Electron microscopy lens aberrations

The resolution in TEM is limited by lens aberrations. In contrast to optical microscopy, where by serially ordering concave and convex lenses, aberrations can be compensated and hence the wavelength of the radiation is resolution limiting in TEM lens aberrations cannot be compensated since concave electron lenses are not feasible. The objective lens is the crucial part for image defining the microscope s resolution. The quality of the objective lens is described by the constants of spherical Cs(" 0.5-3mm) and chromatic aberration Cc ( 1-2 mm). Recently, microscopes equipped with complex correctors for the spherical aberration have become available. ... 

A very important reeent advanee in eleetron microscope development is in the correction of lens aberration. From the very start of electron microscopy development, the importance of lens aberration as a limiting factor of the spatial resolution of the electron microscope has been recognised [3.41, 3.42]. Ways to correct these aberrations were proposed a long time ago and put into actual microscopes with partial success at least about 30 years ago [3.43]. The main idea is to compensate the positive spherieal aberration of the round lens with the negative aberration of multipole lenses. 

Three factors may limit the resolution of an image the diffraction limit, lens aberrations and noise. Noise can be a problem in scanning microscopy. A very bright source is required if rapid, TV scan rates are to be used at high resolution. In CTEM noise is a problem for radiation sensitive polymers (Section 3.4.4) because a limited number of electrons can be used to form the image. 

Aberration correction using exit wave restoration or commercial aberration correcting lenses generally only compensate the coherent objective lens aberrations up to either third or fifth order spherical aberration. For these microscopes the information limit resolution is no longer determined by the coherent objective lens aberrations but by chromatic aberrations coupled to the stability of the electron source, lens current and power supply. Correction of the remaining chromatic aberration is seen by many as the next step in the future of high-resolution electron microscopy. ... 

Chromatic aberration is limited by using monochromatic radiation, that is, electrons which all have the same energy. Modern microscopes have highly stable accelerating voltage and lens currents, but there is an intrinsic energy spread of about 1 eV in electrons leaving the source. The chromatic limit to resolution due to this is only 0.2 nm at lOOkeV, not important as a limit for polymer microscopy. 

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