We report recent progress in the construction of a new aberration-corrected photoemission electron microscope. The correction element in this instrument is a hyperbolic electron mirror, correcting for chromatic and spherical aberration. We present results from a numerical simulation combining a trajectory and wave-optical analysis, and we discuss the calculation of the modulation transfer function in this approach. First images on nano-structured metal and semiconductor surfaces have been obtained with the new instrument and are also presented.

Experimental and numerical results for an aberration-corrected photoemission electron microscope

http://ieeexplore.ieee.org/iel5/11219/36119/01717160.pdf

The spherical and chromatic aberrations of a converging electron mirror are of opposite sign from those of electron lenses. This important property makes it possible in principle to compensate the aberrations of electron lenses by means of an electron mirror and to design electron microscopes based on a corrected optics system incorporating an electron mirror. In this paper the properties of the hyperbolic electron mirror are calculated, and the conditions for simultaneous correction of spherical and chromatic aberrations are worked out for several types of electron microscopes. The hyperbolic mirror field is a rotationally symmetric potential field between two electrodes. The electrodes are shaped as equipotential surfaces of the hyperbolic field, except for an aperture on the axis of the positive electrode for entrance and exit of electrons. The effect of the aperture is to create a thin diverging aperture lens at the termination of the hyperbolic field. The properties of the mirror are calculated analytically. The problem of separating the electron beam incident on the mirror from the beam returning from the mirror without impairing the image quality is solved by means of magnetic deflecting fields located at image planes. The mirror corrections can be applied to either magnetic or electrostatic lenses. The parameters for correction of aberrations are calculated for systems using electrostatic lenses. With appropriate polarity of the accelerating voltage and the lens and mirror voltages the calculations apply to ion imaging systems as well.

A theoretical study of the hyperbolic electron mirror as a correcting element for spherical and chromatic aberration in electron optics

All lenses, whether for light or for electrons, have aberrations that limit their performance. In light optics the invention of the achromat over 200 years ago solved the problem of correcting spherical and chromatic aberrations; however, correcting aberrations of electron lenses remains a challenge. Spherical and chromatic aberrations constitute a fundamental barrier to improving resolution and reducing probe size in various types of electron-optical instruments. Recently, efforts have intensified to reduce probe size in microanalysis and lithography, and to improve resolution in the new emission microscopes being built for synchrotron light sources. The experiments described in this report show how an electron mirror can achieve simultaneous correction of spherical and chromatic aberrations.

Experimental and numerical results for an aberration-corrected photoemission electron microscope

References

A theoretical study of the hyperbolic electron mirror as a correcting element for spherical and chromatic aberration in electron optics

Simultaneous Correction of Spherical and Chromatic Aberrations with an Electron Mirror: An Electron Optical Achromat

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