Chemically Amplified Resist Research Articles

Effects of acid generator anions on radiation-induced decomposition and dissolution kinetics of chemically amplified resists

Chemically amplified resists (CARs) are widely used in lithography for manufacturing semiconductor devices. To reduce the occurrence of stochastic defects in CARs, increased acid generator concentration is required. In this study, we investigated the effects of acid generator anions on the radiation-induced decomposition of acid generators using electron pulse radiolysis and γ-radiolysis methods. Their effects on the dissolution dynamics of poly(4-hydroxystyrene) (PHS) films were also investigated using contact angle measurement and quartz crystal microbalance methods. Triphenylsulfonium trifluoromethanesulfonate, triphenylsulfonium nonafluoro-1-butanesulfonate, triphenylsulfonium 4-toluenesulfonate, and triphenylsulfonium salicylate, were used as acid generators or photodecomposable quenchers. The anions showed minimal effect on the decomposition of the acid generators and photodecomposable quenchers; however, they influenced the surface free energy, dissolution kinetics of the PHS films, and water penetration into the PHS films. In particular, the effect of salicylate on the dissolution kinetics of PHS films is significant.

Dynamics of ionized poly(4-hydroxystyrene)-type resist polymers with tert-butoxycarbonyl-protecting group

The imaging reactions of resist materials used for nano-patterning have become radiation-chemical reactions, with the shortening of wavelengths of the exposure light sources in lithography systems. The most widely used patterning materials in industrial lithography are chemically amplified resists (CAR). Understanding the deprotonation mechanism of ionized polymers (radical cations) is important for acid generation in CARs. In this study, the dynamics of radical cations in poly(4-hydroxystyrene) (PHS)–type resist polymers, partially and totally protected by tert-butoxycarbonyl (t-BOC) groups, are investigated using a combination of electron pulse radiolysis experiments, acid yield measurements, and quantum chemical calculations. The t-BOC(oxy) group exhibits π-electron-donating behavior in the monomer cation but changes to electron-accepting behavior in the polymer cation, owing to the interaction between substituents. The destabilization of radical cations due to decreased intramolecular charge resonance may contribute to the high deprotonation efficiency of t-BOC-capped PHS polymers.

Aqueous Developable and CO2‐Sourced Chemical Amplification Photoresist with High Performance

AbstractSeeking high‐performance photoresists is an important item for semiconductor industry due to the continuous miniaturization and intelligentization of integrated circuits. Polymer resin containing carbonate group has many desirable properties, such as high transmittance, acid sensitivity and chemical formulation, thus serving as promising photoresist material. In this work, a series of aqueous developable CO2‐sourced polycarbonates (CO2‐PCs) were produced via alternating copolymerization of CO2 and epoxides bearing acid‐cleavable cyclic acetal groups in the presence of tetranuclear organoborane catalyst. The produced CO2‐PCs were investigated as chemical amplification resists in deep ultraviolet (DUV) lithography. Under the catalysis of photogenerated acid, the acetal (ketal) groups in CO2‐PC were hydrolysed into two equivalents of hydroxyl groups, which change the exposed area from hydrophobicity to hydrophilicity, thus enabling the exposed area to be developed with water. Through normalized remaining thickness analysis, the optimal CO2‐derived resist achieved a remarkable sensitivity of 1.9 mJ/cm2, a contrast of 7.9, a favorable resolution (750 nm, half pitch), and a good etch resistance (38 % higher than poly(tert‐butyl acrylate)). Such performances outperform commercial KrF and ArF chemical amplification resists (i.e., polyhydroxystyrene‐derived and polymethacrylate‐based resists), which endows broad application prospects in the field of DUV (KrF and ArF) and extreme ultraviolet (EUV) lithography for nanomanufacturing.

Aqueous Developable and CO2-Sourced Chemical Amplification Photoresist with High Performance.

Seeking high-performance photoresists is an important item for semiconductor industry due to the continuous miniaturization and intelligentization of integrated circuits. Polymer resin containing carbonate group has many desirable properties, such as high transmittance, acid sensitivity and chemical formulation, thus serving as promising photoresist material. In this work, a series of aqueous developable CO2-sourced polycarbonates (CO2-PCs) were produced via alternating copolymerization of CO2 and epoxides bearing acid-cleavable cyclic acetal groups in the presence of tetranuclear organoborane catalyst. The produced CO2-PCs were investigated as chemical amplification resists in deep ultraviolet (DUV) lithography. Under the catalysis of photogenerated acid, the acetal (ketal) groups in CO2-PC were hydrolysed into two equivalents of hydroxyl groups, which change the exposed area from hydrophobicity to hydrophilicity, thus enabling the exposed area to be developed with water. Through normalized remaining thickness analysis, the optimal CO2-derived resist achieved a remarkable sensitivity of 1.9 mJ/cm2, a contrast of 7.9, a favorable resolution (750 nm, half pitch), and a good etch resistance (38 % higher than poly(tert-butyl acrylate)). Such performances outperform commercial KrF and ArF chemical amplification resists (i.e., polyhydroxystyrene-derived and polymethacrylate-based resists), which endows broad application prospects in the field of DUV (KrF and ArF) and extreme ultraviolet (EUV) lithography for nanomanufacturing.

A Novel Process to Reduce Roughness in Chemically Amplified Resist (CAR) for Next-Generation Lithography

A Novel Process to Reduce Roughness in Chemically Amplified Resist (CAR) for Next-Generation Lithography

Chemical Structure-Physicochemical Property Relationships of Copolymers Utilizable for Negative-Tone Photoimaging via Chemical Amplification.

We demonstrate an understanding of different physicochemical properties of copolymers induced by systematic changes in their structural parameters, i.e., the chemical structure of the comonomer unit, composition, molecular weight, and dispersity. The terpolymers were designed to be implemented in a chemically amplified resist (CAR) to form negative-tone patterns. With two basic repeating units of 4-hydroxystyrene and 2-ethyl-2-methacryloxyadamantane as monomers for conventional CARs, the pendant group of the third methacrylate comonomer was varied from aromatic, aliphatic lactone to lactone rings to modulate the interaction capability of the copolymer chains with n-butyl acetate, which is a negative-tone developer. Along with these structures, the monomer composition, molecular weight, and dispersity were also controlled. Physicochemical properties of the synthesized copolymers having controlled structures, i.e., dissolution behaviors and quantified Hansen solubility parameters, surface wetting characteristics, and surface roughness, which can be important properties affecting patterning capability in high-resolution lithography, were explored. Furthermore, the feasibility to use experimentally determined Hansen solubility parameters of the copolymers for the prediction of pattern formation using a coarse-grained model was assessed. Our comprehensive studies on the correlation of the structural parameters of the copolymers with final properties offer fundamental avenues to attain effective designs of the complex CAR system toward the lithographic process to achieve a sub-10 nm dimension, which is close to a single-chain dimension.

Understanding the onset of EUV resist chemical stochastics

Abstract The ability of chemically amplified resists to transfer an aerial image at increasingly smaller dimensions is critical to EUV lithography success at increasingly smaller process nodes [Bisschop and Hendrickx, Extreme Ultraviolet (EUV) Lithography X (SPIE) 10957 37 (2019)]. Stochastic inhomogeneities in resist exposure and patterning have been studied, which include photon shot noise and resist surface roughness. However, previous work has indicated that inhomogeneities and defectivity are present in multicomponent resists beyond those predicted by random statistics [Jablonski et al. (Santa Clara, CA) p. 302 (2004); Woodward et al. Advances in Resist Materials and Processing Technology XXIV (SPIE) 6519 416 (2007); Fedynyshyn et al. Advances in Resist Technology and Processing XXIII (SPIE) 6153 387 (2006); Fedynyshyn et al. J. Vac. Sci. Technol. B, 24, 3031 (2006); Kohyama et al. Advances in Patterning Materials and Processes XXXVI (SPIE) 10960 218 (2019)]. This is thought to be due to self-segregation of components in the multi-component chemically amplified resist. The results in this paper show that the most critical part of the resist chemical segregation occurs during the spin coating process after a significant amount of the solvent has evaporated, but while there is still enough solvent to enable molecular mobility within the resist.

Optimization of chemically amplified resist formulation based on simple random sampling and kernel density estimation

Formulation optimization plays an important role in the research and development of chemically amplified resist (CAR). However, the CAR profile after development process is influenced by multiple resist parameters and process conditions, so it is hard to determine the optimal CAR formulation in the multivariate problem. An optimization method for the CAR formulation is developed. The simple random sampling is applied to each CAR parameter’s value range independently, and the combinations of these samples from different parameters are used in the simulation of lithography profiles. Kernel density estimation is applied to analyze the simulation results. Then the CAR formulation is optimized based on the probability density distribution from the analysis results. The verification results show that the proposed optimization method can greatly improve the stability of the CAR formulation and thus generating acceptable critical features’ sizes of the CAR profile.

Synthesis of a PCST-containing acrylic polymers and its application in negative-tone photoresist

The process design and synthesis of new acrylate photoresist is still a challenge due to the limited resolution and sensitivity of resist. In this report, acrylate derivatives with different p-chlorostyrene (PCST) contents were prepared. The effects of different monomer contents on the film-forming properties and sensitivity of the polymers were discussed. The introduction of PCST significantly improves the thermal stability and film-forming properties of the polymers, and reduces the polydispersity (PDI) of the polymers to a certain extent, facilitating the achievement of high sensitivity and resolution of chemically amplified resists (CARs). Under optimized conditions, a final linewidth of 105 nm can be obtained. This new acrylate photoresist with good thermal stability and high resolution is also expected to be used in the next generation of lithography.

Study and comparison of resist characteristics for different negative tone electron beam resists

In this work, four types of negative electron beam resists are investigated. Electron beam lithography (EBL) experiments were conducted using EBL system ZBA23 (Raith) with the variable-shaped electron beam cross-section at 40 keV electron energy. Important electron beam resist characteristics such as sensitivity, dissolution rate, aspect ratio and sidewall developed profiles in the chemically amplified resist (CAR) SU-8 2000, non-CARs ma-N 2410 and ARN-7520, and inorganic negative resist HSQ XR-1514 are studied and compared. This study was motivated by the selection of a suitable resist for practical use such as large area gratings fabrication for optoelectronics.

Stochastic defect generation depending on tetraalkylhydroxide aqueous developers in extreme ultraviolet lithography

Patterning targets in leading-edge technologies such as extreme ultraviolet lithography (EUVL) are starting to push present photoresist materials (e.g. chemical amplification resists) to their physical limits. The appearance of randomly occurring (stochastic) photoresist-based defects in these stringent patterning targets has become one of the main concerns in EUVL. To obtain possible clues to understanding these stochastic defects, the effect of developer solutions (alkyl chain length of tetraalkylammonium hydroxide) on stochastic defects was investigated. This paper was built on our previous work in which we investigated the dissolution dynamics of three types of typical EUV photoresist processed in developer solutions with different alkyl chain lengths. Using the same materials, we found from EUV patterning experiments focusing on contact hole (CH) patterns that the long-alkyl-chain developer solution, i.e. tetrabutylammonium hydroxide, was effective in mitigating stochastic defects in acryl-type and hybrid-type photoresists (the latter being more commonly utilized for EUVL). (147/150)

Development of Selenonium PAGs in EUV Lithography toward High Sensitivity Achievement

Extreme ultraviolet (EUV) lithography at 13.5 nm has become an essential technology for the mass production of state-of-the-art semiconductor integrated circuits for use in devices such as smartphones. Chemically amplified resists (CARs) are primarily used for their production, and several performance requirements have to be met to achieve fine patterning. The requirements are known for simultaneous achievement of resolution, low line edge roughness (LER), high sensitivity, and low outgassing. As photoacid generators (PAGs) are ingredients for determine CARs sensitivity, high absorption of EUV light, acceptability of the secondary electron from matrix polymer, and high decomposability (high acid generation efficiency) are required in EUV lithography. Most onium-salt structures that have been proposed for use as EUV PAGs possess a sulfur atom at its center of cation. In this study, we focus on selenonium salts possessing selenium atoms at its center of cation, which exhibit strong absorption of EUV light, and evaluate their utility in EUV lithography.

Grazing-Incidence Soft-X-ray Scattering for the Chemical Structure Size Distribution Analysis in EUV Resist

In extreme ultraviolet lithography (EUVL), it is required to develop EUV resist which has low line width roughness (LWR) for the further miniaturization of circuit pattern. In order to reduce LWR, it is necessary to analyze and control the chemical-components spatial distribution in the resist thin film. We have reported that the measurement of chemical-components spatial distribution in the resist thin film coated on the Si3N4 membrane using the method of the transmission-mode resonant soft X-ray scattering (RSoXS). In this study, in order to analyze the chemical-structure-size distribution in the resist thin film on a Si wafer under similar condition adapted to the resist-coating actual process, we examined the grazing-incidence RSoXS (GI-RSoXS). A chemically amplified resist (CAR) and a non-CAR were spin-coated on silicon wafers, which had varied film thickness of 20, 50, and 100 nm. The scattering profile of each sample was measured at the incident photon energy of 280 and 296 eV. As a result, it is suggested that the chemical-structure-size distribution in the resist thin films depends on the resist film thickness. It is confirmed that the GI-RSoXS method is very effective to evaluate the chemical-structure-size distribution of resist thin films.

Effect of Alternative Developer Solutions on EUVL Patterning

This work explores the application of alternative developer solutions (“developers”) with the aim of understanding their potential effectivity in the reduction of resist-based stochastic pattern defects (or “stochastic defects”) in extreme ultraviolet lithography (EUVL). Specifically, the application of a quaternary ammonium hydroxide type aqueous developer; ethyltrimethylammonium hydroxide (ETMAH) in comparison to the industry de facto standard aqueous alkali tetramethylammonium hydroxide (TMAH) developer was investigated. Focusing on EUV exposed contact hole (CH) patterns on a typical chemically amplified resist (CAR), the effect of these developers on stochastic defects were assessed. As a result, patterning investigations showed that the lithographic performance of the CAR developed in ETMAH is comparable to the those obtained with TMAH. In situ resist dissolution analysis using the high-speed atomic force microscope (HS-AFM) confirms this as it showed that the rate of CH formation during resist dissolution in both developers are relatively the same. Moreover, it was also understood that compared to the commonly used alkali developer concentration of 0.26N, a lower ETMAH developer concentration of 0.20N resulted in stochastic defect margin improvement, while maintaining lithographic performance. In situ resist dissolution analysis showed an obvious slowing down of CH pattern formation rate at 0.20N concentration, suggesting the possibility that of over-dissolution at the higher concentration condition, translating to an increase in merging CH defects. The results from this study show the advantages of further pursuing optimal developers for EUVL. This is especially significant as these findings indicate how optimal developers mitigate resist-based stochastic defects while maintaining lithographic performance.

Progress in EUV Photoresists for High-Resolution Patterning

The high-NA extreme ultraviolet (EUV) lithography scanner, which will push the resolution limit to single-digit nm half pitch in a single exposure step, is predicted to be introduced for the most advanced technology node in a few years. To successfully print such fine patterns with enough sensitivity and low roughness, entirely new photoresist platforms superior to traditional organic polymer-based chemically amplified resists (CAR) are essential. We have been pursuing novel high-resolution photoresist material under the assumption that incorporating inorganic atoms and precise control of molecular sizes and structures are the key solution to print the finest featured patterns without stochastic printing failures. Herein we overview our recent progress in metal-containing photoresist materials and report our preliminary results of deep ultraviolet (DUV) patterning with a new organic single-component molecular glass photoresist that possesses ultimate material heterogeneity.

Accelerated Diffusion Following Deprotection in Chemically Amplified Resists.

Polymeric chemically amplified resists (CARs) are critical materials for high-throughput lithographic processes. A photoactivated acid-anion catalyst changes the polymer's solubility via a deprotection reaction, which enables pattern development through selective dissolution. To capture observed reaction kinetics, reaction-diffusion models employ a catalyst diffusivity that is accelerated by reaction. However, the microscopic origin and factors contributing to this phenomena remain unclear. Herein, we employ detailed atomistic molecular dynamics simulations to examine the impact of protecting group removal and material relaxation on catalyst mobility. We report data on polymer density, catalyst dispersion, excess free volume, and segmental dynamics with increasing time/extent of deprotection. We then propose simple kinetic Monte Carlo algorithms that can describe both molecular dynamics simulations of deprotection reactions and experimental data.

Electrostatic effect on the dissolution kinetics of poly(4-hydroxystyrene) in alkaline aqueous solution

The effects of pH and ionic strength on the dissolution behavior of poly(4-hydroxystyrene) (PHS) in an alkaline solution were studied to gain a fundamental understanding of the dissolution of chemically amplified resists (CARs). Tetramethylammonium hydroxide, which is a standard developer in recent lithography, and its corresponding salts were used. Dynamic light scattering (DLS) and quartz crystal microbalance (QCM) methods were used to study the state of PHS chains in solution and the dissolution behavior of PHS films, respectively. In DLS, a higher pH and ionic strength caused PHS chains to become more dispersed owing to an increase in the number of dissociated acidic groups and the resulting increase in repulsive force. The results of the QCM method showed that the diffusion of the polymer into the liquid phase from the film corresponds to an increase in the number of dissociated acidic groups, but did not correspond to the magnitude of transient swelling.

Study on deprotonation from radiation-induced ionized acrylate polymers including acid-generation promoters for improving chemically amplified resists

The demand for improved performance of chemically amplified resists (CARs) is continually increasing with the development of extreme ultraviolet lithography. Acid-generation promoters (AGPs) increase the sensitivity of CARs by increasing the initial acid yield immediately after the exposure process. However, the detailed mechanism of acid-yield enhancement has not been clarified yet. Deprotonation from the ionized polymer (i.e. radical cations) is an important reaction to assess acid generation. In this study, we investigated the dynamics of the radical cations of methacrylate polymers and the effect of an AGP on deprotonation from the radical cations formed by ionizing radiation. We clarified that the promotion of deprotonation by the AGP is more effective for the polymer with lower deprotonation efficiency. In addition, a molecular-level approach using density functional theory and molecular dynamics calculations were carried out.

Chemically amplified resist CDSEM metrology exploration for high NA EUV lithography

Background: The chemically amplified resist (CAR) has been the workhorse of lithography for the past few decades. During the evolution of projection lithography to extreme ultraviolet lithography (EUVL), a continuous reduction in feature size is observed. Also, a reduction in resist film thickness (FT) is required to prevent large aspect ratios that lead to pattern collapse. A further reduction in resist FT, into an ultrathin film regime (<30 nm resist FT), is expected when advancing to high NA EUVL. This brings along associated challenges with (1) resist critical dimension scanning electron microscope (CDSEM) metrology and (2) resist patterning performance. Aim: Assessment of metrology challenges and patterning limits of a CAR working in this ultrathin film regime. Deconvoluting the metrology and patterning effect on the determination of the unbiased line width roughness (uLWR). Approach: Patterning a CAR at different nominal resist FTs on two different underlayers to quantify the changes in CDSEM image quality and resist patterning performance with the resulting uLWR changes. Results: The CDSEM image signal-to-noise ratio (SNR) depends on resist FT and the underlayer. The uLWR increases with a reduction in resist FT but scales differently on the two underlayers. Conclusions: A relationship between CDSEM image SNR and uLWR is found. The SNR and uLWR scaling difference on the two underlayers, as well as the uLWR dependency on SNR was determined to be a metrology effect. The general uLWR increase for a reduced resist FT was determined to be a patterning effect.

Understanding the Exposure Process in the Extreme Ultra Violet Lithography.

Although being the optical lithography, the extreme ultraviolet (EUV) lithography with 13.5-nm wavelength is very different from the deep ultraviolet (DUV) lithography with 193-nm wavelength. Hence, the understanding of the complex detailed EUV mechanisms to cause a chemical reaction in chemically amplified resists (CARs) is required to develop EUV resists and exposure process. In this paper, for organic, metal-organic and metal-oxide resists, the electron-scattering model of exposure mechanisms needs to include the elastic and inelastic mean free paths. On top of that, Dill's parameters of DUV and EUV resisters from the photo-generated reaction are discussed to indicate the physical and chemical characteristics. For CAR and EUV resists, Dill B parameter is large than Dill A and B parameters.