An Introduction to FIB-SIMS Using the fibTOF

FIB-SIMS

FIB-SIMS is most useful for high resolution chemical imaging, at spatial scales or for materials where alternatives such as an SEM with EDS become difficult to use. FIB-SIMS inherently produces three-dimensional images, and it is very well suited to obtaining depth profiles from small areas (especially if quantification is not essential). FIB-SIMS can be used to obtain chemical images at lateral spatial resolutions of tens of nanometers, with depth resolutions of less than 10 nanometers. Excellent contrast and sensitivity is obtained for light elements such as boron and lithium, and for halogens such as fluorine and chlorine.

FIB-SIMS Technique

FIB-SIMS refers to the use of secondary ion mass spectrometry (SIMS) in conjunction with a focused ion beam (FIB) microscope. The FIB provides a source of energetic primary ions, which interact with the sample being analyzed to produce secondary ions (e.g. when imaging or milling a sample with the FIB beam). The secondary ions, which contain information about the chemical composition of the sample at the location of the FIB beam, can be collected and identified by a mass spectrometer such as TOFWERK’s fibTOF. Together, a FIB beam and secondary ion mass spectrometer allow high spatial resolution 2D and 3D chemical images or depth profiles to be obtained with a resolution and sensitivity that can be better than that of an EDX instrument.

Why TOF?

‘TOF’ in the fibTOF product name refers to ‘time-of-flight’. The fibTOF mass analyzer measures the time-of-flight of secondary ions given the same energy, over a fixed distance, and converts this time to a mass to charge ratio for a mass spectrum. The advantage of this technology over alternatives, such as a quadrupole or magnetic sector mass analyzer, is that it collects information about the entire mass spectrum all the time. With this technology there is no need to scan over mass to charge ratios. This has the advantage when milling nanoscale structures (or depth profiles in thin films) that there is no danger of missing an important mass peak because your specimen has been used up.

How do FIB-SIMS and the fibTOF differ from other SIMS implementations?

  • Dedicated secondary ion mass spectrometers generally use an ultra-high vacuum (i.e. less than 1×10-9 mbar) whereas FIB chambers usually operate at a higher pressure and this has some consequences for FIB-SIMS. The residual gases in the FIB microscope chamber (mostly water vapor, nitrogen and oxygen) interact with the surface of the sample, increasing the secondary ion yield for many species detected in positive ion mode. Although this boost to the signal is often useful, it does mean that the observed SIMS signal depends on the chamber history, and also in a non-linear way on the beam current and scan pattern (because of a competition between sputtering of the target surface and adsorption of residual gases).
  • The fibTOF is a time-of-flight secondary ion mass spectrometer, but this terminology (TOF-SIMS) is also used to refer to a particular technique in which slow primary ions are used to sputter only a fraction of a monolayer – so-called static SIMS. This is useful when observing organic materials, as there is then a better chance of detecting molecular ions or recognizable fragments. In normal operation using a gallium ion beam on a FIB-SEM microscope with the fibTOF (sputtering at least nanometers of the target – so-called dynamic SIMS) we do not expect useful yields of molecular ions from the target.
  • FIB-SIMS is usually carried out on so-called ‘dual beam’ FIB-SEM microscopes. These microscopes can be configured to become powerful platforms for scientific research involving gas injection systems for FEBID/FIBID deposition of nanostructures or the use of a an array of analytical detectors such as EDX, EBSD, XRF. This is in contrast to dedicated SIMS instruments, which perform SIMS excellently, but can’t do much else.

Spatial resolution

The spatial resolution achievable in a FIB-SIMS image depends on the spot size of the primary ion (FIB) beam, the energy of the beam, the nature of the sample, as well as the secondary ion yield. In general, the best possible lateral resolution of a FIB-SIMS image will be some tens of nanometers – larger than the best possible FIB spot size. Depth resolution is better at lower primary beam energies, because this reduces both the penetration depth of the primary ions and the extent to which mixing occurs at interfaces within the sample.

Sample preparation

As is usual for high vacuum charged article microscopy, the sample must be prepared to have at least a conductive surface, otherwise the accumulation of charge on the sample will led to distortions in the image. The fibTOF is even more sensitive to this effect than SEM images: local potentials of even a few tens of volts will result in severe degradation of the SIMS signal, and for some materials alternative sample preparation methods may be required. Sometimes, it is possible to use an electron flood-gun or even the electron microscope beam to compensate for charging.

How can I learn more about the interaction of primary ions with my sample?

FIB-SIMS performance depends on how the focused ion beam interacts with the sample, because this determines how many atoms and ions are sputtered from the sample. A list of freely available codes supporting the simulation of the scattering of primary ions within a target, and in some cases also the estimation of sputter yields (but not secondary ion yields) can be seen at IonBeamCenters.eu. TOFWERK staff have in the past used SRIM to make estimates of the best possible spatial resolution for a SIMS image.

Other FIB-SIMS and fibTOF Questions

Can I measure both positive and negative secondary ions?

Yes, but not at the same time. To switch between measuring positive and negative secondary ions, the electric potentials within the fibTOF have to be altered which requires a few minutes. If the same beam conditions are chosen, and the target allows two nearby depth profiles to be made, it is possible to overlay the positive and negative ion results. Bombardment by the primary ion (FIB) beam will in general produce both negative and positively charged secondary ions, although this depends on the material matrix and the elements of interest.

What primary ions should I use?

This depends on your sample, what you want to do, and what primary ions (FIB) you have available. In general, tradeoffs must be made between the achievable spot size (which affects the SIMS spatial resolution), the beam current (which as well as affecting the spot size, also controls how much material can be sputtered in a given time), and the energy and mass of the primary ions (affecting sputter yield and secondary ion yield). In general, if spatial resolution is a priority, fast light ions are preferred (Ga+ and Ar+ make good general purpose beams with similar sputtering effects). If the yield of molecular ions is important, heavier slower primary ions are preferred. Your microscope vendor can provide further information about the tradeoffs.

Where can I learn more about (FIB)-SIMS?

For a cutting edge academic perspective, the International Conference on Secondary Ion Mass Spectrometry is held every two years.

Further Reading

Whitby, et al. High spatial resolution time-of-flight secondary ion mass spectrometry for the masses: a novel orthogonal ToF FIB-SIMS instrument with in situ AFM. Adv. Mat. Sci. Eng., 2012. DOI: 10.1155/2012/180437

Stevie, F.A  Focused Ion Beam Secondary Ion Mass Spectrometry (FIB-SIMS) pp 269-280 in Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques and Practice Springer (2005) ed. L. A. Giannuzzi and F A Stevie, ISBN: 978-0-387-23313-0

Pillatsch et al. FIBSIMS: a review of secondary ion mass spectrometry for analytical dual beam focussed ion beam instruments. Progress in Crystal Growth and Characterization of Materials, 2019. DOI:10.1016/j.pcrysgrow.2018.10.001

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