Directed Self-Assembly of Cylinder-Forming Block Copolymers

Oct 2, 2009 - Directed Self-Assembly of Cylinder-Forming Block Copolymers: Prepatterning Effect on Pattern Quality and Density Multiplication Factor...
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Directed Self-Assembly of Cylinder-Forming Block Copolymers: Prepatterning Effect on Pattern Quality and Density Multiplication Factor Lei Wan and XiaoMin Yang* Seagate Research Center, 1251 Waterfront Place, Pittsburgh, Pennsylvania 15222 Received May 8, 2009. Revised Manuscript Received September 3, 2009 We demonstrate a method, employing prepatterns based on hydrogen silsesquioxane (HSQ) resist, for fabricating highly dense dot arrays of perpendicular block copolymer (BCP) cylinders with improved quality or increased density multiplication factors compared to conventional SiOx-based chemical prepatterns (dot arrays of exposed SiOx surfaces surrounded by polystyrene (PS) brush matrix). Due to the higher quality and simpler preparation procedures, HSQ-based chemical prepatterns were able to induce better BCP patterns, including nearly perfectly ordered and well registered arrays of perpendicular poly(styrene-b-dimethylsiloxane) (P(S-b-DMS)) cylinders. On prepatterns with identical chemical properties and patterning features, graphoepitaxy can induce higher density multiplication than chemical prepatterning.

1. Introduction The semiconductor industry aggressively pushes to resolve smaller critical dimensions; however, conventional “top-down” lithography is approaching technical and cost limits. By combining “bottom-up” self-assembly with “top-down” lithographic patterned templates, directed self-assembly of block copolymers (BCPs) provides a potential route to further shrink critical dimensions while maintaining the advantages of conventional lithography.1-3 Many methods, including chemical prepatterning4-6 and graphoepitaxy,7-9 have been proposed for directing BCP self-assembly in thin films. Graphoepitaxy is usually carried out in linear grooves or substrate steps. Chemical prepatterning requires lithographically patterned substrates with features nearly matching the domain structures of BCPs, and it is able to create high-quality BCP patterns. A recent innovation in the prepatterning method directs BCP self-assembly on sparse prepatterns (density multiplication).10-14 Compared to conventional electron-beam (e-beam) lithographical patterning, BCP lithography yields improved quality and can significantly reduce lithographic writing time through density multiplication.10,14 The conven*Corresponding author. E-mail: [email protected]. (1) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505–2521. (2) Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152–1204. (3) Segalman, R. A. Science 2008, 321, 919–920. (4) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602. (5) Ouk Kim, S.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411–414. (6) Stoykovich, M. P.; Muller, M.; Kim, S. O.; Solak, H. H.; Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Science 2005, 308, 1442–1446. (7) Segalman, R. A.; Yokoyama, H.; Kramer, E. J. Adv. Mater. 2001, 13, 1152– 1155. (8) Cheng, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Vancso, G. J. Appl. Phys. Lett. 2002, 81, 3657–3659. (9) Sundrani, D.; Darling, S. B.; Sibener, S. J. Langmuir 2004, 20, 5091–5099. (10) Ruiz, R.; Kang, H.; Detcheverry, F. A.; Dobisz, E.; Kercher, D. S.; Albrecht, T. R.; de Pablo, J. J.; Nealey, P. F. Science 2008, 321, 936–939. (11) Cheng, J. Y.; Rettner, C. T.; Sanders, D. P.; Kim, H.; Hinsberg, W. D. Adv. Mater. 2008, 20, 3155–3158. (12) Bita, I.; Yang, J. K. W.; Jung, Y. S.; Ross, C. A.; Thomas, E. L.; Berggren, K. K. Science 2008, 321, 939–943. (13) Tada, Y.; Akasaka, S.; Yoshida, H.; Hasegawa, H.; Dobisz, E.; Kercher, D.; Takenaka, M. Macromolecules 2008, 41, 9267–9276. (14) Yang, X.; Wan, L.; Xiao, S.; Xu, Y.; Weller, D. ACS Nano 2009, 3, 1844.

12408 DOI: 10.1021/la901648y

tional chemical prepatterning method developed by Nealey et al.15 transfers a positive-tone resist pattern to the underlying imaging layer and produces prepatterned dot arrays consisting of exposed SiOx surfaces surrounded by a polymer brush matrix. The SiOx-based chemical prepatterns have been employed to direct self-assembly of cylinder-forming poly(styrene-b-methyl methacrylate) (P(S-b-MMA))10,13,14 and sphere-forming poly(styrene-b-dimethylsiloxane) (P(S-b-DMS))16 with density multiplication. Recently, very thin negative-tone e-beam resist hydrogen silsesquioxane (HSQ) patterns on top of an imaging layer were also demonstrated as chemical prepatterns for directed self-assembly of lamella-forming P(S-b-MMA).11 Topographical prepatterns with the same patterning features as BCP microdomains can also serve as the sparse prepatterns for inducing BCP (sphere-forming P(S-b-DMS)) self-assembly,12 which differs from the typical graphoepitaxy approach. In all of these cases, the prepatterns were lithographically fabricated with the microdomains aligning to the BCP and led to long-range ordered and well registered BCP patterns. The directed BCP self-assembly behavior is highly dependent on the properties of the prepatterns. For instance, directing BCP self-assembly on prepatterns with the same length scale as the BCP can lead to degraded BCP pattern quality because of dimensional mismatch,4,17 an undesirable interfacial interaction between the prepattern and the BCP.15 While directing BCP self-assembly on sparse prepatterns, we hypothesize that the pattern quality and the density multiplication factor may also vary on different prepatterns. Therefore, it is worth investigating the effect of the prepatterns on BCP selfassembly. Our goal is to develop a method to achieve defect-free longrange ordered and well registered hexagonal BCP dot arrays, which may serve as resist patterns for fabricating nanoimprint lithography templates in patterned media applications.18,19 Both (15) Edwards, E. W.; Montague, M. F.; Solak, H. H.; Hawker, C. J.; Nealey, P. F. Adv. Mater. 2004, 16, 1315–1319. (16) Xiao, S.; Yang, X.; Park, S.; Weller, D.; Russell, T. P. Adv. Mater. 2009, 21, 2516. (17) Peters, R. D.; Yang, X. M.; Nealey, P. F. Macromolecules 2002, 35, 1822– 1834. (18) Ross, C. Annu. Rev. Mater. Res. 2001, 31, 203. (19) Terris, B. D.; Thomson, T. J. Phys. D: Appl. Phys. 2005, 38, R199.

Published on Web 10/02/2009

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cylinder-forming and sphere-forming BCPs are capable of thin film dot patterning.20 Usually, BCP films are used as etch masks for transferring patterns to underlying substrates.21 Patterned media application presents stringent requirements for BCP patterns because of the narrow size distribution and tight spacing tolerances needed in order to achieve magnetic recording.22 Therefore, the sphere-forming BCP films may become undesirable choices because of the low aspect ratio (height/width) of the spherical microdomains.23 The residue of the majority block beneath the spheres may further degrade the pattern quality and the placement accuracy during the following pattern transfer. However, cylinder-forming BCP films are more useful in patterned media applications due to the higher aspect ratio and nonresidue of the perpendicular cylinders, and have been used as an etch mask for pattern transfer.24-26 In this work, we investigated the directed self-assembly of cylinder-forming P(S-b-MMA) and P(S-b-DMS) on various hexagonally prepatterned substrates including SiOx-based chemical prepatterns and HSQ-based prepatterns with different heights (HSQ dots on PS brush grafted substrate).We compared the BCP pattern quality on SiOx-based and HSQ-based chemical prepatterns and examined the density multiplication behavior of P(S-b-MMA) on HSQ-based prepatterns with different heights.

2. Experimental Methods Materials. Polished 150 mm diameter SiÆ100æ wafers with a 300 nm thick thermal oxide layer were purchased from Silicon Quest International and used as substrates for the deposition of films. All block copolymers were purchased from Polymer Source Inc. and used as received, including two P(S-b-MMA)s with (1) number-average molar mass, Mn = 53.8 kg/mol (corresponding to a bulk equilibrium cylinder period, L0 ∼ 28.6 nm), polydispersity (PDI) = 1.09, styrene block volume fraction, f = 0.70 and (2) Mn = 39.0 kg/mol (L0 ∼ 24.0 nm), PDI = 1.07, f = 0.70; and one P(S-b-DMS) with Mn=17.5 kg/mol, (L0 ∼ 24.0 nm), PDI=1.07, f = 0.7. The monohydroxyl-terminated polystyrene, Mn=6.0 kg/ mol, was used as the chemical imaging layer for modifying the SiOx surface. The e-beam resist ZEP520 was purchased from Nippon Zeon. HSQ was from Dow Chemicals. All other chemicals were purchased from Sigma-Aldrich. Sample Preparation. The following is a description of the self-assembly process. (1) Preparation of oxidized silicon wafers with the polystyrene brush treatment: The SiOx substrates were precleaned using O2 plasma ashing at 100 C for 5 min, spincoated with monohydroxyl-terminated polystyrene (PS brush), and then annealed at 170 C for 6-8 h under vacuum. The wafers were then soaked in toluene for 15 min and rinsed with isopropyl alcohol (IPA) and blow dried with N2. The grafted PS brush thickness was ∼2-3 nm. (2) E-beam patterning: The exposure was performed using a Vistec VB6 HR direct write vector beam lithography system operating at 100 keV with a maximum write speed of 25.5 MHz operating at a beam current range of 0.31.0 nA. CAD data were transposed into Vistec format with CATS software from Transcription Enterprises. For ZEP520 resist, a ∼45 nm thick layer was applied and baked at 180 C for 5 min then (20) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525–557. (21) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401–1404. (22) Richter, H. J.; Dobin, A. Y.; Lynch, R. T.; Weller, D.; Brockie, R. M.; Heinonen, O.; Gao, K. Z.; Xue, J.; Veerdonk, R. J. M. v. d.; Asselin, P.; Erden, M. F. Appl. Phys. Lett. 2006, 88, 222512. (23) Segalman, R. A. Mater. Sci. Eng., R 2005, 48, 191–226. (24) Black, C. T.; Guarini, K. W.; Milkove, K. R.; Baker, S. M.; Russell, T. P.; Tuominen, M. T. Appl. Phys. Lett. 2001, 79, 409–411. (25) Xiao, S. G.; Yang, X. M.; Edwards, E. W.; La, Y. H.; Nealey, P. F. Nanotechnology 2005, 16, S324–S329. (26) Ramanathan, M.; Nettleton, E.; Darling, S. B. Thin Solid Films 2009, 517, 4474–4478.

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e-beam exposed to write designed patterns at the dose of 40-100 mC/cm2. The wafer was developed for 15 s using an IPA soak in an ultrasonic cleaner then rinsed with deionized (DI) water for 30 s. For HSQ resist, a layer with the desired thickness was applied and the wafer was baked at 90 C for 5 min followed by e-beam exposure at a dose range of 400-1000 mC/cm2. Then HSQ resist was developed in 10% potassium hydroxide for 15 s and rinsed with DI water. (3) Surface treatment and resist strip: (a) ZEP520 resist. The patterned surface was etched by O2/Ar (1:1 in volume) reactive ion etching (RIE) at 100 W for 3-5 s in order to remove the ZEP520 residue and also remove the PS brush layer in the patterned holes. A chemical contrast dot pattern (SiOx exposed dot areas surrounded by a PS brush matrix) was formed on the substrate, and after that the ZEP520 resist was stripped by soaking in EBR (2-(1-methoxy)propyl acetate (PGMEA) solution) at 60 C for 1 h, sonicated for 20 min, rinsed with EBR, and then blown dry with N2. (b) HSQ resist. This step is not necessary. (4) Preparation of the BCP film: Thin P(S-b-MMA) or P(S-b-DMS) films were spin-coated with thicknesses of ∼1.2-1.5L0 and annealed in a vacuum oven at a temperature range of 160-170 C for 12-18 h to reach their equilibrium state. For the P(S-b-MMA) film, the PMMA blocks were removed by flood exposing the wafer to deep ultraviolet (DUV) for 10 min and then soaking in acetic acid for 1 min. The P(S-b-DMS) film was first treated by high-angle CHF3/O2 (10:1 in volume) ion beam etching (IBE) for 15 s, and then the PS blocks were removed by O2/Ar (1:1 in volume) RIE for 25 s. Metrology. The resist and BCP film thickness was determined using a KLA-Tencor profilometer. Scanning electron microscopy (SEM) characterization was conducted using a NOVA SEM system (FEI). The ZEP resist patterns were over coated with a 1 nm thick tantalum layer, to dissipate charging, before SEM imaging. 2D Fourier transforms were obtained from SEM images using FTL-SE.

3. Results and Discussion The PS brush was grafted on SiOx substrates, and then either a ∼45 nm thick ZEP520 resist film or a HSQ film with a thickness of ∼5, ∼15, or ∼30 nm was spin-coated onto the substrates, e-beam exposed, and developed to form hexagonal patterns. The SiOxbased chemical prepatterns were formed by treating the ZEP520 resist patterns to a brief dose of O2 plasma and then stripping the resist.5 The ∼5 nm thick HSQ dots were directly used as chemical prepatterns after being developed, since HSQ resist cross-linked and formed a hydrophilic silica-like material after e-beam exposure.11 The HSQ patterns, with pillar height comparable to the further coated BCP film thickness, could be considered as topographical prepatterns.12 We then applied a spin-coated BCP film and annealed it in vacuum. Two cylinder-forming P(S-bMMA)s (L0 ∼ 28.6 nm, corresponding to an areal density of 910 gigadots per square inch, Gd/in2, and L0 ∼ 24.0 nm, corresponding to an areal density of 1.3 Td (teradots)/in2) and one cylinderforming P(S-b-DMS) (L0 ∼ 24.0 nm) were employed. The prepatterned dots (either the exposed SiOx surface or HSQ) preferentially interact with the PMMA or PDMS block, and the PS-grafted background area slightly preferentially to the PS block. The PMMA block in P(S-b-MMA) films and the PS block in P(S-b-DMS) films were then selectively removed while leaving the PS matrix with cylindrical pores and PDMS cylinder posts, respectively. Direct Self-Assembly of Cylinder-Forming P(S-b-MMA) on SiOx-Based and HSQ-Based Chemical Prepatterns: Prepatterning Effect on Pattern Quality. It has been demonstrated by other groups10,13 and our prior work14 that nearly perfectly ordered and well registered hexagonal dot arrays of cylinder-forming P(S-b-MMA) with pattern periodicity Lp10-1). A quality improvement of the P(S-b-MMA) (L0 ∼ 24.0 nm) patterns was observed when directed on the thin HSQ dot patterns with Ls = 24 nm (Figure 2g) or Ls = 48 nm (Figure 2h). The BCP pattern quality was improved in positioning accuracy and pore size uniformity, and had fewer defects (∼10-2). The procedures for coating and annealing BCP films were identical to those used with the two prepatterns. For instance, P(S-b-MMA) (L0 ∼ 28.6 nm) was spin-coated on both prepatterns from its 1.5% toluene solution at spin rate of 3500 rpm, resulting in ∼40 nm thick films. Then the BCP films were annealed in vacuum at 170 C for 18 h. On both prepatterns, the P(S-b-MMA) (L0 ∼ 24.0 nm) films were ∼30 nm in thickness and annealed in vacuum at 165 C for 18 h. Therefore, the BCP pattern quality only depends on the quality of the chemical prepatterns in terms of the patterning quality and chemical affinity to the BCP blocks, particularly referring to the interaction between the hydrophilic dots and PMMA or PDMS block. Upon e-beam exposure, HSQ resist cross-links and forms a silica-like material, which has a composition similar to that of a SiOx surface. On both chemical prepatterns, surface and interfacial forces between the substrate surface and the two blocks of the 12410 DOI: 10.1021/la901648y

Figure 2. Top-down SEM images of developed ZEP520 resist patterns with Ls = 24 nm (a) and Ls = 48 nm (b), and developed HSQ resist patterns with Ls = 24 nm (c) and Ls = 48 nm (d). (e-h) Top-down SEM images of P(S-b-MMA) (L0 ∼ 24.0 nm) films selfassembled on prepatterns defined by the patterns corresponding to the features on the left. The insets in (c, d) are the corresponding tilted SEM images showing ∼5 nm thickness of the HSQ dots. The insets in (e-h) are 2D Fourier transforms.

BCPs were precisely engineered to direct the assembly of nanodomains into thermodynamically stable long-range ordered and registered patterns.15 However, several differences can be addressed: (1) The HSQ dots are ∼5 nm in thickness, while the exposed SiOx dots are ∼2-3 nm below the PS brush layer. In both cases, entropy loss associated with extended chain configurations due to the concave or convex patterned dots is expected but the difference is difficult to quantify. (2) Due to the better e-beam resolution over the ZEP520 resist, HSQ usually forms higherquality patterns, especially with sub-25 nm pitches (shown in Figure 2a and c).27 The BCP pattern quality may be improved by using resists with higher resolution and/or better e-beam exposure system. It should be mentioned that our results are based on currently commercially available resists and our e-beam system. (3) Furthermore, the homogeneous O2 plasma treatment may also damage the uniformity of the prepatterned dots while transferring the ZEP520 resist pattern to the PS brush layer. (27) Yang, X.; Xiao, S.; Wu, W.; Xu, Y.; Mountfield, K.; Rottmayer, R.; Lee, K.; Kuo, D.; Weller, D. J. Vac. Sci. Technol., B 2007, 25, 2202–2209.

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Figure 3. Top-down SEM images of P(S-b-DMS) (L0 ∼ 24.0 nm) films self-assembled on SiOx-based chemical prepatterns with Ls = 24 nm (a), and on ∼5 nm thick HSQ dot patterns with Ls = 24 nm (b) and Ls = 48 nm (c). The insets show the 2D Fourier transforms. (d) Top-down SEM images of P(S-b-DMS) (L0 ∼ 24.0 nm) films self-assembled on a substrate partially prepatterned by ∼5 nm thick HSQ dots with Ls = 24 nm. The inset in (d) is a tilted SEM image of a P(S-b-DMS) film. (e) Schematic cross section diagram of a P(S-b-DMS) film on ∼5 nm thick HSQ dot patterns.

When preparing the sparse prepatterns, the e-beam exposure doses were chosen to be the correct doses for patterns with a pitch Ls ∼ L0. This created a prepatterned dot size commensurate to the BCP cylinders. Although the 48 nm pitch ZEP520 resist pattern has overall reasonably good quality, the uniformity of the individual dots is still affected by the lower resist resolution and the O2 plasma treatment. (4) Argon gas is usually mixed with oxygen gas for the O2 plasma treatment in order to get good directionality. The physical milling by argon can make the bare SiOx surface more hydrophobic. (5) The hydrophilicity of the bare SiOx surface also decreases because of possible ZEP520 resist residue contamination during the resist strip process. (6) In addition, the O2 plasma etch time is critical, since either underetching or overetching will not produce an optimum surface treatment to form good chemical prepatterns. Therefore, we observe BCP pattern quality improvement on HSQ-based chemical prepattern at high density, with a simplification of the overall process. However, HSQ resist requires a ∼10 times higher e-beam exposure dose than ZEP520, resulting in a correspondingly longer e-beam writing time. A high-resolution negative-tone resist with higher sensitivity than HSQ would be desirable in the prepatterning process. Direct Self-Assembly of Cylinder-Forming P(S-b-DMS) on SiOx-Based and HSQ-Based Chemical Prepatterns: Influence of Prepatterns and χ on Pattern Quality. Although the quality of the 24 nm pitch cylinder-forming P(S-b-MMA) pattern has been improved using HSQ-based chemical prepatterns, Langmuir 2009, 25(21), 12408–12413

the BCP patterns still cannot meet the device-level quality requirement because of their limitation of a low Flory-Huggins interaction parameter (χ) value of 0.06.28 P(S-b-DMS) with a higher χ value, 0.26,29 is thus introduced. P(S-b-DMS) has been demonstrated to form sphere arrays of PDMS block with a density as high as 4 Td/in2.16 Moreover, the PDMS block can serve as a robust etch mask because of the high Si content.30 Currently, most of the dot patterns using P(S-b-DMS) are based on sphere-forming ones.31 Compared to cylinder-forming BCP films, the sphereforming BCP films are less suitable for use as etch masks. The self-assembly of the ∼30 nm thick cylinder-forming P(S-b-DMS) (L0 ∼ 24 nm) film was directed on both types of chemical prepatterns with Ls = 24 nm (∼ L0) and Ls = 48 nm (∼ 2L0) (Figure 3). Hexagonal patterns of perpendicular PDMS cylinders can be obtained with SiOx-based chemical prepatterns (Ls = 24 nm) (Figure 3a). However, the pattern is poorly long-range ordered, exemplified by many small domains not registered to the prepattern (some of them are marked by white outlines). Connected cylinders are also observed. Nearly perfectly ordered and well registered P(S-b-DMS) (L0 ∼ 24.0 nm) patterns formed at a length scale of several micrometers on ∼5 nm thick HSQ dot (28) Russell, T. P.; Hjelm, R. P.; Seeger, P. A. Macromolecules 1990, 23, 890– 893. (29) Nose, T. Polymer 1995, 36, 2243–2248. (30) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551–575. (31) Ross, C. A.; Jung, Y. S.; Chuang, V. P.; Ilievski, F.; Yang, J. K. W.; Bita, I.; Thomas, E. L.; Smith, H. I.; Berggren, K. K.; Vancso, G. J.; Cheng, J. Y. J. Vac. Sci. Technol., B 2008, 26, 2489–2494.

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Figure 4. (a-c) Top-down SEM images of developed HSQ resist on PS-grafted SiOx substrates with Ls = 87 nm. The insets in (a-c) are the corresponding tilted SEM images showing that the thicknesses of the HSQ posts are (a) ∼5 nm, (b) ∼15 nm, and (c) ∼30 nm. (e-f) Top-down SEM images of P(S-b-MMA) (L0 ∼ 28.6 nm) self-assembled on the prepatterns defined by the corresponding HSQ patterns above. The insets in (d-f) show the 2D Fourier transforms.

patterns with Ls = 24 nm (Figure 3b). Figure 3c shows long-range ordered and registered P(S-b-DMS) (L0 ∼ 24 nm) cylinder arrays on the thin HSQ patterns with Ls = 48 nm, containing connected cylinders (indicated by arrows). Nearly perfectly ordered arrays of perpendicular PDMS cylinders were observed on the prepatterned area (Ls = 24 nm), while mostly parallel cylinders formed on the nonpatterned area (Figure 3d). The tilted SEM image (inset in Figure 3d) shows that the PDMS cylinders are approximately 25 nm in height (aspect ratio, height/width, ∼2), have no PS block residue underneath, and have relatively vertical sidewalls (marked by black dotted lines). The conical crowns of the posts may be caused by the O2 RIE process for removing the PS block. The microdomain morphology (cylinder or sphere) cannot be easily determined by SEM images. However, only cylinders can form a linear morphology (parallel cylinders) on a homogeneous nonpatterned surface and pillars (perpendicular cylinders) on a patterned surface, while spheres always form separated dots on patterned and nonpatterned surfaces.12,21 The surface tensions (γ) of PDMS, PS, and PMMA are 19.9, 40.7, and 41.1 mN/m, respectively.32 The perpendicular P(S-b-MMA) cylinders are identical through the film thickness because of the very small difference between surface tensions of PS and PMMA. PDMS preferentially segregates at the air/BCP interface due to its much smaller surface tension. Thus, usually parallel PDMS cylinders form on homogeneous substrates.33 Here, we have shown the formation of perpendicular PDMS cylinders on the prepatterned substrates. The suggested P(S-b-DMS) film structure is shown schematically in Figure 3e. The film was covered by a thin layer of PDMS film because of PDMS’s low γ. Therefore, the P(S-b-DMS) film is first treated by CHF3/O2 IBE to remove the PDMS surface layer before removing the PS block by O2 RIE. Direct Self-Assembly of Cylinder-Forming P(S-b-MMA) on HSQ-Based Prepatterns with Increasing Heights: Comparison between Chemical and Topographical Prepatterns Relative to the Density Multiplication Factor. While directing (32) Chan, C. M. Polymer Surface Modification and Characterzation, 1st ed.; Hanser Publishers: Munich, 1994. (33) Jung, Y. S.; Ross, C. A. Nano Lett. 2007, 7, 2046–2050.

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BCP self-assembly on sparse prepatterns, the lithographical writing time is approximately inversely proportional to the density multiplication factor. Previously, we tried to guide the self-assembly of P(S-b-MMA) (L0 ∼ 28.6 nm) on SiOx-based chemical prepatterns with Ls ∼ 3L0. Perpendicular cylindrical pores were observed in the P(S-b-MMA) (L0 ∼ 28.6 nm) film, but they lacked long-range ordering.14 Similar undesirable results were obtained using ∼5 nm thick HSQ dot patterns with Ls = 87 nm (∼ 3L0) (Figure 4d). Perpendicular cylindrical pores would only form hexagonal arrays locally within small domains, and thus, the arrays were neither long-range ordered nor registered to the prepatterns. The results indicate that both chemical prepatterns failed to guide the 9 times density multiplication. The domains in the BCP film, shown in Figure 4d, are separated by dotted lines. The pitches of the BCP patterns (Lp) are approximately consistent with L0 in all of these different domains, which indicates that these domains are not formed because of multiple lattice configurations. If the lattice configurations12,14 formed were Æ2 1æ, Æ2 2æ, or Æ3 1æ instead of Æ3 0æ, then the Lp values would be 32.8, 25.1, or 24.1 nm, respectively. The thickness of the HSQ dots was increased from ∼5 nm (Figure 4a) to ∼15 nm (Figure 4b) and finally to ∼30 nm (Figure 4c) simply by using thicker HSQ resist films. For all three cases, the correct e-beam exposure doses were predetermined for a pitch of 29 nm, although Ls = 87 nm, in order to get the dot sizes commensurate to the perpendicular PMMA cylinders. The experimental results show that, on the HSQ dot patterns with a thickness of ∼15 nm, P(S-b-MMA) (L0 ∼ 28.6 nm) locally self-assembled with well ordered and registered patterns designated as region I in Figure 4e with Lp ∼ 29.0 nm. The other regions designated as II in Figure 4e possess no long-range ordering. Nearly perfectly long-range ordered and registered P(S-b-MMA) (L0 ∼ 28.6 nm) patterning was obtained on the ∼30 nm thick HSQ dot patterns (Figure 4f). High magnification SEM images of P(S-b-MMA) (L0 ∼ 28.6 nm) films guided by the ∼30 nm thick HSQ dot patterns with Ls ∼ 3L0 and Ls ∼ 4L0 are shown in Figure 5. After the removal of the PMMA blocks, some prepatterned HSQ posts became visible inside the cylindrical pores. In Figure 5c, the HSQ posts (indicated by circles) are shown in every ninth cylindrical pore, which indicates Langmuir 2009, 25(21), 12408–12413

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material, template, and experimental conditions. Cylinder-forming BCP could form defects more easily than lamella-forming BCP, since the latter only forms arrays in one direction laterally while the former forms arrays in two directions. Sphere-forming BCP can perform pitch multiplication more easily on homogeneous polymer brush grafted substrates than cylinder-forming BCP, since only one block will attach to the substrate when the former self-assembles while the latter usually requires a neutral surface to form perpendicular cylinders.34 HSQ dot patterns with three different thicknesses were employed. The ∼5 nm thick prepattern works as a chemical prepattern, and the ∼30 nm thick one can be considered as a topographical prepattern since the thickness is comparable to the BCP film thickness (∼40 nm). The ∼15 nm thick HSQ dot pattern is considered as an intermediate prepattern. With identical chemical properties and patterning features, our results show that a topographical prepattern can induce higher density multiplication than a chemical prepattern. On topographical prepatterns, the positions of the BCP microdomains are physically fixed on every prepattern post, which decreases the possibility of defect formation. Every topographical post can also serve as a crystallization nucleus, thus fastening the BCP self-assembly process. Therefore, it is likely that topographical prepatterns can induce higher multiplication factors than chemical prepatterns for cylinder-forming BCP. Figure 5. Top-down SEM images of P(S-b-MMA) (L0 ∼ 28.6 nm) on top of the ∼30 nm thick HSQ dot patterns with Ls = 87 nm (a) and Ls = 114 nm (b). (c) and (d) are magnified images from (a) and (b), respectively. The circles indicate the BCP cylindrical pores with HSQ posts in them. (e) Schematic cross section diagram of a P(Sb-MMA) film on ∼30 nm thick HSQ dot patterns with Ls = 3L0.

a ternary dimensional frequency multiplication. The image also shows that only one lattice configuration, Æ3 0æ, with Lp ∼ 29.0 nm, is observed, while other lattice configurations were restricted by the narrow dimensional latitude, 27 nm < Ls < 31 nm.14 The ternary dimensional frequency multiplication was achieved on whole prepatterned areas, whereas frequency quadrupling, shown in Figure 5d, could only be obtained within a 1  1 μm2 area. The proposed cross-sectional diagram of a P(S-b-MMA) film on ∼30 nm thick HSQ dot patterns (Ls ∼ 3L0) is shown in Figure 5e. Compressed BCP chains are expected along the HSQ posts. Using very thin HSQ dots as a chemical prepattern on a neutral underlayer, Cheng et al. were able to demonstrate perfect spatial frequency quadrupling with lamella-forming P(S-b-MMA) (L0 ∼ 28.5 nm).11 The achievable pitch multiplication factor was more than 4 for the sphere-forming P(S-b-DMS) using PS brush or PDMS brush functionalized HSQ posts as topographical prepatterns,12 or using SiOx-based chemical prepatterns.16 The maximum defect-free pitch multiplication may depend on

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4. Conclusion In summary, we demonstrated quality improvement for P(S-b-MMA) (L0 ∼ 24.0 nm) and P(S-b-DMS) (L0 ∼ 24.0 nm) perpendicular cylinder patterns on HSQ-based chemical prepatterns in comparison to those on SiOx-based chemical prepatterns, while both types of chemical prepatterns lead to similarly good patterns using cylinder-forming P(S-b-MMA) (L0 ∼ 28.6 nm). For the first time, nearly perfectly ordered and well registered arrays of perpendicular P(S-b-DMS) (L0 ∼ 24.0 nm) cylinders were obtained, which are more suitable for highly dense dot patterning. While directing the self-assembly of P(S-b-MMA) (L0 ∼ 28.6 nm), our results also show that, with identical chemical properties and patterning features, topographical prepatterns can induce higher multiplication than chemical prepatterns. Our study indicates that the prepatterning process plays an important role in achieving high-quality dot patterning at higher densities and with greater density multiplication factors. Acknowledgment. L.W. would like to thank Seagate Technology for providing the internship opportunity. The authors would like to thank the Seagate Pittsburgh Research team especially Keith Mountfield for CAD work, Carl Seiler for experimental support, and Shuaigang Xiao and Yuan Xu for helpful discussions. (34) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458–1460.

DOI: 10.1021/la901648y

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