TOF‐SIMS: Data Acquisition - mikee9265/SIMS-Wiki GitHub Wiki

An ideal instrument would produce the ultimate in mass resolution, lateral resolution, sensitivity, and signal to noise simultaneously. In the real world, the analyst must choose between different modes of use for the instrument. These are naturally instrument specific.

In many laboratories where routine TOF-SIMS analyses are performed with either a TRIFT or Reflectron instrument, the choice of primary ion for almost any analysis is Bi_{3</sub⁺. This small cluster produces higher ion yields, especially for molecular species and large fragments, and higher ion efficiency (the most organic ions and fragments you can get before damaging the sample) than any atomic ion. The secondary ion currents produced in instruments using this primary ion generally exceeds those produced by atomic ions, even though the atomic ions can be provided with higher primary ion currents under comparable conditions. Better lateral resolution may be obtained with Bi3⁺⁺, but the spectra are affected since this ion sees twice the acceleration voltage and impacts the sample with twice the energy. Large atomic ions such as Bi⁺ and Au⁺ in some cases may produce better inorganic ion yields, but this difference is usually not significant. Some laboratories have also found organic materials that are better analyzed with atomic ions, but there are no reports of methodical exploration of such observations in print. The atomic ions will tend to penetrate samples to a greater extent, so that these will produce more ions (generally atomic) coming from a greater depth within the sample, another consideration when choosing between cluster and atomic primary ions.}

Large cluster ions have their advantages, especially when it comes to the analysis of many organic materials, since for organic surfaces, the passing of the static SIMS limit does not destroy the organic signals. Further, larger molecules and fragments may be lifted off the surface intact and with less internal energy, sometimes making the detection of larger molecular ions and fragments more successful. The spectra produced are different relying as they do only on ionization mechanisms that do not require the use of an ionizing primary beam.

The largest clusters using Ar or H₂O are more problematic as a primary ion source in instruments and analytical modes relying on the primary ion pulse width for mass resolution, since they are not pure ion beams (there is a range of cluster sizes included in each pulse) and so cannot be easily narrowly focussed in time or space. They can produce rich spectra, and if those spectra give information about a sample that, say, Bi₃⁺ does not, then it may be better to use the large cluster source. For instruments that do not rely on short primary ion beam pulses such as the J series and hybrid OrbiSIMS, these beams are more attractive primary ion sources.

C₆₀⁺, considered an “intermediate” cluster ion, has the advantage that it can be focused into beams a few microns in size, and being a single well defined species can be produced with reasonable pulse lengths that allow for decent mass resolution. Focusing the beam well, though, reduces the primary ion current, making imaging analyses more difficult. Without imaging abilities, the uses of C₆₀⁺ are somewhat limited, and, of course, it cannot produce images that rival those that can be produced using Bi₃⁺.

When cluster beams are being used to profile samples as well as to be primary ion sources for analysis as when 3D analyses are being performed, one other issue affecting the ideal cluster size is the nature of the sample. Pure organic samples are usually best analyzed using large cluster beams. However, when samples contain both organic and inorganic materials, large cluster beams will rapidly sputter the organic materials while very slowly etching the inorganic materials. This leads to severe distortions of 3D profiling results in particular. These are situations where intermediate clusters such as C₆₀⁺ or Ar and H₂ with smaller cluster sizes (~500) will have an advantage. They may produce some degradation of the organic materials with depth, but the difference in their ability to etch organic and inorganic materials is much smaller.

TOF-SIMS instruments requiring a pulsed primary ion beam do not produce their best mass resolution and their best lateral resolution simultaneously. Better mass resolution is generally preferred in cases in which high lateral resolution is not needed. Peaks at the same nominal mass are better separated, and the exact mass of a peak can be determined with better accuracy. The more accurate the measurement of the mass, the more limited are the possible ion empirical formulae that a given peak may represent. Obtaining the best mass resolution involves compromising the spectrometer transmission (using lower voltages to increase flight times or inserting an aperture—the contrast diagram), so the analyst has to decide if these extra measures are really necessary.

The analysis of large surface areas up to or even beyond the static SIMS limit is best performed with a broader analysis beam. A small probe can only sample large sample areas. The ion beam spot will be smaller than a pixel when high lateral resolution modes are used over large areas. Since only a fraction of the imaged area will actually be analyzed, the instrument will effectively bore a small hole in the middle of each pixel. As a rule, the beam size should be at least two times the pixel size in order to ensure even irradiation of the surface. If you are going to need to broaden the beam anyway, you might as well take the higher mass resolution that can go with it.

Many analytical situations call for high mass resolution. Surface cleanliness assessments can often be performed with high mass resolution, and for trace metal analysis, high mass resolution is essential. High mass resolution should be chosen in cases where a contaminant to be identified is widely dispersed. Surfaces formed via bond failure are often reasonably homogeneous over large areas, and ideally analyzed at high mass resolution. Even in cases where good lateral resolution is needed, if there are multiple areas available, it is often worthwhile to do a high mass resolution analysis in one area and a high lateral resolution analysis in a second. The high mass resolution can be critical to understanding the significance of the high lateral resolution images.

At the point at which the area of the sample that is to be analyzed is significantly larger than 0.5 mm on a side, it is preferable to raster both the ion beam and the stage. The software stitches together each separate raster into the larger stage raster. The images can have many more pixels than a typical analysis, and the pixels may be quite large. With the primary ion beam spread over such a large area, the static SIMS limit becomes less of an issue. High data rates, though, ensure enough data is captured for each pixel in order to give the ion images sufficient contrast and significance without taking up an inordinate amount of instrument time. Too high a secondary ion current can damage the Trift’s detector, and produce intense peaks whose actual intensities are unknown due to dead time effects in the Reflectron. The EDR (extended dynamic range) method developed for IonTof's Reflectron instruments can be quite useful for these analyses.

Not only does the choice of the highest possible lateral resolution mode sharply reduce mass resolution (often so significantly that even inorganic and organic species cannot be distinguished) but it also means a reduced primary ion beam current and, therefore, a much reduced data rate. The best lateral resolution also requires fresh tuning before each analysis. The delayed extraction method available in IonTof instruments allows the use of high lateral resolution ion gun modes with reasonable mass resolution at the expense of some sensitivity and the loss of much of the low end of the spectrum below ~390 amu.

The other problem with high-resolution measurements is a result of the static SIMS limit. As noted above, the pixel size needs to be significantly smaller than the primary ion beam spot size. The raster size available is of necessity small. The ion beam current, needed to create useable secondary ion images, is now funneled into a small raster area with the result that most analyses will significantly exceed the static SIMS limit. Analysis of inorganic species may not be severely affected, but the analysis of most organic materials will be difficult. Substances such as methyl silicones and perfluoropolyethers may still be imaged, even with the damaged spectra produced with small raster sizes, but most other organic materials will be too heavily damaged before sufficient contrast is obtained in the ion images. If the sample is thick, and a large cluster beam is available to clear away the damage the primary ion beam produces, the static SIMS limit is less of an issue.

When using the ultimate lateral resolution of the TOF-SIMS, given that little information about organic materials will be produced, one is directly competing with the Auger in terms of the types of analyses you can perform. The analyses are still much faster than the Auger, but the results are also less quantifiable, at least without standards. The method is still useful in many cases because of the shorter analysis times, even if you have an Auger available, and Auger cannot work with insulating samples.

Both IonTof and Physical Electronics instruments have modes in which submicron ion beams are produced with excellent pulse times and, therefore, good mass resolution. The ion beam currents are less than what one can obtain with high mass resolution and much larger spot sizes, and the lateral resolution is many times worse than the ultimate lateral resolution than can be obtained with these tools. Nonetheless, this compromise mode turns out to be extremely useable. The raster sizes that are small enough to accommodate the small primary ion beam spot sizes are large enough so that in many more situations, useful information about the organic materials present at the surface are produced before the sample becomes too damaged for further analysis. The data rates are sufficient for the analyses to be typically completed in tens of minutes rather than the much longer times that would be needed with the far smaller ion currents available at ultimate lateral resolution conditions. The previously mentioned delayed extraction mode is another form of compromise, sacrificing some signal for added mass resolution with small sopot beams. The method also has the advantage of being far less sensitive to sample topography.

Best practices have been detailed for obtaining results that are repeatable (measurements are obtained with stated precision by the same team using the same measurement procedure, the same measuring system, under the same operating conditions, in the same location on multiple trials), reproducible (measurements are obtained with stated precision by a different team using the same measurement procedure, the same measuring system, under the same operating conditions, in the same or a different location on multiple trials) and reproducible (measurements are obtained with stated precision by a different team, a different measuring system, in a different location on multiple trials) (Spool and Linney 2024).

The most important of these practices enable analysts to get repeatable results.

• Normalization schemes may be used (selecting a suitable substrate peak, or using the total ion signal) but regardless of whether one is used or not, it is important to keep the primary ion current the same from run to run. Because there is often dead time affecting either the numerator (the signal being used for quantification) or the denominator (the signal or signals being used for normalization), or both, and likely to different degrees, differences in the primary beam current will generally contribute to irreproducibility. Faraday cups and/or external standards can be used to assure a consistent current.

• The acquisition setup should be standardized. This includes all the instrument settings, and acquisition settings.

• When analyzing insulating samples, care should be taken to minimize and standardize the use of charge compensation beams (low energy electrons and/or ions) including both their energies and flux.

• Especially if using relatively high primary ion currents and larger raster sizes in order to detect weak signals, keep your spot size significantly larger than your pixel size. Smaller spots can lead to the analysis of a smaller area within each pixel, and will not as attended address the full area that is being rastered.

• In cases where samples are not flat, control yje macroscopic topography as much as possible.

• In TRIFT instruments, take care to keep your analyzer voltages optimized. The 2nd micro-channel plate (MCP) degrades with usage, requiring stepped up voltages over time to keep secondary ion detection efficiencies unchanged.

• An ANOVA Gage R&R study can be invaluable to establish quantification schemes for the TOF-SIMS technique.

The following points help with getting reproducible results.

• Thoroughly document the analysis protocol that you will share between sites and personnel.
• Train personnel.
• If using “Faraday Cup” measurements to control primary ion flux and dose, be aware that current instruments do not come with holders having true faraday cups. Measurements depend on polarity, a sure sign that secondary electrons are escaping during current measurements. Measurements often depend on the holder used because the degree to which secondary electrons are captured varies from one holder to the next. Even the location of the beam within the cup can affect the measurement (try to stay well centered). Instruments such as the M6 with faraday cups built into the ion source help with repeatability, but beware possible differences from instrument to instrument. Run the same sample with the multiple instruments to be used to establish the method and any offsets resulting from differences in current measurements.

Replication of quantification can readily be managed between different models of TOF-SIMS is quite doable.

• Create an analysis protocol for each instrument.
• Establish repeatability (Gage R&R is ideal for this, see above) at each location/instrument.
• When using internal standards, correlation work may be needed.
• Run the same samples on all instruments. You need varying levels of the measured species.
• Establish a correlation calculation, if needed, that gets all the results onto the same scale.
• Verify that your correlation is consistent over time.
• If each instrument passes Gage R&R, the correlation should stand the test of time.

Replication across techniques can be trickier. When the different technique has been used to produce standards for the SIMS analysis, this replication process should be doable. Beware of how differences in analysis depth of the different techniques may affect the results. If the samples are not homogeneous within the analysis depth, then the techniques may legitimately get different results.