We demonstrate that a multi-technology approach enables the accurate characterization of the thickness and optical properties of Amorphous Carbon (α-C) films used in semiconductor manufacturing. Because the material is found to be highly birefringent, with its measured refractive index and extinction coefficient depending strongly upon the polarization and angle-of-incidence of the optical probe beam that is used, conventional single angle-of-incidence Spectroscopic Ellipsometry (SE) has insufficient information-content to detect the ordinary and extraordinary refractive indices with sufficient accuracy. On the other hand, Beam Profile Reflectometry® (BPR®) is particularly strong in this case because it measures the actual reflectances for both polarization components (rather than just the difference between them) at multiple angles-of-incidence. Furthermore, the fact that BPR is a single-wavelength technique means that no assumptions must be made about the optical dispersion of the film and an absolute measurement can be made. It is then possible to combine this with other, spectral technologies to obtain ordinary and extraordinary n and k at all wavelengths. We show that, to first order, the birefringence can be modeled by assuming a single “anisotropy parameter”, the ratio between the ordinary and extraordinary refractive indices (or, more correctly, the complex dielectric functions). Using this approach in combination with a simple Bruggeman effective-medium dispersion model enables robust characterization of these films for production.
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Leng, J., Opsal, J., Pois, H. et al. Multi-Technology Measurements of Amorphous Carbon Films. MRS Online Proceedings Library 872, 175 (2005). https://doi.org/10.1557/PROC-872-J17.5