2.3.1.4 Barrier Layers

In addition to the already described materials which are used as dielectrics, semiconductors, and interconnects, there has to be another material type introduced which protects the other materials from metallurgical reactions and diffusion. This is required to prevent for instance poisoning of certain regions in semiconductor layers and consequently destruction of the required material properties [31]. The diffusion into dielectrics would cause additional traps in the surface region of the dielectrics and yield high coupling capacitances, higher leakage, and reduced break down voltages [148]. Materials which offer these required capabilities are called barrier materials. They can be either metals, dielectrics, or compound materials but they have to be chemical robust and stable in terms of chemical and thermal burdens. Common materials are silicides for their compatibility to the Si regions in the lowest level to protect the Si from the materials which would change the doping profiles. Common materials to protect the dielectrics are nitrides, phosphides, and borides.

Barrier layers are also often used for contacts to provide a good conducting interface from the semiconductors to the via metal. As Figure 2.8 depicts, there are a couple of layers necessary to provide an appropriate transition from a semiconductor layer, which is mainly Si, to the via of the first metalization layer, which is mainly W. In between, there are a couple of barrier layers such as a silicide layer to contact the Si region and a Ti layer which comes from the deposition of TiSi$_2$ . On top of those there is a TiN layer which actually protects the W layer from the via to diffuse towards the Si regions.

Similar protective coatings are necessary for the metal Cu. This is because the Cu atoms show a high diffusibility and a rather high solubility especially in Si and $\mathrm{SiO_2}$ . Industrial experience has shown that good protective properties can be observed with special alloys, nitrides, phosphides, and borides like ${\mathrm{TiN}}$ , ${\mathrm{TiSi_2}}$ , ${\mathrm{Si_3N_4}}$ , ${\mathrm{TaN}}$ , Mn-Si-O compounds, as well as CoWP [32,158,159], NiMoP [160], and NiMoB [159].