Multimode objective lens for momentum microscopy and XPEEM: Theory

The strong electric field between the sample and the extractor is at the heart of cathode lenses and a crucial factor for high resolution. However, fields in the range of 3-10 kV/mm can be a source of complications. Local field enhancement at sharp edges or microscopic protrusions of cleaved samples can lead to field emission or flashovers. In addition, slow background electrons drawn into the microscope column contribute to space charge effects. A novel objective configuration, optimized by ray-tracing simulations at energies from a few eV to 6 keV, significantly reduces the field at the sample. One or more annular electrodes concentric to the extractor can shape the electric field in front of the sample. The formation of a 'gaplens' reduces the field to values below the 1 kV/mm range. Tuning the field to zero is advantageous for 3D structured samples. Retarding fields repel slow electrons, suppressing space charge effects. The properties of the different lens modes are investigated using ray tracing and determination of aberration coefficients. Despite its much lower electric field, the gaplens mode exhibits smaller aberrations and enables larger fields of view for both momentum and real space imaging. At electric fields as low as 1200 and 880 V/mm, the accessible solid angle interval in the gaplens mode is three times larger than in the extractor mode (with a start energy of 100 eV and a k-resolution of 10^-2 Å^-1). Due to the elimination of space charge effects in the retarding field mode, XPEEM resolutions in the range of 25 nm are predicted. The ray tracing results are confirmed by the spherical and chromatic aberration coefficients of the real-space and k-space images.