In-Lens Band-Pass Filter for Secondary Electrons in Ultrahigh Resolution SEM.

Ivo Konvalina,Stanislav Krátký,Eliška Materna Mikmeková,Ilona Müllerová,Filip Mika

doi:10.3390/ma12142307

Ivo Konvalina, Stanislav Krátký + Show 3 more

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https://doi.org/10.3390/ma12142307

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Abstract

Scanning electron microscopes come equipped with different types of detectors for the collection of signal electrons emitted from samples. In-lens detection systems mostly consist of several auxiliary electrodes that help electrons to travel in a direction towards the detector. This paper aims to show that a through-the-lens detector in a commercial electron microscope Magellan 400 FEG can, under specific conditions, work as an energy band-pass filter of secondary electrons that are excited by the primary beam electrons. The band-pass filter properties verify extensive simulations of secondary and backscattered electrons in a precision 3D model of a microscope. A unique test sample demonstrates the effects of the band-pass filter on final image and contrast with chromium and silver stripes on a silicon substrate, manufactured by a combination of e-beam lithography, wet etching, and lift-off technique. The ray tracing of signal electrons in a detector model predicate that the through-the-lens detector works as a band-pass filter of the secondary electrons with an energy window of about 3 eV. By moving the energy window along the secondary electron energy spectrum curve of the analyzed material, we select the energy of the secondary electrons to be detected. Energy filtration brings a change in contrast in the image as well as displaying details that are not otherwise visible.

Highlights

Today’s scanning electron microscopes (SEMs) use an immersion objective lens (OL) to improve image resolution
We focus on the microscope Magellan 400 FEG with Elstar column [20]
The summarizing results of the band-pass filter simulations are plotted into one graph that best describes how the filter works. It is the dependence of the emission energy of secondary electrons (SEs) detected by the the-lens detector (TLD), which takes into account the cosine distribution of emission on the positive sample bias (Figure 5)

Summary

Introduction

Today’s scanning electron microscopes (SEMs) use an immersion objective lens (OL) to improve image resolution. A strong magnetic field penetrates to the sample region and decreases the aberration coefficients of the objective lens. The second effect of the magnetic field near the sample is a collimation of signal electrons to the optical axis. In this way, a large part of the signal electrons are guided into the OL, where they are processed further and detected. The final contrast of the SE images can vary with intensity and distribution of electrostatic and magnetic fields in the area where the signal electrons are moving [1].

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