The below image is a 150um x 170um block from an ASIC design file produced automatically by fully open-source tools such as yosys.
The apparently unused sections have a repetitive pattern, presumably some kind of filler cells.
What is the purpose of filling the area with cells rather than just leaving it blank? Something to do with routing or layout?
I'm not an ASIC designer, but on a quick search found What is the use of adding filler cells to a design? which contains answers such as the following which sound reasonable. I.e. to meet the manufacturing requirements of the fabrication process:
Filler cells are used to fill any spaces between regular library cells to avoid planarity problems. They are need when the density of the required metal or layer has not meet the foundry or fab requirement. Thus, you need to add it whether it is low or high frequency.
Use of filler cells create symmetrical metal density that is manufacturable.
