Fabrication of MOSFET

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is widely used in digital logic circuits and as memory devices, because these application requires very high packing density for the memory. Manufacturing is very important task and is directly related to performance of resulting chip. The circuit designer must have the knowledge of different manufacturing process so as to optimize the design with respect to different manufacturing processes in order to improve the design. 

Silicon is preferred for MOS fabrication

Because silicon is very easy to oxidize. We just need to increase the temperature and keep the silicon in an oxygen ambient and oxidation will occur. Also, silicon has a great affinity for oxygen, so it will form silicon dioxide, SiO2 layer easily and this silicon dioxide has an excellent insulating, masking and dielectric  properties.  

Now, in the following steps we will discuss the different fabrication steps of MOSFET on silicon substrate. 

Steps:

1. Thermal Oxidation

We have considered n type silicon substrate. The first step in the fabrication of MOSFET is the growth of a thick field oxide on silicon substrate as shown in figure a and b.  

2. Photolithography

After the thick field oxide is grown, next step is photolithography. Photolithography is defined as the selective removal of oxide. To dope the substrate selectively, some regions will require oxide and some other region will not require. For that reason firstly the entire silicon dioxide surface is covered with a layer of photoresist.

The areas where we want to dope, we will expose those areas to ultraviolet (UV) light and other regions are protected with mask. The exposed photoresist area becomes soluble and this area can be etched by using some etching solvents in the next process and then it can be doped.

Positive photoresist material- The type of photoresist which is initially insoluble and becomes soluble after exposure to UV light is called positive photoresist.

Negative photoresist material- The type of photoresist which is initially soluble and becomes insoluble (hardened) after exposure to UV light, called negative photoresist.

After the region is etched then the entire surface is covered with gate oxide which is thin and high-quality oxide layer. This thin oxide layer will eventually form the gate oxide. 

On top of this thin oxide layer, next layer is formed which is a polysilicon layer. Polysilicon is used both as gate electrode material for MOS transistors and also as an interconnect medium in silicon integrated circuits.

After the layer is deposited, the polysilicon layer is selectively etched to form the interconnects and the MOS transistor gates. In next step, for the doping purpose, the thin gate oxide area which is not covered by polysilicon is also etched away. This exposes the bare silicon surface through which the silicon substrate is doped to form source and drain junctions. 

3. Diffusion or Ion implantation

The process of doping is called as diffusion or ion implantation. 

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 2.4(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

in order to dope the substrate selectively, you have to keep oxide in some regions and remove the oxide from some other regions. Wherever the oxide is present, it will act as a mask against doping, so those portions will not get doped. The other regions from where you have removed the oxide, it will get doped when subjected to doping. Ok? Now, this selective removal of oxide is done by a technique called photolithography