We recommend using an Intermetallic crucible when e-beam evaporating aluminum because of the crucible material’s unique properties. 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 material spill-over and crucible cracking. FABMATE® is also an acceptable crucible liner for e-beam evaporating aluminum and is typically less expensive than Intermetallic crucible liners. The tradeoff between extra runs and added expense will vary per user.
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 ⅔ 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