it prevents copper from diffusing into surrounding silicon or dielectric materials.
What is CMP, and what role does it play in dual-damascene?	Chemical-mechanical polishing — it removes the excess copper deposited above the trenches, leaving only the inlaid wires.
What is the wire pitch of the lowest metal layers (M1/M2) at a leading-edge node?	Perhaps ~30 nm.
What is 'low-k dielectric' and why is it used between metal layers?	A dielectric with relative permittivity below ~2.5 (versus 3.9 for SiO₂), used to reduce capacitive coupling between adjacent wires and speed signal propagation.