ToF-SIMS: Overview

SIMS is an analytical techniques carried out under ultra-high vacuum (UHV) conditions. The process is initiated by bombarding the surface of a solid sample with a pulsed energetic primary ion beam (1-50 KeV) over a designated area. Both atomic and molecular ions are emitted from the outer layers, but only a small percentage of the emitted fragments are positive or negative ions. It is these secondary ions that are extracted from the sample and accelerated into the field-free analyser with a common energy. The lighter ions move through the analyser more rapidly than the heavier ions, providing mass separation of the ions. When the secondary ions strike the detector their masses are determined from the time it took them to travel through the analyser. This analysis cycle is repeated at high frequency to generate the complete mass spectrum with high dynamic range. ToF-SIMS has a unique range of characteristics:

• Parallel detection of all ions, organic and inorganic
• Unlimited mass range
• High mass resolution at full transmission
• High lateral and in-depth resolution
• High sensitivity in the ppm/ppb range

ToF SIMS is carried out in static mode, that is to say, the primary ion beam is maintained at a very low fluence typically less than 10^12-15 ions/cm² (sample dependent). This very low fluence ensures that secondary ions are not emitted from an area damaged previously by another primary ion, resulting in the emission of elemental secondary ions and molecular fragments. The structure and composition of these fragments is directly related to the structure of the surface from which they were emitted. Importantly, analysis of the type and amounts of secondary ions emitted from a sample under static SIMS conditions provides information about the atomic and molecular surface structure of the samples being analysed.

Mass Resolution

The ToF analyser has high mass resolution capabilities, often over 10,000 (m/Δm). This high resolution enables the positive assignment of peaks in complex systems through the ability to distinguish species whose masses only differ by a few millimass units, e.g. the identification of glutamic acid and lysine amino acids in an adsorbed protein film. Both have secondary ion fragments with a nominal m/z of 84, but separation of the two ions is possible with ToF-SIMS.

Sampling Depth

The sampling depth of SIMS is in the range of 10 to 20Å (inorganic materials), since only the particles in the outermost region of a sample have sufficient energy to overcome the surface binding energy and leave the sample. The sampling depth of most other surface analysis techniques, XPS for example, is typically ≥ 50Å.

Detection Limits

Due to its high transmission and parallel detection of all masses the ToF analyser is capable of high sensitivities. Depending on the samples, detection limits can be as good as part per billion, but more typically they are in the part per million.

ToF-SIMS Applications

SIMS provide a wide range of unique capabilities, and as such enables a wide range of applications, for the characterisation of both the surface and the bulk of a sample. Some brief examples are listed below:

• Detection of contaminants (surface and/or bulk)
• Characterization of surface modification treatments
• Examination of adsorbed/immobilized biomolecules
• Determination of surface structures and chemical imaging of patterned surfaces
• Chemical ion mapping of biological samples
• Determination of isotopic ratios (surface and/or bulk)
• Measurements of diffusion profiles due to environmental conditions (experimental or otherwise)
• Examination and characterisation of corroded surfaces and corrosion products
  
  

ToF-SIMS in the Department of Materials:ION-TOF ToF-SIMS V

Instrument specifications:

• Primary ion beam 25keV Bi (Bi⁺, Bi₃⁺, Bi₃⁺⁺)
• Sputter ion beams 0.5 to 2keV O₂⁺, Ar⁺, or Xe⁺
• Cluster ion beam Ar_n⁺
• Flood gun charge compensation for insulating samples
• Time-of-flight mass analyser
• Sample size up to 6cm by 5cm
• Heating and cooling stage (-150°C to 500°C) in load lock and main chamber
• Vacuum suitcase for transfer of air sensitive samples