Through-Thickness Vertically Ordered Lamellar Block Copolymer Thin

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Thru-thickness Vertically Ordered Lamellar Block Copolymer Thin Films on Unmodified Quartz with Cold Zone Annealing Monali N. Basutkar, Saumil Samant, Joseph Strzalka, Kevin G. Yager, Gurpreet Singh, and Alamgir Karim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04028 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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Thru-thickness Vertically Ordered Lamellar Block Copolymer Thin Films on Unmodified Quartz with Cold Zone Annealing Monali N. Basutkar, † Saumil Samant, † Joseph Strzalka, $ Kevin G. Yager, # Gurpreet Singh, † Alamgir Karim †* †

$

Department of Polymer Engineering, University of Akron, Akron, OH 44325, United States

X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States #

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

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Abstract Template-free directed self-assembly of ultrathin (~10’s nm) lamellar block copolymer (l-BCP) films of high-interfacial area into vertically oriented nanodomains holds much technological relevance for fabrication of next-generation devices from nanoelectronics to nanomembranes due to domain interconnectivity and high interfacial area. We report for the first time, the formation of full thru-thickness vertically oriented lamellar domains in 100 nm thin polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) films on quartz substrate, achieved without any PMMA-block wetting layer formation, quartz surface modification (templating chemical, topographical) or system modifications (added surfactant, top-layer coat). Vertical ordering of lBCPs results from the coupling between a molecular and a macroscopic phenomenon. A molecular relaxation induced vertical l-BCP ordering occurs under a transient macroscopic vertical strain field, imposed by a high film thermal expansion rate under sharp thermal gradient cold zone annealing (CZA-S). The parametric window for vertical ordering is quantified via a coupling constant, C (= v.∇T), whose range is established in terms of a thermal gradient (∇T) above a threshold value, and an optimal dynamic sample sweep rate (v ~ d/τ), where τ is the lBCP’s longest molecular relaxation time and d is the Tg,heat-Tg,cool distance. Real-time CZA-S morphology evolution of vertically oriented l-BCP tracked along ∇T using in-situ Grazing

Incidence Small Angle X-ray Scattering exhibited an initial formation phase of vertical lamellae, a polygrain structure formation stage, and a grain coarsening phase to fully vertically ordered lBCP morphology development. CZA-S is a roll-to-roll manufacturing method, rendering this template-free thru-thickness vertical ordering of l-BCP films highly attractive and industrially relevant. KEYWORDS: cold zone annealing, block copolymer, vertical lamellae, in-situ. 2 ACS Paragon Plus Environment

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The ability of block copolymers (BCPs) to spontaneously microphase separate and self-assemble into well-defined nanostructures having periodicities in the range of 5 nm – 100 nm makes them highly attractive as potential candidates for next-generation technologies.1–15 In addition to the self-assembly, confinement effects play a very important role in ultrathin films.16–24 Particularly, directed self-assembly (DSA) aimed at highly ordered vertical microdomains of lamellar-BCPs (l-BCPs) has attracted much attention due to their advantages of higher surface area to volume ratio compared to cylindrical-BCPs (c-BCPs) as well as dual domain interconnectivity. Such high aspect ratio vertically oriented microdomains with smooth sidewall profile25 would be potentially useful in applications such as ion conducting membranes for batteries,26,27 nanowire grid polarizers,28–30 nanolithography1,31 and ultrafiltration.32,33 In this regard, polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) is a well-studied BCP due to the photo-degradability of the PMMA block. However, the strong wetting tendency of PMMA block on semiconductor industry standard smooth silicon oxide surface drives ultrathin films of lamellar PS-b-PMMA to orient parallel to the substrate. Thus, it is particularly challenging to vertically orient lamellar microdomains of PS-b-PMMA, even more so than c-BCP PS-b-PMMA. Template free ultrathin films of vertically oriented l-BCP morphology can be particularly important as a general method for transferring lamellar patterns onto the underlying substrate by methods such as reactive ion etch (RIE) directly for device applications. Consequently, alternate strategies employed to obtain vertical lamellae (of mostly PS-PMMA BCP) in ultrathin films include chemical surface modification via neutral substrate brushes and/or top coats,34–39 inducing nanoscale roughness,40– 42

and graphoepitaxy.43,44 Of these, most methods involve complex chemical modification of the

surface/substrate and multi-step fabrication processes. In addition, very few methods have the capability to be translated into continuous processes such as roll-to-roll (R2R) fabrication for


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producing the desired l-BCP morphology over sizable areas, which hinders their scale-up to device fabrication beyond the laboratory scale. Magnetic field has previously been used for directing nanomaterial assembly to create scalable vertically ordered c-BCP nanostructures for organic electronics and transport membrane applications.45–48 Alignment of c-BCPs using strong permanent magnets in conjunction with a thermal field demonstrated microstructure control by coupling the magnetic contrast in the system to the intermaterial dividing surface between the BCP microdomains.49–51 In addition, Zhang et al.52,53 had observed that in c-BCP PS-PMMA systems, surface and bulk cylindrical microdomains orient vertical at relatively “lower” temperatures, as compared to annealing at “higher” temperatures wherein they observed completely laying down parallel cylinders by the traditional oven annealing method. The temperature at which a transition in the surface pattern morphology is observed depends upon the molecular mass of the BCP, however a base PMMA wetting layer nevertheless existed in all cases. However, in cases of l-BCPs, this temperature induced vertical ordering has not been observed previously on smooth surfaces owing to the higher energetic penalties associated with overcoming the larger attractive surface forces due to larger contact area of the wetting lamellar block with the substrate. In this work, we demonstrate a template-free approach towards rapid fabrication (2 – 4 min.) of highly ordered vertical lamellar PS-b-PMMA microdomains in l-BCP films on quartz (silicon oxide) by a one-step continuous, dynamic thermal gradient process of Cold Zone AnnealingSharp (CZA-S). More uniquely, CZA-S accomplishes this without the need for any surface pretreatment, chemical modification or templating of the quartz substrate, rendering it useful more generally for continuous large-area manufacturing of l-BCP films with existing roll-to-roll technology.54 We determined that a requisite sharp thermal gradient (∇T = 48 °C mm–1) of the 4 ACS Paragon Plus Environment

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CZA-S process enables the formation of full thru-thickness vertical lamellar morphology in PSb-PMMA diblock copolymer films, despite PMMA attraction to quartz surface. The surface energies, γ, of PS, PMMA and UVO treated quartz are γPS = 40.7 mJ m–2, γPMMA = 41.1 mJ m–2 and γQuartz = 75.7 mJ m–2.55 Although the overall surface energies of PS and PMMA are not significantly different, the individual polar and dispersive components of γ are the driving forces in determining the surface morphology (γPMMA,polar = 11.5 mJ m–2, γPMMA,dispersive = 29.6 mJ m–2, γPS,polar = 6.1 mJ m–2, γPS,dispersive = 34.5 mJ m–2). PMMA having a higher polar component (-C=O) preferentially wets the quartz (SiO2) substrate surface after the UVO cleaning treatment while PS gets preferentially attracted towards air surface. We have engineered CZA-S with an enhanced temperature gradient to overcome this strong PMMA quartz wetting behavior. The real-time study of the evolution of vertical lamellar morphology by Grazing Incidence Small Angle X-ray Scattering (GISAXS) under the CZA-S process on template-free, non-modified substrates provides new fundamental insights into the physical mechanism of vertical ordering in l-BCPs, thereby guiding the design of future DSA processes targeting such morphologies. Figure 1a is a schematic of the CZA-S set-up to create vertical orientation in l-PS-b-PMMA thin films. An ~1 mm narrow hot wire heater (nickel−chrome wire covered with a ceramic insulation of 3 mm outer diameter) powered with a high current source is used for heating. Aluminum cold blocks cooled by circulating low molecular weight PDMS oil at 10 °C using a chiller system are placed at a distance of 0.5 mm on both sides of the hot wire. The l-BCP film sample is translated over the temperature gradient curve that is created by the CZA-S cold-block/hot-wire/cold-block assembly. The parameters determining the l-BCP morphology orientation are the symmetric thermal gradient (∇T), the maximum temperature of the thermal zone (Tmax), and the velocity (v) at which the BCP film is translated over the heated region. Previously, Singh et al.56 and Samant 5 ACS Paragon Plus Environment

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et al.57 have shown that with a sufficiently high ∇T ( ≥ 30 °C mm–1), a high degree of vertical

ordering can be achieved in cylindrical morphology PS-b-PMMA (c-BCP) films, primarily attributed to the rapid expansion of the films over the hot zone and the resulting strain gradient forcing the BCP domains to orient vertical to the surface. However, the vertical ordering of lBCPs is more demanding as compared to c-BCPs, owing to the complete substrate wetting characteristics of the lamellar PMMA microdomains. Consequently, the l-BCP did not orient vertically at the ∇T ≈ 30 °C mm–1 with other CZA-S parameters remaining constant, for

comparable c-BCP and l-BCP total molecular weight. A larger fraction of higher viscosity PMMA coupled with 100% contact area with substrate in parallel lamellar orientation makes the

l-PS-b-PMMA more challenging to orient than the c-PS-b-PMMA. Notably, in the right range of CZA-S speed with ∇T ≥ ∇Tthreshold (~45 °C mm–1), it overcame the PMMA wetting forces and

viscosity effect to rapidly induce vertical order in the l-BCP film as described below.

Figure 1b and Figure 1c depict the AFM topographical patterns for 100 nm thin l-PS-b-PMMA films subjected to a ∇T of 35 °C mm–1 and 48 °C mm–1 respectively after CZA-S processing. The

temperature gradient is altered by changing the gap between the hot wire and cold blocks while keeping Tmax constant. The sample translation velocity is held constant at 20 µm s–1. As observed, a gradient of 35 °C mm–1 in a CZA-S annealed sample induces an island and hole morphology indicative of a parallel lamellar orientation in the l-BCP thin film whereas a sharper gradient of 48 °C mm–1 produces vertically oriented lamellae. Essentially, as the film is heated above Tg along the CZA-S ∇T curve, the film is exposed to a thermal shock associated with rapid thermal expansion of the BCP film. The kinetic constraints in the plane of the film lead to this initial expansion in the film-normal direction to orient the polymer chains within the film. This vertical pull therefore favors vertical orientation of lamellae in order to minimize the stress in the 6 ACS Paragon Plus Environment

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system that is created by the CZA-S assembly. The larger ∇T increases the film expansion rate, thereby generating a high vertical stress/strain field. Under this holding stress/strain field, the CZA-S velocity needs to then provide a sufficient transient period for the disordered PS-PMMA chains to relax and molecular level lamellar domain ordering to occur. The vertical (out-ofplane) film expansion driven holding forces must also exceed any PMMA wetting forces. Apparently CZA - Sharp induces such a high enough strain gradient in the l-BCP film at 48 °C mm–1 at 20 µm s–1. The macroscopic rapid film expansion phenomenon is analogous to a study by Beaucage et al.58 wherein an abrupt normal thermal expansion of a homopolymer film occurs from a thermal shock when the film is rapidly brought to a temperature above Tg. For GISAXS, the X-Ray beam was incident on the polymer surface at different angles ranging from 0.14 ° to 0.2 °. When the angle of incidence (θ) is smaller than the critical angle (θc) of the polymer film (θc,BCP = 0.156 for this PS-b-PMMA system with the photon energy of 7.35 keV), total external reflection takes place to reveal the surface morphology of the film. At angles greater than the critical angle for PS-b-PMMA film, the X-Ray beam penetrates the film surface to reveal the internal morphology of the film. The greater the angle of incidence of the X-rays, the greater the penetration depth into the film.59 The two-dimensional GISAXS image illustrates the BCP film structure, both in-plane (vertical morphology) and out-of-plane (parallel morphology). Figure 1d and Figure 1e are the 2-dimensional GISAXS data at ∇T of 35 °C mm–1 and 48 °C mm–1 respectively of the bulk structure (θ = 0.18°) of the CZA-S processed l-BCP

films. The data can be integrated over the regions of interest shown, to obtain intensity profiles in the qy (Figure 1f) and qz (Figure 1g) planes in order to quantify the structures. The l-BCP film annealed with a shallow ∇T of 35 °C mm–1 exhibits no distinct peaks in the qy plane (X-Y plane of the scattering geometry) but shows a clear correlation in the qz plane validating the out-of7 ACS Paragon Plus Environment

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plane parallel lamellar orientation arising from PMMA block substrate wetting. For the l-BCP film annealed over a sharp ∇T of 48 °C mm–1, the in-plane structural correlation (vertical

lamellar morphology) leads to Bragg peaks in the qy plane, the domain spacing of the lamellae can be calculated as Lo = 2π/qy*. In addition to the first order Bragg peaks, one can also observe third order peaks, demonstrating that the vertical lamellar structure shows high degree of spatial correlation within the plane of the film. Only first and third order peaks are prominent because symmetric lamellae result in the absence of even order peaks.60 Integrated intensity profile in the qz direction does not show any distinct Bragg peak confirming that there is no parallel l-BCP component, and that the vertical ordering is near perfect. Internal morphology of CZA-S annealed l-BCP thin films The through-thickness integrity of vertical lamellae in another film processed by CZA-S at a sharp ∇T of 48 °C mm–1 can be verified by sequential ablation of the top surface of the PS-b-

PMMA film (real space) followed by AFM imaging after each step. For this purpose, we

employed a method demonstrated by Albert et al.61 Figure 2a reveals the topography of a 100 nm thin film when subjected to successive UVO exposure cycles. It is observed that the vertical lamellar morphology induced by CZA-S persists throughout the thickness of the film, where vertical lamellae can be seen even at a film thickness of 34 nm from the substrate, i.e. slightly less than 1Lo of this PS-b-PMMA system. (Lo = 37 nm). This result is of particular interest because unlike previous studies, we can achieve vertical lamellar orientation in 100 nm PS-bPMMA thin films without any PMMA wetting layer at the substrate interface without the use of any substrate pretreatment/modification.


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The above result of depth fidelity of vertical lamellae (thru-thickness) can be corroborated by GISAXS analysis of an identically CZA-S processed film at ∇T of 48 °C mm–1 (without surface

ablation). The vertical streaks in the qy plane (Figure 2b) indicative of an in-plane vertical lamellar morphology in GISAXS are observed at higher incident angles where the entire film thru-thickness is fully probed, with the third order peaks being retained prominently (Figure 2c), supporting the AFM results of 100% through film thickness vertical lamellae. Furthermore, the absence of peaks in the qz direction confirm an absence of parallel component of the BCP

lamellae. Real space cross-sectional TEM imaging of the CZA-S processed l-BCP film obtained by focused ion beam (FIB) shows a highly oriented vertical lamellar structure throughout the 100 nm thin film (Figure 2d). Finally, corroborating fitted through film scattering length density (SLD) depth profile measured by X-ray reflectivity (XRR) in Figure 2e corresponds to homogeneous PS and PMMA distribution throughout the film thickness direction, as expected for fully vertical lamellae. Notably, there is no parallel PMMA block layering at the BCP film/quartz interface confirming the presence of completely vertical lamellae throughout the film from substrate surface upwards. (If the data is fitted to a substrate wetting PMMA layer, the fit is disrupted, as shown in Supporting Information Figure S2.) We can therefore conclude that high thermal gradient CZA-S is a robust annealing method for orienting ~100% of lamellar PSPMMA microdomains vertical to the oxide substrate without involving the complexities of substrate pre-treatments. Analogy of transient film expansion to short time static thermal (oven) annealing Kinetics of static thermal annealing of the 100 nm thin films of PS-b-PMMA in a vacuum oven at 210 °C (Tmax of CZA-S) provides insights into the vertical orienting mechanism of the CZA-S dynamic process. Figure 3 (AFM) shows that as the film is given a large step-jump in 9 ACS Paragon Plus Environment

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temperature (thermal shock) from ambient to 210 °C in a pre-heated uniform thermal annealing vacuum oven, a vertical lamellar structure develops within 2 min. (at least at the film surface if not internally as well), from its quench disordered as-cast state. This effect is analogous to the CZA-S induced rapid thermal expansion of the BCP film above its Tg as the film encounters the thermal gradient, leading to a vertical strain field producing a vertical lamellar orientation in a time frame comparable to the longest relaxation time of the BCP chains that allows for lamellar domains to form in a dynamic fashion. However, unlike CZA-S, the rapid thermal expansion in the BCP film caused by uniform oven annealing induces spatially inhomogeneous vertical l-BCP orientation with numerous defects (Figure 3(b)). As the uniform oven annealing time is increased, as shown in Figure 3(c-d), the inhomogeneity decreases and short-range ordered space filling vertical lamellae is observed. With further oven annealing (Figure 3(e-h)), the vertical nanostructure improves in long range order but the surface morphology transitions to increasing fraction of large smooth area regions of island and hole features indicative of increasing parallel lamellar orientation.62–64 This is attributed to the substrate and air interface driven wetting of PMMA and PS block respectively, when chain relaxation process transforms to diffusive behavior at longer times. It is therefore evident that the spatially uniform vertical lamellar morphology observed by CZA-S is the effect of the sharp dynamic thermal gradient field. Effect of CZA-S Sweep Rates on Vertical Lamellae Ordering The surface wetting dynamics of PS and PMMA blocks that drives parallel lamellae order in oven annealing at long times also competes with the CZA-S dynamics and the BCP chain relaxation dynamics to induce a net lamellar orientation that can be parallel, vertical or a mix of the two (Figure 4a). In order to quantify the conditions of vertical orientation of l-BCPs through CZA-S dynamics, we compare the CZA-S exposure traversing time from T=Tg,BCP-heating to 10 ACS Paragon Plus Environment

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T=Tg,BCP-cooling on the gradient. It is useful to compare this exposure time to the longest relaxation time (τ) of the l-BCP. As shown later, the sharp ∇T region on the heating side plays the most

important role in creating vertical lamellar morphology that is further developed as the sample is translated along the remaining temperature gradient curve. Therefore, τ at 180 °C (upper quartile

temperature on sharp gradient) is obtained by extrapolation of Beaucage’s experimental data.58 τ ~ 4 min at this temperature. The data is corrected for molecular weight from a reference 130 kg mol–1 PMMA system. As a first order of approximation, it is assumed that the relaxation dynamics of PS-b-PMMA thin films are the same as the higher viscosity PMMA material. At low CZA-S sweep rates (v = 0 – 15 µm s–1), the sample exposure time > τ so that polymer chain relaxation and diffusion dynamics dominate, leading to the equilibrium surface wetting morphology. At these speeds, even if the high temperature gradient induced a vertical microdomain orientation on sharp heating gradient, around Tmax where the gradient is shallow, the low translation rates provide sufficient time for the polymer chains to relax and preferential substrate/surface wetting of the blocks takes place producing notable fraction of parallel lamellae.58,52 Depending upon the translation speed, the percentage of vertical lamellae population changes (Figure 4b). At the other extreme of very high translation rates (v = 40 – 60 µm s–1), the sample thermal exposure time on heating gradient, t ≪ τ, so that the CZA-S dynamics is too fast to induce complete microphase separation producing a mixed, poorly developed morphology. Thus,

there exists an optimum sweep rate range at which complete vertical orientation with minimum defects takes place, above and below which poor ordering or mixed orientation of lamellar microdomains is observed. The % vertical values in Figure 4b indicate that the CZA-S translation speeds of 20 µm s–1 to 35 µm s–1 are most effective in inducing vertical microdomain 11 ACS Paragon Plus Environment

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orientation. The sweep rate of 20 µm s–1 produces the highest vertical orientation (~100%) with minimum defects, corresponding to ~3.6 min. across the Tg,heat-Tg,cool distance to generate vertically ordered lamellae with high orientation control by CZA-S. As in vertically ordered cPS-PMMA BCPs,56 the coupling hypothesis for maximum vertical orientation, i.e. the dynamic CZA-S Tg,heat-Tg,cool annealing time (~3.6 min.) should match the longest molecular relaxation time of the polymer (τ = 4 min.), holds valid in l-PS-PMMA BCPs as well. It is worth noting that the timescale for morphology transition from a completely vertical lamellar orientation to a mixed lamellar orientation is comparable in case of oven annealing and CZA-S. In both the cases, this transition takes place as the l-BCP film is annealed for more than τ (= 4 min.), the longest polymer chain relaxation time. We quantify this dynamic thermal behavior by establishing a ‘coupling constant’ (C) between molecular relaxation rate (~1/v) and macroscopic film thermal expansion rate (~∇T) such that: C = v.∇T

(1)

Cmin and Cmax define an operating parametric range of the dynamic CZA-S process that essentially quantifies the vertical ordering phenomena due to the abrupt thermal expansion of the film under tension and the relaxation rate of the microstructure. In case of the particular l-BCP system under study, we define this range as 1.0 °C s –1 < C < 1.7 °C s –1, above and below which the dynamic thermal field is insufficient to produce complete vertical lamellar morphology. In-situ structure evolution of vertically oriented lamellar morphology in CZA-S by GISAXS Real-time structure evolution of the l-BCPs forming vertical lamellae under the effect of CZA-S gradient by GISAXS yields novel insights into the molecular ordering dynamics associated with


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this process. Analogous to recently demonstrated in-situ vertical ordering of 1 µm thick c-BCP films under dynamic thermal gradient CZA-S and postulating an energy-based ordering pathway for the process,65 we illustrate the ordering dynamics pathway in vertically oriented 100 nm thin l-BCP films. Notably the main difference is that the l-BCPs do not lose much orientation in the shallow gradient region in the vicinity of Tmax, presumably due to the low residence time of the lBCP in this region providing higher stability of lamella to re-orientation. In-situ GISAXS was performed at an incident angle (θ = 0.15°) above the polymer film critical angle (θc,BCP = 0.115° for the photon energy of 10.82 keV) using the same in-situ CZA-S setup used in our c-PS-PMMA study.65 The X-ray (λ = 1.1458 Å) exposure time was minimized to 0.2 s for each measurement to avoid beam damage to the sample. Data was recorded every 1 mm (50 s at 20 µm s–1) of sample translation along the gradient zone from 75 °C on the heating side to 58 °C on the cooling side (below the Tg of both, PS and PMMA blocks on each side) as shown in Figure 5a. Precise correlation between GISAXS image data point and instantaneous local spot temperature under CZA-S was enabled by imaging T-profile with an IR camera. CZA-S exposure timescales above Tg of both blocks for ordering of l-BCPs was documented. Integrated intensity line cuts for each GISAXS image along the sample plane qy and corresponding line cuts as a function of CZA-S temperature are illustrated in Figure 5b. For the purpose of focusing on the evolution of structure, the line cut intensities are scaled arbitrarily. A plot illustrating CZA-S annealing time and corresponding temperature within the gradient is shown in Figure 5e for convenience. We observe four regimes of ordering of l-BCP by CZA-S along the spanned temperature range as described below, similar to previously documented c-BCP ordering.65


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Regime 1 – Quenched-Disordered state (25 °C < T < 150 °C): As the l-BCP thin film sample is prepared by flow coating, the rapid solvent evaporation leads to a kinetically trapped weakly microphase separated structure. This trapped, poorly ordered structure in the as-cast 100 nm thin film is indicated by a very weak scattering pattern until the l-BCP sample reaches the Tg on the heating side of the gradient. Once the sample crosses the Tg of both the blocks (Tg for the PS block is 107 ºC and for the PMMA block is 120 ºC), we start to observe a weak, broad scattering peak indicating the evolution of origins of a vertically lamellar microdomain structure due to the mobility imparted to the polymer chains at temperatures above Tg. Regime 2 – Initial Vertical Ordering State (150 °C < T < 190 °C): As the sample moves further into the heating zone along the linear sharp gradient, we observe the appearance of an isotropic ring that eventually begins to fade accompanied by the sharpening of the peak in the qy plane. This region is the most crucial zone of the CZA-S assembly. The CZA-S utilizes the linear region on the heating front with a high thermal gradient at temperatures lower than 190 °C to induce vertical microdomain orientation in l-BCPs by virtue of the transient vertical thermal induced strain gradient effect in the film, analogous to short time oven annealing observations reported in Figure 3(b-d). Regime 3 – Strain Relaxation to Polygrain State (190 °C < T > 190 °C): As the sample traverses the sharp ∇T zone and approaches Tmax (~ 205 °C), it reaches a broad temperature gradient (∇T ~

0-20 °C mm–1) zone extending from about 190 °C on the heating side to about 190 °C on the

cooling side. In this regime, the sample resides only for about 54 s and the qy peak is the only prominent peak, with a decreased full width at half maximum (FWHM i.e. peak width). However, the peak is still broad with a low intensity suggestive of a mixture of different


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alignments of poorly developed lamellar domain spacing with ill-defined grains. Once the ordering has occurred on the sharp ∇T side, the high energetic barrier to transition to another

morphology prevents the l-BCP from readily flipping or changing its orientation. This leads to

trapping of the vertically oriented l-BCP microdomains in a metastable state. In our previous study based on in-situ structure evolution of c-BCPs by CZA-S,65 we had identified Regime 3 as the one dominated by preferential substrate wetting induced ordering that leads to horizontally oriented cylinders near the substrate owing to a broad ∇T. However, in this case of l-BCP systems, we do not observe any peak in the qz direction indicating the absence of any

horizontally oriented lamellae. This disparity in the ordering dynamics and structural orientation is attributed to the difference in the residence time of the sample in this zone. At the higher CZAS speeds, the l-BCP samples reside in this zone for a maximum time of 54 s (~1/4th of that for the c-BCPs that form parallel wetting layer) that prevents polymer chain wetting relaxation dynamics to dominate over the previously imposed vertical strain gradient even in this broad ∇T region.

Regime 4 – Grain Growth for Highly-Ordered State (190 °C > T > 107 °C): As the l-BCP thin film sample further crosses the Tmax region and enters the sharp gradient cooling zone, the Bragg peaks in the qy plane become progressively sharper suggesting an enhancement in the in-plane ξ of the vertically ordered lamellar domains. This implies that as the sample passes through the cooling zone, grain growth phenomenon takes place and the vertical lamellar grains get well aligned as the sample is being cooled at a constant rate. We observe that the FWHM decreases continuously until the sample reaches a temperature below Tg of both blocks, following which the GISAXS pattern does not evolve further and the structure is frozen. As mentioned in the previous section, ξ increases with time according to a power law given by ξ ~ tn at a fixed 15 ACS Paragon Plus Environment

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annealing temperature.66 In our case, a gradient cooling mechanism drives the grains to coarsen steadily as the sample vitrifies, as opposed to a sudden quench in traditional annealing methods. The grain coarsening process may be analogous to the one observed by Harrison et al.67 wherein small grains collapse into the grain boundaries between larger grains for annihilation of dislocations and coarsening of grains in spherical BCPs. In a similar way, as the l-BCP grains formed in the first three Regimes of CZA-S are subjected to gradient cooling, they undergo defect annihilation and grain growth mechanism in Regime 4. In order to quantify the ordering, we tracked the evolution of Bragg peak corresponding to qy = 0.0168 and obtained fits to the GISAXS intensity curves. The curve fits were then used to obtain FWHM values. Scherrer analysis was performed on the peak widths to calculate ξ (Figure 5c). Corrections are required in order to account for the grazing incidence experimental geometry by subtracting the broadening contributions arising from the X-ray energy bandwidth, instrumental beam divergence, geometric smearing of the beam along the beam direction and the detector pixel size.68 ξ = 2πK / Δq

(2)

Δq = (B/2)(4π/λ)cos(θ)

(3)

where, λ is the X-ray wavelength (1.1458 Å), θ is the Bragg angle, B is the peak width (FWHM of the Bragg peak), and K is the Scherrer constant (~ 0.94). The in-situ GISAXS final correlation length (61.39 ± 2.1 nm) for the 100 nm thin film is consistent with the correlation length calculated from the ex-situ surface AFM images (68.39 ± 7.2 nm), suggesting uniform vertical lamellar distribution throughout the film. Since AFM images correspond to the surface grain sizes whereas GISAXS measures bulk grain sizes, there is a minor variation in numbers. 16 ACS Paragon Plus Environment

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Additionally, GISAXS probes a much larger area of the sample (~0.2 mm x 8 mm) as compared to AFM. In addition to an increase in grain size, we also observe that the qy* value decreases from 0.0173 at Tmax to 0.0168 below Tg and remains constant thereafter (Figure 5d). This implies an increase in the Lo of the l-BCP system as the sample is being translated over the cooling zone which can be attributed to the increase in the interaction parameter χ (increased segregation power) on the cooling side of the CZA-S temperature gradient. (Note that this counters the thermal contraction upon cooling). Once the sample crosses Tmax, the χ value for our system increases from 0.036 to 0.038 as the temperature decreases from 205 °C to 107 °C (calculated based on ref 69 and plotted as Supporting Information Figure S3). For our l-BCP system, N = 646.45, therefore 23.27 < χN < 24.57 as the temperature decreases on the cooling front, corresponding to an intermediate segregation regime70,71 for which: ΔLo ~ aN(1/2 + a)(χ1a ‒ χ2a) + α(ΔT)Lo

(4)

ΔLo / Lo,205°C = ((χ107°C a ‒ χ205°C a) / χ205°C a) + α(ΔT)

(5)

ΔLo / Lo,205°C = 0.038 ‒ 0.007 = 0.031

(6)

where, a = average statistical segment length of PS and PMMA (0.71 nm), α = average linear coefficient of thermal expansion (70 x 10-6 °C-1).69 This implies an overall Lo change resulting from an increase in Lo due to the enthalpic contribution of the first term on the RHS and a decrease in Lo arising from the thermal contraction behavior of the BCP upon cooling, given by the second term on the RHS. The overall experimental value of % ΔLo/Lo (2.5 %) agrees well with the theoretically predicted value (3.1 %), attributing the increase in Lo to the increased χ that


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dominates over the thermal contraction effect. Therefore, in this regime, as the temperature decreases, χ increases and the peak positions (qy*) shift towards a lower q (higher Lo) until the domain spacing remains constant at Lo = 37 nm. This Lo is consistent with that obtained by traditional annealing methods. A similar trend has been observed in previous studies demonstrating a decrease in Lo with an increase in temperature72–74 where simulations by Forrey et al.75 predict the relaxation of BCP chains to a more Gaussian-coil conformation at these temperatures. As the sample is cooled below the Tg on the cooling side, it crosses Regime 4 and the decreased mobility of polymer chains restricts further grain growth and domain evolution as the structure does not undergo any further ordering. In conclusion, we have demonstrated a rapid one-step template free method to yield nearly full vertical ordering of lamellae in ultrathin PS-b-PMMA lamellar films through our dynamic thermal annealing technique, CZA-S. A sharp ∇T of 48 °C mm–1 is essential for achieving such a

high degree of vertical lamellar microdomain orientation. A 25% broadening of the CZA-S gradient instead induces parallel lamellar morphology. The high thermal gradient needs to be selectively tuned with CZA-S sweep rates for controlling the polymer chain relaxation dynamics for vertical order. Structural evolution mechanisms and ordering dynamics of l-BCP thin films

under the effect of CZA-S ∇T probed using in-situ GISAXS are reported for the first time, with four identified regimes leading to vertical ordering of l-BCP thin films and highlighting the role of each zone of the CZA-S assembly in nanostructure formation. With an increasing scientific and technological demand for through thickness vertically oriented lamellar BCP ultrathin films for etchable pattern transfer into substrates, this work demonstrates a facile roll-to-roll compatible processing technique for high degree of vertical orientation and provides


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fundamental insights into the molecular mechanisms and dynamics of ordering underlying the CZA-S process.


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Figure 1. (a) Schematic of the CZA-S process for the directed self-assembly of l-BCP thin films. (b, c) AFM images of CZA-S annealed PS-b-PMMA 100 nm thin films at varying ∇T: (b) 35 °C mm–1 and (c) 48 °C mm–1. (d, e) GISAXS images of Figure 1b and Figure 1c, respectively with corresponding (f) in-plane and (g) out-of-plane integrated intensity profiles.


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Figure 2. (a) AFM images of CZA-S annealed (Tmax = 210 °C, ∇T = 48 °C mm–1, sample translation velocity = 20 µm s–1) PS-b-PMMA 100 nm thin film subjected to sequential ablation from the top surface to give film thickness: (i)100 nm, (ii) 85 nm, (iii) 75 nm, (iv) 58 nm, (v) 40 nm and (vi) 34 nm. (b) GISAXS images indicating surface (θ < θc,BCP) and bulk (θ > θc,BCP) morphologies of the 100 nm thin film with angle of incidence θ as: (i) 0.14°, (ii) 0.16°, (iii) 0.18° and (iv) 0.2° such that increasing θ corresponds to deeper morphologies (closer to the substrate). (c) In-plane integrated intensity profiles of GISAXS images in Figure 2b. (d) Cross-sectional TEM image of the film showing through-film vertical lamellar morphology. (e) X-ray Reflectivity profile of the vertically oriented l-BCP film. Inset: SLD profile through the film thickness.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

Figure 3. AFM images of oven annealed 100 nm thin films of PS-b-PMMA at 210 °C at time intervals of 0 min, 2 min, 3 min, 4 min, 5 min, 10 min, 15 min, 30 min, 2 h and 24 h, respectively from a–j.


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Figure 4. (a) AFM images of 100 nm thin films of PS-b-PMMA with CZA-S sweep rates in the decreasing order i.e. increasing Tg,heat-Tg,cool annealing time from i–xii (Tmax = 210 °C, ∇T = 48 °C mm–1). (b) Histogram of changing vertical lamellae composition with CZA-S sweep rates.


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Figure 5. (a) Two-dimensional GISAXS scattering patterns at incident angle 0.15° (θc,BCP = 0.115°) showing in-situ nanostructure evolution of lamellar PS-b-PMMA 100 nm thin film swept over the CZA-S temperature gradient (∇T ∼ 48 °C mm–1) at a translation speed of 20 μm s–1. The intensity scale is constant for all images as indicated. Inset: Surface topography AFM micrograph of the in-situ annealed film. (b) One-dimensional integrated intensity profiles (taken along qy at qz = 0.025 Å–1) of in-situ GISAXS patterns shown in Figure 5a as a function of CZA-S temperature (q*vertical indicates peak position). The profile intensities are offset for clarity. (c) Plot showing the evolution of peak width (FWHM) and orientation correlation length (ξ) of vertically oriented lamellae with respect to CZA-S annealing time. (d) Plot of evolution of Lo within the CZA-S temperature gradient as obtained from the intensity profiles. (e) CZA-S annealing time and corresponding temperature within the gradient.


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ASSOCIATED CONTENT Supporting Information CZA-S thermal gradient profile plotted from the temperature profile captured by the IR camera, X-ray Reflectivity fit for the vertically oriented l-BCP film, image analysis for calculating the inplane orientation correlation length by AFM, graph showing variation of χ within the in-situ CZA-S thermal gradient, and selective etching of PMMA microdomains for creating block copolymer thin film membranes, are provided. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present address S.S. and G.S.: Intel Corporation, 2501 NW 229th Ave, 636 Hillsboro, OR 97124. A.K.: Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204. Notes The authors declare no competing financial interest. Acknowledgements


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This work was supported by the National Science Foundation (NSF) via Grant DMR - 1411046. GISAXS measurements were carried out at the Advanced Photon Source, which is supported by the U.S. Department of Energy (DOE) Office of Science User, under Contract No. DE-AC0206CH11357. Research was also carried out at the Center for Functional Nanomaterials (CFN) and the National Synchrotron Light Source, Brookhaven National Laboratory, which are supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. The authors are grateful to Ed Laughlin for help with fabricating the customized CZA-S assembly. M.N.B. would like to thank Dr. Guangcui Yuan and Dr. Sushil K. Satija for help with the X-Ray Reflectivity measurements at National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) and Kim Kisslinger for help with the cross-sectional TEM imaging at CFN. M.N.B. would also like to thank Dr. Arvind Modi, Dr. Sarang Bhaway, Dr. Danielle Grolman, Namrata Salunke and Asritha Nallapaneni for fruitful discussions. References 1.

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Figure 1. (a) Schematic of the CZA-S process for the directed self-assembly of l-BCP thin films (b, c) AFM images of CZA-S annealed PS-b-PMMA 100 nm thin films at varying ∇T: (b) 35 °C mm–1 (c) 48 °C mm–1 (d, e) GISAXS images of Figure 1b and Figure 1c, respectively with corresponding (f) in-plane and (g) out-ofplane integrated intensity profiles 215x297mm (96 x 96 DPI)

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Figure 2. (a) AFM images of CZA-S annealed (Tmax = 210 °C, ∇T = 48 °C mm-1, sample translation velocity = 20 µm s-1) PS-b-PMMA 100 nm thin film subjected to sequential ablation from the top surface to give film thickness (i)100 nm (ii) 85 nm (iii) 75 nm (iv) 58 nm (v) 40 nm (vi) 34 nm (b) GISAXS images indicating surface (θ < θc,BCP) and bulk (θ > θc,BCP) morphologies of the 100 nm thin film with angle of incidence θ as (i) 0.14° (ii) 0.16° (iii) 0.18° (iv) 0.2° such that increasing θ corresponds to deeper morphologies (closer to the substrate) (c) in-plane integrated intensity profiles of GISAXS images in Figure 2b (d) Cross-sectional TEM image of the film showing through-film vertical lamellar morphology (e) X-ray Reflectivity profile of the vertically oriented l-BCP film. Inset: SLD profile through the film thickness 217x297mm (96 x 96 DPI)


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Figure 3. AFM images of oven annealed 100 nm thin films of PS-b-PMMA at 210 °C at time intervals of 0 min, 2 min, 3 min, 4 min, 5 min, 10 min, 15 min, 30 min, 2 h and 24 h respectively from a - j 261x132mm (96 x 96 DPI)


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Figure 4. (a) AFM images of 100 nm thin films of PS-b-PMMA with CZA-S sweep rates in the decreasing order (Tmax = 210 °C, ∇T = 48 °C mm-1) (b) Histogram of changing vertical lamellae composition with CZA-S sweep rates 276x289mm (96 x 96 DPI)


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Figure 5. (a) Two-dimensional GISAXS scattering patterns at incident angle 0.15° (θc,BCP = 0.115°) showing in-situ nanostructure evolution of lamellar PS-b-PMMA 100 nm thin film swept over the CZA-S temperature gradient (∇T ∼ 48 °C mm-1) at a translation speed of 20 µm s-1. The intensity scale is constant for all images as indicated. Inset: Surface topography AFM micrograph of the in-situ annealed film (b) One-dimensional integrated intensity profiles (taken along qy at qz = 0.025 Å−1) of in-situ GISAXS patterns shown in Figure 5a as a function of CZA-S temperature. The profile intensities are offset for clarity (c) Plot showing the evolution of peak width (FWHM) and orientation correlation length (ξ) of vertically oriented lamellae with respect to CZA-S annealing time (d) Plot of evolution of Lo within the CZA-S temperature gradient as obtained from the intensity profiles (e) CZA-S annealing time and corresponding temperature within the gradient 492x367mm (96 x 96 DPI)


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For TOC Only 34x17mm (300 x 300 DPI)


Through-Thickness Vertically Ordered Lamellar Block Copolymer Thin

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