Metal Coating

Metallic coatings can be used to boost the secondary electron emission of the specimen as well as rendering it conductive. Aluminum, gold, platinum, chromium, tungsten, tantalum, and palladium are common metal used to coat specimens. The two most common methods of coating are thermal evaporation and sputter coating. With thermal evaporation, there is the risk of radiant thermal damage to the specimen. Also, the metallic particles may retain enough heat to burn into the specimen. Sputter coating is generally the preferred method of specimen coating. Sputter coating takes place in a vacuum chamber. The specimen to be coated is loaded upon the anode. A vacuum is generated. Prior to coating, the vacuum is compromised with an inert gas (usually argon). When a high tension is applied to the cathode where the metal source resides, the argon gas molecules are attracted to the cathode. The ionized argon strikes the metallic target, knocking loose metal grains, which are attracted to the anode. Due to the directional randomness of the argon/target collisions, and omnidirectional coating is achieved upon the specimen.

There is discussion in the literature about coating versus decoration. Sputter coating of gold and gold/palladium mixtures is referred to as decoration. When a metal grain from such a source strikes the surface of a specimen creating islands of coating. Metals such as chromium, tantalum, and tungsten tend to stick where they land upon the surface of the specimen. Therefore, they are coatings.

Grain size of the metal produced is also important. Smaller grains provide better resolution. This is because they obscure less specimen detail. Au and Au/Pd grain sizes are about 2-2.5 nanometer. Cr and W can produce grain sizes on the order of –1.5 nanometer. Thus, Cr and W coatings can generate higher resolution images.

In the discussion of signal generation, it was stated that the amount of backscattered electrons increases with increasing atomic number. It was also stated that the BSE signal lacks high-resolution information. From these two statements, what can be predicted about gold vs. chromium coatings? Chromium, having a lower atomic number than gold, generates fewer backscattered electrons. This makes it a better coating for high-magnification use. It takes a special field emission (in-the-lens) SEM to resolve the coating grains. The average SEM cannot take advantage of the increased resolution of the Cr coat.

Once prepared, the sample should be stored in a vacuum desiccator. This prevents hydration to atmospheric humidity levels and reduces oxidation of the metal coating. Most specimens can be stored indefinitely with little appreciable degradation.