Energy-filtering characteristics of double-aperture-lens (DAL) retarding-field analyzer

A newly established double-aperture-lens (DAL) retarding-field analyzer is a simple and accurate electron-energy analyzer for evaluation of the electron-beam energy spread ( Δ E ) in the next-generation electron gun. In this study, we discussed the energy-filtering characteristics of DAL analyzer in comparison with a conventional single-aperture-lens (SAL) analyzer, using surface-charge calculations, electron-trajectory simulations, and DAL defocus experiments. We examined how the lateral beam offset ( Δ R ) affects the transmission and cutoff behavior, and the shape of differential spectra. While DAL behaves as an ideal high-pass filter for small offsets ( Δ R ≤ 2 μ m ), providing an accurate Δ E , SAL exhibits a strong Δ R dependence of the effective cutoff energy, which broadens the differential spectrum and causes the apparent Δ E to increase with beam radius. For larger offsets, however, electrons entering the DAL that overcome the saddle-point barrier can be intercepted by the second aperture, producing low-pass-like attenuation on the high-energy side and narrowing the differential spectrum, which was also confirmed by experiments. These results show that the DAL improves robustness to incident-position variation of the electron beam only within a limited offset range, and that reliable Δ E metrology requires careful control of beam size and alignment.