Aluminum can be evaporated successfully via e-beam or thermal evaporation. It is important to note that aluminum has the tendency to alloy with refractory metals causing cracks in metal crucibles and boats.
Here are two methods for thermally evaporating aluminum.
One approach is to use 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 when using this method.
The second recommendation is 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/ºCx10-6) and thermal conductivity of 70 (W/mºK)/25ºC. 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.
As with thermal evaporation, we recommend using an intermetallic crucible when e-beam evaporating aluminum because of the crucible material’s unique conductive and lubricious characteristics. A key process note is to consider the fill volume in the e-beam application because we find that the melt level of Aluminum in the crucible directly affects the success of the crucible liner. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth. The outcome is cracking in the crucible. This is the most common cause of crucible liner failure. Placing too little material in the crucible or evaporating too much material before refilling can be detrimental to the process as well. When the melt level is below 30%, the e-beam is likely to strike the bottom or walls of the crucible which immediately results in breakage. Our recommendation is to fill the crucible between 2/3 and ¾ full to prevent these difficulties. While our customers report success in running aluminum out of intermetallic crucibles, others claim that FABMATE® crucibles last longer. It is important to mention that most FABMATE® crucibles are less expensive than intermetallic.
There is a risk of aluminum carbide contamination in films if higher powers are used during evaporation. High beam power causes increased thermal load on the crucible and carbon from the liner can infiltrate the aluminum melt. Evidence of this occurrence is generally a transparent, yellowish film that covers the surface of the melt. The evaporation rate will plummet and the natural reaction is to increase the power. However, this only exacerbates the problem. Applying lower beam powers from the beginning of the process will help to minimize this threat.
Crucible liners should be stored in a cool, dry place and always handled with gloves or forceps.
Category: Deposition Materials
Sub-Category: Evaporation Pellets, Pieces and Wire