Split manufacturing
Beginner
Making a chip in two factories, so the one that isn’t trusted never sees the whole design.
Novice
Building a chip in two factories: an untrusted one makes the transistors and lower wiring layers, and a trusted one adds the upper wiring layers that complete the connections.
Expert
The lower part is called the front end of line (FEOL) and the upper part the back end of line (BEOL). Security depends on how much of the hidden wiring can be inferred from the FEOL. Proximity attacks exploit the fact that placement tools put connected cells near each other; defenses include placement perturbation, wire lifting and layout obfuscation.
Explained in Defense (Industries).