Thermal and chemical loading effects in non selective Si/SiGe epitaxy

Rapid thermal reduced pressure chemical vapour deposition of non-selective Si and SiGe epitaxy is used for the base formation in the fabrication of high performance heterojunction bipolar transistors. The performance of the device is very sensitive to the base stack (thickness, Ge composition…). Thus, kinetics must be carefully controlled. This paper reports on the influence of the substrate on the kinetics of the epitaxial deposition. This so-called loading effect is found to be due to two significant effects, thermal and chemical. The thermal effect comes from a differential heating, depending on the emissivity of the substrate and on the deposition tool. This effect can induce a variation in Si and SiGe growth rates when the growth is carried out in the surface-kinetics limited regime, where the process is very sensitive to the temperature. The chemical effect depends on the polycrystalline silicon/total surface ratio and on the pad size. For the first time, these two effects are experimentally and unambiguously separated and quantified. We show that in our conditions, the thermal effect is the most important but chemical effect, which is usually even not considered, also plays a significant role.