Frequently Asked Questions
*How do e-Guns work?
Electrons are emitted from a Filament and formed into a beam by a split cathode assembly. The electrons are then accelerated towards an anode assembly. The beam is injected into a magnetic field bending the beam in a radius allowing it to strike a water-cooled crucible assembly holding the desired evaporant material.
*How do I know what liner material to use?
Liner material should have higher vapor pressure then evaporation material. A 30% margin or more is ideal. Liners do pick up some cooling, and in some cases a material with the same vapor pressure can be used. Liners with close vapor pressure curves are more likely to produce contamination of the final film.
*How to you prevent stray electrons?
There is no guarantee for eliminated “secondary electron“ penetration of substrate due to physics. However, Thermionics has developed ways to significantly reduce electron damage by designs of Thermionics’ e-Guns™ “electron beam” sources.
The evaporation system design considerations can also improve the occurrence of electron damage.
One advantage is Thermionics’ strong magnetic field, extending to the rear of the source. A longer e-Gun™ source is better to reduce secondary electrons. Thermionics’ rotary multi-pocket sources are better in this respect.
The other beneficial measure is the angle of incidence of the electron beam as close to normal (90°) as possible at the pocket. Thermionics’ magnetic design and beam geometry significantly improves the impingement angle of the beam to the pocket. It provides 90° arrival and continuation to the bottom of the pocket.
Throw distance is another measure. The farther away the substrate is from the source, the lesser the chance for “secondary electrons” to reach the substrate. Rate and emission current (power density of incoming beam) are a factor. High Voltage is also a factor when considering secondary electron management. A lower high voltage 7kV to 8kV for instance, still acceptable for most metals, preferable for dielectrics produces “slower” electrons.
*How do I evaporate Au?
For Gold, the best liners are Coated Graphite and Intermetallic (Ti doped BN). You should not sweep during coating. You can sweep during material conditioning, but once the material is fully melted, just place the beam in the center of the pocket. This will give the best stability. You can overpower/overheat the material when using a liner. Watch for oscillation in the light emission from the source. If you get a strobe light effect, the material is unstable, and spitting will occur (reduce power). It is best to have a couple of welding lenses to look at the melt with. A #5, #8 and #10 lens will allow viewing of practically any material during evaporation. For this process, a #8 should work fine. Using a CG liner, blow the liner off with an inert gas prior to loading. Loadthe material to no more than 80% volume of the liner.
Coated graphite:
*Chemical resistivity. Materials such as aluminum will not react as fast, forming carbides.
*Particulate reduction. Graphite is messy and the coating “seals” the graphite. Evaporating for example- Gold. Straight graphite you will often get a piece or flake of graphite floating around in the melt. Not a real problem as it won’t evaporate.
*Reduced surface area. Glassy carbon -CG liners have a very smooth surface. This reduces contact and reduces mechanical stress from the material forming to the surface. As the material expands and contracts from heat the smooth surface helps reduce breakage.
Insulating liners:
BN and Al2O3 liners are insulating. They must be soaked at low power for several minutes prior to evaporation. When you first apply beam current the beam will strike the center of the crucible and then build a surface charge that causes the electrons to “skip” to the edges of the liner (blue luminescent you are seeing). As the beam continues to hit the center and skip to the edges it will gradually create heat on the liner. Once the liner is heated adequately it will begin to conduct electrically and the beam will stabilize. Run the source at low power 30-40 mA for 3 to 10 minutes -or- until you see the blue haze stop and the beam stabilize in the center of the crucible.
*How do I evaporate Al?
Aluminum (Al) is highly reactive when molten. Al will react with all liner materials. Al also getters Oxygen to form oxide layers if any oxygen is present during deposition. Aluminum Oxide (Al2O3) has a much higher melting point and vapor pressure when compared to Al (2,045C M.P., 1550C V.P. for Al2O3 compared to 660C M.P., 1,010 V.P. for Al). Further Al2O3 is an excellent insulator compared to Al which is a fairly good conductor. These properties make electron beam evaporation of Al more complicated than most metals. When evaporating Aluminum DO NOT SWEEP the beam! Sweeping the beam produces more heat closer to the liner / material interface. This will greatly speed up the reaction process. You can sweep to initially melt down the charge if needed (larger crucibles will require this- a 2.2cc typically will not). Place the beam as close to the center of the crucible as possible during evaporation. Slowly ramp power up and down. It should take 1-3 minutes to reach evaporation and after finishing the run, you should come back to a low power level (20-30 mA) and soak for 30-60 seconds to “cool down” the melt. This will reduce liner breakage from expansion.Do not overfill liner. Fill the liner 50%-70% (when melted). Aluminum will “climb” the walls of a liner and if it spills over and touches the water cooled crucible the liner will break. In larger sources you can use a liner inside a liner to eliminate this possibility. Overfilling a liner is the #1 cause of failure with AL.
