Dose for 10,000 EUV photons within different diameters. An estimated 2000 photons would be expected in the outermost 10% of the feature width. The table displayed gives a more accurate estimate between the specified inner and outer diameters. According to the Poisson distribution, the standard deviation is the square root of the photon number expected value, or √2000≅45 photons. In a large population of features approaching the normal distribution, one out of a billion features would be more than six standard deviations away from the mean, i.e., 6*√2000/2000≈13%.