Photoemission microscopy with microspot-XPS by use of undulator radiation and a high-throughput multilayer monochromator at BESSY

Abstract

We present a new experiment for photoelectron microspectroscopy by use of undulator radiation, which has been set up at the beamline U2 at the Berlin electron storage ring BESSY 1. This approach employs a non-imaging simulated hemispherical electron energy analyser attached to an imaging photoemission electron microscope (FOCUS IS-PEEM) with integrated microarea selector. The photoemission microscope exhibits a lateral resolution of 25 nm (with 4.9 eV UV-excitation), while the resolution with incident synchrotron radiation in the soft X-ray range is about 100–120 nm (mainly due to chromatic aberrations). Photoemission microscopy as well as photoelectron microspectroscopy of selected areas on the sample surface were performed by using the third harmonic of the direct undulator beam which was monochromatized and refocused by a two-element multilayer optic in the 70–95 eV energy range. The multilayer monochromator operating at near-normal incidence consists of a concave spherical multilayer mirror (r=650 mm) and a plane multilayer grating (blazed grating 1221 L/mm, blaze angle 0.8 deg). Both elements were coated with a Mo/Si multilayer of equal d-spacing (20 doublelayers, d=10.5 nm) to enhance the reflectivity for EUV radiation at near-normal incidence angles. The characterization of the individual optical components by EUV reflectometry shows a peak reflectance of about 47% at a photon energy of 95 eV in the case of the focusing multilayer mirror while the first order diffraction efficiency of the multilayer blaze grating was measured to be up to 32%. The evaluation of the photoelectron spectra measured with this set-up displays that the spectral resolution of the incident radiation is better than 0.7 eV, while it is about 2–4 eV in the case of a two-multilayer-mirror configuration. Analysis of the surface topography and the chemical composition of inhomogeneities of thin evaporated layers on a mesoscopic scale are the main applications of this experiment.