Characteristic X-ray excitation in thin films by 4–50 keV electron bombardment

Abstract

X-ray excitation by electron bombardment has been used to analyse thin films, the main objective being non-destructive thickness determination. In a specially designed, but otherwise conventional, electron probe the thin film targets were irradiated with 4–50 keV electrons. Al, Mg, Cu and Zn films evaporated and/or sputtered onto aluminium and copper substrates were measured and the influence of the excitation energy on the sensitivity was studied in the thickness range from 20 to 5000 Å. Typical sigmoid-shaped calibration curves are found, provided X-ray excitation by the primary electrons is the dominating process. Good agreement can then be expected with calculations based on the depth distribution of characteristic X-ray generation. This is not true, however, of film-substrate systems in which X-ray excitation by electrons backscattered from the substrate becomes comparable with direct excitation ( e.g. Al on Cu). Artificial enhancement of the ionization density by thin surface layers is demonstrated and applications of this effect are discussed with particular reference to energy-dispersive measurements.