We recently had an interesting question from a student at one of the local universities. The student wanted to know what differences he might expect if he were to change from evaporating Aluminum in his Electron Beam Source to Gold when depositing on to a plastic substrate?
Gold (Au), like aluminum (Al), is an excellent electron beam evaporation material. However, it has a much higher melting temperature, (1052°C vs. 660°C for Al) and a higher vapor pressure (1132°C at 10-4 vs. 1010°C for Al). Unlike the more complex Titanium Diboride-Boron Nitride (Intermetallic) crucible liner necessary for evaporating Aluminum, an Aluminum Oxide (Al2O3) crucible liner is usually recommended for electron beam depositions of Au; although Boron Nitride (BN) or Vitreous coated Graphite liners will perform equally as well.
Evaporated thin films of Gold are typically quite “soft” and may not exhibit good adhesive characteristics to the substrate. It may be necessary to heat the substrate to produce films with greater density and better adhesion. However, this may cause problems with plastic substrates, depending on the specific organic composition. If the plastic substrates are biaxially oriented, such as Polyvinylidene chloride or Polyethylene Terephthalate, or have low melting/softening temperatures, they may distort from the combination of reflected heat associated with the line of site electron beam source, latent heat of condensation of the gold vapor on the substrate surface added with any deliberate substrate heating involved. Maintaining a proper balance between film density/adhesion and substrate distortion may be difficult, depending on the physical characteristics of the substrate being utilized.