Actinic mask imaging: Recent results and future directions from the SHARP EUV Microscope - eScholarship

Actinic mask imaging: Recent results and future directions from the SHARP EUV Microscope Kenneth A. Goldberg,“ Markus P. Benk,a Antoine Wojdyla, Iacopo Mochif‘ Senajith B. Rekawaf‘ Amaud P. Allezy,a Michael R. Dickinson,“ Carl W. Cork,a Weilun Chao,a Daniel J. Zehmf‘ James B. Macdougallf Patrick P. Naulleauf’ and Anne Rudackb aLawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA USA 94720; bSEMATECH, 257 Fuller Rd. Suite 2200, Albany, NY USA 12203 ABSTRACT The SEMATECH High Numerical Aperture Actinic Reticle Review Project (SHARP) is a synchrotron-based extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP has been operational and serving users since June, 2013, and in eight months, SHARP has recorded over 71,000 high-resolution images. Exposure times are 5 to 8 seconds, and 8 or more through-focus series can be collected per hour at positions spanning the entire mask surface. SHARP’s lossless coherence-control illuminator and variable numerical aperture (NA) enable researchers to emulate the imaging properties of both current and future EUV lithography tools. SHARP’s performance continues to improve over time due to tool learning and upgraded capabilities, described here. Within a centered, 3-um square image region, we demonstrate an illumination power stability above 99%, and an average uniformity of 98.4%. Demonstrations of through-focus imaging with various illumination coherence settings highlight the capabilities of SHARP. Keywords: extreme ultraviolet, EUV, mask, microscope, coherence, zoneplate, imaging 1. INTRODUCTION The SEMATECH High-NA Actinic Reticle review Project (SHARP) microscope is the successor to the SEMATECH Berkeley Actinic Inspection Tool (AIT) and occupies the same synchrotron, bending magnet beamline at Lawrence Berkeley National Laboratory’s (LBNL) Advanced Light Source (ALS). SHARP was created by an industry/govemment partnership to provide advanced research and development capabilities. It utilizes an all-EUV “actinic” illumination and imaging system. Like AIT, the SHARP microscope uses off-axis Fresnel zoneplate lenses to project EUV images with high magnification (typically 900gl) and diffraction-limited quality. Yet SHARP is a ground-up re-design of the AIT concept and achieves 150 times higher power with the same source due to an efficient illuminator design and a reduction in the number of mirrors by two. SHARP is used to study nearly every aspect of EUV mask technology, including programmed and native defect detection and sensitivity, mask cleaning and repair strategies, through-focus defect printability, horizontal-vertical (HV) bias arising from off-axis illumination, optical proximity correction and pattern enhancement, multilayer and absorber architlecsture, multilayer surface roughness and pattern line-edge roughness, phase—shifiing mask architectures, and more. 1.1 The role of SHARP A host of inspection technologies have contributed to progress in EUV mask research,° including tools to improve defect detection and verify mitigation. Clearly the need for mask metrology tools offering high scanning speed, and other benefits will persist. Yet EUV photomasks are highly tuned to the wavelength and illumination profiles used in lithography. Non-actinic (that is, non-EUV-wavelength) inspection technologies, such as scanning electron-microscopy (SEM), photo-electron microscopy (PEEM), atomic-force microscopy (AFM), and deep-ultraviolet inspection techniques miss subtle optical effects that can only be accurately characterized at the operational wavelength. This may be especially true for the approaching, ever smaller pattern design rules (at and below 16-nm) and for so-called phase- defects resulting from small bumps and pits in the multilayer coating or subtrate. SHARP is a unique research prototype, designed to provide a window on EUV lithography mask research at current or future technology nodes. Its high specifications and current availability enable it to participate in ongoing strategic photomask decision-making. SHARP is designed for use with a narrow-band, narrow-divergence source, and it is installed at a synchrotron light source where it can be accessed by participating industrial or academic, research teams.