Physical Cell - Filler Cells

Today, we are going to discuss the Physical cells one by one throughout the series. These cells are special cells used in the Floorplan or Placement or Routing stage. So, it will full fill it intended role. 

 

What is Physical cell?

The physical cells are special-purpose cells. These cells are not available in the netlist. We called it from the library. These cells don’t perform any logic functioning but these cells are used to avoid the disability of layout.

Types of Physical cells

• Filler Cells 
• Tap Cells 
• Decap Cells 
• Boundary cell or Endcap Cells

Filler Cells:

• Filler cells are used to establish the continuity of N well and the implant layers on the standard cell rows. 
• It is connected to between the VDD and VSS. 
• It is used to fill empty space between cells and used to complete the connection of power rails. 
• The size of the cells is varying as per the space available between the cells. 
• Filler can be single height or multicell height cells. 

Figure: Different Size of Filler Cells

When we placed filler cells?

After the ROUTING stage, we have to clear the following stage.

• DRC and LVS of the design must be cleared. 
• Timing optimization must be done along with another optimization. 
• The issues of Antenna Effect must be resolved. 
• DFM of the design must be done.

Then, we have to add the filler cells in the design.

But why we need Filler cells?

We need the filler cells for continuity of N well and implant layers, so at the time of mask creation, we get the single continuous mask for N-well and implant. If there is any discontinuity in N well and implant, then there is a chance of DRC error and most important among all, there will be less process variation. If we don’t keep continuous, then more process variation and change the dimension due to photolithography and proximity effect. Also, we will get continuous VDD and VSS.

So, continuity of the Well and Implant is important.

 

At this note of discussion, we wrapped up this topic and we will meet soon with a new Physical Cell. Thank you. Have a nice day!!!