Fin field-effect transistors (FinFET) are promising devices in small-scale integrated circuits, such as 22 nm technology and below, for their high drive currents and low chip-area usage. To further improve the drive currents of FinFETs, semiconductor materials having high electron mobility and hole mobility may be used in FinFET structures.
Germanium is a commonly known semiconductor material. The electron mobility and hole mobility of germanium are greater than that of silicon, hence making germanium an excellent material in the formation of integrated circuits. However, in the past, silicon gained more popularity since its oxide (silicon oxide) is readily usable in the gate dielectric of metal-oxide-semiconductor (MOS) transistors. The gate dielectrics of the MOS transistors can be conveniently formed by thermal oxidation of silicon substrates. The oxide of germanium, on the other hand, is soluble in water, and hence is not suitable for the formation of gate dielectrics.
With the use of high-k dielectric materials in the gate dielectrics of MOS transistors, the convenience provided by the silicon oxide is no longer a big advantage, and hence germanium is reexamined for the use in integrated circuits. Recent studies of germanium focusing on germanium nano-wires, which are used in FinFETs, have been reported.
A challenge faced by the semiconductor industry is that it is difficult to form germanium films with high germanium concentrations or pure germanium films. Particularly, it is difficult to form high-concentration germanium films with low defect densities and great thicknesses, which germanium films are required for forming FinFETs. Previous research has revealed that when a silicon germanium film is epitaxially grown from a blanket silicon wafer, the critical thickness of the silicon germanium film reduces with the increase in the percentage of germanium in the silicon germanium film, wherein the critical thickness is the maximum thickness the silicon germanium film can reach without causing excess defects. For example, when formed on blanket silicon wafers, the critical thickness of a silicon germanium film having a 20% germanium percentage may be about 10 nm to about 20 nm, which thickness is still not adequate for forming FinFETs. To make things worse, when the germanium percentage increases to 40, 60, and 80%, the critical thicknesses are reduced to about 6-8 nm, 4-5 nm, and 2-3 nm, respectively. Accordingly, it is not feasible to form germanium films on blanket silicon wafers for the purpose of forming FinFETs.