Effects of accelerating voltage and thickness on the signal-to-noise ratio in Electron Energy Loss Spectroscopy (EELS)

The advantages of medium accelerating voltages (200-400KV) for EELS have been considered to be an improved jump ratio of edges and an improved signal to background ratio for an increased range of thiknesses. However, very little experimental data of the Signal to Noise Ratio (SNR) as a function of incident electron energy (E o ) can be found in the literature. The interest in performing such measurements is to investigate the effect of accelerating voltage on the detectability limits in EELS since SNR can be related to the minimum detectable fraction. In this paper, measurements made on NiO samples show an improved SNR with increasing E o at a constant thickness value. Changes in the angular distribution of scattering are considered to be responsible for the improved SNR. The NiO sample studied was prepared by evaporation on a copper supporting grid. Uniform areas of different thicknesses (t) caused by the superposition of the film were analysed. Values of mean free path (λ) for NiO were found in the literature. All analyses were carried out with a Philips CM30 and a GATAN 666 Parallel EEL spectrometer (PEELS) coupled to a Link AN10000 MCA. Spectra were transfered on to an external computer for SNR calculations.