Nanospectroscopy Beamline Description

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.