Physical vapour deposition is a process that has been in use for over a hundred years and is used widely within current technology and manufacturing.

The process involves feeding a substrate into a vacuum chamber containing two powerful magnets where a controlled gas is added and the magnets pull atoms from the substrate. The atoms collide with each other in their gaseous state before condensing into a plasma and drying into a thin film on the substrate. The final product is a thin but durable coating which is very useful across a variety of applications. The choice of target material to coat depends on the application.

Applications include:

	Semiconductors - electronics incorporate essential components which have been produced with tantalum sputtering targets. These include microchips, memory chips, print heads, flat panel displays as well as others.
	Glass Coating - Sputtering targets are used to produce low-radiation coated glass (Low-E glass) which is commonly used in building construction with its ability to save energy, control light, and aesthetics.
	Solar Cell Coating - Third generation, thin-film solar cells are prepared using sputter coating technology.

 

Metals: Many metals are good electrical conductors but may have other unsuitable properties for the intended application. Headlight housings and CDs, aluminium is preferred for high optical reflection. SEM requires a good electrical conductivity using a very thin coating, a coating material that will not tarnish and possess a very fine grain structure. Greater resolution is possible with higher atomic number metals.

Oxidisation: Most oxidising metals are not well regarded for SEM coatings except for chromium, 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.

Coating thickness: Thick specimen coatings can hide small structures and very thin layers can form islets which may eventually merge. These islets are the graininess that may be seen at very high magnifications. 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 chromium, followed by iridium, platinum, gold/palladium amalgam and gold. Carbon is very fine but has too low an atomic number (soft) for high resolution.

Mounting: Most targets are held by a cover or are crimped around the edges, if not, it must be glued. The 'glue' also must be electrically conductive. ‘Professional mounting’ uses a few slivers of indium wire on the support plate, cover with the target and then either place in an oven with a flat weight on top of the target, or use a smoothing iron to heat the gold and melt the indium. A piece of lens tissue on the target will protect it. A temperature a little higher than melting point of indium is required (156.6°C) for a short time. Alternatively silver conductive paint, preferably a few small drops of paste is workable. Conductive paste can also be made with silver powder and commercial epoxy. Only spot gluing is required but keep weight on the target while drying/curing. 

Sputter coating for SEM

Ultimate spatial resolution attainable in an SEM depends on several factors, and most importantly the average atomic number of the specimen. As a guide, the atomic number of the coating element is averaged with that of the specimen.

For example, carbon (evaporated) onto a biological sample may have an average atomic number eight. A biological specimen coated with gold (79) could be given an average atomic number of 43. It is not practical to evaporate uranium which is toxic and would also oxidise during the process. The most popular target metals for SEM are from the platinum group of elements, palladium, iridium and platinum.

Gold is most used for sputter targets in conventional SEM. Finer thickness 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 (chromium 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 these targets are generally 0.3mm thick - which makes them expensive.

 

See here for a full list of Sputter Coaters

See here for a full list of sputter targets