Introduction to Spherical Aberration Correction in Electron Microscopes


Microscopy was revolutionised in the 1930s by using electrons to illuminate specimens, creating images at much higher magnifications than optical microscopes allowed. But electron lenses are inherently poor compared to optical lenses and soon the design of electron microscopes had improved to the state where the performance was mainly limited by spherical aberration Csa feature of all round lenses that causes image distortion and limits the resolution.

In a perfect lens all rays that emerge from a point object P are brought to focus at a conjugate point P´ in the image plane. The image is therefore a true representation of the object being imaged. 

In reality high-angle rays (the solid lines in the figure) are brought to a premature focus by lens aberrations so that the point object P appears blurred in the image plane. This phenomenon is called spherical aberration

Although proposals to correct spherical aberration were made as early as 1947, the realisation of spherical aberration correction kept physicists busy for another 50 years. Among the first people to show that spherical aberration could be overcome were Ondrej Krivanek and Nicklas Dellby at Cambridge working with Mick Brown in the Microstructural Physics group in 1997 

(First working quadrupole/octupole probe Cs corrector:  Krivanek O.L., Dellby N., Spence A.J., Camps R.A., and Brown L.M. (1997) “Aberration correction in the STEM”, in: Inst. Phys. Conf. Ser. 153 (Proceedings 1997 EMAG meeting) Ed. Rodenburg JM, 35)

The idea behind the current spherical aberration corrected electron microscopes is to introduce a corrector that produces negative spherical aberration. This then combines with the positive aberration of the objective lens to give a total of zero spherical aberration. There are two approaches both using multipole lenses: the so-called quadrupole-octupole (QO) corrector and the hexapole corrector.
Fig1: Multipole lenses. The z-axis, along which the electrons travel, is into the page. The geometry of the field lines is sketched in on the quadrupole.

The SuperSTEM microscopes are fitted with Nion QO correctors. The image aberration measured in terms of shifts in beam offsets, tilts and changing of defocus is recorded and translated into computer-controlled adjustment of all the quadrupole and octupole currents - in effect introducing the negative of the microscope’s aberration. 


Fig2: Schematic trajectories of axial rays in x and y through the QO aberration corrector (z is along the optic axis). C2 is the final condenser lens and OL is the objective lens. The beam cross section is shown below at four places through the corrector.


In an aberration corrected microscope all rays are more or less brought to a common focus. This results in a sharper image. 

The SuperSTEM microscopes achieve a resolution of 1Å or better; that's a millionth of the size of a single human hair! 

With aberration correction all electrons are focused within the region of interest, which in our case is the size of a single atom, so that it is possible to determine not just how individual atoms are brought together to form a bulk solid, but the chemical identity of those atoms as well.

 


A more detailed discussion of aberration correction and its history can be found here:

Aberration correction past and present (Phil. Trans. R. Soc. A 28 September 2009 vol. 367)

Imaging at the picoscale (materialstodayVolume 7, Issue 12, December 2004, Pages 42-48)

SuperSTEM the highest resolution microscope in the world (azom.com, 2003)






 website contact: dorothea@superstem.org