Aluminum has the tendency to alloy with refractory metals causing cracks in metal boats and crucible heaters. During the thermal evaporation process, cracking is inevitable and boats will need to be replaced quite often. We recommend using a thin width, thick gauge, high current tungsten boat such as our EVS20A015W. We have been successful with evaporating aluminum in this particular boat and are able to deposit more than 3,000 angstroms per run with rates greater than 30 angstroms per second. We are able to get multiple runs (about 4) with two ¼” X ¼” pellets in the boat during each run. Generally, the current draw from the boat is quite consistent run-to-run. It is crucial not to disturb the boat when venting the chamber to re-load with more pellets as the boat becomes brittle after the first evaporation. Even though the boats will need to be replaced frequently (a thicker boat only prolongs inevitable cracking), they are usually inexpensive. However, down-time experienced while switching out the boats should be considered.
Another option would be to use a shielded, tantalum crucible heater with a tall intermetallic crucible. Intermetallic crucibles are composed of 50% titanium boride (TiB2) and 50% boron nitride (BN). Its density is 2.77 g/cc, with a thermal expansion coefficient (CTE) of 7 (in/in/°Cx10-6) and thermal conductivity of 70 (W/m°K)/25°C. This material combination works well with aluminum because the material is both lubricious and electrically conductive. The crucible is both strong and conductive, yet its lubricious properties prevent the material spill-over and crucible cracking. Great care must be taken when installing the heater to prevent the outer shields from becoming warped which can cause a short in the heater, causing the welded joints to fail. The heater should be centered between the contacts and the outer shielding clear of the leads.
Correct - Crucible heater centered, Incorrect - Crucible heater off-center,
outer shielding clear of leads shielding in contact with leads/inner shielding
Some of our production customers have indicated they can get 10-15 runs out of the intermetallic crucibles. These users are very experienced in thermally evaporating aluminum in high volumes. Therefore, they have refined their processes so it is possible that these results may not be typical in all applications. An example of a heater-crucible set-up would be our EVCH1 or EVCH10 paired with EVC9INTSPL01 if using one of our vacuum systems.
Category: Evaporation Sources
Sub-Category: Evaporation Pellets, Pieces and Wire