The kind of precision and accuracy tolerances you need in these machines basically make it so that everything has to be exactly right. The 200mm mask won't have a place to "fit correctly" in the 300mm machine and so you won't be able to get the precision and accuracy you need. Remember: a silicon chip is actually made of many layers, which means many masks (1 mask per layer). Additionally, the method by which 200mm masks and 300mm masks are actually used in the etching process can vary significantly because of physics that I honestly don't have a phenomenal insight into.

Making larger wafers drives prices up, because the process of making a monocrystal larger and larger, but I don't think that's a major cost factor except for super cheap chips. The minimum cost of a 200mm wafer is about $2/in^2 and the minimum cost for a 300mm wafer is about $3/in^2, in case that's helpful.

The pad size for external wires might dominate the design for extremely simple designs, however couldn't a combination of these factors help?

* Shrink the average node size 1-2 times (E.G. 180nm to 90 or 45nm)

* Combine popular combinations of products into a single package that can have interlinks cut by laser or skipped at the metal traces stage?

* Generally consolidate similar products within fewer variations?

The slightly greater cost of the physical wafer is supposed to be offset by the increased machine throughput and the die shrinks possible, as well as ideally fewer defects by using newer processes. An extremely inexpensive set of processes for 300mm wafers would also be a good target for long term bulk semiconductor production.

Masks are not whole wafer , except in some packaging cases. The mask illuminates a 25*35 area, for both 300mm and 200mm.

Masks are not a significant expense for most processes. Nor is the bare wafer. It is all the process steps that drive the cost.

300mm qafer

I assume there's other equipment still in use on the line that can only handle 200mm wafers, otherwise it'd make sense to upgrade to 300mm and take the extra capacity increase - the masks are stepped across the wafers anyway, there's no reason in principle they can't just add more chips to fill in the extra area if they have the ability to process 300mm wafers.