Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a very high sensitivity method for elemental and molecular analysis of the extreme surface (< 1 nm). It is also possible to obtain a profile down to 20 µm by alternating analysis and abrasion cycles with an Argon cluster (GCIB) for organic materials or a Caesium (Cs+) and Oxygen (O2+) gun for inorganic materials.
All materials compatible with ultra-high vacuum (10-10 mbar) can be analysed with this technique.

The ToF-SIMS can detect trace elements down to ppb in extreme surface (< 1 nm) and up to 20 µm in profile mode on all types of samples compatible with vacuum.

A pulsed source of primary mono or multi-atomic ions (Ga+, Bin+, Au+, C60+, ...) with an energy of a few keV bombards the surface of the sample. The interaction between the material and these ions gives rise to different types of emission. The ToF-SIMS is interested in positive and negative secondary ions. 

In static mode, it uses a total primary ion dose of less than 1012 ions per cm2 , i.e. less than one primary ion per 1000 atoms of the surface, to acquire a spectrum. ToF-SIMS is therefore a soft ionisation method that allows surface molecular analysis.

The secondary ions formed are then focused and accelerated with the same kinetic energy in the time-of-flight analyser, which separates them according to their m/z ratio with very good mass resolution (ΔM/M > 10,000 at mass 28). The resulting mass spectra represent the intensity of the secondary ions as a function of their travel time, which is proportional to the square root of the m/z ratio.

Using a scanning device for the primary ion beam, it is possible to map the different elements present with sub-micron resolution.

By alternating acquisition and abrasion sequences, a compositional profile can be drawn with nanometer depth resolution.

MASS SPECTRA

Acquisition of the mass spectrum to a depth of < 1 nm
The secondary ions, positive and negative, are mostly mono-charged (z = ±1), those multi-charged appear only in the spectra of monoatomic samples (e.g. aluminium). The spectra are mass-calibrated and analysed qualitatively on the basis of the atomic elements and their molecular combinations.

IONIC IMAGES

Mapping of individual elements and molecular compounds with submicron resolution
The primary ion beam is reduced to a small diameter spot and scans the surface to be imaged. The secondary extraction and mass analysis optics are fixed. The image is reconstructed by synchronising the secondary signal with the primary beam scan. The lateral resolution of the images depends on the size of the micro-beam (from 100 nm to 3 µm in diameter depending on the analysis conditions).

DEPTH PROFILES

Acquisition of a mass spectrum at different depths down to 20 µm
They are obtained by alternating analysis and abrasion sequences. The abrasion rate varies according to the density and chemical nature of the abraded layers; it must be measured for each medium traversed. It is therefore necessary to calibrate the instrument by using deposits of known thickness or by measuring the depth of craters using optical (interferometry) or mechanical (Talystep) methods.

ToF-SIMS technical specifications 

• Primary ion source: Bi+, Bi3+, Bi3++
• Signal detected: secondary ions
• Elements detected: from H to 10,000 D
• Type of analysis: elemental, molecular and semi-quantitative
• High sensitivity: < ppm... ppb or femtomole
• Lateral resolution: 100 nm to 3 µm
• Depth resolution: nm
• Charge neutralization for insulator analysis (Flood Gun)
• Coupling of guns (analysis/abrasion) for profiling: O2, Cs, Argon clusters

ToF-SIMS strenghts

• Analysis of all solid samples compatible with ultra-high vacuum
• Extreme surface analysis < 1 nm
• Very low detection limits
• Elemental and molecular identifications
• Isotopic sensitivity
• Depth profiles of multilayers
• 3D imaging