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Wafers used for making semiconductors are round -- but this wastes quite a few chips around the periphery of the wafer in the fabrication process. Wouldn't it make sense to make the wafer as a square or rectangle instead?

Is there some aspect of the lithography process that requires that the surface be round?

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  • \$\begingroup\$ Google is autocompleting "why silico..." to this question. \$\endgroup\$ – Eugene Sh. Mar 18 '15 at 17:14
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    \$\begingroup\$ If you know how to make high-purity, single crystal rectangular ingots of silicon, I'm sure the manufacturers would like to talk to you. \$\endgroup\$ – Dave Tweed Mar 18 '15 at 23:02
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As the wafer material is drawn up out of the molten silicon, it is spun in order to produce a single uniform silicon crystal via the Czochralski process. It is this spinning that produces the round profile of the wafer itself.

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The manufacturing process results in a cylinder of silicon. The manufacturers could just square it off and return the trimmings to the pot, but as the whole process evolved to handle round wafers they just don't.

So the direct answer to "Is there some aspect of the lithography process that requires that the surface be round?" is "the machines are designed to accept round wafers, so that's what the wafer makers deliver. The next machines need to work with existing round wafers, so ......" and on it goes.

Some manufacturers will make smaller chips in the corners, it really depends on compatibility and economics. I have heard of foundries specializing in custom imaging elements (which can be quite large) adding smaller imaging devices for "free" around the perimeter. "Free" in this case being far less money than usual.

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