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Experimental and simulation-based investigation of He, Ne and Ar irradiation of polymers for ion microscopy

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ABSTRACT

Secondary ion mass spectrometry (SIMS) on the helium ion microscope (HIM) promises higher lateral resolution than on classical SIMS instruments. However, full advantage of this new technique can only be obtained when the interaction of He+ or Ne+ primary ions with the sample is fully controlled. In this work we investigate how He+ and Ne+ bombardment influences roughness formation and preferential sputtering for polymer samples and how they compare to Ar+ primary ions used in classical SIMS by combining experimental techniques with Molecular Dynamics (MD) simulations and SD_TRIM_SP modelling. The results show that diffusion coefficients for He, Ne and Ar in polymers are sufficiently high to prevent any accumulation of rare gas atoms in the polymers which could lead to some swelling and bubble formation. Roughness formation was also not observed. Preferential sputtering is more of a problem, with enrichment of carbon up to surface concentrations above 80%. In general, the preferential sputtering is largely depending on the primary ion species and the impact energies. For He+ bombardment, it is more of an issue for low keV impact energies and for the heavier primary ion species the preferential sputtering is sample dependent. For He+ steady state conditions are reached for fluences much higher than 1018 ions/cm2. For Ne+ and Ar+, the transient regime extends up to fluences of 1017–1018 ions/cm2. Hence, preferential sputtering needs to be taken into account when interpreting images recorded under He+ or Ne+ bombardment on the HIM.

No MeSH data available.


Concentration profiles of F and C at a fluence of 1018 ions/cm2 for He, Ne and Ar bombardment of PTFE: a) for 1 keV, and b) 30 keV impact energy. The results have been obtained by SD_TRIM_SP.
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Figure 12: Concentration profiles of F and C at a fluence of 1018 ions/cm2 for He, Ne and Ar bombardment of PTFE: a) for 1 keV, and b) 30 keV impact energy. The results have been obtained by SD_TRIM_SP.

Mentions: Compared to data found in literature, the trends for surface concentrations of the different species agree qualitatively. For PMMA irradiated with 4 keV Ar+ ions, XPS gives a ratio of carbon to oxygen of 9 indicating a large enrichment of carbon compared to oxygen. This ratio is larger than the factor 2 predicted by SD_TRIM_SP, but in both situations the largest enrichment is obtained for carbon [44]. Hydrogen cannot be detected by XPS, so no comparison is possible for this element. In general, in SD_TRIM_SP sputtering depends on the surface binding energy which is calculated depending on sample composition, but the emission of volatile compounds and chemical reactions taking place in the sample under ion irradiation are not taken into account. The latter include reactions between oxygen and carbon as well as the change of hybridisation for carbon. These different aspects can lead to some discrepancy between experimental and simulated values. For PS, this becomes apparent for the increase of the C sp2 concentration compared to the C sp3 concentration due to the lower binding energy of the latter. Experimentally, the opposite is observed [45–46]. This can be explained by the inability of SD_TRIM_SP to change the hybridisation of a given element. However, hydrogen concentrations around 20% and global carbon concentrations are in good agreement with experimental findings [47]. The polymer samples are not only modified at the surface, but the rare gas irradiation induced also damage in the bulk of the material. The implantation at different energies for a fluence of 1018 ions/cm2 is shown for PTFE and PE (Fig. 12 and Fig. 13). For PTFE at 1 keV, the sample composition is changed most at the maximum implantation depth for He. For Ne and Ar bombardment the change in sample composition is comparable and far less than for He. It is also much closer to the sample surface due to the larger size of the primary ions. For Ne and Ar at 30 keV, no major change in sample composition can be observed, but some changes are observed down to a depth of 600 nm for He bombardment. The implantation probably goes along with chain scission and other damage, which cannot be verified by SD_TRIM_SP simulations. For PE, the difference in mass of the sample species produces a much larger change in concentration for He. At 1 keV, the hydrogen concentration increases above 80% at the main implantation depth of helium. This is opposite to the sample surface concentration which is largely enriched in carbon. A similar behaviour is observed for Ne and Ar bombardment, but it decreases with the mass of the primary ion. At 30 keV the implantation depth is largely increased, leading to a smaller change in composition for He bombardment. Only a small enrichment in hydrogen is observed at the maximum implantation depth. For Ne and Ar bombardment, the changes are more important and hydrogen concentrations at the maximum implantation depth are comparable to the hydrogen concentration at 1 keV He bombardment. Results for PS and PMMA are not shown but they are similar to PE and PTFE.


Experimental and simulation-based investigation of He, Ne and Ar irradiation of polymers for ion microscopy
Concentration profiles of F and C at a fluence of 1018 ions/cm2 for He, Ne and Ar bombardment of PTFE: a) for 1 keV, and b) 30 keV impact energy. The results have been obtained by SD_TRIM_SP.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979758&req=5

Figure 12: Concentration profiles of F and C at a fluence of 1018 ions/cm2 for He, Ne and Ar bombardment of PTFE: a) for 1 keV, and b) 30 keV impact energy. The results have been obtained by SD_TRIM_SP.
Mentions: Compared to data found in literature, the trends for surface concentrations of the different species agree qualitatively. For PMMA irradiated with 4 keV Ar+ ions, XPS gives a ratio of carbon to oxygen of 9 indicating a large enrichment of carbon compared to oxygen. This ratio is larger than the factor 2 predicted by SD_TRIM_SP, but in both situations the largest enrichment is obtained for carbon [44]. Hydrogen cannot be detected by XPS, so no comparison is possible for this element. In general, in SD_TRIM_SP sputtering depends on the surface binding energy which is calculated depending on sample composition, but the emission of volatile compounds and chemical reactions taking place in the sample under ion irradiation are not taken into account. The latter include reactions between oxygen and carbon as well as the change of hybridisation for carbon. These different aspects can lead to some discrepancy between experimental and simulated values. For PS, this becomes apparent for the increase of the C sp2 concentration compared to the C sp3 concentration due to the lower binding energy of the latter. Experimentally, the opposite is observed [45–46]. This can be explained by the inability of SD_TRIM_SP to change the hybridisation of a given element. However, hydrogen concentrations around 20% and global carbon concentrations are in good agreement with experimental findings [47]. The polymer samples are not only modified at the surface, but the rare gas irradiation induced also damage in the bulk of the material. The implantation at different energies for a fluence of 1018 ions/cm2 is shown for PTFE and PE (Fig. 12 and Fig. 13). For PTFE at 1 keV, the sample composition is changed most at the maximum implantation depth for He. For Ne and Ar bombardment the change in sample composition is comparable and far less than for He. It is also much closer to the sample surface due to the larger size of the primary ions. For Ne and Ar at 30 keV, no major change in sample composition can be observed, but some changes are observed down to a depth of 600 nm for He bombardment. The implantation probably goes along with chain scission and other damage, which cannot be verified by SD_TRIM_SP simulations. For PE, the difference in mass of the sample species produces a much larger change in concentration for He. At 1 keV, the hydrogen concentration increases above 80% at the main implantation depth of helium. This is opposite to the sample surface concentration which is largely enriched in carbon. A similar behaviour is observed for Ne and Ar bombardment, but it decreases with the mass of the primary ion. At 30 keV the implantation depth is largely increased, leading to a smaller change in composition for He bombardment. Only a small enrichment in hydrogen is observed at the maximum implantation depth. For Ne and Ar bombardment, the changes are more important and hydrogen concentrations at the maximum implantation depth are comparable to the hydrogen concentration at 1 keV He bombardment. Results for PS and PMMA are not shown but they are similar to PE and PTFE.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Secondary ion mass spectrometry (SIMS) on the helium ion microscope (HIM) promises higher lateral resolution than on classical SIMS instruments. However, full advantage of this new technique can only be obtained when the interaction of He+ or Ne+ primary ions with the sample is fully controlled. In this work we investigate how He+ and Ne+ bombardment influences roughness formation and preferential sputtering for polymer samples and how they compare to Ar+ primary ions used in classical SIMS by combining experimental techniques with Molecular Dynamics (MD) simulations and SD_TRIM_SP modelling. The results show that diffusion coefficients for He, Ne and Ar in polymers are sufficiently high to prevent any accumulation of rare gas atoms in the polymers which could lead to some swelling and bubble formation. Roughness formation was also not observed. Preferential sputtering is more of a problem, with enrichment of carbon up to surface concentrations above 80%. In general, the preferential sputtering is largely depending on the primary ion species and the impact energies. For He+ bombardment, it is more of an issue for low keV impact energies and for the heavier primary ion species the preferential sputtering is sample dependent. For He+ steady state conditions are reached for fluences much higher than 1018 ions/cm2. For Ne+ and Ar+, the transient regime extends up to fluences of 1017–1018 ions/cm2. Hence, preferential sputtering needs to be taken into account when interpreting images recorded under He+ or Ne+ bombardment on the HIM.

No MeSH data available.