Secondary ion mass spectroscopy: Sensitive compositional analysis at the micro- and nanoscale

When it comes to elemental analysis of low concentrations or light elements, secondary ion mass spectroscopy (SIMS) is the technique of choice for materials characterization. SIMS not only allows the detection of element traces down to the parts per million (ppm) level, it also enables the detection of all elements of the periodic table, isotopes, and small ion clusters.

In this on-demand workshop with Dr. Fabián Pérez Willard, Carl Zeiss Microscopy GmbH, and Douglas Runt, Carl Zeiss SMT, Inc., gain insight into SIMS and discover the new ZEISS SIMS portfolio, including cost-efficient add-ons for ZEISS FIB-SEMs, as well as an innovative solution offering the current world record in SIMS lateral resolution. Selected applications in steel failure, battery, and solar cell research illustrate the use of the different ZEISS SIMS solutions.

Read on for highlights from the live Q&A session or register to watch the webinar at any time that suits you.

Q: Is it possible to quantify SIMS results?

FW: SIMS quantification is difficult because the SIMS signal depends strongly on the matrix effect. This means, depending on the surroundings of an ion, the probability for this ion to be produced changes. For example, I could have a low concentration of a certain ion, let's say ion A in matrix B, and a higher concentration of this ion A in matrix C, and still have a lower signal for the second case because of this strong matrix effect. This makes quantification of SIMS data difficult, although it is still possible by using a standard. If I have reference samples, then I am able to quantify the SIMS signal.

Q: Can SIMS be implemented in a 3D manner, using the focused ion beam scanning electron microscope (FIB-SEM)?

FW: We cannot combine SIMS with FIB-SEM tomography in the classical way. The reason for this is that the surface we want to analyze with SIMS needs to be at a shallow angle with respect to the detector, almost parallel to the detector. If we were to take a cross-section, the angle would be 90 degrees to the original sample surface, and therefore, the SIMS signal would be very, very weak. You could do 3D work by producing cross-sections, as we did for the solar cell at a very shallow angle, and just cutting in a similar manner at this very shallow angle. It would always then be possible to perform a SIMS map on the freshly cut surface. It would not be an automatic process and would require a lot of manual intervention by the user.

Q: How do SIMS in the helium ion microscope compare to dedicated SIMS instrumentation?

DR: Where you have a dedicated SIMS system you can get very high sensitivity and very high signals. You do lose a lot of the lateral resolution, and you also lose the correlative abilities. So, what you are going to be receiving back on a dedicated system is just your elemental, or your chemical compositions.

What we're able to provide with the helium or neon SIMS is the ability to correlatively review the area of interest with a high-resolution image, as well as being able to recognize and analyze the elemental analysis of that same area. We can still get that high resolution in the elemental analysis, but we can do it without the same kind of sample prep and create a much faster time-to-results with higher resolutions.

Q: Can you detect organic or biological samples via SIMS?

FW: One prerequisite for SIMS is that we need to have a conducting sample. If the sample is non-conductive, like very often tissue and materials like this are, then we can use or combine the SIMS measurement with a flat CAM. This makes it possible to do SIMS. The ion species we are using, neon or gallium, are quite light, so we will not be able to remove large clusters out of the samples. Typically, we can cover the whole periodic system with the gallium ion, and a big portion of it with the lighter ions, but that's a limitation if you're interested in measuring larger CH clusters, which might be the case in a typical lifetime application.

Q: Can SIMS analyze liquid and gaseous samples?

FW: We perform our SIMS experiment under vacuum conditions, so the samples need to be vacuum compatible. We can analyze liquid samples if we freeze them first. We could then do SIMS on the frozen sample. It’s called an experiment under cryogenic conditions.

Q: How many liters of nitrogen or helium would be needed?

DR: To create the neon beam or the helium beams, we are really extracting a single atom of either helium or neon. We are actually leaking a very small amount of helium or neon in order to create that beam. The consumption of helium for a standard year of usage of a helium ion microscope is usually less than 100 milliliters of helium or neon, depending on how heavy you use it, or what your application sides are. We can usually use the same bottle of helium or neon for a number of years, regardless of what your application is.

Q: Can you apply SIMS to the nanoparticles that are made from organic compounds and to the nanomaterials that are found on the surface?

FW: It depends, of course, on the size of the nanoparticles. The resolution we can achieve with the SIMS solutions is of the order of 35 nanometers for the gallium FIB, and much better for the neon FIB. If the particles are larger than this scale, then it would be possible to make a measurement on both FIBs.

Q: Can you use SIMS to analyze sodium in situ? Are there any possible contaminants to be aware of?

FW: Sodium is a good ionizer, so sodium would be an ideal element to look at with SIMS. Of course, the problem with sodium is that it is present on almost any surface, but the advantages here with the ion beam is that you could do depth profiling. You could remove parts of this initial sodium signal, that gets deeper into the sample, and then stop to measure the real sample that might have been exposed to air and therefore has already some sodium covering the surface.

Q: Is it possible to quickly switch between negative and positive ion modes with TOF-SIMS and get a pseudo real-time spectrum in one go?

FW: For the TOF-SIMS atom — this is the atom we use on the FIB-SEM instruments — we can indeed switch between positive and negative ions. This is the same for the helium ion microscope SIMS add-on. However, the switching takes a bit of time, and what we typically do is we acquire a full spectrum, either in positive or in negative mode. We are not switching between the acquisition of its spectrum.

Q: Does using silicon wafers reduce the surface charges of the samples?

FW: Yes. If you are thinking of spraying nanoparticles on a substrate, silicon would be a nice substrate, because it is conductive. It would make the measurements easier, and also flat, which is an advantage.

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