Nanospectroscopy Beamline Description

Overview


In order to achieve high spatial resolution and maximize collection efficiency, the beamline was designed to convey the highest possible flux density at the microscope focus. Target requirements were met by combining an insertion device with a moderate energy resolving power monochromator, together with micro -spot refocusing of the photon beam. In order to match the typical working conditions of the microscope (field of view 2 μm to 10 μm), the beamline strongly demagnifies the source, providing homogeneous illumination of the sample area which is imaged by the microscope.

Beamline Layout

The light source is the middle-point between the two undulator sections 1.1 and 1.2 with phase modulator electromagnet . At 10 m from the source middle position, the pinhole (PH) sets the beamline angular acceptance and stops unwanted radiation from the undulator. The toroidal mirror (TM) demagnifies the source by a factor of 8 in the horizontal plane and 5.3 in the vertical. The entrance slits are located at the horizontal (HS) and vertical (VS) foci of the toroidal mirror. The HS slit becomes the virtual source for all of the following mirrors in the horizontal plane. The light is then dispersed by the monochromator, which also determines a further vertical demagnification by a factor 1.7. After the exit-slit (ExS), a retractable plane mirror allows switching operation between the two branches of the beamline. The refocusing mirrors are two bendable mirrors arranged in a Kirkpatrick-Baez geometry. They are located in dedicated vacuum chambers. On the beamline first branch, the demagnification introduced by these elements is 11.5 in the horizontal direction and 5 in the vertical direction. For the second branch, the demagnification factors are 13.9 and 7.6, for the horizontal and the vertical direction respectively.


Insertion device

The Nanospectroscopy beamline shares the insertion device with the Elettra Free Electron Laser (EuFEL). Based on the Sasaki Apple II scheme, the photon source consists of two identical undulator sections (each with 20 poles and period 10 cm) and a phase modulation electromagnet arranged in an optical klystron configuration. Documentation is provided here (EPAC_2216.pdf; EPAC_2322.pdf; EPAC_2349.pdf). This enables the two undulators to be properly phased, thus effectively doubling the undulator length and the useful flux. During the storage ring FEL operation, this configuration offers the benefit of an increased laser gain. The insertion device is able to provide elliptically polarized light (circular left and right as well as linear horizontal and vertical as special cases) in a spectral range extending from below 40 eV to 1000 eV, with high brilliance (using the first, third and fifth undulator harmonics). We emphasize the undulator capability of helicity inversion, which allows performing XMCD (X-Ray Magnetic Circular Dichroism) measurements. The optimum phase for measurements requiring circular polarisation can be found here.

 


Monochromator

The beamline monochromator uses the Variable Line Space (VLS) plane grating architecture. In this way, a large energy range can be covered with a relatively small change in the resolving power. Only two gratings are used to cover the energy range from 50 eV to 1000 eV. This is achieved by coupling the rotation angle of the grating with that of the preceding plain mirror. The first grating has 200 lines/mm and covers the energy range from 50 to 250 eV, while the second grating (400 lines/mm) covers the range from 200 to 1000 eV. The calculated resolving power for the two VLS plane gratings is reported on the left hand panel of the figure. The full line is for the 200 lines/mm grating and the dotted one for the 400 lines/mm grating. The graph shows the measured spectrum of the nitrogen 1s to 1π* absorption from which we deduce a resolving power of 4000 at 400.8 eV, in agreement with the calculations.

The monochromator has been recently upgraded to solve long term stability issues and improve mechanical and optical operation. Our tests demonstrate that the instrument is now extremely stable. A report on the instrument performance is available here.


Beamline flux

The experimental flux curves for a storage ring energy of 2.0 GeV are shown below for linear horizontal and circular polarisation (corresponding to the undulator phase set to 0 mm and 35 mm, respectively), normalized to 200 mA ring current. The data were collected with a photodiode inserted after the exit slit, which was opened to 10 μm (best energy resolution). In the case of the horizontal polarisation, the beamline delivers more than 1012 ph/s in an energy range extending from 50 to 600 eV. The maximum value of 1.8 x 1013 ph/s is reached at 145 eV. When considering the photon flux on the sample, the above figures have to be decreased by a factor of 0.7 for the first branch and 0.5 for the second one.


Refocusing

The refocusing of the photon beam is of crucial importance for the collection efficiency of the microscope. Practically, the micro-spot homogeneity and size is greatly affected by the residual aberrations and slope errors of the refocusing optical elements. The company S.E.S.O. designed and manufactured from Glidcop™ all bendable elliptical mirrors installed in the two branches of the beamline. Initially polished to the nominal profile with an accuracy of 10%, these mirrors are then bent by applying unequal moments to their ends. The obtained mirror profile approximates up to the 4th polynomial order the desired ellipse. The metrological characterization of the mirrors revealed residual slope errors of around 1 μrad RMS. By varying the bending torque, the focal distance can be changed up to 40% around its nominal value.

The expected performance in terms of micro-spot size has been calculated after considering the total demagnification factor of the beamline (equal to 92 along the horizontal direction and 45 along the vertical). For the ideal case of a monochromatic source (for the ID two sections, 562 μm x 73 μm FWHM in the horizontal and vertical directions respectively), a micro-spot size of 6.1 μm x 1.6 μm is obtained. By imaging the secondary electrons with the SPELEEM, we measured a FWHM spot size of 25 μm x 2 μm at hv = 140 eV. Keeping into account the angle of incidence (16°) of the photon beam onto the sample, the spot size in the plane normal to the beam is 7 μm x 2 μm, in good agreement with the expected value. When trying to illuminate a field of view of 10 μm, the illumination becomes already inhomogeneous, showing striations which reflect the slope errors of the mirrors.


End station


The schematics diagram of the SPELEEM microscope is illustrated in the figure on the right hand side: (1) main chamber; (2) preparation chamber with load lock; (3) image column; (4) illumination column; (5) beam separator; (6) connection to the beamline. There are valves between the main chamber and the beamline, between the main chamber and the preparation chamber, between the preparation chamber and the load lock, and between the main chamber and the beam separator.

The Main Chamber (1)

Overall, there are eight ports pointing at the sample, which sits in front of the objective lens. One horizontal port connects the main chamber to the beamline, another allows illumination of the sample with Hg-lamp.The sample temperature cn be read through the objective lens using an optical pyrometer. The other six ports are generally used for e-beam evaporators (two inclined 45° from above, two 45° from below, one from top, one from bottom). The main chamber is pumped by a Varian Starcell ion pump (300 l/s) coupled with a titanium sublimation pump, assuring a base pressure in the low 10-10 mbar range. Alternatively a magnetic bearing turbo pump with vibration insulation can be used. In the latter configuration, the microscope can be operated using the main chamber as a gas flow reactor, up to a maximum pressure of 1·10-6 mbar when the beamline is open. This value can be increased to 1·10-5 mbar when the microscope is isolated from the beamline (LEEM operation mode). Available facilities are a gas line with precision leak valves, e-beam evaporators (Focus - Omicron GmbH), a quadrupole mass spectrometer, a magnetization stage, and a sample parking stage.

The preparation chamber (2)

A small preparation chamber allows simple treatments such as annealing and exposure to gas. A small heating facility allows annealing and flash up to temperatures of about 2000 K. An Ar ion gun facing the sample is provided for sputtering.  There are also two precision leak valves for traetent with gases. The sample preparation chamber is equipped with a load-lock that allows to transfer samples in 45 minutes from air to a vacuum of better than 10-8 mbar. The preparation chamber is pumped through theairlock turbo or with a dedicated ion pump . Base pressure is in10-9 mbar range.

The image column (3)

The image column consists of imaging optics (transfer lens, field lens, intermediate lens, projector lens), the imaging energy analyzer, a double projector, and a chevron channel plate/screen detector (Burle). Images are taken with a video camera (PCO SensiCAM).

The illumination column (4)

The illumination column consists of an LaB6 electron gun and capacitor lenses. It provides the electron beam used to probe the sample in LEEM and µ−LEED operation. It illuminates an area of about 80 µm on the sample with a maximum total current of about 100 nA. Apertures can be used to illuminate smaller regions (20, 5, 2µm).

The Beam Separator (5)

The beam separator connects the image column and the electron gun with the main chamber. It acts as a magnetic prism which separates incident and reflected beams when the microscope is operated in LEEM or LEED modes.

The Connection to the Beamline (6)

It hosts the beamline diagnostics, a Au coated Mo mesh to measure I0, a photodiode and a phosphor screen.


References

  1. Microfocussing VLS grating-based beamline for advanced microscopy;
    D. Cocco, M. Marsi, M. Kiskinova, K. C. Prince, T. Schmidt, S. Heun, and E. Bauer;
    Proc. SPIE 3767, 271 (1999)
    10.1117/12.371126
  2. High lateral resolution spectroscopic imaging of surfaces: The undulator beamline "Nanospectroscopy" at Elettra;
    A. Locatelli, A. Bianco, D. Cocco, S. Cherifi, S. Heun, M. Marsi, M. Pasqualetto, and E. Bauer;
    J. Phys. IV 104 (2003) 99 - 102.
    10.1051/jp4:200300038
  3. Kirkpatrick-Baez elliptical bendable mirrors at the Nanospectroscopy beamline: metrological results and x-rays performance;
    A. Bianco, G. Sostero, and D. Cocco;
    Proc. SPIE 4782, 74 (2002);
    10.1117/12.450983
  4. New optical setup for the generation of variable spot size on third generation synchrotron beamlines;
    T. Moreno, R. Belkhou, G. Cauchon and M. Idir;
    Proc. SPIE 5921, 59210F (2005);
    10.1117/12.622942
  5. Photoemission electron microscopy with chemical sensitivity: SPELEEM methods and applications;
    A. Locatelli, L. Aballe, T.O. Menteş, M. Kiskinova, E. Bauer;
    Surf. Interface Anal. 38, 1554-1557 (2006).
    10.1002/sia.2424
  6. Cathode lens spectromicroscopy: methodology and applications;
    T. O. Menteş, G. Zamborlini, A. Sala, A. Locatelli;
    Beilstein J. Nanotechnol. 5, 1873–1886 (2014) [Published 27 Oct 2014];
    doi: 10.3762/bjnano.5.198;

 

Last Updated on Thursday, 15 November 2018 10:31