  Focused ion beam (FIB) microscopy suffers from source shot noise - random
variation in the number of incident ions in any fixed dwell time - along with
random variation in the number of detected secondary electrons per incident
ion. This multiplicity of sources of randomness increases the variance of the
measurements and thus worsens the trade-off between incident ion dose and image
accuracy. Time-resolved sensing combined with maximum likelihood estimation
from the resulting sets of measurements greatly reduces the effect of source
shot noise. Through Fisher information analysis and Monte Carlo simulations,
the reduction in mean-squared error or reduction in required dose is shown to
be by a factor approximately equal to the secondary electron yield. Experiments
with a helium ion microscope (HIM) are consistent with the analyses and suggest
accuracy improvement for a fixed source dose, or reduced source dose for a
desired imaging accuracy, by a factor of about 3.
