Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Dark-Field Image Contrast in Transmission Scanning Electron Microscopy: Effects of Substrate Thickness and Detector Collection Angle

Published

Author(s)

Taylor J. Woehl, Robert R. Keller

Abstract

An annular dark field (ADF) detector was placed beneath a specimen in a field emission scanning electron microscope operated at 30 kV to calibrate detector response to incident beam current, and to create transmission images of gold nanoparticles on silicon nitride (SiN) substrates of various thicknesses. Based on the linear response of the ADF detector diodes to beam current, we developed a method that allowed for direct determination of the percentage of that beam current scattered to the ADF detector from the sample, i.e. the transmitted electron (TE) yield. Collection angles for the ADF detector region were defined using a masking aperture above the detector and were systematically varied by changing the sample to detector distance. We found the contrast of the nanoparticles, relative to the SiN substrate, decreased monotonically with decreasing collection angle and increasing substrate thickness. We also performed Monte Carlo electron scattering simulations, which showed quantitative agreement with experimental contrast associated with the nanoparticles. Together, the experiments and Monte Carlo simulations revealed that the decrease in contrast with decreasing collection angle was due to a rapid increase in the TE yield of the low atomic number substrate. Nanoparticles imaged at low collection angles ( 50 nm) showed low image contrast in their centers surrounded by a bright high-contrast halo on their edges. This complex image contrast was predicted by Monte Carlo simulations, which we interpreted in terms of mixing of the nominally bright field (BF) and ADF electron signals. Our systematic investigation of collection angle and substrate thickness effects on ADF t-SEM imaging provides fundamental understanding of the contrast mechanisms for image formation, which in turn suggest practical limitations and optimal imaging conditions for different substrate thicknesses.
Citation
Ultramicroscopy
Volume
171

Keywords

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Monte Carlo simulations, Scanning transmission electron microscopy (STEM), transmission scanning electron microscopy (t-SEM)

Citation

Woehl, T. and Keller, R. (2016), Dark-Field Image Contrast in Transmission Scanning Electron Microscopy: Effects of Substrate Thickness and Detector Collection Angle, Ultramicroscopy, [online], https://doi.org/10.1016/j.ultramic.2016.08.008 (Accessed June 19, 2024)

Issues

If you have any questions about this publication or are having problems accessing it, please contact reflib@nist.gov.

Created August 6, 2016, Updated May 9, 2019