XRF is a respected and commonplace technique that is well known for its ability to provide quick, accurate and precise elemental analysis within a diverse range of applications. It is especially useful for the industrial production control of building materials, metals, slags and petrochemical products.
When working with these applications, it is expected that samples will be homogenous, flat and generally infinitely thick in terms of the depth of x-ray penetration.
Homogeneity is essential as the detected x-rays will often originate from the first few microns below the material’s surface. As it is essentially a surface analysis technique, XRF can also be utilized in the analysis of coatings, thin films or layers on a substrate, even though these samples are not homogeneous by nature.
In this instance, the characteristic line intensity differs with the thickness of the layer that contains the elements. This principle is shown in Figure 1, below.
Figure 1 - Thickness and bulk analysis with XRF
Coatings are used in the manufacturing sector to improve and enhance product characteristics, helping them to achieve improved performance with reduced costs. These coatings are often applied in order to improve wear and corrosion resistance, with examples of coatings on metal substrates including zinc (Zn), tin (Sn), titanium (Ti) or zirconium (Zr).
Protective coatings on metal may also be oxides or nitrides such as TiN. In such circumstances, it is crucial that at least one of the elements in the coating can be detected via XRF. For example, within a TiN coating the element present would be Ti, so the coating’s thickness could be ascertained by measuring this titanium.
It is worth noting however that the substrate does not need to be a metal. Coatings can be applied on glass as well, with a further example being the application of a silicon (Si) coating on plastic or paper.
Additionally, layer analysis is not restricted to a single layer. A stack of layers present on a substrate can also be analyzed, so long as there is a single unique element within each layer. Multi-layer applications are common in the semiconductor and electronics industries.
This article specifically focuses on two applications – zirconium coatings on aluminum and analysis of CIGS solar cells.
This application utilized the ARL QUANT’X EDXRF spectrometer – a compact, benchtop analyzer that possesses a generously sized sample chamber to allow for analysis of irregular or larger samples. It is fitted with the most advanced Silicon Drift Detector (SDD) and a 50 kV, 50 W rhodium or silver target X-ray tube.
The peak-to-background for the elements from F to Am has been optimized by a set of specialist filters. This ensures that the ARL QUANT’X spectrometer can be easily adjusted for each application or element range.
Additionally, the instrument can be fitted with a camera which can help the user to position the sample. It also includes a series of collimators which can help in adjusting the beam diameter.
Zirconium on Aluminum
Zirconium conversion coatings are often applied to aluminum in order to enhance corrosion resistance. Zirconium is generally preferred over chromate, as chromate can pose a risk to health and the environment.
A calibration was set up using 20 standards, accommodating Zr coating thicknesses from 2 to 20 mg/m2. The Zr Kα line allows for an analysis in air, whilst the measurement time equals 60 s live time. To improve the signal-to-background ratio a thick palladium (Pd) primary beam filter has been used.
The calibration curve itself is illustrated in Figure 2, below. As can be seen, a strong correlation coefficient of 0.993 has been acquired. The average error (RMSE) across the whole thickness range is around 0.3 mg/m2.
Figure 2 – Calibration curve for Zr coating thickness on Al
CIGS Solar Cells
Copper Indium Gallium Selenide (CIGS) is a direct bandgap semiconductor that is common in the manufacturing of solar cells. CIGS absorbs sunlight very well, so smaller amounts of material are required when compared with other semiconductor materials, leading to the widespread production of thin-film photovoltaics.
Figure 3 (below) shows the structure of a CIGS thin-film solar cell.
Figure 3 – Typical CIGS multilayer
CIGS solar cells can be analyzed without the need for sample preparation other than cleaning the cell’s surface, when this is required. Table 1 (below) illustrates the excitation conditions that can be used for this application which can be run in air as no light elements are present.
In order to generate element characteristic peak intensities of several thousand counts, a measurement time of 30 s per condition is used.
Table 1 – EDXRF excitation conditions for CIGS analysis
In order to ascertain the thickness and elemental composition of each layer, a thin film quantification method based on fundamental parameters (FP) (included with the Thermo Scientific WinTrace QUANT’X EDXRF software) was used.
Information about concentration or thickness is not required, and calibration is conducted using bulk samples of Cu, Ga2O3, SeO2, Mo and In. Thin film standards for each element were also used, at a concentration of ~50 μg/cm2 each.
Table 2 (below) shows the results from a typical CIGS solar cell in terms of analysis and repeatability.
Table 2 – CIGS analysis results
This cell was not covered with a transparent front contact, so this explains why there was no Cd or Zn detected (see Figure 3). The instrument’s repeatability was tested by analyzing the sample, 10 times.
The repeatability of the layer thickness determination was exceptional, with a relative error of 0.5% or less. Relative errors on the concentrations for all elements are all below 1% (when present) except for Ga which was 1.5%.
As has been demonstrated here, the use of a benchtop EDXRF analyzer like the ARL QUANT’X can combine easy sample preparation with precise, reliable performance. This system can work with both straightforward coating applications as well as more complex, multi-layer stacks.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.
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