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Research paper : Innovative electron microscope for light-element atom visualization (Y. Sato et al.)−175−Synthesiology - English edition Vol.4 No.3 (2012) However, assuming that the application of this corrector to our microscope at 30–60 kV in Triple-C project, it was predicted that the spatial resolution would be limited by the residual six-fold astigmatism, which is fundamentally inevitable in the case of this double-hexapole system. Therefore, we started to search for a completely new method to enable the correction of both spherical aberration and high-order geometrical astigmatisms up to six-fold, and finally developed an innovative correction system consisting of triple magnetic dodecapoles and transfer lenses[15]-[17].Figure 4 shows the construction and calculated electron-beam trajectories of the conventional double-hexapole type and our new triple-dodecapole type (named as Delta type) correctors[17]. For the calculated trajectory of the conventional corrector (a), six-fold symmetry is more obvious in the higher angle region at the end of the second hexapole. This is caused by the six-fold astigmatism that is inevitably generated by the combination of three-fold magnetic fields between the double hexapoles as described above. Such six-fold astigmatism is also generated between the first and second dodecapoles of the Delta corrector, and between the second and third dodecapoles as well. In this case, however, the astigmatisms can be completely canceled out by carefully adjusting the azimuth of the magnetic field in each dodecapole. As shown in Fig. 4 (b), the calculated trajectory for the Delta corrector (b) is almost circular even at higher angles at the end of the third dodecapole, indicating that the residual six-fold astigmatism is negligible. 3.3 Chromatic aberration correctorThrough the Triple-C project, we have developed a new chromatic aberration corrector for TEM in addition to a Delta spherical aberration corrector. As described in subchapter 3.1, the degree of electron-beam broadening due to chromatic aberration is proportional to E/E, which implies that spatial resolution is more seriously affected at a lower E. Therefore, chromatic aberration correction of the objective lens significantly contributes toward improving spatial resolution and image quality at 30 kV, even when the microscope is equipped with our new low-voltage CFEG. In this project, we developed a completely new system for chromatic aberration correction, for which two thick quadrupole fields are applied to generate a combination concave-lens effect[17]-[19].Figure 5 shows the calculated electron-beam trajectory of our new chromatic aberration corrector consisting of two thick dodecapoles and several transfer lenses. Each dodecapole of this corrector is used to superimpose magnetic and electrostatic quadrupole fields, where the deflection sensitivity of an electron beam depends on the acceleration voltage in a different manner compared to the magnetic field of the objective lens. The two-fold stigmatism introduced at the first dodecapole is canceled at the second dodecapole, and the entire system functions as a concave lens with negative chromatic aberration. We installed this corrector in the microscope and examined its performance by slightly changing the acceleration voltage from 30 kV. We found that the defocus remained unchanged even when the acceleration voltage was changed by ± 0.025 kV, which confirms that the chromatic aberration was successfully corrected. 4 Integration of elemental technologies4.1 Construction of low-voltage electron microscopesWe constructed experimental systems of low-voltage electron microscopes by integrating the elemental technologies described above and then examined their performances. Two microscopes with different constructions optimized for their use were developed separately, enabling us to efficiently proceed with their examination and reach the stage of Fig. 4 Construction and calculated electron-beam trajectories of spherical aberration correctors(a) Double-hexapole type, (b) Delta (triple-dodecapole) type Fig. 5 Construction and calculated electron-beam trajectory of new chromatic aberration corrector(a) Double-hexapole typeElectron beamHexapoleTransfer lensElectron beamDodecapoleTransfer lens(b) Triple-dodecapole “Delta” typeSecond hexapoleFirst hexapoleSecond dodecapoleFirst dodecapoleThird dodecapole50100(mrad)50100(mrad)First dodecapoleSecond dodecapoleSpecimenObjective lensObjective mini-lensDodecapoleTransfer lens100 mrad50 mrad

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