Using X-Ray Photoelectron Spectroscopy to Characterize Low-Emissivity Glass Coatings

The overall structure of a multilayered low-emissivity (low-E) coating was studied using the Thermo Scientific K-Alpha XPS (Figure 1) through high-quality sputter depth profiling. In order to control radiative heat flow, low-E coatings of metal or metal oxide layers are deposited on a glass substrate.

These coatings are deposited on glazing units fixed in building to aid control of room temperatures thereby minimizing the heat loss from the interior during the winter or to control entry of heat via the glass during the summer.

In both cases, the operation principle is similar. The coating reflects the longer wavelength infrared radiation and allows shorter wavelength light to pass through (Figure 2).

Figure 1. The Thermo Scientific K-Alpha XPS

In general, the thickness of the coating ranges from 100 to 200nm. The integrity and composition of the coating are two important parameters for both the efficiency of the glass and its visual appearance. Therefore, there is a need for a method, which performs both routine analyses and failure investigation.

Figure 2. An example of a use of low-E glass to control room temperature by controlling transmission of infrared radiation from the sun

The real layer structure of a coated sample can be examined with respect to the expected layer structure using XPS. Using the surface sensitivity of the technique, it is possible to achieve excellent depth resolution.

The detection of the chemical state of the individual elements present is also possible, which enables the analysts to check, for instance, the occurrence of unwanted oxidation of one of the metal components. During failure analysis, the layer that includes delamination can be checked with the help of the same depth profiling process.

Experimental Procedure

A sample of low-E glass can be depth profiled with the help of the K-Alpha, using 500eV argon ions with > 1μA beam current. The rapidly acquired snapshot spectra were collected at each level in the depth profile, which reduces spectral acquisition time between ion etches.

The integrated argon ion source is completely aligned and controlled by computer, and provides excellent ion flux at low energies as well. In addition, K-Alpha is integrated with a simple turn-key charge compensation system, which not only facilitates the analysis of insulating samples but also keeps analysis conditions constant throughout the profile.

A rotating stage was mounted with the sample (Figure 3). The best possible depth resolution can be achieved by rotating the sample during the Ar ⁺ etch cycles.

Both azimuthal and off-axis (compucentric) rotation can be carried out using the Thermo Scientific Avantage data system, which helps in achieving several profiles on the same sample without removing the sample from the instrument.

Since the experiment was designed to use "multi-phase" etching, the required time can be lowered by setting up the etch time and individual region acquisition times per level or group of levels. It is also possible to edit the parameters during the experiment.

Figure 3. The K-Alpha sample holder fitted with the 3 cm diameter rotating stage for depth profiling

Results

Figure 4 shows the results of the depth profiling of a low-E glass coating. K-Alpha's live reflex optics system was used to capture the image of the etch crater following the depth profile. This image illustrates the high quality of the etch crater.

Figure 4. Depth profile of low-emissivity glass. The image of the etch crater taken with the sample viewing system of K-Alpha. The depth scale was calibrated to a known standard

The two silver layers represented in red in the profile indicate the critical components of the low-E glass. No visible degradation of the depth resolution was observed on the second, deeper layer in comparison with the first silver layer. This shows that the K-Alpha is capable of sputtering complex multi-component films without degrading the depth resolution throughout the profile.

Except the silver layers, most of the coating consisted of zinc oxide, tin oxide and silicon nitride. The silver layers are deposited with the nickel layers to prevent oxidation. The silicon nitride and ZnO layers were also found to have aluminum and chromium in small concentrations.

The ability to measure chemical state information in terms of depth is one of the main aspects of an XPS depth profile. The presence of a minimum of two chemical states of nickel was observed in the multilayer coating, and the states had been designated as nickel oxide and nickel metal. The nickel oxide was found at the interface with the tin oxide layers.

Conclusion

A low-E glass coating was depth profiled with the help of the K-Alpha. This technique helps in identifying the overall structure of the coating, both elemental components and their chemical environment with respect to depth. It ensures accurate interfacial characterization, thanks to the excellent depth resolution of the data.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).