Your choice of target material depends on the application. For headlight housings and CDs, aluminium is preferred for high optical reflection. Most of our SEM using customers require (a) good electrical conductivity using a very thin coating, (b) the coating material should not tarnish and (c) possess a very fine grain structure. Greater resolution is possible with higher atomic number metals.
- Many metals are good electrical conductors (including Cu, Ag, Al), but they may not have other suitable properties. A thick specimen coating would hide small structures in the same way an increasingly thick layer of snow will hide landscape features.
- Very thin layers of sputtered (or evaporated) metals form islets which eventually merge. These islets are the graininess that may be seen at very high magnifications. Most oxidising metals are not well regarded for SEM coatings; the exception is Cr, which gives possibly the finest deposit of any metal. The target and coated specimens should be kept in a non-oxidising atmosphere for long term storage. This could be under vacuum or, more cost efficiently for large specimen collections, in a dry nitrogen gas desiccator. (See our page E12).
- Ultimate spatial resolution attainable in an SEM depends on several factors, and especially the average atomic number of the specimen. As a guide, the atomic number of the coating element is averaged with those of the specimen. So carbon (evaporated) onto a biological sample may at best reach the average atomic number eight. A biological specimen coated with gold (79) could, somewhat arbitrarily, rate an average atomic number of 43. It is not practical to evaporate uranium which is toxic and would oxidise. The most popular target metals for SEM are from the platinum group of elements (Pd, Ir, Pt).
The finest coatings are achieved by the simultaneous evaporation of carbon platinum; or tungsten (requires an electron gun). The finest coating using a sputterer is with Cr, followed by Ir, Pt, Au/Pd amalgam and Au. Carbon is very fine, but too 'soft' (low atomic number) for high resolution. Gold is most used for conventional SEM and is perfectly satisfactory. Finer deposits are only required when using magnifications over ~40k; for that usually a FESEM or a TEM is required. Gold and gold/palladium sputter well using base model sputter coaters running on a rotary vane pump only. Some other metals can also be sputtered from these simple instruments. However, oxidising metals (Cr especially) require a high vacuum pumped system and several of the heavier metals are much better sputtered by a larger instrument with a more generous power-supply. Iridium is a very brittle metal and we can only produce these targets at least 0.3mm thick - which makes them expensive.