Physical Principles of Electron Microscopy: An Introduction to TEM ...

Chapter 7
RECENT DEVELOPMENTS
7.1 Scanning Transmission Electron Microscopy
In Section 1.6, we noted that a field-emission STEM was the first instrument
to successfully image single atoms in the late 1960’s. Several years later, a
“dedicated STEM” (operating only in scanning mode) was manufactured by
the Vacuum Generators Company in the UK. A cross-sectional diagram of a
substantially modified version of this instrument (at IBM Watson Research
Laboratories) is shown in Fig. 7-1. Because there is no need for a viewing
screen inside the lens column, its electron gun is located at the bottom of the
instrument (with some advantage in terms of mechanical stability) and the
electron detectors are on top (making it easier to modify the detectors or add
new
ones).
Most STEM images are recorded in dark-field mode, using an annular
dark-field (ADF) detector located just above the specimen. Its design is
similar to that of a Robinson or solid-state BSE detector used in the SEM
(Fig. 5-10) but it collects electrons that are transmitted through the specimen
and scattered within a certain angular range (�min to �max), determined by the
inner and outer diameters. By making �min large, only large-angle (mainly
elastic) scattering is collected. According to Eq. (4.15), its intensity is
proportional to Z 2 . Therefore the ADF image contains strong atomic-number
contrast and not much diffraction contrast (most of the diffracted electrons
have a scattering angle less than �min and pass through the central hole in the
detector). Although the signal is weak, the annular detector design provides
high collection efficiency, as all azimuthal angles of scattering are recorded
(unlike the case of dark-field imaging in a conventional TEM).