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In this study we examine the effects on lithographic performance due to repetitive cleans on a mask patterned for use in extreme ultraviolet lithography (EUVL). Exposures were carried out at the SEMATECH Berkeley micro-exposure tool (MET) on both a cleaned mask and a reference (uncleaned) mask with the same mask architectures. The performance is measured against the process window for lines and spaces, line edge roughness (LER), and contact size variation measured using scanning electron microscopy (SEM). Mask properties such as surface roughness were used as metrics of the cleaning process effects. We also introduce a new method involving the correlation in LER of a single line from exposure to exposure at the same dose and focus. If mask cleaning were to introduce significant damage to either the capping layer or the absorber, we might expect an increase in LER correlation from exposure to exposure of the same feature, as uncorrelated effects due to the resist cause a second order change depending on aerial image contrast loss. We look at these metrics on the same mask used in previous cleans studies, now for a 50X and 75X cleans.

In this study, the impact of repetitive cleaning of EUV masks on reflectivity, surface roughness and lithographic performance was evaluated. Two masks were fabricated and patterned with the same layout using commercially available EUV blanks; one was subjected to 33 cleaning cycles and the other was kept as a reference. Wafers were patterned using both masks on the SEMATECH Berkeley 0.3 NA micro-field exposure tool (MET), and the data was used to determine process latitude and line edge roughness at regular intervals between cleaning cycles. Additionally, mask surface roughness and EUV reflectivity were also measured. After a total of 33 cleaning cycles, minimal degradation was observed in lithographic performance compared to the reference mask, as well as surface roughness and reflectivity.

Several high-performing resists identified in the past two years have been exposed at the 0.3-numerical-aperture (NA) SEMATECH Berkeley Microfield Exposure Tool (BMET) with an engineered dipole illumination optimized for 18-nm half pitch. Five chemically amplified platforms were found to support 20-nm dense patterning at a film thickness of approximately 45 nm. At 19-nm half pitch, however, scattered bridging kept all of these resists from cleanly resolving larger areas of dense features. At 18-nm half pitch, none of the resists were are able to cleanly resolve a single line within a bulk pattern. With this same illumination a directly imageable metal oxide hardmask showed excellent performance from 22-nm half pitch to 17-nm half pitch, and good performance at 16-nm half pitch, closely following the predicted aerial image contrast. This indicates that observed limitations of the chemically amplified resists are indeed coming from the resist and not from a shortcoming of the exposure tool. The imageable hardmask was also exposed using a Pseudo Phase-Shift-Mask technique and achieved clean printing of 15-nm half pitch lines and modulation all the way down to the theoretical 12.5-nm resolution limit of the 0.3-NA SEMATECH BMET.