Electron Nano-Imaging by Nobuo Tanaka

Author:Nobuo Tanaka
Language: eng
Format: epub
Publisher: Springer Japan, Tokyo

9.3 Principle of Image Formation in STEM

Figure 9.2 illustrates the basic structure of STEM which was developed by Crewe (1966). Electrons extracted from a cold FEG are merged into a probe smaller than 1 nm by an electrostatic gun lens , a magnetic condenser, and an objective lenses L1 and L2. The probe is scanned on a specimen surface by two reflection coils. Electrons emitted from the exit surface of the specimen enter electron detectors (D, Dʹ). Time-sequence signals are obtained for mapping the intensity on a viewing screen, previously a cathode ray tube (CRT) , but now a liquid crystal panel or a plasma-display one . The system is essentially similar to that in television . When one picks up secondary electrons (SE) or backscattered electrons at the same side as electron incidence, ordinary SEM images (Reimer 1985) are obtained even in STEM. Recent developments of SE imaging using aberration-corrected STEM produced imaging of single atoms (see Chap. 11 and Zhu et al. 2009).

The magnification of STEM images is determined by the ratio of a size of the viewing screen to a scanning area on the specimen surface, which is changed by the current applied to the reflection coils above the objective lens.

The resolution of STEM images is determined by the following factors:(1)Intensity of scattered electrons, detection efficiency , and conversion efficiencies of electron/light/electric signal.

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