Photoacid Generator Research Articles

Dual nonionic photoacids synergistically enhanced photosensitivity for chemical amplified resists

In this paper, 4,5-dimethoxy-2-nitrobenzyl methacrylate (MONMA) was copolymerized with 2-hydroxyethyl methacrylate (HEMA) and tert-butyl methacrylate (TBMA) to obtain the terpolymer with dual sensitivity of ultraviolet (UV) light and acid, P(MONMA-HEMA-TBMA). The nonionic photosensitive ester, 4,5-dimethoxy-2-nitrobenzyl p-toluene sulfonate (MONS), is designed and synthesized as the prime photoacid generator (PAG) because of excellent photoacid generating capability of MONS and the similar substituted structure between MONS and MONMA. The ester groups in MONS PAG small molecules and the MONMA units of P(MONMA-HEMA-TBMA) disintegrate under UV irradiation to generate free diffused sulfonic acids and localized carboxylic acids, respectively. Both free and localized acids facilitate the subsequent acid-catalyzed hydrolysis of TBMA adjacent to MONMA units to induce exponential growth of carboxylic acids. Therefore, the hydrolyzed polymers containing high-content carboxylic acids become soluble in the alkaline developer. Hydrophilic HEMA is introduced to the photoresist resin to improve the interfacial affinity between the resist and the substrate. Dual synergy effect from free photoacid generator MONS and polymerized photoacid unit MONMA enhances photoacid generation and accelerates acid proliferation, which enables the decreased exposure dose and lower content of free photoacid generator MONS. A series of novel chemical amplification photoresists with various methacrylate monomer ratios are designed and synthesized to optimize the photolithographic effect of photoresist resin. The UV photolithography results demonstrate that the distinct exposed patterns are obtained at a low exposure dose using the photoresist formulation with P(MONMA50-HEMA19-TBMA31) and MONS of 9:1 w/w.

Push–pull biphenyl–based iodonium salts: Highly sensitive one-component photoinitiators for photopolymerization under UV–visible LEDs

Diaryliodonium salts play an important role as highly efficient photoinitiators in the field of photopolymerizations. They can rapidly photodecompose to produce free radicals and super acids; thus, can initiate highly efficient photocuring of acrylic and epoxy monomers and resins. However, the absorption of commercialized diphenyliodonium salts is poorly matched with popular LED light sources, and the participation of sensitizers is usually required to achieve efficient photoexcitation. Here, we synthesized five new phenyl biphenyl iodonium salts with different electron-pushing and electron-withdrawing substituents, e.g., methoxy and CN, at the biphenyl substituent. Our results demonstrate that all synthesized iodonium salts have strong absorption in the range (λmax = 258–352 nm; εmax = 21,300–29,900 M−1 cm−1). The photochemistry of these synthesized iodonium salt is strongly influenced by substituents on the biphenyl ring. All iodonium salts can undergo rapid photolysis under light excitation, with higher quantum yield of photoacid generation reaching 0.25 and can quickly initiate cationic photopolymerization of epoxy monomers. Furthermore, free radicals are generated during photolysis, which can successfully trigger free radical polymerization. These iodonium salts with good absorbance in the UVA band demonstrate their potential as excellent photoinitiators under 365–405 nm LED excitation.

Mechanisms of acid generation from ionic photoacid generators for extreme ultraviolet and electron beam lithography.

Photoacid generators (PAGs) are important components of chemically amplified resists. The properties of PAGs directly affect the sensitivity of photoresists, line edge roughness, and resolution. Understanding the photoacid generation process in extreme ultraviolet (EUV) and electron beam (EB) lithography is helpful for photoresist design. However, the microscopic mechanisms remain largely unclear and the large variety in the molecular structure of PAGs presents a challenge to overcome. In this work, we investigate the microscopic processes of photoacid production of ionic PAGs for EUV and EB lithography. The PAG dissociation pathway is found to depend on the molecular structure and conformations. The processes of photoacid production and by-product generation are also revealed. The results contribute to a better understanding of the photochemical reactions in EUV and EB lithography, providing insights into the molecular design of novel PAGs and photoresists.

Visible photons for the regioselective nucleophilic ring opening of epoxides

Herein, we introduce a visible-light promoted green, metal-free method for the regioselective nucleophilic ring opening of epoxides by exploiting arylazo sulfones as PhotoAcid Generators (PAGs).

Microscale manipulation of bond exchange reactions in photocurable vitrimers with a covalently attachable photoacid generator.

Vitrimers are polymer networks with covalent bonds that undergo reversible exchange reactions and rearrange their topology in response to an external stimulus. The temperature-dependent change in viscoelastic properties is conveniently adjusted by selected catalysts. In these thermo-activated systems, the lack in spatial control can be overcome by using photolatent catalysts. Herein, we advance this concept to locally manipulate bond exchange reactions on a single digit microscale level. For this, we synthetize a linkable non-ionic photoacid generator, which is covalently attached to a thiol-click photopolymer. UV induced deprotection of the photoacid yields a strong immobilized sulfonic acid species, which is able to efficiently catalyze transesterification reactions. Covalent attachment of the formed acid prevents migration/leaching processes and enables a precise tuning of material properties. As proof of concept, positive toned microstructures with a resolution of 5 μm are inscribed in thin films using direct two-photon absorption laser writing and subsequent depolymerization. In addition, the possibility to locally reprogram bulk material properties is demonstrated by performing a post-modification reaction with ethylene glycol and carboxylic acids. The Young's modulus is varied between 3.3 MPa and 11.9 MPa giving rise to the versatility of the newly introduced catalysts for creating light processable and transformable materials.

Synthesis and properties of a series of sulfonate ester photoacid generators

A total of six sulfonate polymeric nonionic photoacid generators (PAGs) have been synthesised and subjected to comprehensive characterization for their application in photoresist materials.

The use of a sulfonium-based photoacid generator in thiol–ene photopolymers for the controlled activation of transesterification through chemical amplification

Herein, we exploit chemical amplification to release –OH groups in dynamic covalent photopolymers on-demand. Via a single photon event, a cascade of reactions occurs, which allows the polymers to flow through thermo-activated transesterification.

Improvement in radiation sensitivity of poly(methyl methacrylate) film dosimeters doped with Black100 via addition of sensitizers

We fabricated poly(methyl methacrylate) (PMMA) films containing Black100 and trichloroacetic acid (TCA), tribromoacetic acid (TBA), or bis(t-butylsulfonyl) diazomethane, a photoacid generator (BTDPAG), as sensitizers and measured their absorption spectra before and after X-ray irradiation. The films containing TCA, TBA, and BTDPAG exhibited absorption peaks attributed to the oxidized form of Black100. In a previous study, radiochromism due to the oxidation of Black100 was not observed in PMMA films containing only Black100. These results indicate that Black100 underwent oxidation by radicals or acids generated from the sensitizers by X-ray irradiation. The sensitivities of the PMMA films containing Black100 and the sensitizers were 4.5–6.4 times higher than those of the films without sensitizers in the dose range of 0.1–1 kGy. Although this is the comparison of sensitivities at different tube voltages and dose rates, by adding sensitizers, the sensitivity of the PMMA films, which are compatible with three-dimensional (3D) printing, was about one-third that of the widely recognized 3D radiochromic dosimeter, PRESAGE.

Nanoscale Vertical Resolution in Optical Printing of Inorganic Nanoparticles.

Direct optical printing of functional inorganics shows tremendous potential as it enables the creation of intricate two-dimensional (2D) patterns and affordable design and production of various devices. Although there have been recent advancements in printing processes using short-wavelength light or pulsed lasers, the precise control of the vertical thickness in printed 3D structures has received little attention. This control is vital to the diverse functionalities of inorganic thin films and their devices, as they rely heavily on their thicknesses. This lack of research is attributed to the technical intricacy and complexity involved in the lithographic processes. Herein, we present a generalized optical 3D printing process for inorganic nanoparticles using maskless digital light processing. We develop a range of photocurable inorganic nanoparticle inks encompassing metals, semiconductors, and oxides, combined with photolinkable ligands and photoacid generators, enabling the direct solidification of nanoparticles in the ink medium. Our process creates complex and large-area patterns with a vertical resolution of ∼50 nm, producing 50-nm-thick 2D films and several micrometer-thick 3D architectures with no layer height difference via layer-by-layer deposition. Through fabrication and operation of multilayered switching devices with Au electrodes and Ag-organic resistive layers, the feasibility of our process for cost-effective manufacturing of multilayered devices is demonstrated.

Photoacid Generators for Biomedical Applications.

Photoacid generators (PAGs) are compounds capable of producing hydrogen protons (H+ ) upon irradiation, including irreversible and reversible PAGs, which have been widely studied in photoinduced polymerization and degradation for a long time. In recent years, the applications of PAGs in the biomedical field have attracted more attention due to their promising clinical value. So, an increasing number of novel PAGs have been reported. In this review, the recent progresses of PAGs for biomedical applications is systematically summarized, including tumor treatment, antibacterial treatment, regulation of protein folding and unfolding, control of drug release and so on. Furthermore, a concept of water-dependent reversible photoacid (W-RPA) and its antitumor effect are highlighted. Eventually, the challenges of PAGs for clinical applications are discussed.

Degradation of modified polystyrenes having degradable units by near‐infrared light irradiation

AbstractModified polystyrenes having tertiary ether linkages or carbonate linkages were degrade by irradiation of near‐infrared light (NIR) and subsequent heating. The polymer films containing a photoacid generator and commercially available photon upconversion nanoparticle (UCNP) were decomposed to form isobutene and/or carbon dioxide by irradiation of NIR and subsequent heating. The thermal decomposition behaviors of the polymers were investigated and discussed in terms of the chemical structures of degradable units. The thermal stability of the polymers was found to be in the order: tertiary ester > tertiary carbonate. We believe that the degradation of NIR and subsequent heating contributes easy recycling of composite materials.

Novel Mechanism-Based Descriptors for Extreme Ultraviolet-Induced Photoacid Generation: Key Factors Affecting Extreme Ultraviolet Sensitivity.

Predicting photolithography performance in silico for a given materials combination is essential for developing better patterning processes. However, it is still an extremely daunting task because of the entangled chemistry with multiple reactions among many material components. Herein, we investigated the EUV-induced photochemical reaction mechanism of a model photoacid generator (PAG), triphenylsulfonium cation, using atomiC-Scale materials modeling to elucidate that the acid generation yield strongly depends on two main factors: the lowest unoccupied molecular orbital (LUMO) of PAG cation associated with the electron-trap efficiency 'before C-S bond dissociation' and the overall oxidation energy change of rearranged PAG associated with the proton-generation efficiency 'after C-S bond dissociation'. Furthermore, by considering stepwise reactions accordingly, we developed a two-parameter-based prediction model predicting the exposure dose of the resist, which outperformed the traditional LUMO-based prediction model. Our model suggests that one should not focus only on the LUMO energies but also on the energy change during the rearrangement process of the activated triphenylsulfonium (TPS) species. We also believe that the model is well suited for computational materials screening and/or inverse design of novel PAG materials with high lithographic performances.

Group transfer polymerization in bulk methacrylates

AbstractGroup transfer polymerization (GTP) is a polymerization method developed to obtain targeted (meth)acrylic polymers in solution at ambient temperatures. In this work, it is employed with methacrylic monomers to obtain low Ð polymers in bulk. In this regard, different initiator systems that exhibit different mechanisms are compared concerning the molecular weight, the polydispersity and double bond conversion of the resulting bulk polymers. The respective systems were chosen carefully to give a broad overview of earlier developed initiator‐catalyst combinations 1‐methoxy‐1‐(trimethylsiloxy)‐2‐methylprop‐1‐ene (MTS) & tetrabutylammonium cyanide (TBACN) and more recently investigated initiating systems MTS & trityl tetrakis(pentafluorophenyl)borate (TTPB) or dimethyl phenyl silane (DMPS) & tris(pentafluorophenyl)borate (BCF). The described initiating systems are applied as a two‐component (2K) system to ensure a homogeneous distribution of the respective initiator and catalyst in the bulk monomer. In addition to the 2K experiments, photochemical initiation is also applied to bulk formulations. Therefore, a photoacid generator (PAG) and MTS is used to trigger the polymerization reaction by irradiation with UV light. A highly controlled photopolymerization method in bulk was developed that way achieving a low polydispersity polymer with high double bond conversion.

New photoacids in microarray synthesis of oligonucleotides

Photochemical activity of thiophene oxime esters and of sulfonium salt tris[4-(4-acetylphenyl)sulfanylphenyl]sulfonium tetrakis(2,3,4,5,6-pentafluorophenyl)boranuide (photoacid generator 290 [PAG 290]) was investigated in a reaction forming a trityl cation. Quantum yields of photoacid phototransformation reactions and of photodetritylation of 5′-O-(4,4′-dimethoxytrityl)thymidine were determined. Then, [(Z)-[(3Z)-3-[cyano(o-tolyl)methylene]-2-thienylidene]amino] 2,4,6-triisopropylbenzenesulfonate (H 365) and PAG 290 were used as detritylation photoactivators in a procedure of oligonucleotide synthesis on a microchip synthesizer prototype, and several oligonucleotides were synthesized. At the stage of oligonucleotide chain extension, the yield was 94–96%. The tested photoacids can be successfully applied to microchip synthesis of oligonucleotides.

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.

Positive-tone chemically amplified photosensitive polyimides with controllable taper angles

Current methods for tuning the taper angle of photosensitive polyimide (PSPI) often require additional equipment or materials, and has limited achievable angle tuning range while maintaining resolution. To overcome these limitations, a positive tone chemically amplified photosensitive polyimide (p-CA-PSPI) was prepared from tert-butylcarbonate-protected polyimide (t-Boc-PI), photoacid generator (PAG), other additives, and solvent. By optimizing the protection ratios of t-Boc-PI resins, the p-CA-PSPI achieved a high film retention rate and wide processing window. The photo-patterning conditions were able to manipulate the sidewall profile and taper angle within the range of 11–53°, maintaining a resolution of 3 μm for patterns with taper angles higher than 28° at 1.5 μm film thickness, by controlling the acid diffusion. The thermally cured polyimide films demonstrated excellent mechanical properties, including a tensile strength of 95.7 MPa, tensile modulus of 2.9 GPa, and elongation at breakage of 24.2%, high thermal stability with a glass transition temperature of 343 °C, decomposition temperature of 409 °C, and good chemical resistance. This novel taper angle tuning method has the potential to optimize the internal stress of semiconductor devices and advance the development of display devices, MEMs, and multifunctional electronics that require a single package containing multiple or unique taper profiles.

High-Resolution Multicolor Patterning of InP Quantum Dot Films by Atomic Layer Deposition of ZnO

This paper presents the high-resolution (>2000 PPI) multicolor patterning of InP quantum dot films using a conventional photolithography process with a positive photoresist (PR). The solvent resistance of the quantum dot (QD) film is achieved by depositing an ultrathin ZnO layer through atomic layer deposition. This is different from previous studies, which lack high-resolution patterning or compatibility with indium phosphide (InP) QDs owing to chemical weaknesses. By employing a positive PR with a photoacid generator, the side-by-side patterning process yields multicolor patterns of red- and green-colored InP-based QDs. Additionally, the stacking of each color QD film is achieved. The patterning process can be used to fabricate QD light-emitting diode devices without degrading their performance. This process can be used not only for thin (<100 nm) QD films, which are used in QD-LED devices, but also for thick (>1 μm) QD films, which can be used in the color-conversion layer with a backlight.

Theoretical Insights into the Solubility Polarity Switch of Metal-Organic Nanoclusters for Nanoscale Patterning.

Metal-organic nanoclusters(MOCs) are being increasingly used as prospective photoresist candidates for advanced nanoscale lithography technologies. However, insight into the irradiation-induced solubility switching process remains unclear. Hereby, the theoretical study employing density functional theory (DFT) calculations of the alkene-containing zirconium oxide MOC photoresists is reported, which is rationally synthesized accordingly, to disclose the mechanism of the nanoscale patterning driven by the switch of solubility from the acid-catalyzed or electron-triggered ligand dissociation. By evaluating the dependence of MOCs' imaging process on photoacid, lithographies of photoresists with and without photoacid generators after exposure to ultraviolet (UV), electron beam, and soft X-ray, it is revealed that photoacid is essential in UV lithography, but it demonstrates little effect on exposure dose in high-energy lithography. Furthermore, theoretical studies using DFT simulations to investigate the plausible photoacid-catalyzed, electron-triggered dissociation, and accompanying radical reaction are performed, and a mechanism is demonstrated that the nanoscale patterning of this type of MOCs is driven by the solubility switch resulting from dissociation-induced strong electrostatic interaction and low-energy barrier radical polymerization with other species. This study can give insights into the chemical mechanisms of patterning, and guide the rational design of photoresists to realize high resolution and high sensitivity.

Effective Optimization Strategy for Electron Beam Lithography of Molecular Glass Negative Photoresist

AbstractAs the crucial dimension (CD) of logic circuits continues to shrink, the photoresist metrics, including resolution, line edge roughness, and sensitivity, are faced with significant challenges. Photoresists are indispensable in the integrated circuit manufacturing industry, and specifically in achieving smaller critical dimensions. In this study, the effects of two categories of photosensitive compounds on lithography performance are explored, through a series of sulfonium salt‐based photoacid generators (PAGs) with diverse reactivity and photodegradable nucleophiles (PDNs) with varying nucleophilicity. The detailed characterization and exposure experiments suggest that the reactive alterations of different PAGs are mostly associated with the amount of phenyl composed of cations in PAGs. The “PDN first, PAG second” strategy, which employs a combination of low reactivity PAG and high reactivity PDN and involves PDN decomposition first and PAG decomposition second in the electron beam lithography process, achieves high sensitivity (100–270 µC cm−2), high resolution (25 nm 1:1 line/space, L/S), and low line edge roughness (LER ≤ 3.3 nm) stripes. This approach outperforms conventional formulations and may provide a potentially effective and useful strategy to improve electron beam photoresists.

Latent and controllable doping of stimuli-activated molecular dopants for flexible and printable organic thermoelectric generators

Conjugated polymers (CPs) are a promising class of materials for organic thermoelectric generators (OTEGs); however, achieving high electrical conductivity through molecular doping while maintaining compatibility with thin-film printing processes remains a huge challenge. In this paper, we present a novel doping strategy using stimuli-activated molecular dopants (SAMDs) based on photoacid generators (PAGs) that can be activated by light of a specific wavelength. We demonstrate that this approach can effectively control the doping efficiency and optoelectronic properties of CP-PAG-blended thin films, resulting in a wide range of electrical conductivities. Our selected PAG molecules enabled efficient printing of the CP-PAG mixed solution and yielded a high thermoelectric figure of merit. To elucidate the mechanism behind this high thermoelectric performance, we systematically investigated the morphologies, microstructures, and energy structures of the PAG-doped CP thin films and performed various comparative tests. We also demonstrate the feasibility of using SAMDs to print flexible OTEG modules on thin polyimide substrates. We believe that our work represents a significant step toward the development of efficient, scalable, and sustainable thermoelectric devices for power generation and waste heat recovery, and highlights the advantages of PAG-based SAMDs for printable organic thermoelectrics.