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

Abstract

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.

---

1. Introduction

Photoacid generators (PAGs) are important components in photoresists, which are essential for advanced lithography.^1–3 PAGs contain photosensitive molecules that produce photoacids upon light/electron exposure, which then catalyze various chemical reactions that change the solubility of photoresists.^4,5 The efficiency of photoacid generation directly impacts the sensitivity of the photoresists and the optimal exposure dose.^6,7 In deep ultraviolet (DUV, 193 and 248 nm) lithography, photoacids are produced from bond cleavage in the singlet excited PAG molecules after light excitation.^8–10 However, this process is different in extreme ultraviolet (EUV, 13.5 nm) and electron-beam (EB) lithography, where direct excitation in the PAG molecule is insignificant.¹¹ A good mechanistic understanding of the dissociation of PAGs and the generation of photoacids is useful for the development of EUV and EB photoresists that have harsh requirements in terms of resolution, line-edge roughness (LER), and sensitivity.^12–14

In EUV lithography, light interacts with photoresist molecules and produces photoelectrons and secondary (low-energy) electrons.¹⁵ It has been proposed that charge transfer may occur among the generated cations and anions.^16,17 In this process, electrons attach to PAGs, causing their dissociation into a neutral radical and an anion.^18,19 It is generally considered that the photoacid can be produced when the anion extracts a proton from another molecule such as a photoresist cationic radical.^11,16,20 By-products are also generated along with the photoacid, which causes chamber contamination.¹⁷ The photoacid generation process during EB exposure is believed to be similar to that in EUV, with the difference that incident high-energy electrons interact with the photoresist and produce low-energy secondary electrons.²¹ So far, these understandings remain largely empirical and a detailed molecular description is incomplete. In multi-scale molecular simulations of advanced photoresists, the detailed process of photoacid generation from PAGs has not been thoroughly investigated.^22–24 For targeted molecular engineering of photoresists, knowledge of molecular dissociation and reaction pathways for photoacid generation is needed.

It has been shown that the quantum yield of photoacids in EUV lithography varies greatly with the choice of PAG.^15,25,26 It is thus likely that the PAGs with different molecular structures may have distinct photoacid generation mechanisms. However, the diverse PAG structures pose a challenge in revealing the full picture. Here, we focus on ionic PAGs, which constitue a major type of PAG used in advanced photoresists. The detailed pathways for photoacid generation after electron attachment and the formation of by-products are studied using density functional theory (DFT) calculations. Depending on the molecular structure and conformations of PAGs, there can be multiple dissociation pathways. The photoacid anion is found to affect the photoacid generation pathway and its energy barrier. These insights are useful for modeling and designing novel PAGs for improved photoresists.

2. Computational methods

We first optimized the conformations of the PAG anion and cation pairs using the M062X-D3 functional;^27,28 this is denoted as conformation 1. The M06-2X functional effectively describes non-covalent interactions and reaction enthalpy, which has been used in calculating the dissociation energy curve of TPS-Tf (an ionic PAG).^{22,23,29–31} To consider the different ionic conformations in solids, we also applied a conformational search approach.³² Molecular dynamics simulations of PAG were performed using the GFN0-xTB method³² at 400 K for 100 ps. Two thousand molecular conformations were selected at 50-fs intervals from the trajectory, which were then optimized at the GFN2-xTB level.³³ The five lowest energy non-equivalent conformers (with an energy difference of >0.5 kcal mol⁻¹ and root mean square displacement tolerance of >0.25 Å) were further optimized using the M062X-D3 functional and designated as conformations 2–6.²⁷

The LANL08(d) basis set was used for I and Sb, while 6-311+G(d,p) was adopted for other elements.^34,35 The integral equation formalism polarizable continuum model (IEFPCM) was applied in all the DFT calculations to approximate the solid environment, with the dielectric constant set to 4.^36–41

The vertical electron affinity (VEA) of PAGs was calculated using the following equation:⁴²

VEA = E(N) − E(N+1) (1)

where E(N) is the energy of the optimized neutral molecule and E(N + 1) is the energy after electron attachment at the optimized geometry of the neutral molecule. The bond dissociation energies (BDE) of PAGs were calculated using the following equation:⁴³

BDE = E(anion) + E(radical) − E(PAG⁻) (2)

Here, E(anion), E(radical), and E(PAG⁻) are the energies of the photoacid anion, the neutral radical, and the PAG after a secondary electron attachment at their optimized structures, respectively. The zero-point correction was considered in the calculation of BDEs.

To calculate the potential energy surfaces (PESs) for PAG anion and cation dissociation after electron attachment, we conducted a flexible structural scanning by increasing the ionic distance. The transition states (TSs) in this study were identified by searching structures with one imaginary frequency, with molecular structures optimized using the M062X-D3/6-311+G(d,p) method.^27,34 Intrinsic reaction coordinate (IRC) analyses⁴⁴ were performed to confirm that each identified transition state is connected to the reactant and product.

To assess the strength of the covalent bonds in the PAG cation, we calculated the Laplacian bond order (LBO).⁴⁵ The LBO between atoms A and B (LBO_A,B) is defined as the integral of the negative part of ∇²ρ in the bonding region (fuzzy overlap space):⁴⁵

where w_A(r) and w_B(r) represent atomic weights for atoms A and B, respectively. ∇²ρ(r) is the Laplacian function of electron density.

The semi-empirical calculations were carried out using xtb (version 6.4.1)³² and the conformation search was performed with the Molclus program.⁴⁶ All DFT calculations were carried out using Gaussian 16 software (B.01).⁴⁷ The LBO was calculated using Multiwfn software.⁴⁸

3. Results and discussion

In EUV or EB lithography, the photoacid anion originates from PAGs and subsequently extracts a proton to form a photoacid (Fig. 1). Here, a total of 25 ionic PAGs were selected, covering the widely used sulfonium salts and iodonium salts.^15,25,26,49

Fig. 1 (a) Schematic representation of photoacid generation in EUV and EB lithography. (b) The molecular structures of photoresists CR4-2BOC (fully protected) and CR4-BOC (partially protected).

3.1. PAG dissociation and bond cleavage

We note that, in an un-exposed photoresist, it is possible that the cation and anion of the PAG molecule exist in separated configurations, or are connected by the ionic bond.⁵⁰ We will discuss these two cases sequentially.

3.1.1. Initially separated PAG cations and anions. If the cation and anion are initially separated, the secondary electron mainly interacts with the cation of PAG, resulting in the cleavage of the S–C or I–C bond. According to the results of cationic radical flexible scanning after attaching secondary electrons (Fig. S1 and S2, ESI†), two mechanisms are identified: the non-spontaneous cleavage mechanism (Fig. 2a) and the spontaneous cleavage mechanism (Fig. 2b).

Fig. 2 Molecular structures of PAGs in the initially dissociated state, belonging to (a) the non-spontaneous cleavage mechanism and (b) the spontaneous cleavage mechanism. The dissociation mechanisms of PAGs in the initially bound state: (c) the dissociation-first mechanism, (d) the dissociation-last mechanism, (e) the one-step dissociation mechanism, and (f) a summary of the mechanisms each PAG is associated with. Blue, red, and green denote the dissociation-first, dissociation-last, and one-step dissociation mechanisms, respectively.

Fourteen of the twenty-five PAGs belong to the non-spontaneous cleavage mechanism, including TPS-Tf, M-TPS-Tf, F-TPS-Tf, I-TPS-Tf, TPS-Nf, TPS-PF₆, TPS-SbF₆, TPS-TSM, TPS-CPDI, PAG-3, PAG-4, PAG-2-Nf, PAG-6-Tf, and PAG-7-Nf. The cations of these PAGs are mainly triphenylsulfonium cation (TPS⁺) and its derivatives (except for PAG-6-Tf and PAG-7-Nf). The cleavage of the S–C/I–C bond in this type of PAG requires activation, with energy barriers ranging from 0.05 to 0.09 eV (Fig. S1, ESI†).

The rest of the PAGs (DPI-Tf, DPI-Nf, BBI-Tf, BBI-Nf, BBI-TSM, BBI-CPDI, PAG-8-Nf, PAG-1-Tf, PAG-5-Nf, PAG-9-Tf, and PAG-10-Nf) belong to the spontaneous cleavage mechanism, in which the cleavage of the S–C/I–C bond is barrierless (Fig. S2, ESI†). The cations in this type are mainly diphenyliodonium cation (DPI⁺) and its derivatives, as well as TPS⁺ derivatives with large substituents (PAG-1-Tf, PAG-5-Nf, PAG-9-Tf, and PAG-10-Nf).

3.1.2. Initially bound PAG cations and anions. If the anion and cation of PAGs are connected by an ionic bond in the un-exposed photoresist, the dissociation is promoted by electron attachment. Upon the attachment of a secondary electron, the ionic distance in the PAG increases (Fig. S3–S27, ESI†). It was previously believed that the dissociation of the anion happens first (denoted as the dissociation-first mechanism). Here, we identified two additional mechanisms: the dissociation-last mechanism and the one-step dissociation mechanism. We found that the dissociation mechanism depends on both the molecular structure and the conformation of the PAG.
The dissociation-first mechanism. The PAG first dissociates into a neutral radical and an anion, followed by the cleavage of the S–C/I–C bond in the radical, producing a phenyl radical and a neutral molecule (Fig. 2c). The PAGs involved to this mechanism are consistent with the PAG cations of the non-spontaneous cleavage mechanism in their initially dissociated state (Fig. 2a). The PAG cation are mainly TPS⁺ and its derivatives.

Specifically, twelve of the twenty-five PAGs (TPS-Tf, M-TPS-Tf, F-TPS-Tf, TPS-Nf, TPS-PF₆, TPS-SbF₆, PAG-3, PAG-4, TPS-TSM, TPS-CPDI, PAG-2-Nf, and PAG-1-Tf), regardless of their ionic conformations, belong to this mechanism (Fig. S3–S14, ESI†). Most of these cations are TPS⁺, featuring a sulfuric center with sulfonium–phenyl bonds.

Depending on the ionic conformation, I-TPS-Tf (conformations 1, 3, 4, 5, and 6), PAG-5-Nf (conformation 1), PAG-6-Tf (conformation 1), PAG-7-Nf (conformation 1) and PAG-9-Tf (conformations 3 and 6) also exhibit the dissociation-first mechanism (Fig. S15–S18, S26, ESI†). These have substitutions in the TPS⁺ cation or have an iodonium center with fluorine substituents on the side groups.

The dissociation-last mechanism. In this mechanism, after the attachment of a secondary electron, the I–C/S–C bond is broken to release a benzene radical, followed by the dissociation of a photoacid anion (Fig. 2d). This roughly corresponds to the PAG cations associated with the spontaneous cleavage mechanism in initially dissociated state.

Specifically, the six iodonium salts (DPI-Tf, DPI-Nf, BBI-Tf, BBI-Nf, BBI-TSM, and PAG-8-Nf; mainly diphenyliodonium salts and its derivatives), regardless of their conformations, belong to the dissociation-last mechanism. The I–C bond breakage in these iodonium salts is barrierless after electron attachment (Fig. S19–S24, ESI†).

Depending on the ionic conformations, PAG-5-Nf (conformations 2–6), PAG-6-Tf (conformations 2–6), PAG-7-Nf (conformations 2–6), BBI-CPDI (conformations 1, 3–5), PAG-9-Tf (conformation 5) and PAG-10-Nf (conformations 4 and 5) can also exhibit the dissociation-last mechanism (Fig. S16–S18 and S25–S27, ESI†). These PAGs include TPS⁺ cation derivatives with phenyl groups substituted, as well as those containing iodonium centers with fluorine substituents on the side groups (excluding BBI-CPDI).

The one-step dissociation mechanism. For some PAGs, depending on the ionic conformations, the anion dissociation and the bond cleavage occur simultaneously. These are I-TPS-Tf (conformation 1), BBI-CPDI (conformations 2 and 6), PAG-9-Tf (conformations 1, 2 and 4), and PAG-10-Nf (conformations 1, 2, 3 and 6), as shown in Fig. 2e. Their cations correspond to the spontaneous cleavage mechanism in the initially dissociated state.

To better understand the relationship between the above mechanisms and the PAG molecular structures and conformations, we analyzed PAG dissociation energies (BDE) (Table S1, ESI†) and the LBO parameters of the S–C/I–C bond in the PAG cation (Table S2, ESI†).

The BDEs of the PAGs associated with the dissociation-first mechanism range from 0.14 to 0.69 eV, which are generally higher than those of PAGs in the dissociation-last mechanism which range from 0.03–0.35 eV. Additionally, the larger the photoacid anion, the stronger the interactions between the PAG cation and anion, leading to higher BDEs (Fig. 3a). We also found a variation in the BDEs depending on the ionic conformation, which explains why some PAGs are associated with multiple mechanisms.

Fig. 3 (a) The BDEs of PAGs for different cations. (b) The LBO values of the S–C/I–C bond in PAG cations. The error bars represent the variations in LBO values from different ionic conformations.

The LBO of the S–C bond for the sulfonium PAGs ranges from 0.6418 to 0.6956 (Fig. 3b). These high values indicate that the S–C bonds are less likely to break. This also explains why they tend to follow the dissociation-first mechanism. When the phenyl group in the TPS⁺ cation (e.g., PAG-5-Nf, PAG-9-Tf, and PAG-10-Nf) is substituted with –CH₃ or dibenzothiophene derivatives, the LBO value of the S–C bond reduces to the range of 0.5940–0.6416, indicating reduced stabilities of the PAG cation. This explains why they can follow the dissociation-last mechanism or the one-step dissociation mechanism instead. The LBO values of the TPS⁺ cation and its derivatives are not impacted by the ionic conformations, which means that the latter influences the mechanisms mainly through BDE, as mentioned above.

On the other hand, the LBO of the I–C bond ranges from 0.2670 to 0.3854, significantly smaller than that of the S–C bond. Therefore, iodonium PAGs tend to exhibit the dissociation-last mechanism. In this case, both substitutions and the ionic conformations impact the LBOs, which aligns with the classifications of PAGs discussed above.

3.2. The generation of the photoacid

The generated anion (either initially dissociated or produced from the PAG molecule upon electron attachment) will then extract a proton from the photoresist to form the photoacid.²⁰ Here, we have used CR4, a widely used glass photoresist, as an example. We consider both the fully protected CR4-2BOC and the partially protected CR4-BOC (containing an –OH group) (Fig. 1b).⁵¹ Kozawa et al. have found that the diphenylsulfide cation (DPS⁺; from hole transfer between the photoresist cation and the DPS generated by PAG) can act as a proton source in anion-bound chemically amplified photoresists (which lacks t-BOC or –OH groups).¹⁷ Here, we found that the –OH (in CR4-BOC) and t-BOC groups (in CR4-2BOC and CR4-BOC) can provide protons more easily in the studied photoresists (see Table S3, ESI†). The calculations show that the proton of the –OH group in ionized CR4-BOC⁺ (from EUV radiation or high-energy electron collision) can be easily captured (barrierless) by the PAG anion (see Fig. S28, ESI†). The CR4-BOC⁺ will then become a neutral radical (CR4-BOC*).

In the case of fully protected photoresists (CR4-2BOC) or when the PAG anion does not have accessible –OH groups surrounding it, the protecting group (t-BOC) acts as the proton source. We identified three photoacid generation pathways, as shown in Fig. 4. We use the trifluoromethanesulfonate (Tf⁻) to illustrate them.

Fig. 4 The pathways for photoacid generation: (a) pathway A, (b) pathway B, and (c) pathway C. (d) The relative potential energies. The molecular configurations are shown in Fig. S29–S31 (ESI†).

Pathway A. The tert-butyl group interacts with the photoacid anion and separates from the ionized photoresist CR4-2BOC⁺, releasing CO₂ (Fig. 4a). This is the rate-determining step with a 0.79 eV energy barrier. Next, the photoacid anion captures a proton from the tert-butyl cationic radical, forming a photoacid and isobutene. The energy barrier of this reaction step is 0.16 eV.

Pathway B. The first step is similar to that in pathway A, except that the photoacid anion and the separated tert-butyl cation form a covalent bond (Fig. 4b). Compared to pathway A, the photoacid anion in the initial configuration is closer to the Cα atom of the t-BOC protective group. The energy barrier of this reaction step is 0.44 eV. The separation of isobutene requires bond breakage, resulting in a large reaction barrier of 1.19 eV in the second reaction step.

Pathway C. The photoacid anion can directly capture a proton from the t-BOC group (0.99 eV energy barrier), forming a photoacid and a neutral photoresist radical (Fig. 4c). Next, CO₂ is released from the photoresist radical. This process is endothermic and absorbs an energy of 0.95 eV. Finally, an isobutene molecule is removed from the photoresist radical with a reaction barrier of 0.91 eV.

Among the three pathways, pathway A is the most probable pathway for proton transfer from the t-BOC group, having the lowest overall reaction energy barrier (0.79 eV) and the most favorable initial configuration (Fig. 4d). In addition, the photoacid anions are found to have an impact on the reaction. Besides Tf⁻, we also investigated Nf⁻, PF₆⁻, and SbF₆⁻, as shown in Table 1. The reaction pathways of Nf⁻ are similar to those of Tf⁻, with an energy difference of ∼0.1 eV in the reaction barriers. However, for PF₆⁻ and SbF₆⁻, there are only two pathways (pathway A and pathway C) instead of three pathways. As such, the photoacid anions are not inclined to bond with the central carbon (Cα) of the tert-butyl cation. In all cases, pathway A remains the most probable one. The photoacid anions primarily affect the second reaction step (proton transfer). In the second step of pathway A, the reaction barrier of PF₆⁻ (0.73 eV) is much higher than those of Tf⁻ and Nf⁻ (0.16 and 0.26 eV).

Table 1 The reaction energy barriers (eV) of photoacid generation for different photoacid anions

		Tf^−	Nf^−	PF6^−	SbF6^−
Note: A convergence cannot be achieved for the transition state (A-TS2) of SbF6^−. IRCs are shown in Fig. S32–S35 (ESI).
Pathway A	Step 1	0.79	0.64	0.70	0.70
	Step 2	0.16	0.26	0.73
Pathway B	Step 1	0.44	0.51	—	—
	Step 2	1.19	1.05	—	—
Pathway C	Step 1	0.99	0.95	2.27	1.89
	Step 2	0.95	0.97	0.89	0.86
	Step 3	0.91	0.86	0.84	0.75

---

Therefore, when all photoresists are fully protected or the PAG anion does not have surrounding –OH groups, the photoresist cationic radical tends to dissociate into CO₂ and a tert-butyl cationic radical, followed by proton transfer that leads to the formation of photoacid. When the –OH groups are present (either from the initial photoresist structure or exposed due to photoacid catalysis), the photoacid may be generated through their proton transfer to the photoacid anion. As we have discussed, the latter process can happen more easily, which is consistent with the experimental result indicating that photoresists with lower protection ratios have larger photoacid yields.⁵²

3.3. Factors impacting the photoacid yield

To further explore the dominant factors that affect photoacid generation, we analyzed the relationship between experimentally reported photoacid yields²⁶ with vertical electron affinity (VEA), BDEs, and the energy barriers for proton transfer. As shown in Fig. 5a, although electron attachment is the first step for PAGs to produce photoacids, there is no strong correlation between the photoacid yield and VEA. This suggests that electron attachment is not the rate-limiting step in photoacid generation. On the other hand, PAGs with smaller BDEs (easier dissociation of the PAG anions and cations) and small proton transfer energy barriers lead to larger photoacid yields compared to the experimental data from Natsuda et al.²⁶ As can be seen in Fig. 5, DPI-Tf and BBI-Tf, having smaller BDEs and proton transfer energy barriers, also exhibit the largest acid yield (55% and 64%, respectively).²⁶ This suggests that in this case, the PAG cations and anions are likely bound by ionic bonds in the un-exposed film. Additionally, the impact of the proton transfer energy barrier indicates that the PAG anion cannot easily access the –OH groups within the photoresist film,⁵¹ likely due to steric hindrance.

Fig. 5 The reported photoacid yield as a function of (a) VEA, (b) BDE, and (c) the energy barriers of proton transfer. The experimental yield data are taken from ref. 26. PAGs with the same anion are marked in the same color.

3.4. The generation of by-products

The generation of photoacids is accompanied by the production of small-molecule by-products. They are a major source of contamination for the vacuum chamber and atomic-precision mirror surfaces. It is thus useful to understand how these by-products are produced.

For sulfonium PAGs, we studied DPS as an example (generated along with the photoacid anion generation for TPS-Tf, TPS-Nf, TPS-PF₆, TPS-SbF₆, TPS-TSM, TPS-CPDI, PAG-3 and PAG-4 in Fig. 2a and c). Experimental studies using high-performance liquid chromatography (HPLC) have shown that the by-products for TPS-Tf are mainly phenyl, DPS, and phenyl-substituted DPS.¹⁷ According to the above discussions, phenyl and photoresist radicals are generated during photoacid production. These radicals can extract hydrogen atoms from DPS, forming phenyl and DPS radicals, which then form covalent bonds with phenyl radicals, resulting in the final by-products (Fig. 6a). The reaction energy barriers for hydrogen extraction are found to depend on its location. For the phenyl radical extracting a hydrogen atom from the DPS, the energy barriers range from 0.64 to 0.86 eV, as shown in Fig. 6b. The lowest energy barrier corresponds to the ortho-position, which is consistent with the strongest signal of ortho-substituted DPS detected experimentally.¹⁷ For the photoresist radicals extracting hydrogen from the DPS, the energy barriers are larger, ranging from 1.55 to 1.88 eV (Fig. 6c), with the hydrogen in the ortho-position of the DPS still being the easiest to extract.

Fig. 6 (a) The by-product generation pathways from DPS and (b) and (c) the corresponding potential energy diagrams; (d) the by-product generation pathways from TIB and (e) and (f) the corresponding potential energy diagrams. The configurations of the transition state are shown in Fig. S36–S38 (ESI†).

For the by-products from iodonium PAGs, 4-tert-butyl-iodobenzene (TIB) is used as an example (generated along with the photoacid anion generation for BBI-Tf, BBI-Nf, BBI-TSM, and BBI-CPDI in Fig. 2b and d).⁵³ During photoacid anion dissociation, the 4-tert-butylphenyl radical is generated instead of the phenyl radical for sulfonium PAGs. The 4-tert-butylphenyl radical or the photoresist radical (from the photoacid production) may extract a hydrogen atom from TIB (Fig. 6d). For the 4-tert-butylphenyl radical, the reaction barriers for extracting hydrogen at the ortho- and meta-position of TIB are 0.73 and 0.85 eV, respectively (Fig. 6e). Similar to the case of sulfonium PAGs, extracting hydrogen atoms from the TIB by the photoresist radical is more difficult, as shown in Fig. 6f (the energy barriers are 1.62 and 1.67 eV in the ortho- and meta-positions, respectively). The 4-tert-butylphenyl radicals then react with each other through covalent bonds to form the final by-products.

4. Conclusions

In summary, the photoacid generation from ionic PAGs was systematically studied for EUV and EB lithography using density functional theory calculations. Based on a thorough analysis of the results, the following conclusions can be drawn:

(1) If the cation and anion are initially separated in the un-exposed photoresist, bond cleavage occurs in the cation upon the attachment of a secondary electron via two mechanisms: the non-spontaneous cleavage mechanism and the spontaneous cleavage mechanism. TPS⁺ and its derivatives tend to exhibit non-spontaneous cleavage mechanisms. On the other hand, TPS⁺ derivatives with large substituents, and the DPI⁺ cation and its derivatives tend to exhibit spontaneous cleavage mechanisms.

(2) If the PAG anion is initially bound to the cation in the un-exposed photoresist, it may be produced from the dissociation of the PAG molecule upon secondary electron attachment via three mechanisms, depending on the molecular structure and conformations of the PAGs. The PAGs containing TPS⁺ and its derivatives tend to exhibit the dissociation-first mechanism, while the PAGs containing iodonium–phenyl bonds tend to exhibit the dissociation-last mechanism. Substitutions and ionic conformations may affect the tendency of ionic dissociation and the difficulty of bond breaking in the PAG cation, leading to changes in their orders, which can also happen simultaneously.

(3) In the absence of –OH groups for fully protected photoresists or when the photoacid anion cannot access –OH groups in its surroundings, photoacid anions can extract protons from the protective groups of the photoresist cations after ionization to form photoacids via three different reaction pathways. The reaction energy barriers for the second reaction step are found to depend on the photoacid anion.

(4) When –OH groups are present (either from the initial photoresist structure or products of deprotection) and are accessible by the nearby PAG anion, the anion can directly capture the proton in the –OH group to form the photoacid.

(5) By-products are also generated from substitution reactions of the radicals (phenyl radical and 4-tert-butylphenyl radical) generated from the cleavage of S–C/I–C bonds in the PAG cationic radicals. The dominant isomers of the by-products can be determined from the energy barriers of the reaction pathways.

Overall, these results provide a comprehensive understanding of the microscopic processes of photoacid generation from ionic PAGs, which can be useful in the photoresist design for EUV and EB lithography.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant number 22090013), the Shanghai Committee of Science and Technology (grant numbers: 21QA1402900 and YDZX20213100002672) and the Shanghai Technical Service Center of Science and Engineering Computing at Shanghai University.

Notes and references

1. N. A. Kuznetsova, G. V. Malkov and B. G. Gribov, Russ. Chem. Rev., 2020, 89, 173–190.
2. H. Ito and C. G. Willson, Polym. Eng. Sci., 1983, 23, 1012–1018.
3. L. Li, X. Liu, S. Pal, S. Wang, C. K. Ober and E. P. Giannelis, Chem. Soc. Rev., 2017, 46, 4855–4866.
4. W. D. Hinsberg, F. A. Houle, M. I. Sanchez and G. M. Wallraff, IBM J. Res. Dev., 2001, 45, 667–682.
5. C. K. Ober, F. Käfer and C. Yuan, Polymer, 2023, 280, 126020.
6. T. Asakura, H. Yamato and M. Ohwa, J. Photopolym. Sci. Technol., 2000, 13, 223–230.
7. T. Kozawa, S. Tagawa, J. J. Santillan, M. Toriumi and T. Itani, Jpn. J. Appl. Phys., 2008, 47, 4465–4468.
8. J. L. Dektar and N. P. Hacker, J. Am. Chem. Soc., 1990, 112, 6004–6015.
9. J. V. Crivello, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 4241–4254.
10. J. F. Cameron, N. Chan, K. Moore and G. Pohlers, Proc. SPIE, 2001, 4345, 106–118.
11. T. Kozawa and S. Tagawa, Jpn. J. Appl. Phys., 2010, 49, 030001.
12. R. A. Lawson, S. J. Blanksby and G. B. Ellison, J. Micro/Nanolithogr., MEMS, MOEMS, 2010, 9, 013016.
13. R. A. Lawson and C. L. Henderson, Microelectron. Eng., 2009, 86, 741–744.
14. J. H. Ma, H. Wang, D. Prendergast, A. Neureuther and P. Naulleau, J. Micro/Nanolithogr., MEMS, MOEMS, 2020, 19, 1–11.
15. D. L. Goldfarb, A. Afzali-Ardakani and M. Glodde, Proc. SPIE, 2016, 9779, 97790A.
16. J. Deng, S. Bailey, S. Jiang and C. K. Ober, Chem. Mater., 2022, 34, 6170–6181.
17. Y. Komuro, H. Yamamoto, K. Kobayashi, Y. Utsumi, K. Ohomori and T. Kozawa, Jpn. J. Appl. Phys., 2014, 53, 116503.
18. J. Deng, S. Bailey, S. Jiang and C. K. Ober, J. Am. Chem. Soc., 2022, 144, 19508–19520.
19. C. Hoelzel, L. Cui, B. D. Naab, J. K. Park, P. Kang, K. Hernandez, S. M. Coley, S. Alexandrescu, R. Rena, J. F. Cameron and E. Aqad, Proc. SPIE, 2023, 12498, 124981V.
20. T. Kozawa, Y. Yoshida and M. U. Tagawa, Jpn. J. Appl. Phys., 1992, 31, 4301–4306.
21. S. Tagawa, S. Nagahara, T. Iwamoto, M. Wakita, T. Kozawa, Y. Yamamoto, D. Werst and A. D. Trifunac, Proc. SPIE, 2000, 3999, 204.
22. M. Kim, J. Moon, J. Choi, B. Lee, C. Jeong, H. Kim and M. Cho, Proc. SPIE, 2017, 10143, 101432E.
23. M. Kim, J. Moon, J. Choi, S. Park, B. Lee and M. Cho, Macromolecules, 2018, 51, 6922–6935.
24. M. Kim, J. Moon, S. Park and M. Cho, Macromolecules, 2020, 53, 4748–4763.
25. S. Grzeskowiak, A. Narasimhan, E. Rebeyev, S. Joshi, R. L. Brainard and G. Denbeaux, J. Photopolym. Sci. Technol., 2016, 29, 453–458.
26. K. Natsuda, T. Kozawa, K. Okamoto, A. Saeki and S. Tagawa, Jpn. J. Appl. Phys., 2009, 48, 06FC05.
27. Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241.
28. T. Lu, F. Chen, S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104.
29. E. Alvareda, P. A. Denis, F. Iribarne and M. Paulino, Comput. Theor. Chem., 2016, 1091, 18–23.
30. B. Khalili, M. Mamaghani and N. Bazdid-Vahdati, Theor. Chem. Acc., 2022, 141, 3.
31. A. Castro-Alvarez, H. Carneros, D. Sánchez and J. Vilarrasa, J. Org. Chem., 2015, 80, 11977–11985.
32. P. Pracht, E. Caldeweyher, S. Ehlert and S. Grimme, ChemRxiv, 2019 DOI:10.26434/chemrxiv.8326202.v1.
33. C. Bannwarth, S. Ehlert and S. Grimme, J. Chem. Theory Comput., 2019, 15, 1652–1671.
34. B. P. Pritchard, D. Altarawy, B. Didier, T. D. Gibson, T. L. Windus, L. A. Curtiss, M. P. McGrath, J. Blaudeau, N. E. Davis, R. C. Binning and L. Radom, J. Chem. Phys., 1995, 103, 6104–6113.
35. L. E. Roy, P. J. Hay and R. L. Martin, J. Chem. Theory Comput., 2008, 4, 1029–1031.
36. L. A. Curtiss, M. P. McGrath, J. Blaudeau, N. E. Davis, R. C. Binning and L. Radom, J. Vac. Sci. Technol., B, 2004, 22, 3489.
37. W. Sang-Aroon and V. Ruangpornvisuti, Int. J. Quantum Chem., 2007, 108, 1181–1188.
38. O. Fliss, K. Essalah and A. Ben Fredj, Phys. Chem. Chem. Phys., 2021, 23, 26306–26323.
39. D. Paul and U. Sarkar, J. Phys. Org. Chem., 2023, 36, e4419.
40. V. Sharma and P. K. Jha, Sol. Energy Mater. Sol. Cells, 2019, 200, 109908.
41. O. Kostko, T. R. McAfee and P. P. Naulleau, Proc. SPIE, 2023, 12750, 127500K.
42. J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer, S. Nandi and G. B. Ellison, Chem. Rev., 2002, 102, 231–282.
43. S. J. Blanksby and G. B. Ellison, Acc. Chem. Res., 2003, 36, 255–263.
44. K. Fukui, Acc. Chem. Res., 1981, 14, 364–368.
45. T. Lu and F. Chen, J. Phys. Chem. A, 2013, 117, 3100–3108.
46. T. Lu, Molclus program, Version 1.12., https://www.keinsci.com/research/molclus.html. Accessed on 23 Aug 2019.
47. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 16, Revision B.01, Gaussian, Inc., Wallingford CT, 2016.
48. T. Lu, F. Chen, S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Comput. Chem., 2012, 33, 580–592.
49. S. Sharma, Y. Ogata, C. Tung, J. M. Blackwell, T. R. Younkin, Y. Hishiro, J. S. Figueroa and A. L. Rheingold, Proc. SPIE, 2009, 7273, 72733N.
50. B. P. Prajwal, J. M. Blackwell, P. Theofanis and F. A. Escobedo, Chem. Mater., 2023, 35, 9050–9063.
51. A. De Silva, J.-K. Lee, X. André, N. M. Felix, H. B. Cao, H. Deng and C. K. Ober, Chem. Mater., 2008, 20, 1606–1613.
52. H. Yamamoto, T. Kozawa, A. Nakano, K. Okamoto, S. Tagawa, T. Ando, M. Sato and H. Komano, Jpn. J. Appl. Phys., 2005, 44, 5836–5838.
53. X.-Z. Niu, R. D. Pepel, R. Paniego, L. Abrell, J. A. Field, J. Chorover and R. Sierra-Alvarez, Environ. Sci. Pollut. Res., 2022, 29, 25988–25994.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp01814a

---