Integration of a Monochromator in the Field Emission STEM

Abstract

It would be very desirable to obtain EELS at 0. l-0.2eV resolution for bandstructure studies at interfaces and defects. [1,2] It has been possible to build spectrometers that perform in this energy range, but it has been difficult to utilize this resolution, because the energy width of the cold field emission source is about 0.3-0.4 eV for the currents needed to do practical microscopy. [3] Numerical methods have been used to sharpen the spectra, showing unambiguously that EELS spectra would benefit greatly if a higher resolution source were available. [4,5]This situation has led one of us (H.W. Mook) to investigate electron monochromator optical designs which would be compatible with the high brightness and current required by the STEM. [6] This work produced a 4cm long monochromator that can be integrated into the field emission source. As shown in Fig. 1, the monochromator optics includes a field lens formed by deceleration of the electron beam from the 3 KeV extraction energy to about 350 eV inside the monochromator, a short Wien Filter velocity dispersive field, 150nm wide Si energy selecting slits, and an acceleration field lens at the monochromator exit.