Polarizer–surface-analyzer null ellipsometry for film–substrate systems*

Single-pass polarizer–surface-analyzer null ellipsometry (PSA-NE) can be used to characterize film–substrate systems, provided that the film thickness lies within one of a set of permissible-thickness bands (PTB). For a transparent film on a transparent or absorbing substrate, the PTB structure consists of a small number of finite-bandwidth bands followed by a continuum band that extends from a film thickness of about half the wavelength of light to infinity. We show that this band structure is a direct consequence of the periodicity of the ellipsometric function ρ (the ratio Rp/Rs of the complex amplitude-reflection coefficients for the p and s polarizations) with film thickness. The PTB for the SiO2-Si film–substrate system at He–Ne laser and mercury spectral lines are calculated. The angles of incidence for PSA-NE on a film-substrate system with known film thickness are easily predicted with the help of a graphical construction in the angle of incidence-vs-thickness ϕd plane. PSA-NE is generally applicable to the determination of both film thickness and optical properties of a film-substrate system. The procedure for its application to the special, but important, case of film-thickness measurement alone, when the optical properties are known, is given and is checked experimentally by the determination of the oxide-film thickness on Si wafers. In an automated form, PSA-NE can be a serious competitor for interferometric reflectance methods.