The proximity effect is a major challenge for electron beam lithography. Patterns in electron sensitive resists (eFilm) are created by exposing the eFilm to a beam of energetic electrons. Like how a projectile may ricochet and hit the same target multiple times, an electron can expose the eFilm multiple times. The additional exposures after the first one arise from uncontrollable mechanisms and are collectively called the background exposure. Severe background exposures cause prints that differ from the layout.