Measurement of mean free paths for inelastic electron scattering of Si and SiO₂

The effects of accelerating voltage and collection angle on the mean free path for all inelastic electron scattering (λ_p), which is an important parameter for determining specimen thickness by using electron energy-loss spectroscopy, were investigated with crystalline Si and amorphous SiO₂. First, thickness of Si film was measured with the convergent-beam electron diffraction method, while thickness of SiO₂ particles was estimated from their spherical shape. Then from electron energy-loss spectra, λ_p was evaluated for Si film and SiO₂ particles by changing the accelerating voltage (100-300 kV) and the collection angle for the scattered electrons. Under the condition of no objective aperture, λ_p for Si film and SiO₂ particles was found to increase with the increase of accelerating voltage and to take values of 180 ± 6 nm (Si) and 247 ± 8 nm (SiO₂) at 300 kV. Also, it was found that λ_p in both cases decreases drastically with the increase of collection angle in the range smaller than 25 mrad, while it tends to take a constant value at the collection angle larger than 25 mrad at 200 kV.