Characterization of the optical properties of PECVD SiN x films using ellipsometry and reflectometry

Abstract

The optical properties of plasma-enhanced chemical vapor deposited (PECVD) silicon nitride (a-SiN x ) films are strongly dependent on the deposition parameters. One of the important applications of a-SiN x is its use as an anti-reflection coating for submicron Deep UV (DUV) photolithography. This application is primarily based on the ability to adjust the SiN x optical functions to the values optimal for the specific exposure wavelength — 248 nm and, potentially, 193 nm — by changing the material composition. Nitride films were deposited on Si wafers in a production PECVD reactor using dynamic deposition. Silane and ammonia gas flows were changed to create a matrix of wafers with changing composition. In this work we characterize the optical properties of the a-SiN x at 248 nm using a multiple wavelength and multiple angle of incidence (MW-MAI) ellipsometer with a UV reflectometer. A new method is suggested which combines the benefits of ellipsometry and reflectometry, and allows reliable measurement of film thickness ( t), refractive index ( n) and extinction coefficient ( k) at the DUV exposure wavelength. This method uses the effective medium approximation (EMA) to represent the optical properties of the SiN x . The limitations of the EMA interpretation of strongly non-stoichiometric SiN x are circumvented by correcting the estimated value of the absorption of the material at exposure wavelength using the measured reflectance. The results were found to be consistent with those obtained using research grade spectroscopic ellipsometry. The latter measurements were interpreted using the EMA and an effective dielectric function (EDF) approximation. The composition of the SiN x measured using the EMA model was found to correlate well with the deposition parameters and the results of Auger analysis of the material.