Imec Shows Updates of High-NA EUV Patterning

Imec’s latest research has made significant progress in EUV processes, masks, and metrology. Particularly, prepared for enabling high-numerical aperture (High-NA) extreme ultraviolet (EUV) lithography.

Accordingly, key achievements came from resist and underlayer development, mask enhancement, and optical proximity correction (OPC) development. Furthermore, on at-resolution field stitching, reduction of stochastic failures, and improved metrology and inspection. Thus, imec demonstrates readiness for transferring the EUV processes into the joint imec-ASML High-NA EUV Lab, built around the first prototype High-NA EUV scanner.

Imec is presenting this breakthrough at the ongoing 2024 Advanced Lithography + Patterning Conference.

Steven Scheer, senior vice president of Advanced Patterning, Process and Materials at imec: “The first High-NA EUV scanner (TWINSCAN EXE:5000) has been assembled by ASML and the first wafers will be exposed soon. In the next few months, the joint imec-ASML High-NA EUV Lab will be operational, and access will be provided to the High-NA customers. The High-NA EUV lab, with the installed equipment and processes, enables an early start of High-NA EUV learning for the customers before tools are operational in their fabs.”

Assembly of a High NA EUV tool in joint imec-ASML High-NA lab at ASML’s headquarters in Veldhoven, the Netherlands. Credit: ASML

In addition, Scheer said, “It has been imec’s role, in tight collaboration with ASML and our extended supplier network, to ensure timely availability of advanced resist materials, photomasks, metrology techniques, (anamorphic) imaging strategies, and patterning techniques. Readiness of these processes for High-NA enablement is shown in more than 25 papers presented at the 2024 SPIE Adv Litho & Patt Conference.” 

Wide Acceptance Among Chipmakers

Field stitching is a key enabler in High-NA: field stitching is necessary because of the anamorphic lens (i.e., a lens with different de-magnification in the x and y directions). Thus, resulting in field sizes of half of the conventional scanner field size. For that reason, imec will share the latest insights that enable at-resolution stitching based on the progress of the work with ASML and the mask shop partners on imec’s NXE:3400C scanner. At-resolution stitching will reduce the need for design changes to cope with the field size reduction. 

On the material and process side, it is clear that metal-oxide resists (MORs) are still in the lead for metal line/space patterns. Thus, imec will present the advancements of MOR in terms of EUV dose-to-yield reduction. The selection of the specific underlayer, optimization of the development process, choice of mask absorber, mask bias, and mask tonality were leading to a more than 20% dose reduction for lines and spaces, without increased roughness or stochastic failures. Also, tip-to-tip dimensions did not have negative impact by these dose reduction activities. The dose reduction work continues and has high level of appreciation by chip makers, since it leads to EUV cost reduction due to the higher scanner throughput.

Best Known Methods

The use of MOR resists with a binary bright field mask for contact hole patterning got an unexpected result. Particularly, it achieved a 6% dose reduction with a 30% local CD uniformity (LCDU) improvement after pattern transfer when comparing to a positive tone chemically amplified resist (CAR) and a binary dark field mask transferred in the same stack. A remaining concern of bright field masks for contact holes is the mask quality and defectivity. Thus, this will need careful investigation to make MOR an option for contact holes. Until then, positive tone CAR resists with dark field masks will be the leading candidates for contact and via patterning in High-NA EUV.

Moreover, high-NA also requires improvements in metrology and inspection, giving the higher resolution (by the high NA) and thinner films (by the reduced depth of focus (DOF)). Imec will present new results on e-beam and deep-UV (DUV) inspection, showing that new best-known methods (BKMs) are in place to find High-NA relevant stochastic patterning failures, such as hexagonal contact holes. In addition, several machine learning techniques (based on denoising SEM micrographs) will be proposed to facilitate the small defect inspection and classification.

Finally, imaging improvements through source-mask optimizations and anamorphic mask OPC (considering the need for stitching) will be presented by imec and partners.