Sample Handling

One of the foremost precepts of medical care is "First, do no harm." That is equally true for sample handling prior to analysis. I cringe every time some one brings me a sample for analysis of contamination and the first thing I spot is a collection of big fingerprints. When I point out that they kind of render the analysis useless, the culprit often asks, "Well, can't you just wash them off?" No, we can't.

I know that this is obvious but contamination can happen in mysterious ways. If the sample has to be cut to fit into the analysis machine, as often is the case, make sure the cutting tool is clean. I know that is obvious too but a tiny amount of adhesive or other organic material can be on the edge of a cutter used to snip a disc. Wipe it off with clean solvent and allow it to dry fully before cutting.

Do not use anti-static bags to transport samples. These may have the appearance of being protective because they protect against ESD but the anti - static material is a coating which is transferred to the surface of whatever touches the bag. If your sample requires surface analysis, it is the anti - static coating which will be the analytical result. Large samples like discs and wafers should be transported in disc or wafer carriers. Smaller samples can be transported in gelpac boxes. If you feel the sample requires wrapping, then plain old grocery store aluminum foil is the material of choice. It is cheap, readily available and does an excellent job of protecting the sample.

If the sample is small or loose and has to be removed from the wafer or disc, do not use cotton swabs, do not use cotton wipes and do not use Scotch or other cellulose desk tapes, be they single - or double - sided. The reasons for each no - no are slightly different. Do not use cotton swabs to remove a sample for three reasons. First is that the swab is a huge haystack of fibers with immense spaces in between. Small particles that can be seen with the naked eye are far down and inaccessible when examined in the SEM. Secondly, cotton swabs charge up like the dickens and it is nearly impossible to coat them to eliminate this charging without also losing the sample to be analyzed. Thirdly, cotton swabs are filthy. If the sample is organic, it will be completely overwhelmed by the cotton oil from the cotton in the swab.

Do not use cotton wipes to remove stains or grunge from containers for analysis for the same reasons as cotton swabs. These wipes are often picked up specifically to obtain samples from the inside of reaction chambers. The samples are often little more than stains on the cotton and are essentially invisible in the SEM. If they are organic then again, the cotton oil makes the foreign material unidentifiable.

If you have the type of sample that you think is crying out for a swab or a wipe, then, there are swabs that are specifically designed for this purpose. They are made by Texwipe for one. They are made of a dense polymer mesh that has no oils or other extractables and are resistant to common organic solvents so that we can wash the contamination sample back out to do the analysis.

Finally, if you have particles that need to be transferred to something, do not use Scotch or other desk tapes for several reasons. First is that these tapes are fairly complex chemically and make it difficult to determine what part of the spectrum is coming from the tape rather than the sample. Secondly, the tape is only sticky on one side and so must be held down somehow for analysis. Third, the tape is non-conductive and so must be coated with gold or something conductive to permit the analysis. This can further confuse the process.

A far superior alternative is double - sided conductive carbon tape. It is available from SPI Supplies to name one source. Since it is conductive, it does not require coating. Since it is double sided, it fastens itself to the sample holder and since it is just carbon, its contribution to the spectrum is very simple.