Wavelength-specific reflections: A decade of extreme ultraviolet actinic mask inspection research

Abstract

Mask inspection is essential for the success of any pattern transfer lithography technology, and extreme ultraviolet lithography ͑EUVL͒, in particular, faces unique challenges. EUV masks' resonant-reflective multilayer coatings have a narrow, wavelength-specific response that dramatically affects the way that defects appear, or disappear, at various illuminating wavelengths. Furthermore, the ever-shrinking size of "critical" defects limits the potential effectiveness of deep ultraviolet inspection techniques over time. Researchers pursuing numerous ways of finding and characterizing defects on extreme ultraviolet ͑EUV͒ masks and have met with varying degrees of success. Their lessons inform the current, urgent exploration to select the most effective techniques for high-volume manufacturing. Ranging from basic research and demonstration experiments to commercial inspection tool prototypes, the authors survey the recent history of work in this area, including sixteen projects in Europe, Asia, and America. Solutions range from scanning beams to microscopy, darkfield imaging to pattern transfer.

An EUV Fresnel zoneplate mask-imaging microscope for lithography generations reaching 8 nm

“…3,4 Prototypes and research tools have been at the forefront of actinic mask research for over a decade, 5 and they may continue to chart the course, reaching forward to explore future nodes and ever-higher resolutions.…”

Scanning coherent diffractive imaging methods for actinic EUV mask metrology

“…4 Phase defects (resulting from bumps and pits in the substrate below the multilayer) are particularly problematic as these are elusive to detect and difficult to repair. 5 Here we report on the results of our EUV mask scanning lensless imaging tool (RESCAN) which uses a scanning coherent diffraction imaging (SCDI) method. 6 SCDI methods such as ptychography have been invented a long time ago but where long considered impractical for experimental use due to the stringent restrictions of early algorithms.…”

Actinic characterization of extreme ultraviolet bump-type phase defects

“…Despite known gaps in our collective understanding of the nature and impact of defects, few experimental tools are now capable of providing direct, aerial-image measurements of real defects, without printing in photoresist. 2 Of particular concern is the class of defects known as phase defects, which are caused by pits, bumps, trapped particles, and other imperfections in the mask substrate, or in the EUV-reflective multilayer-coating itself. Defects of this category may be only tens of nm wide and 1 nm tall, at the surface of the multilayer coating, yet they can cause significant disruption in the reflected EUV-light field, especially when imaged out of focus.…”

“…2 Other tools include the extreme ultraviolet microscope (EUVM), developed by the University of Hyogo, 4 and the EUV imaging system at Aachen. 5 The EUVM has demonstrated the ability to image patterned phase-shifting features, including lines and pointlike defects.…”