Ellipsometry spectroscopic

Ellipsometry is an optical technique that detects the change of the polarization state when light is reflected from a surface. For rather simple systems like transparent films on reflecting substrates, film thickness and refractive index can be determined with high accuracy. More complicated samples (e.g., multilayer structures or layers with a graded index of refraction on a reflective carrier) can be characterized with a sufficient set of independent experimental data obtained for multiple angles of incidence and/or multiple wavelengths (spectroscopic ellipsometry). With a liquid cell, ellipsometry can be performed also in aqueous environments. 

FIGURE 4.21 Poly(Al-isopropylacrylainide-co-iV-(l-phenylethyl) acrylamide) film thickness versus temperature during first and second heating/cooling cycles in distilled water (heating/cooling rate 1 K/min). Source Cordetro et al. [49]. Reproduced with permission of Royal Society of Chemistry. 

Beyond the working principle described so far, ellipsometry can also provide laterally resolved information. This is possible in a scanning process but also directly by imaging ellipsometry. The latter approach is illustrated, for example, by the work of Schmaljohann et al. [50] who exemplified the advantages of this technique for a micropattemed thermoresponsive coating. 

5 Quartz Crystal Microbalance with Dissipation Monitoring 

FIGURE 4.22 QCM-D data of a PNiPAAm-based copolymer film at tempa-atures around the phase transition. Solid lines correspond to data obtained ramping up temperature, while dashed lines correspond to ramping down temperature. Source Alf et al. [51]. Reproduced with permission of John Wiley Sons. 

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. 

1990 Roseler et al., 1993). However the spectrally multiplexing procedure requires a photometric technique to be applied. To circumvent an absolute calibration of the intensity scale and to avoid experimental errors due to different optical paths and particularly, different irradiated areas, the evaluation should be based on quotients of intensities which were obtained under identical conditions except the polarization states. 

Starting with radiation of equal field strength for all the four measurements, the coefficients Kp and can be taken as (half) the side lengths of the box which circumscribes the ellipse in the reflected beam (see Fig. 6.4-4). Since intensities I = EE are measured, one has 

The wave components in the 0° plane and in the 90° one are phase shifted by Sp and respectively. The resulting difference Sp-S = A acts on the quotient Q2 = 7(45°)//(135°) whose components are the intensities measured at the intermediate polarization azimuths 

In order to improve the evaluation in the range where the cosine function is quite insensitive, an additional phase shift is introduced by placing a retarder in the reflected beam. Usually an internal-reflection prism is employed to produce a phase shift somewhere in the range from 30° up to 150°. Since the refractive index does not change much within the transparent spectral range, the phase shift produced by such a retarder is almost achromatic (Korte et al., 1988). For limited spectral ranges multilayer devices are also suitable (Rdseler and Molgedey, 1984). Provided a phase retardation of exact 90° is applied, one finds with the quotient Q 2 of 7 (45°) and 7 (135°) 

Among spectroscopic techniques, ellipsometry is unique in that it is capable of giving useful information at only one wavelength, as the previous examples show. Most of the ellipsometry literature is, in fact, still dominated by single-wavelength applications. However, it is only in its wavelength- 

Moreover, ellipsometry has the added operational advantage that, because the technique is affected only by surface layers and has a high degree of discrimination against the optical properties of the medium, measurements on such adsorbed dye layers can be made with the dye also present in the supernatant liquid, as has been shown in the author s laboratory [20]. An application of this technique to the detection of Rhodamine B as a metalplating additive has recently been published [21]. 

The main experimental technique applied in this chapter is SE. Several textbooks were written on SE [73,114-118], Therefore, only some basic concepts are described. SE examines the relative phase change of a polarized light beam upon reflection (or transmission) at a sample surface. In Fig. 3.4 the setup of an ellipsometry experiment is shown. Upon model analysis of the experimental data, the DFs and thicknesses of the sample constituents can be extracted. Two different experimental approaches have to be distinguished, standard and generalized ellipsometry. 

When neither s-polarized light (light polarized perpendicular to the plane of incidence) is converted into p-polarized light (light polarized parallel to the plane of incidence) nor vice versa, standard SE is applied. This is the case for isotropic samples and for uniaxial samples in the special case, where the optical axis is parallel to the sample normal, for example (0001) ZnO [119]. 

Standard SE determines the complex ratio p of the reflection coefficients for p-polarized and s-polarized light 

A[ and B denote the intensities of the incident and reflected light beam, respectively. and A are the ellipsometric parameters, where tan is the magnitude and A the phase of p [116]. 

In the general case, when s-polarized light is converted into p-polarized light and/or vice versa, the standard SE approach is not adequate, because the off-diagonal elements of the reflection matrix r in the Jones matrix formalism are nonzero [114]. Generalized SE must be applied, for instance, to wurtzite-structure ZnO thin films, for which the c-axis is not parallel to the sample normal, i.e., (1120) ZnO thin films on (1102) sapphire [43,71]. Choosing a Cartesian coordinate system relative to the incident (Aj) and reflected plane waves ( ), as shown in Fig. 3.4, the change of polarization upon reflection can be described by [117,120] 

The reflected light is decomposed into its perpendicular (s-direction) and parallel (p-direction) components (with respect to the plane of incidence) and their amplitudes and phase shift are measured, as detemiined by the following relationship [29]  

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With the development of multichaimel spectroscopic ellipsometry, it is possible now to use real-time spectroscopic ellipsometers, for example, to establish the optimum substrate temperature in a film growth process [44, 42]. 

Vedam K 1998 Spectroscopic ellipsometry a historical overview Thin Solid Films 313/314 1-9... 

The film thickness and retractive index were calculated using spectroscopic ellipsometry. X-ray photoelectron spectroscopy (XPS) was used for composition analysis. Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) was used to investigate the depth profiles of the film. 

De Souza et al. (1997) used spectroscopic ellipsometry to study the oxidation of nickel in 1 M NaOH. Bare nickel electrodes were prepared by a series of mechanical polishing followed by etching in dilute HCl. The electrode was then transferred to the spectroelectrochemical cell and was cathodicaUy polarized at 1.0 V vs. Hg/HgO for 5 minutes. The electrode potential was then swept to 0.9 V. Ellipsometry data were recorded at several potentials during the first anodic and cathodic sweep. Figure 27.30 shows a voltammogram for Ni in l.OM NaOH. The potentials at which data were recorded are shown. Optical data were obtained for various standard materials, such as NiO, a -Ni(OH)2, p-Ni(OH)2, p-NiOOH, and y-NiOOH. 

Spectroscopic ellipsometry has also been applied in the characterization of compositionally graded a-SiC H alloys [353], where the flow ratio z —... 

Layadi et al. have shown, using in. situ spectroscopic ellipsometry, that both surface and subsurface processes are involved in the formation of /xc-Si [502, 503]. In addition, it was shown that the crystallites nucleate in the highly porous layer below the film surface [502, 504], as a result of energy released by chemical reactions [505, 506] (chemical annealing). In this process four phases can be distinguished incubation, nucleation, growth, and steady state [507]. In the incubation phase, the void fraction increases gradually while the amorphous fraction decreases. Crystallites start to appear when the void fraction reaches a maximum... 

Spectroscopic Ellipsometry Porosimetry (EP). In general, ellip-sometry takes advantage of the change of polarization of a polarized light beam after reflection from a surface. From the parameters (T and A), obtained... 

Spectroscopic ellipsometry is a non-destructive, interface sensitive, in situ technique for interface characterization. Time resolved ellipsometric spectroscopy was used to determine the mechanism of electrochemical deposition of photoresists on copper electrodes under potentiostatic, anodic conditions. Nucleation of photoresist deposition occurs randomly. During the early stages of nucleation the semi-spherical particles are separated by about 100 A. The deposits tend to grow like "pillars" up to 50 A. Further growth of the "pillars" lead to coalescence of the photopolymer deposits. 

Tomkins HG, McGahan WA (1999) Spectroscopic ellipsometry and reflectometry A user s guide. Wiley, New York http //www.woollam.com (see tutorials)... 

Colins RW, Ferlauto AS, Ferreira CM, Chen C, Koh J, Koval RJ, Lee Y, Pearce JM, Wronski CR (2003) Evolution of microstrucutre and phase in amorphous, protocrystalline, and microcrystalline silicon studied hy realtime spectroscopic ellipsometry. Sol Energy Mat Sol Cells 78 143-180... 

An ozone treatment (10 minutes at room temperature) of the HF-etched SiC surface before the metallization step was introduced as a very convenient processing step to produce Schottky diode gas sensors with an increased stability and reproducibility. The use of spectroscopic ellipsometry analysis and also photoelectron spectroscopy using synchrotron radiation showed that an oxide, 1-nm in thickness, was formed by the ozone exposure [74, 75]. The oxide was also found to be close to stochiometric SiO in composition. This thin oxide increased the stability of the SiC Schottky diodes considerably, without the need for any further interfacial layer such as Ta or TaSi which have been frequently used. Schottky diodes employing a porous Pt gate electrode and the ozone-produced interfacial layer have been successfully operated in both diesel exhausts and flue gases [76, 77]. 

Adsorption of putidaredoxin on gold electrodes has been studied using dynamic spectroscopic ellipsometry and differential capacitance measurements [307]. In Ref. 307, a method for the measurement of metal surface optical perturbation during protein adsorption at a constant potential has been described. The method is based on the concept that the charged transition layer develops between the electrode substrate and the adsorbate. 

The present methods inferring the polymer conformation from data on T, p, 0, and tuns must be refined further. For example, spectroscopic ellipsometry which should allow T, p, 0, and t to be determined simultaneously might be a powerful tool. 

One of the easy and effective approaches for quantifying the polymer volume fraction within films in situ is to use in situ spectroscopic ellipsometry (SE) [49,118, 119, 144], The measurements should be performed in a thermostated cell (Fig. 7) with full control over the solvent vapor atmosphere p/po, where po is the solvent vapor pressure at saturation and p is the actual pressure, which can be adjusted by a combination of the saturated vapor flow and dry nitrogen flow [118, 119], or by the difference between the temperature 7j of the polymer sample and the temperature 72 of the solvent vapor [49, 114, 144],... 

Recently, the adhesion of MF microparticles on a cellulose film in air as well as in liquid media was characterised using AFM. The cellulose film was made by dissolving cotton powder in N-methylmorpholine-N-oxide (NMMO) solution, followed by spinning on a silicon wafer. Spectroscopic ellipsometry was employed to measure the film thickness, and AFM was also utilised for characterising the film roughness and material distribution (Figure 20). The cotton cellulose film was also... 

The uniformity of such an OVPD film of Alq3 is shown in Fig. 9.6. Analysis by variable angle spectroscopic ellipsometry (VASE) confirmed the surface was smooth across the entire substrate area with thickness deviation of +1.7%, a standard deviation, a, of 1.0% only. Atomic force microscopic analysis of such a typical film revealed RMS values to be 6 A, i.e. thickness differences in the range of a single monolayer only, irrespective of deposition rate [20-22]. 

Tompkins, H.G. and McGahan, W.A. (1999). Spectroscopic Ellipsometry and Reflectome-try A User s Guide, Wiley, New York. 

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