Fundamental Processes

The interpretation of ion-beam assisted deposition effects requires some understanding of the mechanism of thin film nucleation and growth and a knowledge of the effects produced by ion bombardment of a solid. From the atomistic approach, a film grows as atoms condense on the surface and diffuse as adatoms on the surface until forming a critical size nucleus or reevaporating. Film growth may take place in islands, layer by layer or in the Stranski-Krastanov or mixed mode where monolayer coverage is followed by island growth. The mode of growth depends on several factors including thermodynamics of the surface interaction, strain and epitaxial relationship. The nucleation and growth processes are reflected in the microstructure developed by the film as illustrated schematically in Fig. 1. At values of the absolute melting temperature of 0.1, the adatoms have low mobility and columnar grains with pipes develop. At 0.3 Tm, high energy boundaries rearrange, while at 0.5 Tm a dense structure can be expected to form. The microstructure formed is thus determined by the physical processes occurring during film growth (3).

The interaction of an energetic ion with a surface can cause a number of effects as illustrated in Fig. 2. High energy ions penetrate the surface and are slowed by inelastic interactions with

Fig. 1 The Movchan-Thornton structure diagram schematically shows the relationship between microstructure and the process variables of substrate temperature (in fraction of melting point in °K) and argon pressure for films deposited by sputtering.
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