# Evaporation theory

One method of generating the vapour flux is through evaporation (heating a solid or liquid source). For evaporation to occur the heating must lead to sufficient vapour pressure (typically between 0.1-1 Pa). This often requires melting of the source, but not necessarily.

The Clausius-Clapeyron equation for solid-vapour and liquid-vapour equilibrium is an approximation (based on the vapour being a perfect gas) that is often used as a starting point to describe the connection between temperature and pressure.

\[ \frac{{{\rm{d}}P}}{{{\rm{d}}T}} = \frac{{\Delta H\left( T \right)}}{{T\Delta V}} \] \[\ln P \cong - \frac{{{\rm{\Delta }}{H_e}}}{{RT}} + c\]The plot below is based on this, and gives the vapour pressure of the material at a given temperature. Click on the line that you want to analyse and use the scroll bar to see how far you can reduce the temperature whilst keeping an effective vapour pressure.

As you can see, a small variation in temperature leads to a large change in vapour pressure. Vapour flux (atom arrival rate per unit area per unit time) is linked to the vapour pressure, so we need very precise temperature monitoring in order to control the vapour flux and hence the film growth rate.

This process is carried out in a vacuum, and the evaporated atoms have relatively low energies (~ 0.1-0.3 eV)