Secondary electron detection for distributed axis electron beam systems

A secondary electron detection scheme for the distributed axis, fixed-aperture system is described. It employs a multi-channel detector array with a through-hole for a primary beamlet on each channel, a field terminator installed between the detector array and sample, and a deflector forming a static transverse electric field between the field terminator and sample. These elements enable detection of the secondary electrons stimulated by the primary beamlet. In order to achieve a high detection rate, small separation of the primary beamlets, and small aberrations, the size and the layout of the through-holes of the field terminator are studied. The equation of motion in an ideal field distribution is analytically solved and the dispersion of the secondary electrons caused by the helical motion in an axial magnetic field and chromatic variation of deflection are calculated. Aberrations are calculated by using numerical simulation. On the basis of these calculations, two types of the field terminator are proposed. One has a single through-hole, which is shared by a primary beamlet and the secondary electrons stimulated by the primary beamlet, per primary beamlet. The other has a through-hole exclusively for a primary beamlet and an extra slot for the secondary electrons, per primary beamlet. Simulations reveal that the former achieves a secondary electron detection rate of 99.7% and aberrations smaller than 4.6 nm, but doesn't enable the separation of the primary beamlet to be smaller than 1000 μm. In contrast, the latter achieves a secondary electron detection rate of 95.0%, aberrations smaller than 9.7 nm. Furthermore, it also enables the separation of the primary beamlet to be as small as 250 μm, the same as in our detector array at this moment.