A diffusion barrier is a thin layer (usually micrometres thick) of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.
Adhesion of a plated metal layer to its substrate requires a physical interlocking, inter-diffusion of the deposit or a chemical bonding between plate and substrate in order to work. The role of a diffusion barrier is to prevent or to retard the inter-diffusion of the two superposed metals. Therefore, to be effective, a good diffusion barrier requires inertness with respect to adjacent materials. To obtain good adhesion and a diffusion barrier simultaneously, the bonding between layers needs to come from a chemical reaction of limited range at both boundaries. Materials providing good adhesion are not necessarily good diffusion barriers and vice-versa. Consequently there are cases where two or more separate layers must be used to provide a proper interface between substrates.
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