Listed below are questions and answers regarding evaporation materials, techniques and proper source selection that may help address issues that apply to your thin film coating process.
Aluminum can be successfully thermally evaporated with several sources including a filament, boat or crucible. The correct source for your needs depends on the power you have available from your transformer, the volume of aluminum to evaporate and the distance from your source to substrate. Generally, a tungsten filament or boat will work very well for most evaporations.
SiO has a tendency of “spitting and streaming” during evaporation which will result in pinhole type defects in most coating applications. One way to reduce the particulates in your vapor stream is to use a “multi-baffled box source”. This type of source positions your evaporant material in two separate cavities, when heated it follows an indirect path through a series of baffles and then out the vertical chimney. Your substrate cannot see the bulk material at any time, this essentially eliminates any chance of spitting and streaming which can cause pinhole type defects. This type of source is pictured, with various configurations, on pages 31 through 34 of our catalog. Also, see our list of SiO related articles available to you on our “Technical Information” page.
One solution is to use an alumina coated boat source. The alumina, which is plasma coated onto the evaporation boat acts as an inert surface and prevents the gold from wetting to it. Also, when the gold melts it forms a sphere resulting in point source performance. This type of source gives long life compared to uncoated sources. A variety of alumina coated sources are pictured on pages 23 through 27 of our catalog.
Another recommendation is to consider using a crucible and crucible heater to hold and heat your material. Crucibles also provide a focused evaporation pattern and are available in several ceramic and metallic configurations.
Your best choice would be to use a tungsten filament or basket. Although nickel has a much higher melting point than aluminum, many tungsten filament and basket configurations are capable of reaching high enough temperatures for both materials. The materials you mentioned are somewhat corrosive when melted, with nickel being the most corrosive. This will lead to the eventual deterioration of the filament or basket. Another alternative would be to use a tungsten basket heater with the appropriate crucible. Although this will require slightly more power, the deterioration will be minimized and both the basket and crucible will provide several uses.
Yes. A “Chrome Plated Tungsten Rod” is a simple and user-friendly way to deposit thin layers of chrome. These rod sources provide good thermal efficiency, regulation of film thickness and elimination of spalling. More information is shown on page 37 of our catalog.