Low-k dielectric

A low-k dielectric is an insulating film used between the wires inside a chip. The k is the dielectric constant, a measure of how strongly an insulator lets neighboring wires affect each other. Ordinary silicon dioxide has a k of about 3.9. Low-k materials get down to about 2.5 to 3, and porous versions go lower. Less coupling means signals move faster, waste less power and interfere less.

The common kind is carbon-doped oxide, often written SiCOH. Its framework is the same glassy network of silicon and oxygen as silica, but some links are replaced with small carbon groups capped with hydrogen. These groups take up room, making the material less dense and a better insulator. It's deposited on the wafer by plasma-enhanced chemical vapor deposition: a gas of silicon molecules carrying carbon groups is broken apart in a plasma, and the fragments settle as a film. For porous versions, a removable organic ingredient is mixed in and then driven off with ultraviolet light and heat, leaving tiny voids.

In a silicon die, it fills the spaces between the many layers of copper wiring. It's mechanically fragile, which makes chips harder to package.