Introduction to Ellipsometer
5. Ellipsometer and Data Processing Software

Ellipsometer measures the Y and D at different incident angle (or even at different wavelength). A general configuration of ellipsometer is illustrated as:

According to the operation mode, ellipsometer can be classified into rotate elements, polarization modulation and null ellipsometer.  Polarization modulation is, in fact, only a way to avoid the rotate components by using polarization modulation devices (it offers the benefit of fast operation and less mechanical vibration).  A null ellipsometer normally need human being to interact to find a condition in which the reflected light is nulled at the detector.  This needs to be implemented by adjusting the azimuth angle of polarizer, wave plate and analyzer.  Even though very accurate, null ellipsometer needs patience and experience to find the nulling condition. Rotating elements ellipsometer has several variants, including rotate polarizer or analyzer (in both case, the wave plate might be absent) and rotating wave plate.  The intensity at the detector side is measured as a function of rotating angle, and be analyzed by Fourier analysis to determine the polarization state change.  They are normally called photonic ellipsomter because the way to measure optical energy.  Because the fast operation, photonic ellipsometer normally can take tens or hundreds of measurements to average off the random error.

Another widely used method to classify the ellipsometer is by the light source.  The abbreviation SWE means single wavelength ellipsometer, which means a "monochromatic" light source -- normally a laser -- is used as light source; while SE is the short name for Spectroscopic Ellipsometer, which means the light source covers a spectrum range.  There are also Discrete Wavelength Ellipsometers which applied several discrete wavelength.

For a bulk material, a measurement at single incident angle is enough to determine the refractive index and extinction coefficient.  For films with known refractive index and extinction coefficient, it can be used to determine the thickness.  However, in reality the measurement itself is not as simple as measuring a bulk material.  Refractive index and extinction coefficient of a film might differ greatly from the bulk materials due to the effect of the substrate on which it grows as well as the growth condition.  Things get more complex with multiple layer stacks.  These changes make the refractive index and extinction coefficient an unknown variable, and makes manual solution of the equations almost impossible.  Therefore, modern ellipsometers need to be equipped with powerful software, not only for the hardware control and data acquisition, but also the data analysis.

The processing of the experiment data (which normally only the records of incident angle, wavelength, Y and D) starts from make a model to describe the film stack.  With a correct model, the software then carries out a minimum searching based on an elevation function (which normally the MSE- mean square error) according to the variable specified.  Because the minimum search algorithm is normally ended up with a local minimal point, the starting point plays an important role in the result, with The modeling process is very important to the correctness of the result.

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