Recent success of spherical aberration (Cs) correctors for TEM and STEM opened a new generation of microscopy. In order to achieve sub Å resolution in experiment, however, it is necessary to use a monochromator to reduce the effect of chromatic aberration due to the energy spread of the electron source. The monochromator is also necessary for the high energy-resolution EELS (HREELS). Three microscopes - sub Å TEM, 1 Å STEM and HREELS - and those combinations are considered to bring the high performance electron microscopes of the next generation. Among those three, 1 Å STEM has the highest technical difficulties from the point of view of electron optics instrumentation. In this investigation, we will describe the main electron optical problems, which we should solve within a few years, to achieve 1 Å STEM.
The main reason, why 1 Å STEM is so difficult, is the spread of the beam due to the second order aberrations of the energy filter used as the monochromator.