Layer-Ordered Organooxotin Clusters for Extreme-Ultraviolet Photolithography

Extreme-ultraviolet (EUV) photolithography, which enables the high-throughput production of well-defined patterns with critical dimensions on the scale of several nanometers, is essential for the fabrication of a highly integrated semiconductor. The full exploitation of EUV lithographic techniques necessitates the development of photoresist (PR) materials with both high EUV sensitivity and a long shelf-life. However, despite notable advances, the available library of EUV PR materials remains limited. Here we report EUV PRs capable of forming preorganized layers consisting of ladder-structured tetranuclear stannoxanes. Single-crystal X-ray structure analyses reveal a close interlayer distance of 8.5 Å through interdigitation of the pseudoaxial butyl chains. The developed EUV PR materials exhibit high solubility in organic solvents commonly used in semiconductor processing, enabling the preparation of PR solutions with superior wettability and uniform film-forming ability on Si wafer substrates. These PR solutions also demonstrate notable resistance to hydrolytic decomposition for as long as 1 month, indicating a long shelf-life. Our PR materials enabled negative-tone patterning processes that involved a solubility decrease upon irradiation. The presence of chromophoric ligands makes our PR materials compatible with conventional UV photolithography, through photochemical reactions involving carbonyl units. In addition, e-beam and EUV lithography could produce fine line patterns of our PRs, with critical dimensions of 20 and 15 nm, respectively. Our research showcases the potential of layer-ordered organooxotin clusters for EUV PR applications.

Soft X-ray absorption and fragmentation of tin-oxo cage photoresists

"Tin-oxo cage" organometallic compounds are considered as photoresists for extreme ultraviolet (EUV) photolithography. To gain insight into their electronic structure and reactivity to ionizing radiation, we trapped bare gas-phase n-butyltin-oxo cage dications [(BuSn)12O14(OH)6]2+ in an ion trap and investigated their fragmentation upon soft X-ray photoabsorption by means of mass spectrometry. In complementary experiments, the tin-oxo cages with hydroxide and trifluoroacetate counter-anions were cast in thin films and studied using X-ray transmission spectroscopy. Quantum-chemical calculations were used to interpret the observed spectra. At the carbon K-edge, a distinct pre-edge absorption band can be attributed to transitions in which electrons are promoted from C1s orbitals to the lowest unoccupied molecular orbitals, which are delocalized orbitals with strong antibonding (Sn-C σ*) character. At higher energies, the most prominent resonant transitions involve C-C and C-H σ* valence states and Rydberg (3s and 3p) states. In the solid state, the onset of continuum ionization is shifted by ∼5 eV to lower energy with respect to the gas phase, due to the electrostatic effect of the counterions. The O K-edge also shows a pre-edge absorption, but it is devoid of any specific features, because there are many transitions from the different O1s orbitals to a large number of vacant orbitals. In the gas phase, formation of the parent [(BuSn)12O14(OH)6]3+ radical ion is not observed at the C K-edge nor at the O K-edge, because the loss of a butyl group from this species is very efficient. We do observe a number of triply charged photofragment ions, some of which have lost up to 5 butyl groups. Structures of these species are proposed based on quantum-chemical calculations, and pathways of formation are discussed. Our results provide insight into the electronic structure of alkyltin-oxo cages, which is a prerequisite for understanding their response to EUV photons and their performance as EUV photoresists.

UV and VUV-induced fragmentation of tin-oxo cage ions

Photoresist materials are being optimized for the recently introduced Extreme Ultraviolet (EUV) photolithographic technology. Organometallic compounds are potential candidates for replacing the ubiquitous polymer-based chemically amplified resists. Tin (Sn) has a particularly large absorption cross section for EUV light (13.5 nm, 92 eV), which could lead to a lower required EUV dose for achieving the desired solubility change (improved sensitivity). However, the fundamental interaction between organometallic materials and higher energy photons is poorly understood. In this work, we exposed n-butyltin-oxo cage dications (M2+) in the gas phase to photons in the energy range 4-35 eV to explore their fundamental photoreactivity. Photoproducts were detected using mass spectrometry. Homolytic cleavage of tin-carbon bonds was observed for all photon energies above the onset of electronic absorption at ∼5 eV (∼250 nm), leading to photoproducts which have lost one or more of the attached butyl groups (Bu). Above 12 eV (<103 nm), dissociative photoionization occurred for the dication (M2+), competing with the neutral loss channels. The photoionization threshold is lowered by approximately 2 eV when one counterion (triflate, OTf- or tosylate, OTs-) is attached to the tin-oxo cage (MOTf+ and MOTs+). This threshold is expected to be even lower if each tin-oxo cage is attached to two counterions, as is the case in a solid film of tin-oxo cages. Addition of counterions also affected the fragmentation pathways; photoexcitation of (MX)+ (X = counterion, OTf or OTs) always led to formation of (MX-2Bu)+ rather than (MX-Bu)+. MOTs+ was much more reactive than MOTf+ in terms of reaction products per absorbed photon. A possible explanation for this is proposed, which involves the counterion reacting with the initially formed tin-based radical.

High-Resolution Lithographic Patterning with Organotin Films: Role of CO2 in Differential Dissolution Rates

Details of the chemistry enabling the patterning of organotin photoresists to single-digit-nm resolution continue to engage study. In this report, we examine the contributions of atmospheric gases to the differential dissolution rates of an n-butyltin oxide hydroxide photoresist. Cryo scanning tunneling electron microscopy (cryo-STEM) produces a micrograph of the latent image of an irradiated resist film, readily distinguishing exposed and unexposed regions. Temperature-programmed desorption mass spectrometry (TPD-MS) and cryo electron energy loss spectroscopy (cryo-EELS) show that irradiated films are depleted in carbon through desorption of butane and butene. Upon aging in air, irradiated films absorb H₂O, as previously established. TPD-MS also reveals a previously unrecognized absorption of CO₂, which correlates to a heightened dissolution contrast. This absorption may play an active role in determining intrinsic patterning performance and its variability based on changes in atmospheric-gas composition.

Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

Monoalkyltin(iv) complexes are well-known catalysts for esterification reactions and polyester formation, yet the mode of operation of these Lewis acidic complexes is still unknown. Here, we report on mechanistic studies of n-butylstannoic acid in stoichiometric and catalytic reactions, analyzed by NMR, IR and MS techniques. While the chemistry of n-butyltin(iv) carboxylates is dominated by formation of multinuclear tin assemblies, we found that under catalytically relevant conditions only monomeric n-BuSn(OAc)3 and dimeric (n-BuSnOAc2OEt)2 are present. Density functional theory (DFT) calculations provide support for a mononuclear mechanism, where n-BuSn(OAc)3 and dimeric (n-BuSnOAc2OEt)2 are regarded as off-cycle species, and suggest that carbon-oxygen bond breaking is the rate-determining step.

A core-shell type alkyl-Sn-oxo cluster of {Sn14As16} bridged by 4-aminophenylarsonate ligands and incorporated with a {Na6} cluster

Herein, we report the largest organotin-oxo arsonate cluster {Sn14As16}, which is made up of a tetramer of alkyl-Sn-oxo subunits linked by four APA ligands and incorporated with a zigzag belt-like {Na6} cluster. This complex shows high water stability and finds potential application as an electrode decoration material in CO2 reduction.

Review of recent advances in inorganic photoresists

The semiconductor industry has witnessed a continuous decrease in the size of logic, memory and other computer chip components since its birth over half a century ago. The shrinking of features has to a large extent been enabled by the development of advanced photolithographic techniques. This review focuses on one important component of lithography, the resist, which is essentially a thin film that can generate a specific feature after an exposure and development process. Smaller features require an even more precisely focused photon, electron or ion beam with which to expose the resist. The promising light source for next generation lithography that will enable downscaling patterns to be written is extreme ultraviolet radiation (EUV), 92 eV (13.5 nm). The review mainly focuses on inorganic resists, as they have several advantages compared with traditional organic resists. In order to satisfy the throughput requirement in high volume semiconductor manufacturing, metal oxide resists with high resolution and sensitivity have been proposed and developed for EUV lithography. The progress of various inorganic resists is introduced and their properties have been summarized.

The current review aims to focus on recent progress and opportunities in inorganic photoresist materials, including their fabrication process, performance and working mechanism.

Assembly of high-nuclearity , -oxo clusters: solvent strategies and inorganic Sn incorporation

A series of unprecedented high-nuclearity tin-oxo nanoclusters (up to Sn34 ) with structural diversity have been obtained. The characteristics of the applied solvents had great influence on the assembly of these Sn-O clusters. Pure alcohol environments only gave rise to small clusters of Sn6 , whilst the introduction of water significantly increased the nuclearity to Sn26 , which greatly exceeds those of the known tin-oxo clusters (≤14); the use of aprotic CH3CN finally produced the largest Sn34 to date. Apart from the nuclearity breakthrough, the obtained tin-oxo clusters also present new structural types that are not found in previous reports, including a layered nanorod-like structure of Sn26 and the cage-dimer structure of Sn34 . The layered Sn26 clusters represent good molecular models for SnO2 materials. Moreover, an electrode derived from TOC-17 with a {Sn26 } core shows better electrocatalytic CO2 reduction activity than that from TOC-18 with Sn34 . This work not only provides an efficient methodology for the rational assembly of high-nuclearity Sn-O clusters, but also extends their potential applications in energy conversion.

Stabilizing γ-Alkyltin-Oxo Keggin Ions by Borate Functionalization

We report a hierarchical self-assembly engineering of tin-oxo clusters from nanosized hydrophobic clusters to a single-layer film of assembled clusters. These clusters are derivatives of the previously reported Na-centered butyltin Keggin ions, but they are bicapped with butyltin and with borate ligands. The formulas γ-[( n-BuSn)₁₄(OCH₃)₁₀(OH)₃O₉(NaO₄)(HBO₃)₂] and γ-[( n-BuSn)₁₄(OCH₃)₁₀(OH)₃O₉(NaO₄)(PhBO₂)₂] were determined from single-crystal X-ray diffraction and bulk solution characterization including small-angle X-ray scattering, electrospray ionization mass spectrometry, and multinuclear and multidimensional NMR (¹¹⁹Sn, ¹³C, and ¹H). Solution characterization confirms that borate functionalization inhibits the solution-phase β-γ Keggin isomer interconversion that was recognized prior for uncapped butyltin clusters, and in this case, the γ isomer is favored. The assembly of the γ-NaSn₁₄BO₃ clusters into a homogeneous Langmuir-Blodgett monolayer is the first step toward creating nanopatterned films for microelectronic devices.

Effect of Oxygen on Thermal and Radiation-Induced Chemistries in a Model Organotin Photoresist

Organotin photoresists have shown promise for next-generation lithography because of their high extreme ultraviolet (EUV) absorption cross sections, their radiation sensitive chemistries, and their ability to enable high-resolution patterning. To better understand both temperature- and radiation-induced reaction mechanisms, we have studied a model EUV photoresist, which consists of a charge-neutral butyl-tin cluster. Temperature-programmed desorption (TPD) showed very little outgassing of the butyl-tin resist in ultrahigh vacuum and excellent thermal stability of the butyl groups. TPD results indicated that decomposition of the butyl-tin resist was first order with a fairly constant decomposition energy between 2.4 and 3.0 eV, which was determined by butyl group desorption. Electron-stimulated desorption (ESD) showed that butyl groups were the primary decomposition product for electron kinetic energies expected during EUV exposures. X-ray photoelectron spectroscopy was performed before and after low-energy electron exposure to evaluate the compositional and chemical changes in the butyl-tin resists after interaction with radiation. The effect of molecular oxygen during ESD experiments was evaluated, and it was found to enhance butyl group desorption during exposure and resulted in a significant increase in the ESD cross section by over 20%. These results provide mechanistic information that can be applied to organotin EUV photoresists, where a significant increase in photoresist sensitivity may be obtained by varying the ambient conditions during EUV exposures.