Well Tap Cells in Physical Design - Team VLSI
Well Tap Cells in Physical Design - Team VLSI
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Well tap cells (or Tap cells) are used to prevent the latch-up issue in the CMOS design. Well tap cells connect the nwell to
VDD and p-substrate to VSS in order to prevent the latch-up issue. There is no logical function in well tap cell rather than
proving a taping to nwell and p-substrate therefore well tap cell is called a physical-only cell. In this article, we will discuss
the structure of well tap cell, the requirement of well tap cell and how to place them in the physical design flow.
Well tap cells have no logical functions, it has only two connections.
A typical structure of well tap layout has shown in figure-1. Well tap cell has no input and output pins, therefore it is called a
physical-only cell.
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Early days there was no concept of well tap cell, Standard cells were designed in such a way that each standard cell had nwell
to VDD and p-substrate to VSS connection within the standard cell. But such a standard cell design had consumed more
area and to save the area, later a concept of Tapless cell has evolved. In a tapless cell, there are no well taping inside the
standard cell, well taping is provided by a separate standard cell which is called a well tap cell. So well tap cell is a part of a
tapless standard cell library. Figure-2 shows the structure of a traditional standard cell and a tapless standard cell.
Well tap cells are used to prevent the latch-up issue in design. how it prevent, has been explained in the article “latch-up
prevention in CMOS” in this blog.
Well tap cells are placed after the macro placement and power rail creation. This stage is called the pre-placement stage.
Well tap cells are placed in a regular interval in each row of placement. The maximum distance between the well tap cells
must be as per the DRC rule of that particular technology library. A typical placement of well tap cells is shown in figure-3.
Well tap cells are generally placed in a straight column in the alternate row as shown in figure and such a pattern is called
checkerboard pattern to provide maximum coverage for well tap. If a macro comes in the path of vertical columns, then the
placement of vertical column shifted alongside macro as shown in the figure.
This placement is performed using the PnR tool command. For ICC and Innovus tool following command have used to place
the well tap cells.
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For ICC tool:
add_tap_cell_array –ignore_soft_blockage true –master_cell_name $tapCell–distance $tapPitch –connect_power_name VDD –
connect_ground_name VSS –respect_keepout-pattern stagger_every_other_row –tap_cell_identifier WELLTAP
Thank you!
standard cell, Standard Cell Library, tap cells, Well Tap Cell
Standard Cell Library for ASIC Design
End Cap Cells in VLSI | Boundary Cells in VLSI
Unknown
March 1, 2021 at 4:42 am
thanks for this. Very informative. I'm wondering if you have ever heard of a scheme where there are two types of tap
cell, one containing only a p-well contact and one containing only an n-well contact?
Reply
Team VLSI
March 2, 2021 at 3:49 am
Hi,
You are most welcome.
Can you please provide some references to me of such design?
I need to check, then we can discuss more on that.
Reply
Believer
January 24, 2022 at 5:07 am
Reply
Unknown
February 27, 2022 at 3:50 am
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2023/5/17 11:30 Well Tap Cells in Physical Design - Team VLSI
Reply
Dhananjay
March 5, 2022 at 11:05 am
Latch up comes due high nwell and pwell resistance, Due which potential difference creates that can on the internal
transistors.
Tap cells provide extra dopping of nwell, which lower the resistance.
Reply
Team VLSI
April 7, 2022 at 3:51 am
Reply
Team VLSI
April 7, 2022 at 3:53 am
Reply
U.Upender
March 19, 2023 at 5:48 pm
Reply
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