This paper introduces a simple physical model to quantitatively explain resist surface charge effect observed in EBM- 9500PLUS, our latest VSB mask writer designed for 7 nm+ generation. The model takes into account secondary electrons drawn to resist surface by an already-existing surface charge, and vertical diffusion of positive charge from resist surface to inner resist. In order to verify the model, we experimentally evaluated the surface charge densities after beam exposure on resists of different thickness (from 80 nm to 300 nm) and different dose sensitivities (from 7 μC/cm2 to 100 μC/cm2). The introduced model successfully reproduced the exposure-dose-dependent and time-dependent behaviors of those surface charge densities experimentally obtained. The model enables us to predict the amount of surface charge, and serves as one of the barometers to select the preferable resist thickness and its dose sensitivity under the pattern density and the required IP accuracy for the given product layouts. Furthermore, although the mechanism of charging had been unclear for a decade or more, the model finally provides a quantitative physical validity of our charge effect correction (CEC) system.
We investigated the contribution ratio of process fidelity and beam accuracy in patterning with the multi-beam mask writing system. A beam pitch-related line edge profile may occur, which impacts on line edge roughness (LER) in the multi-beam writing system. The printability of beam image into the final etched pattern depends on the mask process, therefore, we need to understand quantitatively the printability of beam placement errors on LER with the actual mask process. We examined how the patterning characteristics are modified in each step of the mask process. The printability of beam placement errors largely depend on the period of errors, rather than the amplitude of errors. These results can optimize the writing strategy in multi-beam mask writing.