|
EUV technology is according to current trend approaching the final development phase in which defect free EUV masks
are of key importance for development and optimization of the lithography process. This task consists of three
contributing aspects- defect free multilayer blank, mask manufacturing process with very low defect formation
probability and availability of repair process for EUV mask.
In comparison to optical mask, development of the repair process for EUV mask is different in several aspects. The fact,
that the TaN absorber is placed on top of Mo/Si mirror is making the process very sensitive to variation of the mask
material, as the etch rate of the mirror is significantly higher, than that of absorber, when no capping layer is present
between the absorber and ML mirror. The presence of the Ru capping layer increases the process window due to
significant selectivity improvement by one or two orders of magnitude, however, the capping layer is very sensitive to
damage by preceding manufacturing processes.
Its thickness and also it chemical purity - lack of modification by incorporation of impurities is crucial for successful
mask repair.
The repair process for optical masks is typically optimized using AIMS for both development and qualification of the
process. The availability of EUV AIMS system is very limited, for what reason we have to rely on other measures
during the process development and use the AIMS for process qualification only, or use correlation between e.g. CD
SEM or AFM measurement and AIMS data for selection and qualification of the repair process.
Also the usage of mask – exposure on the scanner is modifying the mask surface. Therefore the impact of the mask
exposure needs to be investigated, when EUV gets in HVM stage.
In the past, the influence of the mask cleaning process on the integrity of EUV mask was investigated, with respect to
several lithography-critical parameters as actinic reflectivity, critical dimension (CD) shift, edge roughness and surface
roughness. The reparability of the mask was so far not in focus, assuming, that mask can be repaired anytime during its
lifetime. This missing item needed for the successful usage of EUV mask needs to be checked and status confirmed
prior start of the HVM.
|
