ToF-SIMS

Principles
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface-sensitive analytical method that uses a pulsed beam of primary ion beams (such as Ar⁺, Cs⁺, O₂⁺, C₆₀⁺, … or microfocused Ga⁺, In⁺, Au_n⁺, Bin_x⁺, …) with several kiloelectronvolts of energy to eject and ionise species from the uppermost layers of the sample. The actual desorption or sputtering of material from the surface is the result of collision cascades or correlated atomic motions in the solid, which are initiated by the primary ion imprinting on the sample surface. A small fraction of the sputtered material is ionised during the emission process. The resulting atomic and molecular secondary ions, characteristic of the surface chemistry, are accelerated into a mass spectrometer, where they are mass analysed by measuring their time-of-flight from the sample surface to the detector.

PHI-EVANS TRIFT 1 instrument	ION-TOF V instrument

There are four different modes of analysis in TOF-SIMS :
1) Mass spectra are acquired to determine the nature of the elemental and/or molecular species present on the surface.
In the mass spectrometry analysis mode (and also in the imaging mode, see part 2), only the outermost layers of the surface are analysed (top ≈ 1 nm). The emitted secondary ions are extracted into the ToF analyser by applying a high voltage difference between the sample surface and the entrance of the mass analyser. They travel through the ToF analyser with different velocities, depending on their mass to charge ratio (E_k = qV =½mv²). For each primary ion pulse, a full mass spectrum is obtained by measuring the arrival times of the secondary ions at the detector and performing a simple time to mass conversion. Mass spectra of positive and negative secondary ions are obtained by inverting the high voltages applied on the sample holder and in the spectrometer column. For optimal performance, ToF-SIMS spectra are generated with very short pulses of primary ions (<1 ns). Indeed, the mass resolution or the resolving power (M/ΔM), a critical parameter for the separation of ions with the same nominal mass but different chemical compositions, is directly related to the width of the primary beam pulse.

2) Chemical images are acquired to visualise the distribution of individual species across the surface.
An image is generated by scanning the finely focused beam across the sample surface. Due to the intrinsic parallel detection of ToF-SIMS, the entire mass spectrum is acquired from every pixel in the image. The local mass spectra and the secondary ion images obtained from their compilation are then used to determine the composition and distribution of sample surface constituents. For technical reasons related to the primary ion beam pulsing, there is a certain trade-off between the best achievable mass resolution and lateral resolution. Therefore, the choice between the mass spectra and the imaging mode is made according to the type of sample and the desired information.

3) Depth profiles are used to determine the distribution of different chemical species as a function of depth from the surface. Recently, organic depth profiles have been found possible thanks to the development of the primary cluster beams (C₆₀⁺, Ar_n⁺) or very low energy reactive primary beams (200 eV, Cs⁺ or O₂⁺).
ToF-SIMS is able of shallow sputter depth profiling. For this purpose, an ion gun is operated in the DC mode during the sputtering phases, in order to remove material, and either the same ion gun or a second ion gun is operated in the pulsed mode during the data acquisition phases. Phases of sputtering and data acquisition are alternated until the complete depth profile is recorded (dual beam mode). With ToF-SIMS depth profiling, all the species of interest are monitored simultaneously, and with high mass resolution. Recently, the acquisition of depth profiles of organic samples with retention of the molecular information has been made possible with the development of primary cluster beams (C₆₀⁺, Ar_n⁺) and very low energy reactive primary beams (200 eV Cs⁺ or O₂⁺).

4) By combination of the high lateral resolution images and the depth profiles, 3-dimensional chemical reconstructions of microstructures are possible.

Advantages
- High mass resolution (M/ΔM > 10000). Different secondary particles with the same nominal masse can be separated (e.g. ²⁸Si⁺, CO⁺, CH₂N⁺ and C₂H₄⁺ all with m/z = 28).
- Mass range of 0 - 10000 m/z. Ions (positive or negative), isotopes, and molecular compounds (including polymers, organic compounds, amino acids) can be detected, identified and, to some extent, quantified.
- Trace element detection limit in the ppm range.
- Lateral resolution better than 0.1 µm in the imaging mode.
- Ultra-shallow depth profiling capabilities (including on organic compounds).
- Retrospective analysis, every pixel of the ToF-SIMS image or 3D analysis represents a full mass spectrum. Using the resulting 3D or 4D data matrix, mappings or depth profiles can be produced retrospectively for any masses and regions of interest (ROI) can be selected via computer processing after the dataset has been instrumentally acquired.

Limitations
- ToF-SIMS works under ultrahigh vacuum (< 10^-9 mbar), which is needed to increase the mean free path of ions liberated in the flight path. Thus, samples have to be compatible with these very low pressures.
- Generally, it does not produce quantitative results (often semi-quantitative).
- The optical capabilities are typically limited, making it difficult to find grains or specific regions of interest for analysis (particularly for the objects < 10 µm)
- Electrical charging may be an issue with some samples, although charge compensation routines are generally sufficient to overcome this problem.
- There is an image shift when changing from positive to negative ion data polarities. Therefore, it is difficult to collect positive and negative ion data on the exact the same spot.
- The quantity of data is very large. The benefit of retrospective analysis has a counterpart. As mentioned before, every pixel of an image produced by ToF-SIMS contains a full mass spectrum for that point. Thus, it may take hours, days or weeks to fully analyse a single data set. Consequently, it is extremely important to have a very clear purpose in collecting ToF-SIMS data, and focus on analysing and interpreting the data that are specifically related to the question at hand. To alleviate this problem, multivariate data analysis methods are also used to reduce the information and extract the most significant variations from the data sets (spectra, images, etc.).

Applications
ToF-SIMS is widely used in material sciences for the study of organic or inorganic materials such as polymers, biomaterials, semi-conductors, metals, catalysts.

Systems
- Two PHI-EVANS TRIFT 1 instruments are available:
The first one has been installed in 1992. Recently, it has been modified and equipped with a C₆₀⁺ ion source (instead of the original fitted Cs+ ion source). A Ga⁺ ion source also equips this instrument.
The second one has been installed in 2002 (it is a second hand instrument which was installed from 1994 in a private US company). This instrument has also been modified by adding a water flooding system¹. This instrument is also equipped with a Ga⁺ ion source.
¹Mouhib T., Delcorte A., Poleunis C. and Bertrand P., Organic Secondary Ion Mass Spectrometry: Signal Enhancement by Water Vapor Injection, 2010, J Am Soc Mass Spectrom, 21, 2005-2010.

- One ION-TOF V instrument has been installed in 2010. This instrument is equipped with four ions sources: O₂⁺ (or noble gas); Cs⁺; C₆₀⁺ and Bi_n⁺.

Access
All our systems are accessible to academic and industrial research teams. For additional information, please contact Claude Poleunis.

General literature (reviews) on the ToF-SIMS techniques and their applications
- Benninghoven A., Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), 1994, Angewandte Chemie International (in English), vol 33 #10, 1023-1043.
- Bertrand P. and Weng L.-T., Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), 1996, Mikrochim. Acta [Suppl.], 13, 167-182.
- ToF-SIMS Surface Analysis by Mass Spectrometry, Edited by Vickerman J.C. and Briggs D., 2001, ISBN: 1 901019 03 9, published by IM Publications (Chichester , UK) and SurfaceSpectra Limited (Manchester, UK)
- McDonnell L.A. and Heeren R.M.A., Imaging Mass Spectrometry, 2007, Mass Spectrometry Reviews, 26, 606-643.
- Mahoney C.M., Cluster Secondary Ion Mass Spectrometry of Polymers and Related Materials, 2010, Mass Spectrometry Reviews, 29, 247-293.