Silicon on sapphire

Silicon on sapphire (SOS) is a hetero-epitaxial process for metal–oxide–semiconductor (MOS) integrated circuit (IC) 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 (complementary MOS) technologies.

Typically, high-purity artificially grown sapphire crystals are used. The silicon is usually deposited by the decomposition of silane gas (SiH
₄) 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.