As EUV lithography advances toward higher numerical apertures, the angular range experienced at the mask increases significantly, approaching and exceeding similar to 12 degrees for high-NA and proposed hyper-NA systems. Under these conditions, conventional periodic Mo/Si multilayers exhibit increasing polarization separation, reflectance variation, and reflected phase error, which can introduce effective wavefront aberrations at the mask. In this work, the angular reflectance and reflected phase behavior of EUV mask multilayers are analyzed for several multilayer designs including periodic Mo/Si, periodic Ru/Si, aperiodic Mo/Si, and hybrid Mo/Si-Ru/Si stacks. Polynomial representations of the reflected phase are used to quantify quadratic and fourth-order terms corresponding to defocus- and spherical-aberration-like contributions. Full 3D electromagnetic lithography simulations using RCWA modeling are then used to evaluate the imaging impact of these multilayer phase effects. Results show that multilayer design strongly influences mask-induced aberrations and polarization behavior at large angles. Aperiodic and hybrid multilayer designs provide opportunities to reduce reflected phase error, polarization dependent phase shift, and multilayer-induced aberrations, supporting improved mask performance for future high-NA and hyper-NA EUV lithography systems.

As extreme-ultraviolet (EUV) lithography advances at 13.5 nm with numerical apertures (NA) of 0.55 and higher, the effects of large incidence angles on multilayer design, wavelength selection, and NA limits are examined. Much of this is driven by EUV multilayer (ML) design, which should be optimized for large-angle applications to achieve maximum angular and spectral bandwidth. Reflectance amplitude and phase-shift analyses of Mo/Si multilayers are performed for angles corresponding to 0.55-0.75 NA EUV lithography, demonstrating the importance of accounting for phase effects. Understanding higher-order phase contributions reveals that defocus and spherical aberration originate from the mask multilayer itself, as confirmed through transfer matrix method (TMM) design and rigorous coupled-wave (RCWA) analysis. A depth-grading approach to large-angle multilayer optimization is introduced, leveraging the EUV optical properties of the constituent materials to improve performance by adjusting the material ratios within the stack. Results are presented for solutions using Mo/Si and Ru/Si multilayers for 13.5 nm high-NA and hyper-NA EUVL. Considering polarization effects starting at high NA-both at the mask object plane and the resist image plane-alternative large-angle Ru/Be multilayer configurations operating at 11.3 nm are also evaluated. The introduction of an 11.3 nm/0.63 NA lithography option mitigates some polarization-induced degradation observed at 13.5 nm/0.75 NA and offers an additional increase in focal depth. Design strategies for depth-graded multilayers for optical coating and mask applications are detailed.

The influence of polarization is becoming more critical in extreme ultraviolet lithography (EUVL) as numerical apertures are increased and pattern dimensions approach the sub-20 nm pitch regime. At the large diffraction angles associated with high-NA EUV systems, the separation between S (TE) and P (TM) polarized reflectance in multilayer mirrors influences both aerial image contrast and image intensity balance. These effects arise from fundamental Fresnel reflectance behavior and are amplified by the angular distributions encountered across the mask pupil in anamorphic EUV systems. The resulting polarization-dependent amplitude and phase variations introduce pitch-dependent and orientation-dependent imaging behavior, complicating optical proximity correction and reducing usable depth of focus, particularly for pitches approaching similar to 15 nm and below. This work examines polarization behavior in EUV lithography through analysis of multilayer reflectance and phase across effective mask reflectance and phase pupils. Results show that polarization-dependent mask reflectance produces significant variations in image intensity and contrast that increasingly limit patterning performance at 13.5 nm as NA increases. Because practical polarization control in EUV systems is inherently difficult, these effects are largely determined by multilayer materials and optical design. An alternative wavelength strategy at 11.3 nm using Ru/Be multilayers is also explored. Compared with the 13.5 nm Mo/Si case at 0.75NA, an equivalent 11.3 nm / 0.63NA alternative may exhibit reduced polarization separation, improved reflectance uniformity, smaller phase variation, and corresponding improvements in image contrast and depth of focus. These results indicate that polarization considerations will play an increasingly important role in future EUV lithography and may influence decisions regarding wavelength, materials, and system architecture for continued scaling.

The viability of next-generation extreme ultraviolet (EUV) lithography wavelengths is evaluated through photon–matter interactions across optical materials and photoresists, rather than by resolution scaling alone. Candidate wavelengths of 13.5, 11.3, 6.6, and 3.13 nm are examined within a unified materials-based framework to compare imaging capabilities, enabling assessment of wavelength-dependent tradeoffs. Photomask and optical multilayers impose primary constraints through refractive index contrast, absorption, dispersion, and achievable large-angle Bragg reflectance. Although 13.5 nm Mo/Si systems define the current baseline, 11.3 nm multilayers may provide improved index contrast, reduced absorption, and increased angular bandwidth, supporting more favorable optical performance. At shorter wavelengths, reduced index contrast and subnanometer layer thickness requirements may impose fundamental limits on reflectance, bandwidth, and manufacturability, which may challenge the practical implementation of 6.6 nm and water-window regimes. These limitations propagate to system behavior. Scatter-driven flare increases rapidly with decreasing wavelength, imposing stringent roughness requirements or motivating reduced-surface optical designs. Polarization effects may limit hyper-NA imaging at 13.5 nm, in which large incidence angles produce significant TE–TM imbalance and image contrast loss. Reduced numerical apertures at shorter wavelengths can mitigate polarization sensitivity, with 11.3 nm potentially providing a more favorable balance between angular performance and polarization behavior. Within the photoresist, increasing photon energy with shorter wavelengths produces higher-energy primary electrons and broader secondary-electron cascades, which can increase stochastic blur and offset optical resolution gains. This identifies new resist design opportunities based on core-level threshold matching, in which the exposure wavelength is aligned just above accessible core-level binding energies to constrain electron cascade transport. Wavelength–material pairings such as 11.3 nm with the Si L2,3 edge and 6.6 nm with the S L2,3 edge represent enabling photoresist opportunities to localize energy deposition and potentially reduce stochastic variability through control of electron dynamics rather than absorption alone. These results establish wavelength selection as a challenge of material convergence across multilayer optics, imaging physics, and photoresist response.

Multilayer masks for extreme ultraviolet lithography (EUVL) are currently structured as repeated pairs of silicon and molybdenum with a patterned absorber on top. This periodic design has performed well during the 0.33 NA era, and with small changes to layer thicknesses, can be expected to maintain high reflection under high-NA (0.55) conditions. Even so, the 3D nature of the EUV mask leads to optical interactions, which worsen as feature sizes shrink, one example being shadowing. In recent years, a great deal of interest has been placed on material changes to the mask absorber with attPSM, low-n, and index-matched absorbers being proposed. In this work we build off the progress in optimizing absorber material by altering the periodic nature of the mask. An aperiodic mask allows each layer to have unique thickness, allowing careful optimization of reflected intensity, phase, polarization, and spectral bandwidth. We show validation of previously presented aperiodic multilayers using Siemens pxSMO. We find for 18 nm pitch lines in the 8x direction a 26% gain in NILS with a similar 19% gain in the 4x direction compared to a periodic multilayer. While results on a horizontal 18 nm pitch metal layer show 10% or higher ILS gains across a wide range of clips when compared to a periodic multilayer.

To achieve higher resolution extreme ultraviolet lithography (EUVL) targeted toward sub-10 nm, reflective projection scanner image numerical apertures (NAi) are being increased beyond the current value of 0.33 to 0.55 and upward of 0.75 as a desirable target. Bragg reflectors using alternating silicon and molybdenum that have heretofore been coated as periodic multilayers cannot achieve desired reflected amplitudes as corresponding 0.25x mask numerical apertures (NAm) are accordingly increased. In addition, transverse magnetic-polarized image modulation decreases with NA, which becomes significant at 0.55 and above. We present here the optimization of non-regular alternating, or aperiodic, silicon-molybdenum multilayer reflective coatings that can achieve improved amplitude and polarization performance through angle as higher-NA EUVL lithography is pursued. Through the use of rigorous EM computation paired with a genetic optimization method, we show that amplitude apodization can be recovered to 60% peak reflectance for NAm values up to 0.2 (corresponding to NAi values of 0.8) while at the same time achieving a transverse electric degree of polarization exceeding 40%. In addition, aperiodic multilayers optimized for spectral bandwidth using refractory metals also show improvement over periodic designs. (c) 2024 Society of Photo-Optical Instrumentation Engineers (SPIE)

To achieve higher resolution extreme ultraviolet lithography targeted toward sub-10nm, reflective projection scanner image numerical apertures (NA(i)) are being increased beyond the current value of 0.33 to 0.55, and upwards of 0.75 as a desirable target. Bragg reflectors using alternating silicon and molybdenum that have heretofore been coated as periodic multilayers cannot achieve desired reflected amplitudes as corresponding 0.25X mask numerical apertures (NA(m)) are accordingly increased. Additionally, TM polarized image modulation decreases with NA, which becomes significant at 0.55 and above. We present here the optimization of non-regular alternating, or aperiodic, silicon-molybdenum multilayer reflective coatings that can achieve improved amplitude and polarization performance through angle as higher-NA EUVL lithography is pursued. Through the use of rigorous EM computation paired with a genetic optimization method, we show that amplitude apodization can be recovered to 60% peak reflectance for NA(m) values up to 0.2 (corresponding to NAi values of 0.8), while at the same time achieve a TE degree of polarization (DOP) exceeding 40%.

As extreme ultraviolet lithography tools with higher image numerical apertures (NAi) are introduced, the range of angles at the multilayer mask stack is also increased. Lithography systems are designed to fulfill the "abbe sine rule," where NAm is related to NAi by the reduction factor. As a result, increases in NAi will increase NAm. High-NA and hyper-NA systems will be implemented with anamorphic optics, 4x in "X," and 8x in "Y" to reduce the necessary angles. Even so, hyper-NA masks may see illumination angles as high as 13.4 degrees, up from 10.8 degrees for 0.33 NA. This represents a challenge for maintaining through-angle mask reflection using the current periodic mask multilayer structures. In addition to the reflectance amplitude, the phase of reflected light, which plays an important role in imaging, is also strongly influenced by increasing angles. The propagation of light through each bilayer in the stack imparts a phase shift based on the incidence angle. This is then accumulated over many layers, inducing phase effects, which are unique to each illumination point. This will become especially true for high-NA and hyper-NA mask applications. While adjustments to the multilayer period are sufficient to achieve acceptable reflectance, periodic multilayers may suffer in normalized image log slope (NILS) and image placement error (IPE) metrics as a result of strong oblique multilayer M3D phase effects. In this work, we present a first principles methodology for the systematic reduction of oblique multilayer M3D effects through the use of aperiodic multilayer design. We find that when used for low-k1, hyper-NA patterning NILS is improved by 40+%, while IPE through pitch is similarly improved. Under high-NA conditions, a 5%-25% NILS gain is found alongside the enhanced depth of focus.

Attenuated phase shifting masks (attPSM) for EUV lithography have shown the potential to improve the aerial image contrast through light and phase modulation and reduced mask three-dimensional (M3D) effects through thinner absorber. However, a robust optical design capable of generating almost identical performance for various patterns and feature sizes needs to be determined. The ability to identify and experimentally verify various mask absorber candidates is both challenging and expensive. Effective media approximation (EMA) identified material candidates have been shown to simplify the determination of material candidates through known optical constants of the constituent elements. An approach to engineer the desired optical properties and experimentally verify EMA modeling technique through multilayer thin films is presented. Mo – Ni multilayer films satisfying EMA requirement are deposited through RF magnetron sputtering. The verification of optical constants for multilayer films with 20%, 50% and 90% Ni volume fractions is performed at visible wavelengths through UV-Vis-NearIR variable angle spectroscopic ellipsometry (VASE). EMA modeled multilayer absorber candidates are shown to have the flexibility in obtaining desired optical properties based on the layout-design requirements.

Pattern transfer in an extreme ultraviolet lithography (EUVL) system requires reflective optical elements illuminated at oblique illumination angles. This, in combination with the three-dimensional effects at the mask, is the source of the so-called mask 3D (M3D) effects that include shadowing, best focus shifts, and contrast fading. Alternative mask absorbers at lower thickness possess the ability to extenuate the M3D effects and improve overall imaging performance. An approach to recognize candidate material combinations as alternative EUVL mask absorbers through dielectric constant modeling by using methods involving Wiener bounds and effective media approximation (EMA) is presented. Using EMA, several stoichiometrically stable binary alloys of refractory metals that may serve as attenuated phase shifting mask absorbers have been studied. The optical properties and thickness of each absorber candidate alloy in a standard EUV mask stack are optimized for imaging performance. The best performing alloys are characterized through rigorous 3D image modeling of near-field intensity and phase at varied illumination angles.

Alternatives to Ta-based absorbers are being considered for next generation lithography nodes to reduce 3D mask effects and to improve image modulation through phase interference. Low complex refractive index (n – ik) materials can provide phase shifting behavior at thicknesses less than those needed for conventional absorbers, essentially acting as attenuated phase shift mask (attPSM) films. Identifying attPSM absorber thickness and consequent phase requires determining optimum phase shift mask reflectance. Imaging with absorbers at high reflectance show better imaging performance. The absorber thickness is determined where the interference effects lead to high absorber reflectivity. Low refractive index (n) materials are therefore desired as candidate attPSM absorbers. Low – n material combinations identified using Wiener bounds and Effective media approximation (EMA) modelling are optimized for NILS and MEEF using absorber reflectivity on line-space and contact-holes patterns. Absorber candidates at optimum thickness for contact holes are compared with conventional Ta-based absorber using reflected nearfield intensity imaging.

As extreme ultraviolet lithography (EUVL) technology progresses towards and below sub-7nm generations, polarization effects will begin to have an impact. Moving to higher NA will introduce polarization effects at all locations: the mask plane, the optics, and the image plane. Contrast losses at a larger angle can occur from the interference at the image plane that will become significant for smaller geometries approaching the EUVL wavelength. Some of these contrast losses can occur at the mask where the polarized reflectance from an EUV mask is affected by the multilayer reflective stack and patterned features. This paper explores the polarization effects that are induced by EUVL masks for sub-7nm lithography. The results show a polarization-dependent induction and attenuation of current in EUVL mask structures, especially as mask pitch decreases below 6 lambda.

Aberrations must be sufficiently controlled to make moving to a higher numerical aperture worthwhile. Traditional isomorphic imaging systems form the same image regardless of their rotation. Likewise, the aberration basis chosen for isomorphic optics is invariant under rotation. Anamorphic optics are not rotationally invariant though-they are only reflection invariant. We have shown in previous reports that a basis composed from a product of Legendre polynomials represents the balanced aberrations of anamorphic optics. Solutions have been presented under the presence of a circular central obscuration. This paper will examine the properties of these aberrations and their effects on image formation through analogies to the well-known Zernike aberrations. It will be shown that ray tracing simulations of the point spread function of an anamorphic optic in Code V matches predictions made by the proposed basis. A system will be described for computing an anamorphic aberration basis in the presence of an arbitrary obscuration. Based on this system we will analyze the effects of using the basis for the wrong type of obscuration.

Optical lithography involves the creation of relief image patterns through the projection of radiation within or near the ultraviolet (UV) visible portion of the electromagnetic spectrum. Techniques of optical lithography, or photolithography, have been used to create patterns for engravings, photographs, and printing plates. The optical configuration for projection microlithography tools most closely resembles a microscope system. The diffraction of light is responsible for image creation in all optical situations. Several properties of optical materials must be considered in order to effectively design, optimize, and fabricate optical components. Optical lithography below 300 nm is made difficult because of the increase in absorption in optical materials. Few transparent materials exist below 200 nm, limiting design and fabrication flexibility in optical systems. The optical characteristics of glasses in the UV are important when considering photolithographic systems containing refractive elements. Optical absorption and luminescence can be caused by a lack of stoichiometry in the fused silica molecular matrix.

This chapter addresses general trends and examples of the evolution in advanced lithography methods beyond traditional single patterning technology. The ability to print multiple exposure passes to meet performance targets in a cost-effective manner has driven the search for the overall best techniques in multiple patterning. The need for continuous device scaling has significantly driven the development of new lithographic patterning techniques and imaging methods. The patterning technique chosen to print a given lithographic layer can have significant impact on the final design style, and vice versa. Considering all aspects and options to imaging approaches, layout decomposition, materials selection, integration scheme, and target performance in multiple patterning schemes, one can conclude that a significantly large number of possible forms of multiple patterning can exist. Integrated patterning essentially consists of forming feature edges by indirect methods. Multiple patterning beyond doubling can offer resolution improvement but at a cost of process complexity and control.

As the extreme ultraviolet (EUV) lithography technology progresses towards and below sub-7nm generations, polarization effects will begin to have an impact. As numerical apertures increase, the consequences at both the mask and the wafer plane need to be understood. Contrast losses at large angles can occur from non-ideal interference at the wafer plane (i.e. TM vs. TE polarization). While such loss of infidelity can be low for near wavelength half-pitch generations, additional image degradation can be attributed to polarization effects for higher resolution generations. Some of this arises from the mask. The polarized reflectance from a EUV photomask is influenced by the multilayer reflective stack as well as the polarizing effect of the patterned features. This paper explores the polarization effects that are induced by EUV masks for sub-7nm lithography generations. From the results, it was found that there is polarization-dependent induction and attenuation of current in EUV mask structures as mask pitch decreases.

Next-generation EUV lithography systems will use anamorphic optics to achieve high-NA. The well-known Zernike circle polynomials do not describe the sixteen primary aberrations of these anamorphic optical systems though. We propose to use a basis which does describe the primary aberrations. We examine the properties of this new basis and how they impact lithographic processes through analogies to isomorphic aberrations. We have developed an application to use the proposed basis in existing lithography simulators. There is an additional importance in EUVL placed on understanding how pupil variation evolves during system operation. Interferometric methods are the de facto standard of pupil phase metrology but are challenging to implement during tool use. We have previously presented an approach to measure both the pupil amplitude and phase variation of isomorphic EUVL systems from images formed by that system. We show how this methodology can be adapted to anamorphic optical systems. More specifically, we will present a set of binary metrology targets sensitive to the anamorphic primary aberrations.

Aberration characterization plays a critical role in the development of any optical system. State-of-the-art lithography systems have the tightest aberration tolerances. We present an approach to image-based pupil plane amplitude and phase characterization using models built with a space-domain basis, in which aberration effects are separable. A polynomial model is constructed between the projections of the image intensity for chosen binary mask targets onto this basis and pupil amplitude or phase variation. This method separates model building and pupil characterization into two distinct steps, thus enabling rapid pupil characterization following data collection. The basis is related to both the transmission cross-coefficient function and the principal components of the image intensity. The pupil plane variation of a zone-plate lens from the Semiconductor High-NA Actinic Reticle Review Project (SHARP) at Lawrence Berkeley National Laboratory is examined using this method. Results are compared to pupil plane characterization using a previously proposed methodology where inverse solutions are obtained through an iterative process involving least-squares regression. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)