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1. Tabletop EUV lithography system for resist characterization

   https://doi.org/10.1117/12.3072539  

   Flores, Ethan; Mohanty, Saurav; Mitchell, Richard; Chang, Chih-Hao (November 2025, SPIE)

   Ronse, Kurt G; Gargini, Paolo A; Naulleau, Patrick P; Itani, Toshiro (Ed.)

   Not AvailableAchieving higher resolution in high-density chip fabrication requires shorter wavelengths and higher-energy photons, making EUV lithography essential. As EUV becomes the standard, there is growing interest in developing photoresists with high sensitivity, contrast, and etch selectivity. However, testing these materials typically requires costly and limitedaccess EUV sources. Tabletop systems offer a more accessible alternative for accelerating resist development. This work evaluates ZEP520A, a commercially available electron beam resist, as a positive photoresist for EUV lithography at a 30 nm wavelength. Characterization was performed using a tabletop high-harmonic generation (HHG) EUV source driven by an ultrafast laser, with dose distribution estimated by modeling the elliptical beam as a 2D Gaussian. A protocol that combined confocal microscopy and atomic force microscopy enabled the extraction of key metrics using a binary resist model, which assumes a critical clearing dose. Regions exposed to doses above this threshold are completely removed during development, resulting in zero remaining thickness, while regions exposed to lower doses retain their original thickness. ZEP520A exhibited a clearing dose of 35.4 mJ/cm² and a contrast of 2.9. Additional grid tests near the clearing dose may further improve accuracy of the initial results. The developed protocol is broadly applicable for testing both positive and negative-tone resists, such as indium hydrate nitrate, and highlights the practicality of tabletop EUV systems for photoresist development. 

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2. Gas assisted electron beam patterning processes

   https://doi.org/10.13023/etd.2024.66  

   Kumar, Deepak (May 2024, University of Kentucky Libraries)

   Radiolysis is a complex phenomenon in which molecules subjected to ionizing radiation form new chemical species. Electron-beam irradiation has proven to be a versatile approach for significantly altering materials’ properties and forms the basis for electron-beam lithography using both organic and inorganic resists. Electron-beam exposure is normally carried out under high vacuum conditions to reduce contamination and allow for unhindered interaction between the electrons and the resist material. Exposure under an ambient gas at sub-atmospheric pressures has been found to provide a distinct mechanism which can be exploited to circumvent some of the challenges associated with material processing and significantly alter or enhance material’s properties. This dissertation discusses the modifications in standard electron beam resist characteristics during gas assisted electron beam pattering. We studied the effect of water vapor pressure on positive and negative tone electron-beam patterning of poly methyl methacrylate (PMMA). For both positive and negative-tone patterning, it was found that increasing the water vapor pressure considerably improved the contrast of PMMA. As expected from electron scattering in a gas, the clearing dosage for positive tone patterning gradually increased with vapor pressure. Also, electron scattering in water vapor yielded a substantially larger clear region around the negative-tone patterns. This effect could be useful for increasing the range of the developed region around cross-linked PMMA far beyond the backscattered electron range. As a result, VP-EBL for PMMA offers a new means of tuning clearing/onset dose and contrast while enabling more control over the size of the cleared region around negative-tone patterns. We provide a novel way to simultaneously tune the emission wavelength and enhance the fluorescence intensity of fluorophores formed by irradiating polystyrene with a focused electron beam under various gaseous environments. We studied the effect of electron dose and gas pressure on the emission spectra and photon yield of irradiated polystyrene film on a variety of substrates. Up to 10x enhancement in fluorescence yield was achieved using water vapor and the peak emission wavelength tuned over a wide wavelength range. Thus, localized electron-beam synthesis of fluorophores in polystyrene can be controlled by both dose and by ambient water-vapor pressure. This technique could enable innovative approaches to photonics where fluorophores with tunable emission properties can be locally introduced by electron-beam patterning. We also studied the effect of ambient gases on contrast and resolution of PMMA on conducting and insulating substrates. E-beam exposures were conducted under vacuum conditions and 1 mbar of water vapor, helium, nitrogen and argon to study their effect on contrast and resolution of PMMA on silicon, fused silica and soda lime glass substrates. On silicon, exposure under water vapor yielded contrast values significantly higher than vacuum exposure, consistent with our previous work. However, exposure under helium yielded slightly improved contrast compared to vacuum exposure. On insulating substrates exposure under helium environment yielded contrast values significantly higher compared to vacuum exposure. The clearing dose was found to increase with the gases’ molecular weight and proton number, consistent with the increase in scattering cross-section. The improved contrast and sensitivity (dose to clear) of PMMA under helium motivated us to study the resolution under various gases. Resolution testing indicated that despite the lower clearing dose, helium still exhibited the best resolution with 25-nm half-pitch dense lines and spaces clearly resolved on soda lime glass. Thus, VP-EBL of PMMA under helium yields higher sensitivity and contrast on insulating substrates without sacrificing resolution. 

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3. Three-Dimensional Nanopatterning Using Extreme Ultraviolet Colloidal Talbot Lithography

   https://doi.org/10.1021/acs.nanolett.6c01662  

   Mohanty, Saurav; Flores, Ethan; Hur, Daniel; Mitchell, Richard; Chang, Chih-Hao (May 2026, Nano Letters)

   While extreme ultraviolet (EUV) lithography has enabled the continued scaling toward high-resolution features, existing processes are limited to patterning of planar two-dimensional (2D) structures. This work demonstrates EUV colloidal Talbot lithography (CTL) for the patterning of 3D nanostructures with 25 nm minimum feature sizes. In this approach, a monolayer of self-assembled nanospheres is utilized as a binary mask and illuminated using a tabletop high-harmonic generation (HHG) EUV source to form a volumetric intensity pattern for proximity-field printing. The interference pattern formation is investigated using finite difference time domain (FDTD) simulations and maintains an adequate fringe contrast within the volume. Experimental results demonstrate the fabrication of 2D nanostructures with tunable unit-cell geometry and 3D nanostructures down to 25 nm using a single exposure. This cost-effective approach enables single-exposure 3D EUV lithography with low hardware requirements and has broad applications in nanophotonics, quantum devices, and advanced materials. 

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4. Extreme Ultraviolet and Beyond Extreme Ultraviolet Lithography Using Amorphous Zeolitic Imidazolate Resists Deposited by Atomic/Molecular Layer Deposition

   https://doi.org/10.1021/acs.chemmater.5c01476  

   Waltz, Kayley E; Zhou, Xinpei; Krull, Xavier; Singh, Somya; Mattson, Eric; Miao, Yurun; Hettermann, Matt; Grodt, Travis; Zhang, Qi; Im, Honggu; et al (November 2025, Chemistry of Materials)

   Amorphous zinc-imidazolate (aZnMIm) resists show potential to meet the demands for next generation high-numerical aperture (high-NA) metal-containing extreme ultraviolet (EUV) resist materials given their ease of deposition by atomic/molecular layer deposition (ALD/MLD) at thicknesses of 20 nm and below. This study demonstrates that aZnMIm thin films, previously identified as high-resolution electron beam resists, can also function as negative tone EUV photoresists. Water development achieves high sensitivity (5 mJ/cm²) but leaves significant residue, while acetic acid development results in poor contrast. A hybrid approach—water followed by acetic acid—enables residue-free development with a sensitivity of 181 mJ/cm². Dry development using 1,1,1,5,5,5-hexafluoroacetylacetone (hfacH) is also possible, but shows lower sensitivity (375 mJ/cm²) compared to wet development methods. EUV photoelectron spectroscopy (PES), reflectometry/EUV absorption, total electron yield (TEY), residual gas analysis (RGA), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were used to investigate the effects of EUV irradiation on aZnMIm resists. Reflectometry experiments reveal an aZnMIm EUV absorption coefficient of 6.2 µm-1, while PES and TEY analyses show that, compared to poly(4-hydroxystyrene) (PHS), a polymer-based reference resist, aZnMIm emits more primary and secondary electrons but generates fewer slow electrons relative to its primary electron emission; its total electron yield is similar to that of poly(methyl methacrylate) (PMMA) resists. When exposed to EUV, aZnMIm predominantly outgasses H2, as determined by RGA. TOF-SIMS measurements demonstrate that high dose EUV exposure only partially fragments the 2-methylimidazole (2MIm) organic linkers, unlike high dose electron beam exposure, which is known to completely degrade them. Additionally, aZnMIm resists show promise for potential beyond-EUV lithography (BEUVL) due to the presence of Zn, which provides higher sensitivity at 6.7 nm compared to other metal ions, like Sn, that are currently used in the best-performing EUV metal-organic resists. TEY measurements demonstrate that aZnMIm emits nearly twice as many electrons as PHS at 6.7 nm. The BEUV TEY of aZnMIm also surpasses that of PMMA, poly(pentafluorostyrene), and poly(4-iodostyrene), with the latter two being known for their high EUV TEYs. This work provides insight into zeolitic imidazolate framework (ZIF)-based EUV and BEUV resists and highlights their potential for both wet and dry development. 

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5. Molecular layer deposition of an Al-based hybrid resist for electron-beam and EUV lithography

   https://doi.org/10.1117/12.2657636  

   Ravi, Ajay; Shi, Jingwei; Lewis, Jacqueline; Bent, Stacey F. (May 2023, Proc. SPIE 12498, Advances in Patterning Materials and Processes XL)

   Guerrero, Douglas; Amblard, Gilles R. (Ed.)

   As lithographic techniques advance in their capabilities of shrinking microelectronics devices, the need for improved resist materials, especially for extreme ultraviolet (EUV), has become increasingly pressing. In this work, we study the molecular layer deposition (MLD) of an Al-based hybrid thin film resist, known as “alucone,” extending our previous research that tested the Hf-based hybrid thin film “hafnicone” as an EUV resist. Alucone is grown at 100 ºC using the metal precursor trimethylaluminum and the organic precursor ethylene glycol. Like hafnicone, alucone behaves as a negative tone resist that can resolve 50-nm line widths, though preliminary data suggest that alucone’s line patterns are more sharply defined than those of hafnicone. Whereas hafnicone’s sensitivity is 400 μC/cm2 using 3 M HCl as the developer, alucone’s sensitivity is not yet as good (4800 μC/cm2 using 0.125 M HCl). Our study of alucone offers new insight into structural features of an MLD film that can lead to desired EUV-responsive behavior. This insight may accelerate the development of vapor-deposited inorganic resists for use in electron-beam and EUV lithography. 

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