ESG Experimental Systems Group
Tutorial - PEEM technique
Photoemission Electron Microscopes (PEEM's) record electrons emitted from a sample in response to the absoprtion of ionizing radiation. The electrons are accelerated by a strong electric field between the sample and the outer electrode of the objective lens, and the image is magnified hundred- or thousand-fold by a series magnetic or electrostatic electron lenses. An electron-sensitive detector records the electron emission.
The first working photoemission electron microscope was built by Brüche in 1932 using ultraviolet (UV) light to image photoelectrons emitted from a metal. The principal design of his PEEM is still used.G. Rempfer, E. Bauer, G. Schoenhense, and B. Tonner besides others developed improved electron optics in the 80s and 90s, which are simular to the optics of Low Energy Electron Microscopes (LEEM's). Today a a spatial resolution below 10 nm can be routinely achieved using UV light and of several 10 nm using X-rays. Aberration correction schemes are about to improve the resolution down to a a few nanometers close to the physical limit of emission microscopies: the mean free path of low energy electrons.
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 hace a much larger energy spread compared to UV excitation.
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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.
The calculated resolution of PEEM-2 and PEEM-3 are compared on the left as function of the transmission of the electron optics, which can be adjusted by the size of the backfocal plane aperture. Using the smallest availabe aperture the transmission of PEEM-2 and PEEM-3 lies at several percent. Without aberration correction PEEM-2 can achieve at best a resolution of a few 10 nanometers while aberration correction improves the resolution by about one order of magnitude down to about 5 nanomaters. Even at high tranmission (large or no aperture) aberration correction produces a resolution well below 100 nanometers. This will greatly improve the ability of the PEEM technique to work with radiation sensitive samples, e.g., polymers.
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PEEM-3 is currently being commissioned at beamline 11.0.1 of the Advanced Light Source. It is available to users without aberration correction, which will be installed in 2008.