|Re-Vap™ coiled filaments are typically three stranded tungsten wires looped into coils. Multistrand filaments are generally used because they offer a greater surface area than single wire filaments. Under these conditions, the evaporant charge should be small compared with the mass of the filament. The filament can hold up to 1 gram of evaporant material which is formed into staple-like shapes and hung on the central helix of the tungsten filament. Upon melting, the evaporant wets to the filament and is held in place by surface tension. Spreading of the molten evaporant across the wire is desirable to increase evaporation surface area. This is accomplished by distributing the initial charge evenly over the entire length of the filament coil. To minimize dripping of the molten material, the filament coil temperature must be increased rapidly to 1200-1500°C. Using this technique, the molten material will climb or cling to the hot wire and vaporize efficiently. Another type of element coil is the filament basket, used to evaporate pellets or chips of materials which either sublime or do not wet the filament wire upon melting. If wetting occurs, the coils of the basket are shorted and the temperature of the source drops.
Resistive Heater Source Assembly
Metal foil boat type resistive elements are yet another choice for small evaporation applications. Metal foil boats are made from thin refractory metal stampings, usually tungsten, molybdenum or tantalum. These boats have dimples which hold the evaporation material. Their miniature size and small capacity make them ideal for small evaporation jobs. Metal foil boats operate at very high temperatures and may cause alloying to occur with certain types of evaporation materials. Wetting of the metal surface by the molten evaporant is desirable in the interest of good thermal contact, however, the molten metal will lower the electrical resistance of the foil in the melt area, thereby causing a drop in temperature. This problem can be eliminated by using a boat which has been coated with a thin layer of aluminum oxide. The oxide coating will not allow wetting of the molten metal evaporant to the metal foil element.
Crucibles have insulating properties which form a thermal barrier between the filament and melt, allowing a uniform melt temperature. Crucible evaporation is very stable because of its uniform heating. A wide range of low to moderate temperature metals like palladium, tin, selenium, arsenic, indium and organic materials evaporate well from crucibles. Crucibles are less prone to failure compared to metal foil boats because of the complete isolation between the evaporant and the heater element, thus eliminating shorting or alloying.