I use a commercial thermal evaporator to deposit aluminium contacts on samples through conveniently located shadow masks. During the process of evaporating aluminium we have found that the baskets get very fragil (in fact, not resusable for a second time), PARTICULARLY when we evaporate at a slow rate (< 1 Angstrom/s), I mean, at low current and for long times, say 200 seconds. In those cases, ususally the basket even breaks during the process. In most cases after opening the chamber we observe a whitish residue. This is not observed when we evaporate other metals, like gold. I do not mention the name of my commercial evaporator because other colleagues using other models and baskets, have found the same problem. Is this an exclusive problem of aluminium? Is it not of sufficient purity?
Just to add that the baskets/crucible are made of tungsten, that the vacuum is about 6e-6 mbar during evaporation, and that eventually we get nice and mirror-like surfaces on the aluminium contacts. The whitish residue stay just in some basket zones.
We sometimes tried to evaporate aluminium in form of wires, beeing held within a spiral-like heating wire instead of a tungsten boat/crucible. Also leads to the destruction of W wire after single use. Also observed increased brittleness.
This might be due to formation of Al-W intermetallic phases.
Aluminium is said to form an alloy with tungsten (I never checked that). If so, it would be a surprise if the structure of the solid would remain untouched. Problems with brittle and breaking tungsten wires of any kind are normal in reflector coating with Al. Sputtering the Al will help and generate new problems. Just get used to it. Don't worry...
Thanks Markus and Heinrich for your quick answers. Either spiral-like baskets, of boat-shaped crucibles break apparently by the point of higher curvature or where they are bended. We will try to get usted, while searching a way to avoid the problem. Acknowledge and thanks again.
This is a typical problem with tungsten. I had the same breakage problem with a tungsten heater filament in an electron evaporator. To me it sounds like you might be having more failures than I would expect. One thought, how close to the basket are the connecting clamps? If you can adjust the position of these you might find a more robust arrangement.
Acknowledge, Martin. Difficult to vary the position of the clamps in our system. I understand we should think of changing the basket material if we wish to circumvent the problem. Thank you very much.
Depends on matrials of basket.white color may be due to oxidation.
Indeed. Initially I was afraid that it was a poor vacuum level but I have discarded that problem, since then it would happen in any evaporation cycle. But it is only -as far as we know- with tungsten basket and evaporated aluminium. Thanks, Benny, for the suggestion!
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