Fabrication of NMOS transistor:-

The process starts with the oxidation of silicon substrate (figure a) in which a relative thick silicon dioxide layer also called as field oxide is created on the surface (figure b). Then, the field oxide is selectively etched to expose the silicon surface on which the MOS transistor will be created (figure c). Following this step, the surface is covered with a thin, high-quality oxide layer, which will eventually form the gate oxide of the MOS transistor (figure d).

On top of the thin oxide layer, a layer of polysilicon (polycrystalline silicon) is deposited (figure e). After deposition, the polysilicon layer is patterned and etched to form the interconnects and the MOS transistor gates (figure f). The thin gate oxide not covered by polysilicon is also etched away, which exposes the bare silicon surface on which the source and drain junctions are to be formed (figure g).

The entire silicon surface is then doped with a high concentration of impurities, either through diffusion or ion implantation (in this case with donor atoms to produce n-type doping). (Figure h) shows that the doping penetrates the exposed areas on the silicon surface, ultimately creating two ntype regions (source and drain junctions) in the p-type substrate. The impurity doping also penetrates the polysilicon on the surface, reducing its resistivity. Once the source and drain regions are completed, the entire surface is again covered with an insulating layer of silicon dioxide (figure i). The insulating oxide layer is then patterned in order to provide contact windows for the drain and source junctions (figure j).

The surface is covered with evaporated aluminum which will form the interconnects (figure k). Finally, the metal layer is patterned and etched, completing the interconnection of the MOS transistors on the surface (figure l).

Mask Layout Diagram:-