A simulation model for electron irradiation induced specimen charging in a scanning electron microscope

Abstract

A numerical model has been formulated to simulate the dynamics of specimen charging in a scanning electron microscope. In this model, the electric field due to imposed boundary conditions and fixed charges is solved by the finite element method. The empirical electron yield data are stored in 'Universal Yield Curves (UYC)'. These UYCs control the generation of secondary and backscattered electrons from various materials. The electrons emitted from electron-solid interactions are tracked using a leapfrog integration scheme. Excess charges generated on the surface of electrically floating solids are assigned to numerical grids using a linear charge redistribution scheme. The validity of the simulation model was verified by measurements in a special setup which consisted of several isolated electrodes in the SEM chamber. Excess currents generated inside each electrode due to electron irradiation were measured simultaneously. Measurements and simulation results are in broad agreement and show that electrically floating electrodes, not directly irradiated by the primary beam, can charge-up if they are irradiated by secondary electrons and backscattered electrons emitted from a nearby electrode. The polarity of charge generation on the electrically floating solid depends on its own material property, and also strongly on the potential distribution in the space surrounding the floating electrode.