It almost certainly refers to 300 mm wafers, which are the largest size used right now. They offer significantly better economics than the older 200 mm wafers or lab experiments done in even smaller (i.e. 100 mm) wafers.

The text in the article supports this:

> This is a commercial 300mm monolithic silicon photonics platform, meaning the technology is ready to scale today, rather than being limited to laboratory experiments.

From the paper

> footprint of 330 × 290 µm2 using the GlobalFoundries 45SPCLO

That’s a 45nm process but the units for the chip size probably should have been 330um? However I’m not well versed enough in the details to parse it out.

https://arxiv.org/abs/2503.19544