TOF‐SIMS: Sample Preparation - mikee9265/SIMS-Wiki GitHub Wiki

Having decided that TOF-SIMS, alone or in conjunction with other methods, will serve to solve a problem, analyze a failure, or assess a set of samples, the analyst needs to choose among the many analytical options the available instrument(s) allows for the collection of data.

Sample Transport

Samples need to get to the SIMS lab without being compromised. Similar to other surface sensitive methods, the TOF-SIMS is most effective when the samples are clean, or at least, when they are not contaminated after the process of interest. Many plastic containers have mold release agents and/or plastic additives that can be mobile and can migrate to enclosed samples. Tapes that may be employed to immobilize samples during shipment will usually have a release layer, most likely silicone based, which can migrate onto the sample surfaces. The analyst should participate in the choice of containers and other materials used to get samples to the lab.

Following a sample’s arrival in the lab, it needs to be mounted in a sample holder specific to the instrument in such a way that data acquisition is optimized.

Sample Mounting

TOF-SIMS is best performed on flat samples. The instruments are designed to optimally extract ions from flat surfaces. There is generally only a distance of a 1-3 mm in the gap between the sample and the extraction optics, so that any macroscopic topography could actually result in a portion of the sample touching the extraction optics. This will, at the least, short the potential difference, hinder analysis and, at the worst, contaminate or damage the system. Macroscopically concave samples are thus dangerous to TOF-SIMS instruments. Convex samples can be more readily accommodated, but generally only a portion of the surface can be analyzed. The sides of any macroscopic feature create a field that will accelerate ions in a direction perpendicular to the extraction axis. Even if such ions make it into the extractor, they are less likely to make it to the detector, and if they do make it to the detector, their time of flight will be different than that of ions with the same mass whose flight is more “normal”. Finally, the intensity of such ions, being a function of the angle of their ejection from the surface, will be greater for this surface than it would be for an ion leaving a flat surface, and so variations in intensity may be driven as much by the topography as by the nature of the sample surface. Some adjustment can be made during the analysis for topography problems, but there are limits to the effectiveness of these methods.

Sometimes mounting the sample properly can reduce the effects of the topography of the sample. In the magnetic recording industry, the magnetic recording heads are small (mm) sized blocks of ceramic, the edges of which are problematic for good ion extraction in TOF-SIMS instruments. The samples can be mounted, however, in a field of such samples. Putting a block of ceramic with the same height directly adjoining the sample of interest alleviates the edge effects. Other samples can be mounted in miniature vises so that the holder’s edges are at the same height as a relatively flat portion of the sample. Convex samples can be placed with their high point thrust into an aperture in the holder, again placing the holder surface at the same height as an area of interest of the sample, thereby straightening the extraction field. When a given sample type requires repeated analysis in a given laboratory, ingenious mounting devices can be designed to eliminate many topographic effects. Many clever schemes for sample mounting have been devised (Reich 2013).

As a last resort, samples can be cut to the size and shape needed. The trick to doing this lies in keeping the surface of interest clean. As with all surface analysis techniques, a surface exposed to cutting fluids or even just the dirty environment of a machine shop may produce results that are a larger function of the sample preparation methods than of the situation that drove the need for the analysis. If the sample is plastic that can be easily cut, careful use of a clean X-Acto knife or similar tool can produce clean samples. Punches and sheers can be kept clean and further can be used without direct contact to the surface to be analyzed. In the case where there is no help but to use machine shop level tools to produce a sample suitable for analysis, the surface of interest can be protected. The area around the surface can be coated with a clean adhesive or a silicone-free double-sided sticky tape. The adhesive or tape and the area of interest can then be covered with a piece of aluminum foil. Aluminum foil is remarkably clean, and will usually serve as an excellent protective layer for the surface of interest while the larger sample is mutilated.

Some samples have a topography that cannot be improved by the method of mounting. Woven fabrics and other samples with an intrinsic topography fall into this category of sample. For a sample such as this, the triple focusing time of flight (TRIFT) is naturally better able than a reflectron to deal with the difficult topography. The system is simply able to handle larger secondary ion velocities parallel to the sample surface. In the IonTof Reflectron systems, a delayed extraction method has been developed that greatly improves the results obtained from difficult topographies. In addition, an “advanced TOF correction” algorithm can be invoked that, given sufficient signal, adjusts the mass calibration for localized areas of the sample to improve mass resolution for the analysis of samples with roughness or height differences that would otherwise compromise the resolution.

Some samples are just too difficult to mount properly. In addition to those samples that have little by way of a flat surface, or any surface of interest that is highest in the direction of interest (another requirement that needs one to mount the sample so it will not run into anything in the instrument), some samples will just be too big and either cannot be cut or are too valuable to be damaged in the course of the analysis. In these cases, another technique is indicated.