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Low energy electron microscopy (LEEM) (Fig.1)utilized low energy,
elastically backscattered electrons for surface imaging at high spatial and temporal
resolution. It mains strength against other surface imaging techniques is that it
can acquire images at real time, under extreme conditions, and with its unique contrast
mechanism, make it favorable in studying dynamic and static processes, such as growth
and decay, segregation, phase transition, structure, morphology and magnetic microstructure
and so on.
LEEM uses a coherent monochromatic electron source. The electrons beam is accelerated
to the microscope potential, typically at 20keV, passed through a series of lens
and deflectors and form a micrometer size, parallel beam hitting the sample.
Two major differences from conventional electron microscope is that LEEM uses cathode
lens as objectives in which the sample is acting as one of the electrode, and the
incident and imaging beams will pass through the same objectives twice, therefore
a beam separate is needed to separate the two beams. The separator basically utilizes
a magnetic field, where electron beams passing through the field will be deflected
by the Lorentz force. There are different beam separator configurations and it determines
the physical geometric configuration of the LEEM. Different configurations also
lead to different degrees of astigmatism and other aberrations, which can be compensated
by other elements. Figure 2 shows a typical cathode lens setup.
After going through the beam separator, the incident beam (red) passes through the
objectives lens, being decelerated and focused onto the sample. The energy of the
electrons before hitting the sample is in the order of 1-100eV. For crystalline
sample, the backscattering electrons will be confined to several specific angles,
the Bragg angles, by diffractions. The beams are reaccelerated by the objective
lens and focused at the back focal plane, where a diffraction pattern is formed.
The beams are deflected by the beam separator and enter the imaging column, where
an aperture can be inserted to select one of the diffraction spots and more lens
and deflectors are used to form the image at a microchannel plate image intensifier.
The image is then picked up by a phosphor screen and a camera.
Spatial resolution of LEEM is limited by the diffraction at the aperture and the
spherical and chromatic aberration of the beam. Calculations show that with ideal
homogeneous accelerating field, LEEM resolution can reach as high as 3nm. The homogeneous
field of a real objective lens is less than ideal. The magnetic objective lens used
in HKUST gives a resolution of 5-7nm depending on the incident energy. Perpendicular
to the surface, atomic height resolution can be achieved by phase contrast mechanism,
which is described below.
Temporal resolution is LEEM is at video rate or higher, depended on the reflectivity
of the sample and the sensitivity and frame rate of the camera in use. LEEM has
high frame rates because it is a non-scanning technique. Electrons reach different
part of sample at the same time and form the image simultaneously. Electrons at
low energy have very high reflectivity. At the incident energy of 10eV, the reflectivity
can get as high as 50%. With the fact that diffraction beams are confined in a very
narrow cone, by picking up the highly reflected diffraction beam we can image at
very high rate. Even at higher incident energy, where the reflectivity is much lower,
we can still image at a slightly lower rate by employing a CCD with long exposure
and low background noise.
As LEEM is an imaging technique based on (Low energy electron diffraction) LEED,
the most obvious contrast mechanism will be coming from the local differences in
diffraction conditions. This arises when there is a periodic local difference due
to local strain, or large scale geometric structure such as different structural
phases. For example, by using the specularly reflected electrons, which is called
bright field imaging, we can observe contrast between the (7x7) reconstruction and
(1x1) structure on Si(111) surface (Fig. 3). In case where
several azimuthally rotated domains structure are found, such as the p(2x1) on the
Si(100) surface, a bright field image will not be able to distinguish between different
domains, a slightly tilted bright field image can be used (Fig. 4),
as the reflected intensity depends strongly on the incident angle of the beam. Furthermore,
contrast can also found by using non-specular diffraction beams and it is called
dark field imaging. It is mainly used when we need to spatially separate different
crystalline phases when small tilt in the bright field image is insufficient to
give enough contrast. One example is in Cr oxide (Fig. 5),
where the (0 ½) spots are used and different part of the complex oxide structure
light up when the spots which the domains represent are selected.
A second contrast mechanism is the phase contrast originated from the interference
between electrons beam waves. One of them is the atomic step contrast, which enable
the LEEM to resolve single atomic height step (Fig. 6). The
contrast comes from the path length difference between the beams reflected from
different surface layers. At low energy, the wave length of electron is comparable
to the length of single atomic height, therefore the phase shift varies greatly
at small change in incident energy and give raise to different interference conditions.
A wave-optical model for LEEM step contrast has been developed. Step contrast is
calculated as the interference of the Fresnel diffracted waves from the terrace
edges which meet at a step. This model allows for the routine identification of
the step sense, i.e., the up and down-sides of a step, by simple visual inspection.
Quantum size contrast is another contrast mechanism that utilizes beam interference.
It is the manifest of the quantum size effect (QSE) in electron reflectivity. Electrons
transmission through the thin film experiences different phase shifts in according
to their energy and the path length they have traveled. The electrons beam reflected
from the interface interferences with beam reflected from the surface of the thin
film. The amplitude and phase of the interference beam modulate with energy and
film thickness and therefore showing contrast between film areas of different thickness
(Fig. 7).
LEEM is an ultra high vacuum instrument. Normal operates at below 1e-10 torr but
can go as high as 1e-6 torr. The pressure is mainly limited by the possible high
voltage discharge between the objective lens and the sample under high pressure
condition. High pressure also limits sample cleanliness, which is important in surface
study and therefore is usually avoided. LEEM can also works under extreme temperature.
Special cooling stage allows cooling with liquid Helium and achieves a temperature
as low as 50K. At the other extreme, sample can be flashed up to 2000K while continue
imaging is attainable at up to 1500K. Imaging is also possible during deposition
of metals and semiconductor or during gas exposure, which make it an extreme valuable
tool in the study of the dynamic processes.
SPLEEM (Fig. 8) is a special version of LEEM. Beside normal
diffraction, phase and topography contrast, it also has magnetic contrast while
preserving the bread and butter of LEEM, its fast imaging rate.
Magnetic domain imaging is possible in SPLEEM as it exploits the spin polarized
dependent on electron reflectivity. The main difference between a conventional LEEM
and the SPLEEM is that SPLEEM employs a spin polarized electron source and in order
to preserve the spin orientation of the electron beam, there is no magnetic lens
in the system. Spin dependent reflectivity originates from the exchange interaction
between the incident electrons and the spin-aligned electrons in the ferromagnetic
samples. Images of different spins can be recorded in real time and the difference
between the images provides a high contrast image of magnetic microstructure at
very fast rate (Fig 9). With a special polarization manipulator,
spin electrons of any azimuth and polar angles can be obtained and therefore magnetic
domain of both in-plane and out-of-plane magnetization can be visualized.
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Fig. 1 Low energy electron microscope in the Hong Kong
University of Science and Technology.
Fig.2 Schematic description of electron
trajectory in the LEEM objective lens.
Fig.3 Diffraction contrast in LEEM bright
field imaging between clean Si(111) (7x7) (dark areas) and Si(111)-In (bright areas)
surface reconstructions. Field of view of this image is 6μm.
Fig.4 Tilted bright field imaging of
Si(100) surface. With (2x1) and (1x2) terraces appearing black and white respectively.
Field of view of this image is 6μm.
Fig.5 Dark field images of Cr oxide grown
on the W(100) surface, and the corresponding diffraction pattern.
Fig.6 LEEM Step phase contrast on Si(111)
(7x7) surface at the imaging energy of 42.5eV and a field of view of 6μm.
Fig.7 Quantum size phase contrast in
a Cu film on the W(110) surface at imaging energy of 8.4eV and a field of view of
6μm.
Fig.8 A spin polarized low energy
electron microscope at the National Center of Electron Microscopy, Lawrence Berkeley
Laboratory of California.
Fig.9 SPLEEM magnetic domain image of a Fe film on the Cu(100)
surface with out of plane magnetization in which domains pointing into and out of
the plane appears black and white respectively. Field of view of this image is 10μm.
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