Untitled - IAP/TU Wien - Technische Universität Wien

Nanostructure Characterisation by Electron Beam Techniques
SE M Simulation Program for dimensional Nano-Metrology
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*Carl.G.Frase@PTB.de
The Scanning Electron Microscope (SEM) is a valuable and versatile instrument for imaging and
characterisation of nanostructures. However, prerequisite for the application as a quantitative dimensional
measurement tool is a proper calibration of the instrument and a physical model of SEM image formation
that definitely correlates the SEM signal profile with the underlying specimen topography [1].
Here, we present the SEM simulation program MCSEM which models the different stages of SEM
image formation and generates SEM grey level images of complex shaped nanostructures. The individual
aspects of image formation are implemented in different modules of the program, i.e. the electron probe
formation, the three-dimensional specimen model, the electron probe solid state interaction, the emission of
secondary electrons, electric fields and the dynamic charge-up behaviour of the specimen as well as
secondary electron ray-tracing in the vacuum above the specimen surface, and different detector models.
The Monte Carlo simulation of electron diffusion in solid state is the core of the program. It is based on
tabulated elastic Mott scattering cross sections by Salvat and Mayol [2] and the Bethe continuous slowing
down approximation in the modification of Joy and Luo [3]. The three-dimensional specimen model is
realized by methods of constructive solid geometry (CSG), combined with the possibility to include
two-dimensional height maps. The electric field is calculated using finite element methods (FEM). The
model includes space charge in dielectrics due to absorbed primary electrons, emitted and reabsorbed
secondary electrons, as well as metals on a floating or fixed potential. A secondary electron ray-tracing
algorithm models the SE trajectories in the electric field. Dynamic charge-up behaviour can be modeled in a
feedback loop, involving an iterative series of field recalculations. Detector models comprise an InLens SE
detector based on electron-optical detector efficiency calculations plus BSE and Transmission SEM (TSEM)
detectors [4].
References
[1] C. G. Frase, D. Gnieser and H. Bosse, J. Phys. D: Appl. Phys. 42 (2009) 183001 (17pp)
[2] F. Salvat, R. Mayol, Comp. Phys. Comm. 74, pp.358-374, (1993)
[3] D.C. Joy, S. Luo, Scanning 11(4), 176-180 (1989)
[4] T. Klein, E. Buhr, C.G. Frase, in: Advances in Imaging and Electron Physics 171, pp. 297-356, (2012)%
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