An investigation of the effects of charging in SEM based CD metrology

Scanning Electron Microscopes are considered the most likely tool for future CD metrology down to 0.1 micron linewidths and below. Charging effects on insulating materials are a long standing problem for Electron Microscopes. The shrinking design rules are making the measurement errors caused by charging more significant. In this paper a model is proposed which incorporates charging effects into a Monte Carlo simulation model. The model stems from the notion of beam induced conductivity, an established phenomenon whereby an insulator becomes conducting for a brief period of time after being hit by a primary electron. The insulator becomes conducting only within the interaction volume of the primary electron. So after multiple scans of the primary beam has occured, it can be expected that because of the transient beam induced conductivity that the resulting charge distribution will be such as to create an equipotential surface where significant primary beam dose has occured. This concept is applied to resist by treating the top region of tile resist as a negatively charged equipotential. The substrate is given a different potential. In general different materials can be expected to have different potentials, One important consequence is that the corners of the resist lines, if they are sharp, have strong electric fields and they repel the beam electron. We calculate the electrostatic fields given the resist geometry, then we calculate the beam deflection caused by this field, we remap Monte Carlo simulation data to fold in this effect, and finally we compare with some experimental data to see if this charging effect can account for the apparent resolution degradation that occurs at the edges of resist lines with scanning electron microscopes.