Physical Cell - Tap Cell

Till now we have seen some physical cells such as Boundary Cells and Filler cells. There are some other cells also. Today we are going to take look on the cell which is part of the basic structure of CMOS.

 

Tap Cells

• A tap Cell is a special kind of non-logic cell with well and substrate tie. 
• It has only physical significance kind of cell used in the design. 
• Tap cells are placed in the regular interval of distance in standard cell rows. 
• The distance between two tap cells is mentioned in the Design Rule Manual (DRM). 
• DRM is manual contains some set of predefined technologies rules given by the foundry for manufacturing.  
• The cell from the library are tap-less, it means that CMOS Structure without body connection. 
• Before starting the placement (or before power planning we placed), we have to insert the cells.

 

*Figure: Tap Cell distribution. (it is taken from Design & Reuse) 

The Purpose of Tap Cells:

To avoid the Latch-up:

It is a kind of condition where the chip turns into a complete failure due to the low impedance path is formed between supply V_DD and ground V_SS.

To know detail about the Latch-up: Texas Instruments White Paper on Latch-up. (Public domain paper)

To reduce the size of the standard cell:

Traditionally, N-well is connected to V_DD and P-substrate is connected to V_SS. But in reality, each cell doesn’t require the body or tap connection. The body connection to the cell has significance. However, standard cells are placed in well-defined rows of uniform height where wells are continuous.  Thus, standard cell, placed in a raw, can share these cells. This kind of topology reduces the size of the cell and allows the designer to accommodate more standard cell. This type of standard cell library is called Tap-less Library.

 

I would like to suggest to readers that read an article- Latch-up Improvement for Tap Less Library Through ModifiedDecoupling Capacitors Cells- to know more on it.

 

At this note of discussion, I wrapped this topic and we will meet soon with a new discussion on our last physical cell. Thank you. Have a nice day!!!!