August 1, 2015.
This work compares and assesses the effectiveness of beta backscatter (BB) and x-ray fluorescence (XRF) for measuring the thickness of gold coatings on two substrates: silicon and an iron-nickel alloy. A set of samples of known gold coating thickness, ranging from 0.5 – 4.0 microns, was measured in each case along with the substrate alone. In BB the number of electrons (beta particles from a very small radioactive source, Pm-147 in this case) backscattered from a sample in a fixed time period is counted. The XRF method uses x-rays generated in the sample from a primary x-ray beam of higher energy (e.g. 50 kV). The spectrum of the outgoing x-rays is collected and all the peaks in that spectrum are identified. The peak intensities for the primary peaks of each element can be quantified and the data summarized. The data from both methods was recorded in Excel and the data plot using IGOR, a data visualization software package. Both methods are capable of measuring coating thickness, BB is simple and easy to use but the compositions of both the coating and substrate must be a known and standards are required in order for calibration. XRF gives intensities for each element in the coating-substrate system that can be seen, however the data takes longer to interpret and it is limited by the energy of the characteristic x-rays from each element. XRF can also identify unexpected elements, e.g. nickel seen between the gold and the silicon in this work. Although we used standards with XRF in this work, there is software that can be used for standardless XRF thickness measurements if the composition of all of the layers and the substrate is known. This suggests potential future research directions.
Other Materials Science and Engineering
Sandia National Laboratory/California (SNL)
This material is based upon work supported by the S.D. Bechtel, Jr. Foundation and by the National Science Foundation under Grant No. 0952013. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the S.D. Bechtel, Jr. Foundation or the National Science Foundation. This project has also been made possible with support of the National Marine Sanctuary Foundation. The STAR program is administered by the Cal Poly Center for Excellence in Science and Mathematics Education (CESaME) on behalf of the California State University (CSU).