Surface analysis with high energy time-of-flight secondary ion mass spectrometry measured in parallel with PIXE and RBS

doi:10.1016/j.nimb.2010.02.045

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 268, Issues 11–12, June 2010, Pages 1714-1717

https://doi.org/10.1016/j.nimb.2010.02.045 Get rights and content

Abstract

The Surrey Ion Beam Centre (IBC) is routinely using focused MeV primary ions to generate two-dimensional molecular maps using time-of-flight secondary ion mass spectrometry (ToF-SIMS) collected simultaneously with particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) spectra. Measurements made with the ToF-SIMS, PIXE, and RBS device with a focused and scanned MeV primary ion beam provide a more complete elemental and molecular evaluation of the target sample’s surface. In this paper, we explore the use of high electronic energy loss by MeV primary ions in the surface region of the target as a method for generating molecular images of the surface. We provide analyses of the relative secondary ion yield of leucine molecules as a function of primary ion velocity using MeV primary ions. We also demonstrate our ability to collect PIXE, RBS, and ToF-SIMS images generated using the same MeV primary ion.

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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Surface analysis with high energy time-of-flight secondary ion mass spectrometry measured in parallel with PIXE and RBS

Abstract

Introduction

Section snippets

Experimental procedure and results

Discussion

Acknowledgments

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Surface Science

Applied Surface Science

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Nuclear Instruments and Methods in Physics Research

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Matrix-free high-resolution imaging mass spectrometry with high-energy ion projectiles

Journal of Mass Spectrometry

Fast-ioninduced erosion of leucine as a function of the electronic stopping power

Physical Review B