Film Growth

Films grow by the continuous nucleation of depositing atoms on previously deposited material. The film growth, as well as the nucleation mode, determines many film properties, such as density, surface area, surface morphology, and grain size. Important aspects of film growth are:

• Continual burial of the surface under newly depositing material

• Substrate surface roughness--initially, and as the film develops

• Surface temperature--initially, and as the film develops

• Adatom surface mobility

• Geometrical shadowing effects (angle-of-incidence effects)

• Reaction and mass transport during deposition, such as segregation effects and void agglomeration

Developing Surface Roughness. On an atomistic scale, surface morphology can vary from very smooth, such as that of a flowed glass surface, to very rough, such as is found with sintered materials (see Fig. 2). Generally, as the film grows the surface roughness increases because some features or crystallographic planes grow faster than others. The roughness may not be uniform over the surface, or there can be local areas of roughness due to scratches, vias, embedded particles, particulate contamination, and so on that lead to variations of the film properties in these areas.

Structure-Zone Model (SZM). Typically the film near the interface is influenced by the substrate and/or interface material, and it takes an appreciable thickness before the film establishes a particular growth mode. After a growth mode has been established, the film morphology can be described by a structure zone model (SZM). The SZM was first applied to vacuum-deposited coatings by Movchan and Demchishin in 1969 (Ref 32). Later the SZM was extended to sputter-deposited films by Thornton (Ref 33), as shown in Fig. 4, and was later modified by Meissier (Ref 34) to include point defect agglomeration and void coarsening with thickness. The details of the condensation processes that determine the film morphology at low temperatures where atom mobility is low are not well understood, although there are a number of factors involved, including:

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