Step Coverage Prediction in Low-Pressure Chemical Vapor Deposition

A mathematical model incorporating simultaneous one-dimensional Knudsen diffusion and chemical reaction is employed to analyze transient behavior during low-pressure chemical vapor deposition (CVD) in features of arbitrary geometry on patterned semiconductor wafers. For a given feature geometry and CVD reaction, the dimensionless model equations reveal that step coverage is controlled by the value of a single dimensionless parameter that represents the ratio of a characteristic deposition rate to a characteristic reactant diffusion rate. Temperature, reactant partial pressure at the feature mouth, and aspect ratio are the process parameters that may be varied to improve step coverage for a given CVD chemistry. The proper directions of change for temperature and reactant pressure are determined by the parameters of the intrinsic reaction rate expression for the particular CVD chemistry of interest. Deposition of amorphous SiO₂ from TEOS (tetraethyl silicate) is considered as a specific example. The model predicts that step coverage improves with decreasing temperature and increasing pressure of TEOS at the feature mouth, in agreement with experimental results. Calculation of reactive sticking coefficients shows that this diffusion-reaction model is consistent with the modification of the line-of-sight model that invokes low sticking coefficients to explain conformal step coverage.