Imaging Spectroscopic Ellipsometry and Spectral Reflectometry with Ellipsometric Contrast

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Imaging Spectroscopi...

Imaging Spectroscopic Ellipsometry and Spectral Reflectometry with Ellipsometric Contrast

Sebastian Funke Niklas Reineking Dirk Hönig

Two Methods for the Optical Characterization of 2D-Materials.

Introduction

Ellipsometry is a sensitive optical method, which has been used
for about a hundred years to derive information about surfaces.
It makes use of the fact that the polarization state of light may
change when the light beam is reflected from a surface. If the surface is
covered by a thin film (or a stack of films), the entire optical system of
film & substrate influences the change in polarization.
It is therefore possible to deduce information about the film properties,
especially the film thickness and optical properties.
Ellipsometry remains a macroanalysis technique, i.e.,
the spot size limits resolution.
Features smaller than the spot are not resolved.

The question is, how to apply ellipsometry to exfoliated 2D materials?

Imaging Ellipsometry

The development of imaging ellipsometry,
which combines the power of ellipsometry with microscopy,
has overcome this limitation. The enhanced spatial resolution of
imaging ellipsometers potentially expands ellipsometry into
new areas of microanalysis, microelectronics, and bioanalytics.

The ep4 is a microscopic imaging spectroscopic ellipsometer with
unique measurement capabilities.
It can provide real-time-ellipsometric enhanced contrast images,
maps of the ellipsometric angles Psi and Delta, or data for very
small regions of interest.
Film thickness and the complex refractive indices of the sample are
determined by computerized optical modeling.

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Setup of an imaging ellipsometer including objective and 2D CCD
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Spectroscopic Imaging Ellipsometry on Exfoliated Micro Crystals

Ellipsometric enhanced contrast by optimized polarizer and analyzer angle at selected wavelength and angle of incidence.

Regions of Interest (ROI) concept for ellipsometry on smallest flakes	Microscopic mapping of Delta and Psi.

Automatic Localization & Identification of microcrystallites

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Spectroscopic reflectometry with ellipsometric contrast for process control applications

Here, we present a setup that allows the acquisition of about 1000 spectroscopic spectra per second with a thickness sensitivity- of less than 0.5 nm. This becomes possible by comparing the sample with a similar reference sample. Due to the orientation of the reference (see Fig. 1) it acts as an ideal compensator
for all wavelengths. Hence no compensator is required. If the sample is equal to the reference, the outgoing light is linearly polarized and can extinguished with a crossed analyzer. If there’s any difference between the sample and reference, the light becomes elliptically polarized, and the detected light flux increases. In order to maximize the rate of data acquisition, the analyzer is set to an off null-position.

During the measurement neither the polarizer - nor the analyzer - angle is changed, so the measurement speed is only limited by the intensity of the light source
and the processing speed of the spectrometer.

2D-Materials