Dispersive white-light spectral interferometry with absolute phase retrieval to measure thin film

Abstract

Abstract:We present a white-light spectral interferometric technique for measuring the absolute spectral optical path difference (OPD) between the beams in a slightly dispersive Michelson interferometer with a thin-film structure as a mirror. We record two spectral interferograms to obtain the spectral interference signal and retrieve from it the spectral phase, which includes the effect of a cube beam splitter and the phase change on reflection from the thin-film structure. Knowing the effective thickness and dispersion of the beam splitter made of BK7 optical glass, we use a simple procedure to determine both the absolute spectral phase difference and OPD. The spectral OPD is measured for a uniform SiO 2 thin film on a silicon wafer and is fitted to the theoretical spectral OPD to obtain the thin-film thickness. The theoretical spectral OPD is determined provided that the optical constants of the thin-film structure are known. We measure also the nonlinear-like spectral phase and fit it to the theoretical values in order to obtain the thin-film thickness.

The effect of silicon substrate on thickness of SiO2 thin film analysed by spectral reflectometry and interferometry

“…The reflectance and interferometric phase (nonlinear-like phase [10,11]) of a thin-film structure, which are measured in a slightly dispersive Michelson interferometer at the same time and which agree very well with theory, are used to determine precisely thickness of the SiO 2 thin film on silicon wafers of two crystallographic orientations and different dopant concentrations. We revealed that the thin-film thickness, which varies with the type of silicon substrate, depends linearly on the wavelength at which minimum in the spectral reflectance occurs.…”

“…The procedure for the interferometric measurement [10,11] is based on recording the spectral interferograms in a dispersive Michelson interferometer with one of its mirrors replaced by a thin-film structure and retrieving the absolute optical path difference (OPD) ∆(λ). The OPD ∆(λ) is used to construct for a chosen mirror position L = L 0 the nonlinear-like phase function δ(λ) given by the relation (2) where t eff is the effective thickness of a beam splitter cube [9][10][11] and δ 2 (λ) is the phase change on reflection from the mirror in the interferometer.…”

“…The OPD ∆(λ) is used to construct for a chosen mirror position L = L 0 the nonlinear-like phase function δ(λ) given by the relation (2) where t eff is the effective thickness of a beam splitter cube [9][10][11] and δ 2 (λ) is the phase change on reflection from the mirror in the interferometer. To compensate for the phase change δ 2 (λ), a next measurement step with the reference sample used instead of the thin-film structure needs to be applied [11].…”

“…The experimental set-up used in the application of spectral reflectometry and interferometry to analyse the effect of silicon substrate on the thickness of SiO 2 thin film is the same as that shown in previous papers [9][10][11]. It consists of a white-light source: a halogen lamp HL-2000 (Ocean Optics) with launching optics, an optical fibre with a collimating lens, a bulk-optic Michelson interferometer with a cube beam splitter made of BK7 optical glass, a metallic mirror connected to a micropositioner, a thin-film structure, a microscope objective, micropositioners, a read optical fibre, a miniature fibre-optic spectrometer S2000 (Ocean optics), an A/D converter and a personal computer.…”

“…The thickness of the thin film is determined from the fit of the recorded spectral interferogram to the theoretical one. More recently, the use of dispersive white-light spectral interferometry was extended for measuring the thin-film thickness utilizing the absolute phase retrieval from the spectral interferogram [10]. However, the results suffered from the systematic phase errors due to the optical components present in the interferometer.…”