Sophisticated analysis made simple
Stratos is an evolution of PANalytical’s market-leading thin film metrology software used in the semiconductor industry. A distinct feature of Stratos is the ability to analyze thin layers by using only bulk standards in the calibration. By doing so, one reduces the dependence on standards that are layered samples, which are often difficult to obtain or certify. Whenever available, layered standards and certified reference materials can still be added for ultimate accuracy. Stratos also makes use of a step-by-step setup guide called the Virtual Analyst which simplifies and optimizes the setup of an application. It suggests the best possible measurement conditions for a specific sample type which can be a tedious and complicated process if done manually.
An important application of Stratos is in the analysis of tin coatings on steel. Tin-coated steel is widely used in the food packaging industry to preserve and increase the shelf-life of food.
This study demonstrates the powerful capabilities of an Epsilon 3XLE, one of PANalytical’s benchtop EDXRF spectrometers, combined with the Stratos software module for determining the mass thickness of Sn coatings on steel using bulk standard instead of costly in-type standards.
The measurements were performed using a PANalytical Epsilon 3XLE EDXRF spectrometer equipped with a 50 kV silver anode X-ray tube, 6 filters, a helium purge facility, a high-resolution SDDUltra silicon drift detector, a spinner and 10-position removable sample changer.
Sample description and sample preparation
The samples analyzed in this study were NIST certified reference materials. A cross section is shown in Figure 1.
Samples of steel substrates coated with tin were fitted into a sample cup with a 10 mm aperture. The mass thickness of tin ranged from 4 to 12 mg/cm2 wherein 1 mg/cm2 Sn is about 1.3 μm in terms of layer thickness.
Figure 1. Sn on steel
Calibration with the Virtual Analyst and measurement details
Based on the expected thickness and composition of the unknown samples, PANalytical’s Virtual Analyst facilitates the setup of the optimum measurement program to analyze multilayer samples. Optimum XRF lines are chosen based on the existing hardware configuration of the instrument.
For this application with mass thicknesses ranging from 4-12 mg/cm2 (equivalent to layer thickness of 5-16 μm), Sn Kα is the optimum line for the analysis of Sn layers.
Samples were analyzed using the measurement condition in Table 1. The total measurement time per sample was only 90 seconds.
Table 1. Measurement conditions used in the analysis of Sn layer on steel
The calibration was set up using only bulk samples - an Omnian standard in the form of a pressed powder and a blank steel substrate which was also provided by Tata Steel International. Figure 2 presents the Sn Kα spectrum and Figure 3 shows the obtained calibration line.
Figure 2. The spectrum of a Sn Kα line
Results and discussion
The obtained XRF results using bulk standards are compared to the certified values. The results reported in Table 2 demonstrate that Stratos is capable of analyzing multilayer samples using only bulk standards with excellent agreement.
Table 2. Comparison of nominal and calculated values
In order to evaluate the instrument repeatability, the same samples were measured ten times sequentially, with loading/ unloading between each measurement. The results reported in Table 3 show good repeatability of the instrument.
Table 3. Results of ten repeat measurements of Sn on steel samples
Figure 3. Calibration line of Sn Kα
This datasheet demonstrates that Epsilon 3XLE, in combination with Stratos software is capable of accurate analysis of layer thicknesses of Sn coatings on steel. Excellent correlation of Stratos results and reference values shows that for this application, the need for in-type layered standards is not necessary. The method established by using Virtual Analyst is fast, non- destructive and permits the automatic selection of optimal measurement conditions for the analysis of thin films and multilayer samples.
The repeatability of the instrument also demonstrates that the Epsilon 3XLE in combination with Stratos is an excellent solution in the production control of Sn coatings in the food packaging industry.