Progress on PEEM3 - An Aberration Corrected X-Ray PhotoemissionElectron Microscope at the ALS Page: 3 of 4
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the lines of the SMART type separator concept . Here the separator is designed to be aberration free to high order
and therefore imposes severe electron optical and engineering challenges. A detailed design was worked out 
and is very similar to the SMART design being a single magnetic unit wound such that there are 8 dipoles on each
pass that employ only edge focusing. The performance of this design depends critically on stringent machining
tolerances and is essentially a non-tunable design. By relaxing the chromatic aberration condition for the separator
we are now proposing to use, the engineering tolerances are more reasonable and it is also tunable.
The transfer optics and projection system are used to magnify the intermediate image at the exit plane of the
magnetic separator onto the CCD detector without distortion. The optical properties of this type of lens with
different geometries of individual electrodes have been studied in great detail by Rempfer . Based on this work,
the transfer and projection system of PEEM3 consists of four electrostatic uni-potential lenses with the last two
lenses having a larger aperture than the first two.
The resolution of the system is dependant on the aberrations of electron optics, the energy and angular spread of
the initial electrons and diffraction effects due to the wave nature of the electron. This has been modeled  with
the resolution defined as the rise-distance of 15% - 85% in intensity of the edge scanning the electron point-spread
function. The resolution for 100% transmission is 50nm with the mirror corrector - a significant improvement from
that of 440nm without correction. The best predicted resolution is 5nm at 2% transmission, as opposed to 20nm at
0.5% transmission for PEEM2.
PROGRESS WITH PEEM3 CONSTRUCTION
The engineering requirements for this instrument are quite demanding. The magnetic shielding requirements are
stringent given that the electrons have very low energy at the sample and electron mirror. It is however the long
travel length (1950mm) of the 20KeV electrons that drives the specified AC magnetic field down to a required value
of <2x10-7 Gauss. In the area where the instrument is to be located, AC field magnitudes of +-2.5 x10-4 Gauss with a
time period of -1Hz are measured. The source is most likely the injector booster for the main storage ring. Triple
mu-metal shielding is used to reduce the fields to the required level. The entire assembly is to be mounted on a
single electron optical table that sits on an epoxy granite table with visco-elastic sheet dampers to reduce floor
vibrations. The mu-metal shields and vacuum tank are supported separately and wrap around the electron optical
table connected only via vacuum bellows. Ultra High Vacuum (UHV) is required for the sample and electron mirror
region to reduce surface contamination. Rather than try to evacuate the entire rather large instrument to UHV we
have adopted localized UHV sections with the electrons passing in and out of them via small conductance limited
holes. The power supplies that supply voltage to the lenses are required to be very stable. An analysis indicated that
the electron optics in regions where the electrons have low energy require <+-2 parts per million (ppm) stability.
Power supplies for the sample, objective lens and the mirror voltages have been custom fabricated.
Constraints imposed by the project complexity and funding have required that the instrument be deployed in two
stages. The first stage is in the PEEM2 mode. The schematic of the electron optics is shown in Figure 2.
Sample Objective Field Lens 1 Transfer Intermediate Projector
00v -1 360 -8530v Lenses Lensl Lens2 Lens1 Lens2
-1082v -11841v -18440v -19540v -19371v
\ dodecapole Irnage plane Various sizes
FIGURE 2. Schematic layout of the electrostatic lenses for the PEEM2 (stage 1) mode of the microscope buildup
Here the separator has been replaced with a two-lens (4f) image transfer system. The image from the objective
section is relayed to the transfer and projection section. The left panel of Figure 3 shows the instrument undergoing
the final assembly. The electron column is visible along with the sample manipulator system mounted in the UHV
sub chamber and surrounded by the triple mu-metal shields. The right panel of Figure 3 shows a closer view of the
UHV non-magnetic sample manipulator and objective lens column. The manipulator is a novel device based on
flexures that allows for 5 degrees of freedom of the sample. The degree of freedom missing is rotation of the sample
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MacDowell, Alastair A.; Feng, J.; DeMello, A.; Doran, A.; Duarte,R.; Forest, E. et al. Progress on PEEM3 - An Aberration Corrected X-Ray PhotoemissionElectron Microscope at the ALS, article, May 20, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc889767/m1/3/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.