MOSFET Fabrication – Nwell, Pwell, Twin Tub and SOI – IC Fabrication process

Mosfet (Metal Oxide Semiconductor Field Effect Transistor) fabrication process is discussed below.

MOSFET can be fabricated in four technologies, they are:-

  1. Pwell process
  2. N-Well Process
  3. Twin Tub Process
  4. SOI (Silicon on Insulator) fabrication process

The main fabrication steps in Mosfet/IC are discussed below:

  1. Wafer fabrication
  2. Layering
  3. Doping
  4. Photolithography
  5. Heat Treatment

Wafer Fabrication

In this process, sand is processed till formation of wafer.

The important specifications of wafer fabrication are

  • Wafer should be of single crystal silicon
  • High purity silicon
  • High ppb (part per billion) – impurities concentration should be ppb.

Silicon is chosen for wafer fabrication as it is a semiconductor and second most abundant material found on Earth.

While fabricating the Silicon from Sand, we have mostly two types/ grades of silicon, they are:

  1. Metallurgical Grade Silicon (MGS), and
  2. Electrical Grade Silicon (EGS)

Metallurgical Grade Silicon:

The MGS Silicon has usually more impurity concentration. It is usually in the range of ppm which is mostly Aluminium (Al) and Iron (Fe).

SiO2 (Solid) + 2C(Solid) Si (Liquid)+2CO(Gas)

The silicon is extremely sensitive to impurity concentration, which effects on the carrier mobilities and lifetime. So, there is further need to purify the MGS.

Electrical Grade Silicon:

The Silicon which is formed in the MGS is combined with HCL (Hydrochloric Acid) produces SiH4 called as silane in addition to SiHCL3 (Tricholoro Silane).

The Tricholoro Silane is further processed by distillation to get purest form of silicon.

SiHCL3 + 2H2     2Si (Solid) + 6HCL (gas)

The Silicon formed in the above steps will be in the form of small crystals. Which is further processed through a process called as Czochralski growth and wafer formation process.

In Cz process, the multiple crystals of silicon is processed in molten state through a seeding, which gives a cylindrical structure of Silicon called as ingot or boule.

This ingot is then sliced into wafers, whose thickness will be in the order of few nanometers.

Once, the slice of wafer is procured, it is processed through CMP (Chemical Mechanical Polishing) process. This has to be done because the wafer will be not in uniform thickness and also it will have impurities on the surface of wafer.

The Chemical and Mechanical polishing is named so, because, the surface of the wafer is rubbed and rotated on a pad. While in this process, a slurry is added which is usually (SiO2 and NaOH).

The SiO2 helps in friction while wafer is rotated on the pad. While, NaOH removes the impurity ions from the surface of the wafer.

Layering

Layering process is used to add silicon oxide and nitride oxide in various fabrication steps in MOSFET.

Layering can classified into two types

  1. Grown: In this type the oxide is grown on the wafer itself. It means, for formation of oxide, a small portion of wafer surface is used to grow the oxide. Wafer is consumed to grow the oxide layer.
  2. Deposition: In this Silicon is not consumed to form the oxide layer.

In Grow process, there are further 2 types, they are

  1. Oxidation and
  2. Nitridation

Before knowing the process involved, lets look at the importance of oxidation.

  1. It acts as a protective layer.
  2. It is used for mask against diffusion and ion implantation
  3. Physical, mechanical and chemical support for MOS
  4. SiO2 is also acts a good dielectric, which can be used as gate oxide.

Silicon is exposed to an oxidant at high temperature forms a thin layer of oxide at the exposed temperature. The formed layer is in the form of SiO2.

The oxidation is again divided into two types:-

  1. Wet oxidation and
  2. Dry oxidation
Wet Oxidation Dry Oxidation
Growth rate is very high Growth rate is very slow
Used for thick oxide Used for thin oxide
Low dielectric constant High dielectric constant
Used for masking, mechanical support and mechanical support Used for formation of cap/gate oxide
Also used for isolation between active devices

The second method in Grown layering is Nitridation:

  • With Nitridation, Si3N4 layer is formed
  • It is preferred because its melting point is high
  • Passivation (means, it is used for glass covering, as the outermost layer for the IC)
  • It also helps in protection from Sodium and Moisture.

The second method in Layering is Deposition:

In deposition technique, we have

  1. CVD (Chemical Vapor Deposition) and
  2. PVD (Physical Vapor Deposition)

The chemical vapour deposition (CVD) is used to deposit Polysilicon / Si3N4 and SiO2 layers.

While sputtering is used to deposit Aluminium metal layer for interconnects.

Photolithography

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