Materials Science and Engineering B76 (2000) 193 – 199 Densification of hybrid silica – titania sol – gel films studied by ellipsometry and FTIR A. Maia Seco *, M. Clara Gonc ¸alves, Rui M. Almeida Departamento de Engenharia de Materiais /INESC, Instituto Superior Te ´cnico, A. Roisco Pais, 1000 Lisbon, Portugal Received 25 October 1999; received in revised form 9 February 2000; accepted 1 March 2000 Abstract Hybrid silica – titania films were prepared via sol – gel processing, by dip-coating on silicon wafer substrates, from organically- modified silane and titanium isopropoxide precursors. These hybrid sol – gel films were then subjected to selected heat treatments at temperatures up to 950°C and their densification was followed by ellipsometry and infrared absorption (FTIR) spectroscopy. The volume fraction of residual porosity was estimated, based on the refractive index data. A clear correlation was observed between the intensity of the infrared spectral shoulder near 1150 cm -1 and the porosity values, for the different heat treatment temperatures. For any treatment up to 950°C, the porosity of the hybrid silica–titania gel films was higher than that of the corresponding inorganic gel films, while both continuously decreased with increasing heat treatment temperature. The residual SiCH 3 groups, whose presence was identified in the hybrid silica – titania films until a temperature near 500°C, tend to be transformed into SiOH (silanol) groups as they are eliminated. Most silanol groups are eliminated at a temperature near 600°C. The hybrid silica–titania films were found to have a more porous and more compliant gel network than the corresponding inorganic films, while their structure appears to exhibit a smaller spread in the SiOSi intertetrahedral angle distribution. © 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Silica – titania films; Hybrid sol – gel films; Ellipsometry; Infrared absorption (FTIR) spectroscopy www.elsevier.com/locate/mseb 1. Introduction For optical communications systems, bulk optics is not convenient in large-scale applications, because of the sensitivity of such systems to temperature changes and mechanical vibrations, whereas integrated optics (IO) is a planar waveguide-based technology which allows the implementation of systems with much smaller dimensions than bulk optics, e.g. centimeters rather than meters. For the fabrication of IO compo- nents, silica-on-silicon is a leading technology at present, using processing methods such as thermal oxi- dation, sputtering, flame hydrolysis, chemical vapor deposition or sol–gel [1]. Sol–gel processing offers advantages over the other methods in terms of compo- sitional tailoring, functionality and cost. In particular, active doping of silica – titania sol – gel planar waveg- uides with cations such as Nd 3 + and Er 3 + has yielded very promising results [1 – 3]. In order to increase the single layer thickness of the sol–gel deposits above a critical thickness of about 0.5 m before cracking occurs during densification [4], it is advantageous to use organically-modified precursors for the preparation of hybrid (organic/inorganic) films. Such precursors, containing non-hydrolizable groups like SiR, where R is an alkyl radical, lead to films with reduced network connectivity and, therefore, increased compliance, allowing easier densification without crack- ing. Whereas the structure [5 – 12] and thermal densifi- cation behavior [13,14] of inorganic silica – titania sol – gel planar waveguides has already been investi- gated to some extent, very little has been published so far about hybrid silica – titania waveguides. Brusatin et al. [15] prepared silica – titania coatings from acid cata- lyzed solutions of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS), mixed with tetrabutoxyti- tanate and they found that these films became purely inorganic and similar to those prepared without MTES, after treatment at 500°C; the optical propagation losses of these waveguides were ca. 0.3 dB cm -1 . * Corresponding author. 0921-5107/00/$ - see front matter © 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 4 4 2 - 6