Multiple interference and spatial frequencies' effect on the application of frequency-domain optical coherence tomography to thin films' metrology

Abstract

Frequency-domain optical coherence tomography (FD-OCT) was proved to be very useful under two conditions: (i) the layered sample scatters so that multiple interferences do not exist and (ii) the interferometer uses collimated beams so that a single spatial frequency is used. We examine the use of FD-OCT for thin films' metrology when multiple interferences and a range of spatial frequencies exist, such as with interference microscopy where the spatial frequencies' content is provided by the numerical aperture (NA) of the microscope objectives. The effect of multiple interferences is found to be reduced significantly by shortening the coherence length such that it is less than the thinnest layer, and the smaller the number of layers, the higher the accuracy. For the spatial frequency effect, it is found that using microscope objectives with NA up to 0.3, the accuracy is still high with the advantage of providing full field operation and high lateral resolution; however, a correction factor has to be introduced for the determination of the thicknesses. An inverse scattering approach is proposed to improve the accuracy and allows the measurement of both thicknesses and dispersion laws and allows better accuracy than polarized reflectometry, in particular in the full field mode using annular lenses. Some experimental results are presented supporting the theoretical predictions.