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ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c01044
Article
Novel Etch-Resistant Molecular Glass Photoresist Based on Pyrene Derivatives for Electron Beam Lithography
Cong Xue †
Zhang Siliang †
Gao Jiaxing †
Cui Xuewen †
Wu Yurui †
https://orcid.org/0000-0002-2012-4399
Guo Xudong *†
https://orcid.org/0000-0003-4949-6214
Hu Rui †
https://orcid.org/0000-0002-8281-9399
Wang Shuangqing *†
https://orcid.org/0000-0002-5632-2290
Chen Jinping ‡
https://orcid.org/0000-0002-7018-180X
Li Yi ‡
https://orcid.org/0000-0003-0726-2217
Yang Guoqiang *†
† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
‡ Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
* Email: scoopguo@iccas.ac.cn.
* Email: g1704@iccas.ac.cn.
* Email: gqyang@iccas.ac.cn.
27 08 2024
10 09 2024
9 36 3758537595
01 02 2024
23 08 2024
14 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Novel t-butyloxycarbonyl-protected molecular glass photoresists with pyrene as the core (Pyr-8Boc and Pyr-4Boc) were designed and synthesized. The thermal stability and film-forming ability were measured to assess their applicability for lithography. Pyr-Boc (Pyr-8Boc and Pyr-4Boc) photoresists were evaluated by high-resolution electron beam lithography (EBL), acting as chemically amplified resists. Pyr-4Boc showed a better lithography performance, achieving 25 nm line/space patterns at the dose of 50 μC/cm2. Under SF6/O2 plasma, the etch selectivity of the Pyr-4Boc photoresist to silicon was 12.3, which is twice that of the commercially available poly(methyl methacrylate) photoresist (950 k). The lithography mechanism of EBL was further investigated. Theoretical calculations of HOMO/LUMO orbital energies, cyclic voltammetry, and fluorescence quenching experiments were conducted to confirm the electron-transfer reactions between the Pyr-Boc and photoacid generator. The study provides an option of high sensitivity and etch-resistant photoresist for EBL.

National Natural Science Foundation of China 10.13039/501100001809 22073108 Youth Innovation Promotion Association of the Chinese Academy of Sciences 10.13039/501100004739 2020035 National Natural Science Foundation of China 10.13039/501100001809 U20A20144 National Natural Science Foundation of China 10.13039/501100001809 22375209 National Natural Science Foundation of China 10.13039/501100001809 22275198 National Natural Science Foundation of China 10.13039/501100001809 22090012 document-id-old-9ao4c01044
document-id-new-14ao4c01044
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pmcIntroduction

With the development of the semiconductor industry, the denser-integrated circuits and smaller crucial dimension (CD) are needed correspondingly as predicted in Moore’s Law.1,2 According to Rayleigh formula,3 the resolution of CD is limited by the wavelength of optical lithography. The wavelength and lithography technologies have evolved from the initial g-line (436 nm) and i-line (365 nm) to 248 nm KrF and 193 nm ArF. The postoptical lithography, such as extreme ultraviolet (EUV), ion beam lithography, and electron beam lithography (EBL) are the most advanced lithography technologies for high resolution.4−12 As the most important material in lithography, the performance of photoresists is directly related to the success of lithography. Ito et al. initially proposed chemical amplified resists (CARs) for 248 nm lithography.13,14 The photoacid generators (PAGs) commonly used in these CARs are ionic iodonium salts and sulfonium salts,15 which have strong 248 nm absorption. Although the stochastic defects and uncontrolled acid diffusion become significant when the CD required lower to sub-20 nm, considering of the trade-off of resolution, sensitivity, and line edge roughness (LER), CARs are still the largest class of photoresist materials used today due to the high sensitivity to afford high wafer throughput.16,17 Research on this class of photoresist materials is ongoing, and attempts are being made to make further improvements in terms of increasing resolution,18 reducing LER,19 etc. Several research groups have conducted systematic reviews of this class of photoresists, and the results of these studies also serve as important references for this work.15,20 Further improvement of the resolution without sacrificing LER and sensitivity by material design is also the direction of future research. In addition, when photoresists are exposed to electron beam or EUV light, a cascade of reactions occurs in the exposure area, resulting in the generation of various types of reactive species.21 Therefore, the lithography mechanism of CARs in EBL and EUV is indeed complex, and further exploration is still required. The study of the lithography mechanism will play an important role in promoting the development and application of new photoresists.

For advanced lithography, the photoresists are supposed to fulfill the requirements of high resolution, high sensitivity, low line width roughness (LWR) or LER. Due to the large-molecular weight and uneven distribution, traditional polymer photoresists face challenges in obtaining high resolution and low LER patterns. The molecular glasses with well-defined structures, small and monodisperse molecular weights, are beneficial for achieving high resolution and low LWR/LER patterns.22−24 The slight modification of the cores or protecting groups can greatly affect the lithography performance. The selection of rigid and orthogonal conformation core with easily cleavable acid-labile groups is conducive to enhance the thermal properties, film formation, and sensitivity of photoresists. Several cores of molecular glasses have been reported, including bulky phenol derivatives,25 fullerene derivatives,26,27 and noria and calixarene derivatives.28,29 These photoresists could achieve 25–32 nm L/S by EUV.29 The synthesis and applications of the molecular glass photoresists have been previously reviewed by several research groups, and the conclusion is that this type of photoresist material will be a strong candidate for next generation of photoresist materials.22,30,31 Based on t-butyloxycarbonyl (t-Boc) groups, our previous reports have developed bisphenol A derivatives,32 9,9′-spirobifluorene derivatives,33 tetraphenylsilane derivatives,34 and adamantane derivatives,35,36 in which the resolution and LER were improved to 22 nm L/S and 3–4 nm LER, respectively. However, due to the low deprotection thermal activation energy, t-Boc groups could improve the sensitivity of photoresists while reducing the etching resistance, which greatly limits its application in photolithography.

After the patterns are precisely formed on the photoresist film, the etching process is conducted subsequently to transfer the patterns to the silicon substrate. Advanced lithography techniques require extremely thin photoresist films to minimize mechanical collapse of photoresists during development.37 Thus, the high etch resistance performance of photoresist is necessary to ensure that it would not be totally consumed before the etching process is completed. The resistance of the photoresist to reactive ion etching is determined by the molecular structure. It was predicted that molecular organometallic resists exhibit significantly better etch selectivity than organic resists due to the introduction of the metal atom.38 Some organometallic and coordinative photoresist materials containing Zr, Sn, Zn, and Fe have also been developed and achieved high resolution.39−41 However, the introduction of metal may cause metal ion contamination, leading to failure of the integrated circuit.42 In addition, several models based on empirical parameters have been established to predict the etching rate of organic photoresists, including Bond Contribution Model,43 Ring Model,44 and Ohnishi Model.45 They revealed that the etching resistance can be improved by increasing the carbon content, the carbon atoms contained in the ring structure, carbon–carbon single bonds, and carbon–carbon double bonds in the structure of photoresists.46,47 However, the commercially available photoresist in EBL, poly(methyl methacrylate) (PMMA), lacks cyclic structures, leading to poor etch resistance performance.

In this work, we reported two novel molecular glass photoresists based on pyrene derivatives with t-Boc groups protected (Figure 1). The core pyrene, linked to multiple benzene rings, provides etch resistance and thermal stability, while t-Boc acts as an acid-sensitive functional group to accomplish a dissolution transition at lower doses. The lithography performance and etch resistance of Pyr-Boc photoresists were evaluated by EBL and etching, respectively. They performed high sensitivity, resolution, and etch selectivity. The lithography mechanism of electron transfer was further investigated by cyclic voltammetry, DFT calculations, and fluorescence quenching experiments under 365 nm excitation.

Figure 1 Chemical structures of Pyr-4Boc and Pyr-8Boc.

Experimental Section

Instruments and Methods

The reagents and chemicals were purchased from commercial sources and used as received without any further purification. 1H NMR spectra were measured with Bruker Fourier 300 MHz or Bruker AVANCE III 400 MHz at ambient temperature. CDCl3 and DMSO-d6 were used as solvents, and tetramethylsilane was used as the internal standard. The molecular weight was determined by a MALDI-FTICR-MS. Thermogravimetric analysis (TGA) was performed in a N2 atmosphere with the heating rate of 10 °C/min from 30 to 500 °C. X-ray diffraction (XRD) curves of the powders were performed at room temperature.

Photoresist Film Preparation

The Pyr-Boc, triphenylsulfonium nonaflate (TPS-PFBS) (5 wt % of Pyr-Boc), and trioctylamine (10 wt % of PAG) were prepared and dissolved in propylene glycol methyl ether acetate (PGMEA) at a certain concentration. It should be noted that the formulations of photoresists have been optimized and selected to obtain a better photolithographic pattern in Supporting Information (Figure S1). The photoresist solution was filtered through a 0.2 μm membrane filter twice and spin-coated on a hexamethyldisiloxane primed silicon wafer with a CEE200X coating machine (Brewer Science CEE). Then, the prebake of 80 °C for 180 s on a hot plate was carried out to remove the solvent. The film thickness was measured by using an ASTSE200-BM spectroscopic ellipsometer (Angstrom Sun). Film roughness was estimated by using an atomic force microscope (AFM).

Electron Beam Lithography Performance

EBL patterning was performed by a Vistec EBPG 5000plus ES, using an exposure energy of 100 keV and a 100 pA beam current. After exposure, the postexposure bake was carried out at 90 °C for 30 s, then developed with 2.38 wt % tetramethylammonium hydroxide (TMAH) solution adding 1 mg/mL polyoxyethylene lauryl ether as a surfactant for 60 s (the surfactant polyoxyethylene lauryl was added to reduce the surface tension of water and increase the solubility of Pyr-Boc during development), and rinsed in pure water for 60 s before drying. The top-view and cross-sectional images were obtained by scanning electron microscopy (SEM) using a Hitachi Regulus 8230. The normalized remaining thickness (NRT) was measured by AFM. The LER was estimated by commercial ProSEM software.

Etching Resistance

An area of 0.5 × 0.5 μm2 rectangular was exposed by EBL. The developed film on the silicon substrate was etched by a mixture plasma of SF6/O2 at −110 °C for 10 s on a Sentech/Etchlab200. After etching, the residual photoresist was removed by ultrasonic vibration in ethyl acetate and tetrahydrofuran for 1 min, respectively.

Photophysical and Electrochemical Properties

UV–vis absorption spectra were measured on a Hitachi U-3900H. The oxidation and reduction potentials were measured by cyclic voltammetry using a CHI660C instrument, with glassy carbon and Pt wire as the working and auxiliary electrodes, respectively. The reference electrode was Ag/AgCl (saturated KCl), and the supporting electrolyte was 0.1 M n-Bu4NPF6. A scan rate of 100 mV s–1 was carried out. The redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured for calibration. HOMO and LUMO energies of molecules were calculated by , . The Gibbs free energy change ΔGet of an electron-transfer reaction can be estimated from ΔGet=Eox (D/D+) – Ered (A–/A) – E00, where Eox (D/D+), Ered (A–/A), and E00 are the oxidation of electron donor, the reduction of electron acceptor, and excitation energy of electron donor, respectively.

DFT Calculations of the HOMO/LUMO Orbital Energy

Gaussian 16 A.03 was used to calculate the HOMO/LUMO orbitals and energies.48 Geometry optimizations and frequency calculations for Pyr-Boc and TPS-PFBS were performed using B3LYP/6-31G(d,p) and m062x/6-31G(d,p) level of theory with empirical dispersion correction DFT-D3(BJ), respectively.49 Then, the single-point energies for Pyr-Boc and TPS-PFBS were calculated by B3LYP/def2TZVP and m062X/def2TZVP level of theory, respectively.50,51

Fluorescence Quenching

Fluorescence quenching experiments of Pyr-Boc were performed by using a Hitachi F-7100 fluorescence spectrometer with excitation at 365 nm. Concentrations from 0 to 9 mM of TPS-PFBS were added to the solution of 1 × 10–5 M Pyr-Boc.

Results and Discussion

Synthesis and Characterization

The synthesis of Pyr-Boc is shown in Scheme 1. Pyr-4Boc and Pyr-8Boc both started from the Suzuki coupling of commercially available tribromobenzene and 4-methoxyphenylboronic acid to give compound 1 in 58% yield. Miyaura-Ishiyama Borylation reaction of compound 1 and bis(pinacolato)diboron gave compound 2 in 91% yield. Then, Suzuki coupling between compound 2 with 1,3,6,8-tetrabromopyrene and 1,6-dibromopyrene was conducted, respectively, giving compounds 3 and 4 in 60 and 96% yields, respectively. The demethylation of compound 3 and followed by t-Boc protection reaction afforded the target compound Pyr-8Boc in a two-step yield of 49%. In a similar way, Pyr-4Boc was obtained in a two-step reaction of 63%. Details of the synthesis process are provided in our previous report,52 the synthesized Pyr-8Boc and Pyr-4Boc are evaluated in Supporting Information (Figures S2–S5).

Scheme 1 Synthesis of Compounds 1, 2, 3, 4, Pyr-8Boc and Pyr-4Boc

Thermal Behaviors and Film Forming Performance

The thermal stability of Pyr-Boc was characterized by TGA (Figure 2a). The initial decomposition temperature for Pyr-8Boc and Pyr-4Boc is 172 and 176 °C, respectively. It proves that Pyr-Boc photoresists can meet the needs of lithography, in which the process of soft bake and postexposure bake require the photoresist to be stable at baking temperature (90 °C).

Figure 2 (a) TGA curves of Pyr-Boc; (b) XRD curves of Pyr-Boc; AFM images of (c) Pyr-8Boc and (d) Pyr-4Boc photoresist film.

It is important to ensure that the surface of the photoresist film is uniform and free of particle defects; high crystallinity is not conducive to the formation of high-quality films. The XRD curves for Pyr-Boc show a broad central peak at 2θ = 20°, and the curve is smoother for Pyr-8Boc (Figure 2b), indicating the amorphous state of Pyr-8Boc and some microcrystallinity of Pyr-4Boc. This may be attributed to the less stereoscopic structure of Pyr-4Boc, but it obviously does not affect their uniform film formation obviously. The root-mean-square (RMS) roughness of the Pyr-8Boc and Pyr-4Boc films was estimated as 0.28 and 0.30 nm in an area of 5 × 5 μm2 (Figure 2c,d), respectively, suggesting that they are suitable for film formation and high-resolution lithography.

EBL Performance of Pyr-8Boc and Pyr-4Boc Photoresists

The sensitivity and contrast of Pyr-Boc in EBL were investigated by measuring the remaining film thickness after development at different exposure doses. Contrast (γ) is defined as the slope in the NRT curves by eq 1(53)1

For positive photoresists, D2 is the minimum dose required to remove the photoresists totally; D1 is the maximum dose at which the thickness of the photoresist remains constant. As shown in Figure 3a, 36 square patterns (1.5 μm × 1.5 μm) were exposed on the photoresist film, with the dose starting at 5 μC/cm2 and gradually increasing at a step of 5 μC/cm2. Then, the thicknesses of the exposed and developed patterns were measured (Figure 3b,c). Figure 3d shows the contrast curves of Pyr-Boc, and the sensitivity and contrast of Pyr-4Boc are 46 μC/cm2 and 4.9, respectively, and 50 μC/cm2 and 5.7 for Pyr-8Boc, respectively.

Figure 3 Sensitivity and contrast analysis of Pyr-Boc in EBL. (a) Electron beam exposure layout for NRT measurements. AFM images of (b) Pyr-4Boc and (c) Pyr-8Boc photoresist patterns developed with 2.38 wt % TMAH (adding 1 mg/mL polyoxyethylene lauryl ether) solutions. (d) Contrast curves of Pyr-4Boc and Pyr-8Boc.

The capability of forming 1:1 line/space (L/S) patterns of the Pyr-Boc photoresists is shown in Figure 4a. To minimize mechanical collapse in small pitch patterns, 34–36 nm film thickness was chosen. For Pyr-8Boc photoresist, the 40 and 30 nm L/S patterns were obtained at the dose of 50 μC/cm2, and the LER parameters were calculated as 4.8 and 4.7 nm, respectively. However, there are some obvious collapses and breaks in the 25 nm half pattern (HP). Compared to Pyr-8Boc, Pyr-4Boc achieves a higher resolution, the 40, 30, and 25 nm L/S patterns were observed at the dose of 45–50 μC/cm2, with 4, 4.2, and 4.6 nm LER, respectively. Details of the LER calculations are provided in Supporting Information in Figures S6 and S7. The larger film thickness facilitates the assessment of the steepness of the Pyr-Boc photoresist sidewall profile; therefore, cross-sectional and corresponding top-down images of lithographic patterns of 40 nm L/S lines with 60–80 nm thickness were investigated (Figure 4b). Both Pyr-8Boc and Pyr-4Boc can achieve steep sidewalls without top-rounding, and there is almost no residue in the trench, although some footing was observed.

Figure 4 (a) SEM images of the patterns with 40, 30, and 25 nm L/S of Pyr-Boc photoresists (film thickness: 34–36 nm) at the dose of 45–50 μC/cm2; (b) cross-section and corresponding top-down patterns of dense lines/space of HP 40 nm for the Pyr-8Boc photoresist (film thickness: 78 nm) and Pyr-4Boc photoresist (film thickness: 62 nm).

Etch Resistance and Pattern-Transfer Capability of Pyr-Boc Photoresists

The relationship between the molecular structure of photoresist and the etching rate can be described by the Ohnishi Model,45 as shown in eq 22

where V, NT, NC, and NO are the etching rate, total number of atoms in a molecule, the number of carbon atoms, and the number of oxygen atoms, respectively. According to the Ohnishi Model, it can be assumed that the proportionality constant is the same, and the etching rate ratio of PMMA,45 Pyr-8Boc, and Pyr-4Boc is calculated to be 2:1.04:1. The developed photoresist film on the silicon substrate was etched by a mixture plasma of SF6/O2, it was found that Pyr-8Boc showed an etch selectivity to silicon of 7.2, and Pyr-4Boc showed an etch selectivity of 12.3, which is twice that of the commercial electron beam photoresist, PMMA (950 k) (Figure 5a). Details of film thicknesses after development, etching, and resist stripping are provided in Supporting Information Figures S8–S10, respectively. Pyr-4Boc exhibits a higher etch resistance than Pyr-8Boc due to its larger percentage of carbon atoms in the molecular structure. Furthermore, cross-section patterns of 60 nm L/S after etching and resist stripping for Pyr-Boc photoresists are shown in Figure 5b,c. The 60 nm HP patterns were transferred accurately to the silicon wafer with an aspect ratio at 2.5:1. Due to the isotropy of the etching process, the transferred pattern showed a distinct concave profile, which can be improved by adding an underlayer on the wafer.

Figure 5 Measurements of etch selectivity and pattern transfer capability. (a) Etch selectivity to silicon of the PMMA (950 k), Pyr-8Boc, and Pyr-4Boc photoresists. The cross-section patterns of dense lines/space of HP 60 nm after etching and resist stripping for (b) Pyr-4Boc and (c) Pyr-8Boc photoresists.

Electron-Transfer Reactions between Pyr-Boc and PAG

In EBL, the photoresist matrix absorbs the high percentage of the radiation energy instead of the PAG due to its majority in CAR composition.54 Thus, the decomposition pathway of PAG becomes different from that of photolytic reactions. The common mechanism involves ionization of the photoresist matrix, the generation of secondary electrons and radical cations, the electron transfer between photoresist matrix and PAGs, the deprotonation of photoresist matrix, and the subsequent acid amplification reactions.55,56 In this work, conventional experimental methods were used to illustrate the electron-transfer process, which has rarely been tested in previous studies.

The redox behaviors of Pyr-8Boc, Pyr-4Boc, and TPS-PFBS in solution were investigated by cyclic voltammetry (Figure 6a). The first oxidation waves of Pyr-8Boc and Pyr-4Boc were estimated at Epox = 1.39 V and Epox = 1.28 V (vs Ag/AgCl), respectively, and the first reduction wave of TPS-PFBS was observed at Epred = −1.41 V (vs Ag/AgCl). Thus, the HOMO energies of Pyr-8Boc and Pyr-4Boc were calculated as −5.71 and −5.6 eV, respectively, and the LUMO energy of TPS-PFBS was calculated as −1.25 eV. It indicates that Pyr-4Boc is a better electron donor than Pyr-8Boc, in agreement with the calculations of Gaussian 16 A.03 (Supporting Information Figure S11). Based on these results, free energy change ΔGet of the electron-transfer reaction, in which Pyr-4Boc and Pyr-8Boc as the electron donor and TPS-PFBS as an electron acceptor were determined to be −0.73 and −0.4 eV, respectively. This means that Pyr-Boc can undergo an electron-transfer process with TPS-PFBS.

Figure 6 (a) Cyclic voltammogram of Pyr-8Boc, Pyr-4Boc, TPS-PFBS, and ferrocene in anhydrous dichloromethane solution at room temperature; (b) UV/vis absorption spectra of Pyr-8Boc (1 × 10–5 M), Pyr-4Boc (1 × 10–5 M), and TPS-PFBS (1.8 × 10–6 M and 1 × 10–6 M, which is 5 wt % of Pyr-8Boc and 5 wt % of Pyr-4Boc, respectively) in PGMEA; fluorescence emission quenching of (c) Pyr-4Boc (1 × 10–5 M) and (d) Pyr-8Boc (1 × 10–5 M) by adding different concentrations of TPS-PFBS from 0 to 9 mM, λex = 365 nm; Stern–Volmer plots (e) for Pyr-4Boc and (f) for Pyr-8Boc.

Both Pyr-4Boc and Pyr-8Boc show absorption at around 365 nm (Figure 6b), whereas PAG TPS-PFBS is transparent in this region. This suggests that TPS-PFBS is unable to undergo photoinduced decomposition by absorbing photons and reaching the excited state when exposed to 365 nm light. Under excitation at 365 nm, the fluorescence emission intensity of Pyr-Boc decreased with the presence of varying concentrations of TPS-PFBS in PGMEA (Figure 6c,d). The fluorescence quenching process can be described by the classical Stern–Volmer eq 3(57)3

where F0 and F are the fluorescence intensities in the absence and the presence of the quencher, respectively; Q is the concentration of quencher, and KD stands for the Stern–Volmer quenching constant. The linear relationship was found in F0/F and concentration of PAG (Figure 6e,f), and KD was calculated to be 16.8 for Pyr-4Boc and 8.3 for Pyr-8Boc, respectively. The fluorescence emission quenching experiments indicate the deactivation of excited state Pyr-Boc by electron transfer, proving that the electron-transfer reaction between Pyr-Boc and TPS-PFBS has taken place. In addition to using 365 nm ultraviolet light excitation, the pathway for Pyr-Boc to reach the excited state can also occur during EBL under high energy electron irradiation, and then the excited state of Pyr-Boc can undergo electron transfer with the PAG.

Proposed Lithographic Mechanism of Pyr-Boc Photoresists in EBL

Based on the aforementioned results and previous literature,58,59 a possible mechanism of electron beam exposure was proposed (Figure 7). Under electron beam irradiation, Pyr-Boc acts as a sensitizer for TPS-PFBS, undergoing ionization processes and transferring electrons to TPS-PFBS, in which the electron-transfer process has been validated using specific experimental methods. Then, the radical cation of Pyr-Boc donates protons to TPS-PFBS through deprotonation. The cation part of TPS-PFBS acts as an electron acceptor and decomposes,59,60 and the anion part obtains protons to form photoacid, which in turn catalyzes the deprotection reaction of t-Boc groups in Pyr-Boc, releasing carbon dioxide, and producing phenol hydroxyl groups. After alkaline water development, the exposed area is removed, leaving the unexposed area to form patterns.

Figure 7 Summary of the lithography mechanism of Pyr-Boc photoresists in EBL.

Conclusions

In summary, two molecular glass photoresists based on pyrene derivatives with t-Boc protecting groups have been synthesized (i.e., Pyr-4Boc and Pyr-8Boc), both of which have good thermal and film-forming abilities and are suitable as positive photoresists in EBL. Using Pyr-4Boc in EBL can achieve a higher resolution (HP 25 nm) with a high sensitivity (50 μC/cm2) and contrast (4.9). The Pyr-Boc photoresists exhibit high etch resistance, in which the etch selectivity of Pyr-4Boc is twice that of commercial PMMA photoresists (950 k). The electron-transfer reactions between the Pyr-Boc and the PAG were detected by conventional experimental methods. The validation of the electron-transfer reaction mechanism enables the explanation of possible reactions during the EBL process. These results show that Pyr-Boc photoresists are compelling candidates for applications in high-resolution EBL with improved performance.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c01044.Optimization of photoresist formulation; characterizations of Pyr-Boc; LER measurements of SEM images; etch selectivity of Pyr-Boc and PMMA (950 k); and DFT calculations of the HOMO/LUMO of Pyr-8Boc, Pyr-4Boc and TPS-PFBS (PDF)

Supplementary Material

ao4c01044_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (22073108, 22090012, U20A20144, 22275198, and 22375209) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2020035). We also thank the National Center for Nanoscience and Technology of China for EBL experiments and etching experiments.
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