Effect of Entrapped Solvent on the Evolution of Lateral Order in Self

Feb 14, 2017 - The level of lateral order is determined by measuring the correlation ... SeguiniNatascia De LeoLuca BoarinoMichele PeregoMichele Laus...
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Effect of Entrapped Solvent on the Evolution of Lateral Order in SelfAssembled P(S‑r‑MMA)/PS‑b‑PMMA Systems with Different Thicknesses Tommaso Jacopo Giammaria,*,†,‡ Federico Ferrarese Lupi,† Gabriele Seguini,† Katia Sparnacci,‡ Diego Antonioli,‡ Valentina Gianotti,‡ Michele Laus,*,‡ and Michele Perego*,† †

Laboratorio MDM, IMM-CNR, Via C. Olivetti 2, 20864 Agrate Brianza, Italy Dipartimento di Scienze e Innovazione Tecnologica, Università del Piemonte orientale ‘‘A. Avogadro’’, Viale T. Michel 11, 15121 Alessandria, Italy

‡

S Supporting Information *

ABSTRACT: Block copolymers (BCPs) are emerging as a cost-effective nanofabrication tool to complement conventional optical lithography because they self-assemble in highly ordered polymeric templates with well-defined sub-20-nm periodic features. In this context, cylinder-forming polystyreneblock-poly(methyl methacrylate) BCPs are revealed as an interesting material of choice because the orientation of the nanostructures with respect to the underlying substrate can be effectively controlled by a poly(styrene-random-methyl methacrylate) random copolymer (RCP) brush layer grafted to the substrate prior to BCP deposition. In this work, we investigate the self-assembly process and lateral order evolution in RCP + BCP systems consisting of cylinder-forming PS-b-PMMA (67 kg mol−1, PS fraction of ∼70%) films with thicknesses of 30, 70, 100, and 130 nm deposited on RCP brush layers having thicknesses ranging from 2 to 20 nm. The self-assembly process is promoted by a rapid thermal processing machine operating at 250 °C for 300 s. The level of lateral order is determined by measuring the correlation length (ξ) in the self-assembled BCP films. Moreover, the amount of solvent (Φ) retained in the RCP + BCP systems is measured as a function of the thicknesses of the RCP and BCP layers, respectively. In the 30-nm-thick BCP films, an increase in Φ as a function of the thickness of the RCP brush layer significantly affects the self-assembly kinetics and the final extent of the lateral order in the BCP films. Conversely, no significant variations of ξ are observed in the 70-, 100-, and 130-nm-thick BCP films with increasing Φ. KEYWORDS: block copolymer, self-assembly, random copolymer, rapid thermal processing, surface functionalization, solvent, lateral order

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The orientation of the nanostructures in BCP thin films with respect to the substrate depends on several parameters. The preferential interactions of one of the blocks with respect to the polymer/air and polymer/substrate interfaces lead to a parallel orientation.14−17 Several methods are reported in the literature to achieve the perpendicular orientation of the nanostructures in BCP thin films, such as solvent annealing,18 application of an external electric field,19,20 a top-coating method,21,22 and surface modification.23,24 Among these methods, surface modification of the substrate to prevent preferential wetting of one of the two components of the BCP represents the most promising solution because it does not require control of

INTRODUCTION The self-assembly of block copolymer (BCP) thin films has emerged as an alternative method to fabricate nanostructures that could be extremely useful for a wide range of nanotechnology applications.1−4 The morphology of the selfassembled nanostructures (spherical, cylindrical, gyroid, and lamellar) is determined by the relative fraction of the blocks that form the BCP chain.5 These nanostructured materials are perfectly suitable for application in microelectronics industries because of their ability to form highly ordered periodic structures with sub-20-nm features.6−10 In this framework, the full exploitation of BCPs as lithographic materials requires the perpendicular orientation of the nanostructures in the BCP films with respect to the substrate.11 Moreover, a high aspect ratio of the resulting templates would be remarkably desirable.12 Finally, fast processing represents a key requirement for the BCP-based technology to be integrated in the conventional process flow of semiconductor industries.13 © XXXX American Chemical Society

Special Issue: Block Copolymers for Nanotechnology Applications Received: November 9, 2016 Accepted: February 2, 2017

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DOI: 10.1021/acsami.6b14332 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces external factors. In this context, the “grafting to” approach has been proposed as an efficient method to modulate the BCP interfacial interactions and, consequently, the nanostructure orientation.25,26 This approach consists of the formation of a brush layer on the substrate by means of end functional random copolymers (RCPs) with tunable composition. This approach was successfully introduced for polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP thin films using a functional hydroxyl-terminated poly(styrene-random-methyl methacrylate) [P(S-r-MMA)] RCP, which is grafted to the SiO2 substrate by a thermally activated reaction between the silanol groups of the substrate and the hydroxyl groups of the RCP.23 PS-b-PMMA represents the most studied BCP system because self-assembly can be achieved by a simple thermal treatment and subsequently the PMMA can be easily removed from the polymeric film to obtain a nanolithographic PS mask.27 Moreover, the chemical similarity with standard photoresist materials permits a straightforward integration into lithographic process flows.7,9,28,29 In this regard, the RCP brush layer, necessary to promote the perpendicular orientation of the cylindrical nanostructures in the BCP film, implies also some technological drawbacks in terms of the pattern transfer efficiency and fidelity because it is necessary to eliminate the RCP layer from the bottom of the pores with an O2 plasma etching process. However, cross-linking of the PS matrix results a side effect of this process and makes lift-off processes difficult, resulting in pore enlargement with respect to the original dimension of the PMMA cylinders.30 The possibility of scaling down the RCP thickness to ∼2 nm while maintaining a perpendicular orientation of the cylindrical nanostructures has been previously demonstrated in 30-nm-thick PS-b-PMMA films.31,32 As previously mentioned, in order to fully integrate the BCP technology in a lithographic process workflow, it is highly desirable to properly control the aspect ratio of the nanostructures formed in the BCP thin film. In a seminal paper by Ham et al., a vertical orientation of both the cylinder and lamellar nanodomains in PS-b-PMMA BCP films deposited on a RCP brush layer was achieved in a narrow range of BCP film thicknesses upon annealing in a conventional furnace at 170 °C. Optimizing the composition of the underlying RCP brush layer, the perpendicular orientation of the cylindrical and lamellar nanostructures was obtained in a relatively broad range of BCP film thicknesses.15 More recently, Han et al. investigated the perpendicular orientation of PS-b-PMMA cylinders thermally annealed at 230 °C for 1 day with film thicknesses ranging from 45 to 900 nm and deposited on 10 nm P(S-r-MMA) brush layers with variable PS fractions.12 The authors observed a perpendicular orientation throughout all of the films up to 300 nm when PS-b-PMMA was deposited on a RCP with a PS fraction of 70%. However, industrial exploitation of these materials requires extremely fast annealing processes both for grafting of the RCP and for the BCP self-assembly processes. Over the last 5 years, several works reported the use of a rapid thermal processing (RTP) machine to obtain the RCP brush layers and to achieve self-assembly of the BCP by operating at high temperatures (Ta > 230 °C) and short annealing times.31,33,34 In this context, the self-assembly of asymmetric PS-b-PMMA BCP thin films, subjected to RTP treatment at 250 °C for 300 s, was investigated in the thickness range between 5 and 400 nm, keeping a RCP brush thickness of around 8 nm. When the BCP thickness is between 20 and 170 nm, cylindrical PMMA

nanostructures perpendicularly oriented with respect to the substrate are reported to propagate through the entire BCP film thickness. Moreover, in the same thickness range, the lateral order of the perpendicular cylindrical nanostructures was found to be independent of the BCP film thickness.35 However, it has been recently demonstrated that solvent retained in the system RCP + BCP has a strong influence on the lateral ordering of the nanostructures at fixed BCP film thickness and depends on the thickness of the RCP. In particular, the solvent retained in RCP + BCP systems consisting of 30-nm-thick PS-b-PMMA films deposited on a 20-nm-thick P(S-r-MMA) brush layer is much higher than that in the case of BCP films deposited on a 2-nm-thick RCP brush layer, and the lateral ordering of the perpendicular cylindrical nanostructures increases as the solvent amount increases.36 In this article, we investigate the influence of the RCP thickness (H) and of the retained amount of solvent (Φ) on the evolution of the correlation length (ξ) in systems consisting of asymmetric PS-b-PMMA (67 kg mol−1, PS fraction of ∼70%) films with different thicknesses (h). First of all, the amount of solvent retained in brush layers of P(S-r-MMA), having thicknesses (H) ranging from 2 to 20 nm, is measured. Afterward, we measured the value of Φ, retained in systems consisting of PS-b-PMMA with thicknesses (h) ranging from 30 to 130 nm, deposited on the different brush layers (2 nm < H < 20 nm). Finally, Φ is compared with the evolution of ξ in the BCP films subjected to high-temperature thermal treatment (250 °C) in a RTP machine to investigate the role of solvent in the self-ordering process.

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EXPERIMENTAL SECTION

Sample Preparation. Silicon (100) substrates with native SiO2 (about 1 cm2 of surface) were used as a support for the BCP selfassembly. Prior to RCP deposition, the samples were cleaned and the surface was properly activated following the procedure reported in ref 36. End-functionalized bromine-terminated P(S-r-MMA) RCPs with different Mn values and constant styrene fraction ( fs) were synthesized according to the procedure reported in ref 31. More details about the different RCP characteristics are reported in Table 1. RCP solutions

Table 1. Characteristics of the P(S-r-MMA) RCPs: NumberAverage Molar Mass (Mn), Degree of Polymerization (N), Styrene Fraction ( fs), and Polydispersity Index (PDI) RCP label

Mn (kg mol−1)

N

fs

PDI

R17 R35 R54 R87 R145 R200 R690

1.69 3.45 5.42 8.70 14.5 19.9 69.0

16.5 33.6 52.9 84.8 138.3 194.5 673.2

0.62 0.62 0.60 0.62 0.61 0.59 0.61

1.19 1.20 1.17 1.19 1.25 1.29 1.19

were spun on the samples to form a 30-nm-thick polymeric film. Grafting was achieved by thermal annealing in RTP at 250 °C for 900 s under a N2 atmosphere. More details about the sample preparation are reported in ref 35. In these experimental conditions, the thicknesses of the grafted RCP films reached the saturation values for all of the Mn values under investigation.31,34 The resulting thicknesses H of the P(S-r-MMA) brush layers ranged from 2 to 20 nm, depending on Mn. Water-contact-angle (WCA) measurements of all of the RCP brushes were performed using an Attension Theta optical tensiometer. In all the analyzed samples, the WCA was 82 ± 1° irrespective of the Mn value. B

DOI: 10.1021/acsami.6b14332 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The cylinder-forming PS-b-PMMA ( fs = 0.71, Mn = 67.1 kg mol−1, and PDI = 1.09) was purchased from Polymer Source Inc. and used without further purification. PS-b-PMMA was dissolved in toluene solutions with different concentrations. The thicknesses of the PS-bPMMA films were varied by spin-coating on the substrates neutralized with RCP brushes, obtaining films with h = 30, 70, 100, and 130 nm. Within this thickness range, the PMMA cylinders effectively propagate through the entire film when the samples are thermally treated at high temperature (250 °C, 300 s, N2 atmosphere).35 The thicknesses of the BCP and RCP films were measured by means of a model M-200U spectroscopic ellipsometer (J. A. Wollam Co. Inc.) using a xenon lamp at 70° incident angle. The measurement of the refractive index was performed following a procedure previously described37 in the spectral range between 210 and 1000 nm. The Cauchy theory was used to calculate the refractive index: n(λ) = A + (B/λ2) + (C/λ4). The parameters A = 1.58, B = 0.00958, and C = 0 are kept constant, while the wavelength λ = 632 nm. The measurements were conducted at room temperature.37 The RCP and BCP films were modeled as smooth homogeneous materials. Solvent Content Determination. The residual toluene in the films was determined by gas chromatography−mass spectrometry (GC−MS) following the procedure previously reported.36,38 The total volume amount of solvent retained in the samples was calculated both by the absolute amount of toluene Mt measured by GC−MS and scaled for the area of the film (μg cm−2) and as the solvent volume percentage in the total volume of the sample (Vr) calculated by the following equation:

bulk of the RCP, NA is Avogadro’s number, and Mn is the molar mass of the RCP.31 A progressive increase of H from 2.4 to 20.3 nm is observed as a function of N. Conversely, σ decreases from 0.94 to 0.20 chains nm−2 as N increases. Interestingly, the trends of H and σ as a function of N follow an exponential law H ∼ Nα with α = 0.56 and a power law σ ∼ N−β with β = 0.4, respectively. These results are in excellent agreement with previous studies obtained by anchoring PS40−43 homopolymers and P(S-r-MMA)31 RCPs onto SiO2 substrates. In addition, the monotonic increase in the film thickness H as a function of N indicates that the entanglement effect is not operating during the grafting process in the range of Mn values under investigation.44 Previous atomic force microscopy analysis reported in our recent works indicates that the roughness of the brush layers is independent of the thickness and grafting density of the RCP brush layers.8,32 The amount of solvent (Φ) retained inside the different RCP brush layers was determined by means of GC−MS following the procedure previously reported.33,36,38 Φ values are expressed in milligrams per square centimeter because, to compare the data obtained by GC−MS on different samples, the amount of toluene in each polymeric film is normalized to the area of the sample. Figure 2 reports the trend of Φ as a

Vr = (Vt /Vtotal) × 100 where Vt = Mt/ρt represents the volume of toluene retained in the film, calculated from the absolute amount of toluene Mt measured by GC− MS and the bulk density of toluene ρt = 0.867 g mL−1, while Vtotal is the total volume of the polymeric material and toluene estimated from the thickness of the film and the area of the substrate.38 Morphological Analysis. Scanning electron microscopy (SEM) was performed using a Zeiss Supra 40 system. In order to enhance the contrast, the PMMA cylinders were partially removed as reported elsewhere.36 Calculation of the orientational correlation length (ξ) was performed by processing several SEM plane-view images of each sample and using a Matlab routine. More details about software analysis are reported in refs 35, 36, and 39.

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Figure 2. Amount of solvent retained (Φ) in the P(S-r-MMA) brush layers as a function of the RCP brush thickness (H).

RESULTS Solvent Retention in P(S-r-MMA) Thin Films. Figure 1 reports the evolution of the brush layer thickness (H) and grafting density (σ) of the RCPs as a function of their degree of polymerization (N). The grafting density is estimated as σ = HρNA/Mn, where ρ is assumed to be equal to the density in the

function of the brush layer thickness H for the samples analyzed immediately after grafting of the RCPs with different degrees of polymerization and washing of the unreacted polymers. Φ progressively decreases with increasing H. In particular, the thinnest RCP brush layer exhibits the highest Φ (41 μg cm−2), while the thickest one exhibits the lowest Φ (7 μg cm−2). Interestingly, the decrease of the retained solvent amount as the thickness of the polymeric layer increases was already observed in other polymeric thin films45 and suggests that the silicon/ polymer interface influences the trapping solvent capability with the solvent molecules selectively located at the silicon/polymer interface, thus ultimately leading to a gradient of solvent inside the films. Solvent Retention in PS-b-PMMA Films. To quantify the initial amount of solvent (Φ) retained in the RCP + BCP system before thermal annealing, a series of samples with variable thicknesses of the BCP film (h) were prepared using the brush layers obtained from anchoring of the P(S-r-MMA) RCP with different Mn values. The thicknesses of the BCP film ranges from h ∼ 30 to 130 nm. These thicknesses were selected because, in this range, the PMMA cylinders effectively propagate throughout the entire BCP film when the samples

Figure 1. Thickness (H) and grafting density (σ) of the P(S-r-MMA) thin films as a function of the degree of polymerization (N) of the RCPs. The grafting process was performed at 250 °C for 900 s. C

DOI: 10.1021/acsami.6b14332 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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layer. With increasing PS thickness, diffusion of the solvent from inside the volume of the PS film to the polymer−air interface becomes more difficult. Consequently, less solvent reaches the surface at a given time, and the amount of solvent retained in the polymeric film increases with the PS thickness.46 Taking into account this phenomenon, it is reasonable to interpret the increase of Φ with the thickness of the BCP thin film as a consequence of the increased amount of solvent trapped in the BCP layer. A plausible explanation could be attributed to the fact that, during spin coating of the BCP onto the brush layers, the solvent is trapped by the RCP + BCP system and concurrently evaporates from the BCP film through the BCP−air interface. The latest process results in an increment of the polymer density at the BCP film−air interface, producing a capping layer of vitrified BCP. The presence of this BCP density concentration gradient creates a barrier that inhibits further evaporation of the solvent from the BCP−RCP system.47−50 Actually, this effect cannot account alone for the different slopes of the curves describing the evolution of Φ as a function of the thickness of the RCP brush layer. Collected data suggest a complex interplay between the RCP brush layer and the BCP film, determining the effective retention of the solvent within the polymeric film. The variation of the slopes of the curves reported in Figure 3 suggests that a differential swelling of the RCP brush layer occurs corresponding to the different thicknesses of the BCP film. In particular, we expect that the swelling degree increases as the thickness of the BCP layer increases, until a saturation value is reached. A neutron reflectometry investigation has been undertaken to confirm this hypothesis. Evolution of Lateral Order in PS-b-PMMA Films with Different Thicknesses. To investigate the evolution of lateral order in BCP films, the samples comprising the RCPs grafted to the SiO2 substrate and the top layer of PS-b-PMMA with different thicknesses were thermally treated in a RTP machine at 250 °C for 300 s. Figure 4 reports representative SEM planeview images of the sample surfaces after annealing, merged with the corresponding color maps obtained by software analysis of the SEM images. The different colors delineate the grain boundaries within the BCP film surface. From a morphological

are thermally treated in RTP to achieve phase separation and ordering.35 Figure 3 depicts the amount of solvent of PS-b-

Figure 3. Initial amount of solvent (Φ) retained in systems consisting of PS-b-PMMA layers with different thicknesses h of 30, 70, 100, 130 nm spun on the P(S-r-MMA) brush layers with initial thicknesses H ranging from 2 to 20 nm.

PMMA thin films with thicknesses of 30, 70, 100, and 130 nm as a function of the initial thickness H of the RCP brush layer. For a fixed BCP film thickness h, the amount of solvent Φ in the RCP + BCP system increases linearly with the RCP brush layer thickness H. At the same time, for a fixed value of H, the value of Φ increases with h. These results indicate that both the RCP brush layer and the BCP film play an important role in retention of the solvent in the RCP + BCP system. In principle, at least three interfaces are involved, namely, SiO2−RCP, RCP−BCP, and BCP−air. Inside such a complex multilayered system, distribution of the solvent across the polymeric film is unknown. The role of the BCP−air interface can be modeled on the basis of previous studies on homopolymer thin films. According to Perlich et al., the total solvent content in the PS films increases with increasing film thickness. This effect was attributed to the fact that, during spinning of the PS solution, the solvent evaporates from the body to the upper part of the film, producing a dense polymer

Figure 4. SEM images merged with the corresponding orientational color maps indicating the grain coarsening of the self-assembled BCPs with thicknesses of h = 30, 70, 100, and 130 nm annealed at 250 °C for 300 s deposited on the P(S-r-MMA) layers with thicknesses H between 2 and 20 nm. D

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penetration into the BCP films. The solvent increases the chain mobility by reducing the Tg values of the two blocks forming the BCP macromolecule below room temperature, thus leading to the self-assembly of periodic nanostructures in the BCP films at room temperature.11,18,51,52 In particular, Sik et al. studied the effect of the solvent vapor pressure on the arrangement of cylindrical PS-b-PDMS films confined within topographically defined trenches.53 At low vapor pressure, disordered cylinders of PS-b-PDMS are obtained, whereas at high vapor pressure, highly ordered nanodomains with PDMS cylinders parallelly oriented to the edge of the trench are observed. The authors ascribed this effect to an increase in the solvent vapor pressure, which progressively swells the polymer film and consequently reduces the Tg value of the BCP, thus ultimately facilitating rearrangement of the polymer chains.53 Similarly, Baruth et al. investigated the evolution of lateral order in thin films of PS-b-PLA with different molar masses during SVA treatments as a function of the solvent concentration retained in the polymeric films.54 They demonstrated that the lateral order of cylinders perpendicularly oriented with respect to the substrate progressively increases with an increase in the solvent concentration inside the BCP thin films. This effect is observed to occur up to a concentration limit that corresponds to achievement of the order−disorder transition, at which a dramatic decrease of ξ occurs.54 Recently, several groups introduced novel solvothermal annealing processes that produce a significant improvement of the nanostructure ordering dynamics. In these processes, the BCP thin films are simultaneously heated and exposed to solvent vapor in order to reduce the annealing time process of the standard SVA approach.18,55 These approaches were successfully introduced with high-χ BCPs such as PS-bPDMS. Because of the strong incompatibility between the two blocks, these BCPs are affected by slow self-assembly kinetics, and consequently standard SVA annealing requires very long processing times. The combination of thermal treatment with the SVA process allows one to increase the chain diffusivity in the BCP thin films, remarkably reducing the annealing time required to promote phase separation and the self-assembly process.56,57 In particular, Park et al. demonstrated58 a methodology to achieve the self-assembly of PS-bPDMS confined in trench patterns in a short annealing time, obtaining nanostructures with high resolution and low defect density. They investigated the effect of the annealing temperature on the PS-b-PDMS nanostructures by applying thermal treatment between 25 and 85 °C for 300 s during the SVA process. A progressive decrease of the defect density was observed as the annealing temperature was increased, in both parallel cylinders and spherical nanostructures. Similarly, in a recent paper, Seguini et al. demonstrated36 that the solvent, initially retained in a system consisting of a 30-nmthick PS-b-PMMA film (Mn = 54 kg mol−1) deposited on a 20nm-thick P(S-r-MMA) brush layer, can be exploited to enhance the lateral order of the hexagonally packed PMMA cylinders that are formed in the BCP films during simple thermal treatment between 160 and 240 °C. In particular, upon thermal treatment at 220 °C for 300 s, correlation length ξ ∼ 540 nm was achieved in the BCP film deposited on a 20-nm-thick brush layer. Conversely, for the BCP film deposited on a 2-nm-thick brush layer, correlation length ξ ∼ 210 nm was obtained by applying the same thermal treatment. Enhancement of the lateral order was ascribed to the different amounts of solvent that are initially retained in the polymeric films. In the case of

point of view, all of the samples exhibit a perfectly homogeneous surface consisting of perpendicularly oriented PMMA cylinders having center-to-center distance L0 = 35 ± 2 nm and diameter D = 17 ± 1 nm, as shown in Figure S1. The images of the first row in Figure 4 refer to the 30-nmthick PS-b-PMMA thin films deposited onto the RCP brush layer with different H values. A significant increase in the lateral order in the BCP film as a function of H is observed, with bigger and bigger grains exhibiting a perfect hexagonal arrangement of the PMMA cylinders. The other rows of Figure 4 refer to the PS-b-PMMA layers featuring thicknesses h of 70, 100, and 130 nm deposited on the RCP brush layers with different H values. In these latter films, the evolution of lateral order as a function of H is quite limited. For example, in the 130-nm-thick PS-b-PMMA films, the level of lateral order is lower than that in 70- and 100-nm-thick PS-b-PMMA films. Quantification of the lateral order was performed by extracting from the color maps of Figure 4 the correlation length values ξ for all of the samples, following a literature procedure.39 Figure 5 reports ξ values as a function of H for the

Figure 5. Correlation length (ξ) of the perpendicular cylindrical nanostructures in PS-b-PMMA films having thicknesses h of 30, 70, 100, and 130 nm annealed at 250 °C for 300 s as a function of the RCP brush layer thicknesses H.

different BCP films. In the case of 30-nm-thick BCP films, the correlation length rises linearly as a function of H and Φ from ξ ∼ 214 nm (H = 2.25 nm; Φ = 91 μg cm−2) to 365 nm (H = 19 nm; Φ = 206 μg cm−2). Conversely, for all of the other BCP films, ξ is almost constant, within experimental error, irrespective of the RCP thickness. The only remarkable difference regards the absolute value of ξ, which, in samples with h = 70 and 100 nm, lies around 230 nm but drops to 160 nm in samples with h = 130 nm.

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DISCUSSION The experimental results reported in the previous sections indicate that the lateral ordering of the cylindrical nanostructures in the self-assembled BCP thin films depends simultaneously on the amount of solvent initially trapped in the RCP + BCP system prior to thermal annealing and on the thicknesses of the RCP and BCP layers. From a general point of view, the use of a solvent to enhance the mobility of the BCP chains and promote the self-assembly in BCP thin films has been fully exploited in the so-called solvent vapor annealing (SVA). In the SVA processes, BCP thin films are exposed to solvent vapors, controlled by adjusting the pressure in the annealing chamber, to enable solvent E

DOI: 10.1021/acsami.6b14332 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the BCP film deposited on a 2-nm-thick RCP brush layer, the residual solvent initially retained in the polymeric film is not enough to guarantee high chain mobility for the entire duration of thermal treatment because most of the solvent is lost during the initial stages of the process. Conversely, in the case of the BCP film deposited on a 20-nm-thick RCP brush layer, the residual solvent retained in the polymeric film is sufficient to sustain the mobility of the polymeric chains for longer times of thermal treatment, thus enhancing the lateral order by a factor of 2.6. It is worth noting that these results were achieved by processing the polymeric films in a RTP system, which allows one to achieve the target temperature in a very short time (approximately 12 s), thus reducing solvent desorption during the heating stage.33 Accordingly, this solvent-assisted rapid thermal annealing process revealed an ability to enhance the lateral order of the nanostructures by exploiting the combined effect of thermal annealing and the solvent content initially trapped in the BCP + RCP system. A similar enhancement in the correlation length from 214 to 365 nm is observed in the present work for the 30-nm-thick BCP films deposited on the RCP as the brush layer thickness is increased from 2 to 20 nm. This corresponds to an enhancement of the correlation length by a factor ∼1.7. Considering that in the present samples the amount of solvent initially retained in the polymeric films before thermal treatment is similar to that described by Seguini et al., the lower enhancement of the correlation length observed in the present case can be tentatively attributed to the different molar masses of the specific BCP under investigation (cf. Mn = 67 kg mol−1 in the present case and Mn = 54 kg mol−1 in work by Seguini et al.). The higher molar mass of the BCP employed in the present study implies a lower chain mobility, thus resulting in a lower correlation length enhancement.59−61 Moreover, a significant increase in the correlation length ξ is observed for the 30-nm-thick BCP films, while for thicker films, the ξ values are almost constant. It is interesting to note that the data for the 8-nm-thick RCP brush layer perfectly agree with the experimental results reported by Ferrarese Lupi et al. They presented a systematic study of asymmetric PS-b-PMMA (Mn = 67.1 kg mol−1) by varying the BCP thickness from 10 to 400 nm at fixed PS-r-PMMA brush layer thickness (∼8 nm) and molar mass (14.5 kg mol−1). For an annealing temperature at 250 °C for 300 s, the correlation lengths ξ of the samples with BCP thicknesses ranging between 30 and 180 nm appear to be scattered around a value of ∼180 nm, revealing the independence of the correlation length with respect to the BCP thickness for a fixed RCP brush layer thickness of around 8 nm.35 Starting from the values of the solvent reported in Figure 3, we calculate the volume fraction Vr of toluene retained in the different samples as a function of the RCP film thickness H. The collected data for each specific combination of RCP and BCP are reported in Table S1. In the 30-nm-thick BCP films, the Vr values range from 17 to 22%. This variation of the residual solvent in the RCP + BCP system is accompanied by a progressive increase in the correlation length, as highlighted in Figure 5. In particular, it is worth noting that the sample with the 30-nm-thick BCP film deposited on top of the 20-nm-thick P(S-r-MMA) brush layer exhibits a solvent volume fraction Vr = 22%. Similar results were obtained in systems consisting of lamellae-forming PS-b-PMMA (Mn = 51 kg mol−1) BCPs having H ∼ 25 nm deposited on 5 nm of a P(S-r-MMA) (Mn = 11.4 kg mol−1, PS fraction 58%) brush layer.38 In samples with

BCP thicknesses ranging from 70 to 130 nm, the values of Vr vary from 12% to 23% by increasing the thickness H of the RCP brush layer. As a consequence, the different evolutions of the correlation length as a function of the RCP thickness in a 30-nm-thick sample compared to the other sets of samples cannot be explained in terms of a reduced concentration of solvent within the polymeric film. From a different perspective, it is well-known that Tg depends on the thickness of the BCP films.62−64 Hsu et al. established a chemistry-specific coarse-grained model to determine the Tg−thickness dependence due to segmental dynamics in free-standing PS and PMMA films. The authors observed the same qualitative dependence of Tg on the film thickness, with a progressive reduction of Tg with decreasing polymeric film thickness. According to their model for 30-nmthick PS and PMMA thin films, the Tg reductions with respect to the bulk values are reported to be ΔTgPS ∼ −8 °C and ΔTgPMMA ∼ −2 °C, respectively. For 70- and 100-nm-thick PS films, the measured Tg reductions are ΔTgPS ∼ −3.5 and −2.5 °C. For PS films thicker than 100 nm, the measured Tg values are basically equivalent to the one of the bulk.65 These data suggest that the chain mobility in the 30-nm-thick BCP layers is slightly higher than those in the other samples because of the reduced thickness of the polymeric film, which affects the TgPS value. However, the effect of the increased chain mobility due to the TgPS reduction is negligible in our system because of the very high annealing temperature (250 °C). It is clear that the solvent retained in polymeric films provides a significant boost to the lateral ordering evolution in very thin BCP films, in accordance with data previously reported in the literature.36 Nevertheless, the results herein reported clearly indicate that the initial solvent reservoir that is present in the RCP + BCP system is not always effective in speeding up the kinetics of the self-assembly process. Actually, in previous experiments, we observed that a large amount of solvent that is initially trapped in the polymeric film is lost during heating of the sample and/or the initial stages of thermal treatment.33,36,38 This fact indicates that not all of the solvent initially trapped in the polymeric film is available during the self-assembly process because a large amount of solvent is lost before ignition of the grain-coarsening process. In principle, a different release of the retained solvent in the initial stage of thermal treatment depending on the BCP film thickness could result in significant variation of the amount of solvent that is present in the BCP + RCP system during the steady state of RTP annealing. Moreover, we have to take into account that with increasing thickness of the BCP films we progressively increase the length of the PMMA cylinders within the polymeric template. If we assume that the kinetics of the BCP chains in phase-separated thin films is driven by cooperative diffusion of the single domains, the graincoarsening process could be envisioned by the motion of quasi-particles corresponding to the PMMA cylinders and arising from the collective behavior of many chains.66−68 In this picture, the higher the thickness of the BCP film, the higher the mass of the quasi-particles involved in the mass-transfer process guiding the grain coarsening. In this respect, the amount of solvent retained in the polymeric films is not sufficient to significantly increase the mobility of the quasi-particles in thick BCP films and to speed up the grain-coarsening process. In a recent work, a technique based on a coarse-grained molecular dynamics method was used to simulate the SVA in cylinder-forming BCP nanodomains in polymeric films having F

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ACS Applied Materials & Interfaces different thicknesses.69 Although the work of Berezkin et al. presents numerous differences compared to our present work, like, for instance, the absence of the RCP to neutralize the substrate and the process used to achieve phase separation (SVA), they demonstrated the possibility of achieving perpendicular ordering of the cylinders in very thin BCP film thickness. Conversely, by increasing the BCP film thickness intermediate, mixed structures of parallel and vertical cylinders are found. According to these data, the BCP film thickness affects the arrangement of the cylinders in the polymeric matrix.69 These results suggest that the final arrangement of the BCP system is determined by a complex interplay between the solvent present in the polymeric film and the thickness of the BCP layer, similar to that observed in our system. From a technological perspective, the data reported in the present manuscript suggest the possibility of enhancing the lateral order in 30-nm-thick BCP films, taking advantage of the solvent naturally retained in the polymeric film, without implementations of complex setups that are usually required with the SVA techniques. In this respect, these experimental findings could be interesting for a wide range of applications where the development of nanopatterned surfaces or nanostructures at low cost is mandatory. Moreover, in BCP films with thickness h ≥ 70 nm, the reduction of the RCP film thickness to 2 nm does not influence lateral ordering in the BCP films. Accordingly, the use of very thin RCP films to neutralize the substrate does not reduce the correlation length but is able to preserve the quality of the cylindrical template for subsequent pattern transfer to the underlying substrate. The data demonstrate that by scaling the RCP thickness from 20 to 2 nm the quality of the BCP template in terms of lateral order is not significantly modified for BCP films with thickness h > 30 nm. Moreover, in the case of the 30-nm-thick BCP films deposited on thin RCP brush layers (H < 7 nm), the level of lateral order progressively decreases, with correlation length ξ values decreasing from 258 ± 8 to 214 ± 10 nm. These results highlight the effectiveness of these ultrathin RCP films in controlling the perpendicular orientation of the nanodomains in BCP films without any detrimental effect on the selfassembly process of these macromolecules.

the amount of retained solvent. For BCP thicknesses of around 70 and 100 nm, ξ values of ∼210 nm were measured, while for a BCP thickness of around 130 nm, the correlation length drops to ξ ∼ 160 nm. On the contrary, the 30-nm-thick PS-bPMMA films deposited on the different brush layers exhibit a progressive increase of the correlation length values from ξ ∼ 214 to 365 nm as a function of the brush layer thickness and solvent content.

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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/acsami.6b14332. SEM images of the self-assembled BCP films and characteristics of the P(S-r-MMA) RCPs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Federico Ferrarese Lupi: 0000-0002-1055-8839 Michele Perego: 0000-0001-7431-1969 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research activity was partially funded by the Project 14IND01 “3DMetChemIT”. This project has received funding from the EMPIR programme cofinanced by the EMPIR Participating States and from the European Union’s Horizon 2020 Research and Innovation Programme. Patent protection related to this work is pending.

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REFERENCES

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CONCLUSIONS In the present article, we performed a detailed study of the role of the solvent in the self-assembly process of PS-b-PMMA thin films deposited on P(S-r-MMA) brush layers. First, we investigated the solvent retention of brush layers having thicknesses H ranging between 2 and 20 nm obtained from the grafting of P(S-r-MMA) with molar masses ranging from 1.7 to 69 kg mol−1. The data collected indicate that the amount of solvent retained in the brush layer decreases from Φ = 41 to 7 μg cm−2 with an increase of H. Then, we analyzed the solvent retention in PS-b-PMMA thin films with different thicknesses h ranging from 30 to 130 nm on the different P(S-r-MMA) brush layers with thicknesses ranging between 2 and 20 nm. The experimental results demonstrated that increasing the thickness of the BCP film and/or of the RCP brush layer results in an increased amount of retained solvent in the RCP + BCP system. Finally, we studied the effect of the retained solvent on the lateral order of the nanostructures in the BCP thin films upon annealing at high temperature in a RTP machine. In samples with thicknesses of around 70, 100, and 130 nm, almost constant correlation length values ξ were observed as a function of the P(S-r-MMA) film thickness and consequently of G

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