Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted

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Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted Benzenes Kazumasa Okamoto, Naoya Nomura, Ryoko Fujiyoshi, Kikuo Umegaki, Hiroki Yamamoto, Kazuo Kobayashi, and Takahiro Kozawa J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09842 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted Benzenes Kazumasa Okamoto,*†‡Naoya Nomura, † Ryoko Fujiyoshi, † Kikuo Umegaki, † Hiroki Yamamoto, ‡

†

Kazuo Kobayashi, ‡ and Takahiro Kozawa‡ Division of Quantum Science and Engineering, Faculty/Graduate School of Engineering,

Hokkaido University, Sapporo 060-8628, Japan ‡

The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047,

Japan

ABSTRACT: Fluorination of resist materials is an effective method used to enhance the energy deposition of extreme-ultraviolet (EUV) light in the fabrication of next-generation semiconductor devices. The dynamics of radical ions is important to understand when considering the radiation-chemistry of the resist materials using EUV and electron beam lithography. Here, the dynamics of the radical anions and cations of benzenes with one or two 2hydroxyhexafluoroisopropyl groups (HFABs) were studied using radiolysis techniques. The formation of dimer radical cations was observed only in the mono-substituted benzene solutions of 1,2-dichloroethane. If the compound contained more than two substituents, it was found to hinder the necessary π-π overlapping. Pulse radiolysis of HFABs in tetrahydrofuran showed a characteristic spectral shift of the radical anion within the region of several hundred

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nanoseconds. From the results of low-temperature spectroscopy and density functional calculations, it is suggested that excess electrons of the 2-hydroxyhexafluoroisopropyl group of the radical anions causes dissociation into neutral radicals.

1. INTRODUCTION Fluorinated polymers containing a hexafluoroisopropanol (HFA) group have been previously developed as base materials of chemically amplified resists (CARs) for lithographies using ArF (193 nm) and F2 (157 nm) excimer lasers. This development is largely due to their relative transparency to these wavelengths and their possession of an OH group with a high pKa that may act as a proton source.1−7 Extreme ultraviolet (EUV, λ =13.5 nm) lithography is anticipated to be one of the most promising technologies for shrinking the pattern size of integrated devices, such as micro-processing units (MPUs) and dynamic random access memory (DRAM) with dimensions of less than 10 nm.8,9

As the thickness of the resist film is less than ten nanometers,

increasing the energy deposition efficiency against the ultra-thin film is important for the formation of a well-regulated pattern. To overcome this problem, the strategy involving increasing the absorption coefficients of the resist has been used.10−13 Fluorinated polymers are a candidate for use in the main composition of EUV resists. In the EUV region, the energy absorption efficiency mainly depends not on (σ, π, ν) → (σ∗, π∗) transitions, as in typical UV absorptions, but rather in the sum of the attenuation coefficients of constituent atoms, similar to the function of an X-ray.14,15 Increasing the number of F atoms in the resist formulation is a strategy used to enhance the absorption of the EUV as the EUV absorption cross-section of a F atom is much larger than that of those typically used in organic resist polymers, namely H, C, and O atoms.16 Hence, HFA substituted benzenes (HFABs) are a candidate for use in EUV


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resiexperimentalst compositions. In an EUV resist, benzene rings can be incorporated into its composition in forms such as polystyrenes, polyoxystyrenes, and novolaks; alicyclic skeletons, however, are currently used in resist polymers for ArF lithography (193 nm).1,2 Furthermore, HFAB has recently been applied towards the fabrication of new synthetic solvents.17 The radiation-induced reaction of fluorinated resists has been suggested to occur as follows:18−20 RF ⇝

RF+● + e−

(1),

AX + e− → products + X−

(2),

RF+● + RF → RF●(-H+) + RF(H+)

(3),

RF(H+) + X− → RF + H+X−

(4),

RF + e− → RF－●

(5),

RF(H+) + RF－● → RF + R●+ HF

(6).

First, base polymers (RF) are ionized by EUV irradiations, producing radical cations (RF+●) and electrons (1). Electrons are scavenged by photoacid generators (AX) such as sulfonium salts, which dissociate into the counter anion of the superacid (X−) (2). Deprotonations of RF+● occur via the proton-releasing group (3). Photoacids (H+X−) are produced via proton transfer in matrices and encounters with X− ions (4). However, the fluorination of the resist decreases the photoacid yield through the formation of the radical anion of RF (RF－●). RF－● partially causes the dissociation of F－ and the production of HF, which has been previously detected.21 Unlike



photolithography, the excited states seem not to play an important role for acid generation. Because the electron recombination is hardly occurred according to Reactions (2) and (5). Acid generation efficiency following exposure to an electron beam decreases with an increasing F atom composition in poly[4-hydroxystyrene-co-4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropyl)styrene] films.18 We have recently reported upon the pulse radiolysis of polystyrenes substituted with an HFA group.20 The transient absorptions of radical anions and the complex between cations and Cl － were assigned in tetrahydrofuran (THF) and 1,2-dichloroethane (DCE). However, the dynamics of the radical ions remained unclear because of the absence of the fundamental study of radiation-induced intermediates of HFABs. In this study, we investigated the dynamics of the radical ions of HFABs using pulse radiolysis, electron spin resonance (ESR), and computational calculations.

2. EXPERIMENTAL AND THEORETICAL PROCEDURE 2.1 Pulse radiolysis experiments Short-lived intermediates of HFABs following the application of ionizing radiation were observed by time-resolved spectroscopy using the pulse radiolysis method. In the sample solutions, fluorinated benzenes with mono- (HFAB) or di- (1,3-HFAB and 1,4-HFAB) substituted into a 2-hydroxyhexafluoroisopropyl group [HFABs (99.8-99.9%), Central Glass] were used as solutes. The molecular structures of the HFABs are shown in Fig. 1. For the selective formation of the radical anions and radical cations, THF and DCE (Sigma-Aldrich) without a stabilizer were used as solvents for the respective mono- and di-substituted HFABs. Solutions were deaerated by Ar bubbling and enclosed in a quartz cell with a 2.0 cm optical path


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length. A nanosecond pulse radiolysis system at the Institute of Scientific and Industrial Research (ISIR), Osaka University was then used.22 Samples were irradiated with an 8 ns electron beam (EB) pulse from a 27 MeV L-band LINAC, and an Xe flash lamp was used as the analyzing light. The doses per shot of the EB were about 270 Gy. A KSCN dosimeter in a quartz cell with a 1.0 cm optical path length was used for the dosimetry per single-shot electron beam. Measurements were performed at room temperature. 2.2 γ-Radiolysis experiments γ For the UV-vis spectroscopy, sample solutions [100 mM HFABs in 2-methyl tetrahydrofuran (MTHF), Sigma-Aldrich] were transferred into Suprasil quartz cells with a 10 mm light path and bubbled with Ar through a PTFE tube for at least 10 min. The sample cells were rapidly cooled under liquid N2 in a Dewar vessel and exposed to γ-rays produced by a

60

Co source at the

Institute of Scientific and Industrial Research (ISIR), Osaka University. The absorption dose was ca. 3 kGy. In the ESR spectroscopy, sample solutions were transferred into quartz sample tubes and were deoxygenated via Ar gas bubbling. γ-irradiation was then carried out at 77 K, similarly to the UV-vis spectroscopy experiments. Next, the sample tubes were transferred to ESR systems (JESRE2X, JEOR at 77 K and EMS micro, Bruker Biospin at 101 K) and the spectra were recorded. 2.3 Computational calculations The optimal electronic structures of the radical anion of HFABs were derived using quantum chemical calculations. Molecular orbital calculations were performed by way of the density function theory (DFT) using the Gaussian 09 package.23 In DFT calculations, the structure of the



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radical anions of HFABs were optimized using the UB3LYP functional and 6-31+G(d,p) basis sets. The charges and spin densities of HFAB were fixed at -1 and 2, respectively. Several initial structures were applied by rotating the HFA groups from the optimized structure of the neutral molecules by 90°. Time-dependent DFT (TD-DFT) calculations were also performed to estimate the absorption wavelength of the radical anion and cation to compare with the absorption spectra in the experiment. Using the polarizable continuum model (PCM), the solvent effects of THF and DCE were taken into account.

3. RESULTS AND DISCUSSION 3.1 Pulse radiolysis of HFAB solutions in THF and DCE The ionization of liquid hydrocarbons results from the application of ionizing radiation such as high-energy EB and γ-ray. From this, pairs of radical cations and electrons are initially produced. Direct observation of these short-lived intermediates has been carried out using the pulse radiolysis method for time-resolved spectroscopic measurements. In the case of a dilute solution sample, the solvent is largely ionized and excited because the energy deposition is nonselective, and thereafter charges and energy are transferred to the accepting solute. THF is one of the typical solvents used for generating the radical anion of the solute as follows:24, 25 THF

⇝

THF●+ +

e−

(7),

e−

+

HFAB →

HFAB●－

(8),

THF●+ +

THF

→

THF(−H)●

+ THF(+H+)


(9).

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Initially, THF is ionized and THF radical cations (THF●+) and secondary electrons (solvated electrons) are produced (7). The solvated electrons in THF are then captured by the HFAB,18 and the radical anion (HFAB●－) is produced (8). On the other hand, hole transfers from THF●+ to HFABs hardly occur, because THF●+ immediately decomposes within an order of picoseconds (9). Reaction of deprotonated THF●+ [THF(−H)●] with e− is also assumed. Figure 2 shows the transient absorption spectra of the HFABs solution in THF obtained at immediately (0), 50, and 150 ns following the 8 ns EB pulse irradiation. Absorption maxima are shown immediately after the EB pulse irradiation at 400 nm (HFAB and 1,3-HFAB) and 420 nm (1,4-HFAB), respectively. The absorption maxima at 400 nm observed in the HFAB solution shifted to 440 nm within the 8 ns EB pulse. This red-shift of absorption maxima was similarly observed in the 1,3- and 1,4-HFAB solutions within 150 ns after the EB pulse irradiations; however, the latter shifts were found to be slower than that of the HFAB solution. These absorption bands can be assigned to the radical anions of HFABs as no absorption maxima at around 400 nm were shown in the irradiated neat liquid THF (Figure S1). Since the absorption intensity of HFABs in THF in the near ultraviolet region increase compared to the neat liquid, the formation of neutral radicals through the HFAB decomposition is expected. These red-shifts are indicative of the molecular relaxation and/or reaction of the radical anion. Figure 3 shows the kinetic traces of the absorptions observed at 400 and 460 nm obtained by nanosecond pulse radiolysis. The absorptions at 400 nm decayed within the 8 ns pulse (time resolution) in the HFAB solution. The kinetic constants of decay were obtained as 3.9×107 s-1 in 1.3-HFAB and 2.7×107 s-1 in 1,4HFAB. On the other hand, absorptions at 460 nm were found to be stable within several hundred nanoseconds. To understand the dynamics of the radical anions in detail, UV-vis and ESR



spectroscopy in low temperature matrices (γ-radiolysis) were also carried out. Details of these analyses will be described later. The formation of the radical cation of HFAB (HFAB●+) was also studied using pulse radiolysis. The primary process of the radiation chemical reaction in the HFAB solution in DCE is suggested as follows: DCE

⇝ DCE●+

DCE + e−

+

e−

(10),

→ DCE●－/CH2ClCH2●

DCE●+ + HFAB →

HFAB●+

+ Cl−

+ DCE

(11),

(12).

DCE is first ionized and produces a DCE radical cation (DCE●+) and an electron (10).26 It was reported that the absorption of DCE●+ was observed at 360 and 550 nm immediately after the EB pulse irradiation. DCE effectively reacts with electrons because halogenated hydrocarbons possess a high electron affinity. This results in the production of DCE radical anions (DCE●－) and the dissociative electron attachment into a neutral radical (CH2ClCH2●) and a Cl anion (Cl−) (11). A hole transfer from DCE●+ to the solute also occurs, and the HFAB radical cation (HFAB●+) is produced (12). Ushida et al. reported the oxidation of aromatic compounds by chloromethyl radicals derived from the dissociative electron attachment of methylene chloride.27 Similar oxidation reactions between CH2ClCH2● and HFABs may occur. Figure 4 shows the transient absorption spectra of the HFABs in DCE obtained by nanosecond pulse radiolysis. In the HFAB solution, two absorption maxima at 500 and 1000 nm were observed within 150 ns after the EB pulse irradiations. Absorption below 320 nm was also observed. These absorptions at 320 and 500 nm were similarly obtained in polystyrenes and benzenes in chlorinated


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hydrocarbons resulting from charge transfer (CT) complexes between the phenyl ring and Cl atom [(HFABδ+…Clδ−)●] or ion pairs between radical cations of phenyls and Cl− (HFAB●+…Cl−).28−31 In the absorption bands in the near-infrared region (NIR), these were identified to result from the dimer radical cation of HFAB [(HFAB)2●+] in which the positive hole is delocalized into two molecules.32−35 The dimer radical cations are stabilized by the charge resonance (CR) interaction; the decay is therefore delayed in contrast with the transients observed at 320 and 500 nm. The reactions are summarized as: DCE●+ + HFAB →

HFAB●+

+ DCE

HFAB●+ + Cl− → (HFAB●+…Cl−) or (HFABδ+…Clδ−)● HFAB●+ + HFAB

(HFAB)2●+

(12), (13), (14).

In the 1,3- and 1,4-HFAB solutions, the absorptions of the DCE radical cation at 360 and 550 nm were observed immediately following the EB irradiation (Figure S2). Next, steep absorptions at 320 nm with a weak absorption around at 550 nm appeared within 50 ns after the EB pulse irradiation. These absorption bands were deemed to be not responsible for the production of DCE (Figure S2) because of the difference of in the absorption wavelength. Therefore, the absorption maxima at 550 nm are assigned to the CT complexes between the phenyl ring and Cl atom or the ion pairs between the radical cations of phenyls and Cl−, similarly to what was found in the HFAB solution. However, the absorptions at 550 nm demonstrated a lower intensity than those obtained in the HFAB solution. The efficient decomposition of the 1,3- and 1,4-HFAB radical cations via deprotonation is suggested as a mechanism, because the absorptions of the neutral radical (λ = 320 nm) derived from the HFABs were clearly observed.36,37 The instability of the radical cations is reflected in the non-formation of a dimer radical cation, as the 9 Environment ACS Paragon Plus


overlapping of π-electrons on the phenyl rings is difficult to achieve due to steric hindrance of the substituents. The yield of the dimer radical cation was enhanced with the HFAB conc. (Figure S3), although the intensity of the visible band was hardly changed. Therefore, we can assign to the near-IR band as the dimer radical cation. The calculated structure of HFAB dimer radical cation obtained using the PCM model in DCE is also shown in Figure 4S. According to the TD-DFT calculation, the CR band of the HFAB dimer radical cation is shown at 1112.82 nm (Oscillator strength = 0.1594). In addition, the stabilization energy of the dimer radical cation was found to be 0.46 eV without solvation using the PCM model. The energy is nearly equivalent to the half of the absorption maxima of the CR band (λMax/2 ≈ stabilization energy)32. Stabilization energy was estimated to be lower (0.18 eV) using the PCM model, because an increase in solvation energy of the monomer radical cation and the neutral HFAB molecule in DCE is larger than that of the dimer radical cation. The deprotonation of the intramolecular radical cation in polyhydroxystyrene (PHS) has already been identified to be delayed to a greater extent than that of the monomer radical cation.38 Therefore, it is suggested that the formation of the dimer radical cation of HFAB induces the slower deprotonation of the radical cation (lower deprotonation efficiency) than the 1,3- and 1,4-HFAB radical cations. 3.2 Dynamics of the radical anions of HFABs The absorption bands of the HFAB radical anions demonstrated red-shifts when pulse radiolysis was applied at room temperature. To clarify the dynamics of the radical anion, UV-vis and ESR spectroscopy in glassy-matrices at low temperatures (> 77K) were additionally carried out. Figure 5 shows the UV-vis absorption spectra in γ-irradiated glassy solutions of HFAB, 1,3HFAB, and 1,4-HFAB in MTHF after irradiation at 77 K in a Dewar vessel filled with liquid N2.


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Reactions in MTHF solutions are similar to those in THF, as shown in Eq. (7-9).39,40 The absorption maxima of the radical anions are shown at 380 nm, accompanying absorptions in the near-UV region. These absorption maxima, though similar to those determined from results of pulse radiolysis, demonstrated a blue-shift by about 20 nm in comparison with those obtained by pulse radiolysis at room temperature. The temperature of the sample cells were gradually increased using a cryostat to loosen the matrices of the solutions, as shown in Fig. 6. Because of the cracking of the solution during heating in the sample cell, measurements could not be carried out above 77 K for the 1,4-HFAB solution. The absorption spectra were found to change with an increasing temperature. Time dependent behaviors were plotted at a constant temperature (96 K in HFAB and 98 K in 1,3-HFAB) and are shown in Fig.6. The time intervals of the recording of the spectra were about 3 min. HFAB and 1,3-HFAB demonstrated negligibly small differences in absorption intensity from 77 to 88 K (HFAB) and 90 K (1,3-HFAB). With a further increase in the temperature, a decrease in the absorption intensities at 380 nm and new absorption maxima at 320 and 440 nm appeared. The red-shifts of HFAB and 1,3-HFAB radical anions from 380 to 440 nm were clearly observed in response to the red-shift of the radical anion absorption band of the radical anions of HFAB obtained by pulse radiolysis. In order to confirm the spectral change in the low temperature matrix, the electronic transition energy was calculated by TD-DFT (Figure S5). The transition of the radical anions before and after the relaxation was compared. The radical anions with the conformation before the relaxation show relatively large oscillator strength around at 380 nm, which was in good agreement with the spectra obtained in the low temperature matrix at 77 K. However, the most stable structure of the radical anions (after the relaxation) did not show the transitions with remarkable oscillator strength. In these results, red



shift of radical anions could not be explained well by the electronic or molecular relaxation of the radical anion. The ESR spectra for these systems were measured after γ-irradiation at 77 K in liquid N2, as shown in Fig. 7. The ESR spectra of the HFAB solutions obtained at 77 K clearly show seven lines due to the six-equivalent fluorines of the HFA group. The signals at 300 and 350 mT represent the H atom. ESR spectra measurements at 101 K were also carried out. ESR spectra were immediately recorded after transfer of the sample tube from the Dewar vessel at 77 K filled with liquid N2 to the sample holder at 101 K. The spectra obtained at 101 K are also shown in Fig. 7. It can be observed that the spectra drastically changed when the temperature was increased to 101 K. A four line-splitting is shown for all of the HFAB, 1,3-HFAB, and 1,4HFAB solutions. The H atom signals are also shown at 310 and 360 mT. In response to the absorption-shift of the UV-vis spectra at a low temperature (Fig. 6), the ESR spectra were subsequently converted to asymmetric forms, and the intensity was found to decrease at

Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted

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