Availability of defect-free reflective mask has been one of the most critical challenges to extreme ultraviolet lithography (EUVL). To mitigate the risk, significant progress has been made on defect detection, pattern shifting, and defect repair. Clearly such mitigation strategies are based on the assumption that defect counts and sizes from incoming mask blanks must be below practical levels depending on mask specifics. The leading industry consensus for early mask product development is that there should be no defects greater than 80 nm in the quality area, 132 mm x 132 mm. In addition less than 10 defects smaller than 80 nm may be mitigable. SEMATECH has been focused on EUV mask blank defect reduction using Veeco Nexus TM IBD platform, the industry standard for mask blank production, and assessing if IBD technology can be evolved to a manufacturing solution. SEMATECH has recently announced a breakthrough reduction of defects in the mask blank deposition process resulting in the production of two defect-free EUV mask blanks at 54 nm inspection sensitivity (SiO2 equivalent). This paper will discuss the dramatic reduction of baseline EUV mask blank defects, review the current deposition process run and compare results with previous process runs. Likely causes of remaining defects will be discussed based on analyses as characterized by their compositions and whether defects are embedded in the multilayer stack or non-embedded.
The force of adhesion of 50 nm diameter diamond-like carbon sphere probes to three quartz substrates was measured
using an atomic force microscope. The force of adhesion was measured prior to cleaning, within 10 minutes after
cleaning, after storage in an N2-purged cabinet, and after storage in an N2-purged vacuum oven. The evaluated cleaning
recipes were SC1-like, SPM-like, and HF-based, each followed by ultra-pure deionized water (UPW) rinse and spin
drying. The measurements were conducted in a Class 100 clean room at approximately 50% relative humidity. In
addition, contact angle measurements were made on three additional quartz substrates using UPW before cleaning, after
cleaning, and throughout N2 storage. The adhesion force increased after cleaning as compared to the pre-cleaned state,
continued to increase until reaching a maximum after 5 days of N2 storage, and then decreased after 26 days for all three
substrates. One substrate was then stored in a vacuum oven for 3 days, and the adhesion force decreased to 46% of the
pre-cleaned state. The contact angle was reduced from over 30° before cleaning to 0° immediately after cleaning.
During subsequent N2 storage, the contact angle increased to 5° or greater after 18 hours for the substrate cleaned with
the HF-based recipe and after 15 days for the substrates cleaned by the SC1-like and SPM-like recipes.
Defect classification and characterization on mask substrates and blanks can be used to the identify defect sources within the tool and process. Defect reduction has been achieved in SEMATECH’s EUV Mask Blank Development Center (MBDC), aided by successful classifications of defect populations. Failure analysis of EUV substrate and blank defects in the MBDC begins with automatic classification of defects detected by M1350 and M7360 Lasertec inspection tools. Two sets of defect images and classification emerge from the two detection tools. The M1350 provides a more variegated set of 13 defect class types, while the M7360 provides eight. During manual review of the classifications, the defect class sets for both tools are often collapsed to only two major classes of interests with respect to production and failure analysis: particles and pits. This leaves various other classes ignored before subsequent characterization steps like SEM classification and composition analysis. The usefulness of tracking and verifying more detailed classes of defect needs to be explored. SEM analysis can be used to validate the relative size comparison yielded from inspection data alone, beyond the calibrated comparison of inspection signals from well-understood polystyrene latex spheres. The accuracy of rule-based defect classification of inspection tool data must be quantified by statistical tracking and validation SEM analysis. Classification of false counts increases as sensitivity of detection tools are increased to ensure the capture of smallest defects. The validity of calling a defect “false” is usually a manual review of pixel images created on the detection tool.
EUV mask blank substrates will be subject to extraordinarily demanding requirements upon flatness, smoothness and
absence of residual defects. To date, no combination of available surface preparation techniques has been able to
produce essentially perfect substrates with zero residual defect populations. A critical problem yet to be resolved
involves small numbers of nanoscale divots and scratches which are generated by the operations used to meet
smoothness requirements. A new non-contact surface sputtering technique known as accelerated neutral atom beam
(ANAB) shows promise for mitigating the divot and scratch defects without increasing surface roughness and without
altering flatness and planarity. This paper describes a mask blank substrate study which has been conducted to
demonstrate the ANAB defect mitigation capability.
EUV mask substrates, made of titania-doped fused silica, ideally require sub-Angstrom surface roughness, sub-30 nm
flatness, and no bumps/pits larger than 1 nm in height/depth. To achieve the above specifications, substrates must
undergo iterative global and local polishing processes. Magnetorheological finishing (MRF) is a local polishing
technique which can accurately and deterministically correct substrate figure, but typically results in a higher surface
roughness than the current requirements for EUV substrates. We describe a new super-fine MRF® polishing fluid
whichis able to meet both flatness and roughness specifications for EUV mask blanks. This eases the burden on the
subsequent global polishing process by decreasing the polishing time, and hence the defectivity and extent of figure
distortion.
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