Extending a 193 nm mask inspector for 22 nm HP EUV mask inspection

Abstract

Reticle quality and the capability to qualify a reticle remain key issues for EUV Lithography. In this paper, we report on recent advancements that extend the capability of a 193 nm mask inspector to meet requirements for the 22 nm HP / 15 nm Logic node. This work builds upon previous work that was published earlier this year, by D. Wack 1 , et. al. Meeting these requirements requires development of a number of novel capabilities for mask inspection, including the use of offaxis illumination, various polarization modes, and use of an optimized absorber stack for EUV masks. In addition, we discuss the challenges of inspecting EUV masks in die-to-database mode, and how tone inversion can be successfully modeled. Lastly, we show that this same 193 nm mask inspector, with the use of proprietary algorithms, can be extended to meet industry requirements for EUV phase defect blank inspection. MOTIVATION FOR STUDYThe semiconductor industry continues to make progress toward developing EUV technology as the leading candidate for next generation lithography (NGL). The overall goal of these efforts is to insert EUV lithography for 22 nm HP high volume manufacturing (HVM) in 2015. This requires significant improvement in many areas of the supply chain, chief among these are mask infrastructure issues: defect review, defect inspection and EUV blank inspection. We report in this paper on recent progress toward meeting 22 nm HP HVM requirements for EUV defect inspection and blank inspection. INVESTIGATIONThere are several potential technology developments that we have studied which can increase the required defect modulation needed to successfully inspect a 22 nm HP EUV mask. These include: mask stack optimization, use of offaxis illumination (OAI) pupils, using various polarization modes in the illumination light path, using multiple focal positions, using multiple passes to enhance defect capture and reduce noise terms, and increasing the numerical aperture of the inspector. We will discuss these and report on their impact toward enhancing defect capture of EUV masks. Mask stack optimizationIn Figure 1, we show a simplified version of an EUV mask blank stack. Working with partners from the semiconductor industry and with various mask shops and blank suppliers, we evaluated a broad range of materials for the capping layer. Included in this list were materials that have reflectivity at 193 nm wavelength ranging between 1% and 60%. Simulations were used to identify candidate materials for further study and investigation, considering not only the impact to mask inspection, but also the impact to EUV lithography and to the mask and blank manufacturing processes. In this way, the original list of potential material choices was reduced to three for more detailed study, including building a test mask and scanning each of these masks to empirically verify the simulation models. The three inspection capping layers