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71st IUVSTA Workshop
Modeling Scanning Electron Microscope Measurements with Charging
J. S. Villarrubia *
National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
* John.Villarrubia@nist.gov
Nanostructured samples that are relevant in semiconductor electronics manufacturing or in nanotechnology
are often comprised of mixed materials, some of which are insulators. Accumulated charges in insulating
regions exposed to the electron beam or to scattered electrons generate electric fields that alter the contrast
and positions of features in acquired images. To interpret the images it is necessary to account for these effects.
For this purpose, the existing
JMONSEL [1,2] simulator has been
augmented with charging-related modeling
capabilities. JMONSEL already
performed Monte Carlo scattering simulations
that included elastic scattering
via the Mott cross sections, inelastic
scattering via the dielectric function
theory approach (when the requisite
material data are available), phonon
scattering, and electron refraction at
material boundary crossings. For simulations
in which charging is important, the sample is represented as a tetrahedral mesh with variable mesh size:
fine in the region of interest and increasingly coarse with distance (Fig. 1). The net charge due to trapped electrons
or holes in each mesh element is tracked. Simulation proceeds iteratively in a cycle that includes finite
element solution of the electric fields followed by a scattering phase with electron transport modified by the
electric field.
The new capabilities have been used to simulate imaging of unfilled contact holes with radius 35 nm to
70 nm through SiO 2 (Fig. 1) with a view to determining the dependence of visibility of the hole bottom upon
hole size and imaging conditions. Results indicate that under appropriate conditions, the SiO 2 develops a beneficial
positive surface potential that saturates at a scan-size dependent amount. This potential helps to extract
electrons from the hole bottom.
Fig. 1. Cutaway of meshed contact hole and surrounding volume. Tetrahedra
that intersect a plane through the axis of the hole are rendered as solids.
Oxide is blue, the vacuum above the hole is green, and (inset) the vacuum in
the hole itself is purple.
References
[1]J. S. Villarrubia, N. W. M. Ritchie, and J. R. Lowney, Proc. SPIE 6518 (2007) 65180K.
[2]J. S. Villarrubia and Z. J. Ding, J. Micro/Nanolith. MEMS MOEMS 8 (2009) 033003.
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