Combined SIMS-SPM instrument for high sensitivity and high resolution elemental 3D analysis
Secondary ion mass spectrometry (SIMS) is a well-established and extremely powerful technique for the chemical analysis of surfaces and thin films. Its main advantages are its excellent sensitivity, high dynamic range, good mass resolution and ability to distinguish between isotopes. Due to its excellent sensitivity and thus low detection limits, SIMS can be used to detect both major and trace elements. SIMS was originally mainly used for depth profiling. Since then, applications have gradually shifted towards 2D and 3D imaging as a result of the dramatically improved spatial resolution thanks to progress made on the instrumental side. As a consequence, new fields of application for SIMS, e.g. in life sciences and nanotechnologies, are emerging. In addition, the possibility of detecting several isotopes in parallel opens up even more horizons, mainly in life sciences, where isotopic labelling is an important investigation technique.
Traditional SIMS 3D imaging is however affected by serious artefacts: while these traditional 3D reconstruction protocols and software assume that the initial sample surface is flat and the analysed volume is cuboid, “real samples” present a surface topography. Furthermore, this changes during the ion bombardment, as the local sputter yields depend on parameters such as the local angle of incidence of the ion beam and the crystal orientation. In addition, the situation is worsened if the sample consists of different materials due to preferential sputtering phenomena. As a consequence, the produced 3D images are affected by a more or less important uncertainty on the depth scale and can be distorted. Finally, significant field inhomogeneities arise from the surface topography as a result of the distortion of the local electric field. These perturb both the primary beam and the trajectories of secondary ions, resulting in a number of possible artefacts, including shifts in apparent pixel position and changes in intensity.
In order to obtain real high-resolution SIMS 3D analyses without being prone to the aforementioned artefacts and limitations linked to ex-situ AFM measurements, we have developed a new high-precision sample stage with an integrated SPM (scanning probe microscopy) system dedicated to the Cameca NanoSIMS. With this unique tool, an in-situ combination of sequential high resolution scanning probe microscopy (SPM) and high sensitivity SIMS becomes possible. In addition to being able to perform real high-sensitivity high-resolution chemical 3D imaging by recording topographical images of the sample surface in-situ before, during and after SIMS analysis, this extremely powerful analytical tool enables SIMS images to be combined with valuable AFM and KPFM (Kelvin probe force microscopy) data recorded in-situ in order to provide an extended picture of the sample being studied.