In this study, the method to achieve the precise CD MTT (critical dimension mean to target) correction in
manufacturing attenuated PSM (phase-shift mask) is investigated. There has been a growing demand for
more precise Mask CD MTT control in recent years. The CD correction method has been developed and
applied to meet the tighter CD MTT specification . However, the efficiency of the CD correction is greatly
affected by the repeatability of the CD measurement. The factors, which can have an influence on the CD
measurement, are the fluctuations of the pattern profile and the electron current of the SEM.
The conventional CD MTT correction method is basically to correct MoSi CD MTT by applying the
additional dry etch for MoSi based on Cr CD value. Therefore, the repeatability of the Cr CD MTT is the
crucial point for the accuracy of the final CD MTT correction. Although the Cr CD MTT is the crucial factor for
the successful CD MTT correction, it has the fluctuation due to the Cr pattern profile. If the Cr pattern profile
has low patterned angle after MoSi etch process, it can cause the focusing error in the CD measurement
using CD SEM. Therefore, a method to improve the reliability of the Cr CD MTT should be developed.
The IS and the normalized Delta CD concepts are adopted to obtain more reliable Cr CD MTT. The IS
refer the variation of the Cr CD MTT according to the difference in CD values with CD measuring thresholds.
The normalized Delta CD is obtained from the correlation of IS and Delta CD. Finally, the normalized Delta CD
is applied to correct the MoSi CD MTT by dry etch process.
The reduction of the Cr CD MTT fluctuation range is achieved by using the new CD correlation process
including IS and the normalized Delta CD. Consequently, the final MoSi CD MTT is improved 60% of range by
using the new CD correlation process.