Ellipsometry advantages

Similar methods directly related to surface films are involved in double-layer capacitance, differential capacitance, and nuclear magnetic resonance techniques, described in the recent literature. As with ellipsometry, advantages are sensitivity in measurement, but equipment requirements limit these techniques to laboratory use, and therefore are mostly for highly theoretical, mechanistic studies. 

The number of measurable layers of a stack is limited only by the optical contrast between the different layers. In practice stacks of ten layers and more can be analyzed by ellipsometry. Further advantages of ellipsometry compared with other metrological methods are the non-invasive and non-destructive character of the optical method, the low energy entry into the sample, the direct measurement of the dielectric function of materials, and the possibility of making the measurement in any kind of optical transparent environment. 

Whereas the XSW technique takes advantage of the standing wave established on the total reflection of X-rays from a mirror surface, a conceptually more straightforward approach is that of simply specularly reflecting an X-ray beam from an electrode coated with the film of interest, measuring the ratio of the intensities of the incident and reflected rays, and fitting the data, using the Fresnel equations, to a suitable model an approach similar to optical ellipsometry. 

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... 

With all its complications and uncertainties, impedance spectroscopy, as seen at the end of the twentieth century, is a growing technique in fundamental electrodic analysis [cf. the seminal contributions of (independently) D. D. and J. R. MacDonald]. Among its advantages is that the necessary equipment is less expensive than that of competing spectroscopic equipment and that it can provide information on any electrochemical situation (e.g., it is not limited by, say, the need for specular reflectance, as in ellipsometry). 

Castle (LI, 12) and McIntyre (5) have shown how X-ray photoelectron spectroscopy (XPS or ESCA) can be applied to corrosion problems and uses of Auger electron spectroscopy (AES) in corrosion have been given by Clough (18, 19) and Thomas et al ( 0). Possible uses of ion beams in corrosion studies were presented by Deamaley (JU). Raman spectroscopy (.22, 23) and ellipsometry (19) are not discussed in detail in this paper, but they offer the advantage of allowing in situ measurements in a wide variety of corrosive environments. 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

One of the most useful experimental methods to be applied to protein adsorption in recent years is the radiotracer technique (Mura-matsu, 1973). Proteins labeled with, 31I and 125I (Brash et al., 1974) and [14C]acetyl derivatives of proteins (Phillips et al., 1975) have been used as tracers. As well as measuring adsorption directly, this method has the great advantage that it can detect exchange between interface and bulk even when the total amount adsorbed does not vary. A technique that has been used to obtain independent measurements of the amount of protein adsorbed by measuring film thickness is ellipsometry (Trurnit, 1953). 

The above discussion reveals how NR can be used to provide direct information concerning both the thickness and the composition via the SLD of the various layers in the interface. This constitutes an advantage over ellipsometry or surface plasmon resonance methods where the value of the refractive index (composition) of the film is usually assumed in order to determine its thickness. However, to use the kinematic approach to directly determine these quantities, the reflectivity has to be measured in a sufficiently large range of so that at least one-half of the longest period interference fringe, corresponding to the thirmest layer in the interface, is observed on the reflectivity curve. The spatial resolution for a film of thickness ris defined as t= 7r/Qz,max, where is the maximum momentum... 

Ellipsometry is therefore a unique kind of spectroscopy in which each datum point contains two pieces of information. Moreover, since these two quantities are ratiometric and derived from the same beam of light, ellipsometry has the important operational advantage of high immunity to noise in the light source. 

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 properties of adsorbed layers at liquid interfaces can be determined either indirectly by thermodynamic methods or directly by means of some particular experimental techniques, such as radiotracer and ellipsometry. For adsorbed layers of synthetic polymers or biopolymers the advantages of the ellipsometry technique become evident as it yields information not only on the adsorbed amount but also on the thickness and refractive index of the layer. The theoretical background of ellipsometry with regard to layers between two bulk phases has been described in literature quite frequently (243). In brief, the principle of the method assumes that the state of polarization of a light beam is characterized by the amplitude ratio Ep E and the phase difference (8 — 8g) of the two components of the electric-field vector E. These two components Ep and E are parallel (p) and normal (s) to the plane of incidence of the beam and given by... 

Reflection spectroscopy, Raman spectroscopy, and ellipsometry complement the various electrochemical methods to study metal deposition. The optical methods can be used for a direct monitoring of the deposition process. The great advantage of optical spectroscopy... 

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. 

In situ FTIR " also had to overcome serious difficulties in its application to electrochemical problems. Unlike ellipsometry, where the wavelengths used are in a region of low solvent absorbance, IR is strongly absorbed by most familiar organic solvents and most particularly by water. This leads inevitably either to thin-layer cells or the development of internal reflection techniques. The former has the advantage of simplicity in interpreting spectral data, but it severely limits the type of electrochemistry that can be carried out. The latter requires not only a suitable high-refractive index substrate, such as Ge or Si, but also an adherent very thin layer of metal as the electrode. Technically this is difficult to fabricate so that the metal layer is continuous, and a substantial lateral resistance is inevitable. 

Another recent innovation is concerned with eUipsometric measurements in the far infrared (Kircher et al. 1997). The principal advantage of eUipsometiy is that the optical constants of a system can be obtained without KK analysis and thus measurements do not have to be extended to o) — 0 and a) oo. Ellipsometry is an extremely successful technique in the near-lR—visible-near-ultraviolet regime where high-quality... 

Whilst ellipsometry is undoubtedly the technique to use to obtain accurate optical constants for surfaces and surface films, it suffers from being both expensive and slow, and consequently has only been of limited use to electrochemists. As a result of these difficulties, several techniques based on simply monitoring of the reflectivity of an interface have been developed. Whilst in theory these are less informative, in practice they have proved much more useful. The major advantages are lower cost and greater speed, making them ideal for kinetic studies. 

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