TOF‐SIMS: 3D Analyses - mikee9265/SIMS-Wiki GitHub Wiki
With the use of atomic and small cluster primary ions, beyond the static SIMS limit one generally found damaged samples from which only ion beam altered surfaces could be measured. The advent of cluster ion beam sources (Mahoney 2013), and in particular massive Ar cluster ion beams, has made possible in depth analyses where the nature of the organic material is largely preserved for further analysis. These ion beams, with low amounts of energy per incident atom and lower velocities, are known to sputter the surface in a completely different way from atomic ions. Instead of the collision cascade, a very different process takes place, one in which the incident atoms do not penetrate the sample very far, in which there are cooperative movements that can propel surface species off the surface intact, and in which very little subsurface damage is produced. The high sputter yields of these clusters efficiently remove the damaged surface before it can build up. It is also quite possible that some of the damaged material is volatile and simply pumps away. In any event, XPS analyses of Ar cluster sputtered organic materials show very little change from that analyzed before sputtering begins.
The large cluster ion sources (C60, massive Ar) produce very different mass spectra from those produced by smaller primary ions. There is generally less fragmentation, and a sometimes very different pattern of peaks is found in the spectra. Unfortunately, the ion beams are generally much less well focused (especially for the massive Ar clusters, which are also difficult to focus in time, making for poor mass resolution). Using these beams alone, one is generally limited to obtaining molecular depth profiles, and at best limited 3D imaging. For this reason, these beams are often used in conjunction with the liquid metal ion source (LMIS) beams that are commonly used in static SIMS 2D analyses as the sputter gun.
There are multiple ways in which a given cluster ion source, used in conjunction with a liquid metal ion gun (LMIG) primary ion source, can produce 3D images. The first is simply to analyze the surface while etching the sample, getting TOF-SIMS data for each successive layer. Generally, one will be analyzing, in these cases, an area of the surface much wider than the depth of the sputter profile. A second method is to use a focused ion beam (FIB) to etch the sample, producing a cross section. You then etch further (“polishing” the surface) using your cluster ion source in order to remove the damage created on the surface of the exposed cross section by the FIB. This is followed by TOF-SIMS analysis of the exposed surface, usually with an LMIG ion source. These steps are repeated until a full 3D data set is obtained, a kind of tomography. This set will have a much lower aspect ratio than that obtained using direct profiling. In either case, you are doing what amounts to a normal TOF-SIMS analysis on surfaces exposed by the cluster source.
Exciting applications of 3D imaging of organic materials, in particular, include the analysis of biological systems from tissue samples to cells. For this work, analysts are constantly seeking high sensitivities, since the number of actual analyte molecules in the small volume of a voxel at the lateral and depth resolutions being attempted is small. TOF-SIMS has the advantage over most competing techniques in that the sample preparation involves getting the sample into a shape and size and condition (ex. frozen) that is vacuum ready, but that chemical treatments are not necessary. It has the disadvantage, mentioned earlier, of orders of magnitude variation in sensitivities for different organic species. The TOF-SIMS has proven its worth in the analysis of lipids and a variety of small metabolites. It has not done well in the analysis of proteins. Experiments with novel ion beams intended to maximize ion yields, especially in biological systems, show that protonation enhanced ion formation is possible (Sheraz et al. 2015).
3D imaging can compound sensitivity problems. Many 3D analyses involve analysis with an LMIG for the lateral and mass resolution it enables, mixed with sputtering using an Ar cluster source. However, the material sputtered with the Ar cluster source is not analyzed. Unfortunately, the LMIG will quickly damage the sample. In organic depth profiling most of the sputtering must be done with the Ar clusters. In a biological system in particular, this can be problematic.
To solve issues of loss of materials during profiling, researchers have developed new TOF-SIMS instruments designed for the analysis of DC sputtered materials.
3D imaging is, of course, subject to the same issues as 2D imaging in the TOF-SIMS. It is no surprise that matrix effects can cause molecular depth profiles and 3D images to fail to produce direct information on species concentrations. The attempts to do 3D imaging have brought some of these issues to the fore. In particular, it has been noted that mixing compounds of different acidities or basicities leads to very nonlinear responses versus concentration. Aside from the possible issues with protonation leading to M+1 ion formation, salt formation changes the ease with which species will desorb intact with sputtering.
Instrument software will generally show 3D images assuming that the surface is flat. If the sample has significant topography, this will obviously lead to distortions in the 3D images. One instrument manufacturer has a built in AFM option and accompanying software precisely to meet this challenge. Analysis of the topography before analysis can be used to adjust the results.
Another problem, one that is well understood for depth profiling inorganic materials, is that profiling a mixed material surface where the different materials have different sputter rates will lead to distorted 3D images. The problem can be particularly difficult for some polymers for which sputtering tends to break up the backbone of the polymer, leading to very high sputter rates (in some cases, simple evaporation of the small molecules produced adds to the material loss). If such a polymer is mixed with less readily sputtered materials, the distortion of the 3D images can be extreme. Topographic analysis before and after the profile can be used to help correct the data, but obviously if the sample is not columnar but instead has mixed phases, this can become quite a complex process, possibly involving topography checks at multiple points in the profile. Sputter rates derived from the topographic analyses need to be assigned to the different regions to allow expansion of the apparently thinner regions with the higher sputter rates to have their depth better match reality. Composite materials with inorganic fillers in an organic matrix represent the ultimate challenge, since inorganic species are barely sputtered by cluster ion beams. Reducing the size of the cluster (with Ar to Ar500+ or so) allows for etching of inorganic species, but then damage to the organic signals turns up in the profiles. This is a challenge that still remains for the method.