ESCA Microscopy:
The First Spectromicroscopy Beamline
Operating at ELETTRA
M. Marsi, L. Casalis, L. Gregoratti, S. Günther, A. Kolmakov, J. Kovac,
D. Lonza, M. Kiskinova
Sincrotrone Trieste, Padriciano 99, I-34012 Trieste, Italy
Abstract. We present ESCA Microscopy, the first X-ray microscopy beamline
operating at ELETTRA. ESCA Microscopy is a scanning photoemission
microscope, based on the use of a Fresnel zone plate to demagnify to a
submicron spot the monochromatized photon beam emitted by an undulator in
the 250–1000 eV energy range. We provide a description of the facility, and a
brief overview of the scientific activity during its first year of operation; ESCA
Microscopy was used by several groups for various experiments in materials
science, in particular studies of multiphase surfaces and interfaces.
1 Introduction
ELETTRA, the synchrotron radiation source commissioned in Trieste (Italy) in 1993,
is today the brightest source of soft X-rays in Europe. This third generation machine,
whose main features are very low emittance and high brightness, offers unprecedented
opportunities for the field of X-ray microscopy. In fact, several ports have been
assigned to spectromicroscopy beamlines, that will exploit with different approaches
and in different photon energy ranges the light emitted by ELETTRA.
We present here some recent results from the ESCA Microscopy beamline, a
scanning X-ray microscope which is the first operational spectromicroscopy facility in
Trieste; it was commissioned in 1995, thanks to a joint effort between Sincrotrone
Trieste and ENI Ricerche. Thanks to the excellent characteristics of the source, ESCA
Microscopy operates now with 150 nm lateral resolution in photoemission mode,
detecting photoelectrons with 0.3–0.5 eV resolution and with data acquisition times
that make it a competitive instrument for surface science applications. The operation
of the instrument in transmission mode is also possible in the 250–1000 eV photon
energy range.
After describing the experimental setup, we will discuss in more detail a series of
experiments concerning the study of the interface between silicon and noble metal
binary layers, which showed the effects of the interplay between alloying and islanding
on the local electronic structure of these semiconductor interfaces. This is one of the
many scientific projects which are being carried out at this facility, proposed in
different domains by researchers from all over Europe.
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2 Experimental Setup
ESCA Microscopy is a scanning photoemission microscope, based on the use of
Fresnel zone plates to demagnify the X-ray beam [1,3]; similarly to other scanning
instruments [4], it has the advantage of decoupling in this way the energy resolution of
the photoelectrons from the lateral resolution, obtained only with the photon beam.
The main components of the instrument, that we will briefly describe in the next
paragraphs, are: the undulator, a spherical grating monochromator, the zone plate, a
sample scanning stage and a photoelectron multichannel analyser.
ESCA Microscopy is installed on an undulator port on the 2 GeV ELETTRA
storage ring. The X-rays are generated by the U5.6 undulator, a 4.5 m long insertion
device with 81 periods (5.6 cm per period), that produces coherent light in the 100–
1500 eV photon energy range [5]. The photons are then dispersed by a high troughput
spherical grating monochromator, providing monochromatic photons in the 200-1000
eV range; the measured resolving power of the monochromator is above 3000 in
normal operation conditions at the Ar L2,3 edge (244 eV) [6].
The heart of the instrument is the focusing optical system, consisting of a Fresnel
zone plate (ZP) and of a pinhole which acts as an order selecting aperture (OSA) to cut
the unwanted diffraction orders. The ZP's have been provided by the IESS-CNR
(Rome) [7]; the ones we used until now have a 100 nm outermost zone, which allowed
us to obtain 150 nm resolution in photoemission mode. Both optical elements can be
moved along the three axes by means of inchworm motors, in order to make their
precise alignement possible.
The microspot is focused on the sample, which is mounted on a scanning stage
where both mechnical stepper motors and piezoelectric positioners are used to obtain,
respectively, coarse (> 1 µm) and fine (down to 5 nm) movements. This makes it
possible to select specific areas of the sample to be studied with the focused photon
beam, and to obtain two dimensional photoemission maps by scanning the specimen in
the xy plane, such as those shown in Fig.1.
Fig. 1. Chemical maps of Au/Ag/Si(111) obtained by scanning the sample with the electron
analyser tuned to energies corresponding to the Ag 3d (left) and Au4f core level (right).
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A multichannel detection hemispherical analyser with 30º acceptance angle collects
the photoelectrons from the sample, which makes it possible to detect photoemission
spectra from the microspot and to obtain chemical maps when scanning the specimen.
A photodiode placed behind the scanning stage to collect transmitted photons, so that
switching between photoemission and transmission mode is very easy (the two
detection modes can be actually performed simultaneously). A sample preparation
chamber, equipped with basic surface science tools such as LEED, AES and an ion
sputtering system, is UHV connected to the microscope vessel. This enables sample
preparation and basic characterization before its transfer to the microscope chamber.
During normal experimental conditions, a flux of 109-1010 photons/second is
conveyed onto the focus spot, the photoemission yield of core levels such as Si2p on
Si(111)7x7 is of the order of 1-10 KHz with 0.3-05 eV resolution, and the acquisition
time of chemical maps such as those in Fig. 1 (64x64 pixels) is about 5-10 minutes.
For experiments in domains like surface science, where the relatively short lifetime of
the sample under study is a problem, having reached these acquisition times means
that photoemission microscopy is no longer only an interesting novelty of great
potential, but an efficient and competitive technique.
3 Research Activity
What are the scientific challenges for ESCA Microscopy? During its first year of
operation, they were mainly directed at providing new information on old problems in
surface science. In particular, the full power of photoemission spectroscopy was used
to help understand systems that, although studied for a long time, were not well known
at the submicron level.
For instance, the study of noble metals deposited on graphite required the
submicron spatial resolution to study the electronic structure of large individual
clusters [8]. Processes that are of fundamental importance in technology, such as
oxidation, are very sensitive to the microscopic structure of the materials: a series of
experiments are being performed at ESCA Microscopy to understand the effects of
grain boundaries in the oxidation of metals (Pb, Sn) [9]. A problem of technological
relevance, as well as fundamental interest, is also understanding the interaction
between active phase and oxide support for supported catalysts; photoemission
spectroscopy can provide very useful information to ascertain whether the support is
chemically modified and how the preparation procedure determines the final chemical
composition of the active phase, but due to the fact that supported catalysts are
particles with size up to hundreds of nanometers, it must be performed with submicron
lateral resolution. Measurements are currently under way to understand the chemical
interaction between the MoO3 supported catalysts and typical supports such as TiO2 or
Al2O3 [10].
In general, ESCA Microscopy is a very valuable research tool when many
chemical species and more than one phase are simultaneously present at a surface. The
interface between Si and binary metal layers (Ag and Au) is a prototype system in that
respect. Although many different techniques were used to study these systems, only
photoemission microscopy made it possible to unambiguously identify the electronic
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structure of the different phases present on the surface, thanks to the capability of
performing ESCA on microspots. We studied two mirror systems, Ag/Au/Si(111) and
Au/Ag/Si(111) [11]. In both cases, after deposition of few layers of the first metal an
annealing procedure produced an interface with two distinct phases, with different
structure and electronic properties: one is the simple √3x√3–R30o structure, where a
single ordered layer of metal atoms covers almost the entire surface, with the
exception of submicron sized metal agglomerates that form the second phase. This
interface represents a biphase substrate for the deposition of the other metal, that
shows a markedly different behavior on the two phases and that was studied
extensively in function of coverage and subsequent annealing temperature: in Fig. 1,
the Ag metal agglomerates on a Au/Ag/Si(111) surface are clearly visible as white
areas in the left image. To demonstrate what information we can get with ESCA
Microscopy, Fig. 2 shows the photoelectron spectra taken after Au was deposited on
the metallic Ag islands (a), and on the √3Ag-Si flat portion of the surface (b). If in the
images one can appreciate the spatial resolution of the instrument, in these spectra we
can see how the electron energy resolution (0.4 eV in this case) makes it possible to
identify shifted components in the core level photoemission yield, which are related to
the different chemical status of the various atomic species present in a portion of the
surface of submicron dimensions.
Fig. 2. Photoelectron energy distribution curves from two different microspots of a
Au/Ag/Si(111) interface. The top spectra (a) were taken on a Ag island, (b) on the flat √3 area.
On the metallic islands the Ag3d core level shows a metallic component, where as
on the √3 area Ag is present in a chemical state that indicates a bonding formed with
Si. The Au4f level, instead, shows again the presence of AuSi reacted species on the
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√3 flat area, and two components (the same reacted one, plus a metallic one),
indicative of a mixed chemical species, on the Ag islands. We would like to stress that
the chemical shift between the two Au components is only 0.9 eV, and that they are
very clearly resolved in our microspot spectra. Spectra like these ones, obtained for
different Au coverages, allowed us to understand that the Au deposited on the √3-Ag
phase produces a new √3 reconstruction, while it creates an alloy with Ag and a strong
bonding with a preexisting Si skin on the islands [11].
These examples clearly show the actual capabilities of ESCA Microscopy; it is
natural to foresee that in the future there will be applications also in the study of
surfaces where patterns on the submicron scale have been intentionally fabricated, i.e.
ESCA Microscopy will be not only a novel technique to provide new information on
old problems, but really a much needed new eye to look at the electronic properties of
artificial nanostructures.
Acknowledgements
ESCA Microscopy is the result of an intense collaborative effort involving many
groups. We would like to thank in particular S. Contarini, L. DeAngelis, C. Gariazzo,
P. Nataletti, N. Minnaja and G. Perego from ENI Ricerche; M. Gentili, M. Baciocchi
and P. DeGasperis from IESS (CNR-Rome); and our collegues from Sincrotrone
Trieste, namely P. Melpignano, D. Morris, R. Rosei, A. Savoia, G. Margaritondo,
A. Abrami, F. DeBona, A. Gambitta, C. Fava, W. Jark, G. Loda, F. Mazzolini,
R. Krempaska, R. Pugliese, F. Radovcic, G. Sandrin and F.-Q. Wei. Special thanks are
due to G. Morrison for many illuminating discussions and for his contribution during
the commissioning of the microscope.
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