Mean Free Path of Electrons in Organic Photoresists for Extreme Ultraviolet Lithography in the Kinetic Energy Range 20-450 eV.

The blur caused by the nonzero mean free path of electrons in photoresists exposed by extreme ultraviolet lithography has detrimental consequences on patterning resolution, but its effect is difficult to quantify experimentally. So far, most mean free path calculations use the dielectric formalism, which is an approximation valid in the optical limit and fails at low kinetic energy. In this work, we used a modified substrate-overlayer technique that exploited the attenuation of the Si 2p core level originating specifically from the native silicon dioxide to evaluate the attenuation of electrons traveling through 2 and 4 nm of photoresist overlayers to provide a close estimation of the inelastic mean free path relevant for photoresist lithography patterning and for electron microscopy. The photoemission spectra were collected in the proximity of the Si 2p edge (binding energy ∼101 eV) using synchrotron light of energy ℏω ranging between 120 and 550 eV. The photoresist films were prototypical chemically amplified resists based on organic copolymer of poly hydroxystyrene and tertbutyl methacrylate with and without triphenyl sulfonium perfluoro-1-butanesufonate photoacid generator and trioctylamine quencher. The inelastic mean free path of electrons, in the range that is relevant for photoresist exposure in extreme ultraviolet lithography (20-92 eV), was found to be between 1 and 2 nm. At higher kinetic energy, the mean free path increased, consistently with the well-known behavior. The presence of the photoacid generator and quencher did not change the mean free path, within experimental error. Our results are discussed and compared with the existing literature on organic molecules measured via dielectric formalism and electron transmission experiments.

SIMS Analysis of Thin EUV Photoresist Films.

This study reports on the application of secondary ion mass spectrometry (SIMS) for examining thin (20-50 nm) chemically amplified resist films on silicon. SIMS depth profiling was carried out using a gas cluster ion beam to ensure minimal sputter-induced damage to the organic constituents of interest. Specific attention concerned the distribution of the photo acid generator (PAG) molecule within these films, along with the photo-induced fragmentation occurring on extreme ultra-violet photo exposure. Positive secondary ion spectra were collected using a traditional time of flight (ToF)-SIMS and the latest generation IONTOF Hybrid SIMS instrumentation equipped with an OrbitrapTM mass analyzer. Tandem mass spectrometry (MS/MS) capability within the OrbitrapTM secondary ion column was utilized to verify that the C19H17S+ secondary ion did indeed have the molecular structure consistent with the PAG structure. The superior mass resolving power of the OrbitrapTM mass analyzer (∼20× of the ToF mass analyzer) along with improved mass accuracy (a few ppm) proved pivotal in the mass spectral and depth profile analysis of these films. This was not the case for the ToF-SIMS experiments, as the mass spectra, as well as the associated depth profiles, exhibited severe molecular (isobaric) interferences.

Electron-induced fragmentation mechanisms in organic monomers and their implications for photoresist optimization for EUV lithography.

Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in photoresist films. Therefore, it is crucial to understand the electron-induced fragmentation mechanisms involved in EUV-resist systems to improve their patterning performance. To facilitate this understanding, mechanistic studies were carried out on simple organic EUV-resist monomers, methyl isobutyrate (MIB) and methacrylic acid (MAA), both in the condensed and gas phases. Electron-stimulated desorption (ESD) studies on MIB in the condensed phase showed desorption peaks at around 2 and 9 eV electron energies. The gas-phase study on MIB showed that the monomer followed the dissociative ionization (DI) fragmentation pathway, under single collision conditions, which opened up at electron energies above about 11 eV. No signs of dissociative electron attachment (DEA) were detected for MIB in the gas phase under single collision conditions. However, DEA was an active process in MAA in the gas phase under single collision conditions at around 2 eV, showing that slight modifications of the molecular structures of photoresists may serve to sensitize them to certain electron-induced processes.

Dielectric Response Spectroscopy as Means to Investigate Interfacial Effects for Ultra-Thin Film Polymer-Based High NA EUV Lithography.

Extreme ultra-violet lithography (EUVL) is the leading-edge technology to produce advanced nanoelectronics. The further development of EUVL is heavily based on implementing the so-called high numerical aperture (NA) EUVL, which will enable even smaller pitches up to 8 nm half pitch (HP). In anticipation of this high NA technology, it is crucial to assess the readiness of the current resist materials for the high NA regime to comply with the demanding requirements of resolution, line-edge roughness, and sensitivity (RLS). The achievable tighter pitches require lower film thicknesses for both resist and underlying transfer layers. A concern that is tied to the thinning down is the potential change in resist properties and behavior due to the interaction with the underlayer. To increase the fundamental understanding of ultra-thin films for high NA EUVL, a method to investigate the interplay of reduced film thickness and different patterning-relevant underlayers is developed by looking at the glass transition temperature (Tg) of polymer-based resists. To minimize the ambiguity of the results due to resist additives (i.e., photoacid generator (PAG) and quencher), it was opted to move forward with polymer-only samples, the main component of the resist, at this stage of the investigation. By using dielectric response spectroscopy, the results obtained show that changing the protection group of the polymer, as well as altering the polymer film thickness impacts the dynamics of the polymer mobility, which can be assessed through the Tg of the system. Unexpectedly, changing the underlayer did not result in a clear change in the polymer mobility at the tested film thicknesses.

Molecular Modeling of EUV Photoresist Revealing the Effect of Chain Conformation on Line-Edge Roughness Formation.

Extreme ultraviolet lithography (EUVL) is a leading-edge technology for pattern miniaturization and the production of advanced electronic devices. One of the current critical challenges for further scaling down the technology is reducing the line-edge roughness (LER) of the final patterns while simultaneously maintaining high resolution and sensitivity. As the target sizes of features and LER become closer to the polymer size, polymer chain conformations and their distribution should be considered to understand the primary sources of LER. Here, we proposed a new approach of EUV photoresist modeling with an explicit description of polymer chains using a coarse-grained model. Our new simulation model demonstrated that interface variation represented by width and fluctuation at the edge of the pattern could be caused by characteristic changes of the resist material during the lithography processes. We determined the effect of polymer chain conformation on LER formation and how it finally contributed to LER formation with various resist material parameters (e.g., Flory-Huggins parameter, molecular weight, protected site ratio, and Tg).