Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
ToF-SIMS provides detailed chemical information from the surface of materials with unequivocal characterization of elements, chemical groups, polymers and surface additives.
ToF-SIMS involves the use of a primary ion beam which is pulsed to produce packets of primary ions. Each primary ion packet impacts the sample surface and generates a packet of secondary ions at a well-defined point in time. These secondary ions are then accelerated to the same kinetic energy by the use of an extraction field. For the same kinetic energy, secondary ions of different mass will have different velocities and, therefore, different flight times through the mass analyser to the time-sensitive detector.
The technique is non-destructive and highly surface specific with detection sensitivity in the ppm range for most elements. ToF-SIMS involves sputtering the sample area of interest with a pulsed beam of bismuth primary ions (Bin+ where n = 1-3). Elemental and molecular fragment ions formed at the surface are mass-analyzed to produce data in the form of mass spectra, chemical images or depth profiles. A feature of ToF-SIMS is the high mass resolution which allows accurate mass analysis for clear identification of empirical formulae of unknown materials.
- Elemental and molecular information from the uppermost 1-2 nm of the surface
- Detailed chemical information and empirical formulae through the use of extensive library spectra and accurate mass analysis
- Detection sensitivity in the ppm range for most elements
- Chemical imaging of elements, their isotopes and molecular species with sub-micron lateral resolution
- Retrospective mass spectral and imaging analysis of complex surfaces using Region Of Interest mode.
High resolution ToF-SIMS ‘resolves’ the problem of overlapping peaks of secondary ions with the same nominal mass and promotes the technique into the level of accurate mass measurement which is often necessary for the identification of secondary ion signals. High resolution is also important for improved analytical sensitivity since the practical detection limits of some species are restricted by mass overlap.
A selected area on the sample can be scanned in spectrum-per-point mode. A mass spectrum is acquired at each pixel point for the field of view and the resulting two-dimensional array of spectra is stored. Retrospectively, a number of operations can be performed, including:
Region-of-Interest (ROI) analysis: For defined regions (any shape and size) within the analysed area, it is possible to sum the mass spectra at all of the pixel points within each defined region i.e. total area spectrum, small area spectrum (e.g. feature of interest).
Imaging analysis: For the analysed area, chemical maps can be created with high spatial resolution for any of the spectral peaks within the mass range. For weak signals, where there is not enough individual peak intensity to produce meaningful images, it is possible to add related peak intensities, thereby improving image statistics.
The retrospective feature is particularly useful since it allows data to be analysed/re-analysed at a later date and ROI spectra from different regions of a sample can be produced or different chemical maps can be created, without the need for re-analysing the sample in real time.
- Characterization of surface additives on polymer materials
- Investigation of stains and rinse residues on semiconductor wafers and devices
- Monitoring of surface cleaning treatments on medical devices
- Chemical imaging of contaminants and additives on polymer films.
Typical Industries using ToF-SIMS
- Medical Devices