DOI: 10.1002/adma.200602708

Photopatternable Imaging Layers for Controlling BlockCopolymer Microdomain Orientation**

By Eungnak Han, Insik In, Sang-Min Park, Young-Hye La, Yao Wang, Paul F. Nealey, andPadma Gopalan*

Block copolymer (BCP) lithography has emerged as apromising strategy to create highly regular and dense dot orline arrays at the sub-50 nm length scale.[1–6] Block copolymerlithography refers to the use of self-assembled domain struc-tures, typically spheres, cylinders, and lamellas in thin-filmform as a template for the addition and subtraction nanofabri-cation processes.[5] Mainly two pattern geometries have beenstudied extensively: dense arrays of dots and dense arrays oflines and spaces. The former can be generated from sphere-forming BCP or from cylinder-forming BCP with domains ori-ented perpendicular to the substrate, and the latter from cylin-der-forming BCP with domains oriented parallel to the sub-strate or lamella forming BCP with domains orientedvertically to the substrate. These periodic arrays based onself-assembly in BCP thin films have been examined for fabri-cation of magnetic storage media, quantum dot arrays, pho-tonic crystals, and nanowire transistors.[4,7–10]

Vertically oriented domain structures have distinct advan-tages in terms of pattern transfer to the underlying substrateand fabrication of high-aspect ratio features.[11] In the case ofperpendicular cylinder or perpendicular lamellae, the orienta-tion of the BCP microdomains can be controlled by solventevaporation, electric fields, directional crystallization,[12–15]

physical constraints (topography), or chemical patterns.[16–19]

Perhaps the most common means to induce perpendicular do-main orientation, however, is to chemically modify and con-trol the interaction between the BCP and the substrate.[20,21]

For example, the substrate can be chemically modified byusing self-assembled monolayers (SAMs) or random copoly-mer brush. SAMs of alkylthiols (on gold) or alkylsiloxanes

(on Si/SiO2) with polar or non-polar terminal groups havebeen utilized to modify the substrate.[22–24] Exposure to differ-ent doses of X-ray in the presence of oxygen can further alterthe polarity and hence the wetting behavior of the SAM re-sulting in a symmetric, neutral or asymmetric BCP morphol-ogy.[25] Mansky and coworkers demonstrated that the interfa-cial energies between the PS and PMMA blocks of P(S-b-MMA) and the substrate can be carefully balanced by graftingan end-hydroxy functionalized P(S-r-MMA) polymer on theSiOx/Si substrate.[20,26,27] Covalent grafting of P(S-r-MMA)having fSt (where fst is the styrene fraction) of 0.58 resulted inthe neutral wetting behavior and the vertical orientation of la-mellar or cylindrical domains. In this original report covalentgrafting is usually achieved through the dehydration reactionbetween the end-hydroxy functionalized P(S-r-MMA) and thenative oxide layer of silicon substrate. Ryu et. al recently re-ported an elegant chemistry to create substrate independentneutral surface through the self thermal crosslinking reactionof benzocyclobutene (BCB)-containing random copolymersat temperatures above 200 °C.[28] The high temperature re-quired for efficient cross-linking, however may limit the utilityof BCB containing random copolymers with respect to ther-mally sensitive monomers. We recently demonstrated the fea-sibility of using a third comonomer (C) in the random copoly-mer brush P(A-r-B-r-C) to allow multipoint attachment to thesubstrate. The composition of the P(A-r-B-r-C) copolymercould be tuned to precisely adjust the interfacial and surfaceenergies of a diblock copolymer P(A-b-B) thin film over thebrush layer.[29] A very thin side-chain grafted random copoly-mer brush layer of P(S-r-MMA-r-HEMA) (PH2 in Fig. 1a)having 0.02 fraction of hydroxyethyl methacrylate (HEMA)(fHEMA) and 0.58 fraction of styrene (fSt) showed ‘neutral sur-face’ characteristics along with fast binding kinetics to thesubstrate.

Here, we present an efficient room temperature surfacemodification chemistry for controlling the orientation of theBCP microdomains on any substrate. Instead of thermalcrosslinking reaction, negative-tone photoresist chemistry wasexploited to generate ultra-thin neutral surfaces and to pat-tern alternate regions with neutral and preferential wettingcharacteristics. The composition of the copolymer was finetuned to tailor the wetting behavior to the top block copoly-mer film and to optimize the photo-crosslinking process.

The versatility of the side-chain grafted random copolymerbrush layer of P(S-r-MMA-r-HEMA) (PH2 in Fig. 1a) systemwe reported earlier,[29] lies in the ability to functionalize the

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–[*] Prof. P. Gopalan, E. Han, Dr. I. In,[+] Dr. Y. Wang

Department of Materials Science and Engineering, University ofWisconsinMadison, WI 53706 (USA)E-mail: pgopalan@wisc.eduS.-M. Park, Y.-H. La, Prof. P. F. NealeyDepartment of Chemical and Biological Engineering, University ofWisconsinMadison, WI 53706 (USA)

[+] Current address: Department of Polymer Science and Engineering,Chungju National University Chungju, Chungbuk, 380-702, Korea.

[**] This work was supported by the Semiconductor Research Corpora-tion, the UW-NSF Nanoscale Science and Engineering Center(DMR-0425880), and the center on Functional Engineered Nano Ar-chitectonics (FENA).

hydroxyl group of HEMA with photo-crosslinkable functionalgroups. In this work photo-crosslinkable acryloyl group wasincorporated by esterification of HEMA moieties in PH2 andglycidyl group was incorporated by copolymerization withGMA instead of HEMA resulting in PA2 and PG2 respec-tively (Fig. 1b and c) as the first examples of photo-pattern-able imaging layers. Both PA2 and PG2 were designed tohave 2 mole % of photo-crosslinkable monomers and fSt of0.58 so as to preserve the neutrality of the brush toward asymmetric P(S-b-MMA) block copolymer.[29]

Efficiency of the photo-crosslinking process for PA2 andPG2 was evaluated in the presence of a photoinitiator and aphotoacid generator respectively. There are a number ofphotoinitiators such as vinyl acrylates or vinyl ethers that canbe used to effectively UV crosslink vinyl groups. In this studywe used vinylacrylate as a photoinitiator as it is structurallysimilar to the reactive groups in PA2.[30] We examined thephoto-crosslinking of vinyl groups in PA2 with and withoutphotoinitiator by measuring the remaining thickness of cross-linked films following UVexposure (254 nm) at room tempera-ture followed by washing with cyclopentanone and tolueneunder sonication to remove any uncrosslinked chains. Spin-casting of 0.3 wt % toluene solution of PA2 produced 13 nm-thick films over piranha cleaned silicon substrate. The remain-ing thickness of the crosslinked PA2 film was approximately2.5 nm (19 % of the original thickness) with UV exposure of800 mJ cm–2. Large increase of UV irradiation energy is not de-sirable as doses > 1000 mJ cm–2 can degrade the PMMA seg-ments in the polymer. Hence, to increase the effectiveness ofphoto-crosslinking process 2 wt % of vinyl acrylate was in-cluded in the PA2 solution as a photoinitiator before exposure.The remaining thickness increased to 3.6 nm (28 % of the orig-inal thickness) after UV exposure for the same duration on in-clusion of photoinitiator. Thickness of the crosslinked films canbe easily controlled by changing the initial concentration ofPA2 solution. For example, 1.0 wt % of PA2 in toluene re-sulted in a 15 nm (30 % of the original thickness) crosslinkedfilm following exposure to 600 mJ cm–2 of UV light. As the re-quired exposure dose for crosslinking PA2 is higher than desir-able for general lithography, an alternate photo-crosslinkable

random copolymer system PG2 having crosslinkable epoxygroups was examined (Fig. 1c). The epoxy groups in the ran-dom copolymer are easily crosslinked with the help of photo-generated acids during UV exposure. Photo-crosslinked PG2films were fabricated by depositing a PG2 solution having2 wt % of BDSDS/PTPDS as a photo-acid generator (PAG)onto the clean silicon substrate.[31] Spin-coating of 0.3 wt % so-lution in cyclopentanone at 4000 rpm gave ∼ 16 nm thick films.Following UV exposure, post-exposure bake (PEB) was doneat 130 °C for 30 min. Any uncrosslinked random copolymerwas removed by repeated washing in cyclopentanone and tolu-ene. Typically a 50 mJ cm–2 exposure resulted in a 6.5 nm(40 % of original thickness) thick photo-crosslinked film. Thecalculated sensitivity of PG2 system was 50 mJ cm–2, which issignificantly higher than the PA2 system.

Photo-crosslinking of spin-coated PA2 or PG2 thin films re-sulted in a modified substrate which showed a constant con-tact angle between 76–78° on all the substrates tested such asglass, indium tin oxide (ITO)-coated glass, silicon, and goldcoated silicon (Fig. 2b). The ultra-thin (2–6 nm) crosslinkedfilms showed superior stability as they could not be removedby heating and sonication in organic solvents for several days.The ultra-thin imaging layer is also a desirable feature for ef-fective pattern transfer.

A 40 nm-thick symmetric P(S-b-MMA) copolymer film[104 kg mol–1; Lo (bulk), ∼ 48 nm; fSt, ∼ 0.5] was spin-coatedover photo-crosslinked PA2 (3.6 nm) or PG2 layer (6.5 nm)(Fig. 2a). Following thermal annealing in vacuum at 190 °Cfor 72 h, the resulting orientation of the domains was exam-ined by scanning electron microscopy (SEM). Vertically ori-ented lamellas were observed by SEM on PA2 modified sili-con and Au substrates (Fig. 2c and d). As expected from ourprevious results[29] the inclusion of small amount of a thirdphotosensitive monomer in the random copolymer did notsignificantly alter the wetting properties of the random co-polymer layers. Chemically acryloxyethyl methacrylate is sim-ilar to methyl methacrylate and the combined fraction of bothmonomers adds up to 0.42. PG2 was equally effective in in-ducing vertical orientation of lamella on both silicon (Fig. 2e)and gold (Fig. 2f) substrate.

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Figure 1. Schematic representation of the a) covalent binding of side-chain hydroxyl containing random copolymer PH2 via dehydration reaction oversilicon substrate, b) self-photo-crosslinking of acryloyl containing side-chain random copolymer PA2, and c) self-photo-crosslinking of glycidyl groupcontaining side-chain random copolymer PG2.

As both PG2 and PA2 are photo-crosslinkable, it is possibleto photo-pattern alternate regions of preferential and neutrallwetting. PA2 film was patterned by exposure to UV irradia-tion (254 nm) through a TEM grid as a photo-mask. Succes-sive washing with cyclopentanone and toluene resulted inpreferential and neutral wetting regions over unexposed andexposed regions, respectively on silicon substrate. The unex-posed PA2 film showed a residual thickness of 0.4 nm andwater contact angle of 32° following the washing step. Fig-ure 3a shows the SEM image of a thin film of symmetricP(S-b-MMA) deposited and annealed on the patternedPA2 film. The exposed regions I (the rectangles in SEM im-age) and the unexposed regions II (the Cu grid region in theimages) of patterned PA2 film resulted in vertical and parallelorientation respectively of the BCP lamellar domains. The

characteristic hole/island morphology for parallel lamella ori-entation is apparent in the SEM micrograph of the unexposedregion (Fig. 3c). Vertical lamella morphology in the exposedregions is clearly observed in the high resolution SEM images(Fig. 3b). Examination of BCP morphology in various regionsof the patterned substrate, i.e., the unexposed and unexposedregion by SEM shows clear transition from vertical to parallellamellas. The width of the transition region itself varied fromsample to sample as it depends on a number of factors such asthe distance between the TEM grid and the substrate, and theinteraction of the grid with the UV light itself.

PG2 cross-links through the acid catalyzed ring openingmechanism which requires a PAG. PG2 film containing 2.5 %PAG was patterned by exposure to 100 mJ cm–2 of UVthrough a mask, followed by a PEB step of 130 °C for 30 min-utes and washing. Depositing a BCP film on the above pat-terned substrate resulted in vertical lamellae instead of holeand island even in the unexposed region indicating the pres-ence of a cross-linked PG2 film even after washing. This ismost likely due to the thermal crosslinking of epoxy groups tothe substrate during the PEB process. Therefore, for effectivepatterning of the PG2 system the PEB step was optimizedalong with the amount of PAG. In order to minimize the ther-mal crosslinking of PG2 to the substrate we increased theamount of PAG in the PG2 solution to 40 wt % and reducedthe PEB to 120 °C for 30 minutes. A thin film of symmetricP(S-b-MMA) was deposited and annealed on the patternedPG2 film. The exposed regions I (the rectangles in SEM im-age) and the unexposed regions II (the Cu grid region in theimages) of patterned PG2 film resulted in vertical (Fig. 3e)and parallel orientation of the BCP lamellar domains respec-tively (Fig. 3f). PG2 system has higher sensitivity and cancrosslink by both photochemical and a thermal process.Therefore, effective patterning of PG2 involves a careful opti-mization of processing conditions so that thermal crosslinkingto the substrate is minimized during the PEB process.

In summary, we have demonstrated two random copolymerbrushes which combine negative tone photoresist chemistrywith the chemistry of neutral surfaces to create patternedBCP morphologies with controlled microdomain orientation.The advantages of our approach are the accessibility to a) sub-strate-independent neutral surfaces for BCPs containing tem-perature sensitive monomers, and b) photo-patterned sub-strates with regions of different interfacial energies resultingin patterned BCP morphologies. The photo-patternable imag-ing layer can be easily combined with topographically definedsubstrates to achieve perfect alignment and registry of lamellaforming block copolymers with an underlying substrate overarbitrary length scales.[32–34]

Experimental

P(S-r-MMA-r-HEMA) (PH2) was synthesized by the previously re-ported method [28]. PA2 was synthesized by the esterification of PH2in anhydrous THF with excess acryloyl chloride using triethyl amineas a base. PG2 was synthesized by a similar procedure as PH2 by

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Figure 2. Shows a) a schematic representation of the procedure used tocreate the chemically modified substrate and investigate the self-assem-bly of block copolymer domains: i) PG2 or PA2 was spin coated on goldor silicon substrate, cross-linked by exposure to UV light and the un-bound polymer was removed by washing with solvent; and ii) symmetricblock copolymer was spin coated on the modified substrate and an-nealed; b) is a bar chart presenting the measured water contact anglesbefore and after modification of Si/SiOx, glass, ITO and gold substrateswith photo-crosslinked PA2 or PG2 thin films. The measured contact an-gle on all the modified substrates is approximately constant. The plan-view SEM images of symmetric PS-b-PMMA diblock copolymer on:c) PA2 modified silicon substrate; d) PA2 modified Au coated siliconsubstrate; e) PG2 modified silicon substrate; and f) PG2 modified goldcoated silicon substrate showing vertically orientated lamellae. The scalebar corresponds to 200 nm.

using glycidyl methacrylate (GMA) instead of HEMA as a comono-mer. The resulting highly viscous mixture was diluted with THF andprecipitated into methanol. The precipitate was filtered and dried un-der reduced pressure to obtain PA2 (Mn 27 000, PDI 1.18) orPG2 (Mn 31 000, PDI 1.24 ) copolymer as a white solid. In general Pstands for polymer, H for HEMA, A for acryloyl, G for glycidyl and2 represents the mole % of H, A or G in the copolymer.

A symmetric poly(styrene)-block-poly(methyl methacrylate)P(S-b-MMA) diblock copolymer (MBw: 104 kg mol–1, fBSt: ∼ 0.5,PDI: 1.05) was purchased from Polymer Source Incorporation andused without further purification.

Prior to film casting, silicon (100) substrates were cut into 1.5 cm2

pieces (or used without cutting), and cleaned in a toluene bath undersonication for 3 min. The substrates were then washed with toluene,acetone, and ethanol and dried in a stream of nitrogen. Following theprecleaning steps, the substrates were piranha cleaned with a mixtureof H2O2 (30 %)/ H2SO4 (70 %) (v/v) at 80 °C for 30 min (CAUTION!May cause explosion in contact with organic material!) and rinsedwith distilled water and dried in a stream of nitrogen. The thickness ofthe native oxide layer was 1.6 nm.

Photo-crosslinked PA2 film was generated by spin-coating PA2(0.3 wt %) solution in cyclopentanone containing 2 wt % of vinyl ac-rylate at 4000 rpm onto the clean silicon substrate followed by UV ex-posure (600 mJ cm–2). Photo-crosslinked PG2 films were fabricatedby spin-coating of 0.3 wt % PG2 solution in cyclopentanone contain-ing 2.5 wt % of BDSDS/PTPDS onto the clean silicon substrate, fol-lowed by UV exposure (100 mJ cm–2), and post exposure bake at130 °C for 30 min. For both PA2 and PG2 any uncrosslinked randomcopolymer was removed by repeated washing in cyclopentanone andtoluene resulting in crosslinked PG2 film. The photo-crosslinking ofPA2 or PG2 films were carried out in a Spectrolinker XL-1500 UVcrosslinker (254 nm) from Spectronics Corporation. On the PA2 orPG2 modified substrate, P(S-b-MMA) was spin-cast from 1.5 wt %toluene solution at 4000 rpm resulting in a 40 nm thick BCP films.The samples were annealed in vacuum at 190 °C for 72 h and subse-quently cooled to room temperature.

PA2 film deposited on silicon substrate by spin-coating from a0.3 wt % solution of PA2 containing 2 wt % vinyl acrylate was photo-patterned by exposure to 600 mJ cm–2 UV through a 200 mesh TEMgrid (PELCO®, Pitch 127 lm; Hole Width 90 lm; Bar Width 37 lm)as a photomask. PG2 film deposited on silicon substrate by spin-coat-ing from a 0.5 wt % solution in cyclopentanone containing 40 wt % ofBDSDS/PTPDS was photo-patterned by exposure to 100 mJ cm–2 ofUV through a 200 mesh TEM grid as a photomask. Post exposurebake was done at 120 °C for 30 min. Uncrosslinked random copoly-mer was removed by repeated washing in cyclopentanone and tolueneresulting in patterned PA2 or PG2 films on silicon substrate.

Thickness of brush layer and the block copolymer film was mea-sured with a Rudolph Research ellipsometer using a helium-neon la-ser (k = 633 nm). Scanning electron microscope (SEM) images ofblock copolymer thin films were obtained with a LEO 1550 VP field-emission SEM.

Received: November 27, 2006Revised: April 19, 2007

Published online: November 21, 2007

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