In this paper, we will report the modeling of behaviors of low-energy secondary electrons (LSEs) in a charge effect correction system (CEC), which compensates image placement errors predicted by writing layout data, on mask blanks with and without a charge dissipation layer (CDL). For the demand of low line edge roughness and high critical dimension uniformity, the more dose-sensitivity of resist is lowered, the more a contribution of LSE is made in charge effect, because the number of primary electrons generating LSEs also increases. In such a situation of advanced lithography, we have tackled challenges of CEC system in terms of blanks with and without CDL. On blanks without CDL, CEC needs to predict the complicated charge distribution including that of LSE which is affected by electromagnetic field of optical system and already-existing resist surface charge. On blanks with CDL, CEC needs to predict LSE charge diffusing on the resist surface. In order to address these challenges, NuFlare Technology (NFT) has developed models of CEC on blanks with and without CDL. In order to verify these models, we evaluated the surface charge effect in the writing on blanks with resist of 100 μC/cm2 dose-sensitivity on electron beam mask writer EBM-9000/9500. Furthermore, as another solution besides CEC and CDL, NFT has developed a charge effect reduction (CER) system which prevents LSEs from re-entering into the resist surface. We have verified CER by comparing the experimental results between EBM9500 and EBM9500PLUS, which is the NFT’s latest variable-shaped beam with CER system.
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.
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