Quantitative Electron Microprobe Analysis of Thin Films on Substrates

Abstract

Monte Carlo simulation procedure is developed for kilovolt electron beam scattering and energy loss in targets consisting of thin films on thick substrates. Such calculations have direct application to the nondestructive quantitative chemical analysis of ultrathin films in the electron microprobe (an electron probe x-ray microanalyzer), utilizing characteristic x-ray fluorescence. Angular elastic scattering is calculated in the electron trajectory simulation with the screened Rutherford expression for cross section, and energy loss between elastic scattering events is calculated with the continuous-slowing-down approximation of Bethe. The contribution to x-ray fluorescence from the film due to backscattered electrons from the substrate is accounted for. For elemental films, the Monte Carlo simulation predicts intensity ratios ki for characteristic x-rays from the film, referenced to standards of thick elemental samples. No film standards are required, and the mass thickness of any elemental film on any substrate can be determined from theoretical calibration curves. The model has been verified by measurements on films of Si, Cu, and Au on Al203 over wide ranges in E0, and t. For alloy films, calibration curves are generated and graphically iterated to provide independent analysis of weight fractions Ci and total mass thickness pt. Films of Mnx,Biy, and Cox,Pty, were successfully analyzed with pt ≤ 100 µg/cm2.