Full custom design of a logic inverter

A set of images depicting key stages in the full-custom design of a logic inverter (refer to section 6.0).  These slides will be shown in lecture 6/7.  The images are digitised 35 mm slides. 

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P-well (diagonal brown shading). 

  • The process illustrated is a p-well process. 
  • This uses an n-type substrate in which the p-channel transistors will be fabricated directly. 
  • The brown shaded region is implanted with p type dopants to form a p-well, in which the n-channel transistors can be fabricated.


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P-well with active regions (green cross-hatching). 

  • The active regions indicate zones where the transistor channels will lie.
  • These regions are protected by a thin coat of silicon nitride while surrounding areas have a thick layer of field oxide grown upon them.
  • The smaller 'active' squares at extreme top and bottom will provide low-resistance connections to the substrate and the p-well, required to ensure proper isolation between devices.


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Polysilicon gate electrodes and polysilicon interconnect (red).

The single vertical strip of polysilicon crossing the two active regions provides:-

  • the two gate electrodes
  • an electrical connection between them, and
  • a route by means of which the input signal may be brought into the cell across the power rails (yet to be provided at top & bottom). 

The polysilicon also helps delimit the regions implanted p+ and n+.


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P+ and N+ implants (yellow and green).

  • These determine the positions of the source and drain in the corresponding devices. 
  • N-channel transistors require n+ implants to form their drain and source
  • P-devices require p+ implants.
  • Note that the polysilicon gate screens the channel region which does not receive these implants.


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Contact windows (black)

  • These indicate regions where an etch process cuts through the insulating layers, linking:
  • metal to semiconductor or:
  • metal to polysilicon.


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Metal 1 (dark blue)

This furnishes:

  • Power rails (Vdd and Vss at top & bottom respectively, and:
  • A link between the two MOSFET drains

The output is coupled from this link to points at top and bottom of the cell by means of a vertical strip of polysilicon.