With shrinkage of device pattern, optical proximity correction (OPC) will be used for EUV lithography, which leads to need sub resolution assist features (SRAF) on EUV mask. Currently, it is difficult to fabricate EUV mask with SRAF of sub-30nm using conventional resist mask process stably. Moreover, it is necessary to improve line width roughness (LWR) of mask absorber pattern for achieving the lithographic specifications beyond hp15nm patterning. In this paper, in order to meet the requirements of Ta based absorber EUV mask with SRAF, mask fabrication process using new structure blank is studied for sub-30nm SRAF patterning and for improved LWR of primary feature. New mask process using new blank with thinner resist and Cr based hard mask was developed. By using new mask process, resolution of absorber pattern was achieved to 30nm for SRAF patterning, and LWR was improved comparing with conventional process.
Damage to minute features of 45nm-node device masks occurred during megasonic cleaning.
Since we were obliged to weaken the mechanical effect of megasonics in order to prevent the collapse of minute
features, we could not obtain acceptable cleaning results.
In order to manage the minute features, there is a need to develop a new mechanical cleaning method that
causes less damage, but does not compromise the ability to remove particles. Cleaning using a two-fluid nozzle
is a promising candidate. We investigated the two-fluid nozzle and compared it with megasonic cleaning, and
we confirmed that the two-fluid nozzle achieved acceptable cleaning results without damaging 45nm-node
device masks. Furthermore, for 32nm-node device masks, we have improved the two-fluid nozzle in terms of
the cleaning energy distribution.
A new clustered configurational photomask cleaning system has been developed. Accepting the clustered configuration, we can be free from the heavy tank photomask cleaner which has a large footprint and has no flexibility for designing a cleaning recipe. Provided we need to introduce a new cleaning process unit, we can substitute the unit by disassembling an old one in the system. We can always keep our photomask cleaner up-to date with the system, and, we can obtain most effective cleaning result by the least efforts and the smallest expense. Using this cleaning system with an optimized cleaning recipe, we have achieved the cleaning result of less than one particle, greater than 0.2 micrometers , detected by KLA Starlight.
The contact-free photomask cleaning using the UV irradiation has been studied. With the UV from the low pressure mercury lump, we can clean a photomask preventing the mask plate from any contacts with chemicals or DIW in the cleaning process steps. We can be free from any residua or waterprints in the photomask cleaning process process steps by UV. In the UV cleaning, we have observed the position selective cleaning effect. The dependence of the cleaning effect on the particle size has also been observed. As a result of a series of experiments, we can conclude that the cleaning effect strongly depends on the heat stored in the contamination. The chromium thinning free cleaning conditions is also examined.
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