Thin Film Substrates
When you try to evaporate titanium evaporation material, your tungsten boat may break. Why would this happen? A material’s evaporation temperature is often regarded as that needed for the material’s equilibrium vapor pressure to be 1E-2 Torr. At that vapor pressure, the deposition rate on a substrate in a system of “normal” geometry is good or high. For titanium, that temperature is ~1,750C. Titanium has to melt and “wet” a boat or crucible in order for efficient evaporation to take place. At this temperature, titanium will be liquid and quickly alloy with a refractory boat, destroying its electrical and mechanical properties. The end result is the boat cracking and falling apart.
A second option for thermal evaporation is to use a shielded tantalum crucible heater with a tall, intermetallic crucible. Thin films of titanium can be evaporated from intermetallic crucibles. However, film thickness may be limited to 500 angstroms, and the crucible may need to be replaced for each subsequent run. Intermetallic crucibles are composed of titanium boride (TiB2) and boron nitride (BN). This material combination works well because the material is both lubricious and electrically conductive. The crucible is both strong and conductive, yet its lubricious properties help prevent 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, resulting in failure of the welded joints. The heater should be centered between the contacts and the outer shielding must be clear of the leads.
Using the heater/crucible set-up involves heat transfer by thermal radiation across the interfaces (heater-crucible exterior and crucible interior-evaporant surface) and by conductivity through the crucible and evaporant. For titanium to reach proper evaporation temperature, the heater and crucible must be higher (in some instances, much higher) in temperature.
But it is possible that the set-up crucible could fail. The crucible may not get hot enough to melt and evaporate the titanium. Overfilling the crucible can also be detrimental to the process. The titanium could creep out over the walls of the crucible and react with the heater causing it to fail.
One downside of the heater/crucible set-up is that liquid titanium is a universal solvent. In other words, it reacts with and destroys almost all crucible materials. Titanium, once diffused through the crucible, can attack the heater as well. The heater/crucible option is more cost-prohibitive than the tungsten boat option. It could fail during the first run or even before if the material does not reach the proper temperature for evaporation. Crucibles should be stored in a cool, dry place and always handled with gloves or forceps.
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