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 calibation. This is achieved by calibrating primarily using bulk standards. By doing so, one reduces the dependence on standards that are layered samples, which can be difficult to obtain or certify. Layered standards can be added to improve the calibration and are referred to as TAGS. Stratos is also able to reference specific TAGS for different sample types to optimize the calibration. Apart from bulk and/or in-type standards, Stratos can still be calibrated using certified reference materials for ultimate accuracy. Stratos also makes use of a step-by-step setup guide, called the Virtual Analyst, that simplifies and optimises the application setup. It does this by making use of the best possible measurement conditions for a specific sample type. In the past this has been a manual, time- consuming and tedious process.
An application of Stratos is the analysis of InxSy buffer layers that are deposited as coating layers on glass components of solar cells. The determination of thickness of layers is of crucial importance. Another important variable is the stochiometry of the S in the compound which can be varied to obtain specific efficiencies.
The analysis of thin films is not straightforward because the concentration of the elements present in the thin films as well as the thickness of the layers affect the intensity nof the fluorescence lines. In addition to this, the interlayer absorption can affect intensities, making the setup of the measurement program more complex. With Stratos making use of the Virtual Analyst these challenges are easily overcome.
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 description and sample preparation
The sample that was analyzed consisted of a glass substrate on which a layer of InxSy was deposited, according to Figure 1. The glass sample was cut into 1x1 cm pieces with a diamond cutter to fit into a stainless steel cup with a 10 mm aperture.
Figure 1. Cross section of a glass sample coated with an InxSy layer
Calibration with the Virtual Analyst
The calibration was set up using three bulk samples. Bulk samples were a mixture of pure element samples and setup samples from the Omnian software package, PANalytical’s solution for standardless analysis. Based on the expected mass thickness and composition of the unknown samples, PANalytical’s Virtual Analyst facilitates the setup of the optimum measurement program to analyze multilayer samples.
Indium and sulfur were measured for 300 and 150 s respectively at peak and background positions, for a total measurement time of less than 8 minutes.
Results and discussion
The analysis of a typical sample is compared with measurements obtained by XRR (X-ray reflectometry), performed on an X’Pert3 MRD - one of PANalytical’s XRD diffractometers. XRR is regarded as an absolute technique and hence a good reference. The results are reported in Table 1 and show the capability of Stratos to analyze coated samples using only bulk standards.
Table 1. Comparison between XRF and X-ray reflectometry results for InxSy layers sample
In order to evaluate the repeatability of the method, ten consecutive measurements - with unloading between the measurements – were performed on one sample with a summary shown in Table 2. Stoichiometry (which is often needed in this analysis) is also shown. The instrument repeatability is shown to be excellent for thickness and stoichiometric measurements, meeting the needs of the industry.
Table 2. Results of layer thickness and stoichiometry for a glass sample coated with InxSy
This data sheet demonstrates that Zetium, in combination with the Stratos software is capable of accurately determining the layer thickness and stoichiometric composition of an InxSy coating on a glass substrate. The method established using the Virtual Analyst is fast, non-destructive and permits the automatic selection of optimal measurement conditions for the analysis of thin films and multilayer samples. Calibration using bulk standards has
been shown to reduce the dependence on in-type standards which is seen as a significant advantage.