Basis of Ellipsometry

An ellipsometer measures the polarization state of light. Applied to electrode surfaces, monochromatic plane-polarized light is incident on the working electrode. The state of polarization of the reflected light is different it is in general elliptically 

In principle ellipsometric measurements are a way of determining film thickness in situ, as a function of redox state, and this was an early driving force behind attempts to apply ellipsometry to electroactive polymers. The thickness of an electroactive polymer film is a highly significant parameter. As a function of the redox state of the material, thickness can be expected to vary with, among other parameters, polymer tertiary structure and solvent and ion populations. Nonoptical methods of determining film thickness are rather unsatisfactory, since they are invariably made ex situ. 

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The Fresnel equations predict that reflexion changes the polarization of light, measurement of which fonns the basis of ellipsometry [128]. Although more sensitive than SAR, it is not possible to solve the equations linking the measured parameters with n and d. in closed fonn, and hence they cannot be solved unambiguously, although their product yielding v (equation C2.14.48) appears to be robust. 

Ellipsometry measures the orientation of polarized light undergoing oblique reflection from a sample surface. Linearly polarized light, when reflected from a surface, will become elliptically polarized, because of presence of the thin layer of the boundary surface between two media. Dependence between optical constants of a layer and parameters of elliptically polarized light can be found on basis of the Fresnel formulas described above. 

Matsumoto et al. [64] investigated gas occlusion in C g crystals by spectro-ellipsometry. They found that some oxygen remains in the voids (associated with polycrystals) in chemisorbed form. This is in contrast with the behavior of Cgg crystals in the presence of He, Ar, H2, or N2, where the spectra changed reversibly with pressure, in accordance with a physisorption model whereby gas molecules enter the voids and are occluded as a quasiliquid. Niklowitz et al. [65] studied the interaction of oxygen molecules with a fullerene surface using electron energy loss spectroscopy and TPD. On the basis of the vibrational excitation behavior, the authors concluded that molecular oxygen was physisorbed on Cgg under the conditions studied (20 K). In other words, the adsorbed molecules were only weakly perturbed by the Cgg substrate. 

In general, Ss f Sp, and therefore if the incident linearly polarized wave, has both s- and p-components, the reflected wave is elliptically polarized. The parameters of the polarization ellipse are determined by the optical constants of both media and can be measured with high accuracy. This principle forms the basis of reflection ellipsometry (see Section 5.1). For historical reasons one often represents the ratio of the reflection coefficients, p, in terms of the ellip-sometric angles, Y and A, as... 

Plasma analysis is essential in order to compare plasma parameters with simulated or calculated parameters. From the optical emission of the plasma one may infer pathways of chemical reactions in the plasma. Electrical measurements with electrostatic probes are able to verify the electrical properties of the plasma. Further, mass spectrometry on neutrals, radicals, and ions, either present in or coming out of the plasma, will elucidate even more of the chemistry involved, and will shed at least some light on the relation between plasma and material properties. Together with ellipsometry experiments, all these plasma analysis techniques provide a basis for the model of deposition. 

With ellipsometry the polarization state of reflected radiation rather than just its intensity, is experimentally determined. Ellipsometry is not so much another experimental technique but a more thorough variety of the traditional ones, whether external or internal reflection. Two results per resolution element, namely the ellipsometric parameters (cf. Eq. 6.4-17) and A, are derived independently from the measurements. These can further be evaluated for the two optical functions of the medium behind the reflecting surface or other two data of a more complex sample. In any case there is no information necessary from other spectral ranges as it is for Kramers-Kronig relations. In comparison to the conventional reflection experiment, ellipsometry grants more information with a more reliable basis, e.g. since no standards are needed. 

Another reason for its relative neglect may be that it deals with properties that are rather unfamiliar, such as complex refractive index and the nature of polarised light, and has a mathematical basis that can seem obscure. To some extent this is the fault of ellipsometrists who have sometimes been content to talk only to other specialists in the field and have not devoted much effort to popularising the technique. Understanding what ellipsometry is and what it can do does require some acquaintance with the basic concepts, and, in particular, with three important parameters that are often ignored. 

Amebrant and Nylander used the technique to study the sequential and competitive adsorption behavior of " C-labeled jS-lactoglobulin with KT-casein on hydrophobic and hydrophilic chromium surfaces. The adsorption was also followed by in situ ellipsometry measurements, providing a basis for comparison of the two techniques as described in Section II. 1. 

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