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 . 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 , 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 . 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.
The next-generation high-NA EUV scanner is being developed to enable patterning beyond the 3-nm technology node. Design and development of the scanner are based on rigorous litho-simulations. It is important to verify key imaging simulation findings by means of aerial image experiments with representative high-NA scanner characteristics. The first ASML-SHARP joint experiment was done with lines and spaces with pitches down to 16 nm wafer scale (1x). The experimental results confirmed the key litho-simulation findings: central obscuration’s impact on high-NA imaging and mitigations of obscuration’s impact using flex illuminations.
Moore’s law drives the doubling of the number of transistors per unit area every 2-3 years. To enable cost-effective shrink of future devices, a new High-NA EUV platform is being developed in a joint collaboration between ASML and Carl Zeiss SMT. The High-NA EUV scanner employs a novel Projection Optics Box (POB) design concept with a numerical aperture of 0.55 that enables 8nm half pitch resolution and a high throughput. The novel POB design concept tackles the limitations in angular acceptance of the EUV multilayer (ML) masks at increased NA, however also has implications on the system design and usage of the tool. The introduction of a central obscuration in the POB reduces the angular load on the ML mirrors inside the POB, enabling a high transmission and therefore high throughput. The obscuration size has been chosen for minimal impact on imaging performance. Furthermore, the High-NA scanner will be equipped with a highly flexible illuminator, similar to ASML’s NXE:3400 illuminator, that supports loss-less illumination shapes down to 20% pupil fill ratio (PFR). In this paper, we will show that High-NA EUV delivers increased resolution and contrast, thereby supporting EPE requirements of future nodes. We will show that the obscuration can benefit the imaging performance of via- and cutmask-layers by blocking the zeroth order light from the pupil, enhancing image contrast. Further contrast enhancement is possible by introducing alternative absorber stacks.