Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) generates secondary ions from the surface of a sample by bombardment with a focused beam of high energy primary ions and then performs Time of Flight Mass Spectrometry of the secondary ions (TOF-SIMS).  These secondary ions are formed from the atoms and molecules of various species present on the surface of the sample. The TOF-SIMS comprises an ion accelerator, where an acceleration voltage can be applied, connected to a high vacuum flight-tube with a reflector (or electrostatic deflection), and an ion detector to capture the intensity of the arriving secondary ions. Secondary ions of analyte species, from the primary ion bombardment, arrive as pulses at the TOF mass analyzer, where they are all accelerated at a specified voltage. The velocity to which the incoming ions get accelerated is inversely proportional to the square-root of their mass to charge ratios (m/z). Hence, for a fixed distance in the flight-tube, the ions with lower ‘m/z’ values reach the detector earlier than the heavier ions having larger ‘m/z’ values. This variation in Time of Flight is used to generate the mass spectra of ions arriving at the detector. The mass spectra are then used to identify the analyte species.

TOF-SIMS can analyze all elements and molecular species upto 10,000 amu. In addition to surface composition mapping, depth profiling is possible by sputtering of surface layers using the ion beam.

Common Uses of TOF-SIMS

• Semiconductor and thin film surface composition analysis and depth profiles
• Research and development of catalyst and adsorbents
• Geological Analysis
• Surface morphology studies of polymers and polymer blends
• Analysis of Pharmaceutical coatings

Advantages of TOF-SIMS

• High sensitivity for trace elements or compounds including dopants and impurities.
• Provides elemental depth profiles from a few angstroms to tens of micrometers from the surface, using sputtering.
• Detects all the elements in the periodic table
• Provides specific compound identification of molecules on a surface
• Can be used for insulators and conductors

 Limitations of TOF-SIMS

• Sample preparation needs care to avoid contamination
• Practical difficulties exist in producing a well-defined short-duration primary ion pulse and accurately measuring the secondary ion flight time

Industrial Applications of TOF-SIMS

• Materials science research
• Geologic research
• Polymer research
• Pharmaceutical research and quality control
• Semiconductor research