All thin film deposition processes consists of three steps:
1. production of the film forming species
2. transport of these species from the source to the substrate
3. condensation and stitching on the substrate
In PVD the first step is either evaporation or sputtering, the second step implies line-of-sight transport if the process pressure is very low and there is a small probability for collisions or a flow transport if the pressure is high. The type of transport influences the actual growth of the film in the third step.
When an atom arrives at the substrate surface and is being adsorbed, it will diffuse on the surface until it is either desorbed or stuck to an energetic favorable site. This surface diffusion is dependent on what energy the atom has upon arrival to the surface and if the substrate is supplied by an additional energy, e.g. by heating or ion bombardment. The energy of the atom is dependent on the pressure in the deposition chamber, a high pressure decrease the energy due to energy losses in collisions. The ion bombardment of the surface is possible in plasma based methods and can be controlled by a negative bias voltage of the substrate with respect to the plasma.
If the atom sticks to another film atom at the surface, a low-mobility pair is created and this increases the probability for yet another atom to stick to them. At a critical number of atoms, or a critical nuclei-size, a nucleus is formed. These nuclei will grow to crystalline islands that will coalesce when meeting each other and finally form a continuous film. Depending on the process parameters the film growth will continue in different ways giving different microstructures. The film can grow layer-by-layer or in 3D-islands or in a combination of these two growth modes.
In PVD the film growth is often columnar, i.e. the crystallites grow in columns with more or less developed grain boundaries between them. The grain boundaries can contain voids and deteriorates most properties of the film, but a real dense, columnar film can have for example excellent tribological properties. A complete dense microstructure in the film is often very desirable. As the dense microstructure is promoted by ion bombardment of the growing film, such films can often be deposited by PVD-methods in high-density plasmas.
Several film growth models for the influence of the deposition condition on the microstructure of the film have been developed. Commonly used are the empirical structure zone models where different growth modes (zones) are identified in a diagram for different temperature to melting temperature ratios (T/T m ). An extensive review of such models was published by John A. Thornton in 1977 and here follows a short summary of this. Movchan and Demchishin made the following classification: Zone 1 appears when T/T m < 0.3 and is characterized by high surface roughness and voided grain boundaries. Zone 2 appears when 0.3 < T/T m < 0.5 and is characterized by a mat, smooth surface and columnar grains with distinct, dense boundaries. Zone 3 appears when 0.5 < T/T m < 1 and are characterized by a bright surface and equiaxed grains. The structure and properties of this zone are close to bulk material. Thornton has proposed an extended model where the influence of the process gas pressure is added into a second axis in the diagram. In this diagram a fourth zone (zone T, transition) can be identified between zone 1 and zone 2. The zone T structure is dense and fibrous without voided grain boundaries.