A sculpted thin membrane of silicon nitride can be used as a phase-mask [1] to induce specific aberrations into an electron beam probe [2]. Specifically, it can be used to correct the inherent spherical aberration of the standard, spherically symmetric lenses that are used in the electron column, by inducing opposite spherical aberration with a thin film corrector. The advantage of this concept is that it enables an immediate and low-cost upgrade to an existing non-corrected electron microscope, by placing the corrector in one of the available aperture ports of the microscope.
We validated this concept by designing and fabricating (by focused ion beam milling) a corrector that was installed in the condenser 2 aperture of a 300 kV FEI Titan scanning transmission electron microscope. The non-corrected spherical aberration of the microscope, with a coefficient Cs=2.7mm, determined the convergence semi-angle of 7.4 mrad for achieving optimal resolution. However, when our corrector was added, this optimal angle was increased to 12 mrad. This correction reduced the probe size from 183 pm to 124 pm, thus enabling to resolve details that could not be observed with the uncorrected microscope. For example, the silicon dumbbells, separated by 136 pm could be resolved only when the corrector was used [3].
The method we present can be further extended to correct other types of aberrations and in addition to probe correction, it may also be also used for image correction.
[1] R. Shiloh, Y. Lereah, Y. Lilach and A. Arie , “Sculpturing the electron wave-function using nanoscale phase masks”, Ultramicroscopy 144, 26-31 (2014).
[2] R. Shiloh, R. Remez and A. Arie, “Prospects for electron beam aberration correction using sculpted phase masks”, Ultramicroscopy 163, 69-74, (2016).
[3] R. Shiloh, R.Remez, P.-H. Lu, L. Jin, Y. Lereah, A. H. Tavabi, R. E. Dunin-Borkowski and A. Arie, “Spherical aberration correction in a scanning transmission electron microscope using a sculpted foil”, Ultramicroscopy 189, 46-53 (2018).
