A Tracer Method for the Thickness Measurement of Thin Bi Films

Abstract

The use of radioactive Bi²¹⁰ as a tracer in thin Bi films prepared by evaporation in a vacuum has resulted in a simple and direct method for the measurement of film thickness in the range 1–300A. The radioactivity of a standard, prepared by evaporating a weighable amount of active Bi on a piece of glass, is compared to that of a specimen of unknown thickness. For films 200A or less in thickness, the ratio of counting rates of the ``unknown'' and standard yields the mass of Bi on the unknown which, together with the film area (measured with a planimeter) and the density of Bi, yields the average Bi film thickness. Two levels of activity occurring in the standards were assigned after an electron diffraction study to a difference in structure of the Bi films. The method is accurate for extremely thin films (probable error is less than 4 percent if counting statistics are carried to a probable error of 3 percent), simple procedures and calculations are used, calibration is continuous, and the specimen is not altered in any way.