Zetium - Analysis of thin layers of Ti/Sn coatings on steel using Stratos

Zetium Introduction 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 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 the analysis of titanium and tin coatings on steel. Tin-coated steel is widely used for food preservation while titanium layers act as a passivation layer which further increases the lifetime of tin-coated steel.

This study demonstrates the powerful capabilities of a Zetium, combined with the Stratos module for determining the mass thickness of titanium on tin coatings on steels. This was achieved using only bulk calibration standards instead of costly in-type standards.

Instrumentation This study was performed using a Zetium wavelength dispersive XRF spectrometer. It was configured with a 2.4 kW Rh anode, SST R-mAX tube and is a fully integrated system with a X-Y sample handler and state-of-the-art SuperQ software, featuring an advanced Fundamental Parameters algorithm. Zetium is engineered for excellence in terms of both analytical accuracy, precision and operational performance.

Sample preparation The Ti/Sn on steel samples were fitted into cups with a 27 mm aperture. A schematic cross section of a sample is shown in Figure 1. In this study, the mass thickness of titanium ranged down to 3.0 mg/m2 which is approximately 0.7 nm in layer thickness. The mass thickness of Sn ranged from 3.0 g/m2 (~ 400 nm) to 4.0 g/m2 (~ 550 nm).

Figure 1. Cross section of the Ti/Sn coatings on steel substrate

Calibration with the Virtual Analyst 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 Zetium instrument. Line selection is based on the response of a line with respect to changes in thickness and composition within the expected concentration and thickness range of the unknown samples.

Figure 2. Intensity profile simulation generated by the Virtual Analyst

Figure 2 shows the theoretical intensity profiles of Sn lines generated using the Virtual Analyst. The plot shows the theoretical intensities of the lines with respect to increasing mass thickness of the Sn layer. Based on the intensity profiles and the target thickness of the layer, Sn Kα and Ti Kα were used as the optimum lines for the analysis.

The calibration was set up using only bulk samples. For Ti Kα an Omnian standard (in pressed powder form) and a stainless steel certified reference material were used as calibration standards. Similarly, another pressed powder Omnian standard was used for Sn Kα as well as a pure Sn solid reference material sample.

Results and discussion The XRF results using bulk standards are compared to the values obtained by wet-chemical analysis. The results reported in Table 1 demonstrate that Stratos is capable of analyzing multilayer samples using just bulk standards.

Table 1. Analysis results of coatings on steel

In order to evaluate the instrument repeatability, the same sample was analyzed twenty times sequentially, with loading/unloading in between each measurement. The results reported in Table 2 show excellent repeatability of the instrument.

Table 2. Results of measurement repeatability on Ti/Sn on steel samples

Conclusions This data sheet demonstrates that Zetium, in combination with Stratos software is capable of accurate analysis of layer thicknesses of Ti and Sn coatings on steel. Furthermore, a good correlation of Stratos results and nominal values was obtained using just bulk standards in the calibration. 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 combination of Zetium and Stratos is an excellent solution for analyzing thin layers of Ti and Sn coatings on steel in the packaging industry.

Zetium - Analysis of thin layers of Ti/Sn coatings on steel using Stratos

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