Orienting Silicon-Containing Block Copolymer Films with

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Orienting Silicon-Containing Block Copolymer Films with Perpendicular Cylinders via Entropy and Surface Plasma Treatment Kai-Yuan Lu,† Ting-Ya Lo,† Prokopios Georgopanos,‡,⊥ Apostolos Avgeropoulos,‡ An-Chang Shi,§ and Rong-Ming Ho*,† †

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, R.O.C Department of Materials Science Engineering, University of Ioannina, University Campus, Ioannina 45110, Greece § Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4L8, Canada ‡

S Supporting Information *

ABSTRACT: Controlling the orientation of nanostructured block copolymer (BCP) thin films is essential for next-generation lithography. However, obtaining BCP with perpendicular orientation remains a challenge because of the surface selectivity to the different blocks. This challenge is especially severe for silicon-containing BCPs which is notorious for its high surface energy difference between constituted blocks. Here, we demonstrate a new approach to achieve perpendicular orientation with high aspect ratio using a combination of architecture effect (entropy effect) and surface air plasma treatment (enthalpy effect). Specifically, perpendicular cylinders of star-block copolymers composed of polystyrene and poly(dimethylsiloxane) blocks can be formed from the bottom substrate to the top surface of the thin film.

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induced neutral layer on the surface of the BCP thin film. By combining the architecture effect (entropy effect) on BCP selfassembly and the air plasma treatment (enthalpy effect) for neutralization of the air/polymer interface affinity, perpendicular cylinder arrays from non-neutral substrate to the oxidativeinduced neutral layer can be achieved as illustrated in Figure 1. Specifically, a cylinder-forming star-block PS−PDMS film is fabricated by spin coating on Si wafer with an intrinsic SiO2 layer (Figure 1A). Note that the PDMS has a high affinity to the air surface. For the interfacial energy on the SiO2 substrate, the interfacial tension of PDMS is higher than that of PS; thus, the PS has a high preference for the substrate.11,20,21 As reported previously by our laboratory, the effect of star-shaped architecture on BCP self-assembly at high temperature is able to overcome the interfacial energy difference on the substrate by lowering the entropy penalty of looping.11 However, a thinlayer of PDMS (in red) (Figure 1B) would be formed at the air surface, resulting in a transition zone between the parallel cylinders from the top and the perpendicular cylinders from the bottom. To reduce the large interfacial energy differences between PS and PDMS at the air surface, a simple air plasma treatment is carried out to create a cross-linked star block of the (PS−PDMS)3 type (from here on it would be referred as starblock PS−PDMS) layer (in pink) (denoted as oxidativeinduced neutral layer). This oxidative-induced neutral layer

INTRODUCTION Alternative high-resolution, high-throughput, and lower cost patterning technologies must be developed to satisfy the demands of “smaller, faster, and cheaper” semiconductor devices.1,2 In the past decades, it has been shown that block copolymers (BCPs) offer an attractive patterning technology because BCP could self-assemble into various morphologies with length scales ranging from a few tens to hundreds of nanometers.3−5 Among all the BCP applications, in particular for silicon-containing BCPs with large χ parameter and high etch contrast,6−8 the fabrication of well-defined nanostructured thin films is appealing because of its potential application in nanolithography.9,10 Recently, we demonstrated an entropydriven approach for the fabrication of BCP thin films on SiO2 substrate with perpendicular cylinders by using star-shaped polystyrene-b-poly(dimethylsiloxane) (star-block PS− PDMS).11 However, owing to the low surface tension of PDMS as compared to that of PS,12,13 PDMS tends to form a wetting layer on the top surface of the thin films.14 Recently, many efforts have focused on the neutralization of the air/ polymer surface of thin films to acquire span-through perpendicular microdomains of BCPs. Examples include the top-coated methods,15−17 the use of an embedded neutral layer,18 and molecular design.19 Here, we propose an alternative route to regulate the surface energy difference between PS and PDMS by using air plasma treatment instead of top-coat approaches. The basic idea is that through plasma-induced oxidative cross-linking reaction air plasma treatment could lead to the formation of an oxidative© XXXX American Chemical Society

Received: October 17, 2017 Revised: November 13, 2017

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DOI: 10.1021/acs.macromol.7b02218 Macromolecules XXXX, XXX, XXX−XXX

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wt % of diblock PS−PDMS, 5 wt % of star-block PS−PDMS) on the Si wafer with 1 μm SiO2 intrinsic layer at room temperature. After drying under vacuum, the thin film was treated with the ion coater (IB3, Eiko Inc.). The volume of the vacuum chamber was about 13 cm in diameter × 10 cm in height. The electrodes were in the form of circular stainless steel plates each of diameter 5 cm, and the electrodes clearance is 3 cm. The sample was placed on the electrode plate and treated with air plasma using lab air containing 20% oxygen and 80% nitrogen by volume for 1 s with 560 V/1−2 mA at 13.3 Pa. After the air plasma treatment, the thin film was then cooled by air for 5 min before thermal annealing. After the formation of oxidative-induced neutral layer, the treated thin film was thermally annealed at 280 °C for 30 min and then quenched using liquid nitrogen. For bottom morphology observation, the thin-film samples were stripped from the SiO2 substrate using 1% HF solution for 30 s and then floated onto the surface of water, finally collected from the top surface of the thin film using silicon wafer. Morphological Observation. Bright-field TEM images were obtained from the thin-film samples without staining due to the significant mass−thickness contrast from the strong scattering of the silicon-containing microdomains. Transmission electron microscopy (TEM) experiments were performed on a JEOL-2100 TEM instrument operating at an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FE-SEM) observations were performed on a JEOL JSM-7401F using accelerating voltages of 5 keV. The samples were mounted to brass shims using carbon adhesive and then sputter-coated with 2−3 nm of platinum. Tapping-mode SPM images of thin-film samples were also obtained. A Seiko SPA-400 AFM with a Seiko SPI-3800N probe station was utilized at room temperature. A silicon tip was used in dynamic force mode experiments with a spring force of 5 N/m and a scan rate of 1 Hz. Reactive Ion Etching Treatment. The oxygen reactive ion etching (RIE) treatment for oxidation was carried out by a RF power of 60 W at the pressure of 60 mTorr for 60 s. The CF4/O2 RIE for etching PDMS was generated by RF power of 50 W for 10 s, at which the involving gas of CF4 and O2 was in a ratio of 2:1 and the pressure was 150 mTorr. Contact Angle Measurement. The First Ten Ångstroms 1000B contact angle analyzer was used to measure the surface contact angle differences between the thickness of 200 nm polymer surface with and without air plasma treatment. Water and ethylene glycol (EG) were used as the solvents, and each solvent droplet was about 80 μL. Ten data points were collected after the contact angle remained unchanged for 30 s to make sure the droplet was at equilibrium state. Focused Ion Beam Experiment. The cross-section thin-film sample was sliced by a FEI Helios Nanolab 400 dual beam focused ion beam (FIB). Before FIB cutting, a Pt layer (∼10 nm) and a C10H8 protection layer (∼200 nm) were deposited on the thin film surface and low accelerating voltage (5 keV) was used to reduce milling damage. The sectioning temperature was controlled below 100 °C to avoid the deformation of the nanostructure (Tg,PS ∼ 100 °C). A crosssection (20 μm wide by 5 μm tall by ∼50 nm thick) sample was cut and attached to an Omniprobe copper liftout grid for TEM imaging. Grazing Incidence Small-Angle X-ray Scattering (GISAXS). GISAXS was conducted at beamline BL23A1 in the National Synchrotron Radiation Research Center (NSRRC), Taiwan. A monochromatic beam energy was 10 kV, λ = 1.55 Å was used, and the incident angle was 0.2°. Scattering intensity profiles were of the scattering intensity (I) versus the scattering vector (q), where q = (4π/ λ) sin(θ/2) and θ is the scattering angle. Surface Analyses. XPS measurements were made using a Thermo VG-Scientific Sigma Probe spectrometer that was equipped with a hemispherical electron analyzer. The operating conditions for XPS were as follows: Mg Kα anode, 15 kV, 7.2 mA; incident angle, 45°; angle of collection, 45°; analysis diameter, 100 μm; sputter crater, 1 mm2; Ar+ ion beam energy, 1 keV; the sputtering rate was 10 nm/min. The data were acquired approximately every 2 nm. SIMS depth profile measurement were accomplished by using ION-TOF TOF-SIMS IV with time-of-flight analyzer. The sputter conditions were as follows: Cs+ ion source, 1 keV, 60 nA, 250 × 250 μm2. The analysis parameters

Figure 1. Schematic illustration of controlled orientation of cylinderforming star-block PS−PDMS. (A) Star-block PS−PDMS is spincoated on a SiO2 substrate and (B) then thermal annealed. (C) The thin-film surface of the spin-coated star-block film is treated by air plasma treatment before thermal annealing. (D) Span-through PDMS perpendicular cylinders can be formed by combining the architecture effect (entropy effect) to overcome the surface energy difference on SiO2 substrate and the air plasma treatment (enthalpy effect) for neutralization of air/polymer interface affinity.

provides a neutralized top surface for the self-assembly of the star-block PS−PDMS (Figure 1C). Consequently, by combining the architecture effect (entropy effect) on BCP selfassembly and the surface plasma treatment (enthalpy effect) for the neutralization of the air/polymer interface affinity, the formation of perpendicular cylinders could be obtained by simple thermal annealing (Figure 1D).

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EXPERIMENTAL SECTION

Materials Synthesis. The synthesis of the PS−PDMS samples was accomplished through sequential anionic polymerization of styrene (Acros Organics, 99%) and hexamethylcyclotrisiloxane (D3), employing high-vacuum techniques as described elsewhere.22−24 After the synthesis of the PS−PDMS(−)Li(+) living diblock copolymer, a removal of the necessary amount of the reactants took place and the remaining mixture was reacted with the necessary chlorosilane coupling agent, trichloromethylsilane (CH3SiCl3), in order to synthesize the star-block copolymer of the (PS−PDMS)3 type. Therefore, two samples were prepared: the diblock copolymer precursor and the corresponding 3arm star-block copolymer. In this study, the PS−PDMS BCPs molecular characteristics are summarized in Table 1. Sample Preparation. The diblock and star-block PS−PDMS films were prepared by spin coating from cyclohexane (C6H12) solution (3

Table 1. Molecular and Structural Characterization of the Diblock and Star-Block Copolymers Synthesizeda sample diblock PS− PDMS 3-arm star-block PS−PDMS 4-arm star-block PD−PDMS

M̅ nPS (kg/mol)

M̅ nPDMS (kg/mol)

ĐM

f PDMSv

bulk state morphology

13.7

7.1

1.03

0.36

cylinder

41.1

21.3

1.07

0.36

cylinder

37.0

40.4

1.09

0.54

lamellae

M̅ n is the number-average molecular weight of the block, Đ is the molecular weight distribution as detected from gel permeation chromatography in THF at 35 °C with PS standards, and f PDMSv is the volume fraction of PDMS calculated from the weight fraction which was obtained from the proton nuclear magnetic resonance experiments in chloroform. Further details on the characterization can be found in the literature from our previous work.25

a

B

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Macromolecules were as follows: Ga+ ion source, 25 keV, 1 pA, 110 × 110 μm2. The data were acquired approximately every 3 nm.

orienting perpendicular cylinders through the whole thin film can be successfully achieved after the air plasma treatment. To further examine the suggested orientation, focused ion beam (FIB) was used to vertically cut the thin-film samples for acquiring cross-section transmission electron microscopy (TEM) images. As shown in Figure 3A, perpendicular cylinders extend from the substrate for the thin-film samples without plasma treatment; however, owing to the thin PDMS wetting layer at the air/polymer interface, parallel cylinders formed at the top of the thin film are observed. Consequently, the mismatch of the differently oriented cylinders (parallel ones from the top and perpendicular ones from the bottom) gives rise to the formation of a transition zone as shown in Figure 3A at which ill-defined transitional textures from parallel to perpendicular ones (such as disclinations marked by arrows) are formed that causes the misalignment and even distortion of the perpendicular cylinders. By contrast, the thin-film sample with the air plasma treatment shows a well-defined perpendicular cylinders spanning through the entire thin film (Figure 3B). To evaluate the uniformity of the represented morphologies acquired from TEM observations, scattering experiments were carried out for the complementary evidence. As a result, the long-range orientation order of the nanostructured thin films were thus investigated using grazing incidence small-angle X-ray scattering (GISAXS). As shown in the Figure 3C, a weak peak can be observed near qypar = 0.252 nm−1 and also a sharp peak at qyper = 0.303 nm−1 in the intrinsic star-block PS−PDMS thin film after thermal annealing, demonstrating the formation of both PDMS perpendicular cylinders and several layers of parallel cylinders at the thin-film surface. The signals in the parallel and perpendicular directions show that the relative q value of qypar:qyper = 1:2/√3, which is consistent with previous studies.11,26 On the contrary, the plasma-treated sample shows only perpendicular cylinders diffraction pattern in both 2D GISAXS and 1D SAXS patterns (Figure 3D). These results indicate that the air plasma treatment indeed provides a significant improvement on the uniformity of orienting star-block PS−PDMS with high-aspectratio perpendicular cylinders. We speculate that the driven orientation is attributed to the formation of an oxidativeinduced neutral layer at the top surface of the film via the air plasma treatment. To examine the surface properties of the oxidative-induced neutral layer, contact angle measurements were conducted on four samples with different air plasma treaetment times. In contrast to the intrinsic thin films, the contact angle analysis shows a dramatic decrease from 102.5 ± 1.0° to 64.4 ± 3.3° for water (Figure 4A blue line) and 82.7 ± 1.0° to 44.4 ± 3.3° for ethylene glycol (EG) (Figure 4A red line) after the air plasma treatement for only 1 s. As the surface is continuously exposed to the air plasma, the contact angle slightly decreases linearly with time. Note that the water contact angle for intrinsic thin film is approximately 102.5 ± 1.0°, which is close to the water contact angle measured on the PDMS homopolymer thin-film surface (as shown in Table S1), suggesting the formation of a thin layer of hydrophobic PDMS wetting layer on the top surface of the thin film. After a shorttime air plasma treatment, the surface property of the samples obviously changes from hydrophobic to hydrophilic due to the reaction of BCP with the radicals generated by the treatment, resulting in the formation of hydrophilic functional groups due to oxidation. Consequently, with further air plasma treatment, the contact angle slightly decreases with time. To examine the stability of the oxidative-induced neutral layer, an aging

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RESULTS AND DISCUSSION Figure 2A shows the surface morphology (the top-view image) of a thermally annealed star-block PS−PDMS BCP thin film

Figure 2. Effect of air plasma treatment on self-assembled morphologies at the air surface. Tapping-mode SPM top-view phase images of star-block PS−PDMS thin film (A) without and (B) with air plasma treatment. The corresponding top-view FE-SEM images of the thin film (C) without and (D) with air plasma treatment after removal of the surface layer by RIE treatment.

obtained by scanning probe microscopy (SPM). No significant variation can be identified, suggesting the formation of a PDMS wetting layer as expected. The surface morphology of the air plasma-treated samples after thermal annealing exhibits no obvious discrepancy as well (Figure 2B) but with a smooth surface as evidenced by the tapping-mode SPM height images (Figure S1). To examine the self-assembled morphology at the interface of BCP and the SiO2 substrate, the thin-film samples were flipped over for SPM observations (see Experimental Section for detailed procedures). As shown in Figure S2A,B, both SPM phase images exhibit hexagonally packed spots due to the entropy effect of the star architecture as reported before.11 To further examine the discrepancies on the morphological textures from the top, a short-time reactive ion etching (RIE) using CF4/O2 mixed gas was carried out to remove the PDMS thin layer followed by O2 plasma to oxidize PDMS and to degrade the PS matrix at the same time. As shown in Figure 2C, randomly oriented stripes are observed from the top of the untreated thin film, suggesting the formation of parallel cylinders. In contrast to the top-view image, the bottom-view FE-SEM image (Figure S2C) shows an array of hexagonally packed dots, suggesting that the perpendicular cylinders formed at the substrate could not propagate from the bottom to the entire thin film. In contrast to Figure 2C, hexagonally packed texture can be observed in the top-view image of the air-plasma-treated samples (Figure 2D), indicating the formation of perpendicular cylinders on the top of the thin film. Combining the observations of the bottomview SEM images (Figure S2D), it could be concluded that C

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Figure 3. TEM cross-section images for star-block PS−PDMS thin film (A) without and (B) with air plasma treatment. 2D GISAXS pattern and corresponding 1D SAXS plot of the star-block PS−PDMS thin film (C) without and (D) with air plasma treatment.

Figure 4. (A) Water and ethylene glycol (EG) contact angle measurements for star-block PS−PDMS thin film before and after air plasma treatment with different processing time. Water contact angle analysis on oxidative-induced neutral layer was demonstrated in advance after the sample was preserved at the atmosphere for 2 weeks (green) and compared with the initial one (blue). (B) The interfacial energy difference between γPS−neutral and γPDMS−neutral versus air plasma treatment time.

experiment was carried out by storaging the treated sample at ambient conditions. As shown in Figure 4A (green line), no significant hydrophobic recovery can be found after 2 weeks, suggesting that the oxidative-induced neutral layer is stable at the atmosphere. Those results indicate that there is no lowmolecular-weight species formed after the air plasma treatment and diffused from the hydrophilic layer to the topmost surface

due to aging process. On the basis of the experimental results, we suggest that an oxidative-induced cross-linking reaction was occurred after plasma treatment (see below for detailed discussion).27−29 For the calculation of the interfacial energies between the oxidative-induced neutral layer and the constituted blocks of PS and PDMS, the Owens−Wendt−Rabel−Kaelble (OWRK) D

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Figure 5. XPS survey spectra of the top surface of star-block PS−PDMS thin film (A) without and (B) with air plasma treatment for 1 s (red line shows the atomic composition of the as-cast thin film, and the blue line indicates the atomic composition of the thin film after thermal annealed at 280 °C for 30 min). The depths profile for the as-cast star-block PS−PDMS topmost thin-film surface (C) without and (D) with air plasma treatment for 1 s.

method30−32 was used (i.e., see Table S2 for details). The interfacial energies of PS or PDMS for the oxidative-induced neutral layer were calculated by using the equation γ12 = γ1 + γ2 − W12 where γ1 is the surface energy of PS or PDMS, γ2 is the surface energy of the oxidative-induced neutral layer, and W12 is the works of the adhesion of PS or PDMS with the oxidativeinduced neutral layer which can be obtained by using the

composition significantly increases because of radical bombardment on the polymer surface during the air plasma treatment. Note that there is no nitrogen peak in the survey scan spectra, suggesting that no nitrogen radicals were involved during the air plasma treatment. As a result, only oxygen radicals need to be included in the reactions with the plasma-treated polymer surfaces. On the basis of these observations, we propose that the air plasma treatment leads to the oxidation of star-block PS−PDMS and the formation of the hydrophilic functional groups (as evidenced by the formation of C−O bonding; see Figure S3 and below for details), and the composition of Si is approximately the same as the untreated thin film before thermal annealing. To further examine the property of the oxidative-induced neutral layer, XPS depth profile analysis was used to acquire the atomic distribution along the depth by sequential argon ion sputtering for removal of the thin film layer by layer. As shown in Figures 5C and 5D, there is a significant increase of the O concentration at the top layer of the thin films after the air plasma treatment, further evidencing the occurrence of oxidation. The thickness of the oxidized layer was estimated to be 2 nm, which is an advantage for the following treatment to remove this oxidative-induced neutral layer. Moreover, secondary ion mass spectroscopy (SIMS) experiments were carried out to obtain the depth profiles from the surface with larger distance. As shown in Figure S4A, the periodic variations are consistent with the observed parallel orientation from the TEM cross-section image (Figure 3A) as illustrated in the inset of Figure S4A. By contrast, after the air plasma treatment, the variations of the Si and O compositions become insignificant (Figure S4B), reflecting that those atoms are distributed uniformly at depth below 2 nm. This observation could be attributed to the formation of perpendicular cylinders as illustrated in the inset of Figure S4B. On the basis of the above experimental observations, it

equation W12 = 2( r1dr2d + r1pr2p ). As shown in Figure 4B, the resultant interfacial energy difference between PS and C PDMS on the surface before air plasma treatment (γ25 PS ° − 25 C ° = 13.0 ± 0.3 mN/m) is much larger than that after the air γPDMS °C − γ25 °C = 1.3 ± 1.2 plasma-treated surface (γ25 PS−neutral PDMS−neutral nN/m), suggesting that the formation of the oxidative-induced neutral layer obviously changes the BCP surface affinity from a PDMS dominant interface to a neutral interface for both PS and PDMS. As the air plasma processing time increases, a slight increase of the interfacial energy difference between PS and PDMS can be observed, indicating that the surface affinity is slightly preferential to PDMS as the radicals continue to bombard and react with the polymer surface. As a result, a short-time air plasma treatment provides an efficient process for the fabrication of the oxidative-induced neutral layer. To examine the chemical variations after the air plasma treatment, X-ray photoelectron spectroscopy (XPS) experiments were carried out. Figures 5A and 5B show the survey scan spectra of the top surface of thin-film sample before and after the air plasma treatment. Note that the carbon 1s (284.8 eV) signal results from both PS and PDMS and the appearance of the oxygen 1s (533 eV) and silicon 2p (103.4 eV) peaks are attributed to the PDMS block. As shown in Figure 5A, there is a significant increase in the Si ratio after the thermal annealing, indicating the formation of a wetting PDMS layer. By contrast, after the air plasma treatment (Figure 5B), the oxygen atomic E

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Figure 6. Proposed mechanism for plasma-induced oxidative cross-linking reaction on the star-block PS−PDMS thin-film surface.

Figure 7. SEM top-view images of the (A, B) star-block PS−PDMS thin films with lamellar nanostructure (A) with and (B) without air plasma treatment. (C, D) Diblock PS−PDMS thin films with cylinder nanostructure (C) with and (D) without air plasma treatment. (E) TEM cross-section images for cylinder-forming PS−PDMS diblock copolymer thin film with air plasma treatment.

difficult to carry out reliable analysis with respect to the chemical composition variations after the air plasma treatment. Note that the possible reactions of PS and PDMS homopolymers from the air plasma or oxygen plasma treatment have been comprehensively studied in the past decades.37−39 Also, according to previous studies,27−29 the plasma-treated PDMS homopolymer would recover into hydrophobic environment easily not only due to the migration of low-molecularweight (LMW) species during the plasma bombardment but also due to the low Tg of PDMS. From the contact angle experiment after 2 weeks of aging process, the increase of contact angle was insignificant, indicating that no LMW species will be formed after the air plasma treatment and the mobility of polymer chains should be low enough to give the recovery. Also, it is noted that the surface can maintain relatively good hydrophilicity. On the basis of the above discussion and experimental evidence, we propose that an oxidative-induced cross-linking should occur after the air plasma treatment. Among all the chemical bonding in the PS−PDMS, the

could be concluded that the air plasma treatment leads to the creation of an oxidative-induced neutral layer on the top surface of the film. The absence of nitrogen peak in the XPS survey scan spectra could be explained by the stability of nitrogen and oxygen from the electron configuration. Note that the bond dissociation energy of nitrogen is 9.8 eV, which is much higher than that of oxygen (5.2 eV). Considering the relatively low ionization energy of oxygen, a low voltage with small ion current (1−2 mA) was thus used to ionize the oxygen and at the same time avoiding the formation of nitrogen radicals to that could cause complexity of the plasma reactions and serve as a diluent to prevent the thin-film surface from strong reactions like the fabrication of SiO2 layer on Si substrate by using helium−oxygen plasma treatment.33,34 With the lowpressure plasma treatment, the formation of unstable oxygen radicals would tend to either donate their lone electron or accept another one from the polymer.35,36 Owing to the extremely small thickness of oxidation-induced neutral layer (approximately 2 nm) formed on the thin-film surface, it is F

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cylinders cannot be formed from the substrate due to the lack of the entropy-driven architecture effect for a diblock copolymer; that is in line with the proposed mechanisms for the perpendicular orientation driven by the combined effects of the air plasma treatment and the architecture of star-block copolymer.

bonding energies of C−Si (3.3 eV), C−C (3.6 eV), and C−H (4.3 eV) require lower dissociation energy compared to the benzene ring (9.2 eV) and Si−O (4.7 eV), suggesting that those bonds could be easily broken to form radicals during the plasma bombardment as illustrated in Figure 6.40−42 Subsequently, superoxide radicals and hydroxyl group could be formed after reactions with the generated oxygen radicals, resulting in hydrophilic surface due to the formation of hydroxyl entities as evidenced by the contact angle analysis.27,43 The silanol can easily react with other PDMS chains containing silanol by hydrolysis followed by condensation to reduce the chain mobility due to the thermal annealing.44 Owing to the instability of those free radicals after the air plasma treatment, oxidative cross-linking reaction would occur while the reactive fragments start to recombine together, forming a robust crosslinked layer and giving the stability of the surface layer with neutral affinity for PS and PDMS. Although the detailed formation of the PS and PDMS radical fragments under plasma bombardment and the corresponding reactions with the oxygen and oxygen radicals from plasma are difficult to be directly evidenced, the suggested mechanisms for the possible routes to create the hydrophilic functional groups and the cross-linking reactions induced by the oxidation should be reasonable rationales. Accordingly, an oxidative-induced neutral layer could be formed by cross-linking of the constituted blocks of the initial diblock copolymer system. As a result, a thin film with span-through perpendicular cylinders could be obtained by selfassembling star-block PS−PDMS copolymer combined with the air plasma treatment under thermal annealing. To further investigate the orientation control of the nanostructured BCP thin film by the air plasma treatment, different PS−PDMS thin-film samples were used for comparison with the star-block PS−PDMS BCPs with cylinder nanostructure. After removal of the surface layer by RIE, the corresponding morphologies were examined by SEM. As shown in Figure 7A, for star-block PS−PDMS BCPs with lamellar nanostructure, striped texture can be clearly identified on the thin film after air plasma treatment, indicating the formation of uniformly perpendicular orientation. Combining with the architecture effect, the perpendicular lamellar nanostructure can be successfully fabricated through the same process as the cylinder one. On the contrary, the untreated sample exhibits featureless morphology, suggesting that parallel lamellar nanostructure were formed on the thin-film surface (Figure 7B). Furthermore, to clarify the discrepancies between the architecture effect and the neutralization effect due to plasma treatment, cylinder-forming PS−PDMS diblock copolymer was used for comparison with the star-block PS−PDMS with cylinder nanostructure. Similarly, after the air plasma treatment, hexagonally packed texture can be observed on the thin-film surface after RIE treatment as shown in Figure 7C, suggesting the formation of the perpendicular cylinder nanostructures. Instead, randomly oriented stripes can be observed on the untreated BCP thin film (Figure 7D). The results demonstrate that the effect of the plasma treatment did create the oxidative-induced neutral layer for the reduction of surface energy difference. Interestingly, the diblock PS−PDMS thin film with the air plasma treatment shows that a welldefined perpendicular cylinders can be fabricated from the thinfilm surface, but parallel orientation cylinders will be formed near the substrate (Figure 7E). These results indicate that the air plasma treatment indeed reduces the surface energy difference and the effect should be universal. Yet, perpendicular

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CONCLUSIONS In summary, the notorious problem about the low air surface energy of silicon-containing BCP can be neutralized through the oxidative cross-linking reaction induced by 1 s air plasma treatment. In addition, perpendicular cylinders of star-block PS−PDMS thin film with high aspect ratio can be produced through the solvent-free thermal annealing. These results clearly demonstrate that the self-assembly of star-block PS− PDMS copolymers confined between the oxidative-induced neutral layer and the substrate provides a facile method to fabricate nanostructured thin films with span-through perpendicular cylinders and high lateral order.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02218. Experimental and instrumentation details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel 886-3-5738349; Fax 886-3-5715408; e-mail: rmho@mx. nthu.edu.tw (R.-M.H.). ORCID

Prokopios Georgopanos: 0000-0002-6394-0628 Apostolos Avgeropoulos: 0000-0002-6203-9942 An-Chang Shi: 0000-0003-1379-7162 Rong-Ming Ho: 0000-0002-2429-7617 Present Address ⊥

P.G.: Helmholtz-Zentrum Geesthacht, Institute of Polymer Research, Max-Planck-Straße 1, 21502 Geesthacht, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Grant MOST 104-2221-E-007-091-MY3. A part of this work (synthesis and molecular characterization of the samples by P.G. and A.A.) was funded by the European Union Seventh Framework Program (FP7/2007-2013), as part of the LAMAND Project (Grant Agreement No. 245565). The research of A.-C.S. is support by the Natural Science and Engineering Research Council (NSERC) of Canada.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b02218 Macromolecules XXXX, XXX, XXX−XXX