Observation of high resolution elemental mapping images using a high sensitivity detector

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1. Development of a dedicated detector for elemental mapping(1) Construction of a detection system : In order to realize a fast elemental mapping, which displays images of elemental concentration, by use of the parallel electron energy loss spectroscopy (PEELS) and the scanning transmission electron microscopy (STEM), a moderate-scale photodiode arrray (PDA) of 35 channel was used to acquire energy loss spectra. Electric circuits and computer programs were also developed to control the elemental mapping system. Spectrum acquisition time for one pixel is 1.2 msec which is 20 times shorter than that of conventional 1024 channel PDA.(2) Measurements of fundamental properties of the detector : A windowless PDA was prepared to decrease the leakage of light from the electron scintillator in transmission from the window to elements of the PDA, and the cross talk was reduced to 4 % of total output current of the PDA.Measurement of the detective quantum efficiency (DQE) of the detector showed that the DQE decreases in a low electron intensity region, where the elemental mapping is usually carried out, due to electromagnetic disturbance. This result indicates that more effective electromagnetic shield is required.2. Achievement of high current density electron probe by spherical aberration correctionIn order to obtain high resolution elemental mapping images, spherical aberration of the probe-forming lens was corrected with a foil lens. It was found, theoretically and experimentally that the foil lens allows a high current density and should be useful for a low intensity spectrum.3. Observation of elemental mapping imagesIn elemental mapping images of boron nitride on a carbon microgrid, the microgrid and the boron intride were clearly discriminated. With nickel oxide, which have excitation edges in a high energy loss and low intensity region, elemental mapping images were observed as a result of the high sensitivity of the detector.