Refractive Index-Ellipsometry

In the case of Langmuir monolayers, film thickness and index of refraction have not been given much attention. While several groups have measured A versus a, [143-145], calculations by Knoll and co-workers [146] call into question the ability of ellipsometry to unambiguously determine thickness and refractive index of a Langmuir monolayer. A small error in the chosen index of refraction produces a large error in thickness. A new microscopic imaging technique described in section IV-3E uses ellipsometric contrast but does not require absolute determination of thickness and refractive index. Ellipsometry is routinely used to successfully characterize thin films on solid supports as described in Sections X-7, XI-2, and XV-7. 

Ellipsometry is a method of measuring the film thickness, refractive index, and extinction coefficient of single films, layer stacks, and substrate materials with very high sensitivity. Rough surfaces, interfaces, material gradients and mixtures of different materials can be analyzed. 

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

Ellipsometry is used to study film growth on electrode surfaces. It is possible to study films at the partial monolayer level and all the way up to coverage of thicknesses of thousands of angstroms while doing electrochemical measnrements. To get nseful data it is important to determine A and j/ for the bare electrode snrface and the surface with a film. These data are processed to derive the film thickness, d, and the refractive index, h, which consists of a real (n) and imaginary part (k), h = n- ik. So ellipsometry gives information on the thickness and refractive index of snrface hlms. 

Ellipsometry is concerned with the measurement of the changes in polarisation state, as well as light intensity, on reflection since these parameters are highly sensitive probes of the thickness and refractive index, rtf, of a surface film. A full treatment of the principles involved in ellipsometric measurements can be found in any one of several excellent reviews (see references). 

Ellipsometry and profilometry Thickness, refractive index, and consolidation behavior during drying and crystallization. Thickness uniformity. 

Wasserman [186] has described the use of both low-angle X-ray reflectivity and ellipsometry for the determination of thickness of Cio-Cig SAMs prepared on surface silanol groups of silicon plates. Ellipsometry is based on the reflection of polarized light from a sample and depends on the sample s thickness and refractive index. X-ray reflectivity measures the intensity of X-rays reflected from a surface (or interference pattern) that is characteristic of the distance between interfaces. The thickness of the SAMs was consistent with fully extended alkyl chains with all-trans conformations and excellent agreement was observed between the two methods. 

Indirect evidence for solvent exchange can also be gained by in-situ ellipsometry. This technique can provide information of film thickness and optical properties as a function of the electrode potential. Figure 2.19 shows that the film swells during oxidation and shrinks during reduction, in agreement with EQCM measurements. A decrease in the refractive index from 1.42 to 1.38 during oxidation also supports the uptake of water, which has a refractive index of 1.33. 

The absorption parameter, k, and the refractive index, n, were measured using variable angle spectrophotometric ellipsometry. The bottom antireflective coating of test solutions were spin coated onto primed silicon wafers and baked to get selected film thickness. The coated wafers were then measured using an ellipsometer to obtain and n values. 

Reference electrode, 1104, 1108, 1113 potential, 819, 874 Refractive index, determination with ellipsometry, 1148. 1151 Reflection coefficient, 1151 Residence time, definition, 1310 Reversal techniques, determination of intermediate radicals, 1416 Reversible adsorption of organic molecules, 969, 970... 

Table 1 Values of interplanar spacing (d 100 ), film thickness (h), refractive index at 550 nm (n and n ), porosity (Vp), areal density of atoms ( 5 %), obtained for as-prepared and treated films by XRD, ellipsometry and RBS. (A as-prepared coating, B washed in ethanol at 298 K, C thermally treated at 430 K in air for 1 h, D thermally treated at 400 K in air for 1 h + sohxlet extraction in hot ethanol for lh, E thermally treated at 400 K for 1 h + ultrasonic extraction in ethanol for 30 min, F calcined at 620 K in air for 1 h (ramp at 10 K/ min)).

No carbon was recorded for the D-treated film. The O/Si composition ratio was found to be 2.08 and is attributed to the extent of condensation as the organic phase has been removed completely. Based on the amount of Si for sample D and assuming a density of 2.3 g cm3 for amorphous SiC>2, the top layer would correspond to a thickness of 154 nm, if a dense layer is assumed. As the actual layer thickness is 458 nm, this would imply a porosity of 66%. Here a considerable discrepancy with the porosity obtained from ellipsometry is evident. In this respect it should be noted that the RBS measurement was done more to the edge of the sample than ellisometry, where the thickness is smaller than in the centre. Further, the refractive index determined with ellipsometry is very accurate. However, the relation of porosity with refractive index depends on the model used. 

In another publication [75] the adsorption behavior of the same materials from dilute solutions on silicon wafers was studied by ellipsometry. A mixture of cyclohexane and toluene (50 50 by volume) was used in order to provide enough refractive index contrast for the measurements and also to inhibit association of the end groups, which would influence the adsorption process. 

Finally, n was determined by spectroscopic ellipsometry. The main drawback with this technique when applied to anisotropic samples is that the measured ellipsometric functions tanlF and cos A are related both to the incidence angle and the anisotropic reflectance coefficient for polarizations parallel and perpendicular to the incidence plane. The parameters thus have to be deconvolved from a set of measurements performed with different orientations of the sample [see (2.15) and (2.16)]. The complex refractive index determined by ellipsometry is reliable only in the spectral region where the sample can be considered as a bulk material. In fact, below the absorption... 

Fig. 2.3. (a) Real and (b) imaginary parts of the refractive index of highly oriented PPV for the parallel and perpendicular components. Continuous line KK analysis. Dot-dashed, line ellipsometry with 4> = 65°. Dotted line ellipsometry with = 70°. Squares inversion of 1Z and T using (2.8) and (2.9) with k = 0. Circles inversion of 1Z and T with (2.8) and (2.9)... 

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

In Spectral Ellipsometry, changes in the state of polarisation of white light upon reflection at surfaces are monitored. This enables separation of the refractive index and physical thickness by modelling of a layer system. To handle correlations between layer thickness and refractive index, adequate layer thicknesses have to be guaranteed to avoid physically unreasonable solutions of the fitting due to local minima. 

Fig. 2 Relative change in the refractive index (gray squares) and relative change in the physical thickness (open cycles) of a Makrolon layer of 170 nm during exposition to different concentrations of 1-propanol, measured by spectral ellipsometry...

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