OPC model accuracy is an important contributor to the EPE budget in the latest lithography nodes. The overall OPC accuracy depends on accurate calibration of the sub-models capturing mask, optical, resist and etch effects. The advent of high-NA (0.55) EUV lithography with anamorphic imaging has further increased the emphasis on accurate aerial image model calibration for computational lithography. In this paper, we study the feasibility of using direct aerial image measurements with the ZEISS AIMS EUV tool for improving OPC model accuracy as well as accurate metrology of mask pattern variability, which are both relevant to EPE budgeting.
AIMS® EUV is a unique tool in the EUV mask infrastructure. It allows qualification of the mask printing performance in the aerial image under scanner equivalent conditions. For emulation of the high NA EUV scanner, ZEISS upgraded the existing 0.33 NA AIMS® EUV platform. The system can now emulate both 0.33 NA isomorphic scanners as well as 0.55 NA anamorphic scanners. We present the concept of AIMS® EUV high NA with focus on the emulation of a wafer defocus in the anamorphic high NA scanner. Besides defect review applications, this enables aerial image based high NA imaging studies.
During the last decade, the introduction of EUV lithography in high-volume chip manufacturing has been accompanied by the development of technological prerequisites for a future support of the node scaling roadmap. As core element, the next generation EUV scanner with an increased NA of 0.55 will be implemented into wafer fabs within the upcoming few years. In addition to its enhanced resolution, the High-NA exposure tool improves image contrast, and consequently reduces local CDU and defect printing on wafer. To take full advantage of this next leap in lithography, the whole infrastructure including EUV photomask technologies and metrology must keep pace with the scanner progress. In this context, actinic EUV mask measurement represents a unique and variously usable way for the qualification of the mask printing performance under scanner-equivalent conditions. The aerial image metrology is targeted to match the corresponding scanner aerial image by means of the emulation of imaging-relevant scanner properties including wavelength, mask-side NA, through-slit chief ray angle, illumination schemes, and aberration level. To qualify High-NA masks of the anamorphic scanner, a methodology was developed that allows the simultaneous measurement of both NA=0.33 and NA=0.55 reticles based on one isomorphic optical projection optics design. Here, we describe the challenges and corresponding solutions combined with two intrinsically diverse emulation types, NA=0.33 isomorphic and NA=0.55 anamorphic, in one single metrology. Special attention is paid to the emulation of the elliptical scanner NA at reticle, the contrast impact due to vector-effects in High-NA scanner imaging, wafer defocus of an anamorphic system for focus-dose process window determination, the pupil obscuration of the High-NA scanner projection optics, and the scanner facetted illumination by means of physical free-form blades, and by a completely digital solution.
Digital Flex Illu is a fully digital solution which provides SMO functionality to the AIMS® EUV system by combining an adaptation of the already built-in system metrology with a powerful algorithm and most importantly, without changing the machine hardware. In this paper, we will present the concept of Digital Flex Illu functionality, its significant advantages in combination with a binary aperture-based illumination concept, together with showing imaging results obtained on the AIMS® EUV prototype system. This digital solution is a paradigm change for the AIMS® EUV usability and final user, it allows ZEISS to guarantee an agile roadmap for the AIMS® EUV with limited development effort and great benefits in sustainability and roadmap scaling.
DUV lithography has successfully adopted both bright and dark mask tonalities. This gives the freedom to chip manufacturers to choose the optimum combination of mask and resist tonality for their product [1]. In EUV lithography, however, there has been a clear preference for dark field masks, driven by the prevalence of positive tone resist processes, and their relative insensitivity to multilayer defects. Future customer nodes, however, may require negative tone (metal-oxide) resist processes [2][3], resulting in a requirement to use bright field masks. Therefore, a deeper understanding of bright and dark field imaging is needed in order to provide guidance to ASML customers in choosing the optimal approach. In this work we consider the fundamentals of bright and dark field imaging based on the diffraction theory of aerial image formation [4]. We will show that bright field imaging has an intrinsic potential for higher optical NILS (normalized image log-slope), especially for isolated features, but with a lower depth of focus. The theoretical results are compared to rigorous simulations. Experimental bright vs dark-field results is also presented for comparison. Wafer based data has been obtained on an NXE:3400 scanner, whilst aerial image measurements have been obtained using the Aerial Image Measurement System for EUV (AIMS® EUV) at Zeiss. These experimental results confirm the theoretical expectations. The main goal of the paper is to draw attention to bright versus dark field comparison for EUV and to kick off more studies in this direction.
Scaling trends in the semiconductor industry towards smaller technology nodes and feature sizes are continuing and first consumer products manufactured with the help of EUV technology are already on the market. Major industrial players have introduced EUV lithography into their production at the 7nm technology node and with the 5nm node being on its way [1], the amount of EUV lithographic layers is expected to rise significantly and implementation of EUV double patterning is anticipated. These developments lead to more strict technological requirements especially for the corresponding EUV but also for the used high-end DUV photomasks in terms of minimum feature sizes and acceptable Edge Placement Errors (EPE). Moreover, photomask defectivity increases dramatically with shrinking feature sizes. This creates significant challenges to the industry, as in particular the most cost intensive EUV photomasks possess the highest numbers of defects. The current industry standard for high-end photomask repair tools is the MeRiT neXT [2]. To face the upcoming challenges an efficient and reliable way to repair future high-end photomasks is inevitable. A corresponding repair tool must address decreased minimum feature sizes and increased pattern complexity on high-end photomasks. In this paper we present our latest results of high-end EUV repairs carried out on the next generation photomask repair tool MeRiT LE. The tool shows improved system dynamics, makes use of a new electron beam column, which operates at a low electron beam voltage down to 400V and enables the repair of next generation ultra-small defects.
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