Kinetic study on heterogeneous nucleation and incubation period during chemical vapor deposition

The kinetics of heterogeneous nucleation during chemical vapor deposition (CVD) is still unclear despite its importance. Nucleation delay is often observed in many CVD processes, which is known as the incubation period (τ_i). In this study, the effects of concentration (C) and sticking probability (η) of film-forming species on τ_i were formulated based on our kinetic model. To discuss the kinetics, τ_i ^-1 with the rate dimension was used and formulated using C and η. Because η onto heterogeneous surfaces (η^hetero) is difficult to evaluate, the study was initiated with η onto homogeneous surfaces (η^homo), followed by a discussion on its reasonability. The formulation was validated using the experimental dataset for SiC-CVD from CH₃SiCl₃/H₂ onto BN underlayers because CVD involves multiple film-forming species with different η^homo ranging from 10^-6 to 10^-2 and thus is a suitable system for studying the effect of η^homo. High-aspect-ratio (1000:1) parallel-plate microchannels consisting of τ_i-involving BN and a τ_i-free Si surface were utilized to separate these film-forming species along the microchannel depth. τ_i was exceptionally long, up to several hours, depending on the CVD conditions. τ_i ^-1 was found to be proportional to Cⁿ, where n is the reaction order. n was quantified as ≈1.6, suggesting the initial nucleation was triggered by the impingement of two adspecies in the second order and lowered possibly by the discrepancy between C in the gas-phase and that actually producing adspecies on the surface. τ_i ^-1 was also found to be proportional to η^homo. The exceptionally long τ_i was likely originated from the significantly lower η^hetero than η^homo and the higher activation energy for η^hetero than that for η^homo.