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As the specification for photomask becomes tighter, it is strongly demanded for achieving precise CD MTT (critical
dimension mean to target) and enhanced defect controllability in photomask fabrication. First of all, it is necessary that
reducing the factors of CD MTT error and introducing the reliable method to correct CD error for accurate CD
requirement of attenuated PSM (phase shift mask). From this point of view, one of CD correction methods which consist
of Cr CD measurement step after resist strip (strip inspection CD: SI CD) and additional corrective Cr dry etch step was
developed. Previous SI CD correction process resulted in accurate CD control within the range of CD MTT. However it
was not appropriate for defect control due to additional resist processes for selective protection of Cr pattern during CD
correction process.
In this study, the method for achieving precise CD MTT by correcting CD error without any resist process is
investigated. It is not suitable for the CD correction process to control CD MTT precisely that Cr etched resist (etch
inspection CD: EI CD) is very vulnerable to E-beam scanning during CD measurement. Otherwise, photoresist after Cr
etch selectively shrinks via UV irradiation under ozone (O3) condition, which drives a reduction of CD MTT error as a
result of accurate CD measurement (UV-irradiation inspection CD: UI CD). Moreover, it is not necessary any resist
process for Cr protection due to UV irradiated resist as enough for a etch barrier. It is a strong advantage of novel CD
correction method. This strategy solves the problems such as both CD measurement error on the EI CD correction
method and defects originated from resist process on the SI CD correction method at once. For the successful
incorporation of UI CD correction method, several items related with CD should be evaluated: accuracy and repeatability
of CD measurement under UI CD, control of CD MTT and CD uniformity, additional corrective etch bias for UI CD,
independence of corrective Cr etch process from UV irradiated resist, isolated-dense CD difference,.. etc. In this paper,
strategy of design for the progressive CD correction method for defect-free photomask and process details will be
discussed.
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