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Extreme ultraviolet (EUV) lithography is going to be inserted into high volume manufacturing (HVM) of 7 nm technology node device. However, the insertion of EUV pellicle is still a major issue. Since particles on the pellicle larger than a critical size can act as killing defects on the final wafer pattern, through-pellicle inspection has to be preceded before applying into the lithography process. Even though non-actinic techniques which use deep ultraviolet (DUV) or e-beam light source can be used for preliminary inspection, actinic technique is still indispensable for the precise aerial image review and defect detection.
In this study, through-pellicle inspection was performed by using EUV scanning lensless imaging (ESLI), which has been developed as an actinic inspection technique. ESLI uses a coherent light source generated by high-order harmonic generation (HHG) and coherent diffraction imaging (CDI) method which reconstructs the object image using the diffracted light from EUV mask. We especially adopted ptychography for large area through-pellicle inspection. Ptychography, multi-shot CDI which uses a series of diffraction patterns with redundancy, enables more accurate object reconstruction compared to conventional CDI method.
We imaged EUV mask which has 128 nm half-pitch (HP) line and space (L/S) patterns. Also, mask inspection performance of ESLI was verified by comparing the reconstructed imaging results with CD-SEM image results. We also assessed defect detectability of ESLI by defect mapping of pellicle with various size defect. Furthermore, through-pellicle EUV mask imaging was executed for the study about acceptable defect size on EUV pellicle. In conclusion, we confirmed the ESLI’s feasibility for EUV pellicle qualification and defect inspection.
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