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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 11: 1613 - 1624

DOI: https://doi.org/10.7536/PC230304

Review

Advanced Design of Block Copolymers for Nanolithography

  • Chen Leilei 1 ,
  • Tao Yongxin 1 ,
  • Hu Xin , 2, * ,
  • Feng Hongbo , 3, * ,
  • Zhu Ning , 1, * ,
  • Guo Kai 1
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  • 1 College of Biotechnology and Pharmaceutical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University,Nanjing 211800, China
  • 2 College of Materials Science and Engineering, Nanjing Tech University,Nanjing 211800, China
  • 3 Pritzker School of Molecular Engineering, University of Chicago, Chicago IL 60637, USA
*Corresponding author e-mail: (Xin Hu);
(Hongbo Feng);
(Ning Zhu)

Received date: 2023-03-09

  Revised date: 2023-05-16

  Online published: 2023-06-12

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Abstract

Directed self-assembly (DSA) of block copolymer (BCP) has been identified as the potential strategy for the next-generation semiconductor manufacturing. The typical representative of the first generation (G1) of block copolymer for nanolithography is polystyrene-block-polymethylmethacrylate (PS-b-PMMA). DSA of PS-b-PMMA enables limited half pitch (0.5L0) of 11 nm due to the low Flory-Huggins interaction parameter (χ). The second generation (G2) of BCP is developed with the feature of high χ. Solvent anneal or top-coat is employed for the G2 BCP to form the perpendicular lamellae orientation. Towards industry friendly thermal anneal, high χ BCP with equal surface energy (γ) is reported as the third generation (G3) BCP. Recently, based on Materials Genome Initiative (MGI) concept, optimized design of block copolymers with covarying properties (G4) for nanolithography is presented to meet specific application criteria. G4 BCP achieves not only high χ and equal γ, but also high throughput synthesis, 4~10 nm half pitch patterns, and controlled segregation strength. This review focuses on the advanced design of G3 and G4 BCP for nanolithography. Moreover, the challenges and opportunities are discussed for the further development of DSA of BCP.

Contents

1 Introduction

2 Highχblock copolymers with equalγ(G3)

2.1 A-b-B block copolymer

2.2 A-b-(B-r-C)block copolymer

2.3(A-r-B)-b-C block copolymer

2.4 A-b-B-b-C block copolymer

3 Block copolymers with covarying properties(G4)

4 Conclusion and outlook

Key words： block copolymer; directed self-assembly; nanolithography; Flory-Huggins interaction parameter; surface energy

Cite this article

Chen Leilei , Tao Yongxin , Hu Xin , Feng Hongbo , Zhu Ning , Guo Kai . Advanced Design of Block Copolymers for Nanolithography[J]. Progress in Chemistry, 2023 , 35(11) : 1613 -1624 . DOI: 10.7536/PC230304

1 Introduction

Over the past few decades, with the continuous shrinking of semiconductor manufacturing processes, the number of chip transistors has increased rapidly, the performance of devices has improved significantly, and the price has dropped dramatically[1⇓⇓⇓⇓⇓⇓⇓⇓⇓~11]. Lithography is an important step in semiconductor micro-nano processing and manufacturing, which transfers the design pattern from the mask to the substrate surface through different wavelength illumination[12⇓⇓⇓~16]. In the lithography process, photoresist is the key material to realize the preparation of fine patterns, but the traditional photoresist does not determine the feature size of the pattern, and the semiconductor process mainly depends on the parameters such as wavelength (λ) and numerical aperture (NA). As the wavelength of the light source approaches the physical limit, the continuation of Moore's law encounters great challenges[17⇓~19].
Block copolymer (BCP) is a special class of photoresist, which can further obtain patterns with smaller feature size by density multiplication through guided self-assembly (DSA) on the basis of existing photolithographic patterns (Figure 1A)[20⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~31]. The method of self-assembly of block copolymers to form regular patterns by physical or chemical means (DSA) has been listed as one of the options for the next generation of advanced process semiconductor manufacturing by the International Roadmap for Devices and Systems (IRDS)[32⇓⇓⇓~36]. Block copolymers can undergo microphase separation under certain conditions to form periodic ordered structures, and their bulk self-assembly is a thermodynamically driven process, which is mainly caused by the thermodynamic incompatibility between blocks[37]. A-B-B diblock copolymers can be microphase separated to form various structures such as Lamellae, cylinders, spheres, and gyroids (Fig. 1b, C). The BCP phase separation is mainly regulated by three parameters: the Flory-Huggins interaction parameter (χ), the molecular weight (N), and the block volume fraction (fA,fB)[38⇓⇓⇓⇓⇓⇓⇓~46]. According to the change of χN, the phase separation of block copolymer is divided into weak phase separation region, intermediate state and strong phase separation region. In the strong phase separation region, the two phases of the block copolymer are completely separated, the phase interface is narrow, and the polymer segment is strongly stretched perpendicular to the phase interface[47~48]. According to the self-consistent mean field theory, microphase separation occurs when χN of symmetric linear A-B-B diblock copolymer (fA=fB) is greater than 10.5, and the phase period L0 of strong phase separation is proportional to χ1/6N2/3, indicating that high χ is beneficial to achieve smaller L 0 [49]~[59].
图1 (a) 热退火密度倍增引导自组装DSA流程图[60];(b) 随着B嵌段体积分数的增加A-b-B二嵌段共聚物体相自组装结构示意图[62];(c) 对称二嵌段共聚物熔体的平均场相图[47]

Fig.1 (a) Schematic of the fabrication process of the chemically patterned substrates and the directed assembly of PS-b-PMMA with density multiplication[60]. Copyright © 2013, American Chemical Society. (b) Schematic illustration of microstructures of diblock A-b-B on increasing the volume fraction of the B block[62]. Copyright © 2014, the Royal Society of Chemistry. (c) Mean-field phase diagram for conformational symmetric diblock melts[47]. Copyright © 1996, American Chemical Society

As the core of guided self-assembly nanopatterning, the design of block copolymer photoresist has attracted the attention of both academia and industry. A typical representative of the first generation of block copolymer photoresists is polystyrene-poly (methyl methacrylate) diblock copolymer (PS-b-PMMA), which has a minimum half-period (0.5L0) of ∼ 11 nm for feature pattern formation by guided self-assembly due to a low interaction parameter (χ = 0.039, 150 ° C)[60⇓⇓~63]. The second generation block copolymer photoresist is characterized by a high interaction parameter (χ), which can be used to fabricate feature patterns below 10 nm. However, due to the large difference in surface energy (γ) between the two blocks, additional solvent annealing or coating processes are required[64⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~88]. In order to solve the above problems, scholars at home and abroad have developed the third generation of block copolymer photoresists, which not only have high interaction parameters but also have close surface energy (high χ and close γ), and are suitable for industrial-friendly thermal annealing process. Recently, based on the concept of the Materials Genome Project, the fourth generation of multifunctional block copolymer photoresists, which endow a single material with multiple covariation characteristics, have been developed, and a block copolymer library has been established by high-throughput synthesis, which simplifies the thermal annealing process by regulating χ and χN to meet the requirements of different application scenarios. In this paper, we focus on the research progress of the third and fourth generation advanced block copolymer photoresists, and discuss the challenges and opportunities in related fields, hoping to provide reference for advanced semiconductor manufacturing.

2 Third generation: high-χ near-γ block copolymer

Compared with the first and second generation block copolymer photoresists, the third generation BCP photoresists not only have high interaction parameters (χ), but also have close surface energies (γ) of different blocks.Layered phase structures perpendicular to the substrate can be formed by using industry-friendly thermal annealing processes without solvent annealing or coating processes, which promotes the development of guided self-assembly.

2.1 A-b-B block copolymer

In 2002, Hillmyer et al. Reported the formation of nanopores by self-assembly of polystyrene-poly (rac-lactide) (PS-b-PLA) diblock copolymer (PLA is the abbreviation of PDLLA), and then used the surface energy (36~41.1 mJ/m2) of PLA similar to that of PS.PS-b-PLA films with vertically oriented lamellar and columnar phases were prepared, and the changes of self-assembled structure with film thickness and annealing temperature were analyzed, which proved that increasing annealing temperature was beneficial to the formation of uniform self-assembled morphology[89][90]. In 2012, Blakey and Whittaker et al. Derived the relationship between the PS-b-PLA interaction parameter (χ) and the reciprocal of temperature χ (T) = 98.1/T-0.112 (χ = 0.119, 150 ℃), and the χ value was significantly improved compared with PS-b-PMMA[91]. When the PS-stat-PMMA was used as a neutral layer to modify the substrate, the copolymer film could form a lamellar phase perpendicular to the substrate after thermal annealing at 100 ℃ for 24 H, which proved that the existence of the neutral layer was beneficial to the self-assembly of the BCP film at a lower annealing temperature.
In 2016, Nealey and Ji Shengxiang explored the influence of stereochemistry on the self-assembly behavior of PS-b-PLA[92]. The steric complexation within the racemic PDLLA block can lead to the formation of pseudo-" ABA "triblock copolymers with PS-b-PDLLA, which indicates that specific interactions between polymer chains can be introduced to guide the self-assembly behavior of block copolymers. In 2018, Ji Shengxiang et al. Further designed and synthesized a polystyrene-poly (glycolide-lactide) (PS-b-PLGA) diblock copolymer, which had a higher χ value than PS-b-PDLLA (0.155, 150 ° C) as determined by variable temperature small angle laser light scattering (VT-SAXS) (Figure 2A)[93]. Because PLGA and PS also have similar surface energy, the guided self-assembly of 2 times density multiplication can be achieved under the thermal annealing condition of 160 ~ 200 ℃ (Fig. 2C), and the vertically oriented lamellar phase pattern can be obtained in a larger range (Fig. 2b).
图2 (a) PS-b-PLGA和PS-b-PDLLA的χ分别与温度倒数的关系图;(b) PS-b-PLGA引导自组装扫描电镜(SEM)图;(c) PS-b-PLGA热退火条件下2倍密度倍增DSA流程示意图[93]

Fig.2 (a) Plots of χ against the inverse of temperature for PS-b-PLGA and PS-b-PDLLA; (b) top-down SEM images of DSA of PS-b-PLGA; (c) schematic illustration of the procedure used to create asymmetric chemical patterns and DSA of lamellae-forming PS-b-PLGA with 2×density multiplication under thermal annealing[93]. Copyright © 2018 American Chemical Society

The introduction of silicon element is an effective method to improve the interaction parameters of block copolymers. However, the surface energy of silicon-containing polymers is usually much lower than that of organic polymers, which increases the difficulty of controlling the vertical orientation of phase domains. In 2015, Hadziioannou and Fleury et al. Designed a poly (1,1-dimethylsilane-cyclobutane) -poly (methyl methacrylate) (PDMSB-b-PMMA) diblock copolymer (Figure 3A), which can control the domain orientation through the film thickness[94]. Thermal annealing of the 40 nm thick PDMSB42-b-PMMA11 film at 180 ° C for 10 min produced a fingerprint-like pattern with a period L0 of 12.7 nm (Fig. 3 B). PDMSB has better etch resistance than PMMA and can be converted into a silicon carbide (SiC) hard mask, which is beneficial to pattern transfer.
图3 (a) PDMSB-b-PMMA的结构式;(b) 热退火10 min后PDMSB-b-PMMA薄膜的3D-AFM相位图[94]

Fig.3 (a) The structure of PDMSB-b-PMMA; (b) 3D-AFM phase views of thermally annealed (10 min) PDMSB-b-PMMA thin films[94]. Copyright © 2014, Wiley-VCH GmbH, Weinheim

In 2016, Hayakawa et al. Proposed the strategy of introducing polar hydroxyl groups to enhance the surface energy of silicon-containing polymers[95]. Polystyrene-b-polymethylvinylsiloxane (PS-b-PMVS) diblock copolymers were prepared by anionic polymerization, and then two hydroxyl-modified copolymers, PS-b-PMHxS and PS-b -PMHxOHS, were obtained by alkene-thiol click reaction (Figure 4A). The bulk self-assembled microstructure of the copolymers was characterized by SAXS (Figure 4B). The introduction of hydroxyl groups improves the surface energy of polysiloxane, which is close to that of polystyrene. Fingerprint-like patterns can be observed after thermal annealing in air at 130 ℃ for only 1 min, and the minimum half-period reaches 8.5 nm (Figure 4C). Compared with PS-b-PMMA, the annealing time of PS-b-PMHxS and PS-b-PMHxOHS is significantly shortened. Subsequently, the research group designed a high-χ near-γ poly (silsesquioxane methacrylate) -poly (trifluoroethyl methacrylate) (PMAPOSS-b-PTFEMA) diblock copolymer by matching the same low surface energy fluoropolymer with the silicon-containing polymer[96]. The strong phase separation of the silicon-containing block from the fluorine-containing block imparts a high gamma value (150 ° C) of up to 0.45 to PMAPOSS-b-PTFEMA. In addition, the surface energy of PTFEMA(25.1 mJ/m2) is close to that of PMAPOSS(28.7 mJ/m2), and an ordered pattern with a half-period of 8 nm is obtained by physical epitaxy guided self-assembly at 150 ℃ for 24 H. However, due to the small surface energy of the two blocks, it is not conducive to complete wetting on the surface of the silicon substrate, resulting in the pattern quality to be improved.
图4 (a) PS-b-PMHxOHS和PS-b-PMHxS的合成路线;(b) BCP在190℃下热退火3 h后的SAXS谱图;(c) PS100-b-PMHxOHS25的扫描电镜图[95]

Fig.4 (a) Synthesis of PS-b-PMHxOHS和PS-b-PMHxS; (b) SAXS profiles of the BCPs collected after thermal annealing at 190℃ for 3 h; (c) SEM image of PS100-b-PMHxOHS25[95].Copyright © 2016, Springer Nature

In 2017, Xu Zhikang, Nealey and Wu Guangpeng et al. Found that the surface energy (42.9 mJ/m2) of aliphatic poly (propylene carbonate) (PPC) was similar to that of PS, and thus designed and synthesized a polystyrene-poly (propylene carbonate) (PS-b-PPC) diblock copolymer (Fig. 5)[97]. The χ value of PS-b-PPC at 150 ° C was determined to be 0.079, achieving a thermally annealed 5-fold density multiplication DSA with a minimum L0 of 17 nm (Fig. 5). The PS-b-PPC film has a higher etch selectivity than PS-b-PMMA and can be pattern transferred by the sequential infiltration synthesis (SIS) method.
图5 PS-b-PPC的合成路线、图案化模板的创建以及热退火条件下5倍密度倍增DSA流程图及其SEM图[97]

Fig.5 Synthesis of PS-b-PPC, schematic illustration of the procedure used to create prepatterned substrates and the DSA of lamellae forming PS-b-PPC system with 5 times density multiplication under thermal annealing, and the top-down SEM images showing DSA[97].Copyright © 2017 American Chemical Society

In 2019, Wei Yayi et al. studied the hydrogen bonding between the active H in the PS-b-PPC structure and the Si-OH on the surface of the silicon wafer (Fig. 6a, B). The hydrogen bonding can control the structure of the molecular aggregate, and can perform thermal annealing DSA without a neutral layer to obtain a fingerprint-like pattern with a vertical orientation of the crystal domain (Fig. 6C). The microphase separation period is 16.8 nm[98]. In addition, PS-b-PPC also has great potential in other silicon-based substrate materials, such as SiO2 and Si3N4. In 2020, Arnold, Xiong Shisheng, Hur and Wu Guangpeng proposed boundary guided epitaxy, which uses the space boundary formed by the surface difference between different components to guide the self-assembly of PS-b-PPC and PPC-b-PS-b-PPC films, greatly reducing the processing difficulty of guided patterns[99].
图6 (a) PS-b-PC的化学结构式;(b) 氢键形成机理示意图;(c) 无中性层PS-b-PC的SEM图[98]

Fig.6 (a) Chemical structure of PS-b-PC;.(b) a schematic diagram of the formation mechanism of hydrogen bonds; (c) top-down SEM images of PS-b-PC with no neutral layer[98]. Copyright © 2019, the Royal Society of Chemistry

In 2019, Ji Shengxiang and Wan Lei et al. Reported polystyrene-polymethyl acrylate (PS-b-PMA) diblock copolymer and PMA-b-PS-b-PMA triblock copolymer (Figure 7 a)[100]. The relationship between the χ value and the reciprocal temperature is χ (T) = (91.3 ± 3.6)/T − (0.148 ± 0.007) (Fig. 7c), which is calculated by the storage modulus at elevated temperature (Fig. 7B). Compared with PS-b-PMMA, the χ value of PS-b-PMA is nearly doubled (0.068 vs 0.039, 150 ℃), and the structure and surface energy of PMA are similar to those of PMMA. An ordered pattern with 6 times density multiplication and a period of 13. 9 nm was obtained by LiNe thermal annealing guided self-assembly process (thermal annealing at 140 ℃ for 12 H), and a transfer pattern with a half period of less than 8. 5 nm was further formed by SIS.
图7 (a) PS-b-PMA、PMA-b-PS-b-PMA及其热退火引导自组装SEM图;(b) 动态储能模量(G')的温度依赖性;(c) χ值与温度倒数关系图[100]

Fig.7 (a) Structure of PS-b-PMA and PMA-b-PS-b-PMA, and SEM of DSA; (b) temperature dependence of the dynamic storage modulus; (c) linear dependence of χ as a function of inverse TODT.[100]. Copyright © 2019 American Chemical Society

In 2019, Hayakawa et al. Developed a strategy to regulate hydrophilicity, hydrophobicity and surface energy by introducing fluorine-containing groups (Fig. 8A), and obtained a high-χ near-γ polystyrene-poly (2-hydroxy-3-trifluoroethylthio) propyl methacrylate (PS-b-PHFMA) diblock copolymer (χ = 0.171, 150 ℃) (Fig. 8B)[101]. By introducing a hydrophobic trifluoroethyl group into the hydrophilic segment to control the surface energy close to PS, the copolymer film self-assembled under thermal annealing conditions to form an ordered pattern below 10 nm (Fig. 8C). Subsequently, the research group used long-chain C8F17 instead of short-chain trifluoroethyl, and the liquid crystal ordering of long-chain fluorine-containing groups can limit the change of polymer entropy, thus reducing the interfacial effect, with the minimum half-period reaching 6. 3 nm[102].
图8 (a) 高χ近γ嵌段共聚物的结构设计;(b) χ值与温度倒数关系图;(c) PS-b-PGMA的TEM图[101]

Fig.8 (a) Concept for designing a chemically tailored high-χ BCP with balanced surface affinities and increased strengths of segregation; (b) temperature dependences of the effective Flory-Huggins interaction parameter; (c) TEM images of PS-b-PGMA[101]. Copyright © 2019, the Royal Society of Chemistry

2.2 A-b- (B-r-C) block copolymer

The random copolymer segment (B-r-C) can decouple the surface and interface properties (γ, Δγ) and thermodynamic properties (χ, χN), and the A-b- (B-r-C) block copolymer has rich structural design, which can achieve higher interaction parameters and close surface energy (high χ near γ). In 2012, Bates and Nealey et al. Used epoxidation to convert polystyrene-polyisoprene (PS-b-PI) diblock copolymer into partially epoxidized PS-b- (PI-r-PIxn) (originally named PS-PIxn) (Fig. 9a)[103]. By changing the degree of epoxidation, the interaction parameters (Fig. 9 B) and surface energy (Fig. 9 C) of the copolymer can be manipulated, respectively, and when the degree of epoxidation of PI reaches 75%, the surface energy of PS and PI-r-PIxn is almost equal at the annealing temperature (40.7 mJ/m2). Based on the experimental results, the relationship between the interaction parameter and the reciprocal of temperature and the degree of epoxidation (xn) was calculated to be χ(T)=(28.6/T-0.02)[1-2.3(xn/100)+3.39(xn/100)2][104].
图9 (a) PS-b-PI环氧化合成PS-b-(PI-r-PIxn);(b) 环氧化程度对嵌段共聚物相互作用参数(χ)的影响;(c) 环氧化程度对PI-r-PIxn表面能(γ)的影响[103]

Fig.9 (a) Epoxidation of PS-PI to PS-b-(PI-r-PIxn); (b) effective interaction parameter (χ) affected by the degree of epoxidation; (c) effect of degree of epoxidation on the surface energy of PI-r-PIxn[103]. Copyright © 2012 American Chemical Society

In 2022, Nealey and Rowan et al. Used polystyrene-polybutadiene (PS-b-PB) diblock copolymer as the parent polymer and developed a new A-b- (B-r-C) block copolymer PS-b-P(B-r-Bthiol) (Figure 10A) by using the alkene-thiol click reaction[105]. The effects of four different structures of thiols and their reaction degree (φ) on the self-assembled structure, γ, Lo, and χ were examined (Fig. 10 B), in which mercaptoethanol (MEA) or 1-mercaptoglycerol (MGA) modified PS-b-P(B-r-Bthiol) exhibited a lamellar phase. Thermal annealing-guided self-assembly was performed using the LiNe flow process (Fig. 10 C), and it was found that the linear ordered pattern was mixed with the fingerprint-like pattern, which was due to the thermal crosslinking (free radical reaction) of the residual double bonds in the PS-b-P(B-r-Bthiol), which hindered the guidance of the template pattern to the copolymer (Fig. 10 d). When 0.1 wt% 2,6-di-tert-butyl-p-cresol (BHT) (radical trap) was added, a significantly improved ordered pattern was obtained in a 1-μm-wide region after thermal annealing of the PS-b-P(B-r-BMEA) film (Figure 10D).
图10 (a) 利用烯-硫醇点击化学由PS-b-PB制备PS-b-P(B-r-Bthiol);(b) 巯基功能化反应程度(φ)对γ、L0和χ的影响;(c) LiNe flow DSA工艺流程图;(d) 未添加和添加0.1 wt% BHT的PS-b-P(B-r-BMEA)引导自组装SEM图[105]

Fig.10 (a) Synthetic scheme of thiol-ene click chemistry to prepare PS-b-P(B-r-Bthiol) from PS-b-PB; (b) effects of degree of thiol functionalization (φ) on γ, L0, and χ; (c) schematic of the LiNe DSA process flow; (d) SEM of DSA of PS-b-P(B-r-BMEA) with and without 0.1 wt% BHT[105]. Copyright © 2022, Wiley-VCH GmbH, Weinheim

In 2018, Satoh and Isono et al. Prepared PS-b- (PMMA-r-modified PMMA) (originally named modified PS-b-PMMA) by converting a small amount of polymethacrylate (PMMA) into polymethacrylamide through ester-amide exchange reaction (Figure 11A)[106]. The introduction of amide structure improves the hydrophilicity of PMMA and its incompatibility with PS chain segments, thus effectively improving the χ value of block copolymers, so that low molecular weight copolymers can also achieve microphase separation (Fig. 11b). At the same time, the lower degree of ester-amidation did not significantly affect the surface energy of the PMMA segment, and the PS-b- (PMMA-r-modified PMMA) copolymer film was thermally annealed at 140 ℃ for 10 min to form an ordered pattern with a minimum half-period of 5.6 nm by physical epitaxy (Fig. 11c).
图11 (a) 酯-酰胺交换反应修饰PS-b-PMMA;(b) 修饰前后对比(无序vs有序,低χ vs高χ);(c) 共聚物薄膜引导自组装SEM图[106]

Fig.11 (a) Ester-amide exchange reaction of PS-b-PMMA with various amines; (b) comparison before and after ester-amide exchange reaction (disorder vs ordered, low χ vs high χ); (c) SEM of DSA of modified PS-b-PMMA[106]. Copyright © 2018, American Chemical Society

2.3 (A-r-B) -b-C type block copolymer

In 2016, Ellison et al. Found that styrene/vinyl-naphthalene random copolymer-poly (methyl methacrylate) ( (PS-r-PVN) -b-PMMA) had a high interaction parameter and a close surface energy (Fig. 12A)[107]. Compared with PS-b-PMMA, the χ values of vinyl-naphthalene (VN) -containing copolymers were significantly increased (Fig. 12 B), and the χ value of the copolymer containing 35 mol% vinyl-naphthalene was more than doubled, so that the characteristic size of self-assembly reached 6.3 nm. The surface energy of PVN is between that of PS and PMMA, and (PS-r-PVN) -b-PMMA can be self-assembled by thermal annealing to form a fingerprint-like pattern with vertically oriented crystalline domains (Fig. 12c).
图12 (a) (PS-r-PVN)-b-PMMA的结构式;(b) 不同聚合物的χ值比较;(c) 轻度蚀刻的SEM图[107]

Fig.12 (a) Structure of (PS-r-PVN)-b-PMMA block copolymer; (b) comparison of the χ parameter of two VN-containing polymers to PS-PMMA; (c) top-down SEM image of lightly etched[107]. Copyright © 2016 American Chemical Society

In 2021, Jung and Kim et al. designed a pentafluorostyrene/styrene gradient random copolymer-polymethylmethacrylate (P (S-r-PFS) -b-PMMA) (Fig. 13A) (originally named P (S-g-PFS) -b-PMMA), which can form a vertically oriented layered phase structure below 10 nm without an upper coating and a neutral layer (Fig. 13b)[108]. The surface energy difference between the block junction region and the gradient random copolymer tail region allows the copolymer film to form a perpendicular orientation on almost any type of surface and self-assemble into a well-ordered pattern on an extreme ultraviolet (EUV) lithography template (fig. 13C).
图13 (a) P(S-r-PFS)-b-PMMA的合成;(b) 无上涂层和中性层引导自组装;(c) 基于EUV光刻模板引导自组装SEM图[108]

Fig.13 (a) Synthesis steps of P(S-r-PFS)-b-PMMA; (b) DSA without top-coat and neutral brush layer; (c) SEM of DSA based on EUV lithography patterns[108]. Copyright © 2021, the Royal Society of Chemistry

2.4 A-b-b-b-c type block copolymer

Compare to that aforementioned block copolymer,The A-B-B-B-C triblock copolymer has three interaction parameters (χAB, χBC, and χAC), three block volume fractions (fA,fB and fC) and three block sequences (A-B-B-B-C, B-B-C-B-A, or C-B-A-B-B),Therefore, it has a richer phase diagram and provides a larger design space. In 2017, Bang et al. added a high-χ intermediate block polymethacrylate (PMAA) between PS and PMMA (Fig. 14). The strong repulsion between PMAA and PS can promote the phase separation between PS and PMMA[109]. The PMAA midblock has a higher surface energy than PS and PMMA, but it does not significantly change the lateral concentration fluctuation (which is responsible for the horizontal orientation), so the vertical substrate orientation can be maintained. The experimental results show that the interaction parameters at 180 ° C are χSM=0.046, χSH=0.314, and χMH=0.071, respectively, which proves that the phase separation of PS-b-PMAA-b-PMMA (SHM) triblock copolymer is mainly driven by the strong repulsion between polystyrene and polymethacrylic acid. An ordered pattern with a minimum L0 of 16.7 nm was obtained by annealing the copolymer film at 180 ° C for at least 1 day.
图14 PS-b-PMAA-b-PMMA结构式及其形成10 nm以下垂直取向层状相[109]

Fig.14 Structure of PS-b-PMAA-b-PMMA toward perpendicularly oriented nanodomains with sub-10 nm features[109]. Copyright © 2017, American Chemical Society

3 Fourth generation: multifunctional block copolymer

In 2022, based on the concept of the Materials Genome Project, Nealey and Rowan proposed the fourth generation of multifunctional block copolymer photoresist by endowing a single material with a variety of covariation characteristics. Compared with the third generation of high-χ near-γ block copolymer, it has the following advantages: (1) starting from a parent polymer, a BCP library was constructed with high throughput through thiol-epoxy "click" chemical modification[110]; (2) On the basis of high χ and near γ, χN can be controlled to optimize the structure and defects of self-assembly; (3) The block copolymer can spontaneously form its own neutral layer, which simplifies the thermal annealing process; (4) For different application scenarios, χ and χN are screened to achieve 0.5L0=4~10 nm patterning.
A series of S-b-G(RB-r-RC) were synthesized in high throughput using polystyrene-poly (glycidyl methacrylate) diblock copolymer (S-b-G) as the matrix polymer by sequential or simultaneous addition of two mercaptans. A library of block copolymers with high χ, controllable χn, and Δγ = 0 was created by varying the species and content of φb and φC, and adjusting the γ of the G(Rb-r-Rc) block and the χ value between it and S (Fig. 15). The S-b-G (TFET-r-2MP) treated by the sequential infiltration synthesis (SIS) method showed a high-contrast fingerprint pattern after the S block was removed by Ar/O2 etching (Figure 16 A), which confirmed the good pattern transfer of S-b-G(RB-r-RC). More interestingly, the secondary hydroxyl generated by click chemistry in the S-b-G(RB-r-RC) forms its own neutral layer with the silicon wafer substrate interface during the thermal annealing LiNe flow process, which eliminates the traditional step of coating the neutral layer and simplifies the thermal annealing process (Fig. 16b). By regulating χ and χN to meet the requirements of different application scenarios, S30-b-G12(TFET-r-2ME)(φ2ME=37 mol%)L0=8 nm) with χ = 0.42 and χN = 20.7 is selected for hot spot magnetic recording (HDMR), and S108-b-G32(MPTS-r-2MP)(φ2MP=75 mol%)(L0=18 nm) with χ = 0.11 and χN = 24.5 is selected for semiconductor manufacturing, which opens up a new direction for advanced block copolymer photoresist design.
图15 (a) 多功能嵌段共聚物A-b-(B-r-C)的设计思路:通过改变B和C获得高χ近γ 嵌段共聚物;(b) 从母体A-b-B’到系列A-b-(B-r-C)的合成示意图;(c) 基于巯基-环氧点击化学合成A-b-(B-r-C)[110]

Fig.15 (a) Design principle for creating a series of BCPs with tunable χN and Δγ = 0 using an A-b-(B-r-C) polymer architecture. By varying the B and C groups, the architecture can form a BCP that has Δγ = 0 at the desired χ value; (b) schematic of the generation of a series of A-b-(B-r-C) polymers from the parent A-b-B'; (c) synthesis of A-b-(B-r-C) via thiol-epoxy click reactions[110]. Copyright © 2022, Springer Nature

图16 (a) 多功能嵌段共聚物自组装薄膜具有增强的刻蚀对比度(使用SIS技术或引入含硅基团);(b) 多功能嵌段共聚物可以形成自身的中性层,免去传统DSA工艺中涂覆中性层的步骤[110]

Fig.16 (a) Schematic of two distinct strategies for enhancing etch contrast of the self-assembled BCP film, using either SIS or silicon-containing thiols; (b) self-brushing DSA process flow leading to DSA with density multiplication[110]. Copyright © 2022, Springer Nature

4 Conclusion and prospect

The combination of bottom-up block copolymer photoresist-guided self-assembly and top-down lithography can further obtain nanopatterns with smaller feature sizes on the basis of established light sources, which is of great research significance and commercial value. Block copolymer photoresists have experienced the development of low-χ-near-γ (first generation), high-χ-far-γ (second generation), high-χ-near-γ (third generation) and multifunctional (fourth generation), and have achieved gratifying research results. In recent years, the third generation of high-χ near-γ block copolymers has decoupled the thermodynamic properties (χ, χN) and surface and interface properties, and achieved efficient preparation of characteristic patterns below 10 nm under thermal annealing conditions. The fourth generation of multifunctional block copolymers has just come out, which endows a single material with a variety of covariant properties, pointing the way for block copolymer photoresist-guided self-assembly. Looking forward to the future, the design and guided self-assembly of advanced block copolymer photoresists will face many challenges and opportunities: (1) The types of the third generation block copolymers are still relatively scarce, while the fourth generation block copolymers based on the Materials Genome Project have just started, and new design ideas are urgently needed; (2) The structure of advanced block copolymers is relatively complex, and the synthesis steps are cumbersome, so it is necessary to develop efficient, stable and accurate synthesis strategies; (3) The research on advanced block copolymer guided self-assembly is still insufficient, and the problems of pattern defects and edge roughness need to be analyzed in depth, so as to realize the industrial application of block copolymer photoresist-guided self-assembly as soon as possible.

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