UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2010 Meeting


Section: Optics, Spectroscopy, Plasma and Lasers


Title:
Spectroscopic ellipsometry


Authors:
Rovena Pascu, Maria Dinescu


Affiliation:
National Institute for Laser, Plasma and Radiation Physics

Atomistilor Str., No. 409, PO Box MG-16, 077125, Magurele, Bucharest, Romania


E-mail
rovenav@gmail.com


Keywords:
SE, WVASE32, Optical Models, EMA, Model Bruggeman


Abstract:
Spectroscopic ellipsometry (SE) is an optical technique which measures the change of polarization upon reflection or transmission; The exact nature of the polarization change is determined by the sample`s properties (thickness, complex refractive index or dielectric function tensor). It is a non destructive technique used frequently for optical and structural characterization of thin films and substrates. These properties derived from Fresnel reflectivity rp si rs, calculated for polarized light in the known planes as p” and “s”, and are dependent from elipsometry measured data f and d by equation tan(f)exp(id)=rp/rs. At every measurement is obtained a pair of two values (f,d), that represent the amplitude ratio f and phase difference d between the light waves in the two mentioned polarized planes; (f,d) spectra are measured by changing the wavelength of light. Recent progress in the field generated a new type of instruments – VASE- Variable Angle Spectroscopic Ellipsometry with advantage in extension the limits of applications of classical configurations like multilayer, anisotropic samples, non-uniform thin films etc. The measurements are limited by hardware capabilities - performance of individual components, global performance of various configurations. As a result, the last generations of SE instruments are focused on improving the precision of ellipsometric parameters, high speed in operation and extension of spectral band from 140 nm and 200 m. For specific applications for monitoring the technological process, in situ real time SE is developed into a customized configuration (characterization of thin film growth, progress diagnoses including etching and thermal oxidation). There are two general restrictions on SE measurements:1.)Surface roughness should be bellow ~ 30% of the probe wavelength, because errors generally increase, although this effect depends completely on the type of instrument (in VASE configuration are equipped with specific compensator or autoretarder); 2) The measurement must be performed at oblique incidence, because at normal incidence the measurements becomes impossible, since ”p” and “s” polarizations can not be distinguished anymore at this angle. The incidence angle is chosen to maximize the sensitivity of measurements, and varies according to the optical constants of samples. As a rule, in VASE operation, there are selections of three angles between 45º – 75 º due to the performance of autoretarder. One feature of SE is the high precision of the measurement and very high thickness sensitivity (~ 0.1 A) can be obtained even for conventional instrument. The technique allows measurement of the optical constants that included information about micro structural properties of surfaces (roughness, voids, dopant concentration, compositions); the simulation are made by effective medium approximation, mainly Bruggeman model. It makes the choice of the host material complex dielectric function equaling the final effective complex dielectric function of the multi – constituent material. The one inherent draw back of the SE technique is the indirect nature of this characterization method; SE data analysis requires an optical model defined by optical constants and layer thickness of the sample. In the case of new materials, it is possible to start the construction of optical model even when the sample structure is not clear. This process is completed by are iterative fitting Levenberg- Marquardt algorithm that minimize an MSE (Mean Square Error) criteria. In the case of VASE systems, the data analysis is made by WVASE32 software that acts manly like a simulator of sample, being very useful in research activities. WVASE 32 included a data base with refractive index for a large number of materials and the fitting process is made using dispersion functions. Because it can be applied also in VUV, it is useful for researches, because many materials present absorption in VUV that ca not be explained by physics or chemically.