The fabrication of MOSFET comprises all the sequence of activities from taking the substrate to making a mosfet. The simplified basic steps for the fabrication of mosfet include the following steps:
Step 1: Substrate
It is the first step in which the substrate or wafer is selected and prepared for making the MOSFET. The mos is basically built over this. The wafer is cleaned and prepared. The substrate is generally silicon.
Step 2: Oxidation
It is the second step in which the silicon dioxide layer is formed or added on the surface of the substrate. It provides isolation between two layers and also protects the underlying material from contamination. There are two types of oxidation. They are Dry oxidation and Wet Oxidation. Silicon dioxide is formed at a lower rate and is of better quality in dry oxidation. On the other hand, it is formed at a higher rate and is of lower quality in wet oxidation.
Step 3: Ion Implementation
It is the process by which the impurity ions are accelerated at a high velocity and physically lodged into the target. The dopants are vaporized, accelerated and then directed towards the substrate.
Step 4: Diffusion
Diffusion is the process of movement of impurity atoms at the surface of silicon into the back of the silicon i.e, from higher concentration to lower concentration. The dopants diffuse from a high concentration source into a Si- substrate.
Step 5: Deposition
Deposition is the formation of thin film on the surface of Si wafer. The thin film material include semiconductor, metal or insulator. They are silicon nitride, silicon dioxide, aluminium, polysilicon. The different ways of depositing material to a substrate include:
- Chemical Vapor Deposition (CVD)
- Low Pressure Chemical Vapor Deposition (LPCVD)
- Atmospheric Pressure Chemical Vapor Deposition (APCVD)
- Plasma Enhanced Chemical Vapor Deposition (PECVD)
- Sputter Deposition
Step 6: Etching
Etching is the process of selective removal of material layer in Si- substrate. Etching is used to remove various portions as desired, or the underlying layer and masking layer as well. It is done by following two process:
- Dry Etching: Using chemically active ionized gases
- Wet Etching: Using Chemicals
Step 7: Photolithography
Photolithography or optical lithography or uv lithography is the process of forming pattern in the substrate block. The components include photoresist material, photomask, and material to be patterned. The process includes:
- Uniformly cover the substrate with photoresist
- Selectively expose the photoresist to uv light through a photomask
- Develop the photoresist to define the desirable pattern
- Etch the exposed part
- Remove the photoresist
Step 8: Isolation
Isolation is required from one active area to another. Generally, isolation is provided by a thick oxide layer (Field Layer).
After all, the Mosfet is prepared and prepared for the measurement of number of parameters related to current voltage parameters and many more.
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