In this article, we delve into the world of VLSI and explore the concept of filler cells. We discuss the purpose and importance of these cells in the design process and their impact on the overall performance of the circuit. Whether you're a beginner or an experienced engineer, this comprehensive guide will provide valuable insights into the role of filler cells in VLSI.
Standard Cell Layout:
The Standard Cells are of equal heights (a.k.a Track) but width may vary. Standard Cells are placed in rows with cells butting against each other. This ensures continuous wells across the entire row. This helps in fabrication and mask design of the design. Such arrangement allows one to run common power lines such as power(VDD) and ground(GND/VSS) 
through the cell array. This ensures VDD/GND rails (follow pins) are fully connected. A desired design is implemented by picking up necessary standard cell from different such rows/column and interconnecting them as per the target functionality.
Standard Cell Layout & Filler Cell:
This tiled structure of standard cells are optimized for area. However Standard cell placement never reaches 100% utilization. Any blank space in the tile structure is filled up with a special cell called Filler Cell or DeCap Cells from the Standard Cell Library. This ensure proper GDS layers to pass DRC and sufficient diffusion and poly densities. The Filler Cell is a empty cell with power and ground rails. Empty means that cell has no functionality. It has physical exsistance that is it has physical layers like diffusion layer etc. Total effective area is calculated by subtracting the sum of all filler cell area from total area of Standard Cells Tile Structure. Chip Finishing for SignOff includes, at the very least: Insertion of Fillers and DeCaps.
Design Automation for Placing Filler Cell :
Filler Cells are inserted after all cells have been placed and the confidence factor of a design in meeting timing is high. Filler cells will fill in all row spaces, which remain open. The PnR Tools in VLSI generally accepts TCL Scripting for automation. The TCL script traverses the standard cell row (from left to right), it checks the adjacent cell edges. If the edges match, the TCL script moves to the next cell. However, if the edges do not match, the script checks if the opposite side of the right cell matches the current cell edge. If it does, the script flips the cell and continues. If neither sides match, then a filler cell is placed in between the cells, to ensure that design rules are satisfied. As power rails (horizontal power lines) are usually built into the standard cells as feed- through. Leaving any space in the row would result in a break in the power line.
Physical Design Aspect :
A set of physical-only cells (without any boolean function) in the form of fillers and decap cells are provided in standard cell distribution as they are required during digital implementation. Fillers Cells are important as they connect the active implants (n+ and p+), as well as n-wells and power rails throughout an entire row. Fillers should come in various distinct widths, where the width is an integer multiple of the metal one routing pitch (track). Fillers consist of dummy polysilicon and diffusion area, which improves density. Some filler cells may include the well-taps which aid in lowering the substrate resistance. Both Tie-low and Tie-high filler cells are provided to avoid direct connections (ESD prevention) to power and ground rails when there is need for a constant input. Finally some decap cells are provided to help mitigate IR drop issues during digital implementation.
Physical Verification (DRC) Aspect:
Commercial P&R tools apparently fix the minimum implant area (MinIA) violations by inserting filler cells at the final design stage. For example, One commercial tool has a utility to define an implant layer group for filler cells, so that each narrow cell can be padded filler cells having the same implant type. Another commercial tool checks and fixes implant area violations according to the rules specified in LEF, during placement and filler cell insertion. Another commercial tool offers a Voltage-threshold-aware filler cell insertion flow according to which users can define the Vt filler cells to be inserted between different Vt regions. For example, users can insert NVT filler cells between NVT and HVT cells, and LVT filler cells between LVT and NVT cells.
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