Diffraction of light limits single exposure projection lithography of periodic arrays of line-space (L/S) patterns to half-pitches (hps) of $k_1\lambda /\mathrm{NA}$. The technology factor $k_1$ depends on the used illumination, mask, photoresist, and on processing conditions. The introduction of extreme ultraviolet (EUV) projection lithography with a wavelength $\lambda =13.5\mathrm{nm}$ and a numerical aperture NA of 0.33 enables the printing of 20-nm hp patterns with a relaxed technology factors $k_1\approx 0.5$. Next-generation EUV systems with numerical apertures of 0.55 and more aggressive $k_1<0.35$ can provide sub-8-nm hp resolution.¹ Comprehensive optimization of all components of the lithographic system is required to push high NA EUV lithography to its physical limits. The increased importance of stochastic effects^2,3 at smaller feature sizes places further demands on scanner and mask to provide high contrast images. Diffraction of light from three-dimensional (3D) features on the mask introduces several important mask 3D effects^{4 – 6} that impact the quality of the imaging, especially the image contrast. This paper employs rigorous mask diffraction and imaging simulation to understand the impact of the EUV mask absorber and to identify the most appropriate optical parameters for high NA EUV imaging.

Extreme ultraviolet (EUV) lithography is being industrialized as the next candidate printing technique for high-volume manufacturing of scaled down integrated circuits. At mask level, the combination of EUV light at oblique incidence, absorber thickness, and non-uniform mirror reflectance through incidence angle, creates photomask-induced imaging aberrations, known as mask 3D (M3D) effects. A possible mitigation for the M3D effects in the EUV binary intensity mask (BIM), is to use mask absorber materials with high extinction coefficient κ and refractive coefficient n close to unity. We propose nickel aluminide alloys as a candidate BIM absorber material, and characterize them versus a set of specifications that a novel EUV mask absorber must meet. The nickel aluminide samples have reduced crystallinity as compared to metallic nickel, and form a passivating surface oxide layer in neutral solutions. Composition and density profile are investigated to estimate the optical constants, which are then validated with EUV reflectometry. An oxidation-induced Al L2 absorption edge shift is observed, which significantly impacts the value of n at 13.5 nm wavelength and moves it closer to unity. The measured optical constants are incorporated in an accurate mask model for rigorous simulations. The M3D imaging impact of the nickel aluminide alloy mask absorbers, which predict significant M3D reduction in comparison to reference absorber materials. In this paper, we present an extensive experimental methodology flow to evaluate candidate mask absorber materials.

The determination of fundamental optical parameters is essential for the development of new optical elements such as mirrors, gratings, or photomasks. Especially in the extreme ultraviolet (EUV) and soft x-ray spectral range, the existing databases for the refractive indices of many materials and compositions are insufficient or are a mixture of experimentally measured and calculated values from atomic scattering factors. Since the physical properties of bulk materials and thin films with thicknesses in the nanometer range are not identical, measurements need to be performed on thin layers. In this study we demonstrate how optical constants of various thin film samples on a bulk substrate can be determined from reflection measurements in the EUV photon energy range from 62 eV to 124 eV. Thin films with thickness of 20 nm to 50 nm of pure Mo, Ni, Pt, Ru, Ta, and Te and different compositions of N i x A l x , PtTe, P t x M o , R u x T a x , R u 3 R e , R u 2 W , and TaTeN were prepared by DC magnetron sputtering and measured using EUV reflectometry. The determination optical constants of the different materials are discussed and compared to existing tabulated values.

BackgroundTo print ever smaller features at high contrast projection lithography technology has evolved to shorter wavelength light and larger numerical aperture (NA). After enabling the extreme-ultraviolet (EUV) wavelength, the industry is looking into increasing the NA. At NA’s much higher than 0.55, new effects such as polarization will start to 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 beyond 0.55 will exacerbate the contrast loss. In addition, the larger incidence angles on mask when the NA increases above 0.55, will further enhance the mask 3D (M3D) effects forcing additional mask changes.AimStudy the potential imaging challenges regarding NA scaling beyond 0.55 and identify if the imaging can still work at this higher NA and whether specific changes to the mask stack are required.ApproachWe study the polarization effects in detail and assess their impact quantitatively for a set of generic building blocks. 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.ResultsThe current periodic Mo/Si multilayer concept is adequate to support 0.75 NA performance. For NA 0.85 aperiodic multilayers would be beneficial. Novel mask absorber materials currently being developed for the 0.33 and 0.55 NA systems are adequate to support 0.75 NA imaging. M3D phase effects slightly increase with NA but can be compensated by adjusting absorber n / k and thickness. Regions of interest in the absorber space remain low-n and high-k. NA 0.75 promises significant normalized image log slope (NILS) gain over NA 0.55 in the L/S pitch range of 20 to 30 nm even when considering M3D effects, existing multilayer periodicity, and no polarizer. For contact holes (CHs), NILS > 2.4 without polarizer can be achieved in both dark and bright mask tonalities for pitches 19 nm and above, even when considering M3D effects, existing multilayer periodicity, and no polarizer. Even if polarization control could provide significant NILS gain for lines and spaces, EUV polarizer may not be worth it in terms of the balance between contrast and throughput. Further validation should be done using a full stochastic resist model in the future. For CHs, the NILS gain possible by polarization is too small to warrant the lost dose.ConclusionsWe identified clear advantages of high NA imaging but found no significant blocking factors for the imaging of the investigated patterns (single pitch lines/spaces and CHs).

Over the recent years, extreme ultraviolet (EUV) lithography has demonstrated the patterning of ever-shrinking feature sizes (enabling the N7 technology node and below), whereas the EUV mask has remained unaltered, using a 70-nm tantalum (Ta)-based absorber. This has led to experimentally observed mask three-dimensional (M3D) effects at the wafer level, which are induced by the interaction between the oblique incident EUV light and the patterned absorber with typical thickness values on the order of several wavelengths. We exploit the optical properties of the absorber material of the EUV mask as an M3D mitigation strategy. Using rigorous lithographic simulations, we screen potential single-element absorber materials for their optical properties and optimal thickness for minimum best focus variation through pitch at the wafer level. In addition, the M3D mitigation by absorber material is evaluated by process window comparison of foundry N5-specific logic clips. To validate the rigorous simulation predictions and test the processing feasibility of the alternative absorber materials, we have selected the candidate single elements nickel and cobalt for an experimental evaluation on wafer substrates. We present the film characterization as well as the first patterning tests of these single-element candidate absorber materials.

<!-- *** Custom HTML *** --> 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).