The PR (Photo Resist) strip process before MoSi etch is a critical step in the manufacture of high-end PSM (Phase Shift Mask). Any impurity on MoSi film before MoSi etch would generate killer defect which leading to mask reject and yield loss. A super-thin opaque MoSi type defect called as MoSi stain in this paper was observed by KLA mask inspection that is almost unrepairable due to the characteristic of large defect size. This defect locates on Qz (quartz) area and adjacent to MoSi pattern of the mask. It was analyzed by SEM (Scan Electron Microscope) and TEM (Transmission Electron Microscope) that the components of this defect are Mo/Si/O/N, which are same as mask MoSi film. The root cause was proved in this paper to be the chemical residuals in PR strip process on Qz area of the mask that was not fully removed by clean process before MoSi etch, hence, those impacted area of MoSi film was not completely etched in MoSi etch process. In addition, the PR strip recipe was also optimized to prevent mask from encountering chemical residual to address this MoSi stain defect from occurring again without any side-effect impact on mask.
It is well known that with the development of advanced semiconductor technology, the phase shift mask (PSM) has been widely utilized in the semiconductor lithography process. Usually, the PSM blank coated with negative chemically amplified resist (NCAR) was used for fabrication of AA (Active area) layer and Poly layer mask which utilized in the logic semiconductor products. Under the electron beam (EB) writing systems, this NCAR resist showed better opaque pattern forming ability for complex OPC pattern and sub-resolution assist bar features[1]. In this contribution, a kind of defect so-called tiny MoSi defect that was captured by KLA inspection tool during NCAR mask fabrication process would be described. This kind of defect randomly scattered over the whole mask and located on quartz surface where photo resist was developed for not exposure by electron beam and Cr/MoSi was removed after dry etching process. In order to figure out this problem, we investigated the root cause of tiny MoSi defect by split condition test and cross validation, especially focused on the interaction between each process unit. According to the verification result, we would propose the possible formation mechanism and modify the recipe based on this understanding. The long-term inspection monitoring result showed that this kind defect density could be reduced from two hundred to less than twenty counts after recipe optimization.
With wafer technology node migrating to 14nm, the noise of wafer critical dimension (CD) control from photomask
become more significant due to a mask thickness no longer thin relative to the dimensions. Therefore, the Opaque
MoSi-On-Glass (OMOG) photomask is widely used for high-end wafer production because its thin structure is an
advantage for 3D electromagnetic field (3DEMF) mask modeling. In other words, it also means the CD control in such
high-end wafer product become more and more important. The absorber layer of OMOG is composed of MoSiN which
has faster etching rate. Although such kind of MoSiN can help repairing dark defect faster, it also bring some extra
etchings what lead to over repair that impact photomask’s CD control and yield.
In order to stabilize the post-repair CD variation, we performed several treatments at pre-repair cleaning and a
post-repair treatment to stabilize film properties. In addition, we also performed compositional analysis by EDX line
scan to compare the compositional differences.
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