The gate of the standard cells put at the border during chip manufacture has a significant likelihood of being damaged. We have a specific type of cell in the common cell library called an end cap cell or boundary cell to prevent these damages at the boundary. The Endcap cells are the only physical cells with no logic, hence these cells are not part of the netlist. Boundary cells have mainly Nwell layers, Implant layers, dummy poly layers, and metal rails.
In addition to protecting the gate damage at the border, boundary cells have several other uses. The definition of boundary cells and their advantages will be covered in this article.
Also read: What is combinational circuit?
What Are End Cap Cells in VLSI?
To ensure smooth integration while unifying partitions/sub-structures at the IP level or IPs at the SoC level, one would have to align with specific boundary requirements. There are specific requirements for good alignments and well rings on the layout boundary from the process team and Fab. Ideally, the designer can draw well-defined rings or, occasionally, the metal around the boundary to satisfy process requirements if they are aware of the alignment w.r.t DRC. All designs must utilize Special Physical Only Cells, which are provided by the process team and library teams to facilitate this task and ensure that the integration work is done correctly by construction.
While some firms build end cap cells with DECAP so that they can function as decoupling capacitors for the site rows where they are situated, other foundries construct end cap cells to aid with DRC alignment. End cap cells are positioned at the top and bottom margins as well as at the extremities of the horizontal site rows (nwell, DRC requirements). Designers make sure that there won’t be any abutment issues with integration at higher levels by positioning these cells on partition/sub-design borders. Each placement row is terminated by placing an end cap cell or boundary cell at each end. To facilitate interaction with other blocks, it has also been positioned in the top and bottom rows at the block level. Some common cell libraries additionally include corner end cap cells at the block’s corner. Boundary cells cannot be moved during optimization since they have fixed attributes.
Benefits And Features Of End Cap Cells
- The end cap cells in VLSI are positioned in the design to guard against manufacturing damage to a regular cell’s gate that is located close to the border.
- They aid in avoiding the boundary’s base layer DRC (Nwell and Implant layer).
- The other block is properly aligned with boundary cells.
- Some standard cell libraries have end caps that also function as decals.
- Cells with boundary caps can only be physical. These cells are positioned on the edge of the core and power domain.
- In the floorplan flow, boundary cap cells are positioned immediately following macro installation. These cells are sequentially arranged in an array
- There is no signal connection in boundary cap cells. Using an estimate of where the PG grid will be inserted, these cells are connected to the power and ground rails.
- To regulate the unbreakable placement, boundary cap cells of varying widths are included in the design.
- Technology is a requirement for boundary cap cells.
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