The operation principles
Our monochromators of the Omega type are purely electrostatic.
The beam is deflected in an S-shaped manner forming an astigmatic image of the virtual source in the selection plane, which is the symmetry plane of the monochromator. There the beam is filtered using a slit, which is arranged parallel to the astigmatic image line.
An array of slits of various sizes is available in the instrument to allow for an optimization of energy width versus beam current suitable for the actual application. The slits are changed using piezo-slides.
Behind the slit the beam is deflected back to the optical axis of the microscope symmetrically to the first half of the monochromator thus completing the Omega. After leaving the monochromator the beam is entirely dispersion-free, even the angular dispersion is compensated.
It is possible to switch the monochromator off and operate the microscope with a straight beam as in ordinary operation mode.
The monochromator reduces the energy width of the electron beam without introducing disturbing aberrations. No decrease of gun brightness has been observed yet, apart from the unavoidable current loss due to the energy filtering process. A typical result for its energy resolution is given in the figure below for a 1.1µm slit.
Due to its purely electrostatic operation principle, its UHV-compatibility and its small number of power supplies (1 Bipolar voltage + 1 piezo driver) the monochromator is best suited for incorporation within a (S)TEM gun head. Of course it can also be used for other types of electron beam instruments and even for ion beam applications.
Zeiss uses the monochromator as patented, developed and manufactured by CEOS for their Libra TEM.