Determination of the thickness of coatings by means of a new XRF spectrometer

We propose a new, compact XRF spectrometer conceived for the determination of the thickness of a coating deposited on a substrate. The analysis technique is based on the detection, at different angles of observation, of the fluorescence emitted by the substrate and absorbed through the coating. With respect to other methods, the measurement is independent of the XRF analysis of the coating, which need not even be fluorescent. Moreover, no calibration of the instrument with standard samples is required. A key feature of the proposed spectrometer is represented by the use of a Silicon Drift Detector (SDD) for the first time in this kind of measurement. The SDD is characterized by a very low output capacitance (of the order of 0.1 pF), independent of the active area of the device, which allows one to achieve a lower electronic noise with respect to a conventional silicon diode of equivalent area and thickness. Based on a Peltier-cooled SDD, a compact spectrometer, characterized by an energy resolution of about 155 eV FWHM at 6 keV (at about −10°C), was realized. The analysis technique and the spectrometer compactness make this instrument suitable for on-line, non-destructive quality control in the semiconductor and mechanical industries. In this paper the measurement technique is described and the first experimental results obtained on samples composed of a substrate coated with a layer of different thickness are presented. Copyright © 2002 John Wiley & Sons, Ltd.