Three-dimensional simulation of top down scanning electron microscopy images

Low voltage scanning electron microscopy (SEM) metrology and inspection are performed by immersing the sample in an electric field; under this condition, when a scanning electron beam images a sample containing insulating features (like oxides and resist), a surface global charge builds up to offset the applied field and a transverse local field will form as a result of the scanning beam. The surface global charge is responsible for the voltage contrast and imaging properties, while local fields degrade image resolution. In this article we describe a simulation approach able to explain the imaging properties of charged surfaces and how resolution is affected by local fields. Using electron ray tracing in the column, the simulation follows both the emitted and primary electron trajectories outside the sample. In addition, Monte Carlo scattering simulation calculates the electron trajectory and charge deposition inside the sample. The resulting charge density is used to calculate the field inside and outside the sample by solving the Poisson equation with the proper boundary conditions. Ray tracing, Monte Carlo scattering simulation; and field equation are then integrated in a self-consistent fashion to form a simulation algorithm capable of explaining SEM imaging and charging. The simulation is applied to a variety of cases regarding both inspection and metrology. The results are compared with experiments. Furthermore a method to calculate surface charging will be given for both insulating surfaces and patterned insulating surfaces on a grounded substrate.