Devesh Thakare

The imaging performance of extreme ultraviolet (EUV) lithography is determined by the optical properties and thickness of the photomask absorber material and the illumination source shape. Optimizing the trade-offs between imaging metrics, such as normalized image log slope, telecentricity error, and best focus variation through pitch (collectively known as mask-3-dimensional (M3D) effects), is crucial to improve the throughput of the EUV lithography process. This study aims to optimize Ru/Ta bilayer photomask absorber stacks and illumination source shapes to mitigate M3D effects using mask diffraction analysis. It intends to raise questions about the conventional absorber reflectivity or induced phase shift-based approach.

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.

BackgroundThe optical properties of the absorber material and its thickness, along with the choice of illumination, govern the imaging performance of an EUV photomask. Consequently, the imaging metrics, viz., normalized image log slope (NILS), telecentricity error (TCE), and best focus variation (BFV) through pitch, exhibit a trade-off. Previous studies have focused on either reflectivity or phase shift induced by the absorber to determine the optimum absorber thickness, which ignores the structure of the patterns on the mask.AimThis simulation study intends to facilitate the optimum absorber thickness selection with an emphasis on diffraction order analysis of the line and space (LnS) pattern and investigates the impact of illumination source shapes for a Ta-Co alloy absorber mask.ApproachWe investigate the behavior of imaging metrics as a function of the absorber thickness using literature-recommended illumination source shapes. NILS, TCE, throughput criterion, diffraction order’s amplitude ratio, and phase difference are the critical parameters when selecting the optimum absorber thickness. A through-pitch imaging performance of the Ta-Co alloy absorber with recommended thicknesses is compared with the reference 60 nm Ta-based absorber using leaf dipole (LDP), inner half leaf dipole (IHLDP), and outer half leaf dipole (OHLDP) for LnS patterns with a target trench of 10 nm and pitch ranging from 20 to 40 nm.ResultsThe proposed absorber thickness optimization methodology validates the reference Ta-based absorber thickness and recommends 52 nm of Ta-Co alloy absorber thickness. IHLDP and OHLDP demonstrate enhanced NILS for the smallest pitch. However, this behavior is inconsistent throughout the pitch. IHLDP exhibits low TCE. However, because of higher BFV, the advantages are not exceptionally beneficial.ConclusionsA holistic approach makes the proposed absorber thickness optimization methodology reliable and generally applicable. The through-pitch simulation outcomes at selected absorber thickness favor a conventional full-leaf dipole for the studied imaging metrics.

BackgroundA plausible approach for mitigating the mask 3-D (M3D) effects observed in extreme ultraviolet (EUV) lithography is to replace the existing mask absorber with alternative materials. Absorbers with a high EUV extinction coefficient k allow for lower best focus variation (BFV) through pitch and reduced telecentricity errors (TCEs).AimWe aim to evaluate Ta-Co alloys as potential high-k mask absorbers from material suitability and imaging standpoints.ApproachWe study the film morphology, surface composition, and stability of Ta-Co alloys in mask cleaning solutions and a hydrogen environment as present in the EUV scanner to assess the material suitability from an experimental aspect. Optical constants for three selected compositions, viz., TaCo, Ta2Co, and TaCo3, were determined from EUV angle-dependent reflectivity measurements. Next, utilizing rigorous simulation software, the imaging performance of Ta-Co alloys is evaluated and compared with the reference absorber. The recommended absorber thickness for Ta-Co alloy absorbers is based upon normalized image log slope (NILS) enhancement, threshold to size, and balancing of diffraction order amplitudes. A 10 nm line and space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm are used for the simulation study using high numerical aperture 0.55 EUV lithography process settings. The primary imaging metrics for through pitch evaluation include NILS, TCE, and BFV.ResultsThe Ta-Co alloys exhibit a higher EUV extinction coefficient k compared with the currently used Ta-based absorber. TaCo and Ta2Co demonstrate smooth surfaces and are stable in a hydrogen environment and in mask-cleaning solutions.ConclusionTa-Co alloys allow for a reduction in M3D effects at a lower absorber thickness compared with a 60 nm Ta-based reference absorber.

The interaction of matter and light can be described based on optical constants, shortly called δ&β. These constants provide the fundamental basis for the design of any optical system. In the Extreme Ultraviolet (EUV) spectral range, however, the existing data for many materials or compounds is very sparse, non-existent or exhibit considerable discrepancies between different sources. This is further complicated since the scaling effects stipulate the optical response of a thin film to differ from bulk. Oxidation, impurities or interdiffusion significantly affect the optical response of a system to EUV radiation. For this reason, the Physikalisch-Technische Bundesanstalt (PTB) is establishing a new database in cooperation with other European partners. This database, designated as the Optical Constants Database (OCDB) can be accessed online freely (OCDB.ptb.de). This data collection shall support further development of various fields from new metrological techniques, like EUV scatterometry to computational lithography in the EUV. This is demonstrated exemplarily here by the interplay between δ&β and the dimensional parameters with respect to a structured TaTeN EUV photomask. It is equally important either direction, to derive structure parameters from the measured EUV scattering as vice versa to predict the EUV response from the geometrical structure. In addition, the impact of varying δ and β on the expected imaging performance will be investigated by simulating typical lithographic image metrics like Critical Dimension (CD), best focus position, image contrast (NILS) and non-telecentricity for the imaging of through pitch L/S and 16 nm vertical Lines with 32 nm pitch in a NA=0.55 scanner for TaTeN mask absorber as typical representatives of high-k absorber materialsand as an example of the effect on imaging simulation.

The imaging performance of a mask in EUV lithography is governed by the optical properties of the absorber material, namely the refractive index n and extinction coefficient k, and by its thickness. The imaging metrics viz. Normalized Intensity Log Slope (NILS), Telecentricity Error (TCE) and Best Focus Variation (BFV) through pitch, exhibit a tradeoff. In addition, the choice of illumination has a significant influence on these imaging metrics. Most of the previous studies have focused on either reflectivity or phase shift induced by the absorber to determine the optimum absorber thickness. The limitation of this approach is that the structure of the patterns on the mask is ignored. This simulation study is intended to facilitate the selection of the optimum absorber thickness with an emphasis on diffraction order analysis and the impact of illumination source shape using a case study of TaCo alloy. The behavior of imaging metrics is investigated as a function of absorber thickness in combination with illumination source shapes recommended in the literature. Maximal NILS, TCE within specified limits, balancing of diffraction order amplitudes with a minimum phase difference, and throughput criterion, are the important parameters that are considered when selecting the optimum absorber thickness. We evaluate and compare the through pitch imaging performance of TaCo alloy with recommended thicknesses, to that of the reference TaBN 60 nm absorber using Leaf shape Dipole (LDP), Inner Half Leaf shape Dipole (IHLDP) and Outer Half Leaf shape Dipole (OHLDP) for Line and Space (LnS) pattern with trench width of 10nm and the smallest pitch of 20 nm. The study confirms that TaCo alloy exhibits improved NILS and lower BFV compared to the reference TaBN 60 nm absorber.

An alternate mask absorber is a generally applicable approach as a mitigation strategy for Mask 3-D effects (M3D) observed in EUV lithography. It is also an efficient solution in a production-worthy environment compared to multilayer modification or Source Mask Optimization (SMO) techniques. Absorbers with a high EUV extinction coefficient k allow for lower Best Focus Variation (BFV) through pitch and reduced Telecentricity Errors (TCE). This study evaluates Ta-Co alloys as potential high-k mask absorbers. It includes an experimental study of film morphology, surface composition, and stability of Ta-Co alloys as well as a theoretical investigation of the imaging performance. The optical constants were determined from EUV angle-dependent reflectivity measurements for three selected compositions, viz. TaCo, Ta₂Co, and TaCo₃. The Ta-Co alloys exhibit a higher EUV extinction coefficient k compared to the currently used TaBN absorber. TaCo and Ta₂Co demonstrated smooth surfaces, were stable in a hydrogen environment, and in mask cleaning solutions. These qualified compositions of Ta-Co alloys were selected for aerial image simulations and compared with a TaBN absorber. The optimized absorber thickness for Ta-Co alloy absorbers is based upon NILS enhancement, Threshold to Size (TtS) and balancing of diffraction order amplitudes. A 10 nm alternate line-space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm were considered for the simulation study using High NA 0.55 EUV lithography process settings. The through pitch imaging performance was evaluated using NILS, TCE and BFV as metrics. Ta-Co alloys allow for a reduction in M3D effects at smaller absorber thickness compared to a 60 nm TaBN absorber.

Novel reticle absorber materials are required for high-NA EUV lithography. TNO and ASML developed an assessment for the compatibility of novel high-NA reticle absorber materials with conditions that mimic the EUV scanner environment^[1]. Four candidate reticle absorber materials were evaluated, TaCo, RuTa, PtMo and Pt2Mo alloys, in a joint research program. For the compatibility tests, dedicated samples with silicon wafer substrates were fabricated. The silicon wafers were coated with a Mo-Si multilayer coating, followed by a Ru capping layer and finally the absorber material. Chemical outgassing tests, in presence of hydrogen radicals, did not show chemical outgassing for the TaCo and PtMo alloys. RuTa and Pt₂Mo alloys were not tested, based upon their elemental composition chemical outgassing is not expected. Next, all four materials were exposed in a hydrogen plasma resistance test equivalent to an EUV exposure of at least 250 thousand wafers** in the EUV scanner. No plasma-induced defects, like blistering or delamination, were found that were related to the intrinsic absorber properties. The RuTa and PtMo alloys were selected for EUV exposure in the EBL2 facility at TNO. Both materials were exposed to an 9.6 kJ/mm² EUV peak dose at an EUV peak intensity of 450 mW/mm² in a hydrogen environment. This EBL2 EUV exposure of 6 hours represents about 1-2 months of EUV dose (at least 150 thousand wafers) in a NXE or EXE scanner with a 300 W source. Both materials showed good performance during EUV exposure. Relevant surface defects and chemical outgassing were not observed. The few observed blisters in the low EUV intensity areas are likely provoked by particle contamination or coating defects. All four absorber materials, TaCo, RuTa, PtMo and Pt₂Mo alloys performed well in the compatibility tests that were executed. Not all compatibility tests could be performed on each absorber material within the scope of our research program. Further testing would be needed to complete the compatibility assessment, including an EUV exposure on a patterned reticle.

Novel mask absorber designs are catching the attention of the EUVL community due to their ability to mitigate mask 3D effects. Material selection is part of such an optimization. We propose several candidates as novel EUV lithography mask absorbers, namely TaTeN, Ru–Ta, and Pt–Mo alloys. The choice of these materials is based on their theoretical performance evaluated by EUV imaging simulation based on their complex refractive index N ( λ ) = n ( λ ) + ik ( λ ) , where the optical constants n and k relate to the phase velocity and the absorption of electromagnetic radiation with a wavelength λ, respectively. The materials are deposited as thin films on Si substrate with an additional Ru layer to mimic the cap of the multilayer mirror on the real mask. The experimental n and k values are determined by analyzing EUV reflectivity data obtained using a 13.5-nm synchrotron EUV radiation. The imaging simulation presented consists of calculating several imaging metrics including non-telecentricity, normalized image log-slope, and threshold-to-size for specific use cases using the novel absorber. It also compares the proposed materials with the reference TaBN absorber. TaTeN shows higher absorption than TaBN and refraction closer to 1, which improves phase matching for a high k absorber. The refractive index of Ru–Ta and Pt–Mo alloys exhibits a large difference from that of air and provides the required phase shift of attenuated phase shift masks. The characterizations of these materials target the requirements of an EUVL mask: durability for mask cleaning, mask lifetime, and etchability for mask patterning. The stability is first tested against several standard mask cleaning solutions by a beaker test for up to 24 h. The samples are also exposed to hydrogen plasma to imitate the working environment in an EUV scanner. Concerning material patterning, chemical reactive ion etch is applied for preliminary tests. A proper etch recipe is found for TaTeN with a good etch rate (about 60 nm / min) and good selectivity to the Ru underlayer (Ru etch is negligible).