Electron‐specimen interactions in low‐voltage scanning electron microscopy

Abstract

Measurements of the electron range R, and the backscattering coefficient η and the secondary electron yield δ at normal and tilted incidence for different elements show characteristic differences for electron energies in the range of 0.5 to 5 keV, compared with energies larger than 5 keV. The backscattering coefficient does not increase monotonically with increasing atomic number; for example, the secondary electron yield shows a lesser increase with increasing tilt angle. This can be confirmed in back‐scattered electron (BSE) and secondary electron (SE) micrographs of test specimens. The results are in rather good agreement with Monte Carlo simulations using elastic Mott cross‐sections and a continuous‐slowing‐down model with a Rao Sahib‐Wittry approach for the stopping power at low electron energies. Therefore, this method can be used to calculate quantities of BSE and SE emission, which need a larger experimental effort. Calculations of the angular distribution of BSEs show an increasing intensity with increasing atomic number at high takeoff angles than expected from a cosine law that describes the angular characteristics at high electron energies. When simulating the energy distribution of BSEs, the continuous‐slowing‐down model should be substituted by using an electron energy‐loss spectrum (EELS) that considers plasmon losses and inner‐shell ionizations individually (single‐scattering‐function model). The EELS can be approached via the theory for aluminium or from EELS spectra recorded in a transmission electron microscope for other elements. Measurements of electron range Rα En of 1 to 10 keV electrons are obtained from transmission experiments with thin films of known mass thickness. In agreement with other authors the exponent n is lower than at higher electron energies.

A new fit to secondary emission yield in the low impact voltage regime: An improvement of Vaughan’s expression

“…21 This data set has a low concentration of data points near δ max , and therefore shows a very poor fit using Eq. (1); (Right) Modified Vaughan fit (solid line, Eq.…”

Topographic and material contrast in low‐voltage scanning electron microscopy

“…In the range 5 to 30 keV of a standard SEM, δ and η increase with increasing tilt angle (Arnal et al 1969, Reimer andSeidel 1968); however, in LVSEM such behaviour is valid only for η (Böngeler et al 1993, Darlington and Cosslett 1972, Koshikawa and Shimizu 1973. The dependence of δ on the tilt angle becomes weaker with lowering of the primary energy and at 1 keV δ is nearly independent of the inclination angle (Böngeler et al 1993, Joy 1987. In the case of BSE, tilt contrast is also influenced by the change of the angular distribution of BSE with the tilt angle.…”

“…At beam energies ≤ 1 kV, however, the contrast mechanism fails for high Z materials (Böngeler et al 1993, Sternglass 1954. This fact, and the change of the angular distribution of BSE at lower voltages, leads to a different appearance of material contrast in LVSEM.…”

“…The tilt contrast originates from the dependence of an electron emission on the local inclination angle of the surface. The dependence of the SE yield on the tilt angle decreases as the beam voltage is lowered (Böngeler et al 1993, Joy 1987, and as a result the tilt contrast, obtained with SE, decreases when the beam voltage is reduced. In addition, as the electron range decreases, the SE signal becomes more sensitive to a presence of any contamination layers on the surface (Katnani et al 1991).…”

“…As the energy of the primary beam is lowered, the range of primary electrons decreases and the secondary electron (SE) yield increases. As a result, the beam-specimen interaction is more localized, charging artefacts decrease, and the depth of specimen radiation damage decreases (Pawley 1984, Reimer 1993. Observations at low primary beam voltages can be routinely conducted in instruments equipped with high-brightness field-emission guns.…”