Comment by addaon

> I assume they don't pattern the unused area

I’m out of date on this stuff, so it’s possible things have changed, but I wouldn’t make that assumption. It is (used to be?) standard to pattern the entire wafer, with partially-off-the-wafer dice around the edges of the circle. The reason for this is that etching behavior depends heavily on the surrounding area — the amount of silicon or copper whatever etched in your neighborhood affects the speed of etching for you, which effects line width, and (for a single mask used for the whole wafer) thus either means you need to have more margin on your parameters (equivalent to running on an old process) or have a higher defect right near the edge of the die (which you do anyway, since you can only take “similar neighborhood” so far). This goes as far as, for hyper-optimized things like SRAM arrays, leaving an unused row and column at each border of the array.