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Optical properties

Optical properties obtained from a VEELS spectrum (SiN).

The determination of optical proerties (dielectric constant, refractive index, band gap, absorption coefficient, reflection coefficient, transmission coefficient) by using VEELS has attracted great interest since miniaturization of integrated circuits is pushed forward. Optical methods do not reach the necessary spatial resolution anymore. Therefore one of our research topics is on the determination of optical properties by means of VEELS.

The most important advantages of VEELS to optical methods are:

  1. The high spatial resolution, which is only limited by the delocalization of the respective energy losses,

  2. The large energy range which can be inspected (1-1000 eV), where as optical methods are limited from infra-red to ultra-violet (1-5 eV) and

  3. The possibility to have access to the scattering angle dependent determination of dielectric properties.

The principle:

Plural scattering is deconvolved from the experimental data and the loss function is calculated. Using the Kramers-Kronig relations the optical constants are deduced.

The critical steps during this procedure are to account for relativistic effects in the experimental data and the exact normalization of the loss function. Conventional software is not able to to both, therefore USTEM has developped its own software during its research.

accuracy of this method

The accuracy of this method is fantastic. If all effects are considered (retardation, finite sample size effects, Cerenkov losses and surface plasmons) two similar layers with slightly different refractive indices can still be characterized (see M. Stöger-Pollach et al., Ultramicroscopy 108 (2008), 5; pp.439-444).

Comparison of two similar silicon nitride layers with slightly different refractive indices. Left: experiment with 60 keV electrons avoiding relativistic effects, Right: experiment with 200 keV electrons and data processing using our own software.

Inquiries for this topic:

Please contact Dr. Michael Stöger-Pollach.