Figure 1. Argon ions (left) and argon clusters (right) impinging on a surface

Monatomic ions are excellent at cleaning and profiling ‘hard’ samples, such as metallic or ceramic samples. Whilst used for sometime in the SIMS community, gas cluster ion beams (GCIB) also termed argon clusters, have recently been employed in the XPS community to gently clean samples and allow the depth profiling of softer materials such as polymers or thin films, whilst retaining the majority of the sample chemistry.

Thes GCIB sources enable depth profiling analysis of organics with minimal loss of chemical information due to ion beam damage, which is crucial in analysis of modern multi-layer structures, such as OLEDs. More information on GCIB sources can be found in [1,2]

Monatomic ions on soft surfaces

When monatomic ions impinge onto soft materials, such as polymers, they typically break the chemical bonds. This means when the sample is analysed with XPS, the data collected displays results from the defective material the monatomic ions have created and not from the actual sample.

Argon Clusters

Clusters aren’t as destructive as monatomic ions. The larger mass of the cluster removes material from the surface of softer materials. The cluster is low enough energy (per atom in the cluster) to keep the chemistry intact, but must be of a sufficent energy per atom for appreciable sputtering to occur [3] and sample roation during etching, together with minimal X-ray exposure improves the depth resolution [4].

Figure 2, shows the damage imparted to polymer PET after sputtering with 500 eV Ar⁺ ions for 30 secs and after ca. 600 sec etching with a 6 keV Ar cluster of 1000 atoms (Ar⁺₁₀₀₀). In the latter case we see the clusters have removed the surface and near surface damage caused my the monatomic ions and ‘recovered’ the true polymer chemistry which etching retains as a function of depth.

Figure 2. C(1s) spectrum of PET etched with 500 eV argon ions for 30 sec, then for 600 sec with 6 keV argon cluster of 1000 atoms.

Applications to oxide films can be found in [5,6].

References

[1] Kratos 20kV GCIB as on our Axis Supra Instrument. https://www.kratos.com/products/arn-ion-source

[2] Thermo MAGCIS source, with 8kV source as on our Thermo NEXSA and K-Alpha systems https://xpssimplified.com/magcis_ion_system.php

[3] Depth profiling organic/inorganic interfaces by argon gas cluster ion beams: sputter yield data for biomaterials, in-vitro diagnostic and implant applications. Surf. Interface Anal., 45 (2013), pp. 1859-1868. Read online at: https://doi.org/10.1002/sia.5333

[4] Sample rotation improves gas cluster sputter depth profiling of polymers. Surf. Interface Anal., 49 (2017), pp. 953-959. Read online at: https://doi.org/10.1002/sia.6250

[5] XPS investigation of monatomic and cluster argon ion sputtering of tantalum pentoxide. Appl. Surf. Sci. 405 (2017) 79-87. Read online at: https://doi.org/10.1016/j.apsusc.2017.02.006

[6] Reduced Preferential Sputtering of TiO₂ using Massive Argon Clusters. Journal of Surface Analysis 20 (2014) pp. 211−215. Read online at: http://www.sasj.jp/JSA/CONTENTS/vol.20_3/Vol.20%20No.3/13_Paper_Counsell_r1.pdf