“In the EUV case, you have a much higher energy photon. it’s much more complicated and it’s really not well understood,” said Gregory McIntyre, director of the Advanced Patterning Department at Imec. “It creates high-energy electrons that quickly cascade into lower energy electrons. Those electrons then interact with whatever they happen to bump in to. With this, there are quite a few unknowns, such as how many electrons are being generated and what are the energies—and more importantly, what kind of chemistries are resulting because of those electrons.”
“As we go to smaller and smaller feature sizes, what we find is that the Gaussian distribution starts to grow a tail and become asymmetric on one side. This growth of the tail leads to the increasing probability of highly unlikely events,” McIntyre said.
Years ago, stochastics and shot noise were not on the radar screen, but the issues began to appear in 193nm lithography. In 193nm, chipmakers use a dose of 10mJ/cm² near the edge of a feature. “If I take a 1nm² area, then over the course of that exposure, 97 photons on average will pass through that area and go into the photoresist. But if I look at this larger volume of 10nm² on a side, I will have 9,700 photons on average,” explained Chris Mack, CTO of Fractilia. So, with an ample number of photons to process a feature, the photon shot noise or variation amounts to only 1%, according to Mack.
In contrast, EUV photons have 14 times more energy per photon than 193nm light. “That means for the same dose, EUV has 14 times fewer photons,” Mack said. “So while in the example above, we had 97 photons exposing a 1nm² area, at EUV there are only 7 photons. The relative uncertainty is 1/square root of the number of photons. For 97 photons, that is a +/- 10% uncertainty. For 7 photons the uncertainty is +/- 40%.”
Source: EUV's New Problem Areas (Semiconductor Engineering, March 2018)