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Energy-Filtered BSE Images


the four materials at 1 keV. It becomes slightly brighter at the lowest landing energies and is the brightest of the four materials at 0.2 keV. Carbon is the darkest of the four materials at 1 keV. It becomes slightly brighter at the lower landing energies; it is the second brightest of the four materials at the landing energy of 0.2 keV. T ese tendencies of reversal have been explained theoretically by the much stronger deviations of elastic Mott cross sections from Rutherford cross sections at ULE [ 4 ]. T is eff ect has been shown experimentally; at very low primary electron energies the BSE yields of heavier materials tend to decrease, and the BSE yields of lighter materials tend to increase [ 5 – 6 ].


Figure 6 : Experimental BSE yields ( η exp ) versus landing energy for carbon, silicon, copper, and gold.


T ere are several possible causes of the deviations of the experimental values from the reference data ( Figure 5 ). One may be due to the diff erence in the sample state because the BSE yield is sensitively aff ected by factors such as cleanliness and fl atness. T ere might be a little bit of contamination on the sample surface due to a hydrocarbon fi lm (re)deposited by the electron beam, even though the vacuum in the chamber was very good, which is on the order of 10 -5 Pa, and the samples were carefully handled. In addition, instrumental factors such as the acceptance capabilities of the detector could aff ect the results. T e complex phenomena occurring inside the chamber such as the generation of SE3 could aff ect the signal intensity as well.


Figure 7 : The reversal of BSE contrast between Au and Si acquired under the following conditions: accelerating voltage was (a) 1 kV, (b) 0.2 kV.


Discussion T e results shown in Figures 5 and 6 correspond reasonably well to the previous studies, except for a few data points. T e relationship of η exp versus keV for each material should correspond to the material contrast in the images. T e material contrast at the landing energy around 1 keV is proportional to atomic number, but it is not proportional at the lower landing energies. For the lower energies it becomes quite complicated. Gold is the brightest of the four materials at the landing energy of 1 keV. It becomes gradually darker at lower landing energies, and it is the darkest of the four materials at the landing energy of 0.3 keV. Copper is the second brightest of the four materials at the 1 keV landing energy. It also becomes gradually darker at the lower landing energies, and it is the second darkest of the four materials at 0.2 keV. Silicon is the second darkest of


24


Conclusion T e material contrast at the landing energy higher at 1 keV is proportional to atomic number as expected, but it is not proportional to the atomic number at the landing energy of 0.2 keV. At the latter landing energy, C and Si were brighter than Cu and Au. T e η exp values nearly correspond to the η ref , below about 0.5 keV, but there are a few experi- mental data points near 1 keV that are higher than the η ref by a factor


of two. Possible causes of the deviation include diff erences in sample state, contamination, the acceptance capabilities of the detector, and/or the complex phenomena occurring inside the specimen chamber.


References [1] I Müllerová , Scanning 23 ( 2001 ) 379 – 94 . [2] D Joy , A Database of Electron-Solid Interactions Revision # 12 – 1 (2012) .


[3] D C Joy et al ., Scanning 18 ( 1996 ) 533 – 38 . [4] L Reimer , Scanning Electron Microscopy , 2nd ed. , Springer Science & Business Media Verlag Berlin Heidelberg , New York , 1998 .


[5] I Müllerová , Scanning 26 ( 2004 ) 18 – 24 . [6] MM El Gomati et al ., Scanning 30 ( 2008 ) 2 – 15 .


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