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.
|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.
|PEEM-3 is available to users
at beamline 11.0.1 of the Advanced Light Source. The microscope is at the moment in operation without aberration correction. An aberration correction upgrade is planned for the future.