Silicon on sapphire (SOS) is a
hetero-epitaxial process for
integrated circuit manufacturing that consists of a thin layer (typically thinner than 0.6 µm) of
silicon grown on a
sapphire (Al
2O
3) wafer. SOS is part of the
Silicon on Insulator (SOI) family of
CMOS technologies. Typically, high-purity artificially grown
sapphire crystals are used. The silicon is usually deposited by the decomposition of
silane gas (SiH4) on heated sapphire substrates. The advantage of sapphire is that it is an excellent
electrical insulator, preventing stray
currents caused by radiation from spreading to nearby circuit elements. SOS faced early challenges in commercial manufacturing because of difficulties in fabricating the very small
transistors used in modern high-density applications. This is because the SOS process results in the formation of dislocations, twinning and stacking faults from
crystal lattice disparities between the sapphire and silicon. Additionally, there is some
aluminum, a p-type
dopant, contamination from the substrate in the silicon closest to the interface.