Evaporation occurs when the vapor pressure of the material is larger than the pressure of its environment. The vapor pressure is temperature dependent, the vapor pressure curves are plotted.
For materials such as water, this can occur at ambient pressure and close to room temperature. Most metals need to be 1) in a vacuum environment (in the chamber mbar), and 2) heated significantly in order to . This means that, in principle, most evaporation has to be done with some thermal element to it. We have two evaporators in the Deposition Lab, which will be briefly explained below.
First, the resistance evaporator, which uses the dissipation of a resistive boat to create temperatures substantial to melt and evaporate (in high vacuum conditions) Au, Ag and Cr by sending several Ampères of current through. This means that the temperature is limited by the dissipated power , resulting in the succesful evaporation of Ag, Au, and Cr.
Secondly, in order to create substantial metals with a higher vapor pressure such that these will evaporate too we have an e-beam evaporator, which uses thermionically emitted electrons from a tungsten filament to heat the metals. The emission currents are typically between 1 - 500 mA (depending on the material), and the beam has an energy of 10 kV. This allows us to melt Au, Ag, Al, Co, Cr, Cu, Ge, Ir, Nb, Pt, Sn, and Ti.
Because this is such a wide variaty of metals, but there are only six slots in our e-beam evaporator, we like to keep the flexibility to change the metals depending on the users' needs. This is the reason that we use liners, which can create complex interplay between electric charging (due to the emitted beam), heat dissipation, high-temperature dynamics such as spitting and creep at the metal-liner interface, running the risk to crack or burn through liners.
The mean free path of the metal vapor atoms is quite large, which is another benefit of having a low pressure environment. This results in anisotropic deposition (contrary to e.g. sputter deposition) and paves the way for shadow evaporation.