Real time ellipsometry and atomic force microscopy (AFM) were used to measure critical nucleation parameters for polycrystalline silicon deposition on an amorphous SiO2layer by rapid thermal chemical vapor deposition (RTCVD) using disilane (5% in helium). A particularly important parameter for selective epitaxial deposition is the time for nuclei to form, the incubation time. Quantitation of the nucleation parameters, such as the nuclei density, nuclei growth rate, nuclei coalescence, and an operational incubation time were determined from the real time ellipsometric measurements and confirmed by AFM. For a substrate temperature of 700 °C and at a chamber pressure of 0.2 Torr, the nuclei densities of 1.4×1010nuclei/cm2, incubation time of 26 s and nuclei layer growth rates of 20 nm/min were obtained. ©1995 American Institute of Physics.