First, my goal is to test the permeation of moisture through a barrier using calcium metal.
I would like to deposit a calcium metal thin film of several hundred nm using electron beam PVD but find little information in literature or industry publications regarding this. Preparation of the deposition source is straightforward, but the process of venting the chamber and preheat of the source material seem more involved.
Is there another deposition method preferred to EBPVD? I have a simple low vac desktop sputtering system available to me, but little else.
I can vent the chamber with nitrogen or argon if necessary but must open the chamber to air after and cannot have the chamber or pumps poisoned resulting in the degradation of high vacuum.
You need to make sure that your residual gas inside the vacuum has negligible moisture and oxygen. E-beam is highly energetic and you will most likely change chemically during the deposition. Thermal vacuum evaporation is usually better. For deposition of calcium, make sure you deposit at less than 1 x10-5 Pa with partial pressure of oxygen and moisture less than 1x10-7Pa. Or else you just deposit calcium oxide.
I have a question. Once you have deposited the film, how are you planning to preserve it? I haven't worked with Ca before, so I may be wrong, but I would think that once you take it out of the chamber, the O2/H2O in ambient atmosphere would be enough to reduce it to powder...Or are you going to deposit your barrier material all around before breaking vacuum?
I plan to implement a removable load lock chamber to move the samples into an argon environment for processing. While Ca is very reactive, it will likely take some time exposed to ambient before becoming completely oxidized. Unless there is a large change in lattice constant resulting in stress in the film, my understanding is that the film will not become a powder.
Once on the chamber walls, I imagine the Ca will quickly oxidize. Has this caused anyone vacuum degradation issues?
I recommend deposit a thick aluminum ~400nm to protect it. And never expose with ambient atmosphere even for a minute. Any deposition of reactive metals which have been oxidized in the wall will degrade your vacuum level creating..often large reservoir of residual moisture/oxygen during pump down. You need a cryo-pump to remove such unwanted residual gas. People sometimes cool the chamber with N2 to freeze or reduce the residual water and oxygen. The calcium needs to be baked inside the vacuum to degas ideally at least 2hours below its evaporation..and then you need to trial evaporate with your shutter closed so that calcium oxide is removed from the surface of your raw calcium. If you see your pressure goes up drastically and then down after stopping the evaporation, below your initial base pressure of evaporation, it could mean hydrogen gas is being released during evaporation of calcium as the result of reaction of hot calcium with residual moisture. Ideally you have a residual gas analyzer attached to your vacuum chamber to monitor the process of evaporation in order to make sure you really deposit calcium not calcium oxide. For electronic devices, the interface between organic and your reactive metal is critical in performance. Even a nanometer of calcium oxide changes the performance drastically compared with calcium. Eg CsF/Ba/Al is the most electron injecting cathode but very very few people actually make it work because of oxidation of barium.
Dear Kai
But calcium oxide's melting point is higher than that of Ca. isn't it? So it may be Ca which is getting evaporated first after breaking the oxide encapsulation. Right?
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