Spectroscopic ellipsometry of very thin tantalum pentoxide on Si I. Karmakov a, *, A. Konova a , E. Atanassova b , A. Paskaleva b a Soﬁa University, Faculty of Physics, Department of Condensed Matter Physics, 5, J. Bourchier Boulevard, Soﬁa 1164, Bulgaria b Institute of Solid State Physics, Bulgarian Academy of Sciences, 72, Tzarigradsko Chaussee Boulevard, Soﬁa 1184, Bulgaria 1. Introduction The need of high dielectric constant insulators for dielectric stacks in dynamic random access memories (DRAMs) and MOSFETs is now well recognized. The conventional SiO 2 gate dielectric has reached its limits due to the high direct tunneling current. High dielectric constant k materials (high-k oxides) such as Ta 2 O 5 , TiO 2 , ZrO 2 , HfO 2 and Al 2 O 3 are thereby being considered to replace SiO 2 , reducing the tunneling by increasing its physical thickness. Successful attempts have been made to apply Ta 2 O 5 as an active dielectric in storage capacitors of DRAMs [1], and with this the ﬁrst application of high-k dielectrics in nanoscale top- down microelectronics has been marked. Unfortunately, metal oxides are thermodynamically unstable with Si. During deposition and post-deposition thermal treatments, reactions occur between the oxide and Si that result in the formation of a thin interfacial layer [2–8]. It degrades the electrical properties and increases the equivalent SiO 2 thickness (deq) of the high-k oxide gate. If this interfacial layer is larger than about 2 nm and is pure SiO 2 , then the beneﬁt of the high-k dielectric is lost. The interfacial characteristics between the gate dielectric and Si substrate are considered to be the most challenging issue for the performance of MOSFETs at all. Even though Si/SiO 2 material system has been extensively studied and the interface is the most well-understood one, the accuracy of reported optical characteristics (optical dielectric function spectra) for the interface layer is still an issue [9]. The optical constants of the interfacial layer were shown to be approximately 0.4% higher than published values for bulk glasseous SiO 2. It is realized that successful integration of high-k oxides into Si further ultralarge-scale integrated technology requires a more detailed understanding of the interface nature. The existence of an identiﬁable interfacial region in the Ta 2 O 5 /Si system was recognized by high-resolution transmission electron microscopy (HRTEM), medium energy ion scattering (MEIS) [10], Rutherford backscattering spectrometry (RBS) and by electrical studies [11]. In a modeling Ta 2 O 5 ﬂat ﬁlms and ﬂat graded interfacial layers were discussed according to the HRTEM and Applied Surface Science 255 (2009) 9211–9216 ARTICLE INFO Article history: Received 23 December 2008 Received in revised form 2 July 2009 Accepted 6 July 2009 Available online 14 July 2009 PACS: 68 68.47.Fg 68.55.Nq 68.55.jd 77.55.+f 78.67.n 78.68.+m Keywords: Spectroscopic ellipsometry Algorithm High-k T 2 O 5 /Si Constituents Depth proﬁle ABSTRACT Variable angle spectroscopic ellipsometry of very thin T 2 O 5 layers on Si and the previously published appropriate algorithm for data interpretation have been successfully applied in terms of accurate characterization of very thin T 2 O 5 /Si systems. The simulation procedure following a simple three and four layered model was used assuming an existence of inhomogeneous interfacial layers. Quantitative determination of the thicknesses and composition identiﬁcation were achieved, both for the top T 2 O 5 layer and for an interfacial layer. The constituents in the interfacial layer and its depth proﬁles were recognized. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +359 2 8161458/359 2 9895917; fax: +3592 9625276. E-mail address: karma@phys.uni-soﬁa.bg (I. Karmakov). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.014