Ellipsometry

Ellipsometry27,60 62) is based on the principle that light undergoes a change in polarizability when it is reflected at a surface. The refractive index of the surface and the reflection coefficient of a system can be calculated from the change in the phase retardation A and the change in the amplitude ratio tan ip. Adsorption of a polymer on a surface gives rise 

The attenuated total reflection (ATR) method measures the reflection coefficients of vertically and horizontally polarized light reflected from a polymer layer adsorbed on a transparent surface63. These coefficients allow the thickness of the adsorbed layer and the polymer concentration in it to be determined. 

In principle, the ATR method would provide information about the segment distribution in the adsorbed layer if light could penetrate in different depths into the layer, but this possibility still remains untested. 

3 Fraction of Adsorbed Segments and Fraction of Occupied Surface Sites 

The infrared band of a particular group in a polymer shifts when some groups of the polymer are adsorbed onto active sites, e.g. silanol groups on a silica surface23. This phenomenon has been used to measure the fraction of adsorbed polymer segments. The fraction of die surface sites occupied by adsorbed polymer segments can also be determined from the frequency shift of 1R band caused by the interaction between functional groups and an active site. 

Ellipsometry involves measurement of the change of polarization that occurs when polarized radiation is reflected from a specular surface. Ellipsometry is an extension of the reflection technique in which the polarization of the reflected radiation rather than just its intensity is measured. Ranges of operation from ultra-violet to infrared, UV/Visible is typical. 

The basic optical components of an ellipsometer are a source (L), a polarizer (P), an analyser (A) and a detector (D). Some additional components, like compensators (C, e. g. quarter-wavelength plate) and modulators (M, e. g. photoelastic modulator) are added in some configurations. Typical configurations are PCSA or PSCA, where S is the sample. 

There are several principles used for measurement null method, photometric method, and interferometry, the first two being the most important 

In null ellipsometry information about the optical system is obtained from the azimuths of P, C, A, the relative phase retardation of the compensator 6q and, in the case of measurements on surfaces, the angle of incidence that reduces the dc or an ac-component of the detected radiation flux to zero. Photometric ellipsometry is based on measurement of the variation of the detected radiation flux as a function of one or more of the above parameters (azimuth angle, phase retardation, or angle of incidence). 

Ellipsometry is a reflectance method in which the change of the state of polarization upon reflection is determined. This change is described in terms of two ellipsometric angles A and 9 given by  

Ellipsometry provided accurate and reproducible measurement of film thickness to correlate with the cooperativity of the polymer. The film thickness for each of the polymer film systems prepared from 2%, 5%, and 10% solutions was recorded in Table 4.3-1, Table 4.3-2, and Table 4.3-3 respectively. Each row represents one sample giving a total of twenty samples. The error was determined by the difference in the experimental and model curves for each sample overall, the error was a small percentage of the film thickness. 

Disappointingly, this promising work was not followed up, which casts doubt on both the efficacy and general utility of the imprinting technique utilised herein, as well as the applicability of ellipsometry for measuring the extent of binding. 

In this chapter we first introduce optical techniques combined with electrochemistry for study of the passive oxide and then we focus on the growth mechanism, composition, and nature of the passive oxide on iron and steels, which were detected by the optical techniques of ellipsometry, Raman spectroscopy, potential modulation reflectance, and photo-electrochemistry combined with electrochemistry. 

Electrochemical impedance measurement is convenient and usually no extra instrument is needed since contemporary potentiostat, which is normally used in an ECP or ECMP system, tends to possess EIS function. However, the thickness of an anodic film cannot be easily determined from the measured impedance or impedance spectra because the electrical resistivity of the film is usually unknown. 

Ellipsometry can measure films from subnanometer to a few micrometers, depending on material properties and wavelength of the light source. It has been widely used for thin film measurement in various applications, from biology to semiconductor, and from solid/solid to solid/liquid interfaces [24,25]. Ellipsometer with electrochemical cell for in situ thin film analysis is available from J.A. Woollam Co., Inc. and has been used in the research on electrochemical deposition [26]. However, in situ measurement of anodic films is more challenging because the films are usually metal complexes with unknown optical properties and difficult to verify with other ex situ techniques. 

To achieve better planarization efficiency than ECP, different modified ECP approaches have been explored. These approaches, including ECP-DI water technique [27], sectorial cathode ECP [28], membrane-mediated ECP [29], and contact ECP (i.e., ECMP) [30,31], work by different mechanisms and exhibit different performances. 

FIGURE 10.16 Flat anodic film (a) produced by ECP with ultraclose electrodes and sectorial cathode (b). 

an anodic film forms (the film can be formed chemically, i.e., by chemical reaction, or electrochemically, i.e., electrochemical processes proceed under mass transport limiting conditions)  

Studies of the role of protein-surface interactions in blood coagulation were done by Vroman 56). The plasma proteins were adsorbed onto various hydrophilic or hydrophobic surfaces. Vroman showed that fibrinogen was an important component of the plasma protein layer adsorbed to the solid/liquid interface. 

A complete review of the early work applying ellipsometry to biomedical problems is available 177 . 

The surface of silicon has optimum optical properties and the adsorption of proteins 

Ellipsometry can follow the interactions between two types of biological macromolecules, the first of those two bound physically to the surface, the other acting from the solution. The binding of conconavalin A to adsorbed mannan 180) and of cholera toxin to adsorbed ganglioside t83) are examples. The adsorption of complement factors to an antibody-coated surface was monitored by ellipsometry and a modification of the same method was used for quantification of migration inhibition of human polymorphonuclear leucocytes 182). Interaction of proteins and cells with affinity ligands covalently coupled to silicon surfaces has been also studied 183). 

The thicknesses of the films were determined using an ellipsometer SE 400 (Sentech Instruments GmbH) under an incidence angle of 70° at a wavelength of 633 nm. Optical film thicknesses were determined assuming a complex refractive index N=n-ik with a real part n= 1.45 and an imaginary coefficient k = 0. The parameters n and k of the gold substrates were obtained by ellip-sometric measurements of the plasma-cleaned films before monolayer formation. 

Upon reflection of polarized light, both the amplitude (i.e. the magnitude of the electric field vector) and the phase might undergo changes. This depends on the complex refractive index N of the material designated 1 according to 

Further understanding of the interaction is most straightforward when only incident light with its electromagnetic vector parallel with the plane of reflection ( p) and with its vector perpendicular are treated. After superposition of the electric field vectors of these two waves the resulting vector describes a circle, provided they were in phase and of the same amplitude. After reflection the mentioned changes 

The Brewster angle is the angle at which reflected light is completely hnearly polarized the reflected and the ditfracted beams are perpendicular to each other. 

There are numerous polarizing optical components available. Their operation is based on birefringence, polarization at mirrors at the Brewster angle, optical dichroism, etc. Precision achieved with sheet polarizers is inadequate, thus devices like Glan-Thompson, Glan-Foucault or Rochon prisms have to be used. 

The symbol s derives from the German term for perpendicular, senkrecht . 

Very thick interfaces have also been measured on the reactive bilayer systems PA 12/ (PS/SMA2) and PA12/SMA by Dedecker et al. [83, 102]. 

However, recently the fundamentals of ellipsometry have been questioned [77, 78]. The assumptions made in the four-layer model are maybe not close enough to the real situation. In the applied fitting procedure, the interface is considered as a uniform layer with a uniform refractive index. In reality, there is certainly a concentration gradient at the interface. This is a weak point of the method, but it is difficult to assess the error induced by this approximation. 

Films of jff-lactoglobulin adsorbed onto hydrophilic gold surfaces from aqueous solution at pH 4.5-10.0 were analyzed by ellipsometric measurements by Liedberg et alP Both the infrared and ellipsometric results indicated a decreasing surface concentration with increasing pH, which corresponded to an increasing net charge of the protein. 

Recently, Al-Malah et alP constructed adsorption isotherms from a temperature study of the apparent equilibrium adsorptive behavior of 

Welin-Klinstroem et al used a null ellipsometer equipped with an automatic sample scanning device for studies of adsorption and desorption of fibrinogen and IgG at the liquid/solid interface on surface wettability gradients on silicon wafers. To follow the processes along the wettability gradient, off-null ellipsometry was used. The kinetics of adsorption and nonionic-surfactant-induced desorption varied considerably between fibrinogen and IgG. In the hydrophilic region, veiy little protein desorption was seen when a nonionic surfactant was used. 

In a recent paper, Arwin et al described the basic theory of off-null ellipsometry in its application to the determination of the surface concentration of adsorbed proteins on silicon substrates. For surface concentrations below 5 ng mm , a linear relation was observed between the square root of the intensity and the surface concentration of the proteins with an accuracy of the order of 3% or better. 

The adsorption of insulin on metal surfaces from aqueous solutions was monitored by in situ ellipsometry by Amebrant and Nylander. Clean (hydrophilic) chromium and titanium surfaces as well as chromium surfaces treated to be hydrophobic were used. The adsorbed amount was found to be higher on the hydrophilic than on the hydrophobic surfaces. 

---

R.M. Azzam and N.M. Bashara, Ellipsometry and Polarized Light, New York North Holland, 1977. 

A quite different means for the experimental determination of surface excess quantities is ellipsometry. The technique is discussed in Section IV-3D, and it is sufficient to note here that the method allows the calculation of the thickness of an adsorbed film from the ellipticity produced in light reflected from the film covered surface. If this thickness, t, is known, F may be calculated from the relationship F = t/V, where V is the molecular volume. This last may be estimated either from molecular models or from the bulk liquid density. 

Smith [113] studied the adsorption of n-pentane on mercury, determining both the surface tension change and the ellipsometric film thickness as a function of the equilibrium pentane pressure. F could then be calculated from the Gibbs equation in the form of Eq. ni-106, and from t. The agreement was excellent. Ellipsometry has also been used to determine the surface compositions of solutions [114,115], as well polymer adsorption at the solution-air interface [116]. 

The detailed examination of the behavior of light passing through or reflected by an interface can, in principle, allow the determination of the monolayer thickness, its index of refiraction and absorption coefficient as a function of wavelength. The subjects of ellipsometry, spectroscopy, and x-ray reflection deal with this goal we sketch these techniques here. 

In ellipsometry monochromatic light such as from a He-Ne laser, is passed through a polarizer, rotated by passing through a compensator before it impinges on the interface to be studied [142]. The reflected beam will be elliptically polarized and is measured by a polarization analyzer. In null ellipsometry, the polarizer, compensator, and analyzer are rotated to produce maximum extinction. The phase shift between the parallel and perpendicular components A and the ratio of the amplitudes of these components, tan are related to the polarizer and analyzer angles p and a, respectively. The changes in A and when a film is present can be related in an implicit form to the complex index of refraction and thickness of the film. 

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. 

A newer and perhaps more useful application of ellipsometry to Langmuir films is their lateral characterization via ellipsometric microscopy [146], A simple modification of the nuU ellipsometer allows one to image features down to 10-/im resolution. Working with a fixed polarizer and analyzer, some domains are at extinction while others are not and appear bright. This approach requires no fluorescent label and can be applied to systems on reflective supports. 

D. D. Eley, ed., Adhesion, The Clarendon Press, Oxford, 1961. E. Passaglia, R. R. Stromberg, and J. Kruger, eds., Ellipsometry in the Measurement of Surfaces and Thin Films, National Bureau of Standards Miscellaneous Publication 256, Washington, DC, 1964. 

ELL Ellipsometry [194, 195] Depolarization of reflected Thickness of adsorbed film... 

Electrolyte adsorption on metals is important in electrochemistry [167,168]. One study reports the adsorption of various anions an Ag, Au, Rh, and Ni electrodes using ellipsometry. Adsorbed film thicknesses now also depend on applied potential. 

Estimate the thickness of a polymer layer from the loop profile in Eq. XI-20. Assume x = 0,Xs = 2.,= 0.01, and N = Ifr. Calculate the second moment of this profile (this is often measured by ellipsometry) and compare this thickness to the radius of gyration of the coil Rg = VN/6. 

In this chapter we review some of the most important developments in recent years in connection with the use of optical teclmiques for the characterization of surfaces. We start with an overview of the different approaches available to tire use of IR spectroscopy. Next, we briefly introduce some new optical characterization methods that rely on the use of lasers, including nonlinear spectroscopies. The following section addresses the use of x-rays for diffraction studies aimed at structural detenninations. Lastly, passing reference is made to other optical teclmiques such as ellipsometry and NMR, and to spectroscopies that only partly depend on photons. 

Tompkins FI G 1993 A User s Guide to Ellipsometry (Boston Academic)... 

A) MEASUREMENT OF THE OPTICAL CONSTANTS OF MATERIALS USING ELLIPSOMETRY... 

Figure Bl.26.15. The Del/Psi trajectory for silicon dioxide on silicon with angle of mcidence ( )j = 70° and wavelength X = 6328 A (Tompkins FI G 1993 A Users Guide to Ellipsometry (San Diego, CA Academic)).

Dielectric constants of metals, semiconductors and insulators can be detennined from ellipsometry measurements [38, 39]. Since the dielectric constant can vary depending on the way in which a fihn is grown, the measurement of accurate film thicknesses relies on having accurate values of the dielectric constant. One connnon procedure for detennining dielectric constants is by using a Kramers-Kronig analysis of spectroscopic reflectance data [39]. This method suffers from the series-tennination error as well as the difficulty of making corrections for the presence of overlayer contaminants. The ellipsometry method is for the most part free of both these sources of error and thus yields the most accurate values to date [39]. 

Ellipsometry measurements can provide infomiation about the thickness, microroughness and dielectric ftinction of thin films. It can also provide infomiation on the depth profile of multilayer stmctiires non-destmctively, including the thickness, the composition and the degree of crystallinity of each layer [39]. The measurement of the various components of a complex multilayered film is illustrated m figure Bl.26.17 [40]. 

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]. 

Azzam R M A and Bashara N M 1977 Ellipsometry and Polarized L/g/rf (Amsterdam North-Holland)... 

Tompkins H G 1993 A Users Guide to Ellipsometry (San Diego, CA Aoademio)... 

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

Amongst other spectroscopic teclmiques which have successfiilly been employed in situ in electrochemical investigations are ESR, which is used to investigate electrochemical processes involving paramagnetic molecules, Raman spectroscopy and ellipsometry. 

Christensen P and Hamnett A 2000 in s/fu techniques in electrochemistry—ellipsometry and FTIR Eiectrochim. Acta 45 2443... 

Kelly M K, Etohegoin P, Fuoh D, Kratsohmer W and Fostiropulos K 1992 Optioal transitions of Cgg films in the visible and ultraviolet from speotrosoopio ellipsometry Rhys. Rev. B 46 4963-8... 

Monolayers of alkanetliiols adsorbed on gold, prepared by immersing tire substrate into solution, have been characterized by a large number of different surface analytical teclmiques. The lateral order in such layers has been investigated using electron [1431, helium [144, 1451 and x-ray [146, 1471 diffraction, as well as witli scanning probe microscopies [122, 1481. Infonnation about tire orientation of tire alkyl chains has been obtained by ellipsometry [149], infrared (IR) spectroscopy [150, 151] and NEXAFS [152]. 

Porter M D, Bright T B, Allara D L and Chidsey C E D 1987 Spontaneously organized molecular assemblies. 4. Structural characterization of normal-alkyl thiol monolayers on gold by optical ellipsometry, infrared-spectroscopy, and electrochemistry J. Am. Chem. Soc. 109 3559-68... 

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. 

The most recently introduced optical teclmique is based on the retardation of light guided in an optical waveguide when biomolecules of a polarizability different from that of the solvent they displace are adsorbed at the waveguide surface (optical waveguide lightmode spectroscopy, OWLS) [H]. It is even more sensitive than ellipsometry, and the mode... 

---