Chapter 11
Cell Libraries, Models, Files & ECO
Cells in the IO-to-Core Gap
No. The area between the IO and the core boundary is reserved for power rings; placing cells there would cause routing issues.
KEY That gap is reserved for power rings - no standard cells allowed.
Physical Cells
End-cap cells: protect cells from fabrication damage, provide row connectivity and define row ends, prevent drain/source shorts, and resolve boundary N-well DRC issues. Well-tap cells: tie VDD and GND to the substrate and N-well to reduce drift and prevent latch-up; placing them at the specified spacing keeps the N-well potential correct and limits substrate resistance. De-cap cells: temporary capacitors between power and ground that counter functional failure from dynamic IR drop and keep distant flops from going metastable. Filler cells: fill empty space and continue N-well and implant-layer connectivity.
KEY Physical cells = end-cap, well-tap, de-cap and filler cells - they protect, connect and stabilise the layout.
The GDSII File
- GDS (Graphic Data Stream) originated at the Calma company - the original GDS in 1971 and GDSII in 1978.
- It is a binary format that captures layout data in a hierarchical manner.
- It stores items such as text labels, geometric shapes, layer details and other 2D/3D layout geometry.
- The file is handed to the fab, which uses the parameters in it to pattern and etch the chip.