Ionized physical vapor deposition of titanium nitride: A deposition model

Abstract

A model for reactive ionized physical vapor deposition of TiN has been developed to predict film conformality and stoichiometry in a high aspect ratio trench. The two additional model components are a radio-frequency (rf) sheath model for the transport of ions to the wafer surface and a feature-scale deposition model to simulate the thickness and composition of TiNx films in a two-dimensional trench. The feature scale deposition model depends critically on the sticking coefficient for nitrogen atoms on TiNx surfaces since this is the primary method by which the Ti+ that is deposited inside deep trenches becomes nitrided. This sticking coefficient has been experimentally determined as a function of TiNx stoichiometry. The simulation results agree with both experimental observations and intuitive concepts for reactive sputter deposition. Namely, the deposition rates decrease slightly when rf bias is applied to the wafer. This decrease is due to resputtering. The model also predicts a decrease in the nitrogen content of deposited films along the sidewalls of trenches, and nitrogen deficient films at the bottom of trenches under metal-mode deposition conditions. The model’s accuracy is verified by measuring the thin film characteristics, especially deposition rate, step coverage, and composition. The deposition model gives an accurate prediction of composition ratio. The deposition model also predicts the deposition rate and step coverage accurately if no bias voltage is applied to the wafer. Although the bottom coverage and the deposition rate are accurately predicted, the sidewall coverage is 30% lower than experimental results for −50 V rf bias. This discrepancy is believed to be due to uncertainties in the angle-dependent sputter yield of TiNx at low ion energy, and the neglect of unthermalized fast neutral Ti atoms.