Titanium nitride

Titanium nitride is titanium bonded to nitrogen, one atom of each. It's a ceramic that looks like gold, is extremely hard and conducts electricity like a metal. You may have seen it as the gold coating on drill bits, where it keeps the cutting edge sharp.

In a chip factory, titanium nitride is laid down as a film a few nanometers thick. One way is sputtering: a titanium target is bombarded with ions in a chamber containing nitrogen gas, and the knocked-off titanium atoms combine with nitrogen as they land on the wafer. For the tiniest 3D shapes, atomic layer deposition is used instead, pulsing a titanium-containing gas and then ammonia, one atomic layer at a time.

In a modern transistor, it's part of the metal gate that sits on the hafnium dioxide insulator. Older transistors used gates of doped silicon, but those didn't work well with the new high-k insulators. Metals like titanium nitride solved that, and chipmakers fine-tune the voltage at which a transistor switches by adjusting its thickness and adding other metals. It's also used as a thin lining that helps tungsten plugs stick and keeps them from reacting with their surroundings.