Utilizing Inhibitor Molecules in Low Temperature CVD to Control Thin Film Conformality, Nucleation, and Surface Morphology

When performed at substrate temperatures < 300°C, the growth of thin films by chemical vapor deposition can be strongly inhibited by the reversible adsorption of precursor, byproduct, or neutral molecule species on the active surface.  The system-dependent microscopic mechanisms include site blocking, associative desorption of precursor, or competitive adsorption.  Under extreme site blocking conditions, the reaction probability of arriving precursor molecules drops as low as 10^-6 per wall collision, a value that affords extremely conformal growth in deep features with aspect ratio > 100:1.  The surface roughness is exceptionally low due to the smoothing effect of precursor re-emission, which mitigates the ‘shadowing’ effect of peaks in the local surface morphology. For precursor molecules that lack the vapor pressure or adsorption enthalpy to reach such values, we add a neutral molecule ‘inhibitor’ to enhance site blocking.  This approach converts a ‘non-conformal’ precursor into one that provides good step coverage in features with aspect ratio ~ 10:1. 

            We derive, based on the diffusion-reaction equation, a conformal zone diagram that predicts regimes of highly conformal CVD as a function of precursor pressure, substrate temperature and feature aspect ratio.

            We show that an inhibitor molecule can be used to control the film nucleation kinetics.  There are two regimes depending on whether the inhibitor molecule binds more strongly to the film material than to the bare substrate surface, or vice versa, such that the equilibrium coverage of inhibitor is large only on the more strongly binding surface.  For strong binding to the film, the density of small nuclei is enhanced, which affords an ultra-smooth film; we give the example of HfB₂ growth on SiO₂ using NH₃ as the inhibitor.  Conversely, when the inhibitor slows the nucleation rate, the deposit consists the sparse distribution of islands in a narrow size distribution that may be useful in photonic or catalytic applications; we give the example of Cu growth using VTMS as the inhibitor.  Variants of the latter also afford highly selective growth, e.g., Cu deposition on a pre-existing metal line with essentially zero Cu nucleation on neighboring oxide. 

            Finally, we describe two methods to afford superconformal growth, meaning that the film growth rate increases with depth in a feature of aspect ratio between 10:1 and 25:1.  These utilize either a high sticking probability, consumable inhibitor, such as atomic H or N produced by a remote plasma source, or an intrinsic rate competition between two reactants.  Superconformal growth leads to excellent filling of deep features with no void space or seam along the centerline.