Photon Shot Noise in Photolithography

IEEE Spectrum says that photon shot noise becomes a major limiting factor in EUV photolithography. The EUV wavelength of 13.5nm is much shorter than the commonly used 193nm, meaning there is much less photons per each Watt of the light power.

"...when Imec engineers began producing experimental features for 5-nm chips last year, they noticed many more defects than they’d expected.

They built rows of trenches of the kind that would form a chip’s wiring once filled with metal and arrays of holes that would become the contacts from the wiring to the parts of the transistors below. But there were “nanobridges” between the trenches, holes that were missing, and holes that had merged with their neighbors, Ronse says. Such random snafus are collectively called stochastic defects.

What causes them? ...They can be caused by what’s called photon shot noise. It’s the fact that there are few photons falling on the chip and you just don’t always get the same number at every spot on the chip.

Another culprit is likely a sort of chemical version of shot noise. The photoactive chemicals in the photoresist may not be perfectly uniformly distributed on the wafer, leaving spots that act as though they are underexposed."

Noisy EUV image