3.6 Basic CMOS Technology

In early days of technology, the control gate of the MOS transistor was made with aluminum instead of polycrystalline silicon. It was difficult to align the metal over the channel precisely; an offset in one direction or other would create a non-functioning of the transistor. To overcome these problems, the poly-silicon gate was introduced. This polysilicon would be deposited before source/drain diffusion. During the diffusion, source and drain regions are self-aligned with respect to the gate. This self-alignment structure reduces the device size. In addition, it eliminates the large overlap capacitance between gate and drain, while maintaining a continuous inversion layer between source and drain. In the case of metal gate process, Al deposition has to be carried out almost at the end of fabrication because further high temperature processing would melt Al. In case of self-aligned poly silicon gate technology, these restrictions are also circumvented.