Null-ellipsometry

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. 

Manual null ellipsometry is accurate but infrequently done, due to the length of time needed to acquire sufficient data for any meaningffil materials analysis. Automated null ellipsometers are used, for example, in the infrared, but are still slow. Numerous versions of last automated ellipsometers have been built. Examples... 

V. I. Beklemishev, V. V. Levents, 1.1. Makhonin, M. S. Minazhdinov, and 1. V. Seletskaia, Examination of Si substitute surface after different chemical pretreatments by means of nulling ellipsometry, J. Electrochem. Soc. 141, 554, 1994. 

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). 

Benesch, J., Askendal, A., Tengvall, P., 2000. Quantification of adsorbed human serum albumin at solid interfaces a comparison between radioimmunoassay and simple null ellipsometry. Colloids Surf. B Biointerfaces 18, 71—81. 

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 earliest ellipsometry measurements (ca 1890) were used to determine the optical functions (refractive index n and extinction coefficient k, or equivalently, absorption coefficient a) for several materials. In the 1940s it was discovered that a single-wavelength nulling ellipsometry measurement could be used to determine the thickness of certain thin films very accurately. Since that time, single-wavelength... 

One of the most useful applications of ellipsometry has been the routine measurement of silicon dioxide (Si02) film thicknesses grown on silicon. Since the refractive index of Si02 is well known, and does not depend significantly on film deposition technique, nulling ellipsometry measurements are usually sufficient to determine film thickness. 

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