Simulation of electron scattering in a scanning electron microscope for subsurface metrology

Abstract

Electron scattering on a flat sample with a subsurface structure was simulated to investigate the signal profile and critical dimension (CD) measured by scanning electron microscope (SEM). The authors modified an electron-scattering simulator, monsel, which was developed by the National Institute of Standards and Technology for applications to line-width metrology using CD-SEM, to simulate a flat tungsten (W) and silicon (Si) pattern under a nanometer-order-thick amorphous carbon (a-C) film and to classify the emitted electrons from the sample according to their generation processes. The simulation result shows that the material contrast between W and Si regions for measuring backscattered electrons (BSEs) is larger than that for measuring secondary electrons (SEs), though the yield of BSEs is lower than that of SEs. The low contrast given by the SE profile is attributed to the contribution of SEs generated from the a-C film by the incident electrons, which becomes an offset component. In contrast, the offset component in the BSE profile (which is also attributed to the a-C film) is much smaller than that in the SE profile. It is therefore concluded that BSE detection is suitable for CD measurement of a subsurface pattern even under a several-nanometer-thick layer. The simulation result also shows that both CD bias (between the top width of the pattern and measured CD) and material contrast increase with increasing irradiation energy for a tapered pattern. This tradeoff relationship (i.e., where both factors increase) indicates that control of the irradiation energy is necessary to obtain an accurate CD measurement of a flat sample with a tapered subsurface pattern. Moreover, in a similar manner to the simulation result, the experimentally measured SE contrast shows a similar dependence on a-C film thickness and irradiation energy.