The use of electron image contrast to measure the thickness and composition of thin films of light binary alloys

Abstract

A backscattered electron image detector has been used in a scanning electron microscope to study simultaneously the image intensity I f of thin sputtered films of copper-aluminium alloys on single-crystal silicon substrates and the image intensity I r of an adjacent, bulk copper reference specimen. The contrast ratio k = I f/ I r for a given film and the reference sample depends on the energy E of the incident electrons. A graph of k vs. 1/ E is used to obtain a characteristic energy E i called the intercept energy which, in agreement with previous work, depends on the thickness of the film but not on its composition. Another feature of a curve of k vs. 1 E is that k tends to a limit as 1/ E increases. This limit depends on the composition of the film to which the curve applies, and thus both thickness and composition can be obtained from a given curve. The present results which cover the low range of atomic numbers ( Z = 13−29) are compared with the results obtained for Cu-W films which cover the atomic number range from Z = 29 to Z = 74. It is confirmed that the thicknesses of the light Cu-Al alloys studied in the present experiments and of Cu-W films can be obtained from a single curve of ln(thickness) vs. ln( E i) and therefore that the method of using electron image contrast to measure thickness is applicable over a wide range of atomic numbers.