Spin on glass (SOG) is a mixture of SiO2 and dopants that is suspended in a solvent solution. It is applied to a clean silicon wafer by spin-coating similar to a photoresist. These films fill narrow spaces in the metal levels while planarizing the surfaces as well. It can also be easily patterned with electrons emanating from a scanning tip.
In order to get the dopant out of the spin on glass and into the silicon wafer, there are two steps. The first is the "pre-dep" in which the dopant leaves the SOG and saturates the silicon surface before being placed in an HF dip. The second is the "drive-in" step, which is performed so that the dopant will diffuse into the silicon.
These are some of performance advantages of spin on glass:
Most commonly used in spin on glass are siloxanes from 35-100 nm in size. They have excellent crack resistance and are main stream in spin on glass technology. Phosphosilicates are used as well in order to improve crack resistance even more.