In this study, we carried out actinic EUV mask imaging using EUV ptychography microscope with an updated algorithm. To improve the illumination probe update and the reconstruction quality, we adopted the regularized ptychographic iterative engine (rPIE) with intensity correction. The amplitude and phase of line and space patterns were reconstructed and verified quantitatively. The demonstrated performance of EUV ptychography microscope will be helpful for mask qualification and development of the advanced attenuated phase-shift masks (attPSMs).
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.
Background: An extreme ultraviolet (EUV) pellicle is necessary to increase the process yield even though the declining throughput is a big concern. However, an EUV metrology/inspection tool for this pellicle has not been commercialized yet.
Aim: The goal of this study is to verify the pellicle/mask inspection feasibility of EUV scanning lensless imaging (ESLI) and verify the impact of contaminants on pellicles depending on their size.
Approach: Through-pellicle imaging was implemented by using ESLI, which uses a high-order harmonic generation EUV source and ptychography. Optical characteristics of various sizes of Fe-contaminated EUV pellicles were evaluated to verify their impact on wafer images.
Results: Large size (∼10 μm) contaminants on the pellicle were found to contribute to the final wafer pattern loss. However, small size (2 to 3 μm) contaminants on the pellicle do not have substantial impact on the wafer image.
Conclusions: The defect detection capability of ESLI for pellicle and mask was confirmed. Therefore, ESLI is useful in applications like pellicle qualification and EUV mask inspection metrology.