Can ionic liquid additives be used toextend the scope of poly(styrene)-block-poly(methyl methacrylate) fordirected self-assembly?

Thomas M. BennettKevin PeiHan-Hao ChengKristofer J. ThurechtKevin S. JackIdriss Blakey

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Can ionic liquid additives be used to extend the scope ofpoly(styrene)-block-poly(methyl methacrylate) fordirected self-assembly?

Thomas M. Bennett,a,b Kevin Pei,a Han-Hao Cheng,c Kristofer J. Thurecht,a,b,d Kevin S. Jack,e and Idriss Blakeya,b,*aUniversity of Queensland, Australian Institute for Bioengineering and Nanotechnology, Brisbane, Queensland 4072, AustraliabCentre for Advanced Imaging, Brisbane, Queensland 4072, AustraliacAustralian National Fabrication Facility, QLD Node, Brisbane, Queensland 4072, AustraliadARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Brisbane, Queensland 4072, AustraliaeCentre for Microscopy and Microanalysis, Brisbane, Queensland 4072, Australia

Abstract. Directed self-assembly (DSA) is a promising approach for extending conventional lithographictechniques by being able to print features with critical dimensions under 10 nm. The most widely studiedblock copolymer system is polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA). This system is wellunderstood in terms of its synthesis, properties, and performance in DSA. However, PS-b-PMMA also hasa number of limitations that impact on its performance and hence scope of application. The primary limitationis the low Flory-Huggins polymer-polymer interaction parameter (χ), which limits the size of features that can beprinted. Another issue with block copolymers in general is that specific molecular weights need to be synthesizedto achieve desired morphologies and feature sizes. Here we explore blending ionic liquid (IL) additives withPS-b-PMMA to increase the χ parameter. ILs have a number of useful properties that include negligible vaporpressure, tunable solvent strength, thermal stability, and chemical stability. The blends of PS-b-PMMA with an ILselective for the PMMA block allowed the resolution of the block copolymer to be improved. Depending onthe amount of additive, it is also possible to tune the domain size and the morphology of the systems.These findings may expand the scope of PS-b-PMMA for DSA. © 2014 Society of Photo-Optical Instrumentation Engineers(SPIE) [DOI: 10.1117/1.JMM.13.3.031304]

Keywords: ionic liquids; block copolymer; PS-b-PMMA; chi parameter; Flory-Huggins polymer-polymer interaction parameter.

Paper 14048SSP received Apr. 14, 2014; revised manuscript received Jun. 19, 2014; accepted for publication Jun. 23, 2014; pub-lished online Jul. 21, 2014.

1 IntroductionDirected self-assembly (DSA) is an approach that combinesphotolithography (top-down) and self-assembly of blockcopolymers (bottom-up) to achieve ordered features withsub-lithographic resolution.1–12 Block copolymers are madeup of least two chemically distinct polymer chains that arecovalently linked. When the respective blocks are incompat-ible, arrays of highly ordered nanostructures can form. Theequilibrium morphology depends on the relative volumefraction (f) of each block, the degree of polymerization (N)and degree of incompatibility, i.e., the Flory-Huggins poly-mer-polymer interaction parameter (χ).13 There are two mainapproaches for carrying out DSA, namely chemoepitaxy andgraphoepitaxy. Chemoepitaxy relies on chemically pattern-ing a substrate, where typically one of the components ofthe pattern preferentially interacts with one of the blocks andsubsequently drives the ordering during self-assembly.14,15

On the other hand, graphoepitaxy uses surface topography,e.g., a printed resist, to guide self-assembly and align thephase-separated domains.16–20

Polystyrene-block-poly(methyl methacrylate), or PS-b-PMMA, is currently the most widely studied system, whereDSA of PS lines with a pitch of ∼24 nm has been demon-strated.21 There is also a significant body of knowledgerelating to optimization of processing and integration with

manufacturing processes.22–25 Nonetheless, PS-b-PMMA hassome limitations in its scalability and compatibility for futurenodes of fabrication because (a) it has a small Flory-Hugginspolymer-polymer interaction parameter (χPS-PMMA ¼ 0.04 atroom temperature),26 limiting the ultimate resolution, and(b) annealing temperatures that are reported are significantlyhigher than the glass temperature (Tg) of typical positive tonephotoresists. As a result, next-generation materials thatcan achieve better resolutions are being actively studied.Recently, we have reported our investigations of a high-χBCP, polystyrene-b-poly(DL-lactide) (PS-b-PLA), whichovercomes some of these issues. Zalusky et al. have shownthat χPS-PLA was equal to 0.217 at room temperature,27 ena-bling much smaller domain sizes. Our initial efforts involvedidentifying the interfacial interactions of PS-b-PLA withsubstrates that were modified with a series of crosslinkedmattes of statistical copolymers of PS and PMMA.28

Lamellae oriented perpendicular to the substrate with domainsizes as small as 8 nm could be achieved. Graphoepitaxy-based DSA was also carried out with this system, whereno resist freezing step was required when using a polyhy-droxystyrene-based photoresist template. This was becausethe spin-coating solvent for the BCP was a nonsolvent forthe photoresist and the annealing temperature required toinduce order was below the Tg of the photoresist.29 Whilethis system was promising, to achieve feature sizes smaller

*Address all correspondence to: Idriss Blakey, E-mail: i.blakey@uq.edu.au 0091-3286/2014/$25.00 © 2014 SPIE

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than 8 nm would require the development of systems witheven higher χ parameters. While there are a number of can-didates that have been reported,30–33 the disadvantage of suchsystems is that the development cycle can be prohibitivelylong. For example, neutral surfaces may need to be identifiedand annealing conditions developed.

Another option for increasing χ utilizes solvent vaporannealing, where solvent vapor is used to swell one orboth blocks in a BCP and under the correct conditionscan tune domain sizes and transition a system from one mor-phology to another.34–38 These morphological changes canthen be kinetically trapped by flash removal of the solventfrom the system. However, a disadvantage of this approachis that this additional step requires additional infrastructure inthe fab (semiconductor fabrication facility) and strict controlover the experimental parameters to achieve reproducibleresults.

The aim of this paper is to demonstrate that simple addi-tion of additives to PS-b-PMMA [Fig. 1(a)] can be used totune structural parameters such as morphology and pitch.Ionic liquids (ILs) were the initial class of additives chosen.ILs are low-melting-point salts which generally consist of alarge organic cation with low symmetry that is paired with aninorganic or organic polyatomic anion that is usually weaklybasic and possesses a diffuse, or protected charge.39 ILs aredefined as salts with a melting point below 100°C, but themajority of ILs are liquid at or below room temperature.40

The completely ionic composition of ILs endows themwith a range of properties, which makes them attractive can-didates for both the replacement of many traditional organicsolvents and as useful materials in a variety of applications.These properties include nonflammibility, high chemical andthermal stability, high conducitvity, neglegible vapor pres-sure, and relatively low toxicity compared to many othercommon organic solvents.41,42 In terms of use as an additivefor BCP formulations, the negligible vapor pressure, tunablesolvent strength, chemical stability, and thermal stabilitymake ILs an attractive option.

Many of the initial reports of BCP-IL mixtures focusedon relatively dilute solutions of BCPs in ILs, where themorphology of micellar structures was studied.43–45 Morerecently, there have been reports on more concentrated sys-tems where it has been demonstrated that blending ILs withBCPs such as PS-b-poly(2-vinyl pyridine),46–48 PS-b-poly(ethylene oxide)49 [PEO], and PEO-b-poly(propyleneoxide)-b-PEO50 can lead to changes in morphology andd-spacing of the phase-separated domains. In this study, weblend 1-ethyl-3-methylimidazolium [emim] bis(trifluorome-thansulfonyl)amide [NTf2] [Fig. 1(b)] with PS-b-PMMA

and investigate the following questions: (i) Can the effectiveχPS-PMMA be increased? (ii) Can the long period/pitch betuned? and (iii) Can the morphology of a system beswitched? These questions are represented schematicallyin Fig. 2, which shows how addition of ILs might help totraverse the BCP phase diagram.

2 Experimental Methods

2.1 Polymer Materials and Characterization

PS-b-PMMA block copolymers were purchased fromPolymer Source Inc. and used without further purification.The dispersity of each block copolymer was assessed byGPC and found to be ≤1.18. The PS-b-PMMA block copol-ymers are designated PS-b-PMMA (X-Y), where X and Ydenote the number averaged molecular weights (Mn) inDa of the PS and PMMA blocks, respectively. The totalMn, dispersity (Đ), volume fraction of polystyrene (fPS),and degree of polymerization (N) are given in Table 1.

Fig. 1 (a) Structure of PS-b-PMMA. (b) Structure of the cation (top) and anion (bottom) of the ionicliquid (IL).

Fig. 2 A schematic of a representative phase diagram for a diblockcopolymer such as PS-b-PMMA,13 showing how the morphology ofthe block copolymer varies with volume fraction of PS (f ) and thedegree of polymerization (N). The block arrows show the anticipatedtrajectories of the system when ILs that are selective for the PMMAblock are introduced, where i) is transitioning from disordered toa lamellar morphology, ii) is tuning the pitch of lamellae, and iii) istransitioning from a cylindrical to lamellar morphology.

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2.2 IL Synthesis and Purification

1-Ethyl-3-methylimidalzolium bis(trifluoromethanelsu-fonyl)amide, emim NTf2, was synthesized via a metathesisreaction between equimolar amounts of emim Cl and LiNTf2 as described previously.51,52 Prior to use, the emimNTf2 was further purified by stirring under high vacuumfor several hours at 60°C and subsequently stored undervacuum in a desiccator to avoid water absorption.

2.3 Silicon Wafer Surface Cleaning

The silicon wafers used in this study were initially cleanedvia sonication in methanol for 3 min, acetone for 3 min, andfinally in deionized water for 3 min, followed by drying in anoven for 10 min at 100°C. The surface of each silicon waferwas then treated with O2 plasma in a reactive ion etcher witha gas flow rate of 10 sccm, a chamber pressure of 50 mTorr,a power of 150 W, and an exposure time of 60 s. Followingthe cleaning processes, all wafers were stored in a classP10000 clean room prior to use.

2.4 Preparation of PS-r -PMMA Underlayers onSilicon Wafers

Statistical copolymers of polystyrene, poly(methyl methac-rylate), and poly(glycidyl methacrylate) (PS-stat-PMMA-stat-PGMA) were prepared, as reported by Keen et al.28

The statistical copolymers (0.5 wt/v%) were dissolved inpropylene glycol monomethyl ether acetate (PGMEA)along with triphenylsulfonium triflate (5 wt% with respect tothe polymer). The solutions were spin coated at 4000 rpmonto silicon wafers. The coated substrates were annealed ona hot plate at 120°C for 60 s and then flood exposed with UVradiation (wavelength¼320–390 nm, dose¼180mJ∕cm2),followed by further annealing on a hot plate at 150°C for300 s. All films had thicknesses between 10 and 20 nmas determined by ellipsometry. The characteristics of thefive statistical copolymers used to produce the underlayersare detailed in Table 2. All copolymers contained 5 mol% PGMA.

2.5 Preparation of PS-b-PMMA/emimNT f 2 Bulk and Thin Films

Thin films of PS-b-PMMA/emim NTf2 were spin coatedfrom PGMEA solutions onto a series of wafers coated withdifferent underlayers. The desired thickness of the filmsstudied was either 0.5 or 1 long periods, L0, of the blockcopolymer being used, which was achieved by modifyingthe concentration of the block copolymer solutions and byadjusting the spin speed of the spin coater. Bulk films wereprepared by solution casting onto Kapton films. Samples

were annealed at 220°C on a hotplate for 300 s and specifiedfor a further 16 h at 200°C under a nitrogen atmosphere.

2.6 Morphology Characterization of Bulk Films viaSmall-Angle X-ray Scattering

The bulk morphology of the block copolymer/IL blends weredetermined using small-angle x-ray scattering (SAXS) onthe Australian Synchrotron SAXS/WAXS beamline. Thebeamline was configured with an x-ray wavelength ofλ ¼ 1.512 Å (8.2 keV) and focused to a 235 μmhorizontal ×140 μmvertical spot. Two dimensional (2-D) scattering pat-terns were recorded on a Dectris-Pilatus 1M detector withan active area of 169 × 179 mm and a sample to detector dis-tance of 3.412 m. The scattering patterns were then radiallyintegrated to give one-dimensional (1-D) scattering profile asintensity (I) versus wave vector [q ¼ 4π sinðθ∕2Þ∕λ], whereθ is the scattering angle and λ is the x-ray wavelength. Eachscattering profile was also corrected for the empty cell scat-tering (Kapton film) using scatter Brain Analysis version1.73 before morphological assignment.

2.7 Differential Scanning Calorimetry

A 1 STARe DSC system (Mettler Toledo, Greifensee,Switzerland) was used to determine the glass transitiontemperatures of each block copolymer/IL mixture.Approximately 10 mg of each mixture was dissolved intoDCM at a concentration of 10 wt. % and solution castinto aluminum DSC crucibles.

The samples were heated at 50°C for 18 h overnight in avacuum oven to remove remaining solvent and then annealedat 200°C for 24 h under a nitrogen atmosphere. Upon coolingto room temperature, each crucible was sealed by crimpingan aluminum lid on top. Indium and zinc were used as cal-ibration standards for the DSC. Samples underwent two heat-ing and cooling cycles from 30°C to 200°C, and the glasstransitions were measured from the second heating cycle.The DSC employed a heating rate of 10°C∕min and a N2

flow rate of 15 ml∕min. All results were processed and ana-lyzed using DSC 1 STARe software version 9.10.

2.8 Morphology Characterization of Thin Films byGrazing-Incidence SAXS

Grazing-incidence small-angle x-ray scattering (GISAXS)measurements of selected thin films of the BCP/IL blendswere acquired on the Australian Synchrotron SAXS/WAXSbeamline. The beamline was configured as described in thesection on SAXS above with the exception that the x-ray

Table 1 Block copolymer characteristics.

Sample Total Mn (kDa) Đ f PS N

PS-b-PMMA (10k-b-10k) 20 1.18 0.53 196

PS-b-PMMA (38k-37k) 74.8 1.08 0.54 732

PS-b-PMMA (96k-36k) 132 1.11 0.75 1281

Table 2 Underlayer characteristics.

Underlayer Total Mn (kDa) Đ Mole (%) PS

U1 23 1.26 75

U2 50 1.41 69

U3 30 1.29 65

U4 46 1.41 62

U5 62 1.58 52

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beam was focused to a 235 μmhorizontal × 140 μmverticalspot and was impinged on the surface of the film at an inci-dence angle of 0.05 deg. The 2-D scattering patterns werethen integrated in horizontal sectors along the qxy plane togive 1-D scattering profiles.

2.9 Scanning Electron Microscopy

The morphologies of selected films were analyzed usinga JEOL JSM-7800F scanning electron microscope (SEM).Films were mounted onto appropriate stubs using carbontape and stored in a vacuum oven at 50°C for 16 h priorto analysis. In a typical setup, SEM micrographs werecollected using an in-lens secondary electron detector witha working distance of 2.7 mm while operating a 1-kV gentlebeam with a stage bias of 2 kV.

2.10 Atomic Force Microscopy

A MFP-3D (Asylum Research) atomic force microscopewas used for all the measurements. The cantilevers usedwere HA_NC (Etalon) from NT-MDT, Russia, having anominal resonant frequency of 140 kHz. All the imageswere obtained by employing tapping mode of the atomicforce microscopy (AFM) in air. The AFM is mounted onan antivibrational table (Herzan) and operated within anacoustic isolation enclosure (TMC, USA). The scan speed

employed for each film was 1 Hz, and the data presentedhere are phase images of the films.

3 Results and Discussion

3.1 Investigation of the Effect of Blended emimNT f 2 on the Bulk Morphology of PS-b-PMMA

The bulk phase morphology of blends of PS-b-PMMAwithvarying amounts of the emim NTf2 was studied using trans-mission SAXS. The SAXS profiles provide information onthe morphology present by interpretation of the relative spac-ings of the diffraction peaks and the interdomain distancesfrom their absolute positions in q space.

3.1.1 Inducing order in a system that is normallydisordered in the bulk phase

Figure 3(a) shows the SAXS profiles (intensity versus q) ofPS-b-PMMA (10k-b-10k) blended with 0% and 5% of emimNTf2. The sample with 0% emim NTf2 exhibited a singlebroad peak, which was indicative of a disordered morphol-ogy. This result was predicted for this system, because χNwas ∼8 at room temperature, which is below the minimumvalue of 10.5 that is predicted by mean field theory to achievea lamellar phase.13

Fig. 3 (a) SAXS profiles of PS-b-PMMA (10k-b-10k) with 0% emim NTf2 (bottom) and 5% emim NTf2(top). (b) SAXS profiles focusing on the q� peak for a series of PS-b-PMMA (38k-b-37k)/emim NTf2blends for volume fractions of IL ranging 0–12%, (c) Plot showing relationship between the long periodof PS-b-PMMA (38k-b-37k) and the emim NTf2 content. (d) SAXS profiles of PS-b-PMMA (38k-b-37k) for0% and 17% emim NTf2.

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On the other hand, the sample with 5% emim NTf2had a SAXS profile that was characteristic of a lamellarmorphology, having Bragg peaks at 2 and 3 times q�.This indicated that for this particular formulation, the ILincreased the effective χPS-PMMA parameter by at least31%, i.e., such that the effective χN was greater than10.5. This was attributed to emim NTf2 being a good solventfor PMMA chains and a poor solvent for the PS.53 Hence,upon forming a block copolymer/IL blend, the emimNTf2 preferentially interacts with the PMMA chains andscreens the PMMA/PS interactions. This has the effect ofdecreasing the compatibility of the PMMA and PS chains,which increases the effective χPS-PMMA parameter.

An advantage of blends of block copolymers with ILs isthat, because of their negligible vapor pressures, the ILs willnot evaporate under typical processing conditions used inDSA, so they do not require specialized vapor annealingchambers, and precise amounts of the IL can be added duringpreparation of the BCP solutions. For example, the blends ofPS-b-PMMA and emim NTf2 could be annealed to 220°C,and the SAXS profile measured at that temperature remainedunchanged. This indicated that the morphology remainedunchanged over temperatures spanning from 23°C to 220°C. Perhaps the most distinct advantage of this approach isthat it provides access to size scales below the 24-nmpitch resolution limit of neat PS-b-PMMA. For example,the long period (pitch) of the PS-b-PMMA (10k-b-10k)with 5% emim NTf2 was 20 nm. Further studies on systemswith smaller block lengths are currently ongoing in our labsto assess the potential of this approach to improve resolutionbeyond 20 nm.

3.1.2 Tuning the domain spacing of lamellae byincreasing the IL content

Figure 3(b) shows a series of SAXS profiles, focusing on q�,for PS-b-PMMA (38k-b-37k) blended with emim NTf2 atconcentrations varying from 0% to 12%. It can be seenthat q� shifts to smaller values with increasing emim NTf2content. This indicates that the lamellar spacing increasedas a function of emim NTf2 content. This is quantified inFig. 3(c), which shows that the long period of the lamellarmorphology was ∼39 nm for the neat BCP and this couldbe tuned up to 50.6 nm, an increase of almost 30%, byblending up to 12% emim NTf2. In this case, the long periodcould be incremented by about 0.9 nm per percent of emimNTf2 in the system. This change in spacing could beattributed to preferential swelling of the PMMA domainwith increasing amounts of emim NTf2. Importantly forDSA applications, this demonstrates that the pitch of a givenBCP can be tuned to a desired size by simply incorporatinga specific volume fraction of emim NTf2 into the formu-lation. For example, a specific advantage may be that aBCP of a given molecular weight could be tuned such thatit can be used for a wider range of pitches, perhaps reducingthe need for extensive libraries of BCPs to cater to differentpitch requirements.

3.1.3 Tuning the morphology of block copolymers byblending emim NTf 2

Figure 3(d) shows the SAXS profiles of PS-b-PMMA(38k-b-37k) with 0% and 17% emim NTf2. The sample

with no emim NTf2 exhibits the characteristic profile fora lamellar morphology, whereas the sample with 17%emim NTf2 exhibits a hexagonal (cylindrical) morphology.This demonstrates that the material could transition betweenmorphologies when a sufficient volume fraction of emimNTf2 was added. Again this concept may also be usefulfor DSA in that it broadens the scope of application of a par-ticular block copolymer sample.

3.1.4 Other factors that may influenceapplicability to DSA

The effect of the additives on the etch selectivity of theblocks is an important factor to consider for the applicabilityusing blends of ILs and BCPs for DSA. While etch resistancestudies are beyond the scope of this article, it might be antici-pated that the high fluorine content would impart the IL witha lower etch resistance than PMMA.54 Hence, incorporationof IL into the PMMA block may decrease the etch resistanceand consequently increase the etch selectivity between thePS and PMMA blocks.

The second factor that is important to consider is how theIL impacts on the glass transition temperature of the blockcopolymer. The DSC thermograms for blends of PS-b-PMMA (38k-b-37k) with emim NTf2 at polymer fractionsof 1.00, 0.90, 0.75, and 0.60 are shown in Fig. 4. Theneat PS-b-PMMA (38k-b-37k) exhibits two glass transitiontemperatures. The first is at 107°C (assigned to PS) and thesecond at 132°C (assigned to PMMA). Upon incorporationof 10% emim NTf2, only a single Tg at 106°C could beobserved, and this decreased slightly to 104°C for IL concen-trations of 25% and 40%. Based on the solubility of PMMAin emim NTf2 and insolubility of PS in the IL,53 these resultswere attributed to the PMMA block being plasticized, andthe PS block remaining essentially unaffected. In supportof this, Susan et al.55 have shown that blends of PMMAhomopolymer and emim NTf2 have Tg values belowroom temperature for similar degrees of incorporation.The fact that the PS Tg remains virtually unchanged is a pos-itive for its application in DSA, because the PS domains arethe domains that will remain after reactive ion etching due totheir higher etch resistance.

Fig. 4 DSC thermograms for blends of PS-b-PMMA (38k-b-37k) withemim NTf2 for polymer fractions of 1.00, 0.90, 0.75, and 0.60.

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3.2 Investigation of the Effect of Blended emimNT f 2 on the Thin Film Morphology andInterfacial Properties of PS-b-PMMA

3.2.1 Identifying a neutral surface

It is well known that that the balance of interactions betweeneach block and the substrate of air interface can have a sig-nificant effect on the orientation of domains in block copoly-mer thin films. For example, if the substrate preferentiallyinteracts with one of the blocks, then for a BCP that hasa lamellar morphology, the domains can orient parallel tothe substrate.56 On the other hand, if the substrate has non-preferential interactions with both blocks, then the domainsorient perpendicular to the substrate.57–63 A concern whenblending an additive, such as an IL, into the BCP is that thesurface energetics may change drastically, necessitating theidentification of new compositions for “neutral” underlayers.

To determine whether blending of emim NTf2 had aneffect on the interaction with the underlayer, Solutions ofPS-b-PMMA (38k-b-37k) blended with 20% IL in PGMEAwere spin coated onto underlayers composed of statisticalcopolymers of PS and PMMA that had PS compositionsranging from 52% to 75%. From our previous studies forthis particular type of underlayer, this range encompassedthe neutral window for PS-b-PMMA.12 Figure 5 showslow-resolution (20 × 20 μm field-of-view) AFM images forthree underlayers, (a) 69% PS, (b) 65% PS, and (c) 62% PS.When the underlayer with 69% PS was used [Fig. 5(a)],dewetting of the film was observed, which was attributedto the surface having poor interactions with both blocks.For the underlayer with 65% PS, there are height variationsof ∼4 nm across the field of view. Finally, for the 62% under-layer, a number of raised features were observed witha height of half of a long period of the polymer, which werecharacteristics of one block preferentially interacting withthe substrate in a film that had a thickness that was incom-mensurate with the long period of the block copolymer.64

Qualitative analysis of these images suggested that the 65%PS may be a neutral surface for the PS-b-PMMA (38k-b-37k) with 20% emim NTf2.

To investigate the morphology of the thin film further, thePS-b-PMMA (38k-b-37k) with 20% emim NTf2 on the 65%PS underlayer was analyzed using GISAXS. In a similarfashion to SAXS, the GISAXS profiles provide informationon the morphology present by interpretation of the relative

spacings of the diffraction peaks. The primary experimentaldifferences are that the samples are thin films and the beam isdirected onto the sample at a grazing angle of incidence. Thissample geometry means that characteristic scattering profilesfor lamellae are observed only when they are orientedperpendicular to the substrate.28 Figure 6(a) shows a 2-DGISAXS scattering profile, where second- and third-orderBragg peaks are evident at integer multiples of q�. Thesepeaks are more evident in the sector average shown inFig. 6(b). This result confirms that the lamellae are orientedperpendicular to the substrate and that the 65% PS under-layer can act as a neutral layer for blends of 20 volume% of emim NTf2 with PS-b-PMMA.

3.2.2 Tuning lamellae spacing in a thin film

Having identified a neutral surface for one of the blends ofemim NTf2 with PS-b-PMMA, further experiments wereconducted to demonstrate the tunability of the pitch and mor-phology in the thin film. Figure 7(a) shows high-resolutionSEM images of PS-b-PMMA (38k-b-37k) with 0% (left) and25% (right) emim NTf2. In both cases, a fingerprint patterncould be observed that was characteristic of lamellae ori-ented perpendicular to the substrate. The darker contrastcorresponds to the PMMA domains and the lighter contrastcorresponds to PS domains. It could also be seen that thesamples with 0% emim NTf2 had a number of cylinder-like defects, but these are not evident in the sample formu-lated with 25% emim NTf2. This indicates that defectivitymay be lower for IL-containing systems, which may bedue to the higher mobility of PMMA chains. However,more systematic analysis is required to support this hypoth-esis. The insets of the SEM images show fast Fourier trans-forms (FFTs) that were calculated from the SEM images,which allowed the long period to be estimated. The samplewith 0% emim NTf2 had a peak corresponding to a longperiod of 39 nm, while sample formulated with 25% ILhad a peak corresponding to 45 nm, confirming that thepitch of the lamellae could be tuned by introducing emimNTf2 into the PS-b-PMMA formulation. However, theincrease observed was significantly less than for the bulkfilms (Fig. 2), i.e., a 6-nm change in the long period corre-sponds to a bulk sample with approximately 5.5% emimNTf2. In addition, it would be expected that for a blendwith 25% emim NTf2, the morphology should transition

Fig. 5 AFM height images (20 × 20 μm field of view) of PS-b-PMMA (38k-b-37k) doped with 20% emimNTf2 on three different underlayers, (a) 69% PS, (b) 65% PS, and (c) 62% PS. Film thickness was ∼1.5times the long period, i.e., ∼60 nm.

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to a cylindrical/hexagonal morphology. One possible explan-ation for this difference is that the IL is partitioning into theunderlayer, which is a statistical copolymer of styrene andmethyl methacrylate. Partitioning of IL into the underlayermay also explain why the same underlayer acts as a neutrallayer for formulations with 0% and 25% emim NTf2. This is

because emim NTf2 that partitions into the underlayerwould be expected to influence the interfacial interactionsthat occur with the blend of block copolymer and IL.Nonetheless, we have demonstrated that it is possible totune lamellar domain size in thin films by blending emimNTf2 with PS-b-PMMA.

Fig. 7 High-resolution SEM of (a) thin films of neat PS-b-PMMA (38k-b-37k) [left] and of PS-b-PMMA(38k-b-37k) doped with 25% emim NTf2 [right], (b) thin films of neat PS-b-PMMA (96k-b-36k) [left] and ofPS-b-PMMA (96k-b-36k) doped with 25% emim NTf2. For all SEM images, the darker contrast corre-sponds to the PMMA domains and the light contrast to PS domains. All samples used an underlayerthat had a 65% PS content. The thickness of the BCP films were 60� 4 nm. The scale bars in theSEM images are 100 nm. The insets to the SEMs is the FFT calculated from the SEM images.

Fig. 6 (a) 2-D GISAXS scattering profile for a thin film of PS-b-PMMA (38k-b-37k) doped with 20% emimNTf2 on an underlayer containing 65% PS. (b) Sector averaged 1-D GISAXS profile. Both exhibitthe characteristic Bragg peaks for lamellae oriented perpendicular to the substrate.

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3.2.3 Tuning morphology in a thin film

Finally, we sought to determine if the thin film morphologyof PS-b-PMMA could be transformed by addition of emimNTf2. Figure 7(b) shows the high-resolution SEMs of PS-b-PMMA (96k-b-36k) thin films with 0% and 25%. The darkercontrast corresponds to the PMMA domains and the lightercontrast corresponds to PS domains. In the absence of emimNTf2, a cylindrical/hexagonal morphology was observed,while the formulation with 25% emim NTf2 had transitionedto a lamellar morphology. The PS-b-PMMA (96k-b-36k)also exhibits some variation in contrast to the PMMAdomains. This can be attributed to the formation of a wettinglayer of PS at the air interface for some of the PMMA lamel-lae. The morphologies are not identical to those observed inthe bulk for the same concentrations of emim NTf2, whichmay again be due to some partitioning of the emim NTf2 intothe underlayer, or could also be a result of a perturbation ofthe phase diagram due to the increasing significance of thepolymer–air and polymer–substrate interfaces in the thinfilms compared to the bulk. Nonetheless, we have demon-strated that it is possible to transition the thin film morphol-ogy of a block copolymer from hexagonal to lamellar byblending with emim NTf2.

The initial work on thin films has focused on PS-b-PMMA with domain sizes as small as 39 nm. To be usefulfor future DSA applications this will also need to be dem-onstrated for BCPs with smaller domain sizes. The antici-pated challenges in achieving this are likely to lie inminimizing the partitioning of the IL into the underlayer,because use of BCPs with smaller domain sizes will alsonecessitate thinner film thicknesses. One way to achievethis may be to use polymer brushes to generate the neutralunderlayer.

4 ConclusionsThis study has investigated the changes that occur to the bulkand thin film morphology of PS-b-PMMAwhen it is blendedwith varying amounts of an IL, 1-ethyl-3-methylimidalzo-lium bis(trifluoromethanelsufonyl)amide (emim NTf2). Itwas found that small amounts of emim NTf2 could increasethe effective χ parameter and could induce order for low-molecular-weight systems, which when neat have χN values<10.5 (i.e., are disordered). In addition, it was demonstratedthat in the bulk it was possible to tune the long period (pitch)of lamellae, or transitions from one morphology to another,simply by adding different amounts of emim NTf2. In termsof thin films formulated with 0% to 25% emim NTf2, it wasdemonstrated that lamella oriented perpendicular to thesubstrate could be obtained using the same underlayer, i.e.,a neutral underlayer optimized for PS-b-PMMA with noadded emim NTf2. Finally, it was demonstrated that tuningof the long period and transitioning of morphologies couldalso be achieved in thin films. The favorable physical proper-ties of ILs, which include negligible vapor pressure andexcellent thermal stability, mean that they can be formulatedwith BCPs in solution, spin coated, and undergo high-tem-perature annealing steps. This can all be achieved withoutconducting additional processing steps and does not requireadditional infrastructure in the fab, which makes the appoachoutlined in this study amenable to integration into currentpreproduction processing steps that have been developedby industry.

AcknowledgmentsExperiments were carried out in part at the Centre forMicroscopy and Microanalysis (CMM), University ofQueensland (UQ); Queensland node of the AustralianMicroscopy and Microanalysis Research Facility, UQ; theAustralian National Fabrication Facility (ANFF) (Qld node)and on the SAXS/WAXS beamline at the AustralianSynchrotron (AS). This research was supported in partunder the Australian Research Councils Discovery ProjectScheme (DP140103118) and Future Fellowship Scheme(FT100100721, for IB) as well as UQs PostgraduateResearch Award (for TB).

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Thomas Bennett completed a BSc(Hons) in chemistry at theUniversity of Queensland, Australia, in 2011. He is currently undertak-ing a PhD on block copolymer/ionic liquid composites for advancedmaterial applications, with Associate Professor Idriss Blakey atthe Australian Institute for Bioengineering and Nanotechnology. Hisresearch interests include polymer and ionic liquid synthesis,block copolymer thin films, and the characterization of these materi-als via small-angle x-ray scattering, atomic force microscopy, andscanning electron microscopy.

Kevin Pei completed the first year of a bachelor of chemical engineer-ing degree at the University of Queensland in 2013. He carried out avacation project at the Australian Institute for Bioengineering andNanotechnology with Prof. Blakey. He is currently continuing hisundergraduate studies.

Han-Hao Cheng received his PhD degree from the Department ofElectrical and Electronic Engineering, University of Canterbury,New Zealand, in 2009. He was a postdoctoral fellow at AustralianInstitute for Bioengineering and Nanotechnology (AIBN), theUniversity of Queensland, St. Lucia, Australia, for developingdirected-self-assembly-based lithography applications for extremeultraviolet lithography in collaboration with Intel Corporation between2009 to 2012. He is now a senior professional officer at the AustralianNational Fabrication Facility, Queensland node.

Kristofer Thurecht was awarded his PhD in polymer chemistry fromthe University of Queensland (2005). He undertook postdoctoraltraining at Nottingham University (UK) and was subsequentlyawarded an 1851 Research Fellowship (2007). He currently workswithin the Centre for Advanced Imaging (CAI) and the AustralianInstitute for Bioengineering and Nanotechnology (AIBN) at theUniversity of Queensland as an ARC Future Fellow and is CI inthe ARC Centre of Excellence in Convergent Bio-Nano Science andTechnology.

Kevin Jack received his PhD from the University of Queensland,Australia in 1996. He undertook postdoctoral positions at Queen’sUniversity, Canada and Bristol University, UK, and was a researchscientist at the Bristol Colloid Centre until 2004. He is currentlya senior research fellow in the Centre for Microscopy andMicroanalysis, the University of Queensland. His research interestsare in structure-property relations in materials and their advancedcharacterization by x-ray, neutron, and NMR methods.

Idriss Blakey is an associate professor and an Australian ResearchCouncil Future Fellow at the University of Queensland, Australia.There he leads research programs that develop functional polymersand nanomaterials for applications that include lithography, biosen-sors and biomedical imaging agents. Prior to this, he was a researchscientist at Polymerat, an Australian biotechnology start-up company(2000 to 2003). In 2001, he was awarded a PhD in Polymer Sciencefrom the Queensland University of Technology.

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