IONTOF GmbH, model IV

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface-sensitive analytical method that uses a pulsed ion beam to analyze ions or molecular fragments in residence on a material surface.  This is an ultrahigh vacuum instrument (typically 10-9 torr) so samples must be resilient under these conditions. Particles removed from surfaces under the ion beam are accelerated into a "flight tube" and their mass is determined by measuring the exact time at which they reach the detector (i.e. time-of-flight, which is proportional to the mass of the impacted particles). ToF-SIMS liberates particles from only one or a few atomic layers on a material surface (nm-scale); this mode of sample-beam interaction is often referred to as “static” SIMS as only surface components are liberated for analysis proximal to the impacting beam.  This is in contrast to “dynamic” SIMS methods that crater into a material under a focused high-energy ion beam and liberate a much higher yield of secondary ions from the cratered volume (typically ~10 microns across) that then permits quantitative analysis. ToF-SIMS is widely used in material science studies of thin films, polymers and organic compounds, and increasingly biological materials.  Polished samples are best for optimal results, as micro-surface topography can affect the production and detection of secondary particles.

Three operational modes are available using ToF-SIMS: 1) surface spectrometry, 2) surface imaging and 3) depth profiling. Analytical capabilities of ToF-SIMS include: 

  • Mass resolution of ~5,000 m/∆m (~ 0.001 amu). Particles with the same nominal mass (e.g. Si and C2H4, both with amu = 28 ) are easily distinguished from one another because there is a slight mass shift as atoms enter a bound state.  ToFSIMS mass spectra are calibrated against 3 or more ions or compounds of precisely known atomic mass.
  • Mass range of 0-10,000 amu; ions (positive or negative), isotopes, and molecular compounds (including polymers, organic compounds, and up to ~amino acids) can be detected. 
    • In practice, intact organic macromolecules are rarely detected, as these are typically cleaved under the ion beam and characteristic suites of masses of functional groups representative of the parent compound will be detected.
  • Sample requirements: samples must be resilient in ultrahigh vacuum (UHV) conditions (~10-9 torr). Polished or flat samples are preferred to minimize screening of signal by sample topography and to minimize charging issues. Samples may be attached to metal planchettes or Si wafers. Compounds in a fluid suspension can also be dropped onto a Si wafer and then analyzed after the fluid has evaporated. Particles or grains can be embedded into flattened Indium foil as this substrate is both malleable and conducting. For highly charging samples, it may be possible to place a small mesh conducting grid (Cu) over areas of interest on a sample. Small samples (mm-scale) are preferred to minimize degassing of sorbed volatile compounds on material surfaces in the UHV chamber. Taking care of sample preparation to start will greatly improve the probability of a successful experiment. Check with ICAL staff to plan how to proceed with your ToFSIMS experiments.
  • Large sample loading area can accommodate up to 8-inch wafers 
  • The Cameca IV ToF-SIMS at ICAL is equipped with Bi and Cs ion guns. A triple bunch of doubly charged Bismuth (3Bi++) provided by a Liquid Metal Ion Gun (LMIG) is used for optimal  mass resolution. A single Bi+ ion beam is used for higher spatial resolution.  A low energy pulsed sputter ion gun (electron impact, EI) is also available to  produce positive Argon ions and negative Oxygen ions (from gas cylinders) or Cs+ from a Cs salt.
  • Once the sample has been introduced into the chamber and the detector has been calibrated, data acquisition can be very rapid producing spectra and maps in a matter of minutes.
  • Trace element detection limits are in the ppm range. 
  • ToFSIMS must be considered a semi-quantitative method that reveals relative abundances of ions or compounds defined by mass. This is largely due to differential liberation of surface compounds (some are more volatile under the beam than others) and the lack of certified standards that can be used for surface analysis. 
    • This is in contrast to “dynamic” SIMS methods that are used to measure trace element abundances at the PPM level as well as isotopic ratios such as oxygen isotope ratios; this is due to the very large production of secondary ions produced in the impacted volume under an intense ion beam. See tutorial on SIMS.
  • The Cameca IV has a secondary electron detector as well for complementary SEM imaging.
  • Seamless acquisition of positive and negative ion mass spectra is possible on identical analyzed areas.
  • Sub-micron imaging is used to map the distribution of any mass number of interest.  This is important to demonstrate homogeneity or heterogeneity of compounds across an analyzed area of interest (typically on the order of 100 microns x 100 microns).
  • Depth profiling capabilities; sequential sputtering of surfaces allows analysis of the chemical stratigraphy on material surfaces (typical sputtering rates are ~100 A/minute).  This is important for documentation of changes of composition of thin films, or degree of alteration or contamination of materials.
  • Advanced charge compensation methods allow analysis of insulating materials. The ToF SIMS uses a pulsed electron flood gun with a current of a few microamps for charge compensation. 
  • Data Reduction:   ToF-SIMS data are analyzed by accessing an extensive library of masses of ions and compounds. Literally hundreds of mass peaks will be produced in a single analysis.  A reconnaissance survey to identify major peaks is typically done. Alternately, survey and maps may be used to determine the presence and distribution of elements or compounds of interest.  DATA REDUCTION CAN BE EXTREMELY TIME CONSUMING AND WILL COMMAND MOST OF YOUR EFFORT IN DOING TOF-SIMS EXPERIMENTS.
  • Retrospective analysis. Every pixel of a ToF-SIMS map represents a full mass spectrum. This allows an analyst to retrospectively produce maps for any mass of interest, and to interrogate regions of interest (ROI) for their chemical composition via computer processing after the dataset has been instrumentally acquired. The advantage here is that data acquisition only needs to be done at the start, and the data can be revisited repeatedly as new insights are revealed.

Please refer to the tutorial Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) for more information on principles and applications.