Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
MeV-SIMS yield measurements using a Si-PIN diode as a primary ion current counter
Introduction
Megaelectronvolt-Secondary Ion Mass Spectrometry (MeV-SIMS) employs charged particles (primary ions) with a kinetic energy in the MeV range in order to eject particles (called secondary ions if charged) from a sample surface. For ions with a kinetic energy in that range, the dominant stopping mechanism is electronic stopping, leading to the increased desorption of high mass molecules [1], [2], [3], [4], [5]. As the electronic stopping power depends on the impinging ion’s energy, charge and atomic number, a study of the secondary ion yield (secondary ions detected divided by the primary ions applied to the sample) as a function of these parameters is essential. Earlier works by Hakansson et al. [6] and Albers et al. [7] have shown strong yield dependence on these experimental parameters for lower mass sputtering and a similar dependence is likely for sputtering high mass molecules as well.
An example of the strong yield dependence on the electronic stopping can be seen in the comparison of two spectra from the amino acid Glycine (m = 76.04 Da) using 10 MeV Cl5+ and 11 MeV O4+ primary ions (Fig. 1). The absolute yield per primary ion hitting the sample achieved by the heavier chlorine ions compared to the oxygen primary ions is about 6 times higher even though kinetic energies are similar. This shows the necessity to investigate the dependency of the secondary ion yields on primary ion characteristics. The results could be used to optimize the MeV-SIMS setup, in order to achieve the best possible efficiency. Also, a deeper understanding of the secondary ion sputtering process can be gained from a yield database spanning over a wide range of primary ion types and energies. This may also offer the future promise of quantitative MeV-SIMS.
The accuracy of any yield measurement depends on the precise and continuous measurement of the primary ion beam current. The method to measure this current and applications of this current measurement approach will be described in the following section.
Section snippets
Materials and methods
A description of the RBI MeV SIMS setup can be found elsewhere [8].
Any investigation of the yield dependence on different experimental factors requires a precise measurement of the primary ion current. As the current is in the attoampere range and the beam should be monitored continuously, current measurement through conventional indirect techniques like Rutherford Backscattering Spectrometry (RBS) or direct current measurement in a Faraday cup cannot be applied. Moreover, direct measurement of
Results
Yield measurements with the new setup show results that agree with the earlier experiments [6]. A strong yield dependence on setup parameters, such as the angle between the sample and extractor, the position of the beam on the sample or the extractor distance from the sample, has been observed. This means any yield measured by this method will always be a function of the detection efficiency of the mass spectrometer and the secondary ion production in the sample induced by the primary ion beam.
Conclusions
A new primary ion counting set-up, in the form of a Si-PIN diode periodically intercepting the beam, has been installed on the MeV-SIMS beam line at RBI. Its ability to measure primary ion beam current down to a few aA has been validated by direct transmission measurements. First yield measurements give insight into the importance of various experimental parameters in the MeV SIMS setup, such as the angle between the sample and the detector, the extractor distance or the distance between the
Acknowledgements
This work has been supported by Marie Curie Actions - Initial Training Networks (ITN) as an Integrating Activity Supporting Postgraduate Research with Internships in Industry and Training Excellence (SPRITE) under EC contract no. 317169, IAEA CRP project (F11019) “Development of molecular concentration mapping techniques using MeV focused ion beams”, and the UKF project ‘Study of modern paint materials and their stability using MeV SIMS and other analytical techniques’ contract no. 4/13. The
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