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Apr 29, 2016 - domain spacing was found to be nearly identical for thermal and solvent ... In this regime, the domain spacing in solvent annealed film...
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Controlling Domain Spacing and Grain Size in Cylindrical Block Copolymer Thin Films by Means of Thermal and Solvent Vapor Annealing Xiaodan Gu,†,# Ilja Gunkel,†,‡,# Alexander Hexemer,‡ and Thomas P. Russell*,†,§ †

Polymer Science and Engineering Department, University of Massachusetts at Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States ‡ Advanced Light Source and §Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Real-time grazing-incidence small-angle X-ray scattering (GISAXS) experiments were used to study the selfassembly of cylinder-forming block copolymers (BCPs) in thin films during thermal annealing and solvent vapor annealing. BCP thin films were annealed in near-neutral solvent vapor for solvent vapor annealing and on a hot plate under an inert gas atmosphere for thermal annealing. The initially ordered films were heated or swollen to induce an order−disorder transition (ODT) and then cooled or the solvent was removed, respectively. The domain spacings of BCPs as determined from in situ GISAXS measurements during solvent removal and cooling were analyzed with respect to the polymer concentration and the reciprocal temperature. Close to the ODT the domain spacing was found to be nearly identical for thermal and solvent vapor annealing. At lower solvent concentrations ϕ and lower temperatures T, the domain spacing was found to increase for both thermal and solvent vapor annealing until structural reorganization in the film was limited by the slow kinetics at solvent concentrations and temperatures close to the glass transition. In this regime, the domain spacing in solvent annealed films was found to be higher than that in thermally annealed films, which is likely due to a significantly smaller diffusion coefficient in the case of thermal annealing. On the basis of an ex situ scanning electron microscopy characterization of annealed block copolymer thin films, we show that the grain size of the cylindrical microdomains can be strongly increased by annealing films close to the ODT. Well below ϕODT and TODT the formation of large grains is kinetically limited. In thermally annealed films the grain size was found to be smaller than that for the solvent annealed films, which was attributed to a smaller diffusion coefficient in the absence of solvent.

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decays. To use BCPs thin films as lithography masks, film thicknesses that are only several periods of the BCP microdomains are required. To obtain a uniform BCP thin film with controlled film thickness over a large area, BCPs are typically spin-coated from solution onto a flat substrate. This rather simple sample preparation process, however, creates a problem: the fast solvent evaporation traps the BCP microdomains into a nonequilibrium and poorly ordered state. To enhance lateral ordering of the BCP microdomains, these films are annealed thermally, in a solvent vapor at room temperature or in a solvent vapor at elevated temperatures. During annealing, the mobility of polymer chains is increased allowing for a rearrangement of microdomains and annihilation of defects.3

INTRODUCTION One of the biggest challenges the semiconductor industry is facing is the difficulty to further reduce the size of transistors by using industrial standard “top-down” photolithographic techniques.1−8 To overcome this challenge, “bottom-up” approaches using the self-assembly of block copolymers (BCPs) have been recognized in the International Technology Roadmap for Semiconductors (ITRS) as a promising strategy to extend Moore’s law.1−10 In the simplest case of a linear diblock, BCPs can self-assemble into various morphologies including spheres, cylinders, gyroid, and lamellae depending on the volume fraction of the two blocks. The size of the domains, ranging from several to a few tens of nanometers, can be simply controlled by varying the molecular weight of the BCP.11 Self-assembled BCP microdomains can serve as highresolution etch masks for pattern transfer to other functional materials.12 Bulk BCP samples with ordered microdomains are not suitable for such pattern transfer processes since the registry of microdomains in the out-of-plane direction rapidly © XXXX American Chemical Society

Received: February 28, 2016 Revised: April 25, 2016

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DOI: 10.1021/acs.macromol.6b00429 Macromolecules XXXX, XXX, XXX−XXX

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solvent vapor. Figure 1c shows a SVA setup, where the solvent reservoir is located inside the chamber and a controlled flow of inert gas is used to adjust the vapor pressure inside the chamber. While this method allows for adjustable vapor pressures over a wider range, it is more difficult to control the rate of solvent removal as compared to the setup shown in Figure 1b. SVA chambers can also be heated to further manipulate the amount of swelling of the BCP film and the BCP chain dynamics.22 Other variants include independent temperature control of the sample and solvent reservoir,23 control of humidity,24 or use of multiple solvents.25 Sample chambers have been equipped with interferometers26 to measure the change in film thickness, with windows to enable grazing incidence small-angle X-ray scattering (GISAXS)27−30 to characterize the lateral ordering of the BCP microdomains in situ and atomic force microscopes to characterize changes in the film surfaces during annealing.31 Both thermal annealing and SVA have been widely used to anneal BCP films to form well-ordered BCP microdomains. Detailed studies that directly compare both techniques by real time techniques, however, have not been reported. Here, we studied both thermal annealing and SVA of polystyrene-blockpoly(2-vinylpyridine) (PS-b-P2VP) (Mn = 51 kg/mol) BCP thin films by real-time GISAXS. We further studied PS-b-P2VP (Mn = 51 kg/mol and Mn = 90 kg/mol) BCP thin films after thermal annealing and SVA by means of ex situ scanning electron microscopy (SEM).

Thermal annealing, being a traditional heat treatment in metallurgy to remove defects, has also been applied to BCPs to enhance the ordering in as-spun films. BCP thin films are heated above the glass transition temperatures (Tg) of the blocks, then held at this elevated temperature for an extended time period, and then cooled to room temperature. PS-bPMMA BCPs, for example, were reported to form ordered microdomains in thin films after annealing at 160 °C for 1 day.9,13 Heating the BCP to the desired temperature can be realized by placing the sample inside a temperature-controlled vacuum oven, on a hot plate, by using radiation heating devices, or even with laser pulses.2,14 Solvent vapor annealing (SVA) also is a very useful technique to anneal BCP thin films, particularly for high molecular weight BCPs, where low chain mobility and thermal stability impede lateral ordering of the microdomains.15 Although SVA has a much shorter history than thermal annealing, it quickly became popular after Kim and Libera,16 and later, Russell and coworkers showed that annealing cylinder-forming polystyreneblock-poly(ethylene oxide)s (PS-b-PEO) BCPs in benzene vapor generates highly ordered arrays of hexagonally packed PEO microdomains with grain sizes of several microns.17−19 In general, there are three different ways of introducing solvent vapor into the atmosphere of the annealing chamber (illustrated in Figure 1). In the first approach, the solvent is

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EXPERIMENTAL SECTION

Materials and Sample Preparation. PS-b-P2VP BCPs with number-average molecular weights Mn and P2VP volume fractions f P2VP of Mn = 23.6-b-10.4 kg/mol and f P2VP = 0.289 as well as Mn = 40b-18 kg/mol and f P2VP = 0.293 as provided by the supplier were purchased from Polymer Source Inc. and used without further purification. The molecular weights of the samples were determined by gel permeation chromatography (GPC) using a polystyrene standard. The number-average molecular weight of the lower molecular weight PS-b-P2VP BCP (Mn = 23.6-b-10.4 kg/mol) is Mn = 51 kg/mol, and that of the higher molecular weight PS-b-P2VP BCP (Mn = 40-b-18 kg/mol) is Mn = 90 kg/mol. The corresponding degrees of polymerization are N = 488 and N = 861, respectively. Anhydrous tetrahydrofuran (THF), toluene, and ethanol were purchased from Sigma-Aldrich and used without further purification. PS-b-P2VP BCPs were dissolved in a mixture of THF and toluene (volume fraction 1:4) to form a 1.5% w/v stock solution. The polymer solutions were spin coated onto plasma cleaned silicon wafers with a native oxide layer to produce uniform thin films having a film thickness of 65 nm. Grazing Incidence X-ray Scattering (GISAXS). GISAXS measurements were performed at Beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, at a constant Xray energy of 10 keV.32 The exposure time to collect each scattering image was 20 s. After two exposures, the sample was moved to a fresh area to minimize beam damage of the sample. The incident angle between X-rays and the sample surface was fixed at 0.14°. Scattering profiles were recorded with a PILATUS 1M detector (Dectris). The sample-to-detector distance was calibrated using a silver behenate standard. Real-Time Measurements of BCP Thin Films during Solvent Vapor Annealing. In order to perform real-time in situ GISAXS experiments during SVA, the BCP sample was placed inside a customdesigned solvent annealing chamber that has been described elsewhere.28 A Filmetrics F20 spectroscopic white light reflectometer was used for film thickness measurements during SVA. Please note that the SVA setup used here corresponds to the one shown in Figure 1c. During swelling the film thickness increased from 65 to 170 nm and then slowly decreased to 65 nm during controlled deswelling at a rate of ∼4.5 nm/min. The experiments were performed at 23 ± 1 °C.

Figure 1. Typical solvent vapor annealing setups. (a) Solvent reservoir inside a closed jar. (b) Solvent vapor delivery by means of a flowing carrier gas (c) Solvent vapor annealing via solvent reservoir inside a chamber and counterflow of inert gas. Blue color represents the BCP thin film, and red color represents the organic solvents.

kept inside a solvent reservoir and placed inside a sealed chamber together with the BCP sample (Figure 1a). Here, the amount of solvent being put in the reservoir, the surface area of the solvent, the volume of the annealing chamber, temperature, humidity, and annealing time determine the vapor pressure inside the chamber and, therefore, the amount of swelling of a BCP thin film. Such a closed chamber only offers indirect control over the solvent vapor pressure within the chamber via a change of the ratio between the surface area of the liquid solvent and the chamber volume.20 Another drawback is that the rate of solvent removal can only be controlled through changing the evaporation rate of the solvent by introducing leaks of different sizes.21 Figure 1b shows a SVA setup, where an inert carrier gas bubbles through a solvent reservoir and then into the annealing chamber. The partial vapor pressure inside the annealing chamber can be controlled by mixing this gas that is saturated with solvent with a second stream of pure inert gas. The relative flow rates of the two gas streams control the vapor pressure of the gas entering the annealing chamber. Subsequent purging of the chamber with pure inert gas at a controlled flow rate allows for a controlled solvent removal. A disadvantage of this technique is that the highest achievable vapor pressure of solvent inside the annealing chamber is lower than in the case of the first approach, since the inert carrier gas dilutes the B

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Figure 2. (a) Representative scattering images of a PS-b-P2VP BCP thin film (Mn = 51 kg/mol) during SVA in THF at different annealing times. (b) In-plane scatting profiles at different annealing times. (c) Domain spacing, fwhm, and film thickness as a function of the annealing time. Real-Time Measurements of BCP Thin Films during Thermal Annealing. In situ thermal annealing of BCP thin films was carried out using a custom-made chamber that is equipped with a heating stage. The temperature of the heating stage was controlled by a feedback temperature controller (Watlow EZ-Zone PM). An inert nitrogen atmosphere prevented the PS-b-P2VP BCP from degradation at elevated temperatures. Kapton windows at the front and back of the chamber allowed X-rays to enter and exit the chamber for the GISAXS experiments. PS-b-P2VP BCP thin films were heated to 290 °C at a heating rate of 10 °C/min, held at 290 °C for 5 min, and then cooled down to room temperature at a cooling rate of 10 °C/min. GISAXS Data Reduction. The data reduction of the scattering images was performed using “Nika”, an Igor (Wave Metrics Inc.) based software package.33 The 2-D scattering data were reduced to 1-D scattering profiles by slicing the 2-D scattering pattern in the in-plane (horizontal) direction close to the reflected beam resulting in plots of the scattering intensity as a function of the scattering vector. The inplane scattering profiles were then fit to Gaussian functions to obtain peak position and fwhm. The background was accounted for in the fitting function as described previously.28 Surface Structure Characterization. The surface structure of the BCP thin films was characterized by a Zeiss Ultra 60 field emission scanning electron microscope (SEM) at the Nanofabrication Facility of the Molecular Foundry, Lawrence Berkeley National Laboratory. Before imaging, the BCP samples were reconstructed in ethanol for 10 min at room temperature and then etched in an oxygen/argon plasma in a reactive-ion etcher for 10 s (10 mTorr, 75 W in Plasma Lab 150 by Oxford Inc.) to enhance the contrast between different blocks.34 Ex Situ Solvent Vapor Annealing. Ex situ SVA experiments of PS-b-P2VP (Mn = 51 and 90 kg/mol) BCP thin films were carried out

using the same SVA chamber as for the in situ SVA experiments. The BCP thin films were swollen to different swelling ratios and annealed for 1 h followed by an instantaneous removal of the solvent vapor. This rapid removal of the solvent vapor was achieved by opening the chamber top cover. The time scale of removal of the solvent is of order 1 s, as noted by the color change in the sample. The surface structure of the annealed BCP thin films was imaged by SEM after reconstruction of the samples in ethanol and subsequent etching in oxygen plasma as described above. Ex Situ Thermal Annealing. Ex situ thermal annealing experiments of PS-b-P2VP (Mn = 51 and 90 kg/mol) BCP thin films were carried out using the thermal annealing chamber described above. The samples were heated to different temperatures and annealed for 1 h. The BCP films were subsequently slowly cooled to room temperature at a cooling rate of 10 °C/min. After thermal annealing, all samples were reconstructed in ethanol and etched in oxygen plasma before imaging by SEM.

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RESULTS AND DISCUSSION In Situ GISAXS during Solvent Vapor Annealing. Figure 2a shows representative 2D GISAXS images of a PS-b-P2VP BCP thin film (Mn = 51 kg/mol) during SVA in THF at different annealing times (corresponding to different amounts of swelling of the film). Detailed 1-D scattering profiles are shown in Figure 2b. The as-spun BCP sample (t0 = 0 min in Figure 2a,b) showed a diffuse reflection with a relatively large full width at halfmaximum (fwhm) ∼6.5 × 10−2 nm−1, which corresponds to a C

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Figure 3. (a) Representative scattering images of a PS-b-P2VP BCP thin film (Mn = 51 kg/mol) during thermal annealing. (b) In-plane scatting profiles at different annealing times (annealing temperatures). (c) Domain spacing, fwhm, and film thickness of the BCP as a function of the annealing time.

an increase of χeff as a result of the lower solvent concentration. At 20 min, the BCP underwent a disorder−order transition as indicated by a sharp drop in the fwhm. χeff continued to increase as solvent was further removed from the swollen film; thus, an increase in the domain spacing was observed until the film vitrified when the Tg of the swollen BCP film was close to room temperature.28 Note that in the GISAXS image corresponding to t7 = 32.5 min (after drying) in Figure 2a the Bragg reflections are stretched in the qz direction compared to the image at t6 = 21.0 min (before drying), indicating that the film thickness decreased (along with corresponding changes in the packing of microdomains normal to the plane of the film) during the drying process. After SVA, the dry film showed ordered cylindrical microdomains with a periodicity of 27.5 nm. In Situ GISAXS during Thermal Annealing. Figure 3a shows representative GISAXS images of PS-b-P2VP BCP thin films (Mn = 51 kg/mol) at different temperatures during thermal annealing. Note that in Figure 3a (see images corresponding to t2, t3, ..., t7) the Bragg reflection does not show any significant stretching in the qz direction at the different temperatures, as observed in the case of the solvent annealed sample in Figure 2a, indicating that the thermally annealed film does not exhibit any significant thickness changes and corresponding changes in the out-of-plane scattering during annealing.

persistence length of 87 nm, as determined by a Scherrer analysis, indicating the sample only had short-range lateral order. The amount of swelling of the thin film was characterized by the swelling ratio (SR), the thickness of the swollen film divided by that of the as-spun (dry) film. After the thin film was swollen to a thickness of 78 nm or an SR of 1.2, drastic changes in both the domain spacing and fwhm were observed due to a significant reduction of the Tg of the swollen BCP, giving the copolymer chains enough mobility to rearrange.28 As can be seen in Figure 2c between 0 and 10 min the domain spacing initially increased, and then it decreased. The drop of the domain spacing is due to the reduction of the effective Flory−Huggins parameter (χeff), as the neutral solvent screens the unfavorable interactions between the different blocks. THF is essentially neutral for PS and P2VP as shown previously based on homopolymer swelling experiments.28 Increasing the SR further led to a continuous decrease of χeff. After ∼10.5 min, at a film thickness of about 140 nm (or SR = 2.1), the BCP underwent an order− disorder transition, as indicated by the sharp increase in the fwhm in Figure 2c. The BCP remained in the disordered state from 10 to 19 min (Figure 2b) as indicated by a broad peak, with the fwhm being larger than 8 × 10−2 nm−1. After 14 min, when the removal of solvent vapor was initiated, the values of the domain spacing increased and the fwhm decreased due to D

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Figure 4. (a) Domain spacing d and fwhm from Figure 2c as a function of the polymer concentration ϕBCP during deswelling. (b) Domain spacing d and fwhm from Figure 3c as a function of the reciprocal temperature 1000/T during cooling. The top axis in (b) shows the corresponding χN values that were calculated according to eqs 2 and 3 using N = 488. The vertical dashed lines in (a) and (b) mark the order−disorder transition (ODT) and the onset of a plateau in the domain spacing (vitrification).

discontinuously at the ODT, in the case of SVA it jumped to a TA larger value: dSVA ODT = 25.1 nm vs dODT = 24.5 nm. The domain spacing continued to increase upon deswelling and cooling while remaining larger in the solvent annealed films. This increase can be explained as follows. At lower solvent concentrations and lower temperatures the unfavorable interactions between different blocks are increased. This leads to an increased chain stretching which, in turn, results in an increase of the domain spacing. Figures 4a and 4b also show that the domain spacing reached a plateau at ϕvitr BCP ∼ 0.71 and 1000/Tvitr ∼ 2.30 K−1 (Tvitr = 162 °C), respectively. At polymer concentrations and temperatures close to the glass transition the segmental mobility of the chains is reduced. This leads to an effective vitrification of the film when the time required for structural reorganization becomes larger than the time scale of the GISAXS experiment. Interestingly, the domain spacing is TA significantly larger for SVA, dSVA vitr = 27.4 nm, than for TA, dvitr = 26.0 nm, which suggests a higher segmental mobility in the case of SVA. While it had been shown previously that reducing the cooling rate leads to an increase of dTA vitr, the resulting change is expected to be considerably smaller than the difference between SVA 35 dTA vitr and dvitr . For a comparison of the results of the in situ GISAXS experiments during SVA and TA shown in Figure 4 in terms of physical parameters, one can introduce an effective interaction parameter χeff. For a BCP in a neutral good solvent χeff is given by the modified dilution approximation:36

Detailed in-plane scattering profiles are shown in shown in Figure 3b at the different annealing times (corresponding to different temperatures). Apart from the differences in the outof-plane scattering the GISAXS images during thermal annealing in Figure 3a are similar to those during solvent vapor annealing in Figure 2a. Starting from broad Bragg rods (fwhm of 9.0 × 10−2 nm−1) for the as-spun morphology the microstructure changed upon heating above the glass transition temperature of both blocks (Tg ∼ 100 °C), as evidenced by changes in the domain spacing and fwhm (shown in Figure 3c). While the domain spacing initially increased, for temperatures above ∼150 °C the domain spacing was found to decrease. The decrease in the domain spacing was accompanied by a significant narrowing of the primary scattering peak. Similar to SVA, the reduction of the domain spacing is a result of the reduced Flory−Huggins parameter χ at higher temperatures. The PS-b-P2VP BCP thin film with a molecular weight of 51 kg/mol underwent an order−disorder transition (ODT) after 23 min at about 280 °C as indicated by a significant increase of the fwhm to a value of 7.5 × 10−2 nm−1. After being held at 290 °C for 4 min, the temperature was decreased at t = 26 min. During the process of cooling, the sample underwent a disorder−order transition. The domain spacing increased due to the increased value of χ at lower temperatures (Figure 4 c). For temperatures below 160 °C, the domain spacing was found to remain unchanged at a value of 26.1 nm within the times scale of the experiment as a result of the lower segmental mobility for temperatures closer to the glass transition of the BCP. In Situ GISAXS during Annealing: Comparison between Thermal and Solvent Vapor Annealing. Figures 4a and 4b show in situ GISAXS data obtained during deswelling and cooling PS-b-P2VP BCP thin films (Mn = 51 kg/mol) from the disordered state. To facilitate the observation of the ODT, the data are shown as a function of the polymer concentration ϕBCP and the reciprocal temperature 1000/T, respectively. As can be clearly seen in Figures 4a and 4b the films underwent an ODT during both SVA and thermal annealing (TA) as indicated by the decrease of the fwhm at ϕODT BCP = 0.454 and 1000/TODT = 1.80 K−1 (TODT = 282 °C). The fwhm remained nearly constant in the ordered state with only a slight increase at ϕBCP ∼ 0.65 in the case of SVA. Comparing the domain spacing for TA and SVA reveals similar values of d ∼ 24 nm only in the disordered state close to the respective ODT’s. While for both SVA and TA the domain spacing increased

β χeff = χϕBCP

(1)

where χ denotes the Flory−Huggins parameter of the neat BCP and ϕBCP denotes the volume fraction of the BCP in the swollen state. The scaling exponent β is given by the modified dilution approximation and varies between 1.3 and 1.6 for BCPs in (near-) neutral solvent.36,37 The Flory−Huggins parameter for (neat) PS-b-P2VP BCPs can be described by the following equation:38 χ = ( −1.79110−4 + 0.478/T )Vref

(2)

Vref denotes a temperature-dependent reference volume defined as follows: Vref = [vPSMSvP2VPM 2VP]1/2

(3)

where vPS and vP2VP denote the specific volumes of PS and P2VP, and MS and MVP are the molar masses of styrene and 2E

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Figure 5. Colorized grains of cylindrical microdomains oriented parallel to the substrate in SEM images of PS-b-P2VP BCP 65 nm thin films after annealing in THF at different swelling ratios (SRs) for 1 h and subsequent instantaneous solvent removal (a)−(d): Mn = 51 kg/mol, (a) SR = 1.30, (b) SR = 1.50, (c) SR = 1.74, (d) SR = 1.94. (e)−(h): Mn = 90 kg/, (e) SR = 1.50, (f) SR = 1.71, (g) SR = 2.00, (h) SR = 2.54. The grains were colorized according to the orientation of cylindrical microdomains.

vinylpyridine, respectively. In the case of TA, where χeff ≡ χ, the interaction parameter can be easily calculated according to eqs 2 and 3 using tabulated values for vPS and vP2VP for the calculation of the reference volume Vref.38 For thermally annealed PS-b-P2VP BCPs at the ODT, Vref = 106.61 cm3 mol−1 and χNODT = 35.5 for N = 488 using TODT = 282 °C as determined from the in situ GISAXS data shown in Figure 4b. The values of χeffNODT for solvent annealed films can be calculated according to eqs 1, 2, and 3 using β = 1.3 given by the modified dilution approximation and ϕODT = 0.454 as determined from Figure 4a. For solvent annealed PS-b-P2VP BCPs we obtain χeffNODT = χ(T)·174.8. To bring the χeffNODT values for SVA and TA into agreement, the temperature T would need to be −65 °C (for β = 1.6 we obtained T = −110 °C). However, the actual temperature during SVA experiments was T = 23 °C. This inconsistency is likely to arise from the temperature dependence of χ in eq 2 not being valid for T = 23 °C. Note that eq 2 was determined based on measurements of PS-b-P2VP BCPs for temperatures between 130 and 180 °C. Comparison of the solvent and thermally annealed PS-b-P2VP BCP films using an effective interaction parameter cannot be done until the interaction parameters are known precisely for temperatures and solvent concentrations applied during annealing. Furthermore, knowing the effective interaction parameter for solvent annealed films would also allow determining the kinetic effects on solvent annealing in terms of the diffusion coefficients of block copolymer chains in the presence of solvent.39,40 Ex Situ SEM Characterization after Thermal or Solvent Vapor Annealing. Figure 5 shows SEM images of PS-b-P2VP (Mn = 51 and 90 kg/mol) BCP thin films after ex situ annealing in THF for 1 h at different swelling ratios, followed by instantaneous solvent removal, reconstruction in ethanol, and etching in oxygen plasma. As can be seen from the SEM images in Figure 5 the grain size of the annealed block copolymer microdomain structure increased with increasing SR until the sample disordered (Figure 5d). Comparing PS-b-P2VP having different total molecular weight at the same swelling ratio, e.g., Figures 5c and 5f, we found a significantly larger grain size in the case of the smaller molecular weight, which is likely due to

faster diffusion for lower molecular weight BCPs. Note that the persistence length of the annealed film increased only slightly at longer annealing times (see Supporting Information, Figure S1). Figure 6 shows SEM images of PS-b-P2VP (Mn = 51 and

Figure 6. SEM images of PS-b-P2VP BCP thin films (Mn = 51 kg/mol and Mn = 90 kg/mol) after thermal annealing at different temperatures and subsequent slow cooling to room temperature. (a) PS-b-P2VP with Mn = 51 kg/mol and (c) PS-b-P2VP Mn = 90 kg/mol after annealing at 190 °C for 1 h. (b) PS-b-P2VP with Mn = 51 kg/mol and (d) PS-b-P2VP with Mn = 90 kg/mol after annealing at 250 °C for 1 h.

90 kg/mol) BCP thin films after ex situ thermal annealing at different temperatures, subsequent reconstruction in ethanol, and etching in oxygen plasma. The lower molecular weight PSb-P2VP BCP (Mn = 51 kg/mol) showed considerably larger grains after annealing at a temperature of 250 °C as compared to annealing at 190 °C. The higher molecular weight PS-bP2VP BCP (Mn = 90 kg/mol) did not show any significant enhancement of the lateral ordering after annealing at 190 °C as compared to the as-spun film. Also, at a higher annealing temperature of 250 °C, the lateral ordering only slightly improved, the grain size being significantly smaller than that of PS-b-P2VP with a molecular weight of 51 kg/mol. At a F

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Figure 7. (a) Persistence length ξ as a function of the reciprocal temperature 1000/T for thermally annealed PS-b-P2VP thin films. The persistence lengths ξ for solvent annealed films are plotted versus the polymer volume fraction ϕBCP in (b) and (c). ξ was determined based on SEM images of PS-b-P2VP thin films (Mn = 51 kg/mol) after solvent annealing (black squares) and thermal annealing (red spheres) and of PS-b-P2VP thin films (Mn = 90 kg/mol) after solvent annealing (blue triangles) from Figures 5 and 6, respectively The vertical dashed lines mark the ODT as determined from in situ GISAXS measurements shown in Figure 4.

temperature of 250 °C, for a time of t = 1 h the PS-b-P2VP BCP diffusion length (D·t)1/2 is ∼10 μm for Mn = 51 kg/mol and ∼30 nm for Mn = 90 kg/mol. Thus, the higher molecular weight PS-b-P2VP BCP chains exhibit a considerable kinetic barrier to structural rearrangement with the diffusion length being only on the order of the BCP domain spacing, whereas the motion of the lower molecular weight BCP chains is not kinetically constrained at a temperature of 250 °C. The block copolymer diffusion coefficients D were calculated according to D = D0 exp[−γ(χNP2VP − (χNP2VP)ODT)] where D0 is the diffusion coefficient of polystyrene with the same degree of polymerization at the same temperature, NP2VP denotes the degree of polymerization of the P2VP block, and γ = 1.2.40 The diffusion coefficient of polystyrene was calculated according to the WLF equation, D = D∞(M,T) exp[(c1c2 ln 10)/(c2 + T − Tg)], where c1 = 13.7 and c2 = 48 K.41 The molecular weight M of the polystyrene homopolymer was accounted for by scaling the calculated values of the diffusion coefficients as D ∼ M−2.3.42 χNP2VP was calculated according to eqs 2 and 3 with NP2VP = 141 (Mn = 51 kg/mol) and NP2VP = 252 (Mn = 90 kg/mol). (χNP2VP)ODT = 10.25 and was calculated using TODT = 282 °C for NP2VP = 141 (Mn = 51 kg/mol) as determined from the in situ GISAXS data shown in Figure 4b. Note that the reconstruction of PS-b-P2VP thin films in ethanol and subsequent etching in oxygen plasma apparently were more effective for solvent annealed films than for thermally annealed films (see for example Figure S1b and Figure 6a). This makes visualization of differences in grain sizes much harder for thermally annealed samples. The difference in reconstruction efficiency for solvent and thermally annealed films is likely due to a smaller thickness of the glassy PS layer at the air interface after solvent vapor annealing in THF. THF, being a near-neutral solvent for PS and P2VP, mediates the differences in surface energies of the two blocks. The grain size was further characterized by determining the persistence length of the orientational order of the cylindrical microdomains. The persistence length ξ of the ordered microdomains was obtained from SEM images of annealed PS-b-P2VP BCP films as described by Harrison et al.43 Briefly, the method used a custom written Matlab (Mathworks Inc.) code to evaluate the local orientational parameter, Ψ(r) = e2iθ(r). The azimuthally averaged pair correlation function of the BCP pattern is then given by g(r) = ⟨Ψ(0)Ψ*(r)⟩. The correlation

function g(r) decreases exponentially with r. Here, the persistence length ξ was determined as the distance r where the correlation function decreases to 1/2 of its maximum value, i.e., g(r)/g(0) = 0.5. Figure 7 shows a plot of the persistence lengths ξ of thermally annealed and solvent annealed PS-b-P2VP BCP thin films as a function of the reciprocal temperature and the polymer concentration for the two molecular weights studied here. At the ODT ϕODT = 0.454 and TODT = 282 °C, respectively, for Mn = 51 kg/mol as discussed above and for Mn = 90 kg/mol ϕODT = 0.343 (data that are not shown here). As can be seen in Figure 7, the persistence length ξ was found to increase with increasing temperature and decreasing polymer concentration until the BCP disordered. This confirms previously published results of separate studies on the effect of thermal44 and solvent annealing45 on the lateral order in BCP thin films. In the disordered state, ξ was on the order of a single microdomain period. While annealing films close to the ODT resulted in the largest persistence lengths for both TA and SVA, ξ was found to be significantly larger in solvent annealed films. Extrapolating the persistence lengths in Figures 7a and 7b to the ODT gives ξODT ∼ 325 nm for TA, whereas ξODT ∼ 1880 nm in the case of SVA, confirming that the presence of solvent vapor facilitates the formation of larger grains. Closer to the glass transition (at higher polymer concentrations), ξ showed a weaker dependence on polymer concentration as can be seen primarily for the higher molecular weight polymer (Mn = 90 kg/mol) in Figure 7c. The change in the ξ scaling is likely due to stronger kinetic constraints to chain motion at lower solvent concentrations, where the system is closer to its glass transition, indicating a strong influence of the chain dynamics on the formation of BCP patterns with large grains.

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CONCLUSION Annealing of BCPs in thin films is necessary for the use of BCP self-assembly to fabricate nanostructured materials with lateral order. During annealing the mobility of polymer chains is enhanced, whether the film is annealed thermally or with a solvent, allowing for the formation, lateral ordering and coarsening of grains of BCP microdomains in thin films. We studied the effects of thermal annealing and (near-neutral) solvent vapor annealing on the formation of cylindrical G

DOI: 10.1021/acs.macromol.6b00429 Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy BES under Contract BES-DE-FG02-96ER45612. Beamline 7.3.3 of the Advanced Light Source is supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. X. Gu acknowledges an ALS Doctoral Fellowship program for providing partial financial support. I.G. acknowledges the support by the ALS Postdoctoral Fellowship program. A.H. was supported by a DOE Early Career Research Program grant. The authors also gratefully acknowledge the assistance of E. Schaible with GISAXS measurements.

microdomains oriented parallel to the substrate and observed a strong impact of the degree of chain mobility on the grain size of the cylindrical microdomains. On the basis of SEM imaging of BCP films after annealing, we characterized the grain size by determining the persistence length of orientational order of the cylindrical microdomains, which was found to increase with either increasing polymer concentration or temperature, with the largest grain sizes close to ODT. The absolute values of the persistence lengths, however, were found to be much larger for solvent annealed films than for thermally annealed films, which is likely due to faster chain dynamics in the presence of a solvent. These results suggest that increasing the temperature or increasing the solvent concentration is a very effective means for enhancing the lateral order of BCP microdomains in thin films. The coarsening of grains is considerably facilitated by the presence of a solvent, resulting in significantly larger grain sizes. Furthermore, the higher mobility of BCP chains in a solvent results in a larger window between ODT and glass transition for solvent vapor annealing to effectively anneal BCP films and to tune the domain spacing. Kinetic effects on the spacing of BCP microdomains due to differences in the chain dynamics are negligible at ODT, where the domain spacing is almost identical for solvent annealed and thermally annealed films. However, the domain spacing in films close to the glass transition was found to be significantly larger in the case of solvent annealing. Solvent annealing appears more efficient than thermal annealing for the fabrication of BCP thin films with longrange lateral ordering. The significant increase of the film thickness observed only during solvent annealing, however, is disadvantageous for applications of BCP patterns as lithography masks. In the swollen state BCP microdomains show high degree of order in the out-of-plane direction that significantly deteriorates upon removal of the solvent. The combination of thermal and solvent vapor annealing might allow for reducing the swelling in the out-of-plane direction and may also improve the packing of BCP microdomains normal to the film after solvent removal.

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REFERENCES

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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.6b00429. SEM images of PS-b-P2VP BCP thin films (Mn = 51 kg/ mol) annealed in THF for 2 and 14 h, respectively, showing that the persistence length only slightly increased at longer annealing times (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.P.R.). Present Addresses

X.G.: Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, CA 94305-4125. I.G.: Adolphe Merkle Institute, Chemin des Verdiers 4, 1700 Fribourg, Switzerland. Author Contributions #

X.G. and I.G. contributed equally.

Notes

The authors declare no competing financial interest. H

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