Abstract
Large window-size membranes for stencil masks are required to increase the throughput of electron projection lithography (EPL) and low-energy electron projection lithography (LEEPL). In this paper, image placement (IP) accuracy and methodology for correcting stress-induced distortions on 4 X EPL masks are addressed. Although the average of local IP errors (| mean | + 3σ) for reference features across an entire 1mm-window EPL mask is 13.4 nm, the average of errors across an entire 4mm-window EPL mask increases to 20.4 nm, which could be reduced to the required budget with further study on EB writing accuracy or IP corrections. In addition we evaluate local IP errors on 4mm-window mask due to pattern gradients by measuring the placement errors at the edge of dense hole arrays. Applying the correction for stress-induced distortions to EB data, we can reduce the placement errors for dense features to 4.6 nm, which is less than the 10 nm budget allocated for 4mm-window EPL mask at the half-pitch features of 45 nm node. For the global IP, only the measurement repeatability of 7.8 nm contributes to the global IP budget measuring all the global position over an entire 4mm-window EPL mask. And we can meet the required global IP budget. Finally, IP accuracy for a single membrane is also presented, showing the IP error is 24.5 nm (| mean | + 3σ), which compares with that of COSMOS type LEEPL mask. Methodology of measuring the position data on a single membrane, however, remains to be developed.
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