Ž . Diamond and Related Materials 10 2001 11321136 Spectroscopic ellipsometric study of tetrahedral amorphous carbon films: optical properties and modelling A. Canillas a,  , M.C. Polo a , J.L. Andujar a , J. Sancho a , S. Bosch a , J. Robertson b , ´ W.I. Milne b a ` Uni  ersitat de Barcelona, Departament de Fısica Aplicada i Optica, A . Diagonal 647, E-08028 Barcelona, Spain ´ b Engineering Department, Uni  ersity of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK Abstract Ž . We present a spectroscopic ellipsometric SE study of ta-C and ta-C:N films obtained in a filtered cathodic vacuum arc system with an improved S-bend filter that yields high values of sp 3 content and mass density. The films consist of an ambientrough- Ž . nessfilminterfacesubstrate structure, as demonstrated by X-ray reflectivity XRR and SE. The optical properties of the films Ž. Ž. have been derived by using four different approaches for the optical function: i Cauchy’s absorbent dispersion formula; ii the Ž . Ž . contribution of three classical oscillators; iii the Forouhi  Bloomer model; and iv the Tauc  Lorentz model. The interface appears to be a mixture of a-Si and ta-C material, due to an amorphisation process originated by the carbon ion bombardment. The comparison of SE with XRR results, as well as the dependence of the fitted parameters with mass density, allows the Tauc  Lorentz model to be applied with confidence.  2001 Elsevier Science B.V. All rights reserved. Keywords: Spectroscopic ellipsometry; Tetrahedral amorphous carbon; Optical properties 1. Introduction Ž . Tetrahedral amorphous carbon ta-C thin films have attracted great interest in recent years due to their extreme diamond-like properties, such as high hard- ness, low friction, chemical inertness, low electrical resistivity, and optical transparency in the infrared and visible ranges. The optical properties of ta-C films have been obtained recently using spectroscopic ellipsometry Ž . SE . From SE data, the thickness and optical proper- ties of thin films can be calculated in different ways: using a direct inversion algorithm method, using an effective medium theory and using an appropriate parameterisation of the dielectric function. Lee et al.  1 have reported ta-C optical spectra obtained by di- rect inversion, as well as an approach based on the  Corresponding author. Ž . Bruggeman effective medium approximation EMA , which allows deduction of the atomic fraction of sp 3 .  On the other hand Xu et al. 2 have derived the optical properties and microstructure of ta-C films using the parameterisation proposed by Forouhi and Bloomer for  amorphous semiconductors 3 . In this paper, we present an investigation of the microstructure and optical properties of ta-C and nitro- Ž . gen-doped ta-C films ta-C:N deduced from SE. The paper focuses on the determination of a reliable optical model to describe the optical properties of these mate- Ž . rials. We have used X-ray reflectivity XRR to confirm the SE results. 2. Experimental Ta-C and ta-C:N films were deposited on c-Si sub- strates in a filtered cathodic vacuum-arc system with an 0925-963501$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 9 2 5 - 9 6 3 5 00 00551-3