The preparation of a novel polymeric sulfonium salt photoacid generator and its application for advanced photoresists

Abstract

With poly(4-hydroxylstyrene) (PHS) as a raw material, a new type of polymeric photoacid generator (PAG) is synthesized with sulfonium perfluoroalkyl sulfonate groups bonded onto part of the benzene rings. The phenolic hydroxyl groups are esterified to increase the solubilities of the polymeric sulfonium salts in organic solvents. Upon irradiation to 254 nm light, the sulfonium units in the polymer effectively decompose to generate sulfonic acid. The photolysis is confirmed by UV spectroscopy and the generation of sulfonic acids which catalyze the decomposition of the t-BOC protection group of the polymeric PAGs. Making use of the polymeric PAGs, positive-tone chemically amplified 248-nm photoresists can be formed by them and ester acetal polymer with high acidolytic activity. The photolithography performance of the photoresist was evaluated using a KrF laser exposure system with high photosensitivity and resolution. This novel kind of polymeric sulfonium salt PAG is applicable for advanced chemically amplified resist materials.

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1. Introduction

Photoacid generators (PAGs) are specific compounds which can yield acids upon irradiation.¹ They occupy an important place in the system of chemically amplified (CA) photoresists, which were invented in the early 1980s by Ito and his co-workers and have become the basis for the current and future research on advanced resist materials due to their high sensitivity.² Generally, CA resists consist of polymer matrix with good plasma etch resistance and small molecular PAGs. When exposed to the light, the PAGs generate a small amount of strong acid which acts as a catalyst for the solubility-differentiating reactions within the irradiated regions of the resist film. To date, several classes of PAGs have been used in CA resists, including ionic and non-ionic PAGs,³ such as sulfonium salts, sulfonates and α-disulfone compounds, among which sulfonium salts are the most widely used PAGs in CA resists for their high photoacid generating efficiency and thermal stability.⁴ But the solubility performance is not satisfactory for the ionic compounds.

Sulfonium salt PAGs are usually used in their monomeric forms. However, the inherent incompatibility between the polymer matrix and the small molecular ionic PAGs leads to problems including PAG phase separation, non-uniform distribution of PAG and photogenerated acid, as well as acid migration during the post-exposure baking (PEB) processes.^5,6 These problems frequently lead to an undesirable premature and non-uniform deprotection reaction in the CA resist film.

A lot of efforts have been done to improve the performance of CA resists. One way is to use molecular glass resist materials, such as calixarene derivatives,^7–10 noria (water-wheel-like cyclic oligomer),^11–13 and other systems.^14–16 These low molar mass materials exhibit better phase compatibility with PAGs as well as better line-edge roughness (LER). Another approach is to increase the molecular size of PAGs. It is proposed that photoacid generating units are incorporated onto the polymer, rather than adding small molecular PAGs into the resist polymers. Several systems with ionic or non-ionic PAG incorporated into polymers have been studied recently.^17–26 The polymeric PAGs based resist systems showed improved lithographic performance, such as faster photo speed, higher stability and lower LER than corresponding blended resists.

In the reported literatures, polymeric PAGs for CA resist were generally synthesized by free radical copolymerization of monomer with PAG units and other monomers. The direct chemical modification on polymer chains as in this work was seldom reported. The only one example we found described a route to introduce aryl sulfonium salt to poly(4-hydroxylstyrene) (PHS) by reacting it with hydroxyl group but without presenting detailed method.²⁷ The direct chemical modification methodology offers advantages over polymerization way. The starting material PHS is easily available commercial product used in deep-UV resists with various molecular weights and narrow molecular weight distribution. This will guarantee the PHS derived polymeric PAGs being of required structure and characteristics. In this paper, based on our previous work on the reaction of phenolic compounds with dimethyl sulfoxide to give sulfonium salt in the presence of hydrogen chloride,²⁸ we designed and prepared a novel type of polymeric PAG with PHS as raw material and sulfonium perfluoroalkyl sulfonate groups bonded onto part of the benzene rings via chemical reactions. The structure and the synthesis of the polymeric PAGs are outlined in Scheme 1. The characteristics of the novel photosensitive polymers were investigated in detail.

Scheme 1 Preparation of polymeric PAG. (i) DMSO, HCl, 0–8 °C, 8–10 h; (ii) sodium triflate or potassium nonaflate, room temperature; (iii) di-tert-butyl dicarbonate, K₂CO₃, ice bath.

In order to investigate the practical application of the polymeric compound as PAG, we tried to compose a positive-tone chemically amplified 248-nm resist by the compound and ester acetal polymer derived from the polymerization reaction of acrylpimaric acid (APA) and 1,4-cyclohexanedimethanol divinyl ether (CHDDE) with ester acetal linkages in the main chain, which can be easily cleft in the presence of photoacid.²⁹

2. Experimental section

2.1 Materials

Poly(4-hydroxylstyrene) was from Maruzen Petrochemical Co., Ltd (Tokyo, Japan). The trade code is MARUKA LYNCUR-M and the M_n is about 2100 with M_w/M_n of 1.6. Potassium nonaflate (98%) was received from Hubei Hengxin Chemical Co., Ltd (Hubei, China). Di-tert-butyl dicarbonate (98%) and propylene glycol monomethyl ether acetate (PGMEA, 99%) were obtained from Aladdin Reagents Company (Shanghai, China). Sodium triflate was prepared by the reaction of sodium hydroxide and trifluoromethanesulfonic acid. Ester acetal polymer p(APA-CHDDE) was prepared by the polyaddition reaction of acrylpimaric acid (APA) and 1,4-cyclohexanedimethanol divinyl ether (CHDDE) in butyl acetate at 120–130 °C under nitrogen. The measured number-average molecular weights generally range from 4000 to 5500 with M_w/M_n of 1.5. 1-Methyl-1′-(4-methylphenyl) disulfone was synthesized and chosen to be the small molecular non-ionic PAG. Triphenyl sulfonium triflate was prepared by the reaction of triflate sodium and triphenyl sulfonium chloride which was purchased from Yingli Company (Changzhou, China). Bromophenol blue (BPB) was from Yinghai Company (Beijing, China). Dimethyl sulfoxide (DMSO), methanol, sodium chloride, concentrated sulfuric acid (98%), dimethyl formamide (DMF), potassium carbonate, petroleum ether, acetyl chloride and ethylene glycol monoethyl ether were obtained from Beijing Chemical Reagents Company (AR, Beijing, China).

All the reagents were used as received.

2.2 Characterization

Fourier transform infrared spectra (FTIR) were measured on Nicolet AVATAR 360 spectrometer. The ¹H NMR and ¹⁹F NMR spectra were recorded on a Bruker Avance III (400 MHz) spectrometer in methanol-d₄ and DMSO-d₆. The thermo gravimetric analysis (TGA) measurements were performed on a METTLER TOLEDO star system instrument with the heating rate of 10 °C min⁻¹ under a nitrogen flow of 20 mL min⁻¹. The ultraviolet-visible spectra were measured on GBC Cintra 10e Spectrometer by forming films on quartz plates or in methanol solution. The photolysis of the polymeric PAGs was evaluated by exposing the polymers in solution and film to a low pressure mercury lamp (254 nm) with an optical intensity (I₀) of 1.0 mW cm⁻². Exposure experiments were carried out on a KrF laser exposure system (ASML Scanner PAS5500/800). The resist film thickness was measured using a Nanometrics Nano SPEC210 profilometer. The top-view scanning electron micrograph (SEM) was obtained with a Hitachi S9220 CD-SEM system. Cross-section SEM micrograph was taken on a Hitachi S4800 field emission SEM system.

2.3 Preparation

Polymer a. HCl gas was generated in situ by heating the mixture of sodium chloride and concentrated sulfuric acid and dried by passing through concentrated H₂SO₄. PHS (18.0 g, 0.15 mol by monomer) was dissolved in 150 mL of methanol in a three-necked flask fitted with a thermometer and the solution was cooled to below 8 °C. A solution of dimethyl sulfoxide (31.5 mL, 0.45 mol) in 20 mL methanol was added dropwise to the flask under stirring. Meanwhile, dry HCl gas was slowly bubbled through the solution and the extra HCl was absorbed by dilute aqueous base. The mixture was stirred for 8–10 h until thin layer chromatography (TLC) indicated the reaction completion. Then most of the solvent and the dissolved HCl were removed under reduced pressure in a rotary evaporator. The rest of the solution was poured into a large amount of distilled water to give white precipitate. The precipitate was collected and washed to neutral with water and then dried overnight at 40 °C under vacuum to afford a fine white powder. The yield was 85.6%. IR (KBr): ν = 3412 (–OH), 2921 (–CH₂–), 1612 (Ar-H), 1511, 1421, 1225, 830. ¹H NMR (400 MHz, methanol-d₄): δ (ppm) 6.0–7.0 (aromatic H), 2.85–3.15 (–CH₃), 1.1–2.3 (CH and CH₂).

Polymer b. To the solution of polymer a (18.5 g, 0.13 mol) in methanol, a solution of potassium nonaflate in methanol was added with about two times amounts of potassium nonaflate over sulfonium chloride. White solid of potassium chloride appeared immediately. After the mixture was stirred vigorously for 24 h at room temperature, the solid was filtered out and the rest of the solution was concentrated and then poured into water. The precipitated solid was collected and washed with warm water to remove the unreacted potassium nonaflate and then dried in vacuum overnight. The yield was 88.3%. IR (KBr): ν = 3409 (–OH), 2922 (–CH₂–), 1612 (Ar–H), 1512, 1424, 1225, 1029, 830, 648. ¹H NMR (400 MHz, methanol-d₄): δ (ppm) 6.0–7.0 (aromatic H), 2.85–3.15 (–CH₃), 1.1–2.3 (CH and CH₂). ¹⁹F NMR (400 MHz, methanol-d₄): δ (ppm) 95.8, 62.1, 55.5, 50.7.

Similar procedures were followed to prepare sulfonium triflate using sodium triflate. The yield was 83.8%. ¹⁹F NMR (400 MHz, methanol-d₄): δ (ppm) 98.6.

Polymer c. Polymer b (10.5 g, 0.05 mol) and potassium carbonate (K₂CO₃, 6.9 g, 0.05 mol) were added into 200 mL of DMF and the mixture was cooled to below 5 °C in the ice bath. Under stirring, di-tert-butyl dicarbonate (21.8 g, 0.1 mol) was added dropwise. A vigorous bubbling occurred immediately. The reaction mixture was stirred for 5–8 h and then neutralized by adding acetic acid. The product mixture was then poured into water. The precipitate was collected and washed by petroleum ether and then dried in vacuum overnight. The yield was 74.7%. IR (KBr): ν = 3368 (OH), 2977, 2925 (–CH₃), 1758 (–C=O), 1513, 1276, 1253, 1148. ¹H-NMR (400 MHz, DMSO-d₆): δ (ppm) 6.0–7.3 (aromatic H), 2.9–3.3 (–CH₃ of sulfonium salt), 1.1–1.8 (aliphatic H).

2.4 Photolysis of the polymeric PAGs

The dilute solution of polymer b in methanol with a concentration of 2.9 × 10⁻⁴ mol L⁻¹ was exposed to a lower pressure Hg lamp (254 nm) with optical intensity (I₀) of 1.0 mW cm⁻². The UV absorbance of the solution was recorded with different exposure doses.

A film of polymer c was formed by coating a dilute solution of the compound in ethylene glycol monoethyl ether on a KBr plate and then baked at 100 °C for 120 s to remove the solvent. The film was exposed to 254 nm light and the FTIR spectra of the film were recorded before exposure and after flood exposure with PEB.

2.5 Photogenerated acid quantity

Bromophenol blue (BPB) was dissolved in DMSO to make a dilute solution with concentration of 5 × 10⁻⁴ mol L⁻¹. With the addition of increasing amounts of p-toluene sulfonic acid, the absorbance at 607 nm decrease proportionally. The calibration curve was obtained by plotting the UV absorbance (A) versus the concentration of the acid (c) and can be described as an equation:

A = 0.259 − 0.0146c (1)

Polymer c films were formed on a quartz plate and then exposed to 254 nm light (lower pressure Hg lamp). The exposed films were dissolved in a small amount of DMSO and then were added a suitable amount of BPB and diluted to 10 mL. The UV absorbance of the solutions was measured and the quantities of the photoacid can be obtained from the calibration curve.

2.6 Photolithography

Photolithographic experiments were performed as following using a resist consisting of ester acetal polymer and polymer c (sulfonium amount: 20%, t-BOC protection ratio: 81%) as an example. P(APA-CHDDE) and polymer c (3 : 1, w/w) were dissolved in PGMEA to make a 12 wt% solution. The solution was filtered through a 0.1 μm polytetrafluoroethylene membrane filter and then spin coated at 3000 rpm for 25 s onto a 8 inch hexamethyldisilazane (HMDS) primed silicon wafer and pre-baked at 100 °C for 60 s to form an approximately 210 nm thick film. The exposures were then performed on ASML scanner PAS5500/800 stepper with KrF excimer laser as light source (NA = 0.68, mask line/space = 1/1). After exposure, the wafers were post baked at 110 °C and then developed in a 2.38 wt% tetramethylammonium hydroxide (TMAH) aqueous solution for 60 s at room temperature, followed by rinsing with de-ionized water.

3. Results and discussion

3.1 Preparations

The condensation of PHS with DMSO in the presence of dry HCl gas gave dimethyl-aryl sulfonium chloride. During the reaction, the temperature was maintained below 8 °C to avoid possible side-reactions. This reaction is an electrophilic substitution reaction and the sulfonium group is on the ortho-position of the hydroxyl group. The chloride salt was converted to the corresponding perfluoroalkyl sulfonate salt via a metathesis reaction. The ¹⁹F NMR confirmed the presence of perfluoroalkyl sulfonate. Into the solution of polymer b in ethanol several drops of silver nitrate aqueous solution was added and no precipitate appeared, indicating that the chloride ions were exchanged to nonaflate ion completely.

In order to investigate the introduction rate of the sulfonium group on the benzene ring, reactions with different amounts of DMSO were conducted. The results were shown in Table 1. Although the amount of DMSO is greatly excessive, the introduction rates are far less than 100%. That's probably because of the polymer chain entanglement. Comparing with the analogous reactions for small molecular compounds,²⁸ which are almost proceeded quantitatively, the polymeric reactions exhibit distinctive properties.

Table 1 The feed ratio, composition and the thermal decomposition data of the PHS–sulfonium chloride

Sample	Feed ratio (mol mol^−1) (PHS/DMSO)	PAG amount^a	Td^b [°C]
a mol%, calculated from the ^1H NMR spectra.b 5% weight loss.
b-1	1 : 1	16.7	245
b-2	1 : 3	25.0	246
b-3	1 : 5	26.3	247
b-4	1 : 10	34.3	245

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The polymer b (PHS–sulfonium perfluoroalkyl sulfonate) is insoluble in commonly used weak polar and nonpolar organic solvents. Performing esterification reaction on the polymer is a convenient method to improve its solubility. We have reported briefly in the proceeding³⁰ the introduction of sulfonium salt groups onto benzene ring of PHS and the try to protect the hydroxyl groups with the common esterification reaction of di-tert-butyl dicarbonate using p-dimethylaminopyridine (DMAP) as catalyst at room temperature. However, we subsequently found that most of the sulfonium salt groups were decomposed by the catalyst even at 0 °C. Other organic base such as triethylamine (TEA) and pyridine were also tried as catalyst and the same results were investigated with only about 4% of the benzene rings left onto which a sulfonium salt group is attached according to the peak around 3.0 ppm in the ¹H-NMR spectra after the reaction. The result is shown in Fig. 1. This is probably because the organic bases can lead to the cleavage of the S–C bond and generate sulfur ylid.³¹ Then we tried to use inorganic base and the esterification reaction can proceed in the presence of potassium carbonate without affecting the attached sulfonium salt group. However, if the reaction was conducted at room temperature, even inorganic bases, such as K₂CO₃, Na₂CO₃ or NaHCO₃ would also lead to the decomposition of the sulfonium salt group. So the reaction had to be kept at a lower temperature and the amount of di-tert-butyl dicarbonate was made excessive to shorten the reaction time. Acetyl chloride was also used to conduct the esterification reaction and similar results were obtained.

Fig. 1 The ¹H NMR spectra of (A) polymer b-1 (sulfonium nonaflate) (methanol-d₄); (B) polymer c (protected by t-BOC) (DMSO-d₆), catalyzed by K₂CO₃, ice bath; (C) polymer c (DMSO-d₆), catalyzed by DMAP, room temperature. Only the region of 0–4 ppm is shown.

The t-BOC protected products of polymer b were prepared with different protection ratios in the presence of K₂CO₃ in organic solvent. The FTIR spectra showed characteristic absorptions at 1758 cm⁻¹ due to C=O stretching. The actual protection ratios can be calculated by their ¹H NMR spectra. Although a complete t-BOC protection for PHS can be achieved easily, the protection ratio is much lower for polymer b. The ratios of sulfonium and t-BOC protection were calculated from the ¹H-NMR spectra and the sum is no larger than 100%, indicating that the protection reaction can only take place on phenolic unit without sulfonium salt under our experimental conditions. It is apparent that the sum would exceed 100% if the protection reaction could take place on phenolic unit with sulfonium salt. That is due to the steric hindrance and electronic effects of the sulfonium groups.

The molecular weight distribution (MWD) of the raw material PHS and reaction products were measured by GPC. For the raw material PHS, the MWD is 1.6. After the introduction of sulfonium salt group (polymer b), the MWD became 1.8. The MWD of polymer c is 2.1, which is a little wider than polymer b. The GPC curves were shown in the ESI.†

3.2 Solubility

The solubility of the polymeric PAG can be increased through modification on polymer molecular structure. The polymer a (sulfonium chloride) is insoluble in common organic solvents other than methanol, ethanol and ethylene glycol. When the chloride was transformed into perfluoroalkyl sulfonate, polymer b can be dissolved in strong polar organic solvents, such as ethanol, DMF, DMSO and DMAc, but still insoluble in weak polar and nonpolar organic solvents. When about a half of the hydroxyl groups were esterified with t-BOC groups, the final product (polymer c), with PAG amount of 25%, become easily soluble in the commonly used solvents for photoresist, such as ethylene glycol monoethyl ether and ethyl lactate (solubility ≥ 10%), and partly soluble in cyclohexanone and butanone (10% < solubility > 1%), but still insoluble in propylene glycol monomethyl ether acetate (PGMEA) (solubility ≤ 1%). When most of the hydroxyl groups were esterified, the polymeric PAG will also become partly soluble in PGMEA. Different counterions were tried and the solubility of nonaflate is better than that of triflate. Acetylate of the phenolic hydroxyl can not apparently improve the solubility of the polymeric PAG.

3.3 Thermal behaviour and storage stability

The TGA curves of PHS, polymer b-1 and polymer c (protected product of b-1 with 48% protection ratio) are presented in Fig. 2. Sulfonium salt PAGs generally are of high thermal decomposition temperature. The thermal decomposition temperature (T_d) of the polymer b was found around 245 °C. Performing esterification reaction for the polymer b can make the polymeric PAG easily soluble in common resist solvents. The t-BOC ester is a good choice since it is also a well known acidolytic protection group besides improving the solubility of the PAG. Therefore, the t-BOC protected product can also function as dissolution inhibitor in resist composition as well as PAG. However, the t-BOC protection group can undergo thermal decomposition at lower temperatures than that of sulfonium group. The decomposition temperature of polymer c (48% protection rate) is 136 °C as shown in Fig. 2. The IR spectra of the sample before and after thermal decomposition are presented in Fig. 3 in which the absorbance of C=O group at 1758 cm⁻¹ decreased remarkably, indicating the thermal decomposition of the protection group. The more protected of the hydroxyl groups, the higher the T_d will be. In addition, the solution of the polymeric PAG in organic solvents can be stored for more than six months without any change.

Fig. 2 The TGA curves of (A) PHS, (B) polymer b-1 and (C) polymer c (sulfonium amount: 16% and t-BOC protection ratio: 48%).

Fig. 3 The IR spectra of polymer c film (A) before baked; (B) baked at 140 °C for 2 min. Only the region from 4000 cm⁻¹ to 1000 cm⁻¹ is shown.

3.4 The UV absorbance of the polymeric PAGs

The UV spectra of PHS film, PMMA film containing 50% (w/w) b-1, PMMA film containing 50% (w/w) b-4 and PMMA film containing 5% (w/w) triphenylsulfonium bromide are presented in Fig. 4. The film thickness is about 7 μm. The UV absorbance of the polymeric PAG is quite different from that of the monomeric PAG with analogous structure and is more similar to that of PHS which shows excellent transparency at 248 nm. After the introduction of sulfonium group on the benzene ring of PHS, redshift appeared in the UV spectra. The minimum absorption moved from 248 nm to around 271 nm because of the presence of sulfonium groups, making the PAG bonded PHS having a stronger absorbance at 248 nm. Therefore, the polymeric PAG can be used for 248-nm CA photoresists.

Fig. 4 UV spectra of (A) PMMA film containing 5% (w/w) triphenylsulfonium bromide, (B) PHS film, (C) PMMA film containing 50% (w/w) b-1, (D) PMMA film containing 50% (w/w) b-4.

3.5 Photolysis and photoacid generating

Fig. 5 shows the UV absorbance change of the solution of b-4 in methanol before and after exposure to 254 nm light with different doses. The absorption band around 330 nm which is due to the sulfonium groups decreased apparently with the increase of the photolysis dose. The mechanism of the photolysis of sulfonium salts to generate acid has been already studied.^32,33 The UV spectra and the change before and after exposure for the PHS bound sulfonium salt are quite different from that for small molecular analogues,²⁸ which are of strong absorption peaks around 250 nm and the absorption peaks dropped quickly after exposed to 254 nm light. The UV spectra of the PHS bound sulfonium salt reflect the characteristic of the PHS skeleton.

Fig. 5 UV spectra change of b-4 solution after exposure to 254 nm light with different exposure dose.

However, the photoacid generating of the polymeric PAG can be confirmed by the acid-catalyzed decomposition of the t-BOC protection. Taking the 55 mol% protected sulfonium nonaflate as an example (sulfonium amount: 16%), a film of the polymeric PAG was formed on a KBr slice and FTIR spectra of the film were recorded before and after exposed to a low pressure Hg lamp (254 nm). After flood exposure (dose = 400 mJ cm⁻²) following by post exposure bake (PEB) at 130 °C for 2 min, the absorption at 1758 cm⁻¹, which is assigned to the C=O group, decreased remarkably, a change very similar to Fig. 3 and the spectrum can be found in the ESI.† For a reference, the unexposed film was also baked at 130 °C for 2 min and the FTIR spectrum showed no apparent change. This demonstrates that upon irradiation to 254 nm light, the sulfonium nonaflate underwent photolysis to generate nonaflic acid which catalyzed the thermal decomposition of the protection group during PEB.

3.6 Photoacid generation efficiency

Bromophenol blue (BPB) is an acid sensitive indicator and its absorbance peak at 607 nm decreases remarkably with the increase of acid.^34,35 A linear relationship exists between the amount of acid added to the indicator solution and the absorbance intensity change at 607 nm and then a calibration curve can be worked out by adding different amount of p-toluene sulfonic acid to the solution of the indicator and then plotting the UV absorbance versus the concentration of the acid. Therefore, the small amount of acid generated by PAG can be determined by the absorbance change of TBPB from the calibration curve. Three samples of polymer c with different sulfonium ratio and protection rate were tested and the results are shown in Table 2. The efficiency represents the quantity of the photoacid per sulfonium salt can generate after fully photolyzed.

Table 2 The photoacid generation efficiency of polymer c

Sample	Composition^a	M (g mol^−1)	Efficiency
a The sulfonium ratio (mol%, left) and the t-BOC protection rate (mol%, right).
1	6.5%, 70%	213.3	0.29
2	16.5%, 83%	270.8	0.32
3	25.0%, 77%	287.1	0.38

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3.7 Application

This polymeric PAG can find many usages in advanced CA photoresist materials. Compared with small molecular sulfonium salt PAGs, main disadvantage of which is bad solubility in common photoresist solvents,³⁶ the polymeric PAG possesses much better solubility and thus can be used in higher content in CA resist compositions. This will provide possibility to increase the photosensitivity of the resist by increasing the loading of the polymeric PAG. It is of much significance for EUV and E-beam photoresists for which as high as possible photosensitivity is needed.

High resolution patterns with lower LER can be obtained in the lithographic experiments of molecular glass resists due to their small molecular size compared to polymeric resists.³⁷ Therefore, molecular glass photoresists are outstanding candidates for resist materials in the next generation lithography. However, the most frequently encountered problem for the resist system is pattern collapse because of the low film strength and glass transition temperature (T_g) of these small molecular resist materials. Using the polymeric sulfonium slat PAG in molecular glass resists can help to overcome this problem by increasing the strength of the resist film as well as photosensitivity.

We have reported the preparation and properties of a series of specially designed ester acetal polymers which show higher acidolytic activity than that of commonly used matrix polymers with acid labile groups pendent on main chain in positive-tone CA resists because the ester acetal linkages in the main chain can be cleft in the presence of strong acid under moderate heating.²⁹ The polymers are highly transparent from 248 nm to longer wavelength and so suitable to form chemically amplified i-line resists and 248-nm resists together with applicable PAGs. In fact, the i-line resist composed of one of the polymers and 2,1,4-sulfonate, which can function as PAG, achieved excellent photolithographic performance.

We have also investigated the lithographic performance of the 248-nm photoresists composed of acetal ester polymers and different types of small molecular PAGs, including ionic and non-ionic PAGs, such as triphenyl sulfonium triflate and α-disulfone compound. In all the lithographic experiments with KrF laser exposure system, the patterns can not be remained after development even under a lower PEB temperature of 100 °C. This probably because that the lower T_g, which are far below the PEB temperature, strengthen the effect the photoacid diffusion during PEB and make the unexposed region dissolved in aqueous base. Besides, the ionic sulfonium salts PAG can result in the decomposition of the acetal polymers within two weeks in the resist solution while the resists containing nonionic PAG remain unchanged for three months.

But when we use the polymeric PAG in the place of the small molecular ionic PAG, the situation was quite different. The acetal polymer and the polymeric PAG were dissolved in PGMEA to make a two-component resist. The lithographic performance was evaluated on a KrF exposure system. With an exposure dose of 24 mJ cm⁻², a clear positive-tone pattern of 180 nm was obtained. The SEM image is presented in Fig. 6. The polymeric sulfonium salt PAG can help to increase the strength of the resist film and decrease the effect of photoacid diffusion. Further more, after stored for more than three months, the resist solution showed no obvious change. The effect of the polymeric ionic PAG is similar to that of nonionic PAG.

Fig. 6 The SEM image of positive-tone patterns with 180 nm line width obtained from the resist composed of p(APA-CHDDE) and polymeric PAG (3 : 1, w/w), (a) top-view, (b) cross-section.

4. Conclusions

In this paper, sulfonium chloride was introduced onto the benzene ring of PHS by a convenient direct reaction in a high rate. After the anion was exchanged into perfluoroalkyl sulfonate and the hydroxyl group esterified with t-BOC group, the polymeric sulfonium salt becomes easily soluble in common solvents for photoresists. The photoacid generating of the polymeric sulfonium salt was confirmed by the deprotection reaction after exposed to light and following PEB. This polymeric PAG has moderate absorption at 248 nm and hence can be used for 248-nm resists, as well as for EUV and E-beam resist materials. A two-component 248-nm resist consisting of acetal polymer and the polymeric PAG was evaluated on a KrF exposure stepper and preliminary pattern of 180 nm was obtained.

Acknowledgements

The authors thank Haiyan Sun of Kempur (Beijing) Microelectronics, Inc., for her assistance in photolithography experiments. This work is supported by National Basic Research Program of China (2013CBA01703) and National Science and Technology Major Projects (2010ZX02303).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02048k

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