Determination of Thicknesses of Thin Films

Abstract

X-ray fluorescence analysis may be used for determination of the thicknesses of thin films. There are two ways to determine the thickness of a thin film, namely direct measurement of the fluorescent intensity of the film, or determination of its absorption. In the latter case the substrate is excited to fluorescence and the absorption of the emerging radiation, after passage through the thin film, is measured. Besides x-ray tubes, radioactive substances may be applied for excitation. Since radioactive sources have considerably lower photon efficiencies, dispersive geometry (i.e., analysis of x-ray fluorescence radiation with crystals) is not applied, and the radiation is collected directly by a counter. Pulse-height discriminators or filters, which allow selective discrimination of the desired radiation, may be used for unequivocal identification of the radiation. The x-ray fluorescence method was applied to investigate very thin nickel-iron films (200–2000 A) for their compositions (about 80% Ni-20% Fe) and thicknesses. Massive iron and nickel samples are frequently used for calibration, where the fluorescent intensities of thin films are calculated theoretically. The thickness of the film in the production of tinplate can be controlled continuously. In this case, the fluorescent intensity of the tin-plated iron sheet is usually measured. Thicknesses in question range from 3000–30,000 A.