Secondary electrons generated and scattered in the substrate contribute to the dose of other nearby features. How close is close? A 100 kV beam of electrons will scatter in silicon to a range around 26 um (the 1/e range that is). You might think that life would be better at low voltage, but it isn’t. What counts is the ratio of primary electrons to secondary electrons. This ratio is higher at high voltage. Well, that’s not exactly true. When the voltage is below 2 kV or so then the electrons will not even penetrate to the substrate, so at low voltage there is no proximity effect at all! Do you think low voltage e-beam is a good idea? Ha ha: no. We leave it as AN EXERCISE FOR YOU to explain why low voltage lithography is a terrible idea.