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Energy-Filtered BSE Images

Figure 2 : General view of the Hitachi SU8230 SEM (left) and schematic of signal detection system using the deceleration mode (right).

by the upper detector, providing mainly topographic information. T e signal detected by the top detector is controlled by an energy fi lter called the “top fi lter.” T is works as a high pass fi lter, allowing only electrons with energy higher than the fi ltering voltage to be detected. T ese BSE electrons are converted to SE3s at the converter plate, and the SE3s are detected by the top detector. When the fi ltering voltage is not applied, SE signals are detected by the top detector mainly in deceleration mode. T is is because SEs have low energy and are easily converged by the electrical fi eld and defl ected to a high elevation angle into the top detector. When an appropriate combination of landing energy, decelerating voltage, and fi ltering voltage is applied, SE signals are suppressed and only BSEs that have enough energy to pass through the top fi lter are detected. Figure 4 shows SEM images of a

Figure 3 : SE + BSE SEM images of mesoporous silica particles acquired at the following conditions: accelerating voltage = 0.5 kV. Original magnifi cations (a) 200,000× and (b) 500,000×. Specimen courtesy of Dr. Toshiyuki Yokoi of Tokyo Institute of Technology.

source has a small energy spread, which reduces the chromatic aberration. T e objective lens in the system is a semi-in- lens type working at a short focal length, which also reduces aberrations. T e system confi guration can accommodate large specimens—up to 150 mm substrates. T e standard vacuum of the specimen chamber is on the order of 10 -5 Pa. T e deceleration function is a standard feature of the Hitachi SU8200 series, allowing high-resolution imaging at accelerating voltages less than 1 kV. Figure 3 shows SEM images of mesoporous silica nano particles that are used as catalyst supports or adsorbents. T ese particles are of amorphous silica, so they are generally susceptible to radiation damage. T erefore, it is necessary to use ULE conditions to reduce sample damage and observe fi ne structure. In Figure 3 nanopore features about 5 nm in diameter were observed at 500 eV landing energy with no beam damage and no charging. T e SU8200 series SEM has three types of SE detectors— the top detector, the upper detector, and the lower Everhart-T ornley detector—which together provide valuable fl exibility for imaging. Figure 2b shows a schematic of the signal detection system in the deceleration mode. Electrons with high energy and also a low elevation angle will be detected


composite fi lm comprised of carbon nanotubes (CNTs) on polytetrafl uo- roethylene (PTFE), which is expected to be used as a protective fi lm for a fuel cell separator fi lm. It is important to measure the distribution of CNTs on the PTFE because the CNT layer aff ects the conductivity of the composite fi lm. T ese images were taken by the upper detector and the top detector with the top fi lter at a landing energy of 0.3 keV. Figure 4a shows the fi ne structure of the sample

observed with the upper detector. In Figure 4b , taken with the top detector, the image contrast between CNTs and the PTFE is greater.

Specimens . For this article the BSE yield (η exp ) was measured at low accelerating voltages and compared with reference data from the literature. T ese measurements were made on carbon, silicon, copper, and gold found on the standards block of an electron probe micro analyzer (EPMA). T ese specimens were suitable for this experiment because of their purity and their fl at surface morphologies. T e carbon coating fi lm on the standards block was removed by mechanical polishing before it was provided because it could aff ect the penetration of primary electrons, especially at ULE, and the escape of BSEs and SEs from the surface. Methods . T e backscattered yield was measured on the standard materials at landing energies from 0.2 keV to 1 keV. To compare with reference BSE yields ( η ref ) listed in reference [ 2 ], the experimental BSE yields ( η exp ) were calculated by the following process. T e relationship between experimental signal-to-noise ratio ( S/N exp ) and theoretical signal-to-noise ratio ( S/N theory ) is calculated as:

(S/Nexp)2 = k(S/NT eory)2, (1) • 2016 May

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