Paper TF-WeM3
A Zone Diagram for Conformal Film Growth by CVD

Wednesday, October 22, 2008, 8:40 am, Room 302

The field of thin film growth using physical vapor deposition has benefited enormously from the mechanistic insights provided by Thornton’s zone diagram, which he introduced to this society in 1974. There has been no direct analogue for the case of chemical vapor deposition (CVD), presumably due to the multiplicity of chemical reactions and the wide range of experimental conditions associated with the use of different precursor molecules. One major advantage of CVD is the ability to deposit films with extremely good conformal (step) coverage, which is a requirement for the additive fabrication of many current and proposed nanoscale devices. We show that all conformal CVD processes share certain kinetic requirements and limitations. Each of these can be represented on a plot of precursor pressure vs. substrate temperature: threshold phenomena, such as the onset temperature for reaction, appear as a vertical or horizontal line; kinetic competitions, such as surface adsorption vs. desorption, appear as exponential slopes. The properties of the precursor molecule and the aspect ratio of the feature to be coated (represented by a trench) then define a polygonal figure on the (P,T) plot which may or may not allow for conformal growth. We further show that many of the kinetic coefficients can be represented in normalized form, which we present as a zone diagram for conformal growth by CVD. The usefulness of this construction is illustrated by analyzing well-known experimental systems, such as W or SiO₂ growth, and the recent results we have obtained for the low temperature CVD of HfB₂, CrB₂, TiB₂ and MgO. For all these systems, the underlying mechanism that affords good step-coverage is surface site-blocking due to adsorbed precursor molecules or reaction byproducts or co-reactants. The experimental results are consistent with the predictions of the zone diagram. We conclude by showing how the site-blocking processes, and thus the degree of conformal coverage, can be controlled through the addition of site-blocking agents, such that a precursor that normally does not afford conformal growth can be transformed into one that coats very deep features.