Surface analysis with high energy time-of-flight secondary ion mass spectrometry measured in parallel with PIXE and RBS
Introduction
Recent measurements using MeV primary ions from a tandem accelerator for performing surface analysis with time-of-flight secondary ion mass spectrometry (ToF-SIMS) point towards the potential of using high energy ion beams to produce molecular images of large molecules (above 1 kDa) with submicron lateral resolution [1]. The intention of the research presented in this paper is to explore the potential of combining PIXE and RBS simultaneously with MeV-ToF-SIMS to make the MeV-ToF-SIMS technique more accessible to the Ion Beam Analysis (IBA) community.
Secondary ion mass spectrometry (SIMS) is the mass spectrometry of ionized particles desorbed or sputtered from a sample’s surface upon impact by energetic primary ion beams. The secondary ions desorbed from the surface make up only a small fraction (on the order of 1 ion per 10,000 neutrals for impinging MeV primary ions [2], [3]) of the total number of particles desorbed from the surface per impact. Typically, in the static imaging mode of operation [4], these secondary ions are then detected and analyzed using the time-of-flight method by applying several thousands of volts to the sample stage and accelerating the ions towards a channel plate detector located behind a grounded screen. Using low energy (keV) primary ions, static-SIMS has become a widely used surface analysis technique for performing detailed chemical analysis. The term “static” is used in the case when less than 1% of the top surface layer receives an ion impact during acquisition of a measurement [5]. Typically this holds for primary ion fluences of less than 1013 ions/cm2. This use of low primary ion fluence lowers the probability of analyzing an already damaged section of the surface. Another popular surface analysis technique, called dynamic-SIMS, uses a higher fluence (greater than 1013 ions/cm2) to sputter away the upper layers to perform both surface and bulk analysis. A known problem in keV ToF-SIMS measurements is that they are sensitive to changing electronic conditions on a sample’s surface, making quantitative analysis with this technique very difficult [6]. This uncertainty could be eliminated if it was possible to determine accurately during data acquisition if changes in the intensity of the mass peaks were due to an alteration in the ionization efficiency, or if the intensity change reflects the actual molecular distribution in the sample’s surface layers. We suggest that there is a distinct advantage gained by using PIXE and RBS simultaneously while collecting the MeV-ToF-SIMS spectrum since the PIXE and RBS signals are not dependent on the surface’s ionization efficiency and do not suffer from the same variations in intensity caused by topographical effects [7]. We explore in this paper some of the basic properties of MeV-SIMS measurements and point to several potential uses for this new analysis technique.
Section snippets
Experimental procedure and results
For this research we used a 2 MV tandem accelerator located in the Surrey Ion Beam Centre (IBC) with a scanning microprobe beamline typically used for PIXE and RBS analysis (a description can be found in [8]). A duoplasmatron source provides a monoenergetic primary ion beam of 16O4+ with kinetic energies reaching as high as 10 MeV and currents of several hundreds of pA’s that is focused to less than 10 μm and raster scanned across the surface of the sample. The microbeam chamber, which is an
Discussion
The results given in this paper suggest that performing MeV-ToF-SIMS, PIXE, and RBS measurements in parallel can improve the sensitivity and quantification of some ToF-SIMS measurements. This will be explored more rigorously in future research using our microbeam facility.
The yield dependence shown in Fig. 2 has been demonstrated in past studies using MeV-ToF-SIMS [3], [11], [12], [13] and the enhanced yield reflects a regime of greater electronic stopping over nuclear stopping for the high
Acknowledgments
The authors wish to acknowledge the European Union for partial funding of this research through the SPIRIT project.
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2016, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and AtomsCitation Excerpt :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 [1–5]. 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.
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2015, Comprehensive Analytical ChemistryCitation Excerpt :High-energy focussed and scanned megaelectron volt primary ions can also be used to generate two-dimensional molecular maps using ToF-SIMS collected simultaneously with PIXE and RBS spectra. Measurements provide a more complete elemental and molecular evaluation of the target sample’s surface [21]. The relatively larger number of PIXE facilities, compared to synchrotron radiation sources, and their more easy access enhance their use for long-range research projects.