SEM: Sample Coater

All non-conducting samples have to be coated with an electrically conducting layer, unless you are operating in VP mode, or unless you carefully adjust the beam voltage so beam current = backscattered plus secondary electron production. As of this writing you have two coating options: gold-palladium and carbon. Coating is done with a Denton coater which works pretty well. Instructions for the two coating types are in the log book next to the coater. A gold-palladium alloy coating has a high atomic number, so it generates a lot of secondary electrons. It’s preferred for imaging surface topography. The plasma sputtering that deposits the coating allows it to cover rough surfaces, even modest holes and pits. Carbon coating is preferred for X-ray chemical analyses and mapping, especially of polished surfaces. Carbon coating doesn’t cover rough surfaces well, and it is usually much thicker than gold-palladium, which is not so great for high-resolution imaging.

The big white box is the coater, with the vacuum pump attached in the back. The box has the vacuum and gold-palladium sputtering controllers and power supplies. The main switch is on the back right.

The gold-palladium sputter coating lid is permanently mounted on the coater and uses a short glass tube.

The carbon coater power supply, taller glass tube, and its own lid are to the right.

Here the lids have been removed.

The gold-palladium sputter lid (left) has been raised and rotated out of the way to the left to allow removal of the glass tube. This reveals the sample holder (small black puck) on a post.

For carbon coating the carbon coater lid and glass tube (right) replace the equivalent sputter parts. Graphite rods are held in a clamp under the carbon coater lid and over the sample, and the power supply provides high current at low voltage to heat and vaporize the carbon. Dangerous! Keep the unit unplugged, off, and current turned down until you are ready to coat. Turn off the current and power to the coater before doing anything else. Warning: after coating, the clamp that holds the carbon rods is HOT!