Ellipsometry porosimetry (EP): thin film porosimetry by coupling an adsorption setting with an optical measurement, high
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Ellipsometry porosimetry (EP): thin film porosimetry by coupling an adsorption setting with an optical measurement, highlights on additional adsorption results A. Bourgeois · Y. Turcant · Ch. Walsh · Ch. Defranoux
Received: 4 May 2007 / Revised: 9 April 2008 / Accepted: 23 June 2008 / Published online: 22 July 2008 © Springer Science+Business Media, LLC 2008
Abstract Ellipsometry porosimetry (EP) is an emerging technique that is well adapted to porous thin films1 analysis; it is non contact and non destructive. EP tools developed at SOPRA, allows us to obtain adsorption isotherms with many different adsorptives at an ambient temperature. EP leads to the same results as classical adsorption experiments (e.g. porosity, pore size distribution . . .), but it also has some particular features leading to new information. For instance, our optical setup (Spectroscopic Ellipsometry) allows us to determine the variation of the thickness of the samples during the adsorption experiment. It is also very sensitive to interfaces; it is thus possible to detect a porosity gradient or to study a bi-layer sample and plot the two corresponding adsorption isotherms at the same time (Bourgeois et al. 2004). For porous thin films with a non porous barrier layer deposited on top, it is also possible to study the lateral diffusion phenomenon in the film (see figure below). In this paper, we will demonstrate a part of the different features of EP for adsorption on porous thin films.
Keywords Gas phase adsorption · Diffusion
1 We have to underline that in optical studies, thin films are defined as thin layers of material deposited on top of a non-porous solid (substrate). It does not refer to a layer of the adsorptive which has been adsorbed on the adsorbent surface.
A. Bourgeois () · Y. Turcant · C. Walsh · C. Defranoux SOPRA-SA, 26 rue Pierre Joigneaux, 92270 Bois Colombes, France e-mail: [email protected]
1 Introduction Spectroscopic ellipsometry is a reference technique for thin films characterization. This optical technique is particularly suited for the determination of the thickness and refractive indices of thin films. To facilitate the study of porous thin films, SOPRA has developed measurement devices that couple the classic spectroscopic ellipsometry measurement with an adsorption tool. As the Ellipsometric porosimeter (EP) is inspired by the traditional adsorption setups (BET, BJH apparatus), the true revolution is that the amount adsorbed during the experiment is not obtained by a weighing process or the measurement of a differential of pressure anymore, but through the evolution of the optical properties (refractive indices) of the porous material. In this configuration, the accuracy of the measurement remains excellent even for very few quantities of matter; layers with a thickness as low as 100 nm can then be characterized, which combined with the size of the optical beam (approximately 1 mm2 ) corresponds to a volume of material of approximately 10−7 cm3 . This original measurement device, allows a non-dest
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