Fundamental understanding and experimental verification of bright versus dark field imaging

DUV lithography has successfully adopted both bright and dark mask tonalities. This gives the freedom to chip manufacturers to choose the optimum combination of mask and resist tonality for their product [1]. In EUV lithography, however, there has been a clear preference for dark field masks, driven by the prevalence of positive tone resist processes, and their relative insensitivity to multilayer defects. Future customer nodes, however, may require negative tone (metal-oxide) resist processes [2][3], resulting in a requirement to use bright field masks. Therefore, a deeper understanding of bright and dark field imaging is needed in order to provide guidance to ASML customers in choosing the optimal approach. In this work we consider the fundamentals of bright and dark field imaging based on the diffraction theory of aerial image formation [4]. We will show that bright field imaging has an intrinsic potential for higher optical NILS (normalized image log-slope), especially for isolated features, but with a lower depth of focus. The theoretical results are compared to rigorous simulations. Experimental bright vs dark-field results is also presented for comparison. Wafer based data has been obtained on an NXE:3400 scanner, whilst aerial image measurements have been obtained using the Aerial Image Measurement System for EUV (AIMS® EUV) at Zeiss. These experimental results confirm the theoretical expectations. The main goal of the paper is to draw attention to bright versus dark field comparison for EUV and to kick off more studies in this direction.