Role of nucleation layer morphology in determining the statistical roughness of CVD-grown thin films

During the chemical vapor deposition of thin films, a molecular inhibitor can be added to control the morphology during the nucleation stage and/or the conformal coating behavior during the growth stage. The authors use this control strategy to determine the separate influence of nucleation morphology and of conformal growth on the final surface roughness, evaluated through the power spectral density of AFM height data. The experimental system is HfB2 deposition from the precursor Hf(BH4)4 using NH3 as the inhibitor. For a nucleation layer consisting of mounds, the low frequency (long lateral range) roughness cannot be reduced by the overgrowth of film, even with the conformal growth conditions. Conversely, when the nucleation layer consists of a dense compact of islands, the low frequency roughness remains low throughout film growth, even when carried out in the nonconformal growth conditions. In all cases, the high frequency portion of the roughness decays in a similar manner, indicating that short-range smoothing mechanism is operative. The sensitivity of the final surface roughness to the morphology of the initial nucleation layer demonstrates that “shadowing” by peaks in the surface height is a strong kinetic driving force for roughening, consistent with previous theory. The use of an inhibitor molecule in CVD provides a means to obtain ultrasmooth films on relatively unreactive substrates, without the need for surface activation and without changing the film composition.