Hafnium dioxide

Hafnium dioxide, also called hafnia, is the metal hafnium bonded to oxygen. In bulk it's a white, very heat-resistant ceramic. In chips, it's used as a film only a few nanometers thick, and that film helped keep transistors shrinking.

The film isn't made from hafnium metal directly. Instead, it's grown on the wafer by atomic layer deposition. The wafer sits in a vacuum chamber, and two gases are pulsed in turn: a hafnium compound, such as hafnium tetrachloride, then water vapor or ozone, which supplies the oxygen. Each pulse coats the surface with a single layer of molecules and then stops, so the film grows by a fraction of a nanometer per cycle. A few dozen cycles build an even film that coats even tiny 3D structures perfectly.

For decades, transistor gates were insulated with silicon dioxide. As transistors shrank, that layer got so thin, just a few atoms, that electrons leaked straight through. Hafnium dioxide has a much higher dielectric constant, about 25 compared with 3.9, so a thicker, leak-resistant layer works just as well. Intel introduced it in its 45-nanometer processors in 2007. It now insulates the gate in nearly every advanced transistor.