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.

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.

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.

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 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.

High-NA EUV lithography is being prepared for the next stage of volume production of state-of-the-art integrated devices. First wafer exposures on ASML's EXE5000 are expected early in 2024. Beyond assessment of the benefit of high-NA by simulation, ZEISS AIMS EUV offers the potential to compare the imaging benefit of 0.55NA to the established 0.33NA, through aerial image analysis of dedicated mask patterns. The recently available capability of high-NA imaging on AIMS (R) EUV was applied to compare options for imec's logic patterning roadmap, specifically for tip-to-tip structures (T2T). Beyond direct comparison of 0.33 and 0.55 NA, low-n absorber was compared to conventional Ta-based absorber. Moreover, in view of anamorphic imaging at high-NA, T2T pattern orientation was compared, i.e., either along the 4X exposure slit direction or along the 8X scan direction. Lastly pattern tonality, i.e., darkfield versus lightfield, were evaluated side by side. The comparisons were made for selected, yet not optimized, dipole-like sources. Beyond normalized intensity log-slope (NILS) for the line-space part, the through-focus analysis comprised ILS and required bias for shrinking T2T size. The results show that 0.55NA provides clear advantages, but their variation among absorber type, T2T pattern orientation and tonality highlight the potential of preferred combinations. Such are suggested as starting points for further optimization.

To print ever smaller features at high contrast, projection lithography technology has been developed to allow use of shorter wavelength light and to increase numerical aperture (NA) from 0.33 to 0.55. After enabling EUV wavelengths, to keep up with the scaling trends the industry would now again like to increase the NA. Since the depth of focus (DoF) is inversely proportional to the square of NA, in hyper NA EUV lithography (EUVL), we anticipate that the total available DoF in the lithography process would be further limited. Therefore, within the constrained DoF budget, it is necessary for lithographers to minimize the unwanted best focus (BF) variations generated from different pitches on a photomask. In this paper we identify how the mask 3D ( M3D) effect induced BF variation through pitch behaves according to changes in the pattern orientation and mask tonality for hyper NA EUVL. We study how various focus shift mitigation strategies can be combined to align best focus and enhance the image contrast for hyper NA EUVL.

The cost of EUV lithography in chip production calls for imaging solutions that increase scanner throughput via dose reduction. By enhancing imaging via alternative low-n mask absorber materials, the imaging mask becomes a knob for dose reduction. Current low-n masks have an absorber thickness of 40-50nm. Thinning down the absorber results in image contrast losses, limiting their dose reduction potential in EUV lithography. Here we present wavefront optimization enabling thinning down low-n mask absorbers to 24-26nm. For single print EUV logic applications, this achieves a dose reduction of 20-30% compared to current low-n and Ta-based mask absorber thicknesses. In this simulation study, we first show that pole-to-pole image shifts drive the observed contrast losses at reduced low-n absorber thickness. Using wavefront optimization to overcome such image shifts, we then demonstrate that the low-n absorber thickness can be further reduced. The imaging potential of thinner low-n masks for current (0.33NA) and future (0.55NA) metal logic applications is evaluated by rigorous simulation overlapping process window (oPW) analysis. We also show that source/mask/wavefront co-optimization can enable a superior oPW depth of focus for the thinner low-n mask compared to the current generation of Ta-based and low-n masks. Finally, we propose available absorber materials with suitable optical properties for the practical implementation of thinner low-n masks and show their imaging strategy can be achieved at full illumination efficiency for single patterning metal logic applications down to a dense pitch of 20nm. In conclusion, our results prove that by employing wavefront optimization, thinner low-n masks provide similar or improved imaging at a much lower exposure dose.

We are on the eve of the next big step in lithography technology with the introduction of high numerical aperture EUV. The change from NA 0.33-0.55 in EUV lithography is an increase of 67%, which is the largest jump in the last decades, and puts tight requirements on focus and edge placement. Moreover, the lithography system has changed from fully isomorphic, i.e. same demagnification in all directions, to an anamorphic system, i.e. the demagnification in scan direction has doubled with respect to the slit direction. At imec we are fostering the ecosystem surrounding the lithography tool. In this paper we focus on the imaging and mask innovations supporting the EUV ecosystem, which are categorized into four areas: novel absorber masks, stitching, mask variability, and innovative imaging solutions. The current drivers of IC manufacturers implementing (high NA) EUV lithography (EUVL) are reduction of the EUV exposure dose and decrease in wafer stochastics. We discuss how these four areas have the potential to deliver in EUVL an increase in productivity, an improvement in the process window and a reduction in stochasticity at wafer level.

Development of High-NA EUV scanners is maturing and reached the stage of first exposures. Due to the anamorphic 0.55 NA optics, High-NA EUV masks are designed at (4x,-8x) magnification compared to wafer scale (X,Y). Consequently, while dimensions further shrink in X-direction on mask, they relax in Y-direction, resulting in asymmetric mask patterns and new mask perceptions. In this paper, we present a CD-based characterization for a variety of generic patterns on a state-of-the-art High-NA EUV mask, with emphasis on feature dimensions which are specifically relevant to High-NA EUV lithography. The mask metrology is done using an Advantest E3650 mask CD-SEM at imec, with image capture and metrology settings optimized for EUV masks. Besides providing insight into achievable pitches, we touch upon CD linearity for line-space patterns on mask, local roughness and non-uniformity at different length scales, and include a simulation to discuss the transfer of mask variability to wafer variability for a dense contact hole case. Another important aspect which we highlight in this study, is related to the effect of CD errors on mask. Namely, because of the anamorphic imaging, an X/Y symmetric CD offset on mask will lead to asymmetric CD errors at wafer level which can no longer be absorbed e.g. by choice of exposure dose. To avoid these asymmetries at wafer level, it is important to make sure that the mask is well targeted. The latter, however, also depends on choices in metrology settings, which may be 'historic defaults' and based on larger dimensions on DUV masks, yet applied to (High-NA) EUV masks. We therefore appeal to mask vendors for a careful verification of metrology settings applied for measurement on (High-NA) EUV masks.

The semiconductor industry faces numerous sustainability and environmental challenges. These include waste and the circular economy, water use, air pollution, global climate change, and energy use. These topics are interdependent. Many processes use process gases that have significant embodied energy. Recovery and recycling of these gases not only reduces operating costs and improves supply chain resilience, but also reduces the total carbon footprint of the process. To this end, Edwards is developing a Hydrogen Recovery System (HRS) capable of recovering EUV process waste hydrogen gas. The HRS purifies and pressurizes the waste hydrogen to meet purity & pressure requirements and recycles the gas directly into the EUV lithography tool. In partnership with imec, Edwards has demonstrated successful recycling into the ASML NXE:3400B EUV scanner installed at imec. Over 9 million standard liters (approximately 850 kg) of hydrogen have recycled with no negative impact to the performance of the EUV scanner, while reducing EUV-related hydrogen consumption approximately 70%. Recovery rates and purity are discussed.

Dual monopole exposure has been proposed1 as a way to improve imaging performance in EUV lithography by reducing the loss of contrast from image fading caused by the image shifts for the two poles of a dipole source. Simulations showed that the imaging advantages can be significant, with more than 15% image contrast improvements predicted. This paper presents experimental wafer data to demonstrate and verify the predicted advantages. The observed imaging enhancements include: • 18% better NILS (Normalized Image Log-Slope) for 28nm pitch patterns. • Better tip-to-tip patterns enabling gaps as much as 3nm smaller than normal patterning. Tip-to-tip LCDU and exposure latitude were improved at the same time, for better overall T2T capability. • Best focus offsets between three pitches: P28, P56 and P96 was reduced from 30nm range to nearly zero. • Smaller LWR (Line-Width Roughness), as much as 20% depending on pitch. • Reduced micro-bridging defects in the "stochastic cliffs" of narrow trenches, as large as a 40X defect density reduction for narrow trenches with 82nm pitch. Our experimental results validate the substantial imaging advantages seen by initial simulations. In addition to verifying dual monopole imaging advantages, our experiments have measured the pole-to-pole image shift δxP2P, an important parameter relating to image contrast which has never been measured before. This important parameter depends on the detailed mask structure as well as the specific shape and location of the source poles. Our measured δxP2P was consistent with simulated expectations.

Wafer CD Uniformity (CDU) and pattern fidelity are useful properties to monitor when considering yield improvement and scaling to smaller dimensions. Besides control of process fluctuations (e.g., focus, dose), wafer stack film thickness uniformity, and image quality (e.g., contrast), quality of mask manufacturing and OPC models are essential to optimize these properties. Therefore, a proper characterization of the written mask dimensions is becoming more and more important, especially as the mask pattern complexity increases through Inverse Lithography Technology (ILT) as well as the need for curvilinear or all-angle designs. Applying contour-based mask quality assessment instead of traditional gauge-based characterization of mask dimensions allows to intrinsically capture mask imperfections like corner rounding (CR) of the absorber for complex shapes which would be hard to measure and categorize by using only a few gauges. Thus, in our study, we examine ways to use contour-based mask characterization methods, including CD and area uniformity, linearity, and CR determination to evaluate mask quality. We present a method and flow to automatically extract contours and determine values for mask CR from top-down mask SEM images. Contour-based metrology and data evaluation is then used to quantitatively address the above-mentioned mask properties of interest. Finally, as an initial approach to investigate impact of mask quality on wafer printing, we apply a generic EUV model to ideal and imperfect masks and compare the simulated contour results. Using realistic mask patterns for lithography modeling and simulation is considered essential to achieve the required accuracy for advanced nodes and technologies.

To print ever smaller features at high contrast projection lithography technology has evolved to shorter wavelength light and larger NA. After enabling the EUV wavelength, the industry is looking into increasing the NA. This study aims to identify EUV specific challenges regarding NA scaling beyond 0.55. We study if EUV imaging can still work at this higher NA and whether specific changes to the mask stack are required. At NA's much higher than 0.55, new effects like polarization will play a role, and larger impact of ultimate mask resolution and material interactions is expected. Already at NA 0.55, a small contrast loss is predicted due to the use of unpolarized light in the scanner. Further increasing the NA will enhance the contrast loss. We study these polarization effects in detail and assess their impact quantitatively for a set of generic building blocks. In addition, the larger incidence angles on mask when the NA increases above 0.55, will further enhance the M3D effects forcing additional mask changes. To enable proper imaging at high incidence angles, new mask architectures, that include changes in the EUV mask absorber and multilayer, will have to be tested using rigorous simulations.

EUV lithography has been implemented in high volume wafer production. Consequently, maximizing yield is of major importance. One key component to achieve optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimize the printing of defects. The carbon nanotube (CNT) pellicle is a membrane consisting of a network of carbon nanotubes, which demonstrates EUV transmission up to 98%. The challenge is to balance the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, high durability in the scanner environment, while maintaining high transmission and low impact on imaging. While our earlier reporting on full-field CNT pellicle exposures demonstrated minimal impact on imaging, the focus of the current paper is on extended exposures on NXE:3400. In the scanner, the EUV light induces a hydrogen plasma that etches the CNTs, resulting in decreasing membrane density and increasing EUV transmission. In this work, we quantify the CNT pellicle etch rate in a real scanner environment and correlate the findings to those obtained in an offline test setup. Our exposures were performed using two different pellicles, with EUV transmission of 89% and 95%, for up to 3000 wafers. Additionally, we demonstrated the effectiveness of pellicle purification prior to mounting on the reticle, which is important to avoid contamination from the as-fabricated CNT pellicle onto the reticle surface. Current ongoing developments focus on further increasing the pellicle durability in the scanner environment. The presented results demonstrate the potential of a CNT-based pellicle at high EUV powers.

Background: In extreme ultraviolet lithography, the printable feature density is limited by stochastic defectivity, which can be reduced by increasing the optical contrast. The photomask induces pole-specific aerial image offsets. Consequently, illumination settings with multiple poles lead to contrast loss and focus offsets between different features. Aim: We aim to mitigate the contrast loss and best focus offsets between different features. Approach: Illumination was decomposed into monopoles. Each monopole was exposed separately using a fraction of the total dose. Each exposure was shifted by its pole-specific image offset to mitigate 3D mask effects. Results: Single monopoles mitigate contrast loss and best focus shifts, but in defocus, they suffer from aerial image shifts and distortions. Multiple aligned monopole exposures conserve these advantages but mitigate the problems in defocus. Because each monopole is exposed with only a fraction of the dose, the throughput penalty is limited to the scanner overhead. Conclusions: A multiple monopole exposure scheme can increase contrast, align the best foci, and mitigate single monopole exposure constraints. Additionally, it offers an improved pattern placement control through dose control knobs.

High-NA EUV lithography will improve resolution by increasing the EUV scanner NA from 0.33 to 0.55. To fully benefit from the resolution gain offered by the better scanner lens, it is key to develop and improve the EUV ecosystem. The role of the ecosystem is to ensure timely availability of the advanced resist materials, photomasks, metrology techniques, OPC/imaging strategies, and patterning techniques. In this context, in parallel to the EXE:5000 0.55 NA EUV scanner manufacturing, imec and ASML, together with our partners, are addressing the main challenges and needs towards High-NA ecosystem readiness. In this paper, we will discuss the key findings from simulations and experimental work to develop the high-NA lithography ecosystem (resist and patterning, mask technology) and highlight the key areas where development is needed.

The goal of this work is to prepare process readiness towards High NA EUV lithography, by using 0.33NA exposures on NXE3400B scanner. We focus on photoresists, underlayers and etch processes mitigation of P24nm Line Space patterns. Etch transfer has been validated for Metal Oxide Resist (MOR). Furthermore, we investigate challenges to accelerate Chemically Amplified Resist (CAR) P24nm Line Space processes. Also, here, promising patterning results have been achieved. Thin film metrology-friendly methods like Atomic Force Microscopy (AFM) have been performed to characterize and improve the CAR-based etch processes.

EUV lithography has recently been implemented in high volume wafer production. Consequently, maximizing yield is gaining importance. One key component to achieve optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimize the printing of defects. The Carbon Nano Tube (CNT) pellicle is a membrane consisting of a network of carbon nanotubes, and demonstrated EUV transmission up to 98%. The challenge is to balance the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, low impact on imaging through scattered light, high durability in the scanner environment, while maintaining high transmission. We report results of the first full-field CNT pellicle exposures on an NXE EUV scanner. We demonstrate handling of the pellicles on the scanner, without breakage, and provides a first assessment of their imaging behavior. Multiple single- and double-walled uncoated CNT pellicles with EUV transmission up to 97.7% were exposed on the NXE scanner at imec, and minimal impact on the imaging is confirmed. In these exposures, uncoated CNT pellicles were used which will not meet the specifications regarding lifetime. Therefore, current ongoing developments focus on CNT coating and durability in scanner environment. The presented demonstration proves the value of a CNT-based EUV pellicle solution.