Discussion
A student aims to deposit a thin metallic film on SiO2 substrate, with an adhesion layer between the metal film and substrate, in a contiguous planar fashion. Island type of growth must be avoided. The student performs an extensive optimization exercise. Which one of the following steps is in the right direction?
Answer: A
The right step is to choose a metallic adhesion layer with very low interfacial energy with the deposited thin film.
Whether a film grows layer by layer or as islands depends on wetting: layer (Frank-van der Merwe) growth is favoured when \(\gamma_{substrate} \ge \gamma_{film} + \gamma_{interface}\). A low interfacial energy between the adhesion layer and the metal film makes the film wet the layer, so it spreads into a continuous planar film instead of balling up into islands.
Choosing the layer only for its match with SiO2 ignores the film, whose wetting is what matters. Raising the substrate temperature and lowering the rate gives atoms time to reach their equilibrium arrangement, which is islands when wetting is poor; and annealing between stages encourages dewetting and agglomeration.
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