Dark-field image contrast in transmission scanning electron microscopy: Effects of substrate thickness and detector collection angle.
An annular dark field (ADF) detector was placed beneath a specimen in a field emission scanning electron microscope operated at 30kV 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 forward 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 inner exclusion 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 inner exclusion angle was due to a rapid increase in the TE yield of the low atomic number substrate. Nanoparticles imaged at low inner exclusion angles (<150mrad) and on thick substrates (>50nm) 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 inner exclusion 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.
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, United States. Electronic address: tjwoehl@umd.edu.
Taylor
T
Woehl
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, United States; Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, CO, United States.
Robert
R
Keller
Contrast; Image simulations; Monte Carlo simulations; Quantitative microscopy; Scanning transmission electron microscopy; Transmission scanning electron microscopy
Microsc Microanal. 2014 Feb;20(1):124-32
24423133
Microsc Microanal. 2005 Feb;11(1):97-104
15683576
Ultramicroscopy. 2016 Aug;167:43-56
27179301
Ultramicroscopy. 2003 Feb-Mar;94(2):89-98
12505758
Microsc Microanal. 2013 Dec;19(6):1688-97
23981287
Ultramicroscopy. 1976 Jun;1(3):239-53
1028192
Micron. 2007;38(4):390-401
16990007
J Microsc. 2011 Jul;243(1):31-9
21155995
Ultramicroscopy. 2011 Jul;111(8):1018-28
21740864
Microsc Microanal. 2010 Dec;16(6):795-804
20961483
Microsc Microanal. 2010 Oct;16(5):604-13
20633317
Phys Rev E Stat Nonlin Soft Matter Phys. 2011 Jun;83(6 Pt 1):061405
21797362
Microsc Microanal. 2014 Feb;20(1):111-23
24331292
J Electron Microsc Tech. 1988 Feb;8(2):193-200
3246607
J Microsc. 2006 Feb;221(Pt 2):84-8
16499548
Nanotoxicology. 2014 Jun;8(4):433-46
23590554
Scanning. 2011 May-Jun;33(3):135-46
21769885