Analysis of Al coatings on thin polymer films using Stratos

This study demonstrates that Zetium, together with Stratos, is capable of analyzing the thickness of Al coatings deposited on top of polymer layers. The homogeneity of the Al layer at different locations of the polymer film can be determined, which makes the proposed approach an ideal tool for quality control purposes.

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. 

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 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 application of Stratos is the analysis of Al coating thickness on thin polymer films. For example, metalized polymer films  are  used in the food packaging industry acting as aroma, gas,  or moisture barriers, as well as protecting the packaged content from light. Metal layer thickness has to be controlled since it affects the (barrier) properties and consequently the quality of the film. The present study shows the analysis of one type of Al coated polymer film.

Instrumentation

This study was performed using a Zetium wavelength dispersive X-ray fluorescence spectrometer. It was configured with a 1 kW Rh anode SST R-mAX tube and is a fully integrated system with an X-Y sample handler and state-of-the-art SuperQ software, featuring an advanced Fundamental Parameters algorithm.

The Zetium is engineered for excellence in terms of analytical accuracy, precision, and operational performance.

Sample description and sample preparation

Al coated polymer films were investigated. Samples consist of an Al layer deposited on a single polymer layer. A schematic representation of  the samples is shown  in Figure 1. The materials analyzed were sampled from different locations across the width and length of a production roll. Samples for the analysis were cut and then placed on a Perspex disk support, and subsequently mounted in a stainless steel sample holder with an aperture of 27 mm diameter.

Figure 1. Schematic description of Al coated polymer films

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Calibration and measurement details

The calibration was set up using a fused bead sample, a bulk setup sample from the Omnian software package. The Al Kα line was measured with a curved PE 002 crystal, a 700 µm collimator, a flow detector, and an X-ray tube setting of 20 kV and 50 mA. In total the measurement time was 40 s; the peak position was measured for 30 s and the background position for 10 s.

Results and discussion

Sheets from different positions within the width of the production roll were analyzed. Sheets were obtained from the edges (position 1 and position 3) and the center (position 2) of the production roll, as visualized in Figure 2.

Figure 2. Schematic illustration showing sampling locations for the polymer sheets and samples 

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Instrument (and sample) precision was evaluated by measuring one sample repeatedly. Results shown in Table 1 show excellent precision of the measurement with a relative standard deviation of 0.2 %.

Table 1: Average values of aluminum layer thickness observed on one sample measured ten times

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One specific sample was suspected of heterogeneity after being analysed by optical density. In order to verify this, a more thorough XRF study was conducted. Results are reported to show XRF capabilities to detect sample heterogeneity.

Reproducibility results obtained on different samples from one sheet (within-sheet), and one sample each from different sheets (between-sheets) are summarized in Table 2 and Table 3. A nominal thickness of 30 nm is expected from the process, so the values observed are in the correct range.

It can be seen in Table 2 and Table 3 that results vary depending on the sampling location on the roll. For position  1 and 3 (edge), the observed average thickness is lower than for position 2 (center). Also the standard deviation for position 1 and 3 is larger than for position 2. These results indicate that the metal deposition process is less homogeneous at the edges of the production roll. In addition to this, the standard deviations of within-sheet measurements are larger than the standard deviations of between-sheets measurements. This is because samples taken from one sheet originate from different positions in the width of the production role, whereas samples taken from different sheets (of the same position) originate from the same relative position in the width of the role. This is also schematically illustrated in Figure 2.

Figure 2. Schematic illustration showing sampling locations for the polymer sheets and samples

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Table 3: Aluminum layer thickness results at different locations (positions 1 to 3). For every location one sample from different sheets was analyzed (between-sheets repeatability). 

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Conclusion

This study demonstrates that Zetium, together with Stratos, is capable of analyzing the thickness of Al coatings deposited on top of polymer layers. The homogeneity of the Al layer at different locations of the polymer film can be determined, which makes the proposed approach an ideal tool for quality control purposes.

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