The determination of thin layer thicknesses with an electron microprobe

Electron microprobe analysis has been performed on W, Nb, Mo, and Al layers of 2 to 300A?thickness deposited on refractory metals. For this thickness range a linear relation is found between layer thickness and characteristic X-ray intensity. The importance of a correct background measurement and its influence on the accuracy of the determination of the layer thickness is demonstrated. Additional effects arising from the backscatter coefficient of the substrate material and the fluorescence excitation within the layer are investigated. It is found that the highest sensitivity in the thickness measurement occurs at primary electron energies of 2.5 to 3 times the excitation energy of the used characteristic X-ray line of the layer material. This experimental finding is discussed in terms of the dependence of the X-ray background intensity and the ionization probability on the energy of the primary electrons.