PEEM tutorial

When X-rays are absorbed by matter, electrons are excited from core levels into unoccupied states, leaving empty core states. Secondary electrons are generated by the decay of the core hole. Auger processes and inelastic electron scattering create a cascade of low-energy electrons of which some penetrate the sample surface, escape into vacuum and are collected by the PEEM optics. A wide spectrum of electrons is emitted with energies between the energy of the illumination and the work function of the sample. This wide electron distribution is the principal source of image aberration in the microscope since electron lenses are chromatic. PEEM is a surface sensitive technique since the emitted electron originate from a very shallow layer although x-rays penetrate much deeper into the material. Most of the signal is generated in the top 2-5 nanometers.

PEEM-2, the 2nd generation microscope at the Advanced Light Source is a conventional not aberration-corrected instrument employing electrostatic lenses. A voltage of between 15 kV and 20 kV accelerates the photoemitted electrons from the sample. The objective lens and transfer lens produce an intermediary image behind a backfocal plane aperture, which is then magnified by two projector lenses. Spatial resolution and transmission (efficiency) of the electron optics van be varied using different backfocal plane apertures with sizes between 15 mm and 50 mm. A cooled charge-coupled device (CCD), fiber-coupled to a phosphor detects the electron-optical image.

Chromatic (electrons with different speed) and spherical aberrations (electrons at different angle) lead to blurring of the image. The hyperbolic field of a curved electron mirror can in principle undo the effect of both types of aberrations in a Photoemission Electron Microscope. This is the idea of aberration-correction, which has been successfully employed in light microscopes and transmission electron microscopes (using different methods). Chromatic aberrations dominate using X-rays because the emitted electrons have a much larger energy spread compared to UV excitation.

The aberration corrected microscope PEEM-3 employs a curved electron mirror to counter the lowest order aberrations of the electron lenses and the accelerating field.A dipole separator magnet directs the electron beam into the mirror and back into the projector optics of the microscope. Four mirror electrodes allows us to fine-tune spherical and chromatic aberration correction and magnification (-1) of the mirror. Backfocal plane apertures between 10 µm and 50 µm can be chosen to optimize resolution and transmission. A three-lens projector optics produce a total optical magnification between 300 and 10000. Electrostatic and magnetic deflectors are used for beam-stearing and shaping.