Like nickel, cobalt has a strong tendency to alloy with refractory metals. Attempting to thermally evaporate cobalt out of a tungsten boat (even a thick gauge boat) has proven futile. As the boat heats up and the cobalt begins to melt, the liquid cobalt alloys with the boat and immediately causes it to become brittle and crack. As a result, there is very little deposition, if any.
Thick gauge tungsten boat (EVS20A015W) after thermal evaporation
We have reported success thermally evaporating cobalt out of an alumina-coated, tungsten boat such as our EVS9AAOW. With this boat, we were able to deposit a film with an average thickness of 3,790 angstroms at an average deposition rate of 6 angstroms per second using (1) ¼” X ¼” cobalt pellet and 272 amps of power. We estimate the average lifetime of the boat to be approximately 1-3 runs. However, applying a constant current will result in a decreasing deposition rate over time. This is due to the small size of the boat and, in turn, the small amount of material used. In order to maintain a deposition rate, higher powers will need to be applied throughout the process. If the boat survives an evaporation run, it is important not to touch or disturb the boat when reloading material. It is recommended to carefully place new pellets into the boat and avoid touching the clamps or applying any pressure to the boat.
Alumina-coated tungsten boat (EVS9AAOW) after thermal evaporation
Category: Evaporation Sources
Sub-Category: Crucibles and Crucible Heaters