Transition metal coatings are of fundamental value for extreme ultraviolet (EUV) optics. Often, the reported optical constants for transition metals in the EUV range exhibit inconsistencies across different publications. Therefore, additional studies to determine optical constants in the EUV range for transition metals are highly relevant to further develop optical components. In our work, the optical constants of scandium, vanadium, chromium, cobalt, nickel, niobium, molybdenum, rhodium, palladium, hafnium, tungsten, rhenium, osmium, iridium, platinum, gold, and zirconium dioxide are determined in parts of the EUV spectral range. The optical constants were determined from angle-dependent reflectance measurements collected using monochromatized synchrotron radiation. We also test capping solutions for thin coatings of scandium, vanadium, cobalt, niobium, molybdenum, hafnium, tungsten, and rhenium. All of our results are presented in comparison with literature data. For many cases, our results present either significant scale deviation or previously unresolved fine-structure in the widely used optical data compilations.

We measured the angle-resolved reflectance of 3 nm and 30 nm TiN films on Si using s- and p-polarized synchrotron radiation over the wavelength range of 36-120 nm. To determine the optical constants, we solved the inverse problem by employing a transfer-matrix method (TMM) forward model with Bayesian Markov Chain Monte Carlo (MCMC) inference. This enabled simultaneous extraction of n, k, and their associated uncertainties. Our results indicate that while the 30 nm film exhibits bulk-like behavior and reduced sensitivity to interfacial effects, the 3 nm film is strongly influenced by the native surface oxide and the interfacial region. In the ultrathin regime, nanometer-scale structural variations produce percent-level changes in the retrieved optical constants. These findings demonstrate that bulk optical constants are not directly applicable to ultrathin TiN films in the EUV/VUV range, where surface and interface contributions significantly modify the total optical response.

The successful introduction of extreme ultraviolet (EUV) lithography to high-volume manufacturing has significantly increased interest in pushing this technology to its limits. Standard Ta-based EUV masks suffer from mask 3D (M3D) effects that limit contrast. This contrast loss is primarily caused by aerial image fading, driven by phase shifts between the aerial images of different point sources in the illumination pupil. These phase shifts are influenced by the thickness and refractive index of the absorber material, leading to contrast loss, feature-dependent Best Focus shifts, and asymmetric process windows. Tuning the mask absorber material offers a promising approach to mitigate performance degradation caused by M3D effects. While previous studies have evaluated the benefits of low-reflectivity low-n masks at 0.33 NA for logic applications, this work extends the investigation to high-NA. We manufacture the 6% reflective low-n mask and evaluate its high-NA printing performance to compare it to a conventional Ta-based mask. A bright field mask tonality is selected to align with existing high-NA integration flows. We present an experimental comparison of EUV imaging performance using an EXE:5000 EUV exposure tool with a 0.55 numerical aperture (NA). The study targets critical patterning building blocks for logic metal (20 nm pitch) and DRAM (28 nm pitch) applications. We compare a 6% reflective low-n mask stack to the Ta-based reference. Key metrics include Exposure Latitude, LWR, Best Focus shift through pitch, and Overlapping Process Window for line/spaces features. Additionally, we evaluate Mask Error Enhancement Factor, 2-bar asymmetry through focus, and contact hole array process window. By experimentally demonstrating the patterning performance and advantages of 6% reflective low-n absorber masks for logic metal and DRAM applications, this study contributes to the broader goal of developing and experimentally verifying an EUV mask roadmap for single print Logic and DRAM patterning in low-, high-, and hyper-NA regimes.

Recently, ASML's EXE:5000 scanner has become operational, which uses a numerical aperture (NA) of 0.55, which is 67% higher than the 0.33 NA of the current generation of extreme ultraviolet (EUV) scanners. This higher NA results in the printing of smaller features, which is essential for advancing semiconductor technology and sustaining Moore's law. However, as NA increases, the incidence angles on the wafer also increase, leading to a stronger variation of the aerial image through focus, thereby reducing the depth of focus (DOF). Due to a limited focus control in the scanner and the finite resist thickness, this reduced DOF could result in a pattern failure. We explore the enhancement of DOF in high-NA EUV lithography, particularly within the framework of logic metal clips and their building blocks, i.e., dense and isolated lines, and tip-to-tip (T2T) features, using a dark field mask and a negative tone resist. We explain through simulations and experiments that the overlapping DOF of pitch 20, 40, and 60 nm horizontal lines and spaces can be significantly enhanced using three strategies: illumination source optimization, application of a hammer head to line ends, and application of a low-n mask absorber. In addition, we clarify how these three strategies significantly improve T2T printing performance in terms of CD control and local CD uniformity. In conclusion, we show that through a rational application of the three optimization strategies, an acceptable overlapping DOF can be achieved.

The introduction of High NA EUV lithography has led to further improvements in resolution; however, the depth of focus (DoF) has decreased, as it is inversely proportional to the square of NA. While the DoF at a specific pitch can be partially improved through source optimization, the limitations become apparent when dealing with mask layouts containing a wide range of pitches due to mask 3D effect (M3D) induced best focus variation through pitch. In addition, this issue shows a strong dependence on the refractive index of the absorber structure and is most pronounced in a Low-n attenuated phase shifting mask (Low-n mask), as demonstrated by both experimental and simulation results. Low-n masks have recently been widely used in industry, as they align the diffraction phase in specific pitch regions, usually optimized only for dense pitches. As a result, the Low-n mask helps mitigate the fading effect by reducing pole to pole (P2P) offset, thus improving the image contrast at dense pitch regions. However, in the case of three-dimensional masks with a pitch-dependent phase offset, it remains challenging to simultaneously achieve well-aligned best focus and high contrast across all pitches. As a compensatory technique for this issue, we previously proposed sub-resolution grating (SRG) insertion in the pattern area, which effectively mitigates the background intensity particularly for a highly reflective Low-n mask. A reduction of background intensity makes any remaining phase error negligeable and resolves issues about image contrast degradation in isolated pitches as well as side-lobe printing issues. As a result, it aligns the best focus and improves the image contrast across a wide range of pitches. In this paper, we move beyond theoretical and simulation-based validation of the SRG by integrating it into mask patterns with vertical line-space (L/S) pattern pitches varying from 20 nm to 200 nm, and horizontal 2-bar configuration, followed by mask fabrication and wafer exposures with High-NA scanner. The wafer exposure results demonstrate that the SRG effectively mitigates several M3D related issues such as best focus variation through pitch, pitch-dependent contrast loss, Bossung tilt, which are consistent with simulation predictions. We also perform additional simulations based on contours extracted from our fabricated SRG mask layout, to further evaluate its advantages, discuss potential challenges, and explore possible solutions.

At advanced nodes, due to the resolution limitations of 0.33NA EUV lithography, metal layers are usually defined as unidirectional (1D) patterns to guarantee patterning fidelity on the wafer. Accordingly, multiple masks are required to print 2D patterns. However, 0.55NA EUV lithography, with its higher resolution capability, enables the opportunity for bidirectional (2D) metal layer patterning [2], which can significantly reduce process complexity and manufacturing costs. In previous studies [1, 10], unidirectional metal layer patterning has been extensively investigated, and by introducing alternative mask absorbers, 0.55NA EUV has demonstrated excellent single patterning quality down to pitch 20nm. In this paper, we focus on evaluating bidirectional random metal layer patterning capabilities using 0.55NA EUV lithography through Source-Mask Optimization (SMO). Inverse Lithography Technique (ILT) will be employed to assess and optimize the patterning performance of complex 2D metal structures under varying degrees of design rule aggressiveness, and the key imaging parameters will be used for the assessment.

High-numerical-aperture (high-NA) extreme ultraviolet lithography (EUVL) is a key enabler for continued logic scaling beyond the 2 nm node. As metal pitches shrink, single-exposure patterning becomes essential to reduce process complexity, improve variability, and control costs. However, standard EUV mask technology imposes limits on achievable resolution, depth-of-focus (DOF), stochastics, and printability for advanced logic features. This paper presents an imaging roadmap for high-NA EUVL, focusing on mask technology enablers and enhancers that support single-exposure metal patterning for future nodes from A14 onwards. We demonstrate improvements from mask tonality selection, novel low-n absorber materials, sub-resolution pattern design strategies, and mask-scanner co-optimization. Rigorous simulations and on-wafer validation experiments using the EXE:5000 scanner at the imec-ASML high-NA Lab validate the impact of these mask innovations on image contrast, depth-of-focus, exposure dose, and stochastic variability.

Background The new high numerical aperture (0.55 NA) extreme ultraviolet lithography (EUVL) machine has been developed, which uses an anamorphic projection system with the demagnification of 4x in x-direction and 8x in y-direction. Due to the unchanged 6 in. mask, 0.55 NA EUVL reduces the exposure field size to half-field (26x16.5 mm2). Therefore, the in-die stitching between two exposures might be needed for applications requiring a larger than half-field size. To achieve in-die stitching in practical applications at advanced nodes, performing model-based optical proximity correction (OPC) is an essential step. Aim To build an accurate OPC model, the interaction effects between two stitching fields require some special considerations, including aerial image interaction, optical proximity effect among the stitching patterns, mask absorber reflection, black border proximity effect, and the stray light from the neighboring fields effect. All these effects must be captured by specific models and corrected during OPC. In this paper, we will study the model accuracy and design decomposition rules at the stitching region and provide a solution from an EDA perspective. Approach In this paper, the in-die stitching effects and solutions are investigated using a Ta-based dark-field mask. To study the model accuracy at the stitching region, various stitching test patterns have been designed and placed on imec 0.55 NA test masks, and the wafer data are collected from the 0.55 NA EUV scanner at the joint ASML-imec High NA EUV Lithography Lab. To enable effective in-die stitching, the impact of design decompositions on the stitching performance is investigated by performing stitching OPC with the built double exposure OPC model. Results The model accuracy has been evaluated using the obtained wafer data, both for single and double exposures in the stitching region. It is important to use a smart cut approach to decompose the design before running OPC. Comprehensive results provide a detailed comparison between double exposure in the stitching region and single exposure through simulation, with particular emphasis on their respective impacts on overall stitching performance. Conclusions In this paper, the in-die stitching effects and OPC solution have been investigated under idealized lithographic conditions, where mask errors, overlay errors between two exposures, and the resist delay effect are not considered. Our initial investigations indicate that double exposure shows promising model accuracy with the existing calibrated resist model (based on single exposure data), and new resist model calibrations might not be necessary for the stitching region. Notably, the wafer critical dimension (CD) in the double exposure region is sensitive to flare values, resulting in wafer CD shifts and requiring accurate flare characterization. The smart cut approach for design decomposition effectively mitigates edge placement error issues across the stitching line. By employing this smart cut approach, the stitching OPC can deliver patterning performance similar to single exposure OPC, with only slightly degraded exposure latitude.

The successful introduction of extreme ultraviolet ( EUV) lithography to high-volume manufacturing has significantly increased interest in pushing this technology to its limits. Standard Ta-based EUV masks suffer from mask 3D (M3D) effects that limit contrast near the tools' resolution limit. This contrast loss is primarily caused by aerial image fading, driven by phase shifts between the aerial images of different point sources in the illumination pupil. These phase shifts are influenced by the thickness and refractive index of the absorber material, leading to contrast loss near the resolution limit, feature-dependent Best Focus shifts, and asymmetric process windows. Tuning the absorber material presents a promising route to mitigate M3D-induced contrast loss effects. In this work, we manufacture and evaluate the printing performance of a bright field EUV mask with a novel low-n absorber with low reflectivity (similar to 3%) and compare it to a Ta-based mask. The experiments were conducted on an NXE:3400 EUV tool with a 0.33 numerical aperture (NA) and focus on the printing quality of building blocks for logic metal (pitch 28 nm) and DRAM (pitch 36 & 34 nm) applications. We assessed the overlapping process window and pattern placement of trenches through pitch, the Mask Error Enhancement Factor, the pattern asymmetry of 2Bars through focus, and the contrast of hexagonal arrays of contacts. By experimentally demonstrating the patterning performance and advantages of a low-reflective low-n absorber mask for logic metal and DRAM applications, this work shows the potential of tuning the EUV mask stack to meet future highvolume manufacturing requirements for single print logic and DRAM patterning applications.

ASML's first High-NA EUV scanners with 0.55 NA optics are operational. The wafer printing performance is being evaluated and enhanced within the ecosystem of the imec-ASML High-NA lab. Due to the anamorphic optics, High-NA EUV masks are designed at (4x,-8x) magnification compared to wafer scale. Consequently, while mask dimensions further shrink to unprecedented resolution in X-direction, they relax in Y-direction, resulting in asymmetric mask patterns, with new mask perceptions and mask challenges which become dependent on pattern orientation. Meanwhile, at stringent dimensions, CD errors at mask must be further reduced. Moreover, due to the anamorphic optics, even a symmetric CD offset at mask leads to an asymmetric CD error on the wafer. In this context, updating mask metrology alignment between different parties in the litho process is critical in order to minimize mask CD errors originating from differences in metrology conditions. In this paper, we present the outcome after matching the mask CD-SEM conditions between imec's and a mask maker's CD-SEM tool, as well as mask metrology alignment with EDA software, since agreement between these parties is essential for achieving a consistent litho process flow from design and OPC, over mask making and metrology, to wafer print. With alignment on optimized settings for metrology on (High-NA) EUV masks, we demonstrate that an excellent match in mask metrology can be achieved. Subsequently, we demonstrate rigorous simulation for a High-NA EUV use-case, based on measured mask contours. Finally, the simulation outcome is compared to High-NA wafer data by means of edge placement evaluation, showing very good correspondence between simulated and measured in-resist contours. This demonstrates how contour-based rigorous simulation and edge placement evaluation are valuable tools for investigating the contribution of the mask to the wafer printing, and assessing the lithographic performance under different exposure conditions, thereby helping to optimize the High-NA EUV lithography process.

Low-n attenuated phase shift mask ( Low-n mask) has been widely used in EUV lithography industry as it aligns the diffraction phases between the zeroth and first orders, enhancing exposure latitude (EL) and boosting throughput. However, it has faced challenges such as side-lobe printing due to its high reflectivity and best focus (BF) variation when used in environments with a wide range of pattern pitches. To mitigate this, conventional sub-resolution assisted features (SRAF) could be inserted, but as the main features' pitch decreases, the space that needs to be inserted in between also decreases by nearly half or more, it will become challenging when scaling to Hyper NA dimensions. We proposed Sub-Resolution Grating (SRG) crossing the main features perpendicularly, which not only suppresses unwanted side-lobe printing but also improves the imaging quality with a better BF alignment over a broader pitch range by effectively aligning diffraction phases. The SRG helps correct issues such as Bossung tilt and critical dimension (CD) asymmetry in non-repeating n-bar patterns, which certainly contributes to CD-based process window (PW) enhancement. This paper demonstrates, through rigorous simulation, that this technology can ultimately overcome the limitations of Low-n mask and be practically applied by verifying CD-based PW using a more realistic Logic Metal layer.

Background: A low-n attenuated phase shift mask (low-n mask) has been widely used in the extreme ultraviolet (EUV) lithography industry as it aligns the diffraction phases between the zeroth and first orders and enhances exposure latitude, as well as boosting throughput. However, the aligned phase is achieved only for a specific pitch, usually the densest pitch. A pitch-dependent phase offset inevitably remains in a mask layout with various pitches. This leads to the challenge of best focus (BF) variation, which is even more pronounced in the case of a low-n mask with a large refractive index difference from vacuum. The BF variation through pitch is more critical at higher numerical aperture (NA), where the available depth of focus is significantly lower than at low NA. In addition, side-lobe printing caused by high reflectivity from the low-n mask is also a concern. Aim: We aim to enhance imaging performance and demonstrate CD-based overlapping process window (oPW) while minimizing BF variation and unwanted side-lobe printing. Then, we aim to provide a potential mask absorber roadmap in hyper NA EUVL from an imaging perspective. Approach: We proposed a sub-resolution grating (SRG) crossing the main features perpendicularly, which not only suppresses unwanted side-lobe printing but also improves the imaging quality with a better BF alignment over a broader pitch range by effectively aligning diffraction phases. Our approach involves a step-by-step validation of unit cell patterns to assess the SRG impact on oPW, beginning with various L/S pitches, followed by 2-bar and 3-bar for both horizontal and vertical orientations. Ultimately, we verify oPW using real logic use cases applicable to high- and hyper NA EUV lithography. Results: Rigorous simulation demonstrates improved oPW for both vertical and horizontal logic metal layers. As for the vertical L/S patterns as the SRGs can cover the entire mask pattern area, all types of low-n masks can be utilized. As for the horizontal L/S patterns, covering the entire mask pattern area with the same orientation of horizontal SRGs may be challenging from the perspective of mask manufacturability. To overcome this constraint, alternative absorber structures could be considered. A higher k absorber gives a better imaging performance with less pronounced BF variation. By combining the advantages of these two, a thin high-k absorber structure with selective SRG insertion demonstrated an oPW higher than our set criteria and showed more than 25% improvement in productivity compared with a Ni-based high-k absorber. Conclusions: The SRG insertion technique, along with proper wavefront optimization, is expected to offer tangible benefits of imaging and enhanced productivity for logic metal layers in high NA and hyper NA.

Standard periodic MoSi multilayer (ML) EUV mirrors have formed the foundation of optical components for EUV lithography and serve as a key enabler for delivering high-power EUV light on wafer. However, the periodic ML mirror used in 0.33 numerical aperture (NA) EUV masks has a limited angular bandwidth, resulting in phase shifts of off-axis rays that lead to reduced normalized image log slope, telecentricity errors, focal shifts, etc. (i.e., mask 3D, M3D, effects). The ML-based M3D effects compound with absorber properties (thickness, composition) and are exacerbated as the range of incident angles on mask increases with high- and hyper-NA (>= 0.55) systems. Aperiodic ML stacks have been proposed to mitigate these effects by enhancing the angular bandwidth and reducing the depth of the effective mirror plane. However, experimental verification of tuned ML mirrors, combined with a calibrated model to predict their performance, is currently lacking. Here, we utilize a modelling-deposition-measurement approach to predict, tune, and verify the angular bandwidth of a series of pseudo-aperiodic ML MoSi mirrors. ML mirrors are produced via state-of-the-art ion beam deposition and the EUV reflectivity (EUVR) is measured with coherent, p-polarized EUV light from a tabletop source. Deviations between the predicted and measured EUVR curves are rectified via inclusion of interface mixing and roughness into the model, which is verified via correlative transmission electron microscopy (TEM) and electron dispersive X-ray spectroscopy (EDS) measurements. Additionally, we leverage TEM-EDS measurements to isolate and gain deeper insight into the role of interfaces and oxide layers on the resulting EUVR as measured on single MoSi bilayers. This coordinated and correlative approach to predict, produce and characterize the various ML stacks results in an improved EUV mirror model that can be used to find optimized ML geometries that enhance ML performance and reduce M3D effects for high-to-hyper NA EUV lithography.

Mask stacks comprising of alternative materials may allow further improvements in EUV imaging. In a strive towards advancement of resolution limits in EUV lithography, such masks are brought up for consideration. In this work, we evaluate a novel Ru/Si multilayer mirror (MLM) mask and compare this novel mask to a traditional Mo/Si MLM mask. Both test masks have identical Ta-based absorbers. Through simulations and experiments at 0.33 NA, we evaluate the line-space (L/S) imaging performance in terms of best focus through pitch and exposure latitude. Additionally, we study the mask 3D effects such as 2Bar asymmetry through focus and pattern shift through focus and compare these metrics to the imaging performance of Mo/Si MLM masks. This work adds understanding to the patterning benefits and of alternative Ru/Si MLM mask stacks in the case of Metal direct print applications for 0.33NA EUV lithography.

Transmission Electron Microscopy (TEM), complemented with Electron Dispersive X-ray Spectroscopy (EDX), was used for direct analysis of a state-of-the-art Ta-based EUV reticle, after limited use for exposure. The obtained cross-sectional view of the EUV mask stack and the subsequent compositional analysis reveal unexpected differences between open multilayer mirror (MLM) spaces and absorber-covered MLM. In the former, the upper Si layer is found oxidized and increased in thickness. The noted effect identifies a new parameter to be added in an appropriate EUV mask model for simulation, which is changing during the lifetime of the reticle. Based on a proposed mechanism, imaging simulation shows an increase in dose-to-size with increasing oxidation. In addition, this oxidation is a likely trigger for deteriorating intra-die CD uniformity. The noted differences in the open MLM are also found in areas of the reticle outside the exposure field, which see no direct EUV light. The number of wafers exposed with the reticle was still far lower than what is considered a typical cumulative reticle use in IC production based on EUV lithography. It is believed that storage conditions for EUV reticles and their history contribute to an accelerated reticle deterioration by this EUV exposure induced oxidation.

Background Projection lithography technology has been developed to allow the use of shorter wavelength light and to increase numerical aperture (NA) from 0.33 to 0.55. After enabling extreme-ultraviolet (EUV) wavelengths, to keep up with the scaling trends, the industry would now again like to increase the NA. As the depth of focus (DoF) is inversely proportional to the square of NA, in hyper NA (>0.55) EUV lithography (EUVL), we anticipate that the total available DoF in the lithography process would be further limited. Aim We aim to improve the imaging performance for a wide range of lines and space pitches by minimizing best focus (BF) variations generated from different pitches on an anamorphic EUV mask. Approach Sub-resolution grating (SRG) is proposed to address the increasingly complex design of sub-resolution assisted feature (SRAF), especially in high-NA or hyper-NA regimes where SRAF insertion becomes challenging. We first identify how the mask 3D (M3D) effect-induced BF variation through pitch behaves according to changes in the pattern orientation and mask tone for hyper NA EUVL. We study how various focus shift mitigation strategies can be combined to align the best foci and enhance the image contrast for hyper NA EUVL. Results Simulation results indicate that for increased NA, BF variations due to M3D effects for vertical lines deteriorate more significantly than for horizontal lines. As mitigation strategies, we presented diverse solutions in the mask and illumination space, leading to the achievement of well-aligned BF with enhanced image contrast for a broad pitch range and various feature types. By using a phase-less binary mask that has an EUV refractive index of n congruent to 1 and a high EUV extinction coefficient k, the phase offset-induced BF variation through pitch can be mitigated, which could be a favorable option for hyper NA where overlapping DoF becomes crucial. Illumination source optimization in conjunction with aberration injection can correct pole-to-pole offset. SRG in the mask design addresses BF variation through pitch and improves normalized image log slope (NILS) for various patterns, wafer critical dimension targets, and a wide range of exposure dose processes using the most simple and straightforward method. This technique could be one of the best complementary techniques for a high-reflective attenuated phase shift mask also known as a low-n mask. Conclusions We identified that SRG aligns BF for various patterns with different pitches, and this can be applied to all absorber thicknesses. In addition, we identified that SRG even improves NILS at certain absorber thicknesses.

Continued device scaling demands improvements in lithographic resolution, which have historically been achieved by reducing wavelength and increasing numerical aperture (NA). With 0.33 NA EUV lithography now in production, current efforts are directed toward high-NA (0.55) EUV systems, aiming to extend resolution capabilities even further for future nodes. As the next logical step beyond 0.55 NA EUV, either reducing the wavelength (<13.5 nm) or increasing the NA (NA > 0.55) presents a pathway toward further scaling. This paper discusses the opportunities and technical challenges of these approaches, evaluating their feasibility and potential impact on imaging performances.