Ultra-High Resolution SEM in a STEM Instrument

In recent years great improvements in the resolution of secondary electron imaging have been made possible by (1) using a high brightness field emission gun and (2) putting the specimen inside the highly excited objective lens pole-pieces in a scanning electron microscope. Hence, a resolution of 0.8 nm of SE images obtained with a 30 kv electron microscope has been claimed and atom-high surface steps have been observed by detecting SE signals. In a STEM instrument the SE signals are collected at the exit surface of the sample. The emitted secondary electrons spiral up around the fringe magnetic field lines of the objective lens before they are collected by the SE detector. Only type I secondary electrons contribute to the image formation signal in this detecting configuration. The collection efficiency may be high due to the effect of the pre-collection magnetic field and the signal-to-noise ratio is good with the use of a field emission gun.