J. De Laet

Determination of the thickness of thin silane films on aluminium surfaces by means of spectroscopic ellipsometry

Abstract The thickness of thin films of non-functional silane bis-1,2-(triethoxysilyl)ethane (H 5 C 2 O) 3 Si-CH 2 CH 2 -Si(OC 2 H 5 ) 3 (BTSE) deposited on aluminium surfaces is investigated using spectroscopic ellipsometry (250–1700 nm). The data processing of the ellipsometry spectra is carried out by means of simulation and regression techniques. New advances in data processing, e.g. multiple sample analysis and determination of thickness non-uniformity, are applied to characterise these thin polymer films realistically. The influence of the concentration of the BTSE solution and the curing of the film is investigated. Optical thickness estimates are corroborated by independent auger electron spectroscopy and transmission electron microscopy analysis.

Development of an optical model for steady state porous anodic films on aluminium formed in phosphoric acid

Porous anodic oxide films on aluminium formed in phosphoric acid (PAA) have been characterized nondestructively by spectroscopic ellipsometry. Compared to previous studies on porous films formed in sulfuric acid, the optical behaviour of PAA films reveals new features which have been attributed to film-substrate interface roughness and optical anisotropy effects. On one hand relatively large interface roughness has been simulated by a graded index model. On the other hand, the implementation of uniaxial anisotropy in the optical model of the PAA film enables to interpret spectroscopic ellipsometry data acquired at multiple angles of incidence in terms of the morphology of the films. More specifically, accurate and physically realistic values are found for the porosity and porous film thickness. Although more difficult to interpret from the optical findings, the thickness of the barrier part of the porous film can also be estimated. The ellipsometry characterizations are confirmed by complementary TEM analysis of various films. Finally, the anisotropy exhibited by the PAA films is in line with recent theoretical predictions of the optical behaviour of arrays of parallel cylindrical capillaries in an isotropic medium proposed by other authors.

Characterization of various aluminium oxide layers by means of spectroscopic ellipsometry

This paper reports the characterization of both barrier type and porous type anodic oxide films on aluminium by means of spectroscopic ellipsometry (SE). In order to show the capabilities of the technique for quantitative determination of the layer characteristics, results based on ellipsometric data are correlated with complementary information from the analytical techniques transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). It is concluded that ellipsometry yields an accurate characterization for the thicknesses and the interfacial properties of both the barrier layer and the porous layer. The porosity of the porous layer, determined with SE, is found to be in good agreement with the results obtained from TEM.

Characterization of aluminium surface treatments with electrochemical impedance spectroscopy and spectroscopic ellipsometry

Abstract The characterization of barrier and porous anodic oxide films on aluminium by means of Electrochemical Impedance Spectroscopy (EIS) and Spectroscopic Ellipsometry (SE) is reported. Both techniques are used to investigate the sealing of the porous film. To determine the accuracy of quantitative characterizations of the anodic films, the EIS and SE data are correlated to TEM results. It is concluded that ellipsometry yields accurate values for the thickness, the growth rate and the interface properties of the barrier film and the porous film. The characterization of the porosity of the porous film is also in good agreement with the results obtained by TEM. It is shown that ellipsometry gives more information than EIS, where information about the pore structure of porous oxide films can not be achieved from measurements in aqueous solutions. For the investigation of the sealing of the porous film, SE is more sensitive, but less quantitative than EIS, even though the potential for quantitative characterization is present.

The use of impedance spectroscopy and optical reflection spectroscopy to study modified aluminium surfaces

Porous-type anodic oxide Al films were investigated by means of two non-destructive, in situ applicable analysis techniques, electrochemical impedance spectroscopy (EIS) and spectroscopic ellipsometry (SE). The porous part of the porous film cannot be characterized with EIS measurements in aqueous solutions due to short-circuiting of the porous oxide by the electrolyte in the pores. By introducing an optical model which matches previous TEM investigations it is shown that SE characterizes the porous film morphology, including the porous part and the barrier part thicknesses, porosity and film-substrate interface roughness throughout the entire growth process. SE also allows us to determine the change in growth rate for films formed at increasing current density.

Characterization of different conversion coatings on aluminium with spectroscopic ellipsometry

Abstract Different types of conversion coating on aluminium surfaces are studied with spectroscopic ellipsometry. The first film type, the porous anodic aluminium oxide film formed in sulphuric acid, can be accurately characterized by a two-layer structure, that takes the porous part of the film, the barrier part and the substrate roughness into account. This enables the growth efficiency and the porosity of the films to be determined under the applied electrochemical conditions. Ellipsometry measurements on the second film type, the phosphate-chromate conversion film, are interpreted with the Sellmeier relation for n and k . It is revealed that this film type has almost identical optical properties with Al 2 O 3 barrier films. The thickness and growth rate of these films are determined accurately. Indications for the presence of a double-layer structure, predicted by other investigations, are found.

Evaluation of interferometric ellipsometer systems with a time resolution of one microsecond and faster

Abstract An interferometric ellipsometer based on a Zeeman laser has been developed. The so-called Zeeman–LePoole ellipsometer (ZLE) is a single wavelength ellipso-reflectometer with a time resolution of 1 μs. The light source used is a Zeeman (two-frequency) He–Ne laser, operating at the 632.8 nm wavelength with a frequency difference of 1 MHz between the two modes. The optical system of the ZLE is based on an interferometric configuration. Two photo-diodes transform the light carrying the optical information from the sample into electrical signals with a frequency of 1 MHz, which are processed with high speed data-acquisition equipment. This new type of ellipsometer was designed for studying fast processes. As a test of the dynamic capabilities of the ZLE, we measured the optical response of a LCD and a Pockels cell, that were externally stimulated with time-dependent voltages. We also report on in-situ electrochemical experiments monitored with the ZLE. The advantages and disadvantages of the ZLE are discussed. The application of two coupled lasers in an interferometric ellipsometry set-up is a novel approach that allows for a variable time resolution and avoids the problem of frequency mixing. This new set-up also has a high potential for application in optical recording.

Non-destructive optical characterisation of chromium conversion layers on aluminium

Abstract Chromium phosphate conversion coatings were characterised by spectroscopic ellipsometry. A two-layer optical model was used for the simulation and regression analysis procedures. The optical constants of the upper layer of the model are described by a dispersion relation. The second layer of the optical model is an interface layer, located between the aluminium substrate and the top layer. The optical model used both the Forouhi–Bloomer and the Sellmeier dispersion relations. In both cases, the optical constants of the top layer seem to correspond to those of chromium phosphate, provided the coatings are sufficiently thick. When thinner coatings are considered, the presence of other compounds or voids modify the optical constants. Nevertheless, an accurate coating thickness was determined for all conversion times.