Simultaneous real-time spectroscopic ellipsometry and reflectance for monitoring thin-film preparation

Abstract

An expansion of the capabilities of high-speed, multichannel spectroscopic ellipsometry (SE) is described that involves simultaneous measurement of the reflectance spectrum along with the two spectra in the ellipsometric angles (ψ, Δ). Previously, a novel rotating-polarizer spectroscopic ellipsometer has been perfected that employs a photodiode array detector for high-speed acquisition of (ψ, Δ) spectra, designed for real-time studies of thin-film growth. For a polarizer angular rotation frequency of ω0, the (ψ, Δ) values at a given photon energy are deduced from the 2ω0 Fourier components of the detector irradiance, normalized by the dc component. A third parameter, the weighted reflectance RA, can be obtained from the dc component and from a calibration based on the known optical properties of the substrate measured prior to film growth. With (ω0/2π)=12.5 Hz, three-parameter data sets, [ψ(hν), Δ(hν), RA(hν); 1.5≤hν≤4.5 eV], can be acquired with a time resolution as short as 40 ms. Although RA provides complementary information to (ψ, Δ), it has yet to be exploited in conjunction with real-time SE until this study. A resilient analysis approach, based on mathematical inversion and least-squares fitting of the real-time three-parameter data sets, is designed to yield the film dielectric functions and thicknesses independently at each time during the early stages of thin-film growth. The three-parameter approach has been applied successfully in studies of amorphous silicon (a-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition, and aluminum and silver films prepared by physical vapor deposition. For the a-Si:H films, RA(hν) exhibits deviations as large as ∼3% from that predicted in modeling (ψ, Δ) alone. The deviations result from light scattering by plasma particulates, and we show how additional information can be extracted from the spectral dependence of the scattering loss.