When an electron beam is used to expose a resist, neighboring regions of
the resist are also partially exposed. This arises from multiple scattering
of the electrons in the resist and by backscattering of the electrons in
both the resist and (mainly) in the substrate beneath the resist. From
various studies1,2 this non-local energy deposition can
be characterized by a number of regions, There is a very intense energy
deposition, which is typically quite narrow and is produced by the direct
incident beam broadened by multiple scattering in the resist. This is
surrounded by an approximate plateau of intensity of about 1-2 orders of
magnitude weaker, which is produced almost entirely by electrons
backscattering from the substrate. The plateau arises from two conflicting
effects: the backscattering yield drops as we move away from the central
beam, but the mean electron energy also decreases. Therefore the stopping
power increases, thus tending to offset the first effect. Finally this
plateau cuts off fairly sharply at a distance approximately equal to the
Bethe range of electrons in the substrate.