JM3 Editor-in-Chief Harry Levinson discusses the importance of written technical publications.

JM3 Associate Editor Emily Gallagher, a principal researcher at imec, interviews Andreas Erdmann, head of the Fraunhofer IISB Computational Lithography and Optics Group. With Hazem Mesilhy and Peter Evanschitzky, Erdmann is the lead author of “ Attenuated phase shift masks: a wild card resolution enhancement for extreme ultraviolet lithography?,” a review paper in the April-June 2022 issue of the Journal of Micro/Nanopatterning, Materials, and Metrology.

Resolution and depth-of-focus (DOF) formulas in projection imaging are reviewed with the help of the three-dimensional point-spread function (3D PSF). The Debye integral is rigorously evaluated for the non-paraxial situation to give a closed-form expression for the axial 3D PSF from which the validity of DOF formula k₃λ / sin² ( θ / 2 ) is confirmed.

Special section editors Anuja De Silva and Yasin Ekinci introduce the Special Section on Non-chemically Amplified Resists for EUV Lithography.

Background: Metal-containing resists entered the mainstream semiconductor industry process flow to mitigate the low absorbance of extreme ultraviolet (EUV) radiation by thin films of organic resists that lead to poor sensitivity and their inability to handle rigors of development and etching conditions.

Aim: The long and rich history of using metal-containing resists in electron beam lithography can offer interesting lessons, pointers, and insights to the relatively newcomer EUV lithography, which is slightly over a decade old.

Approach: Electron beam lithography has been enjoying a considerable amount of freedom in the choice of resist materials for close to 50 years; especially the use of metal-containing resists to attain not only single digit nanometer resolution, higher sensitivity, and etch resistance but also lower line-edge roughness. Here, we make a comprehensive historical review of the progress made in the patterning of metal-containing resists in electron beam lithography and derive insights that can be potentially useful in EUV patterning.

Perspectives: Small molecular weight resists are proven to be crucial for achieving higher resolution with low line-edge roughness. Simplifying process flow by reducing etch-stack-layers is conceivable with metal-containing resists, along with direct-patterning of functional materials for heterogeneous integration. Efficient contact hole patterning at tighter pitches may be incumbent on progress in positive-tone resist research.

Background: Non-chemically amplified resist (n-CAR) shows great potential as a unique lithographic material because it avoids some disadvantages of CAR, such as post-exposure instability and acid diffusion. Furthermore, since toxic and flammable developers are widely used in semiconductor manufacturing, the implementation of innovative environmentally friendly water-developable photoresists is of interest.

Aim: A unique n-CAR, which could be developed with an environmentally friendly developer, was prepared for electron beam (e-beam) lithography (EBL) and extreme ultraviolet lithography (EUVL).

Approach: A polymer containing radiation/photosensitive sulfonium triflate group (PSSF) was synthesized and characterized by infrared, H1 NMR, and gel permeation chromatography. The lithography performance of the PSSF photoresist was evaluated by EBL and EUVL. The patterns were analyzed with scanning electron microscope and atomic force microscope.

Results: The PSSF photoresist can be used in EBL and EUVL. Post-exposure bake had no significant effect on the resolution of photoresist. Development in water should be kept at an appropriate time of 30 s to obtain the repeatable and high-resolution patterns. It shows a similar sensitivity to polymethyl methacrylate but higher contrast.

Conclusions: The PSSF acts as n-CAR, and 20 nm line patterns and 35 nm 1:1 line/space patterns were achieved in EBL and EUVL, respectively. It can be developed in pure water with high contrast (γ  =  5.49).

A new class of negative-tone resist materials has been developed for electron beam and extreme ultraviolet lithography. The resist is based on heterometallic rings. From initial electron beam lithography studies, the resist performance demonstrated a resolution of 40-nm pitch but at the expense of a low sensitivity. To improve the sensitivity, we incorporated HgCl₂ and HgI₂ into the resist molecular design. This dramatically improved the resist sensitivity while maintaining high resolution. This improvement was demonstrated using electron beam and extreme ultraviolet lithography.

Metal oxide photoresists (MORs) have received attention for use in extreme ultraviolet (EUV) lithographic patterning, in which more established chemically amplified resists suffer from long blur lengths and low EUV absorptivity. To meet the demand for next-generation semiconductor devices, a thorough understanding of the physics behind MOR patterning is required to predict the outcome of arbitrary patterning schemes and to determine optimal process conditions. However, the ability to model and predict MORs through bake and development is still in early stages. We provide a modeling framework and associated analytical expressions that describe the behavior of MORs that have so far been described by kinetic Monte Carlo simulation approaches. We further extend the aforementioned analytical expressions to directly predict dose-to-gel transition energies for wet-developed MORs for arbitrary bake temperature and bake duration. Finally, we briefly compare our methodology using an in-house simulation program to patterning results from an experiment using Fractilia MetroLER™.

A negative tone heterometallic ring resist (HRR) based on a supramolecular assembly [ NH₂(allyl)₂ ] [ Cr₇NiF₈(piv)₁₆ ] with previously demonstrated resolution down to sub 10-nm lines is evaluated in terms of its flexibility to be processed either “wet” (spin cast and solvent developed) or “dry” (deposition and development by vacuum sublimation). The implemented sublimation hardware fits easily in the wafer load-lock chamber of extreme ultraviolet and electron beam exposure systems dedicated to research and development activities and allows for HRR films to be uniformly deposited or developed in the same vacuum environment. The HRR shows a sublimation rate dependence on temperature that obeys a Clausius–Clapeyron relation, with thermal stability up to 275°C. Flood exposures of the HRR show identical sensitivity between wet- and dry-deposited films, whereas contrast degradation is observed when dry development is initiated by increasing the temperature prior to system pump down. A modified sublimation setup allows for the dry development of exposed HRR samples inside the electron beam tool without breaking vacuum. In this case, nominally patterned 25 nm L/S are identically resolved at 30 keV for wet- or dry-developed HRR.

Implementation of extreme ultraviolet (EUV) lithography in high-volume semiconductor manufacturing requires a reliable and scalable EUV resist platform. A mechanistic understanding of the pros and cons of different EUV resist materials is critically important. However, most material characterization methods with nanometer resolution use an x-ray photon or electron beam as the probe, which often cause damage to the photoresist film during measurement. Here, we illustrated the use of non-destructive infrared nanospectroscopy [or nano-Fourier-transform infrared spectroscopy (nano-FTIR)] to obtain spatially resolved composition information in patterned photoresist films. Clear evidence of exposure-induced chemical modification was observed at a spatial resolution down to 40 nm, well below the diffraction limit of infrared light. With improvements, such a nano-FTIR technique with nanoscale spatial resolution, chemical sensitivity, and minimal radiation damage can be a promising candidate for the fundamental study of material properties relevant to EUV lithography.

Guest Editors Manufacturing Data Analytics Bertrand Le-Gratiet and Serap A. Savari introduce the Special Section on Manufacturing Data Analytics.

On-product overlay (OPO) is an important indicator of device yield. In this work, we show that stressed thin films used in semiconductor manufacturing can be an important contributor to OPO at multiple length scales. Depending on the stress level, film thickness, and the mask design, the overlay impact can be a few nanometers for the exposure of the next lithography layer. A predictive compact model based on pattern density is developed to accurately predict this overlay impact. The model is then verified using short-loop dual damascene wafers with stress split. The predictive model opens a new opportunity for model-based mask correction during optical proximity correction to increase the overlay margin for subsequent lithography exposures.

Extending 0.33 NA extreme ultraviolet single patterning to 28-nm pitch becomes challenging in stochastic defectivity, which demands high-contrast lithographic images. The low-n attenuated phase-shift mask (attPSM) can provide superior solutions for individual pitches by mitigating mask three-dimensional effects. The simulation and experiment results have shown substantial imaging improvements: higher depth of focus at similar normalized image log slope and smaller telecentricity error values than the best binary mask configuration. In this work, the exploration of low-n attPSM patterning opportunity for pitch 28-nm metal design is investigated. Using generic building block features, the lithographic performance of the low-n attPSM is compared with the standard binary Ta-based absorber mask. In addition, the impact of mask tone (bright field (BF) versus dark field) on the pattern fidelity and process window is evaluated both by simulations and experiments. The results indicate that BF low-n attPSM provides the best patterning performance. Consequently, the BF low-n attPSM patterning performance is assessed with an actual imec N3 pitch 28-nm random logic metal design. The wafer data indicate BF low-n attPSM enables good patterning fidelity, as well as good overall process window with high exposure latitude (∼20 % ).

The development of optical lithographic technology made important contributions to miniaturization. In optical lithography, it is critical to maintain high uniformity and high resolution of patterning on a silicon substrate by exposing the substrate to ultraviolet (UV) light. However, lens contamination limits the uniformity of the exposed UV light, and the effect of lens contamination on the critical dimension is increasing as electronic devices become smaller. Lens contamination can be generated by turbulence of clean air (CA) and it gradually accelerates over time. In this work, we suggest the extreme clean dry air (XCDA) shield system to reduce lens contamination in optical lithography. We have measured and analyzed the contaminants of a practical lithography lens through time-of-flight secondary mass spectrometry, and the expected contamination mechanism is also shown. Also, we simulated turbulence of CA in practical lithography based on the shield k-epsilon turbulence model. Turbulence simulations not only quantitatively showed the effects of XCDA and CA on lens contamination but also demonstrated that the lens could be directly purged with the XCDA shield system to prevent turbulence of CA. The XCDA shield system reduced the degree of contamination by 33% compared with the conventional level. We believe that the proposed system will provide high efficiency in the optical lithography industry.

Characterizing chemical changes in photoresists during fabrication processes is critical to understanding how nanometric defects contribute to film stochastics. We used nanoprojectile secondary ion mass spectrometry (NP-SIMS) to evaluate the nanoscale homogeneity of components in positive-tone extreme ultraviolet resists. NP-SIMS was operated in the event-by-event bombardment/detection mode, where a suite of individual gold nanoprojectiles separated in time and space stochastically bombard the surface. Each impact ejects secondary ions from a volume 10 to 15 nm in diameter and up to 10 nm in depth allowing for analysis of colocalized moieties with high spatial resolution. Individual partially exposed extreme ultraviolet resists were analyzed after light exposure, postexposure bake, and development. Results showed an expected increase in protonated quencher versus exposure dose, while after development, we observed increased abundance in the remaining film. The latter, we attribute to poor solubility in the developing solvent. Examining the photoacid generator (PAG), we found decreased PAG cation abundance versus exposure dose in the exposed films, likely due to photodecomposition of the PAG cation. Moreover, after the development, we observed decreased homogeneity of PAG ions, which we attribute to preferential extraction caused by ion-exchange interactions with the developer. We found that the insoluble moieties persisting on the surface after the development were relatively rich in the protecting group, likely due to uneven deprotection of the polymer. Overall, NP-SIMS allows to characterize the resist at the nanoscale and identify conditions that lead to defect formation.

Here, we present a methodology for identifying and characterizing nanoscale sites in extreme ultraviolet (EUV) photoresists that deviate from the mean composition by 3σ. The methodology is based on nano-projectile secondary ion mass spectrometry (SIMS) operating in the event-by-event bombardment detection mode. Nanoscale analysis is achieved by probing the surface stochastically with a suite of individual nanoprojectile impacts in which each nano-projectile samples a volume that is 10 to 15 nm in diameter and up to 10 nm in depth. For each impact, the coemitted secondary ions are collected and mass-analyzed, allowing for the analysis of colocalized moieties. We applied this technique to study the fundamental processes occurring in partially developed positive-tone EUV resists, which simulate a critical problem in EUV resists, incomplete resist removal, and production of line edge features. Such features erode device yield and have been the focus of many previous studies. Using NP-SIMS, we examined the changing molecular composition in the partially developed resist and isolated measurements with a probability below 0.3%. Grouping measurements based on the number and type of detected molecular species allowed for the identification of rare sites on the surface that deviate from the mean composition. The mass spectrometry measurements showed that both the photoacid generator (PAG) cation and anion displayed decreased homogeneity on average with increasing exposure dose. The effect was more pronounced in the sites with probabilities below 0.3%, where the measured intensity of the PAG-related ions in these sites was more than twofold larger than the mean. Thus, we attribute these nanoscale sites to aggregations of PAG within the top 10 nm of the remaining film. These results suggest that identifying and characterizing the molecular composition of rare sites may be important in defect production and film stochastics.

With the introduction of high-numerical aperture (NA) extreme ultraviolet (EUV) lithography, the thickness of layers in the lithographic stack will scale owing to reduced depth of focus and etch budget. The consequence of thinning down underlayers for EUV lithography has been scarcely investigated. In here, we assessed the readiness of nine state-of-the-art underlayers, spin-on and dry deposited, in thickness series down to 4 nm nominal. Preliminarily, the coating quality of these underlayers was evaluated. Thickness uniformity across 300 mm wafer ranged from about ±0.5 nm to < ± 0.05 nm depending on the coating technique employed. Surface roughness of the underlayers varied from as much as 0.63 nm to as low as 0.062 nm but was not impacted by thickness scaling. Film density and total surface energy varied by <10 % with thickness. EUV lithography of dense lines/spaces arrays of pitch 28 nm was carried out using a positive tone chemically amplified photoresist. Dose-to-size and exposure latitude changed by < ± 5 % when thickness of underlayer was decreased. Failure free process was at most 1 nm smaller for thinner underlayer than it was for the thinnest version of each type. The unbiased linewidth roughness increased consistently but limitedly (<5 % ) when thinner underlayers were used, mainly due to a reduction in the correlation length. By calculating the power spectral density of the blanket underlayer we can pinpoint this effect to a reduction of correlation length of the underlayer own surface roughness. Finally, Z-factor calculations demonstrated that overall photoresist performance depended more significantly on the specific underlayer type (±12.6 % ) than it did on underlayer thickness (±8 % ). All these results indicate that most of underlayers investigated had limited impact on the properties as well as the patterning performance when scaling in view of high NA EUV.