The terminals of the fabricated devices must be made available for making contacts to the outside world and also individual devices must be connected by interconnects. In early days aluminum (Al) was universally used for contacts and interconnects. Windows were opened in required places using photolithography and Al deposited over the entire wafer. This deposition was done in a vacuum chamber using the e-beam evaporation technique where e-beams were used to vapourize Al pallets kept in a crucible and these Al vapours got deposited on the samples. Next the interconnect patterns were defined by etching out the metal where it was not required.
With passing time, as the number of devices and the interconnects increased, it was found that single layer of metal is not enough to produce all the required interconnects. Hence the focus shifted to multi-level metalization, where individual metal layers are deposited one on to top of the other with each layer separated from the other adjacent layers by dielectrics and at required places connected through holes known as vias in the dielectric.
The dielectric is typically SiO2 deposited by the CVD technique which requires a high temperature of the order of 1400Â°C Al is having melting point of 667Â°C obviously is not suitable for multilevel metalization. Heavily doped poly silicon simply poly is now universally used for multilevel metalization.