Samples destined for viewing on the scanning electron microscope (SEM) are often coated with a thin layer of metal beforehand (sputter coated). Liz Girvan (Microscopy Otago) explains the purpose of sputter coating and shows how it’s done.
Point of interest: Look out for the metal-coated fly in this clip.
Sputter coating is popping a really thin layer of metal on top of our sample surface. The layer we’d use is between 1 and 10 nanometres, so it’s nice and thin, it’s not going to cover up any of the sample, but it’s going to add a nice wee layer so the sample becomes conductive.
If the sample’s not conductive, we won’t see anything, so what will happen is the electrons that land on the sample and go through the sample can’t get away to earth so they build up inside the sample and cause artefacts, which we call charging.
So we would get our sample, attach it to a holder, so it’s attached quite firmly. We’d load it into the sputter coater, shut everything up and create quite a high vacuum in the coater. We would then pass a voltage over the top of the coater where the metal target is sitting, and that creates a little electric field inside the coater itself. We then bleed gas into the chamber, and the gas molecules get all excited and they kind of hoon around and knock little bits of the target material off. Those little bits of the target also move around inside the chamber quite randomly, and they find their way and land on top of the sample, creating quite a thin layer.