The Polarization state of light reflected from a surface and its variations are measured by ellipsometry.
Basics:
Ellipsometry is an optical technique for the investigation of the dielectric properties (complex refractive index or dielectric function) of thin films.
It has applications in many different fields, from semiconductor physics to microelectronics and biology, from basic research to industrial applications. Ellipsometry is a very sensitive measurement technique and provides unequalled capabilities for thin film metrology. As an optical technique, spectroscopic ellipsometry is non-destructive and contactless.
Upon the analysis of the change of polarization of light, which is reflected off a sample, ellipsometry can yield information about layers that are thinner than the wavelength of the probing light itself, even down to a single atomic layer. Ellipsometry can probe the complex refractive index or dielectric function tensor, which gives access to fundamental physical parameters and is related to a variety of sample properties, including morphology, crystal quality, chemical composition, or electrical conductivity. It is commonly used to characterize film thickness for single layers or complex multilayer stacks ranging from a few angstroms or tenths of a nanometer to several micrometers with an excellent accuracy.[1]
We report the development of a new ellipsometry technique which is especially well suited to the measurement of the dielectric constants of high reflectivity metals. Modulation methods are used to permit the measurement of a differential reflectance parameter which does not decrease in sensitivity in the high reflectivity limit as do the usual reflectance parameters measured with standard techniques. In addition the ellipsometer is faster than previous methods, being capable of recording the relevant data in a few minutes as the wavelength is swept continuously over a range of several thousand angstroms. The precision of the dielectric constants so measured is 1% in most cases. The polarization modulation technique, upon which the ellipsometer is based, is presented in detail, with emphasis on its wider applicability to any problem involving the study of dichroic phenomena. [2]
The application of ellipsometry of the study of the adsorption behavior of proteins and synthetic macromolecules at the air-water interface has been investigated. It is shown that for macromolecules the amount adsorbed per unit area, Γ, as determined by ellipsometry, only has a well-defined physical meaning if the refractive-index increment remains constant up to high concentrations present in the adsorbed layer. It has been found experimentally that this conditioned is fulfilled for proteins. The ellipsometric Γ values of some protein agree satisfactorily with those obtained by two independent techniques has been used to investigate the adsorption from solution of κ-casein, bovine serum albumin, and polyvinyl alcohol. For bovine serum albumin, Γ reaches a plateau value of 2.9 mg/m2 for concentrations ≥ 0.05 wt%. The thickness of the adsorbed molecules. For κ-casein, Γ steadily increases with increasing centration and multilayers are formed. The technique provides interesting information on conformational changes in adsorbed macromolecules, on the rate of the process, and on the conditions under which these occur. [3]
Light emitted by separate atoms and molecules is always polarized. Nevertheless, any macroscopic source of the light consists of huge number of such separate emitters and the direction of the electric field at any moment of the time is not predictable. Such light is called unpolarized or natural light. Using light polarizer (polarization filter) we can suppress the component of the light polarized in one direction and transmit only the component polarized in perpendicular direction. Behind the polarizer the light will be plane-polarized. In general case, the totally polarized light consists of two perpendicular plane-polarized components. Depending on the amplitude of these two waves and their relative phase, the combined electric vector traces out an ellipse and the wave is said to be elliptically polarized. Elliptical and plane polarization can be converted into each other by means of birefringent optical systems. Animation shows two waves: one of them are linear polarized wave and the other one is the circularly polarized wave. The electric field vector of linearly polarized electromagnetic wave (marked in blue) oscillates only in one direction. In circularly polarized wave the end of electric field vector (marked in red) moves like a coil. Ellipsometry is a non-destructive optical technique, which deals with the measurement and interpretation state of polarized light undergoing oblique reflection from a sample surface. Linearly polarized light, when reflected from a surface, will change its state to elliptically polarized because of presence of the thin layer of the boundary surface between two mediums. Dependence between optical constants of a layer and parameters of elliptically polarized light can be found on basis of Fresnel formulas . The green line of mercury lamp or laser beam is used in ellipsometry as a source of light. The wide-band light of incandescent lamp can also be used for spectroscopic measurements. The laser has a higher power which gives a higher signal to noise ratio for better imaging at a wavelength where the sample is transparent. Animation shows two linear polarized waves incident on the surface. The wave, which reflects from a thin film of a sample, becomes circularly polarized wave, while the other wave reflected from a substrate does not change the state of polarization. [4]
Ellipsometry is a unique optical technique of great sensitivity for in situ non-destructive characterization of surface (inter-facial) phenomena (reactions) utilizing the change in the state of polarization of a light-wave probe. Although known for almost a century, the use of ellipsometry has increased rapidly in the last two decades. Among the most significant recent developments are new applications, novel and automated instrumentation and techniques for error-free data analysis. This book provides the necessary analytical and experimental tools needed for competent understanding and use of these developments. It is directed to those who are already working in the field and, more importantly, to the newcomer who would otherwise have to sift through several hundred published papers. The authors first present a comprehensive study of the different mathematical representations of polarized light and how such light is processed by optical systems, going on to show how these tools are applied to the analysis of ellipsometer systems. To relate ellipsometric measurements to surface properties, use is then made of electromagnetic theory. Experimental techniques and apparatus are described and the many interesting applications of ellipsometry to surface and thin-film phenomena are reviewed. This reference work is addressed to researchers and students with a strong interest in surface and thin-film physics and optics and their applications. It is a must for libraries in the fields of solid state physics, physical chemistry, electro-chemistry, metallurgy and optical engineering. [5]
Conclusion:
Ellipsometry and polarized light is ineresting to students and researchers who have a big interest in optics and surface and thin-film physics and their use. It is a very important subject for everybody in fields of physical chemistry, optical engineering, solid state physics, electro-chemistry and metallurgy.
References:
[1] http://en.wikipedia.org/wiki/Ellipsometry
[2] ”An Improved Method for High Reflectivity Ellipsometry Based on a New Polarization Modulation Technique” by: Jasperson, S. N.; Schnatterly, S. E.
[3] ”Ellipsometry as a tool to study the adsorption behavior of synthetic and biopolymers at the air–water interface” by: J. A. De Feijter, J. Benjamins, F. A. Veer
[4] http://physics-animations.com/Physics/English/ell_txt.htm
[5] ”Ellipsometry and polarized light” by: R. M. A. Azzam , N. M. Bashara
Special thanks goes to http://www.selco.com.au/ for “Ellipsometry and Polarized Light” sugestion.
