In semiconductor manufacturing, accurate measurement of shapes and sizes of fabricated features is required. These measurements are carried out using critical dimension scanning electron microscope (CD-SEM). Positions of edges are often unclear because of charging. Depending on the SEM setup and the pattern under measurement, the effect of charging varies. The influence of measurement conditions can be simulated and optimized. A Monte Carlo electron beam simulation tool was developed, which takes into account electron scattering and charging. CD-SEM imaging of silicon dioxide lines on silicon was studied. In the experiment, changes in the beam voltage were found to result in contrast tone reversal. The same effect was also found in simulations considering charging. The time dependence of contrast variation was studied. A good agreement between simulation and measurement was found. The simulation software proved reliable in predicting SEM images, which makes it an important instrument to optimize settings of electron beam systems.