Mosfet fabrication

 Photolithography is the key point of  MOSFET  fabrication process , it is used to transfer a pattern from a photomask to the surface of a semiconductor wafer.

1-Start with a p-type wafer as a substrate

The process begins wit P-type silicon wafer , typically dopant with a small amount of boron creating positive charge holes .

2-Grow a layer of silicon dioxide(SiO₂)

After creating p-type as a substrate grow a layer of silicon dioxide (SiO₂) on the surface of wafer, silicon dioxide is done by heating the wafer in the oxidation environment , causing silicon wafer  react with oxygen atoms.

3-Cover the silicon dioxide(SiO₂) layer with photoresist

Apply a photoresist layer on silicon dioxide  photoresist is material sensitive to light of a specific wavelength usually ultraviolet wavelength.

4-Expose specific areas to light through a photomask

Photomask is placed at the top of  the photoresist , The photomask contains the desired pattern or design, with opaque areas where light is blocked and transparent areas where light can pass through.

As shown in the following figure that the ultraviolet light passes through the transparent part of the mask , Lenses is used to collect the light hitting the photoresist layer.

5-Etch the exposed areas

put the silicon wafer in etching solution to remove silicon dioxide layer in the target area that were exposed by photoresist

6- Remove photoresist layer 

After etching , remove the rest of photoresist layer by using chemical solution, that dissolves the  photoresist removing all the residue.

7-Doping the wafer to create n+ regions

A n-type doping is performed by introducing elements like phosphorus or arsenic into the exposed areas. This creates n+ regions, where there is an excess of electrons (negative charge carriers). This doping step changes the conductivity of specific regions of the wafer, allowing the creation of components such as transistors.

8-Final Steps

Keep repeating this process by adding new materials such as metal interconnects or additional dielectric  creating metal wires , gate , etc . until you complete the fabrication of Mosfet. 

conclusion 

Photolithography is essential process in fabrication any circuit on silicon wafer , by creating specific patterns using ultraviolt light.