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. S.E.S.O. designed and manufactured from Glidcop™ the 4 bendable elliptical mirrors installed in the two refocusing stages of the "Nanospectroscopy" 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 characterisation 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 is readily 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 140 eV monochromatic source (for the ID two sections, 562 μm x 73 μm FWHM), 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 (horizontal and vertical directions, respectively), at hv = 140 eV. Due to the grazing 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. Changing the horizontal refocusing mirror roll allows one to slightly increase the spot in the vertical direction up to about 5 μm. At 10 μm the illumination of the field of view becomes already inhomogenous, affected by striations which refect the slope errors of the mirrors.
Left: the photon beamspot
Right: intensity profiles across the vertical and the horizontal directions.
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