The argon cluster gun, an innovation for XPS profiles of coated glasses

The evaluation of the impact of a treatment or an ageing on the elemental composition of extreme surface, sub-micrometric or micrometric of a material, is most often carried out by a profiling in depth using a source of ion abrasion.

The "XPS community" relies on two different depth profiling methods using either a monoatomic argon source or a cluster argon source.

The importance of the choice of the abrasion source, especially its impact on the elemental quantification, concerns mainly light alkaline metals as well as polymers, which diffuse during abrasion. Several industrial fields such as energy conversion and storage, building, medical..., are interested in the distribution of alkaline and alkaline-earth metals on the glass surface. This information is a key element in the manufacturing process: it allows to link the macroscopic properties of these materials to their atomic or molecular architecture, the mastery and control of which condition the performances (properties of use often sought: anti-reflection, corrosion resistance, adhesion, anti-fouling, electrochromism, hydrophilicity, electrical properties, aesthetics, ...). To access these properties, TESCAN ANALYTICS offers advanced characterization techniques to perform XPS or ToF-SIMS profiles using argon cluster sources.

The XPS spectra obtained are made up of peaks whose position (energy) and intensity constitute the chemical signature of the constituent elements of the sample/material analyzed.

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XPS has the advantage of quantifying the concentration of elements present on the first 10 nanometers of the surface. However, this value can be distorted when a diffusion of light elements is induced by the ionic abrasion used for chemical profiling. Thus, the ability of the argon cluster gun to limit this diffusion, allows access to quantification data sufficiently reliable to provide detailed answers to the problems posed by industry concerning corrosion, alteration, aging, surface segregation ...

The use of an argon cluster source helps to limit the diffusion of light elements during the profiles and gives access to more reliable elemental quantification data.

Chemical profiling by XPS (X-ray photoelectron spectroscopy) is obtained by alternating cycles of analysis (surface irradiated by X-ray photons) and abrasion (surface scoured by argon ions: monoatomic or clusters). The quantification data thus obtained as a function of the abrasion time (depth) allows access to the depth distribution (over a few tens of nm to a few µm) of the atomic concentrations of the constituent elements of the analyzed material and of surface contamination.  

TESCAN ANALYTICS has an expertise of more than 20 years in the use of XPS on all types of materials, insulating or conducting... With the latest generation instruments, our team of experts works with all industrial sectors.

Figure 1: XPS spectrum of the extreme surface of soda-lime glass


Figure 2: Sodium profile: monoatomic source Ar⁺ vs clusters Ar

The XPS spectra of the extreme surface (Figure 1), collected with a monochromatic AlK_α source highlight the presence of soda-lime glass elements (sodium, oxygen, calcium, silicon, and magnesium) and the glass surface treatment (in this case: potassium). 

The depth distribution of sodium, the element most often impacted by diffusion phenomena, was followed by profiling with an Ar⁺ monoatomic source and with an Ar₅₀₀⁺ argon cluster source (Figure 2).

The sodium distribution profiles are significantly different depending on the abrasion gun used:

• With the monoatomic source, the sodium concentration measured in the glass volume (after 500 seconds of abrasion) is close to 4.5 atomic %. This concentration is clearly lower than the composition expected for a soda-lime glass ([Na] around 9 atomic %). Numerous studies in the literature describe a decrease in the sodium concentration near the extreme surface when the abrasion is performed with a monoatomic Ar⁺ source. This decrease has been explained by the accumulation of positive charges in the region close to the extreme surface, thus repelling the highly mobile Na⁺ ions in the glass matrix, creating a sodium depleted zone.
• With the Ar cluster source, the sodium concentration measured at the plateau is close to 9.2 atomic %, in perfect agreement with the expected composition.

These results indicate a negligible migration of sodium during profiling, when the abrasion is performed with the Ar cluster source. This reduction in migration could be explained by the difference in energy of the Ar ions in the two profiling modes: while each argon ion carries an energy of 5 keV with the monoatomic source, the energy carried by each atom constituting the cluster is 40 eV, i.e. 125 times lower.
 

In this example, two different depth profiling methods were used: monoatomic Ar⁺ sources and Ar₅₀₀⁺ clusters. Significantly underestimated Na concentrations were measured with the Ar⁺ monoatomic source, highlighting the limitations of depth profiling with this source, due to migration induced by abrasion. With the Ar cluster source, the sodium concentration measured at the core of the glass is in perfect agreement with the expected value.

These results indicate that XPS coupled to the argon cluster source is the optimal method for a reliable quantification of alkalis in the core of glasses.

In other works, it was shown the utility of the XPS for the measurement of the rate of covering of a substrate of biological nature by a waterproofing treatment.

For more glass analysis applications with XPS or our other techniques, ask us for information.

Other complementary techniques can be used to obtain depth distribution profiles of glasses such as ToF-SIMS coupled with the argon cluster source and EDR technology.