Germanium on sapphire

This paper explores the potential of germanium on sapphire (GeOS) wafers as a universal substrate for System on a Chip (SOC), mm wave integrated circuits (MMICs) and optical imagers. Ge has a lattice constant close to that of GaAs enabling epitaxial growth. Ge, GaAs and sapphire have relatively close temperature coefficients of expansion (TCE), enabling them to be combined without stress problems. Sapphire is transparent over the range 0.17 to 5.5 µm and has a very low loss tangent (a) for frequencies up to 72 GHz. Ge bonding to sapphire substrates has been investigated with regard to micro-voids and electrical quality of the Ge back interface. The advantages of a sapphire substrate for integrated inductors, coplanar waveguides and crosstalk suppression are also highlighted. MOS transistors have been fabricated on GeOS substrates, produced by the Smart-cut process, to illustrate the compatibility of the substrate with device processing. © 2008 World Scientific Publishing Company.