ToF-SIM & XPS: pitfalls to avoid when analyzing glasses

ToF-SIM & XPS: pitfalls to avoid when analyzing glasses

CHAUVET Elodie, COUSY Florian, DE PUYDT Yves, LE ROUX Cynthia, RAMOS GARCIA David, ROUKOSS Charbel


One of the recurrent analytical problems encountered in ToF-SIMS is the semi-quantification of element chemical concentrations whose intensity reaches the saturation level of the detector. This is the case, for example, for alkalis in inorganic glasses or silicon in electronic components.
One way to solve this problem is to reduce the intensity of the primary ions to avoid saturation of the detector. However, this solution is accompanied by a loss of sensitivity on trace elements, which is one of the strong points of the ToF-SIMS technique. This compromise is therefore not an analytically acceptable option.
To solve this problem, a new technology patented and marketed by IONTOF extends by a factor of 100 the dynamic range of counting secondary ions while maintaining a linear response of the detection system: EDR technology for ‘Extended Dynamic Range’.

• without EDR, the silicon profile is in the saturation zone, its intensity is thus underestimated which induces a loss of information.
• with EDR, the saturation zone has been pushed back for silicon, its profile is no longer saturated and those of other ions like SiO+ remain unchanged.

The importance of the choice of the etching source, especially its impact on the elemental quantification, concerns mainly light alkaline metals in glasses, which diffuse during profiling. 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, whose mastery and control determine 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 of the extreme surface (Figure 1), collected with a monochromatic AlKα 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 etching gun used:
• With the monoatomic source, the sodium concentration measured in the glass volume (after 500 seconds of etching) is close to 4.5 atomic %. This concentration is clearly lower than the expected composition 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 etching 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 etching 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 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.