Ellipsometry of single-layer antireflection coatings on transparent substrates

The complex reflection coefficients of p- and s-polarized light and ellipsometric parameters of a transparent substrate of refractive index n2, which is coated by a transparent thin film whose refractive index n1=n2 satisfies the anti-reflection condition at normal incidence, are considered as functions of film thickness d and angle of incidence ϕ. A unique coated surface, with n1=n2 and film thickness d equal to half of the film-thickness period Dϕ at angle ϕ and wavelength λ, reflects light of the same wavelength without change of polarization for all incident polarization states. (The reflection Jones matrix of such coated surface is the 2 × 2 identity matrix pre-multiplied by a scalar, hence tanΨ = 1,Δ = 0.) To monitor the deposition of an antireflection coating, the normalized Stokes parameters of obliquely reflected light (e.g. atϕ=70∘) are measured until predetermined target values of those parameters are detected. This provides a more accurate means of film thickness control than is possible using a micro-balance technique or an intensity reflectance method.