Novel Star Polymers as Chemically Amplified Positive-Tone

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Novel Star Polymers as Chemically Amplified PositiveTone Photoresists for KrF Lithography Applications Xiangfei Zheng, Changwei Ji, Jingcheng Liu, Ren Liu, Qidao Mu, and Xiaoya Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05335 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Industrial & Engineering Chemistry Research

Novel Star Polymers as Chemically Amplified Positive-Tone Photoresists for KrF Lithography Applications Xiangfei Zheng †, Changwei Ji †, Jingcheng Liu *†, Ren Liu †, Qidao Mu ‡, Xiaoya Liu *† †

The Key Laboratory of Synthestic and Biological Colloids, Ministry of Education and School

of Chemical and Material Engineering, Jiangnan University, No 1800 Lihu Avenue, Wuxi, China. ‡

Suzhou Rui Hong Electronic Chemicals Co., Ltd., NO.501 Minfeng Road, Suzhou, China.

S Supporting Information

KEYWORDS：star copolymer; photoresist; KrF Lithography; resolution

ABSTRACT ： Xanthate-mediated reversible addition–fragmentation chain transfer (RAFT) methodologies have been applicable to preparation of branched or star polymers. In this paper, novel star copolymers have been synthesized through xanthate-mediated RAFT polymerization with p-acetoxystyrene, tert-butyl acrylate. Fourier transfer infrared, nuclear magnetic resonance spectra and gas chromatography analysis indicated the polymerization was successful between both of the monomers and the star RAFT agent. The intrinsic viscosity and Zimm branching factor (g′) was used to confirm the copolymers architecture. The ultraviolet absorbance of the copolymer solutions indicated that the copolymer was suitable for use as a krypton fluoride (KrF) laser photoresist. Moreover, the photolithography performance of the positive-tone chemically amplified photoresist was evaluated. The results indicated that the photosensitive based on the star copolymer was higher than the linear one, and the pattern resolution was around 200 nm at a low exposure energy.

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1. INTRODUCTION Chemical amplification (CA) mechanism1 help to increase quantum yields (the number of molecules transformed per photon absorbed2), improve photochemical reaction efficiency, and enhance photoresist sensitivity.3,4 A CA resist mainly consists of polymer resin, a photo-acid generator (PAG), a solvent, a base quencher, and other additives.5 In exposed areas, PAG undergoes photolysis to release photo-acid, which is used to catalyze the acid-labile group on the polymer and form a soluble component. The acid-labile group inhibits the dissolution of photoresists in unexposed areas.6 CA resist based on acid deprotection is the main foundation for 248nm, 193nm resist systems that are mainly used in deep ultraviolet (DUV) lithography technology for integrated circuit device manufacturing.7 However, a balance exists between the resolution, line edge roughness (LER), and sensitivity of CA resists, as shown in the following formula: resolution3 × LER2 × sensitivity ≈ constant.8 The main challenge of this technology is to increase the resolution and sensitivity of photoresists with lower LER to meet the advanced lithgraphy.9 Polymer resin is the backbone of resists, contributing to all aspects of resist performance and character. However, most of the polymer resin are now still based on linear structures.10 The linear polymers prepared through free radical polymerization contain bulky and compact molecules, resulting in a broad molecular weight distribution. The varying dissolution behaviours of various molecular weights could result in a reduction in the resist resolution. Additionally, the sensitivity of resist would decease if less photosensitive groups are present in the molecular chains. Critically, molecular chain entanglement and extended coil dimensions are the main causes of poor resolution and LER.12 Thus, novel polymers that meet the demands for resists must be developed. Fortunately, numerous valuable studies have attempted to overcome the aforementioned issues. In the preceding few decades, additional architecture such as branched polymers,13,14

molecular

glasses,15,16

polymer-bound-PAG,17-19

and

non-chemically

amplified20-22 have been introduced as photoresist components.


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One study incorporated branched architecture into photoresists,23 which benefited from the unique structure and special performance in terms of three-dimensional irregular macromolecules with abundant functional groups, weak chain entanglement, high solubility, and low viscosity. Chatterjee et al.24 firstly reported a kind of photodegradable hyperbranched polyacetal and applied in positive photoresist. Kudo et al. 25 synthesized hyperbranched polyacetals through An+ B2 ‐type polyaddition. The polyacetals were used as candidate resists for extreme ultraviolet lithography. Although branched polymers were demonstrated to improve the sensitivity and resolution of resists, polymer heterogeneity caused by broad molecular weight distribution may also contribute strongly to resist features.26,27 Thus, introducing well-defined branched polymers with low heterogeneity is likely to improve the dissolution rate and resolution of resists.28-30 RAFT techniques has been extensively investigated to synthesize branched polymers, since RAFT method can be applicable to most kind of monomers and tailored to ambient temperatures. For example, Martina Stenzel-Rosenbaum et al. synthesized polystyrene stars by RAFT polymerization at different temperatures.31 Bailing Liu et al. prepared hyperbranched poly(methyl methacrylate) via the one-pot method through RAFT polymerization.32 Helen T. Lord et al. used RAFT polymerization to synthesised microgel star polymers.33 Recently, a novel method called xanthate-mediated RAFT method has been designed to prepare branched and star polymers.34,35 Star polymers with branched architectures have been utilized in life sciences and nanotechnologies.36 However, few reports about star polymers were introduced to the research field of KrF lithography. In our previous research, linear tetrapolymer P (ASM-TBA-St-CA) have been synthesized and applied in KrF resist, the pattern resolution was 250nm.37 In this study, to distinguish from the linear tetrapolymer, novel star copolymers with a low polydispersity index (PDI) were synthesized through xanthate-mediated RAFT polymerization methodologies. The results indicated that the resist based on star copolymer possess higher photosensitivity, and the resolution reached 200 nm.

2. EXPERIMENTAL SECTION 2.1 Materials


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Tert-Butyl Acrylate (TBA, 98%, Vita-chemical); p-acetoxy styrene (ASM, 98%， Vita-chemical); 2,2-Azobis (isobutyronitrile) (AIBN, 99%, Aladdin), sodium methoxide in methanol (5.4M, Aladdin); Triocylamine (98%, Aladdin); Methanol (Electronic grade, Suzhou Rui Hong); propylene glycol methylether acetate (PGMEA, Electronic grade, Suzhou Rui Hong); Di-t-butylphenyl iodonium camphor sulfonate (DTBPI-CS, EL, Toyo Gosei).37 Star RAFT agent (CTA) was synthesized according to the references.38 2.2 Synthesis of P (ASM-co-TBA) Firstly, monomers TBA and ASM, initiator AIBN, star RAFT agent CTA and solvent PGMEA were added to a 100-mL single-necked round bottom flask. Subsequently, the flask was cycled between vacuum and nitrogen three times. The polymerization was take place at 70 °C with magnetic stirrer for 20 h. In order to prevent the chain transfer reaction, the flask was then plunged into ice water after the reaction. Table 1 lists the reactant ratios. The polymer solutions were dropped into ultrapure water three times to obtain P (ASM-co-TBA) powder. The copolymers were baked at 65 °C overnight to remove the water. Monomers conversion and copolymer composition is presented in table 1. 2.3 Alcoholysis of P (ASM-co-TBA) P (ASM-co-TBA) powder and CH3ONa were dissolved in a flask with methanol. Then the alcoholysis reaction take place at 75 °C for distillation. The distilled methyl acetate and methanol were supplemented hourly with equivalent methanol. After the polymer solutions had been cooled, a white P (HS-co-TBA) product was obtained through precipitation into ultrapure water. This synthetic approach to branched copolymers is presented in Scheme 1 (see Table S2 for hydrolysis degree of P (HS-co-TBA) in the supporting information).

Scheme 1. Synthetic approach to branched P (HS-co-TBA) copolymers


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Table 1. Polymerization results for P (ASM-co-TBA) copolymers with various feed ratios of CTA a

Samples b

Mole feed ratio

Monomers

Polymer

conversion c

composition d

ASM

TBA

X

ASM,%

TBA,%

ASM:TBA

P (ASM-co-TBA) 0

65

35

0

84.6

60.1

72.3: 27.6

P (ASM-co-TBA) 1.2

65

35

1.2

79.7

68.3

68.4 : 31.6

P (ASM-co-TBA) 2.5

65

35

2.5

82.1

69.7

68.6 : 31.4

P (ASM-co-TBA) 3.8

65

35

3.8

66.1

46.6

72.4 : 27.6

P (ASM-co-TBA) 5.1

65

35

5.1

53.5

43

69.8 : 30.2

a

Polymerization conditions: [M]: AIBN:CTA = 200:1:X, in PGMEA at 70 C for 20 h; b The subscript

of the sample P (ASM-co-TBA) 0-5.1 is the different CTA ratio;

c,d

determined by GC.

2.4 Resist processing For the resist formulation, 3.95 g of the P (HS-co-TBA) polymer and 2.7 w.t. % of the PAG were dissolved in PGMEA; 0.2 w.t. % of triocylamine was added to control the acid diffusion effects during post exposure baking (PEB). Subsequently, 0.2 and 0.1 µm teflon filters were used to remove the particle in the resist solution. Then spin coating the resist solutions on silicon wafers to obtain resist film. The pattern was achieved according to the lithography process: soft bake (SB) at 90 °C for 60 s; exposured by using KrF light source; PEB at 130 °C for 90 s; development in 2.38 w.t. % of tetramethylammonium hydroxide for 60 s; rinsed with deionized water. 2.5 Measurements Fourier transfer infrared (FT-IR) spectra of sample films coating on a KBr disk were collected on an ABB BOMEN FTLA 2000-104 spectrometer. Monomers conversation calculation is carried out by area normalization method by using Shimadzu gas chromatography (GC) systems at 250 ℃. Nuclear magnetic resonance (1H-NMR) spectra of copolymers dissolved in DMSO-d6 were conducted on a Bruker Avance Digital 400 MHz


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spectrometer. UV absorbance of the polymer solution was conducted on TU-1901 spectrometer. The weight molecular weight (Mw), molecular weight distribution (PDI), hydrodynamic radius (Rh) and intrinsic viscosity ([η]w) was determined by Wyatt MultiAngle

Light

Scattering

(MALS)

detectors

equipped

with

Gel

Permeation

Chromatography (GPC) systems and viscosimetric in N, N-Dimethylformamide (DMF) at 25 ℃. Thermal properties were measured by Mettler-Toledo DSC822e. The glass transition temperature (Tg) was conducted under nitrogen flow with the heating rate 20 °C /min, and thermogravimetric analysis (TGA) was performed with a heating rate of 10 °C /min under nitrogen flow. The film thickness was measured by Profilometer of Dektak XT brand. The photospeed (E0) was performed on Litho Tech Japan open frame exposure system UVES-2000 at 248nm. The resist pattern was achieved by exposure at 248 nm light source on an ASML PAS5500-350 stepper. Scanning electron microscopy (SEM) images were obtained on Hitachi S-4800 with an electron voltage of 0.5 kV. Atom force microscopy (AFM) images were obtained through scanning resist pattern via a diamond needle (cantilever) tip on DP15/Hi’Res-C/AIBS (MicroMasch, USA).

3. RESULTS AND DISCUSSION 3.1 Characterization of P (HS-co-TBA)

Figure 1. FTIR spectra of copolymer P (ASM-co-TBA) and P (HS-co-TBA)

FTIR spectra of P (ASM-co-TBA) and P (HS-co-TBA) are shown in Figure 1. The results are similar to the ones correspond to tetrapolymers which have been reported in our previous work. 37 The characteristic absorption peaks corresponded to ASM and TBA units were observed


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clearly in the spectra. P (ASM-co-TBA): ν (C=O, ASM):1763cm-1, ν (C=O, TBA):1719cm-1, ν (C-O, ASM):1012cm-1, 911cm-1; P (HS-co-TBA): ν (C=O, TBA):1719cm-1, ν (-OH): 3500cm-1. The characteristic peaks for P (HS-co-TBA) are clearly visible in the 1H-NMR spectra: δ 0.85–1.6 ppm (–CH2, –CH3), δ 6.3–7.2 ppm (ArH, ASM), δ 8.9 ppm (–OH). Additionally, compared with P (HS-co-TBA) 0, the peaks at δ = 4.4 and 4.6 ppm were observed, which was correspond to the protons marked with c and d, respectively (figure 2). This analysis confirmed that xanthate-mediated RAFT polymerization had been successful.

Figure 2. 1H-NMR spectra of P (HS-co-TBA) copolymers with various CTA feed ratios.

3.2 Polymerization results for P (HS-co-TBA) The effects of CTA feed ratio (X ranging from 0 to 5.1) on copolymerization were investigated, the parameters such as Mw, intrinsic viscosity, hydrodynamic radius are listed in Table 2. For copolymers obtained through RAFT copolymerization, Mw ranged between 8600 and 113400 g/mol, and PDIs varied from 1.43 to 1.52. The Mw decreased with increasing X, which was ascribe to less repeat units and lower molecular weight on each arm. The results can been demonstrated by the Rh of copolymers decreased from 7.8 to 2.0 nm. Additionally, [η]w decreased due to less molecular chain entanglement. Mark–Houwink–Sakurada (MHS) exponent (α), intrinsic viscosities and Zimm factor (g′) have been introduced to characterize the copolymers architecture.39 The α and g′ values corresponded to branched/star copolymer is less than linear copolymers at the same molecular weight due to their compact structures. In present study, α values


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decreased from 0.569 to 0.439 (Figure 3) as X increasing. This was attributed to the branching effect of star polymers. In addition, the value of g′ was lower than 1. All of the results confirmed the formation of star copolymers.

Table 2. Results of polymerization of P (HS-co-TBA) copolymer with various feed ratios of CTA Mw Sample

×10

3

[η]w

PDI

(mL/g)

αa

Rh /nm

g′ b

Tg (°C)

g/mol P (HS-co-TBA) 0

113.4

1.52

30.26

0.569

7.8

1

166.1

P (HS-co-TBA) 1.2

21.6

1.66

10.51

0.518

3.2

0.79

134

P (HS-co-TBA) 2.5

15.6

1.51

6.98

0.482

2.5

0.59

128.9

P (HS-co-TBA) 3.8

12.5

1.43

7.51

0.456

2.4

0.63

123.9

P (HS-co-TBA) 5.1

8.6

1.46

6.08

0.439

2.0

0.58

121.6

a

Mark–Houwink–Sakurada exponent;

b

g′ = [η]b /[η]l, where [η]b and [η]l are the weight-average

intrinsic viscosities of star and linear polymers with identical molecular weights, respectively.


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Figure 3. GPC curves (up) and MHS plot (down) for the copolymers of P (HS-co-TBA) with various CTA feed ratios. The resulting α values were determined based on the slope.

3.3 Thermal properties of P (HS-co-TBA) The factors such as chemical composition, the free volume have an effect on the Tg of copolymers.39 Figure 4 presents the DSC curves of the P (HS-co-TBA) copolymers. The Tg appears to decrease with increasing the CTA loadings. Polymers with star architecture have more chain ends, leading to an increase in free volumes, which result in a reduction in Tg.39 In addition, more star architecture shortens the molecular chain length, and this always reduces the Tg because of less restricted chain mobility. Copolymers with less repeat units per arm have a lower molecular weight, resulting in a reduced hydrogen bonding force among the -OH and C=O groups, which also reduces Tg.

Figure 4. DSC curves of the copolymers of P (HS-co-TBA) with various CTA feed ratios.


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TGA curves of the P (HS-co-TBA) copolymers were shown in Figure 5. It has been reported that the tert-butyl ester bonds of TBA decomposed at around 190 °C.37 Therefore, it can be determined that TBA was successful incorporated into molecular chain The second stage, where the decomposition temperatures were between 210 and 500 °C and the residual polymers were subjected to further decomposition, could be attributed to the decomposition of the poymer backbone and other side chain groups. Tmax corresponding to the first stage of decomposition decreased as CTA loadings increased. Tmax corresponding to the second stage also decreased because of the shorter arm length.39

Figure. 5. TGA curves of P (HS-co-TBA) copolymers with various CTA feed ratios.

3.4 Sensitivity and contrast measurements The absorption of each polymer solution is lower than 0.4 (see Figure S1 for UV absorption spectra in the supporting information), which is suitable for use in KrF lithography. Photoresist film was prepared according to the resist processing. After irradiation with a KrF excimer laser, acids were generated from the PAG. The acid-catalyzed deprotection reaction occurred in exposed regions during the PEB process, which converted the tert-butyl ester to the carboxyl group responsible for the development of contrast. Figure 6 depicts the contrast curves of the resist materials based on the P (HS-coTBA) copolymers. All resists featured a low thickness loss when the exposure dose was low because of the dissolution–inhibition effects of TBA, who was hydrophobic. Subsequently, the thickness loss enhanced exponentially with increasing doses, because an increasing number of TBA repeat units and PAG decomposed, resulting in a faster dissolution effect. The contrast curves in Figure 6 indicate that the sensitivity increased when the CTA loadings increased (E0 decreased from 4.8 to 3.2 mJ/cm2 are presented in Table. S3 in the


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supporting information). Compared with PR1 based on the linear copolymer P (HS-coTBA) 0, the resist corresponded to star copolymers possessing higher photosensitivity. A possible reason for these results is that more acid-labile tert-butyl groups were distributed on the arms, after the exposure and PEB procedure, the photo generated acid (derived from the photodecomposition of the PAG) catalysed the hydrolysis of the tertiary ester in the TBA to form a carboxylic acid, thereby improving the probability of a reaction with the developer. Another reason could be that the molecular chains became shorter and molecular weight became lower per arm, result in weaker chain entanglement and intermolecular force, which would contributed to the dissolution of resist in developer.

Figure 6. Contrast curves for photoresists containing various copolymers; lg E: logarithm of the exposure energy. PR1-5 correspond to the samples P (HS-co-TBA) 0-5.1, respectively.

3.5 Resolution measurement Figure 7 presents the patterns obtained from the PR5 containing the copolymers corresponding to P (HS-co-TBA)

5.1.

SEM and AFM were used to analyze the pattern

resolution. The photoresist revealed 0.2 and 0.25 µm line-space patterns at 3000 Å of the resist formulation exposed at 12 mJ/cm2. Compared with our previous work (resist based on linear tetrapolymer P (ASM-TBA-St-CA))37, PR5 possessed higher resolution. All imaging results indicated that the star polymers have a potential application for lithograph materials.


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Figure 7. (a), (b) SEM images; (c), (d) two-dimensional views; (e), (f) three-dimensional views of 200nm (left) and 250-nm (right) line-space features. The patterns obtained from PR5 corresponding to P (HS-co-TBA) 5.1 exposed at 12 mJ/cm2.

4. CONCLUSIONS A novel chemically amplified positive-tone photoresists for KrF lithography based on a star polymer backbone was synthesized and its performance was examined. The intrinsic viscosity and Zimm branching factor (g′) of copolymers with CTA were lower than the linear analogue, which confirm formation the star architecture. The ultraviolet absorbance of the copolymers indicated that the copolymer was suitable for use as a krypton fluoride (KrF) laser photoresist. Moreover, the photolithography performance of the positive-tone chemically


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amplified photoresist was evaluated. The photosensitive based on the star copolymer was higher than the linear one, and the pattern resolution was around 200 nm at a low exposure energy.

ASSOCIATED CONTENT S Supporting Information The Supporting Information is available free of charge. Further details about UV absorption spectra of P (HS-co-TBA) copolymer solution; The results for molecular weight and PDI of P (ASM-co-TBA) with different feed ratio CAT; Hhydrolysis degree of P (HS-co-TBA) copolymers and sensitivity and contrast of photoresists (PDF). AUTHOR INFORMATION Corresponding Author: E-mail: [email protected]; E-mail: [email protected] ORCID Xiangfei Zheng: 0000-0001-7496-8741 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Postgraduate Research & Practice Innovation Program of Jiangsu Provence (No.KYCX217-1434), the Fundamental Research Funds for the Central Universities

(No.JUSRP51719A),

and

the

China

Postdoctoral

foundation

project

(No.2016M601712). Very thanks for Suzhou Rui Hong Electronic Chemicals Co., Ltd. and 13th Research Institute of China Electronics Technology Group Corporation for the Lithographic evaluations.


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Novel Star Polymers as Chemically Amplified Positive-Tone

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