VLSI (FABRICATION PROCESS OF NMOS)

 FABRICATION OF NMOS:

step 1:

  • processing starts on single-crystal silicon of high purity on which required p impurities are introduced as the crystal is grown. such wafers are about 75 to 150mm in diameter and 0.4 mm thick.
step 2:
  • A layer of silicon dioxide (SiO2) typically 1 micrometer thick is grown all over the surface of the wafer to protect the surface, acts as a barrier to the dopant during processing, and provide a generally insulating substrate.
step 3:
  • the surface is covered with photo resistance which is evenly distributed over the surface.
step 4:
  • the photo resistant layer is exposed to ultraviolet rays through masking which defines the region which diffusion is to takes place together with transistor channels.
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step 5:
  • these areas are readily etched away together with the underlying silion dioxide layer.
step six:
  • the remaining photoresist is removed and a thin layer of the oxide layer is deposited over the total surface.

step 7:
  • after depositing the thin oxide layer upon which a polysilicon layer is deposited on which gate structure is formed.Further photo resist coating and masking allows the poly silicon to be patterned and then the thin oxide is removed to expose areas into which n-type impurities are to be diffused to form the source and drain.
step 8:
Thick oxide  (SiO2) is grown over all again and is then masked with photo resist and etched to expose selected areas of the poly silicon gate and the drain and source areas where connections are to be made. (contacts cut)
step 9:
  • the last is making the metal contacts. This metal layer is then masked and etched to form the required interconnection pattern.

















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