Evaporation boats

Evaporation boats are made of tungsten because tungsten has an extremely high melting point.

There are many boat geometries from RD Mathias. We have used two types: trays with dimples and wire coils. The thickness of the boat (or wire) determines the temperature at a given voltage. For example, if you supply a fixed voltage (say 100 V) across a thin boat and a thick boat, the thinner boat will be hotter.

One issue with tungsten is that it reacts with iron. This makes thermal evaporation of iron troublesome. E-beam evaporation is the best solution. If ebeam is not available, it is possible to use a tungsten wire coil. You have to use thin wire for the temperature to get hot enough. We have not succeeded to get a steady rate of iron, instead, we open the shutter after a burst of fast rate had settled down near 1 angstrom per second.

We have not had any success using alumina coated boats (tungsten with a rough, white coating). The thermal conductivity is significantly reduced by the alumina coating. It is much harder to melt materials, and the chamber reaches undesirable temperatures. Remember that photoresist cannot survive beyond 200 C.

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