Photomasks carry the structured information of the chip designs printed with lithography scanners onto wafers. These structures, for the most modern technologies, are enlarged by a factor of 4 with respect to the final circuit design, and 20–60 of these photomasks are needed for the production of a single completed chip used, for example, in computers or cell phones. Lately, designs have been reported to be on the drawing board with close to 100 of these layers. Each of these photomasks will be reproduced onto the wafer several hundred times and typically 5000–50 000 wafers will be produced with each of them. Hence, the photomasks need to be absolutely defect-free to avoid any fatal electrical shortcut in the design or drastic performance degradation. One well-known method in the semiconductor industry is to analyze the aerial image of the photomask in a dedicated tool referred to as Aerial Imaging Measurement System, which emulates the behavior of the respective lithography scanner used for the imaging of the mask. High-end lithography scanners use light with a wavelength of 193 nm and high numerical apertures (NAs) of 1.35 utilizing a water film between the last lens and the resist to be illuminated (immersion scanners). Complex illumination shapes enable the imaging of structures well below the wavelength used. Future lithography scanners will work at a wavelength of 13.5 nm [extreme ultraviolet (EUV)] and require the optical system to work with mirrors in vacuum instead of the classical lenses used in current systems. The exact behavior of these systems is emulated by the Aerial Image Measurement System (AIMS™; a Trademark of Carl Zeiss). With these systems, any position of the photomask can be imaged under the same illumination condition used by the scanners, and hence, a prediction of the printing behavior of any structure can be derived. This system is used by mask manufacturers in their process flow to review critical defects or verify defect repair success. In this paper, we give a short introduction into the lithography roadmap driving the development cycles of the AIMS systems focusing primarily on the complexity of the structures to be reviewed. Second, we describe the basic principle of the AIMS technology and how it is used. The last section is dedicated to the development of the latest generation of the AIMS for EUV, which is cofinanced by several semiconductor companies in order to close a major gap in the mask manufacturing infrastructure and the challenges to be met.
In today's economic climate it is critical to improve mask yield as materials, processes and tools are more time and cost involved than ever. One way to directly improve mask yield is by reducing the number of masks scrapped due to defects which is one of the major mask yield reducing factors. The MeRiT™ MG 45, with the ability to repair both clear and opaque defects on a variety of masks, is the most comprehensive and versatile repair tool in production today. The cost of owning multiple repair tools can be reduced and time is saved when fast turnaround is required, especially when more than one defect type is present on a single mask. This paper demonstrates the ability to correct repair errors due to human mistakes and presents techniques to repair challenging production line defects with the goal of maximizing mask repair yield and cycle time reduction. KEYWORDSMeRiT TM MG 45, AIMS TM 45-193i, mask repair, defect repair, electron beam repair, repair yield INTRODUCTIONAs the complexity and cost associated with photolithography masks continues to grow, it is more important than ever to optimize the yield of the mask shop. This saves the money invested in the raw materials of the mask, but more importantly, the time and expense involved with the labor and tool costs required for producing the mask. While much time and effort goes into reducing the number of defects introduced to the mask as it travels through the production line, completely eliminating these defects is impossible and their presence on the final product is a major factor in reducing yield. Therefore it is critical to have excellent defect repair capabilities in order to increase overall mask yield.The ability to successfully repair defects that were previously classified as non-repairable increases the repair yield and leads to an increase in mask yield. The MeRiT™ MG 45 electron beam mask repair tool utilizes the newest technology available for mask repair providing advantages over other techniques that have already been presented [1,2]. It has been shown that the MeRiT™ MG 45 can deposit material that matches the optical properties of advanced phase shifting material (PSM) masks, filling a large void in existing repair capability [1,3]. Nearly unlimited inspection and the ability to return to a repair site multiple times are possible without degradation of the mask's optical properties. This is especially important as challenging defects require more skill to repair so they may be approached conservatively. Even mistakes due to human error can be corrected. In both cases, the result is an increase in mask yield. This paper presents examples of both opaque and clear defect repair and expands on the topic of returning to a repair site in order to further improve mask yield. A MeRiT™ MG 45 installed at a customer site was used to perform all the repairs and analysis was carried out to quantify the repair results with an on-site AIMS™ 45-193i. The ability to perform multiple repair attempts on a single site without degradation of the optica...
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