Editor-in-Chief Harry Levinson introduces new guidelines for papers related to metrology and masks.

Starting from Weierstrass’ approximation theorem, Zernike polynomials are obtained by a few straightforward steps involving only the recast of the aberration function as a double sum in the polar coordinates followed by the weighted orthogonalization of a power series. The origin of the name Fringe Zernike polynomials is also explained.

Background: Micrometer and submicron-scale shape memory devices are typically fabricated by standard lithographic techniques or focused ion beam milling. It was previously shown that submicron-scale structures can be directly formed on metal surfaces by focused electron beam irradiation in a single fabrication step, without resist layers or etching.

Aim: Our aim was to apply the method of a focused electron beam irradiation to a surface of a shape memory material to check if the fabricated structures would display any shape memory effects.

Approach: Ni₄₀Ti₆₀ thin film, which was obtained by dual magnetron sputtering on a heated Si substrate, was irradiated by a focused electron beam in point mode. After the irradiation, thin film was heated to 100°C and irradiated points were measured by atomic force microscopy both before and after the heating.

Results: After heating, the volume of the formed structures decreased by up to 88% due to their partial reversion back to a flat surface.

Conclusions: These results present a new method of fabrication for shape memory devices. As only a one-way shape memory effect is achieved in our work, its immediate applications appear to be limited, but we hope that this work will open possibilities for new types of shape memory devices at the nanometer scale.

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).

Two earlier publications showed that mask defectivity contributes to the stochastics of the EUVL-printed image on wafer. The present contribution gives more insights into the methodology and resist models used therein. In addition, an extended study of two types of mask roughness is presented, comprising mask absorber line-edge roughness and rippling of the multilayer mirror on the mask. For both, it is shown that contributions to stochastics are larger than expected purely from normalized intensity log-slope considerations. As a second topic, printability of local defects is readdressed at smaller pitches and more state-of-the-art illumination settings, in preparation to a genuine study of mask defectivity contribution to wafer printing stochastics at high-NA EUV lithography. First results for one-dimensional mask patterns indicate an influence of the anamorphic characteristic of high-NA imaging, showing a different behavior for vertical and horizontal orientations.

Background: Explaining imaging phenomena in EUV lithography requires more than a single point of view. Traditionally, the diffraction characteristics of EUV masks are analyzed in terms of the amplitude and phase of diffraction orders that are generated by the absorber pattern.

Aim: We propose a complementary perspective to view the EUV mask absorber openings as waveguides.

Approach: Comparisons between RCWA simulations and analytical solutions to waveguide equations are performed to prove that EUV mask absorbers behave as a waveguide.

Results: This perspective can explain phenomena left unexplained by conventional analysis of far-field diffraction orders.

Conclusions: The waveguiding effect in EUV mask absorbers explains the need for low refractive index and high extinction materials. The waveguide perspective explains why attenuated phase shift masks behave differently for EUV than our traditional understanding would suggest.

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

The low-n attenuated phase-shift mask can strongly improve extreme ultraviolet imaging performance; it enhances contrast by mask 3D mitigation and a phase-shift effect while simultaneously reducing the required exposure dose. The latter happens because the low-n mask gives optimum contrast at more open mask bias values than its Ta-based counterpart. Here, we experimentally verify the imaging physics of the low-n mask. We show that optimum exposure latitude (EL) with the low-n mask is obtained at more open mask bias values compared to the Ta-based reference mask. This leads to dose reductions exceeding 30% for pitch 38-nm regular contact holes (CHs). Initial local critical dimension uniformity (LCDU) data for hexagonal CHs pitch 38 and 40 nm show 15% LCDU improvement with the low-n mask compared to the Ta-based reference. A 16-nm dense lines show a substantial EL increase and dose reduction with the low-n mask compared to the Ta-based case; this can be even further improved by combining the novel mask absorber with asymmetric illumination. As the low-n masks studied here have absolute reflectivities in the range 8% to 15%, side-lobe printing should be carefully monitored. Initial experimental data for pitch 120-nm CHs and simulations on P32 metal clips, show no signs of side-lobe printing. Careful monitoring of stochastic side-lobe printing for various use cases is recommended.

Background: EUV lithography has been introduced for semiconductor fabrication, which makes maximizing yield and throughput increasingly important. One key component is the use of a high-transmission pellicle to keep particles out of the focal plane and thereby minimize their impact on imaging. Imec initiated the development of a promising pellicle approach based on a network of carbon nanotubes (CNT), which has the advantage of many tunable structural parameters to form a pellicle membrane. A balance between membrane robustness and particle nonpermeability on one side and low EUV absorption and membrane scattering on the other must be found. The membrane scatter is important for EUV flare effects during wafer printing.

Aim: The experimental measurement of the flare must be determined as a function of the tunable CNT structural parameters. However, this measurement can be very challenging for the low-flare requirements involved.

Approach: The EUV scatter measurements on CNT-based pellicle membranes have been performed and optimized in a stand-alone irradiation setup at RWTH Aachen University. The measurement results were compared to flare simulations using a CNT cylinder model, which is used to improve the experimental measurements.

Results: With this approach, the flare of pellicles with different CNT structures and network parameters are investigated, as well as CNT pellicles that incorporate protective coatings.

Conclusion: The proposed flare measurement procedure can be used to test for acceptable scattering levels for EUV imaging applications.

The aim of our work is to investigate deposition conditions to further optimize the reflectivity of Mo/Si multilayers (MLs) for reflective coatings of extreme ultraviolet mask blanks. Dark-field transmission electron microscopy (TEM) measurements imply interfacial roughness values of 80 to 150 pm. Bright-field TEM images indicate intermixed layer thicknesses of 0.4 to 1.8 nm. We present reflectivity calculations including these two ML imperfections and compare against prior empirical results. Both interfacial roughness and intermixing are predicted to lower the maximum reflectivity. For example, interfacial roughness of 400 pm lowers the maximum reflectivity by ∼2  %  . Smoother interfaces with sub-100 pm allow recovery of ∼1.5  %   of the reflectivity. Mo/Si intermixing is predicted to lower the maximum reflectivity by up to 6% relative to an ideal ML. Reflectivity could be recovered by ∼3  %   by reducing the intermixing depth by only 20% to 30%. We demonstrate ways to reduce roughness or intermixing by ion beam deposition (IBD). Ion bombardment simulations provide estimates of the atom energy distribution arriving at the mask blank surface during Mo and Si deposition and of stopping depths of each atom into the underlying layer. Key IBD parameters to reduce the deposition energy, and hence the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure can together reduce the intermixing depth by at least 20% to 30%. Bright-field TEM measurements of MLs deposited at various deposition conditions confirm the intermixing predictions.

The availability of metrology solutions, one of the critical factors to drive leading-edge semiconductor devices and processes, has been confronted with difficulties in advanced nodes. For developing new metrology solutions, high-quality test structures fabricated at specific sizes are needed. Electron-beam direct-write lithography has been utilized to manufacture such samples. However, it can encounter significant-resolution difficulties and may require complicated process optimization in sub-10-nm nodes. Therefore, we investigate the feasibility and patterning control of metrology test structures fabricated by helium ion beam (HIB) direct milling and HIB direct-write lithography, where HIB has the sub-nm resolution in nature. Results show that features down to 5 nm are resolvable without any resolution enhancement technique by HIB direct milling. For HIB direct-write lithography, features down to International Roadmap for Devices and Systems 1.5-nm node are also resolvable without optimization from the lithography simulation. Furthermore, patterns beyond the 1.5-nm node can be achievable with the help of the proximity effect correction technique. Preliminary results demonstrate that HIB direct milling and HIB direct-write lithography can be a promising alternative for fabricating pit-type programmed defects (PDs) and bump-type PDs, respectively. In conclusion, HIB is suggested to be a potential tool to fabricated test structures for developing advanced metrology solutions in sub-7-nm nodes.

An etching process is demonstrated for removing noble metal from microstructures to restore their original function after being characterized by scanning electron microscopy (SEM). Using neither aggressive acids nor high temperatures, the etching method gently removes gold/palladium alloys from complex three-dimensional microstructures, preserving their structural form. To explore the efficacy of the etching process, polymeric photonic crystals and monolithic microstructures were fabricated, metal coated, etched, and then structurally and optically characterized. Metal coating substantially diminishes the optical functional and transmission of the microstructures. SEM imaging performed throughout a series of metal sputtering, etching, and resputtering shows that the etching process does not significantly alter the form of a microstructure. Measurements of optical transmission using a scanned-optical-fiber system confirm that the etchant removes the metal and restores the optical properties of the microstructures.

Extreme ultraviolet (EUV) pellicles are in high demand for improving the yield of EUV lithography. However, when the EUV pellicle membrane is destroyed inside the scanner, machine availability is significantly affected. The deflection of EUV pellicle membranes needs to be thoroughly studied, because when the membrane deflects beyond its limit, the membrane contacts the scanner components and leads to failure. To propose guidelines for sustaining the EUV pellicle membrane during lithography, the deflection of the EUV pellicle membrane with a range of mechanical properties should be investigated. We verified the feasibility of the finite element method (FEM) analysis by comparing the analysis results with the experimental results. Subsequently, the impact of the mechanical properties on the deflection of a full-sized (110  mm  ×  143  mm) membrane was investigated using the FEM analysis. The residual stress showed 6.28 and 2.9 times higher impact on the deflection compared to the Poisson’s ratio and elastic modulus, respectively. Finally, the deflection results for 84 different mechanical properties are presented. The residual stress was determined to be a crucial and controllable parameter. The presented guidelines can be used as a design rule for developing EUV pellicle membranes.

Background: The metal-oxide resist (MOR) is a promising type of nonchemically amplified resist (CAR) for EUV lithography. This family of resists shows some advantages over the conventional CARs. Even though a prior MOR model exists, no documented references for the application described could be found in open literature to develop a physically rational lithographic model that can accurately simulate and predict lithographic results for these resists.

Aim: Increase the fundamental understanding of this class of resists by creating a model that also considers these aspects of the resist.

Approach: Model the metal-oxo clusters of the MOR as nanoparticles with an effective radius that operate under excluded volume.

Results: We show the possibility to include both the packing noise effect, as well as accurate roughness (characteristics) predictions by utilizing power spectral density (PSD) plots.

Conclusions: Varying the calibrated model parameters has a clear effect on the overall roughness of the resist lines and is reflected in the PSD behavior. In contrast to experimental data, changing the resist film thickness did not result in a change in PSD behavior.

Extreme ultraviolet lithography (EUVL) has been adopted into high volume production for advanced logic device manufacturing. Due to the continuous size scaling requirement for interconnect fabrication, EUVL with self-aligned double patterning (SADP) formation has attracted substantial research attention. The current challenge in EUV SADP is the pattern transfer process from lithography to mandrel formation. In this step, the target critical dimension (CD) of the feature needs to shrink by half from the lithography CD during the etch process. The increasing aspect ratio during this etch potentially deteriorates the pattern validity and the line edge roughness (LER). In addition to these challenges, EUVL has a fundamental bottleneck due to stochastic effects, which can lead to device degradation by defect formation and edge placement error (EPE). LER of the line and space pattern is one of the main contributors to EPE. Effective methods of LER reduction in both process and integration are needed in order to reduce pattern variation and boost device performance. In our study, we examine a technique to reduce LER on the EUV SADP line pattern. This technique involves the surface modification on the spin-on carbon (SOC) layer in the patterning stack and tone inversion process. We had found a trend between surface hydrophobicity of the SOC and the EUV SADP LER performance. The condition that increased the hydrophobicity of the SOC resulted in a lower LER performance after tone inversion. The tested conditions include direct current superposition (DCS) function with H₂ plasma, fluorocarbon plasma, and the combination of DCS with H₂ plasma and trimethylsilane dimethylamine deposition. On 20-nm pitch EUV SADP, this technique shows 26% of LER improvement from lithography to SADP formation. PSD analysis recorded about 6% and 30% of the LER improvement at the correlation length of >200  nm and 200 to 30 nm, respectively. A demonstration of this technique for a further scaling to 15-nm pitch also shows an LER reduction of 30% from lithography to SADP formation.