Barrier Layers

A barrier layer is a film deposited between the silicide layer (which is formed over the silicon substrate as an interconnection path or contact) and the metallization layer.

The high diffusivity of silicon through silicide layers can be a problem in silicon-silicide-metal systems since, with heat treatment, these can exhibit excessive interdiffusion between the metal and the silicon through the silicide layer. This silicon-metal interdiffusion can result in loss of system integrity. Barrier layers are therefore used in preventing this interdiffusion problem.

Aside from excellent interdiffusion barrier properties, films used as a barrier layer between the metal layer and the silicide/silicon layers must exhibit excellent adhesion to both the metal layer and the silicide layer. Furthermore, a barrier layer must have low electrical resistivity, and must be compatible with the wafer fabrication process.

Metals alloys are often used as barrier metals in wafer fabrication. One commonly used metal alloy for this purpose is that of titanium (Ti) and tungsten (W), i.e., 10 wt.% Ti, which exhibits a resistivity of 50-80 micro-ohm-cm. In this alloy, W acts as the barrier itself while Ti just serves as a 'glue'.

Ti-W barrier metal is usually formed by single-target alloy sputtering, a process that's also compatible with the formation of the silicide film used as contact layer. Ti-W barriers can withstand thermal processing at 400-500 deg C for 30 minutes. The typical thickness for such barriers ranges from 450 to 1,800 angstroms.

Other materials explored for use as barrier layer include metal nitrides, carbides, and borides.

Titanium nitride has recently been confirmed to be superior to the Ti-W barrier metal. Titanium nitride can exist either as Ti2N or TiN. Stoichiometric TiN exhibits low electrical resistivity (50-100 ohm-cm) and very good barrier properties. Tungsten nitride films, on the other hand, have been found to be excellent barriers to interdiffusion between GaAs and metals such as gold, silver, and aluminum.

See Also: Metallization; Thin Films; Silicides

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