Test: NMOS & CMOS Fabrication

 nMOS fabrication process is carried out in thin wafer of a single crystal with high purity.

p impurities are introduced as the crystal is grown. This increases the hole concentration in the device.

Above a layer of silicon dioxide which acts as barrier, insulating layer is provided upon which other layers may be deposited and patterned.

The photoresist layer is exposed to ultraviolet light to mark the regions where diffusion is to take place.

In nMOS device, the gate material could be metal or polysilicon. This polysilicon layer has heavily doped polysilicon deposited by CVD.

In nMOS fabrication, the bulk substrate used can be either bulk silicon or silicon-on-sapphire.

In nMOS fabrication, etching is done using hydroflouric acid or plasma. Etching is a process used to remove layers from the surface.

The polysilicon layer consists of heavily doped polysilicon deposited by chemical vapour deposition.

 Diffusion is carried out by heating the wafer to high temperature and passing a gas containing the desired ntype impurity.

Contact cuts are made in the desired polysilicon area, source and gate. COntact cuts are those places where connection has to be made.

The metal layer is masked and etched to form interconnection pattern. The metal layer was formed using aluminium deposited over the formed surface.

Boron is used to suppress the unwanted conduction between transistor sites. It is implanted in the exposed regions.

CMOS technology is used in developing microcontrollers, microprocessors, digital logic circuits and other integrated circuits.

Some of the properties of CMOS are that it has low power dissipation, high packing density and low noise margin.

In CMOS fabrication, nMOS and pMOS are integrated in the same chip substrate. n-type and p-type devices are formed in the same structure.

P-well is created on n substrate to accommodate n-type devices whereas p-type devices are formed in the ntype substrate.

 Oxidation process is carried out using high purity oxygen and hydrogen. Oxidation is a process of oxidizing or being oxidised.

Light sensitive polymer is used to form the photoresist layer. Photoresist is a light sensitive material used to form patterned coating on a surface.

The photoresist layer is exposed to ultraviolet light to mark the regions where diffusion is to take place.

Few parts of photoresist layer is removed by treating the wafer with basic or acidic solution. Acidic solutions are those which have pH less than 7 and basic solutions have greater than 7.