Formation of Thin Films

- Jan 23, 2018-

1. Coagulation is the result of the mutual adsorption of the matrix molecules and the evaporating atoms

In vacuum, when certain energy of evaporating atom impinges on the surface of the substrate, energy exchange with the substrate molecule is effected by the force field of the substrate surface, and physical adsorption and chemisorption occur between the evaporating atom and the substrate molecule. Physical adsorption is any two Vander Waals forces close to each other between molecules, and chemical adsorption is the chemical bond between atoms. Not all of the evaporation atoms can adsorption with substrate molecules, some of the atoms keep most of the energy of their own, and the collision happened after reflection, even those evaporated atoms adsorbed he still can be transferred on the substrate surface or by desorption and leave the surface of the substrate. In addition, on the surface of the substrate, there is an association between the evaporated atoms and the same type of atoms adsorbed on the surface of the substrate. The process of evaporating atoms adsorbed by the substrate is called agglomeration.

The ratio of the condensed atom to the incident atom is called the coacervation coefficient of the evaporation atom. Because of the different attraction between atoms, the agglomeration coefficient of the same type of evaporation atoms for different substrates is different. 

Each evaporated atom has different critical temperature of the film formation for different substrates and the temperature of the substrate is lower than the critical temperature of the film, it may be condensed. This phenomenon can be interpreted as: When the temperature of the substrate is too high, the evaporating atoms impinging on the substrate surface are either reflected or desorbed. In the film forming process (existing film forming basis), although the temperature of the substrate exceeds the critical temperature, the film can continue to grow, and its crystallization state will be changed. In the early stage of the films formation, the film is in amorphous state or vitreous body, with the increase of temperature, the film gradually to the metastable transition, if this trend continues, it will be transformed to stable, finally into crystalline films.

2. Formation process of thin film

At the beginning of deposition, the substrate surface is uniformly absorbed some evaporation of atoms, the interaction between the atoms of the evaporation gather in some nuclei which grow, migration, merger, formation of a number of small islands (large crystal nuclei gathered together), the basal surface of these islands further growth similar to one another, are connected to form a large island. With the deposition, the heights of these islands and small islands are increasing, and finally a network structure with ditches is formed on the whole base surface. The result of continuous evaporation, the evaporation atoms are also continuously nucleated and island in the trenches, and eventually the complete ditches are gradually filled. When the ditches and holes in the surface of the base are filled, the evaporated atoms continue to build up all kinds of films on these structures.

Under the electron microscope, it can be seen that the conventional thermal evaporation films are all cylindrical structures. The direction of the cylinder is the same as that of the film growth, but perpendicular to the film interface, and there are many gaps between these cylinders. Measuring the total gap in the film by filling density:

Filling density (P) = the volume of the solid part of the film (cylinder) / the volume of the film (cylinder + pore)

The filling density is a function of the substrate temperature and in most cases, the filler density increases with the increase of the substrate temperature. In general, increasing the evaporation rate can increase the filling density, and the effect of the evaporation rate is particularly significant when the film is lower. The packing density also increases with the increase of film thickness. This is probably due to the change of the structure state of the film during the film-forming process. It may also be due to the blockage of some part of the film pores in the film formation process.