Thin Film Growth : Introduction
Thin films don't grow as perfect slabs of bulk like material except in exceptional circumstances (epitaxy). The first few atomic layers of a thin film usually grow as islands of the depositing material centred on nucleation sites (Volmer - Weber growth).
These islands grow until they touch (the percolation threshold), this percolation threshold typically occurs between 5 and 15nm.
The thin film properties at thicknesses around or below the percolation threshold will be very different from the bulk properties.
Click here to see an animated thin film growth sequence (330 kB).
The classic picture of thin film microstructure is shown here - the structure zone model of Thornton. There are two controlling factors :-
This diagram is for sputtered thin films and shows the second axis as pressure.
In this case the sputtered material leaves the sputter source with relatively
high energies (several electron volts). As the sputtering pressure is increased
then more scattering occurs and the arrival energy of the sputtered material
- the ratio of growth temperature to melting temperature (of the thin film material).
- the amount of added energy.
For evaporated films the arrival energies are low and the thin film structure would
follow the high pressure side of this diagram. If ion or plasma assist is added
then the energy per arriving atom increases and the thin film structure will be
improved and move towards the low pressure side of this diagram.