Optical characterization of HfO 2 by spectroscopic ellipsometry: Dispersion models and direct data inversion Jordi Sancho-Parramon a, ⁎ , Mircea Modreanu b , Salvador Bosch c , Michel Stchakovsky d a Ruđer Bošković Institute, Bijenička 54, Zagreb 10000, Croatia b University College Cork, Tyndall National Institute (TYNDALL), Lee Maltings, Prospect Row, Cork, Ireland c Universitat de Barcelona, Martí i Franquès 1, Barcelona 08930, Spain d HORIBA Jobin Yvon, Thin Film Division, Chilly-Mazarin 91380, France Abstract Hafnium oxide (HfO 2 ) has attracted much interest as high-k material of choice for gate oxide replacement in future CMOS technologies and for its use in optical coating technology. The determination of optical properties, like refractive index and bandgap, is focus of intense research, since the optical constants of HfO 2 depend on the physical microstructure and the deposition methods and conditions. In the present study optical characterization of very thin HfO 2 films deposited by plasma ion assisted deposition and annealed at different temperatures is carried out. The characterization is performed using ellipsometric measurements in the spectral range from 1.5 to 8 eV and by using the Tauc–Lorentz and Cody– Lorentz dispersion models. In addition, direct inversion of the ellipsometric data is also carried out. The combination of the Cody–Lorentz model with Urbach tail results in the best description of the data and enables to determine meaningful parameters. On the other hand, the direct data inversion is shown to be useful to provide additional information like the presence of subgap absorption peaks and points out features associated to the crystallinity of the material. © 2008 Elsevier B.V. All rights reserved. Keywords: Hafnium oxide; Spectroscopic ellipsometry; Optical properties; Tauc–Lorentz; Cody–Lorentz 1. Introduction Hafnium dioxide (HfO 2 ) is one of the most promising candidates to replace silicon dioxide (SiO 2 ) as gate oxide in CMOS technology, due to its high dielectric constant and thermal stability in contact with silicon [1]. Besides, since HfO 2 is transparent down to approximately 250 nm, it has been used as high refractive index material in UV and visible interferential coatings [2]. Other applications of HfO 2 are found in gas and magnetic sensors [3,4]. Due to this high technological interest, a lot of research efforts have been conducted in order to connect the physical properties of the material with the manufacturing conditions, aiming to find the optimal deposition process and parameters for a given application. One of the most useful techniques in HfO 2 characterization is spectroscopic ellipsometry (SE). This technique is generally used to obtain an accurate determination of HfO 2 films optical properties such as refractive index, absorption coefficient and bandgap [5] or as a metrology tool for thickness determination of very thin HfO 2 films [6]. In addition, it has been shown that the shape of the dielectric function above the bandgap is correlated with HfO 2 crystallinity, enabling the use of SE as structural characterization tool [7]. SE has been also suggested as a way to detect film defects by analysis of small absorption features below the HfO 2 bandgap [8]. Successful application of SE requires an appropriate modelling of the sample under investigation. In particular, the availability of a dispersion model that can flexibly account for the energy dependence of material optical properties is a critical issue. In this paper, the suitability of Tauc–Lorentz and Cody–Lorentz dispersion models for describing ellipsometric spectra of HfO 2 films is analyzed. Samples with different film thickness and annealed at different temperatures are character- ized with these models. The validity and limitations of the models are assessed by statistical evaluation of fitting quality, Available online at www.sciencedirect.com Thin Solid Films xx (2008) xxx – xxx TSF-24554; No of Pages 6 ⁎ Corresponding author. E-mail address: j.sancho.parramon@gmail.com (J. Sancho-Parramon). www.elsevier.com/locate/tsf 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.007 ARTICLE IN PRESS Please cite this article as: J. Sancho-Parramon, et al., Optical characterization of HfO 2 by spectroscopic ellipsometry: Dispersion models and direct data inversion, Thin Solid Films (2008), doi:10.1016/j.tsf.2008.04.007