The optical and microstructural properties of tetrahedral amorphous carbon (ta-C) thin films prepared by filtered cathodic vacuum are (FCVA) have been investigated as a function of the ion energy. The films were characterized by spectroscopic phase modulated ellipsometry (SPME) for the first time. The optical property of the ta-C layer was derived from the Forouhi and Bloomer amorphous semiconductor model. An effective medium approximation and a linear regression analysis have been used to determine the microstructure of thin ta-C films on silicon. The microstructure of these films deposited on silicon wafers was simulated by a four-layer model consisting of a roughness layer, a ta-C layer, a graded ta-C:Si layer and the silicon substrate. The graded layer consisting of the mixture of ta-C and silicon simulates the carbon ion impinging/diffusion into the surface of the silicon substrate. The complex refraction index, N = n - ik over the range of 250-900 nm, for ta-C had been determined. The Tauc (optical) band gap as a function of ion energy was also studied.