Information about layers growth (ex- or in-situ) and optical properties of the samples can be derived, using the ellipsometry method. It is fundamental in the field of multi-step surface modification, swelling phenomena, and “in situ” monitoring of adsorption processes at interfaces.
The null-ellipsometry is a successive implement, which eliminates many system errors, occurring by a small-scale sample misalignment. As light source, a red He-Ne laser with a wavelength 632.8 nm is used. The polarizer and the compensator generate elliptically polarized light, which reflects from the substrate surface. For measurements in air, the incident angle normally is set to 70°, appropriate for silica wafers with native SiO2 layer. After the sample surface is reached, the reflected light passes through the analyzer and drops onto a photo multiplier, which operates as a detector. The control of the ellipsometer and the data evaluation requires special software. The experimental data can be presented as relative amplitude ratio (tanΨ) and relative phase shift (Δ)- both related to the Fresnel reflection coefficients (R) for p and s polarized light, which are function of the layer properties:
λ- wave length, Φ0- angle of incidence, dj- thickness of the layer (j= 0,1, 2..); NS, nj, n0 are optical constants of the substrate, layer, and surrounding media, respectively.
The ellipsometric quantities Ψ and Δ can be derived from positions of the polarizer and analyzer which lead to zero intensity of the beam at the detector:
Ψ = A, Δ = 2p + 90°
fig. 1 Schematic depiction of an ellipsometic set-up.
Instead studying the film growth after processing in air conditions, the null-ellipsometry is a sensitive technique for evaluation of thin films swelling and adsorption kinetics under appropriate conditions. Since most of the adsorption and surface processes take place at solid/liquid interfaces, the ellipsometric measurements were additionally adapted and optimised to every particular system. An example is plotted below.
fig. 2 In situ measured adsorption kinetics of polystyrene latex nanoparticles, functionalised with -SO3H groups (d=15 nm), adsorbing onto PAA-P2VP brush at pH 3; I-brush swelling: II-nanoparticles adsorption.
Below, two ellipsometers operating at constant wavelength of 632.8 nm are pictured. Both devices have possibilities for sample mapping via X-Y mobile stage.
fig. 3 Null-ellipsometers in a polarizer-compensator-sample-analyzer mode:
Left: Multiscope- Optrel, Berlin, with imaging option and possibilities for kinetic liquid state experiments:
Accuracy Δ±0.005°, Ψ±0.005°;
Right: SE 402 (Sentech Instruments GmbH, Berlin) with Auto Collimating Telescope (ACT) for fine positioning:
Accuracy Δ±0.06%deg;, Ψ±0.03°
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