Diffusion barrier
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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.
[edit] Selection
While the choice of diffusion barrier depends on the final function, anticipated operating temperature and service life are critical parameters to select diffusion barrier materials. Many thin film metal combinations have been evaluated for their adhesion and diffusion barrier properties. Aluminum provides good conductivity, adhesion and reliability because of its oxygen reactivity and the self-passivation properties of its oxide. Copper also easily reacts with oxygen but its oxides have poor adhesion properties. As for gold its virtue relies in its inertness, and ease of application; its problem is its cost. Chromium has excellent adhesion to many materials because of its reactivity. Its affinity for oxygen forms a thin stable oxide coat which prevents further oxidation and provides inertness to corrosive environment. Nickel, Nichrome, tantalum, hafnium, niobium, zirconium, vanadium, and tungsten are a few of the metals combinations used to form diffusion barriers for specific applications. Conductive ceramics can be also used, eg. tantalum nitride, indium oxide, copper silicide, and titanium nitride.
See also barrier metal.
