Spectroscopic ellipsometry over the spectral range from 700 to 3000cm−1and from 1.5 to 3.5 eV is used to simultaneously determine phase and microstructure of polycrystalline hexagonal and cubic boron nitride thin films deposited by magnetron sputtering on (100) silicon. The results are obtained from a single microstructure-dependent model for both infrared and visible-light thin-film anisotropic dielectric functions. The optical behavior of highc-BN content thin films is described by an effective medium approximation. We obtain the amount ofh-BN within highc-BN content thin films. A thin oriented nucleation layer between the silicon substrate and the highc-BN content layer is demonstrated. The preferential arrangement of the graincaxes within theh-BN thin films are found to be dependent on the growth parameters. The results from the infrared and visible spectral range ellipsometry model are compared to each other and found to be highly consistent. ©1997 American Institute of Physics.