The X-Ray Microscopy Facility Project at the ESRF
Jean Susini and Ray Barrett
European Synchrotron Radiation Facility,
BP220, F-38043 Grenoble cedex, France
Abstract. A beamline dedicated to X-ray imaging and spectro-microscopy in the 0.26.0keV energy range is under construction at the ESRF. This beamline is installed on a
low beta straight section equipped with three phaseable undulators and will consist of
two branch lines: the first, a scanning microscope including various contrast modes;
the second, an imaging full-field microscope using Zernike phase contrast. The
scanning microscope will be equipped with 2 fixed-exit high resolution
monochromators (crystal/multilayer monochromator and plane grating monochromator). Both microscopes will use high resolution zone plates as focusing
elements. The design of this beamline is discussed with emphasis on the optical layout
and on heat load management.
1 Introduction
Third generation synchrotron sources produce a beam of unprecedented quality: the
extremely low emittance coupled with high brilliance together with the versatility of
new insertion devices, offer the capability to control brightness, spectrum,
polarisation, coherence and size of the beam. This means that X-ray microscopy
techniques which have been extensively used in the soft X-ray region [1] can now be
extended, with the anticipation of very high performance to higher photon energies.
This will enable new investigations: study of thicker specimens, access to K
absorption edges of elements of major interest in the biological and materials
sciences, in particular from Potassium to Chromium, access to M and L edges of
heavy metals (i.e. Au or Ag) for specimen labelling, and the use of X-ray
fluorescence for trace element mapping.
To exploit these characteristics an X-ray microscopy beamline is being built
at the European Synchrotron Radiation Facility, whose source properties offer the
required coherence and brilliance [2]. Approved in 1993, the project started at the
end of 1994, the first beam will be delivered in December 1996 and the beamline
will be open to external users by the end of 1997. Scanning and imaging microscopes
will be built on two independent branch lines. In parallel, collaboration contracts
have been placed between the ESRF and two European laboratories for the
fabrication of high energy zone plates [3–6].
After a brief overview of the ESRF source properties, the conceptual design
of the X-ray microscopy beamline will be presented. The problem of building a soft
X-ray beamline which should cover an energy range (0.3–6 keV) on a high energy
machine will be addressed. Finally, the optical layout will be described in more
detail.
2 The Source
The ESRF is a high energy machine with an electron energy of 6.02 GeV and
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emittances (1% coupling) of εh = 4 x 10 mrad (horizontal) and εv= 4 x 10 mrad
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J. Susini and R. Barrett
(vertical). Currently, two types of straight sections, low or high β, are available. The
considerations taken into account in the choice of the source type are summarised in
Table 1.
Table 1. Considerations involved in choice of the straight section.
High β
Straight section
Low β
Optical
functions
β =27.0m, β =13.3m
h
v
β =0.58m, β =3.14m
h
v
Source
parameters
σ =330µm, σ =23µm
h
v
σ’ =12.1µrad,σ’ =7.5µrad
h
v
- Total power can be
limited by the use of an
aperture.
- Symmetric beam
- Requires small optics.
σ =59µm, σ =11µm
h
v
σ’ =83µrad, σ’ =8.2µrad
h
v
- Better phase-space matching
- Wider beam @ 30m
∅lower power density
Pros
∅possibility for simultaneous operation of
the two branch lines
- Asymmetric beam
- Requires longer horiz. deflecting optics
- Phase-space mismatch
- High power density
Cons
5 10 19
On-axis spetral brilliance (ph/s/mm 2 /mrad 2 /0.1%bw)
N = 20
4 10
K=4
2100W
19
N = 60
K=1.5
887W
K=2.0
1580W
K=1.0
394W
K=1.8
n=3
K=0.8
252W
K=1.5
n=3
3 10 19
K=5.5
4000W
K=0.6
142W
2 10 19
x5
K=0.4
63W
x5
1 10 19
x5
x5
0
0
1000
2000
3000
4000
5000
6000
7000
Energy (eV)
Fig. 1. On-axis spectral brightness vs. energy for various K values.
The integrated total power emitted by the IDs is given.
The X-Ray Microscopy Facility Project at the ESRF
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A number of imaging experiments require at least partially coherent illumination.
Therefore, the main criterion for the choice of the source is the flux per coherent
phase-space volume which can also be expressed as the number of wave modes
contained in the photon beam. The low beta straight section, ID21, of the ESRF
storage ring has been chosen as the source best suited for our applications due to the
higher phase-space density (proportional to λ/σ.σ’). The principle disadvantage of
this source is the large horizontal beam divergence which gives rise to a very flat
elliptical beam shape which does not match the spherical acceptance of the zoneplate.
The ID21 straight section is 4.8m long and is equipped with three identical
1.6m long, 80mm period, phaseable insertion devices (ID) allowing the number of
poles to be changed between 20, 40 and 60. The first harmonic is tuneable from 0.2
to greater than 4.5 keV corresponding to a gap range from 15 to 80mm. Due to the
flexibility of the ESRF strategy of constructing undulators in three independent
sections, the total emitted power can be maintained at a level which can be easily
handled by the first mirror without affecting significantly the coherent brilliance : a
4.8m device would be used for the highest energy experiments, while a single 1.6m
ID would be used for soft X-ray experiments. In diffraction limited imaging
conditions, the factor 9 in the peak brilliance lost by the fact than only 1 ID is used is
largely compensated by the effect of the lateral coherence width increase with
wavelength at a given distance. In all cases, the total power through an aperture of
5x5 mm2 placed at 28m from the source is always maintained below 700W.
3 Optical Layout
The main design features of the optical layout of the beamline are :
- Preservation of the high coherent brightness of the source.
- Scanning (STXM) and imaging (TXM) microscopes are to be built on two
independent branch lines which could be used simultaneously : the STXM
requires only a fraction of the beam (corresponding to a few wave modes) with
no loss of performance, the remaining being used by the TXM. Wide bandpass, horizontally deflecting, X-ray multilayers are used to steer the beam on the
TXM branch line. The Bragg angle of 4 degrees allows a large physical
separation between the two endstations.
- On the STXM branch, a small pinhole aperture (5µm to 50µm) will be used as a
secondary source, set at about 1m from the zone-plate. This aperture and the
microscope are mechanically linked to the same support in order to minimise
relative movements (mechanical vibrations) between the two components. Even
in an experiment requiring spatial coherence, the optimum amount of phasespace to accept is rather more than a single mode and involves a trade-off of
high coherence for diffraction limited focusing- against flux -which is required
for observation of weak contrast phenomena and fluorescence experiments.
Moreover, an overfilling of the aperture used as secondary source and the zoneplates allows the effects of beam instability to be minimised. The aperture size
and the distance between the pinhole and the microscope are variable to
accommodate a wide variety of microscopy, spectro-microscopy and related
coherence experiments over a large energy range.
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J. Susini and R. Barrett
The beamline will be operated in windowless mode in order to maximise the
photon flux at the lower energies and to minimise the degradation of the
inherent coherent brightness of the source.
3.1 Total Power Management by Use of two X-Ray Mirrors
It is important to limit the heat load on those optical components which strongly
influence the energy resolution, namely the focusing mirrors and the two
monochromators. Consequently, a 2-bounce horizontally deflecting mirror device
consisting of two parallel silicon mirrors is used for :
i) Power filtering: 90% of the unwanted part of the spectrum is absorbed by
the first mirror. The maximum integrated power is damped from 700W to
less than 50W after two reflections.
ii) Suppression of the higher-order harmonics of the insertion device: the cutoff angle can be tuned from 5 to 20 mrad allowing harmonic rejections
better than 10-3 for any energy ranging from 1 to 6 keV. At these small
grazing angles the position of the exit beam is effectively fixed.
iii) Separation of the bremsstrahlung radiation from the synchrotron radiation:
the combination of the mirror device with a bremsstrahlung stop and a
collimator allows a tremendous reduction of the shielding required for the
downstream beam transport. If the “pink” beam is efficiently “cleaned” and
collimated there is no shielding required downstream of the collimator. This
strategy has not only an obvious financial impact upon the beamline design
cost but also allows relatively free access to the microscope during the
experiments.
Furthermore, horizontally deflecting grazing incidence mirrors have several
advantages :
i) preservation of the vertical emittance of the source which determines both
the degree of coherence of the photon beam and the energy resolution of the
downstream monochromators.
ii) The photon beam in the horizontal plane is very wide. Therefore, the heat
load along the mirror length is much more homogeneous than it would be in
the case of a vertically deflecting mirror and, the thermal gradient along the
mirror length is minimised.
The two first mirrors are side water cooled. A gallium-indium layer between the
mirror and the copper block provides both a good thermal contact and an efficient
mechanical decoupling. The first mirror which removes much of the unwanted
power, is 130 mm thick while the mirror is cooled along a 10 mm strip on the top.
Finite-element thermo-mechanical simulations show that this cooling geometry
allows the thermal deformation to be kept below 10 µrad rms for a maximum
absorbed power of 700 W.
The X-Ray Microscopy Facility Project at the ESRF
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3.2 Layout of the STXM Branchline
The optical design was influenced by several constraints:
For spectroscopic applications the very wide energy range cannot be covered with a
single device, therefore 2 high resolution monochromators are planned. It is intended
that the operations necessary to change between these two basic operating modes
(see Fig. 2) should be automated as much as possible and allow imaging over the full
spectral range to be achieved for the same sample with a minimum of manual
realignment.
Fig. 2. Various optical configurations of the beamline
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J. Susini and R. Barrett
3.2.1 Crystal Monochromator (2.0–6.0 keV)
A fixed exit 2-crystal monochromator which can be equipped with Silicon and
multilayers will be used in the 2–6 keV range (Fig. 3). The Bragg angle is tuneable
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from 3 to 70 degrees. The energy resolution is about +5.10 for Silicon and 10 –
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10 for multilayers. The optics will be water cooled. In order to ensure a good
The X-Ray Microscopy Facility Project at the ESRF
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stability of the outgoing beam during an energy scan, the monochromator is
positioned as close as possible to the entrance aperture of the microscope (about 1
m). In this operation mode the source is imaged onto the pin-hole plane via a
sagitally focusing cylindrical mirror in the horizontal plane and a vertically focusing
spherical mirror.
3.2.2 Plane Grating Monochromator (0.2–1.5 keV)
The grating monochromator design (see Fig. 4) associates plane gratings, which are
tuned by a simple rotation, with a vertically focusing spherical mirror (VFM). In this
design it is the source (conjugated by the VFM) which acts as the entrance slit of the
monochromator. The pinhole apertures for the STXM act as the exit “slits” of the
grating monochromator and are fixed at 8.23m from the VFM. In the horizontal
plane, a sagitally focusing mirror (HFM) is used to image the source onto the same
pinhole apertures. Three holographic gratings have been optimised to cover the full
energy range [7]. In order to achieve a good overall efficiency, in particular, at high
energies, the deviation angle has been set to 6 degrees. Assuming slope errors of the
order of 1µrad on each of the vertically deflecting components the average calculated
resolving power of the PGM should be around 6000 over the operating energy range.
3.2.3 Stigmatic Design
The exit pinhole of the grating monochromator, used as secondary source for the
scanning microscope, being fixed over the full energy range, the different focusing
mirrors must have the correct radius of curvature. This is particularly critical because
the short distance between the zone-plate and the source (pinholes) makes the
microscope performance very sensitive to astigmatism [8]. The maximum tolerable
longitudinal separation between the vertical and horizontal sources (corresponding to
the foci of the focusing mirrors) is only 10cm. Therefore, the vertical focusing mirror
is bendable (bimorph mirror) in order to match its image plane exactly with those of
the horizontally focusing mirrors. A beam position monitor will be set at the
secondary source plane [9].
3.2.4 The Scanning Microscope
The STXM is conceived to work with zone plate type optics which currently offer
the best proven performance for this type of application. The development of high
resolution zone plates capable of efficiently focusing X-rays up to 6keV has been
addressed in two different collaborative research programs established with laboratories at King’s College London and Göttingen [3-6]. Clearly a single zone plate
will be incapable of providing efficient focusing over the entire spectral range
available and one of the challenging aspects of the microscope design has been to
facilitate the remote exchange of the zone plate allowing close to optimum focusing
conditions regardless of the operating energy. The X-ray microscope will be placed
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in an environmental chamber allowing operation in air, helium or vacuum (10 –10
mbar). The entire microscope can be moved along the beam axis relative to the fixed
exit pinhole which acts as the secondary source. This movement allows the pinhole–
zone plate distance to be varied from 0.3 to 1.5 m and allows the illumination
conditions of the zone plate to be adapted to the experiment. Taking into account the
mirror reflectivities, monochromator band pass and undulator characteristics the
9
7
estimated photon flux in a 50nm probe is of the order of 10 –10 ph/s.
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J. Susini and R. Barrett
Fig. 4. Parameters of the plane grating monochromator and associated focusing mirrors
The X-Ray Microscopy Facility Project at the ESRF
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The wide spectral operating range of the microscope is attractive for spectromicroscopy. Whilst in its simplest form this might consist of taking multiple images
of a single sample region at different incident energies, an interesting extension is to
perform highly spatially resolved XAS scans on small regions of the sample. The
spatial resolution of this mode is potentially limited by the probe size, convergence
and the sample thickness, but requires careful mechanical design due to the energy
dependence of the zone plate focal length.
The sample will be scanned using a combination of piezo driven flexure and mecha2
nical stages giving a total scan area of 10 x 10 mm . The current aim is to approach
pixel rates of 1 kHz. A manual sample rotation will be available, primarily for
fluorescence mode imaging, but with the possibility of future upgrade to motorised
movement for micro-tomography measurements
The microscope design is intended to offer maximum flexibility for the use of
various different detector types. Currently it is planned for absorption measurements
to use alternatively, proportional gas detectors, PIN photodiodes and avalanche
photodiodes. A high energy resolution Germanium solid state detector will also be
available for fluorescence measurements. A standard sample holder has been
designed upon similar principles to that currently used on the full field imaging
microscope at the ALS [11]. This kinematically mounted system should allow
regions of interest to be identified and recorded on a standard light microscope prior
to transfer into the X-ray microscope and rapidly aligned to the probe scan. It is
intended for the same holder to be used on the TXM allowing rapid transfer between
the two microscopes. The microscope will be controlled using essentially standard
ESRF VME based electronics running OS9 with a user interface running on Unix
workstations.
3.4 Layout of the TXM Branchline
The main difficulty of the implementation of a TXM branchline using the ID21
source is associated with the relatively small emittance and the relatively large
energy range. The consequent problems of creating a suitable condenser system for
the microscope are discussed elsewhere [10]. Zone plate optics will be used for the
objective lens with similar characteristics to those planned for use in the scanning
microscope. It is probable however, for reasons of imaging dose, that a stronger
emphasis will need to be placed upon the efficiency of these zone plates, potentially
at the expense of resolution. It is planned initially to use a channel cut Si 111
monochromator for this branch thus allowing narrow bandpass imaging to be
performed. The microscope will enable absorption contrast (bright field), dark field
and Zernicke phase contrast imaging to be performed. The estimated flux at the
detector plane for bright field imaging with a Si 111 monochromator is of the order
10
of 10 ph/s. Cryo-cooling is foreseen to reduce the effects of beam damage and thus
permit multiple imaging of dose sensitive samples such as required in tomographic
measurements. It is anticipated that the primary detectors to be used will be CCD
cameras. It is probable that in the longer term two such detectors will be available
depending upon the imaging energy to be used. In the case of the lower energies this
will probably be a back-thinned CCD using direct conversion whilst for higher
energies (to preserve dynamic range) a phosphor coupled system is envisaged.
4 Conclusions
The conceptual design of the X-ray microscopy facility at the ESRF has been
described. Priority has been given to the spatial resolution (100–50nm), resolving
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J. Susini and R. Barrett
9
power of the monochromators (E/∆E §±IOX[DWWKHVDPSOHSRVLWLRQ –
7
10 ph/s in a probe of 50nm diameter for the STXM) and flexibility (trade-off of
energy resolution for flux, multiple contrast modes, two microscopes). The beamline
will exploit the potential of the imaging applications in the 2–6 keV energy region
which are, so far, relatively unexplored. In particular, the access to the K and L
edges and emission lines of the medium elements will offer new capabilities: higher
penetration depth, fluorescence contrast. For many samples, cryo-microscopy will be
necessary to cope with the very high flux and the related radiation damage.
Acknowledgements
The authors are very grateful to R. Baker, G. Marot and R. Guetta for their for their
very active involvement in the design of the beamline components, E. Delcamp and
F. Polack for the optimisation of the parameters of the grating monochromator. We
thank C. Buckley and B. Niemann for a number of very constructive discussions.
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