Analytical Potential of EDS at low Voltages

Abstract

This paper addresses some of the principles underpinning chemical microanalysis of bulk specimens in the scanning electron microscope using low beam voltages ( ≤5 kV) and energy dispersive x-ray spectrometry. The advantages of this relatively new method lie primarily in the strong dependence of the sample penetration range on the electron beam energy (∼10x from 5 kV to 20 kV). This directly yields superior spatial resolution and sensitivity for the analysis of sub-micron particles, surface films and at interfaces. The correspondingly shorter x-ray escape range reduces the importance of the complex matrix absorption and fluorescence terms; the parameters for which may not always be known accurately. A useful x-ray signal can only be generated with a beam energy which is at least 1.3x the excitation energy for the relevant characteristic x-rays. Low voltage fluorescent yields are relatively low and depend strongly on overvoltage (U < 3) and low voltage x-ray signals are relatively weak. These considerations lead to the development of a radical new instrument which produces 9000 counts per second from an Aluminium target with a < 2 nm probe containing 0.2 nA of current at 5 kV, fast submicron resolution x-ray maps ( < 0.1 µm is possible), and close to 1 nm electron image resolution at 1 kV. The greatly improved (15x) EDS sensitivity and superior electron and x-ray imaging are being used to extend, to explore and to exploit more fully the superior performance envelope of the SEM with EDS at low voltages.