Energy-Filtered BSE Images
where k is the detective effi ciency estimated using electron trajectory simulations and the SE yield of the conversion plate material [ 3 ]. S/N exp can be obtained from:
S/Nexp = Sdiv Imean – IDC (2)
where I mean is the mean level, I DC is a background level which describes a level when the beam is turned off , and S div
is a standard deviation of beam intensity. T ese parameters were obtained from a histogram of actual SEM images. S/N theory is the signal-to-noise ratio when all the signal is detected, and it can be converted to the number of electrons per pixel ( N e ) by:
S/NT eory = Ne √Ne = √Ne. (3)
By applying the formula (2) and (3) to the formula (1), N e can be calculated by:
N 1 Imean – IDC Sdiv
e = k ηexp = Ipτ KIpτ = ( e ( Results Figure 5 shows the comparison of η exp and η ref for
Figure 4 : BSE images of CNT/PTFE composite fi lm acquired at the following conditions: accelerating voltage = 0.3 kV, deceleration voltage = 0.5 kV, and fi lter voltage 0.7 kV. (a) Imaged with upper detector and (b) imaged with top detector. Specimen courtesy of Prof. Yoshiyuki Show of Tokai University.
Figure 5 : Experimental BSE yields ( η exp ) versus landing energy compared with reference data ( η ref ) for four elements: (a) carbon, (b) silicon, (c) copper, and (d) gold.
2016 May •
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(a) carbon, (b) silicon, (c) copper, and (d) gold at the landing energy between 0.2 and 1 keV. In all four materials, the η exp values follow the η ref fairly closely at the lower landing energies. However, there are a few data points that show a deviation from the reference data at the higher landing energy. Figure 6 shows the η exp of each material as a function of the landing energy. T e η exp values for Cu and Au tend to be higher at the landing energy of around 1 keV and then tend to decrease as the landing energy becomes lower. T e η exp of Au is the highest at the landing energy of 1 keV, but it becomes the lowest of the four materials at the landing energy of 0.2 keV. T e decrease in the η exp of Cu with beam energy is more gradual than that of Au. In contrast to Cu and Au, the η exp of C and Si tend to be lower at the higher landing energy; these values tend to increase slightly when the landing energy becomes lower. T e η exp of Si is higher than that of C at the landing energy between 0.2 to 1 keV, and it is the highest of the four materials at the landing energy of 0.2 keV. T e η exp of C is the lowest at the landing energy of 1 keV, but it becomes higher than that of Cu and Au at the landing energy of 0.2 keV. To confi rm these eff ects with actual imaging, a test specimen was used consisting of a piece of bare Si substrate next to another piece of Si substrate with a 50 nm thick fi lm of Au deposited on it. Figure 7 shows the BSE images taken at the landing energies of 1 keV and 0.2 keV. At the landing energy of 1 keV, Au appears brighter than Si, whereas at the landing energy of 0.2 keV the contrast was reversed. T ese images correspond with the graphs shown in Figures 5 and 6 .
23 ) 2 .
T e η exp values were calculated by: eNe
Imean – IDC Sdiv
) 2 , (4) (5)
where I p is the probe current, e is the electron charge, and τ is the dwell time. T e Ip was measured with a faraday cup. T e η exp values were compared with the η ref reference data.
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