Ellipsometry measures the complex reflectance ratio, , of a system, which may be parametrized by the amplitude component and the phase difference. The polarization state of the light incident upon the sample may be decomposed into an s and a p component the s component is oscillating perpendicular to the plane of incidence and parallel to the sample surface, and the p component is oscillating parallel to the plane of incidence. The amplitudes of the s and p components, after reflection and normalized to their initial value, are denoted by and , respectively. The angle of incidence is chosen close to the Brewster angle of the sample to ensure a maximal difference in and. Note that the right hand side of the equation is simply another way to represent a complex number. Since ellipsometry is measuring the ratio or difference of two values rather than the absolute value of either , it is very robust, accurate, and reproducible.
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By rotating the analyzer and polarizer and measuring the change in intensities of light over the image, analysis of the measured data by use of computerized optical modeling can lead to a deduction of spatially resolved film thickness and complex refractive index values. Due to the fact that the imaging is done at an angle, only a small line of the entire field of view is actually in focus.
The line in focus can be moved along the field of view by adjusting the focus. In order to analyze the entire region of interest, the focus must be incrementally moved along the region of interest with a photo taken at each position. All of the images are then compiled into a single, in focus image of the sample. In situ ellipsometry[ edit ] In situ ellipsometry refers to dynamic measurements during the modification process of a sample.
This process can be used to study, for instance, the growth of a thin film,  including calcium phosphate mineralization at the air-liquid interface,  etching or cleaning of a sample.
By in situ ellipsometry measurements it is possible to determine fundamental process parameters, such as, growth or etch rates, variation of optical properties with time. In situ ellipsometry measurements require a number of additional considerations: The sample spot is usually not as easily accessible as for ex situ measurements outside the process chamber. Therefore, the mechanical setup has to be adjusted, which can include additional optical elements mirrors, prisms, or lenses for redirecting or focusing the light beam.
Because the environmental conditions during the process can be harsh, the sensitive optical elements of the ellipsometry setup must be separated from the hot zone. In the simplest case this is done by optical view ports, though strain induced birefringence of the glass- windows has to be taken into account or minimized. Furthermore, the samples can be at elevated temperatures, which implies different optical properties compared to samples at room temperature. Despite all these problems, in situ ellipsometry becomes more and more important as process control technique for thin film deposition and modification tools.
In situ ellipsometers can be of single-wavelength or spectroscopic type. Spectroscopic in situ ellipsometers use multichannel detectors, for instance CCD detectors, which measure the ellipsometric parameters for all wavelengths in the studied spectral range simultaneously.
Ellipsometric porosimetry[ edit ] Ellipsometric porosimetry measures the change of the optical properties and thickness of the materials during adsorption and desorption of a volatile species at atmospheric pressure or under reduced pressure depending on the application.
Compared to traditional porosimeters, Ellipsometer porosimeters are well suited to very thin film pore size and pore size distribution measurement. Magneto-optic generalized ellipsometry[ edit ] Magneto-optic generalized ellipsometry MOGE is an advanced infrared spectroscopic ellipsometry technique for studying free charge carrier properties in conducting samples.
By applying an external magnetic field it is possible to determine independently the density , the optical mobility parameter and the effective mass parameter of free charge carriers.
Without the magnetic field only two out of the three free charge carrier parameters can be extracted independently. Applications[ edit ] This technique has found 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 unequaled capabilities for thin film metrology.
As an optical technique, spectroscopic ellipsometry is non-destructive and contactless. Because the incident radiation can be focused, small sample sizes can be imaged and desired characteristics can be mapped over a larger area m2.
Advantages[ edit ] Ellipsometry has a number of advantages compared to standard reflection intensity measurements: Ellipsometry measures at least two parameters at each wavelength of the spectrum. If generalized ellipsometry is applied up to 16 parameters can be measured at each wavelength. Ellipsometry measures an intensity ratio instead of pure intensities. Therefore, ellipsometry is less affected by intensity instabilities of the light source or atmospheric absorption. By using polarized light, normal ambient unpolarized stray light does not significantly influence the measurement, no dark box is necessary.
No reference measurement is necessary. Both real and imaginary part of the dielectric function or complex refractive index can be extracted without the necessity to perform a Kramers—Kronig analysis. Ellipsometry is especially superior to reflectivity measurements when studying anisotropic samples.
Spectroscopic Ellipsometry: Principles and Applications / Edition 1
Spectroscopic Ellipsometry for Photovoltaics
Spectroscopic Ellipsometry: Basic Concepts