Enabling non-actinic EUV mask inspection using carbon nanotube pellicle

Wafer fab mask inspection is facing new challenges in the era of Extreme Ultraviolet Lithography (EUVL). Mask qualification methods in use for ArF lithography are reticle verification based on wafer prints or defect inspection on Deep Ultraviolet (DUV) mask inspection tools. The latter is the industry preference in most cases. To implement this established methodology for EUV masks with pellicle, the pellicle must have transmission that enables inspection for defect detection. In this work we test the viability of a DUV mask inspection platform to image and inspect an EUV mask with a Carbon Nanotube (CNT) based pellicle [1]. Pellicles are used to keep particles off the patterned side of the photomask. For DUV and longer exposure wavelengths, polymers with transmission above 99% are routine. For EUV wavelengths, identifying a transparent, mechanically robust film that is compatible with EUV exposure environments has been challenging. Commercially-available EUV pellicles for lithography are p-Si based[2]. This pellicle type achieves the critical protection of the mask but induces a significant productivity loss due to EUV light absorption. The critical need for a highly-transmissive pellicle will increase with advanced nodes that put more pressure on uniformity specifications and throughput. In this arena, CNT based pellicles exceed the required transmission for High-Volume Manufacturing (HVM) and show promise towards production adoption [3]. DUV light absorption is relevant for mask inspection. DUV light is absorbed by the p-Si pellicle, while the CNT has high transmission in this wavelength region as well, making mask inspection of EUV masks with pellicles possible on existing DUV inspection tools, such as Applied Materials Aera5TM. In order to understand the impact of CNT pellicle on DUV inspection, a controlled study was performed by Applied Materials in cooperation with imec. The study consisted of various system measurements and extensive application qualification. Aera5TM inspection results on a Programmed Defect Mask (PDM) with a full field CNT pellicle will be presented. The results with pellicle are comparable to the reference no-pellicle inspection and meet the same detection sensitivity. Detection optimization Best-Known Methods (BKM) developed on EUV masks without pellicles proved to be effective. These include, but are not limited to, aperture shaping, defocus and polarization. Moreover, the imaging and main system components performance were examined, and no impact was observed.