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MOSFET Fabrication Technology
Self-aligned Gate versus Conventional

In the self-aligned gate technology, the polycrystalline silicon gate is defined first, and then drain and source doping is performed using the ion implantation technology.  In the conventional processing of MOSFET, the source and drain doping is completed first by thermal diffusion or ion implantation, and then Gate metal is defined, where the precision of the Gate alignment with the underlying chanel (or the source and drain edge) is dictated by the precision of mask alignment of the lithography (which always produce some overlap between G and S and D).

The following applet compares the two technology.  

The source and drain doping by ion implantation in the self-aligned gate technology addresses the Lightly-Doped Drain (LDD) as well.