Linewidth measurements from secondary electron (SE) images were compared to those from backscattered electron (BSE) and low-loss electron (LLE) images. With the commonly used 50 % threshold criterion, the lines appeared larger in the SE images. Since the images were acquired simultaneously by an instrument with the capability to operate detectors for both signals at the same time, the differences cannot be explained by the assumption that contamination or drift between images affected the SE and BSE or LLE images differently. Simulations with JMONSEL, an electron microscope simulator, indicate that the nanometer-scale differences observed on this sample can be explained by the different convolution effects of a beam with finite size on signals with different symmetry (the SE signals characteristic peak vs. the BSE or LLE signals characteristic step). This effect is too small to explain the >100 nm discrepancies that had been observed in earlier work on different samples. However, additional modeling indicates that those discrepancies can be explained by the much larger sidewall angles of the earlier samples, coupled with the different response of SE vs. BSE/LLE profiles to such wall angles.
Microscopy and Microanalysis
Metrology, Backscattered Electrons, Low Loss Electrons, Secondary Electrons, JMONSEL, Electron Beam Modeling, Measurement, Critical Dimension