Refractive index measurement of silicon thin films using slab optical waveguides

Measurement of the real part of the refractive index (n_f′) of silicon thin films using a slab optical waveguide (SOW) is discussed. Upon deposition of thin films on a SOW, the transmittance decreases with periodic oscillation owing to the multiple reflection. It is clarified from a simulation of a four-layer SOW that the period of oscillation is reduced when n_f′ increases. In this paper, we discuss a method of determining the n_f′ from correspondence of the minimum film thickness (d₀) in which the oscillation valley appears as a calculation result of transmittance for the TE₀ mode to d₀ in an experiment in which the refractive index and sizes of SOW are known without considering higher modes or the TM mode. Good agreement is observed in comparison of the result of the measurement with that of the ellipsometer at a wavelength of 632.8 nm. Therefore, it can be shown that the distributions of n_f′ of a-Si:H and SiN thin films at wavelengths from 400 to 800 nm are obtained in the same manner. In order to achieve higher accuracy of the measurement, a SOW with a larger refractive index is used.