The use of pure aluminum leads to a diffusion of silicon into the metal. The semiconductor reacts with the metallization at only 200–250 °C. This diffusion of silicon causes cavities at the interface of both materials which are then filled by aluminum. Thus leads to spikes which can cause short circuits if they reach through the doped regions into the silicon crystal beneath.
The size of these spikes depends on the temperature at which the aluminum was deposited onto the wafer. To avoid spikes there are several possibilities. A deep ion implantation - contact implantation - can be introduced at the location of the vias. Thus the spikes do not reach into the substrate.
The disadvantage is that there has to be an additional process step, furthermore the electrical properties change because the doped regions are enlarged.
Instead of pure aluminum an alloy of aluminum and silicon can be used (silicon 1–2 %). Because the aluminum now already contains silicon there will be no diffusion out of the substrate. However, if the vias are very small, the silicon can drop out at the contact area and result in an increased resistance.
For contacts with a high quality a separation of aluminum and silicon is essential. A barrier of different materials (e.g. titan, titan nitride or tungsten) is deposited. To avoid an increased contact resistance at the interface of titan in silicon a thin layer of titan silicide is used.
Barrier layer between aluminum and silicon