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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.


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Surface sputtering vs swelling for different diffusion coefficients for helium irradiation of PE.
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Figure 6: Surface sputtering vs swelling for different diffusion coefficients for helium irradiation of PE.

Mentions: The simulations show that the implantation profile and concentrations of rare gases inside the sample largely depend on the size of the implanted species (Fig. 5). For helium, swelling of the sample is observed for diffusion coefficients up to 1.67 × 10−12 cm2 s−1 (Fig. 6). The transition from swelling to sputtering occurs at a diffusion coefficient of 1.67 × 10−11 cm2 s−1. For diffusion coefficients of 1.67 × 10−11 cm2 s−1 and 1.67 × 10−10 cm2 s−1, implantation profiles with maximum concentrations which do not exceed 20% and 3% are observed. For larger diffusion coefficients, the helium concentration drops almost to 0%, i.e., it diffuses almost completely out of the polymer sample. For Ne+ and Ar+ bombardment, no swelling of the polymer sample is observed but implantation profiles with maximum concentrations of 55% and 68% are obtained. For a diffusion coefficient of 1.67 × 10−10 cm2 s−1 the concentration is reduced to about 2% for both primary ion species and for even larger coefficients the Ne and Ar concentrations are almost reduced to 0. When comparing this to the experimental diffusion coefficients, which lie in between 1.67 × 10−8 and 1.67 × 10−5 cm2 s−1, it shows that almost no rare gas will accumulate in the polymers during He+, Ne+ and Ar+ bombardment. The out-diffusion of rare gas species during sample irradiation allows also to explain why no sample swelling has been observed in the experimental study. Compared to this, the diffusion coefficient for He in silicon at 300 K is of the order of 1.67 × 10−16 cm2 s−1 [36], hence several orders of magnitude below the values for polymers. Thus, sample swelling and blistering as well as bubble formation are only of concern for the analysis of inorganic samples by HIM. For polymer samples, the rare gas species can diffuse out of the sample and analysis results are not altered by this process. Fig. 5 and Fig. 6 show only the results for the ion irradiation of polyethylene, but the same observations have been made for the other polymers investigated in this work.


Experimental and simulation-based investigation of He, Ne and Ar irradiation of polymers for ion microscopy
Surface sputtering vs swelling for different diffusion coefficients for helium irradiation of PE.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

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

Figure 6: Surface sputtering vs swelling for different diffusion coefficients for helium irradiation of PE.
Mentions: The simulations show that the implantation profile and concentrations of rare gases inside the sample largely depend on the size of the implanted species (Fig. 5). For helium, swelling of the sample is observed for diffusion coefficients up to 1.67 × 10−12 cm2 s−1 (Fig. 6). The transition from swelling to sputtering occurs at a diffusion coefficient of 1.67 × 10−11 cm2 s−1. For diffusion coefficients of 1.67 × 10−11 cm2 s−1 and 1.67 × 10−10 cm2 s−1, implantation profiles with maximum concentrations which do not exceed 20% and 3% are observed. For larger diffusion coefficients, the helium concentration drops almost to 0%, i.e., it diffuses almost completely out of the polymer sample. For Ne+ and Ar+ bombardment, no swelling of the polymer sample is observed but implantation profiles with maximum concentrations of 55% and 68% are obtained. For a diffusion coefficient of 1.67 × 10−10 cm2 s−1 the concentration is reduced to about 2% for both primary ion species and for even larger coefficients the Ne and Ar concentrations are almost reduced to 0. When comparing this to the experimental diffusion coefficients, which lie in between 1.67 × 10−8 and 1.67 × 10−5 cm2 s−1, it shows that almost no rare gas will accumulate in the polymers during He+, Ne+ and Ar+ bombardment. The out-diffusion of rare gas species during sample irradiation allows also to explain why no sample swelling has been observed in the experimental study. Compared to this, the diffusion coefficient for He in silicon at 300 K is of the order of 1.67 × 10−16 cm2 s−1 [36], hence several orders of magnitude below the values for polymers. Thus, sample swelling and blistering as well as bubble formation are only of concern for the analysis of inorganic samples by HIM. For polymer samples, the rare gas species can diffuse out of the sample and analysis results are not altered by this process. Fig. 5 and Fig. 6 show only the results for the ion irradiation of polyethylene, but the same observations have been made for the other polymers investigated in this work.

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.


Related in: MedlinePlus