Third-order aberration theory of Wien filters for monochromators and aberration correctors

Third-order aberrations at the first and the second focus planes of double focus Wien filters are derived in terms of the following electric and magnetic field components – dipole: E₁, B₁; quadrupole: E₂, B₂; hexapole: E₃, B₃ and octupole: E₄, B₄. The aberration coefficients are expressed under the second-order geometrical aberration free conditions of E₂ = −(m + 2)E₁/8R, B₂ = −mB₁/8R and E₃R²/E₁ − B₃R²/B₁ = m/16, where m is an arbitrary value common to all equations. Aberration figures under the conditions of zero x- and y-axes values show very small probe size and similar patterns to those obtained using a previous numerical simulation [G. Martínez & K. Tsuno (2004) Ultramicroscopy, 100 , 105–114]. Round beam conditions are obtained when B₃ = 5m²B₁/144R² and (E₄/E₁ − B₄/B₁)R³ = −29m²/1152. In this special case, aberration figures contain only chromatic and aperture aberrations at the second focus. The chromatic aberrations become zero when m = 2 and aperture aberrations become zero when m = 1.101 and 10.899 at the second focus. Negative chromatic aberrations are obtained when m < 2 and negative aperture aberrations for m < 1.101. The Wien filter functions not only as a monochromator but also as a corrector of both chromatic and aperture aberrations. There are two advantages in using a Wien filter aberration corrector. First, there is the simplicity that derives from it being a single component aberration correction system. Secondly, the aberration in the off-axis region varies very little from the on-axis figures. These characteristics make the corrector very easy to operate.     

Development of a high energy resolution electron energy-loss spectroscopy microscope

We have developed a high energy resolution electron energy-loss spectroscopy (EELS) microscope, which can take spectra from specified small specimen areas and specified small reciprocal space areas to investigate detailed electronic structures. The EELS microscope is equipped with retarding Wien filters as the monochromator and the analyser. The filters are designed to achieve a stigmatic focus. The energy resolutions are 12 meV and 25 meV for cases without and with a specimen, respectively. Spatial and momentum resolutions are 30–110 nm in diameter and 1.1 nm⁻¹ in angular diameter, respectively. EELS spectra are presented to show the performance of this instrument.     

A new 200 kV Ω-filter electron microscope

A new 200 kV Ω-filter electron microscope was developed under a project supported by a Grant-in-Aid for Specially Promoted Research of the Ministry of Education, Science, Sports and Culture of Japan. The performance of the microscope is described.