Resists are needed to advance extreme ultraviolet (EUV) lithography. In EUV resists, due to the high energy of the incident photons, most of the chemistry arises from the emitted primary and secondary electrons and not the EUV photons themselves. Because the electrons are playing a leading role in EUV patterning, initiating chemical transformations, it is important to characterize their generation, transport, and energy distribution. In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on critical resist properties: EUV absorption, electron emission, electron attenuation length (EAL), and energy distribution of emitted electrons. Total electron yield provides information on the conversion of absorbed EUV photons to electrons, and photoelectron spectroscopy provides information on energy distribution of generated electrons. The EAL reveals the distance that the electrons can travel in a resist film, which is related to the electron blur. Correlations between the obtained experimental values are discussed. We explore how different elements or functional groups change the yield, EAL, and energy distribution of emitted electrons, aiming to understand how to control the electron cascade. (c) 2024 Society of Photo-Optical Instrumentation Engineers (SPIE)

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques. Here, we report on the size distributions of both pure and doped droplets collected from single-shot x-ray imaging and produced from the free-jet expansion of helium through a 5 μm diameter nozzle at 20 bars and nozzle temperatures ranging from 4.2 to 9 K. This work extends the measurement of large helium nanodroplets containing 109-1011 atoms, which are shown to follow an exponential size distribution. Additionally, we demonstrate that the size distributions of the doped droplets follow those of the pure droplets at the same stagnation condition but with smaller average sizes.

Extreme ultraviolet (EUV)-induced radiation exposure chemistry in organotin-oxo systems, represented by the archetypal [(R-Sn)12O14(OH)6](A)2 cage, has been investigated with density functional theory. Upholding existing experimental evidence of Sn-C cleavage-dominant chemistry, computations have revealed that either electron attachment or ionization can single-handedly trigger tin-carbon bond cleavage, partially explaining the current EUV sensitivity advantage of metal oxide systems. We have revealed that tin atoms at different parts of the molecule react differently to ionization and electron attachment and have identified such selectivity as a result of local coordination chemistry instead of the macro geometry of the molecule. An ionization-deprotonation pathway has also been identified to explain the observed evolution of an anion conjugate acid upon exposure and anion mass dependence in resist sensitivity.

The combination of area-selective deposition (ASD) with a patternable organic monolayer provides a versatile additive lithography platform, enabling the generation of a variety of nanoscale feature geometries. Stearate hydroxamic acid self-assembled monolayers (SAMs) were patterned with extreme ultraviolet (λ = 13.5 nm) or electron beam irradiation and developed with ASD to achieve line space patterns as small as 50 nm. Density functional theory was employed to aid in the synthesis of hydroxamic acid derivatives with optimized packing density to enhance the imaging contrast and improve dose sensitivity. Near-edge X-ray absorption fine structure spectroscopy and infrared spectroscopy reveal that the imaging mechanism is based on improved deposition inhibition provided by the cross-linking of the SAM to produce a more effective barrier during a subsequent deposition step. With patterned substrates composed of coplanar copper lines and silicon spacers, hydroxamic acids selectively formed monolayers on the metal portions and could undergo a pattern-wise exposure followed by ASD in the first combination of a patternable monolayer with ASD. This material system presents an additional capability compared to traditional ASD approaches that generally reflect a starting patterned surface. Furthermore, this bottoms-up additive approach to lithography may be a viable alternative to subtractive nanoscale feature generation.

Metal-organic systems have shown great promise as EUV resists. They have demonstrated good sensitivity and etch resistance while maintaining high resolution and low line edge roughness, making them a potential pathway to modify the tradeoff between resolution, line edge roughness, and sensitivity common to organic chemically amplified resists. In particular, tin-based systems have attracted significant interest and the two known families of fab-ready metal organic resist are based on organotin compounds. Part of this interest derives from the high EUV absorption cross-section of tin, but an equally important driver is the unique chemistry of the element, which affords a multiplicity of coordination environments and a tin-carbon bond stable with respect to hydrolysis, yet sensitive to cleavage by ionizing radiation. Realizing the patterning potential promised by these empirical properties will require a better understanding of the fundamental chemistry behind them, and has already motived several academic and industrial investigations. In this contribution we continue our previous work to develop a deeper understanding of tin-carbon bond chemistry with quantum chemistry using the well-known Sn12 “football” cluster archetype. We demonstrate the consistency between our computations and experimental data. And then we move on to explore a more detailed description of ionization and electron attachment induced chemistry. Investigations of electronic structure would shed light on what chemical reactions can happen subsequently.

The success in the miniaturization of the electronic device constituents depends mostly on the photolithographic techniques. Recently, to achieve patterning at the sub-10-nm node, extreme ultraviolet (EUV) lithography has been introduced into high volume production. Continued scaling of EUV via increased numerical aperture to achieve nodes at 3-nm and below requires the development of fundamentally new patterning materials and new characterization methods. Current EUV-resist film thicknesses are in the 20- to 40-nm range, and further thickness reduction is required for the next generation. Therefore, interfaces become exceedingly important, and the properties of the resist film would be dominated by top and bottom interfacial effects. X-ray photoelectron spectroscopy (XPS) combined with standing-wave excitation (SW-XPS), a fairly new method in the EUV lithography field, previously had been largely applied in multilayers and superlattices for characterizing the composition and electronic structure of buried layers and interfaces as a function of depth. We applied the SW-XPS method to organic/inorganic photoresists to provide depth-selective information on their structural and chemical conditions of as a function of temperature, EUV exposure, different underlayers, and other fundamental parameters. As a first attempt, we perform an SW-XPS feasibility study on self-assembled monolayer (SAM) films after exposure to an electron beam. By SW-XPS, we determined that the interface between the Al2O3 underlayer and the SAMs is smooth, with a mean roughness of about 0.2 nm. Moreover, we determined that the SAM chains are, on average, tilted by similar to 30 deg off the sample normal. The SW-XPS results also suggest that the SAM is not a perfectly aligned and uniform monolayer, with some areas having thickness higher than a single monolayer. We demonstrated that SW-XPS can provide useful information on ultrathin materials with high potential for being used as a characterization method of organic/inorganic photoresists. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)

The BaAl 4 prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr 2 Si 2 family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron systems. The MA 4 family of materials (M = Sr, Ba, Eu; A = Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninteresting metals. We predict the exemplary compound BaAl 4 to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the k z direction and quadratic dispersion along the k x / k y direction resides on the rotational axis with C 4 v point group symmetry. An extremely large, unsaturating positive magnetoresistance in BaAl 4 despite an uncompensated band structure is revealed, and quantum oscillations and angle-resolved photoemission spectroscopy measurements confirm the predicted multiband semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl 4 as a topological semimetal, casting its prototype status into a role as a building block for a vast array of topological materials.

The absorption of an EUV photon by a thin film resist leads to the emission of a photoelectron as well as several secondary electrons with low kinetic energy. The “universal curve”, used in X-ray photoelectron spectroscopy, indicates that the low kinetic energy electrons may travel tens to hundreds of nanometers before losing their kinetic energy via initiation of chemical reactions. The distance that the electrons are able to travel in the resist is directly related to the resultant “blur” of the aerial image. Thus, identifying how to measure and influence the distance traveled by the secondary electrons is extremely beneficial to the resist community. In this work, we utilize several model polymer materials to investigate the impact of specific chemistry groups on the secondary electron attenuation length (EAL) – the thickness of resist material required to reduce number of emitted secondary electrons to 1/e of initial. The EAL measures the distance the secondary electrons can travel in a resist film, which is directly related to the electron blur. Possibilities to gain additional information on electron penetration depth in resist films will also be discussed.

New resists are needed to advance EUV lithography. Tailored design of efficient photoresist is impossible without fundamental understanding of EUV induced chemistry. The absorption of an EUV photon by a thin film resist leads to emission of primary and secondary electrons. The electrons may travel up to tens of nanometers before losing their kinetic energy via collisions which initiate chemical reactions. The "blur" of an aerial image is directly related to the distance that electrons are able to travel and initiate chemistry in the resist. Thus, identifying how to measure and influence the absorption of EUV photons, emission of electrons, and distance traveled by the secondary electrons is extremely beneficial to the resist community. In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on three critical resist properties: EUV absorption, electron emission, and the electron attenuation length (EAL). EUV absorption dictates the efficiency of the film to absorb photons. Total electron yield (TEY) provides information on the conversion of absorbed EUV photons to electrons, whereas photoelectron spectroscopy (PES) provides information on energies and abundance of generated electrons. The EAL corresponds to the thickness of a material required to reduce the number of emitted electrons to 1/e of the initial value. The EAL reveals the distance the electrons can travel in a resist film, which is directly related to the electron blur. Correlations between the obtained experimental values is discussed.

Slow electrons (with energy below 10 eV) play an important role in nature and technology. For instance, they are believed to initiate solubility change in extreme ultraviolet resists. Depending on their mobility, such secondary electrons can lead to image blur and degradation of patterning resolution. Hence, it is important to characterize the transport of slow electrons by measuring parameters such as the effective attenuation length (EAL). We present a technique that allows for prompt characterization of EAL in polymer films. In this experiment, slow electrons are generated in a substrate upon absorption of x-ray photons. The attenuation of electron flux by a polymer film is measured as a function of film thickness, allowing for the determination of EAL for slow electrons. We illustrate this method with poly(hydroxy styrene) and poly(methyl metacrylate) films. Furthermore, we propose an improvement for this technique that would enable the measurement of EAL as a function of electron kinetic energy.

Nematicity, where rotational symmetry is broken while translational symmetry is conserved, is prevalent in high-temperature superconductors. In particular, nematic quantum critical point has been universally found near the optimum doping of the superconducting dome of several iron-based superconductor families. In such a regime, evidence for strong nematic fluctuations have been observed. As the precursor to this order, nematic fluctuations emerge before nematicity, providing favorable ground to study how nematic order modifies the electronic structure in the absence of structural distortion. Here we use spatially resolved angle-resolved photoemission spectroscopy to investigate the correlation between the onset of nematic fluctuations and electronic structure in an optimally doped BaFe2(As1-xPx)(2) (x similar to 0.3) superconductor. We reveal a strong spatially varying anisotropy of the Fermi surface on a length scale of tens of microns with strong correlation between the changes in the hole and electron Fermi pockets, consistent with the variations expected in the presence of fluctuating nematic order. These results provide direct evidence for spatial nematic fluctuations in the optimal doping regime of iron-based superconductors.

In this preliminary computational chemistry study, we report excitation selectivity in a model tin-oxo molecular resist. Upon impact ionization, organic side chains connected to 6-coordinated tin atoms (located near the charge balancing ligands) are preferentially destabilized. Upon electron addition, conversely, side chains connected to 5-coordinated tin atoms (located on the central belt) are destabilized. Inferring from the binding energies, the ionization induced processes likely have a smaller spatial extent than electron attachment induced process.

In EUV lithography, radiation chemistry is largely different from DUV. Engineers have explored self-assembled monolayers (SAM) in the context of lithography and some of their properties could be utilized in EUV. We study SAMs and their interaction with substrates using quantum chemistry. Interface chemistry between resists and underlayers is playing an increasingly important role in EUV. it is conceivable that the resist molecules near the interface are susceptible to activation by electrons originated in the substrate. For their monolayer nature and spontaneous ordering, SAMs can be used for engineering interface properties in a predictable manner. Understanding the physical and chemical processes at the interface in the presence of SAMs would be vital for better modelling their effects on imaging. To address these questions, quantum chemistry is used to understand the properties of SAMs, such as their packing order. The surface electronic structure is also studied to elucidate the physical properties at the interface that could impact EUV dose.

EUV photon absorption by a resist film leads to emission of a photoelectron and several low kinetic energy secondary electrons. The “universal curve”, used in X-ray photoelectron spectroscopy, suggests that the low kinetic energy electrons may travel tens to hundreds of nanometers in solids until they inelastically scatter. The fact that electrons travel long distances before they may initiate chemical reactions ultimately result in blur of the aerial image, reducing the contrast and subsequently resolution of the resist. In this work, we will present an experimental approach to determine secondary electron attenuation length (EAL) – the thickness of resist material required to reduce number of emitted secondary electrons to 1/e of initial. The EAL describes how far secondary electrons can travel in a resist film and is directly related to the electron blur. Possibilities to gain additional information on electron penetration depth in resist films will also be discussed.

In extreme ultraviolet (EUV) lithography, chemistry is driven by secondary electrons. A deeper understanding of these processes is needed. However, electron-driven processes are inherently difficult to experimentally characterize for EUV materials, impeding targeted material engineering. A computational framework is needed to provide information for rational material engineering and identification at a molecular level. We demonstrate that density functional theory calculations can fulfill this purpose. We first demonstrate that primary electron energy spectrum can be predicted accurately. Second, the dynamics of a photoacid generator upon excitation or electron attachment are studied with ab-initio molecular dynamics calculations. Third, we demonstrate that electron attachment affinity is a good predictor of reduction potential and dose to clear. The correlation between such calculations and experiments suggests that these methods can be applied to computationally screen and design molecular components of EUV material and speed up the development process. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

The success in the shrinking of the electronic device constituents depends mostly on the photolithographic techniques. For next generation lithography, in order to achieve the desired downscaling patterns (<10 nm), extreme ultraviolet (EUV) radiation must be used and new materials must be developed. Standing Wave X-ray Photoelectron Spectroscopy (SWXPS), a fairly new method in the EUV lithography field, is an ideal method for characterization such new materials. For example, X-ray photoelectron spectroscopy (XPS) combined with standing-wave excitation can provide depth-selective information on the structural and chemical conditions of the photoresits as a function of temperature, exposure, or other parameters. We performed a SW-XPS feasibility study on self-assembled monolayer (SAM) films after exposure to electron beam. SW-XPS determined the semi-quantitative chemical profiles of the SAM layer with sub-nm accuracy including the roughness/interdiffusion of both interfaces. We demonstrated that SW-XPS can provide indispensable information useful for understanding the depth composition of films as well effects of irradiation (e-beam or EUV) on the latest ultrathin photoresists.

In Extreme Ultraviolet (EUV) lithography, chemistry is driven by secondary electrons. A deeper understanding of these processes is vital to targeted engineering of materials. As electron interactions are non-discriminative, studying these processes directly in condensed phase with experiments is extremely challenging. Proxy experiments such as gas phase experiments and solution phase experiments are only viable to a limited subset of materials, limiting their use for large scale material screening. First principles quantum chemistry calculations have been adopted by various industries for materials development and investigation. We demonstrate that such calculations can be used to model processes involved in EUV radiation chemistry. We can reproduce experimental results and predict dose to clear with such calculations. In this article, we first demonstrate that primary electron energy spectrum can be predicted accurately. Secondly, the dynamics of a photoacid generator (PAG) upon excitation or electron attachment is studied with ab-initio molecular dynamics calculations. Thirdly, we demonstrate that electron attachment affinity is a good predictor of reduction potential and dose to clear.

In EUV, photoelectrons and secondary electrons play indispensable roles in the chemistry of photoresist. An accurate understanding of electron related processes provides foundation for targeted engineering of resists and other EUV materials. As chemistry is initiated by secondary electrons, acquiring the electron energy distribution inside an actual photoresists is important for improving the efficiency of chemical activation. We demonstrate that condensed phase photoemission spectroscopy can be used as a tool for interrogating electrons in resist and electrons owing from underlayers to resists. The electron energy distribution, albeit different from that measured with condensed phase photoemission spectroscopy, can be recovered computationally. The computational approach involves Monte Carlo simulations using the energy resolved scattering mean free path and the photoemission energy spectra as inputs.