Probing Self-Assembly of Cylindrical Morphology Block Copolymer

Article pubs.acs.org/Macromolecules

Probing Self-Assembly of Cylindrical Morphology Block Copolymer Using in Situ and ex Situ Grazing Incidence Small-Angle X‑ray Scattering: The Attractive Case of Graphoepitaxy Mireille Maret,*,† Raluca Tiron,*,‡ Xavier Chevalier,¶ Patrice Gergaud,‡ Ahmed Gharbi,‡ Céline Lapeyre,‡ Jonathan Pradelles,‡ Vincent Jousseaume,‡ Guillaume Fleury,§ Georges Hadziioannou,§ Nathalie Boudet,∥ and Christophe Navarro¶ †

Université Grenoble Alpes, CNRS, SIMAP, F-38000 Grenoble, France Université Grenoble Alpes, CEA, LETI, , MINATEC Campus, F-38000 Grenoble, France ¶ Lacq Research Center, Arkema, 64170 Lacq, France § Laboratoire de Chimie des Polymères Organiques, Université Bordeaux-CNRS UMR 5629-ENSCPB, F-33607 Pessac, France ∥ Université Grenoble Alpes, CNRS, Institut Néel, F-38000 Grenoble, France ‡

S Supporting Information *

ABSTRACT: The self-assembly of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer (BCP) films, leading to hexagonal arrays of perpendicular PMMA cylinders in a PS matrix, and its ordering kinetics were investigated using ex situ and in situ grazing incidence small-angle X-ray scattering (GISAXS). The ex situ measurements have provided accurate information about the structural changes in self-assembled BCP films with multiple processing parameters (PMMA removal process, film thickness, annealing treatment). Such structural information proves essential since BCP films are envisioned as template materials for density multiplication in nanolithography. Moreover, the temperature-dependent GISAXS measurements indicate that phase-separation starts around 140 °C and annealing up to 240 °C is required to form homogeneous self-assembled films within the short baking times required by industry. Furthermore, the guiding effect of an hydrogen silsesquioxane (HSQ) line grating commensurable with the polymer period on self-assembly (the so-called graphoepitaxy approach) was studied on silicon wafers presenting jointly patterned and unpatterned areas. When the line grating is aligned with the direction of the incident X-ray beam, the GISAXS pattern of the graphoepitaxial BCP film reveals the formation of a single domain extended over large areas. In contrast, the pattern of the BCP film self-assembled on the neighbor unpatterned area, comes from randomly oriented domains. Besides, the line grating does not only guide the BCP self-assembly, but as shown, also increases its ordering kinetics, demonstrating the high potentialities of graphoepitaxy.

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INTRODUCTION Self-assembly of block copolymers (BCP) is a powerful and low-cost route to generate templates for the fabrication of nanoscopic elements for CMOS industry as well as for ultrahigh density magnetic storage media. By tuning the composition and molecular weight of BCP, microdomains arrays with cylindrical, lamellar, or spherical morpholophy are spontaneously formed, but their random orientations considerably reduce their potential applications. For 10 years, a huge research effort has been devoted to improve the long-range ordering of BCP self-assembly.1 Several methodologies for guiding BCP self-assembly were investigated, such as the use of reconstructed sapphire surfaces,2 or that of one or twodimensional periodic prepatterns generated by e-beam lithography surfaces.3−5 Coupling directed self-assembly with a first lithography level, the so-called graphoepitaxy method, allows the multiplication of the initial prepattern density with a possible resolution down to 10 nm. However, the different © 2014 American Chemical Society

steps required to generate BCP’s templates (thermal annealing, selective etching of one phase and pattern transfert by plasma) have to be compatible with the current processes used in the CMOS industry. Therefore, to implement graphoepitaxy in large scale industrial processes, a structural characterization of the self-assembled films not only at a nanometric level but also over large areas is required. Moreover the influence of the guiding pattern on the BCP self-assembly has to be investigated. In this way, the grazing incidence small-angle Xray (GISAXS) technique is well suited since it brings an accurate and statistical information about the size, the morphology of patterned features and their spatial distribution. Compared to atomic force macroscopy (AFM) and scanning electron microscopy (SEM), GISAXS has also the great Received: July 15, 2014 Revised: September 15, 2014 Published: October 7, 2014 7221

dx.doi.org/10.1021/ma501453k | Macromolecules 2014, 47, 7221−7229

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keV. The horizontal and vertical widths of the beam focused at the sample position were equal to 300 and 150 μm, respectively. The samples were positioned on a 6 circle-goniometer, allowing an accurate alignement of the patterned area with respect to the incident beam direction. The GISAXS patterns were recorded on a XPAD 2D pixel detector (960 × 560 pixels, 130 μm pixel size)22 placed at a distance of 3700 mm from the sample. The intense incident and reflected beams were stopped by a 2 mm wide Ta strip. A schematic view of the GISAXS geometry is shown in Figure 1. A photomultiplier with a

advantage to probe both the surface morphology and the internal structure of films. GISAXS is becoming an almost routine technique for studying lamellar and cylindrical diblock copolymers2,6−12 and recently cylindrical triblock copolymer using resonant soft X-ray scattering.13 Furthermore, in situ GISAXS measurements were implemented to record the structural and morphological changes during thermal annealing14,15 and in solvant vapor,16−20 and recently to study the selective growth of Co nanoparticles on lamellar BCP.21 In this work, we have investigated thin films of a polystyreneblock-poly(methyl methacrylate) (PS-b-PMMA) copolymer (BCP) self-assembled on unpatterned and patterned Si wafers using the GISAXS technique. The hexagonal array, consisting of perpendicular PMMA cylinders in a PS matrix, produces typical Bragg rods in the GISAXS patterns with an enhanced contrast after PMMA removal (as attested by the higher order rods). For unpatterned wafers, ex situ measurements have led to an accurate description of the changes in the cylindrical morphology with PMMA removal process, film thickness, thermal balance and complementary in situ measurements information about the ordering onset and change in the polymer period with annealing temperature. The graphoepitaxy approach was implemented through a hydrogen silsesquioxane (HSQ) line grating which guides the orientation of the hexagonal array. The optimization of the grating parameters with respect to the intrinsic polymer period was already studied from a detailed analysis of SEM images using Delaunay triangulation which allowed to locate the disclinations and quantify thus the number of defects.5 Nevertheless, such approach has a limited resolution, is restricted to a few squared micrometer areas and concerns only the film surface. As reported in this paper, the GISAXS measurements on graphoepitaxy guided PS-b-PMMA films would allow to characterize a single hexagonally packed cylinder domain on large areas.

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Figure 1. Schematic view of the GISAXS geometry when probing directed self-assembly of a PS-b-PMMA copolymer film on an HSQ line grating.

EXPERIMENTAL SECTION

The polystyrene-block-poly(methyl methacrylate)(PS-b-PMMA) diblock copolymer used in this study has a number-average molecular weight equal to 46 kg/mol for PS and 21 kg/mol for PMMA with a dispersity index of 1.11. PS-b-PMMA was dissolved in propylene glycol monomethyl ether acetate and spin-coated on the top of a 6 nm-thick PS-r-PMMA brush layer with a styrene volume fraction, f PS, of about 70%, as previously described.5 A rapid thermal annealing up to 240 °C, followed by 10 min dwelling before cooling, leads to the phase separation into an hexagonal array of perpendicular cylinders of PMMA with a lattice period, L0, around 38 nm. The investigated copolymer films have thicknesses ranging between 30 and 60 nm. The line grating was generated with e-beam lithography using an HSQ (hydrogen silsesquioxane) negative tone resist spin coated on Si wafers. The period of the line array is equal to 248 nm with a line width, L, of 62 nm, a line height, h0, of 50 nm and a trench width, S, of 186 nm. These values are commensurable with the polymer period such as the spacing between two dense rows of cylinders, a0 equal to (√3/2)L0, leading to a quasi zero-defect block polymer self-assembly directed on the line grating.5 The patterned area with a size of 10 × 1 mm2 was centered with respect to the sample size (10 × 10 mm2), namely for the GISAXS measurements the lines of 10 mm length were precisely aligned with the X-ray beam direction. The whole surface of these samples was treated with the same brush layer. Thus, the morphology of the BCP self-assembled on the line grating could be directly compared with that observed on the unpatterned neighbor area. The grazing small-angle X-ray scattering measurements were performed on the CRG-BM02 beamline at the European Synchrotron Radiation Facilities (ESRF) in Grenoble using a photon energy of 9.8

removable kapton foil was placed before the beam stop chamber to adjust sample position and measure rough reflectivity curves to check the Si critical angle. To minimize air scattering the whole path of the scattered beam was under vacuum apart from the nearest region around sample. For the ex situ studies under air, the sizes of unpatterned and patterned wafers were of 20 × 10 mm2 and 10 × 10 mm2, respectively. In order to get rid of the PMMA degradation under beam exposure, most of samples were measured after removing PMMA which had also the advantage to enhance the contrast between the cylinders and the matrix since the electronic densities of PMMA (387 nm−3) and PS (340 nm−3) are very close. For the in situ GISAXS measurements during self-assembly of BCP, the samples were located under a polymer dome and heated under nitrogen. Nevertheless, the effect of the enhanced degradation of PMMA at temperatures around 200 °C was overcome using short counting times and changing regularly the irradiated area. The incidence angle was chosen between the critical angles of the polymer (0.13°) and the Si substrate (0.18°). With the incident beam propagating along the x direction, the GISAXS images were recorded in the (qy, qz) plane where qy and qz are the components of the scattering vector, related to the in-plane angle 2θf and out-plane angle αf.23 All the GISAXS patterns, shown hereafter, were normalized with respect to the incident beam intensity monitored by a front photomultiplier. Complementary scanning electron microscopy measurements were performed using a CD-SEM H9300 from Hitachi operating at 800 V accelerating voltage, and X-ray reflectivity measurements on a Xpert Panalytical with a Cu Kα X-ray source. 7222


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SELF-ASSEMBLED BCP ON UNPATTERNED SURFACES Ex Situ GISAXS Measurements. To check the stability of self-assembled PS-b-PMMA films under synchrotron radiation (SR), GISAXS patterns were recorded as a function of the SR exposure time for a 40 nm thick BCP self-assembled film. After 0.5 min beam exposure, the GISAXS image in Figure 2a

copolymer, L0, is deduced using the following general relation (1/dhk2) = (4/3)((h2 + hk + k2)/L02), where dhk is the spacing of rows (hk). A value of 38.8 nm was thus found. After 90 min exposure (Figure 2b), the (10) rod intensities increase and two other symmetrical rods can be recorded which are located at scattering vectors equal to √3q10 and thus assigned to (11) reflections. This evolution is attributed to an enhancement of the electronic density contrast between the matrix and cylinders, due to PMMA degradation under the photon beam. However, while a long exposure to beam induces somewhat PMMA removal, it does not change the copolymer period which can be attributed to the stiffness of the PS matrix, comparatively more beam-resistant. The signature of higherorder reflections, namely the (20) and (12) ones, (already present after 0.5 min) is also indicated as spots especially along the Yoneda peak of the BCP film at qz = 0.26 nm−1. It is worth noting that compared with these two reflections, no (11) spot was present after 0.5 min. As discussed afterward, the absence of a specific reflection can be explained by the existence of a minimum in the form factor at the corresponding scattering vector. Thus, for the further experiments and to gain a deep knowledge of the BCP structure, PMMA was removed prior to the GISAXS experiments allowing the measurement of higherorder Bragg rods of the 2D hexagonal array. Three different strategies to remove PMMA were employed: acetic acid surface reconstruction of 10 s, O2 plasma etching of 50 s, and acetic acid reconstruction for 10 s followed by a short O2 plasma etching of 8 s. We use the term “acid acetic reconstruction” since acetic acid treatment of PS-b-PMMA films does not lead to a complete removal of PMMA without further plasma treatment but reconfigures the PMMA domains through the

Figure 2. GISAXS patterns for self-assembled PS-b-PMMA films recorded after a beam exposure time of 0.5 min (a) and 90 min (b).

exhibits two weak symmetrical Bragg rods which are assigned to the (10) reflection of the 2D hexagonal lattice formed by the PMMA cylinders. From its scattering vector, q10, the lattice parameter of the hexagonal array, i.e. the period of the

Figure 3. GISAXS patterns recorded for PS-b-PMMA films after acetic acid surface reconstruction (a), O2 plasma etching (b), acetic acid + plasma etching (c) (counting time = 1200 s). (d−f) Corresponding simulated 2D |F(q)|2 form factors of cylindrical holes to explain the extinction of peculiar hl rods observed in the GISAXS patterns. (g−i) Corresponding SEM images. 7223


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Table 1. Main Structural Parameters of the Etched BCP Films Extracted from both GISAXS Patterns and SEM Images as a Function of the Etching Treatmenta GISAXS

SEM

reflectivity

treatment

L0, nm

D, nm

H, nm

L0, nm

D, nm

t, nm

σ, nm

acetic acid 10 s O2 plasma 50 s ac. acid 10 s + plasma 8 s

38.9 38.8 38.8

15.5 18.3 20.5

53 23 50

36.9 37.1 37.4

18.9 20.6 21.4

55 40 48.5

2 1.3 1.3

a L0 is the polymer period; D and H are the diameter and depth of cylindrical hole. t and σ are the thickness and roughness of the PS-b-PMMA film deduced from low angle X-ray reflectivity.

swelling of the PMMA chains.24 Figure 3 shows the GISAXS patterns of 56 nm thick self-assembled BCP films corresponding to these three PMMA removal treatments. The higher order Bragg rods (20) and (12), occurring at scattering vectors of 2q10and √7q10 are now well-defined after removing PMMA. The coexistence of these reflections shows that the BCP films consist of randomly oriented hexagonal domains. For the three differently treated samples, the polymer periods are very close (Table 1), indicating that the methods of PMMA removal impact neither the PS matrix and in this case nor the hexagonal structure of BCP. Besides, this analysis emphasizes the importance of the PMMA removal methodology on the final BCP structure dimensions. While all successive rods are observed for the sample treated with O2 plasma (Figure 3b), the (12) rod is missing for sample treated only by acetic acid and the (20) rod for sample treated with acid and then etched with O2 plasma (Figure 3c). The extinction of such rods is due to the form factor of the cylinders10 which for a low cylinder size dispersity (expected in self-assembled BCP) presents a series of minima whose positions are related to the cylinder diameter. Parts d−f of Figure 3 show the 2D |F(q)|2 factors of cylinders calculated within the distorted wave Born approximation (DWBA) applied to an hexagonal array of cylindrical holes.25 The parts d and f factors are obtained using a cylinder diameter (D) of 15.5 and 20.5 nm, respectively, which justify the absence of the (12) and (20) rods in the patterns a and c. For the film after acid and plasma etching, the part e factor was obtained with an intermediate diameter of 18.3 nm yielding no rod extinction in the investigated q-range. It is worth noting that these diameters are smaller than the ones deduced from a rather rigorous analysis of the SEM images (Figure 3g−i), given in Table 1. In particular, for the sample etched only with plasma, the diameter determined by SEM should lead to the extinction of the (20) rod, that was not observed. A plausible explanation about such differences could be related to the high energy electron beam inducing a crosslinking of the PS matrix. This exposure probably induces a shrinkage of the hexagonal array due to the decrease of free volume inherent to the cross-linking step, leading to a decrease of the polymer period and conversely to an increase of the hole size. As a matter of fact, the period deduced from the (10) rod position is found larger than the one deduced from the SEM image (Table 1), and this deviation holds for the three samples. Moreover, the modulation of the intensity along the (10) rods is very different between the three differently etched films. In case PMMA is completely removed, the height of cylindrical holes should be equal to the BCP layer thickness. The heights of the BCP etched films were deduced from X-ray reflectivity measurements. For the three BCP films, the best fits of the reflectivity curves shown in Figure 4a−c correspond to the values of thickness and roughness listed in Table 1, including a

Figure 4. Experimental and simulated reflectivity curves for the same BCP films as those measured by GISAXS: (a) After acid acetic surface reconstruction, (b) O2 plasma etching, and (c) acid acetic + plasma etching. (d−f) Corresponding vertical cross sections along the (10) rod (crosses) extracted from the GISAXS patterns in Figure 3 and best fits obtained with FITGISAXS software (red line). The Yoneda peaks of the BCP film and Si substrate are indicated by green and blue arrows, respectively.

6 nm thick brush layer with a roughness of 0.5 nm. As expected, O2 plasma chemistry does not only result in PMMA removal but also etches the PS matrix leading to thinner BCP films. Simulations of the intensities along the (10) rod calculated at qy = 0.18 nm−1 and averaged in a narrow bandwidth of qy (0.017 nm−1) are shown in Figure 4d−f. The most striking feature is actually the low frequency of oscillations in the vertical cut found for the sample etched only by plasma during 50 s which is well represented by an empty cylinder height of 23 nm. Such height is about the half of the film thickness, indicating the presence of PMMA in the lower halves of cylinders. This result is in agreement with an erosion rate of PMMA twice larger than the one of PS, already reported with this particular plasma chemistry.27,28 For the sample treated only by acid, the Yoneda peak attributed to the BCP surface (Figure 4d) is strongly attenuated and somewhat closer to the Yoneda peak of the Si surface in comparison with that observed for the two other samples (Figure 4e,f). Since the acetic acid is a good solvent for PMMA and not for PS, the PMMA chains are dissolved but the covalent bonds with PS are unbroken.26,29 Therefore, PMMA remains on the surface: (i) generating some roughness which attenuates the intensity of the BCP Yoneda peak (Figure 4d), (ii) leading to a Yoneda peak closer to the Si Yoneda peak 7224


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Figure 5. GISAXS patterns for BCP films self-assembled after (a) 2 min, (b) 10 min and (c) 30 min annealing at 240 °C (counting time = 600 s). (d) Intensity profiles along qy calculated around qz = 0.28 nm−1 using an integration width of 0.04 nm−1. (e) Variation of the polymer period (red squares) and the full width at half-maximum of the 10 rod (blue circles) with annealing time.

30 min, in which PMMA was removed prior to the GISAXS experiments. With respect to Figure 3, the images were recorded after moving the XPAD detector from 11 mm along the y direction to reach higher qy wavevectors. After only 2 min, the 2D hexagonal array is already well established, however the presence of a weak ring indicates that ordering is uncomplete through the whole film. Compared to the three other films annealed during longer times, the shorter frequency of the oscillations along the (10) rod reveals a thicker film, and the lessening of the Si Yoneda peak suggests a larger roughness of the BCP/Si interface. From the intensity profiles along qy calculated at qz = 0.28 nm−1 (i.e., at the Yoneda peak of Si (Figure 5d), the intensity for the 2 min annealed films is significantly weaker than for the other films, confirming incomplete ordering. After 30 min of annealing, the (10) rods become asymmetrical; this effect is really enhanced at the Yoneda peak, revealing a distribution in the polymer period with a marked increase of larger periods. The analysis of the (10) rods (positions and widths) leads to the changes with annealing time displayed in Figure 5e. A continuous increase of the average period is thus observed with annealing time likely related to a degradation of the hexagonal array of perpendicular PMMA cylinders after lengthly annealing. However, a deep understanding of such variation would deserve complementary studies. Because of the limited lateral coherence length of the beam, the size of the ordered domains cannot be directly deduced from the full widths at the half-maximum of the (10) rods. Nevertheless, the variation of fwhm with annealing time suggests that the average size of the ordered domains is the largest one after 10 min. This series of measurements have thus shown that the structural properties of the hexagonal array depend finely on the heat balance. In Situ GISAXS Measurements. To overcome the radiation damage of PMMA much faster above 200 °C than at room temperature, in spite of nitrogen atmosphere, the

correlated to an electronic density of PMMA larger than the one of BCP and (iii) increasing the BCP film thickness as shown below. While a good fit of the vertical cross-section was difficult to achieve below 0.45 nm−1(Figure 4d), beyond it the oscillations are rather well described with a cylinder height of 53 nm. For the sample treated with acid and then etched with plasma (Figure 4e), the oscillations are well represented with a cylinder height of 50 nm, i.e., slightly lower than the value obtained for the film not etched by plasma. It turns out that acetic acid etching and a final plasma treatment allow to remove PMMA remaining on the film surface improving greatly surface flatness. When PMMA was removed from the BCP film, no change in the GISAXS pattern after 1 h beam exposure was observed. Finally simulations of the Bragg rods have brought useful information about the morphology of holes and the efficiency of the different etching processes as regards the final cylinder structure obtained through BCP self-assembly. The effect of the BCP film thickness on the polymer period was probed on two samples with thicknesses equal to 30 and 60 nm; in this case, PMMA was removed by acetic acid treatment. When decreasing the thickness by a factor two, L0, deduced from the (10) rod positions, decreases from 38.8 to 35.1 nm. Such behavior was not observed for a PS-b-PMMA/PMMA mixture leading also to vertically oriented PMMA cylinders while the film thickness was increased by a factor three.10 Here, the observed increase of the average period with the film thickness remains unclear even if the surface field induced by the PS-r-PMMA brush layer could be responsible of such behavior. At the end, the effect of heat balance on the 2D-hexagonal array was examined by measuring four PS-b-PMMA films quickly heated up to 240 °C and held at this temperature during different times before cooling. Figure 5 shows the GISAXS images for BCP films annealed during 2, 10, 15, and 7225


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interface. A smaller period at the air/BCP interface established at the onset of self-assembly was also found from other in situ experiments, such as the one performed during heating up to only 150 °C. In this case, such a difference remains even after cooling down to room temperature. Another series of in situ time-resolved measurements were performed at 240 °C. After reaching this temperature at a heating rate of 35 °C/min, GISAXS patterns were recorded each 10 s, and at each time after moving the sample by step of 0.5 mm to reduce beam exposure. Changes in the polymer period with annealing time up to 15 min were thus recorded on different sample area. Figure 7 shows the pattern recorded after

GISAXS patterns were recorded using short counting times of 10 s and moving the sample in the beam by steps during the annealing treatment. Several in situ measurements were performed using the same heating rate of 35 °C/min but varying the maximal annealing temperature. As an example, Figure 6a,b shows two typical GISAXS patterns taken at 170 °C

Figure 7. GISAXS pattern recorded after 5 min annealing at 240 °C.

5 min annealing for a sample area, not previously exposed to the beam. Since the (10) and (10̅ ) spots measured along the two Yoneda peaks occur at the same 2θf angles, the polymer period of the self-assembled film is rather homogeneous through the whole thickness already after 5 min annealing time. The polymer period deduced from the spot positions changes weakly between 5 and 10 min, while beyond 10 min it starts to increase significantly as shown in the Supporting Information. Such behavior is in agreement with the ex situ GISAXS data shown in Figure 5e.

Figure 6. GISAXS patterns of a 60 nm thick BCP film recorded during annealing at (a) 170 °C (sample position: y + 1 mm) and (b) 187 °C (position: y + 2 mm), marked by stars on the temperature profile (counting time=10s and αi = 0.14°). (c, d) Variations of the horizontal profiles as a function of annealing temperature, measured around the Yoneda peaks of BCP and Si, respectively (corresponding integration area are sketched by the dotted red and blue rectangles in patterns a and b). L0: polymer periods close to the air−polymer (c) and polymer−Si (d) interface (see also text).

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EX SITU GISAXS STUDY OF DIRECTED SELF-ASSEMBLY OF BCP The effect of the HSQ line grating on 60 nm thick BCP films is illustrated in Figure 8. The GISAXS patterns were collected both on the patterned area when the beam was centered on the line grating and parallel to the lines (Figure 8a) and on the unpatterned area after moving the substrate sideways from a few millimeters of the patterned area (Figure 8b). Both patterns were obtained after removing PMMA by acetic acid wet treatment. Figure 8a reveals a strong contribution of the line grating with a series of broad rods whose spacing is equal to 2*π/(L + S), and especially the absence of the (11) rod of BCP which is well observed for BCP self-assembled on the neighbor unpatterned area (see Figure 8b). This last feature indicates that the line grating forces the self-assembly of block copolymer such as the [10] direction of the 2D hexagonal array be parallel to the grating lines, as illustrated in the SEM image (Figure 8c), and it is true on the whole patterned surface (∼10 mm2). In contrast, the presence of both (10) and (11) rods for the unpatterned area indicates the formation of randomly oriented domains as observed by SEM (Figure 8d). The streaks of the line grating, resulting from the intersection of the reciprocal lattice rods with the Ewald sphere, are markedly smeared out along qy and qz mainly due to the reduced lateral coherence length and energy spread of the incident beam. The presence of semicircles additional to the one defined by the intersection of the Ewald sphere with a 1D lattice of rods (dashed red semicircle in Figure 8a) comes from

during heating and at 187 °C during cooling from a maximal temperature of 210 °C. Because of the weak electronic contrast between PMMA and PS, in situ self-assembly of the diblock copolymer is only revealed by spots, at the levels of the two Yoneda peaks (i.e., for exit angles, αf, equal to 0.14 and 0.185 deg), assigned to the (10) and (10̅ ) reflections. Moreover, the variation of the spot positions with αf suggests the existence of different periods at the air−polymer and BCP−Si interfaces. In Figure 6c,d, 30 horizontal profiles deduced from the GISAXS patterns and calculated at 0.14 and 0.185 deg are stacked up as a function of the annealing temperature. Such representation reveals both the onset of long-range ordering occurring around 140 °C (i.e., above the glass transition of 120 °C) and the change in the polymer period with the annealing temperature and through the film thickness. The few periods given in Figure 6c,d are really characteristic of the intrinsic state of the ordered BCP (i.e., not damaged by repetitive 10 s beam exposures), since they correspond to patterns recorded just after moving the sample from 1 mm perpendicularly to the beam. Thus, it appears that at 170 °C the period is smaller at the air/BCP interface than at the BCP-Si interface, while after passing the maximal temperature of 210 °C the inverse holds at 187 °C and remains after cooling down to 70 °C. Interestingly, the increase in the polymer period at the air/BCP interface is more or less compensated by a decrease at the BCP−Si 7226


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Figure 8. GISAXS patterns for BCP films self-assembled jointly on the line grating (a) and on a neighbor unpatterned area (b). The dashed semicircle represents the intersection of the Ewald sphere with the reciprocal lattice of the grating. Corresponding SEM images (c, d) and horizontal linecuts calculated at qz = 0.35 nm−1. (e, f) Experimental data (crosses) and best fits of the (10) rod (red curves).

period observed on the lines indicates that the formation of a single defect-free hexagonal array of cylinders would be achieved through a subtle balance between the film thickness and the height of the grating lines. For the polymer selfassembled on the unpatterned area, the 10 rod is well described by a single Gaussian corresponding to a period of 35.65 nm, i.e., very close to the value extracted for the graphoepitaxy guided polymer in the trenches. Finally, we probed the influence of the line grating on the ordering kinetics of BCP thin films. The GISAXS patterns of copolymers self-assembled on patterned and unpatterned area of the same Si wafer were thus recorded after two different annealing treatments, namely 220 and 240 °C for 10 and 15 min, respectively. Figure 9 shows the intensity profiles along qy extracted from the GISAXS patterns. It turns out that thermal annealing at 220 °C for 10 min is sufficient to reach the equilibrium period of the graphoepitaxy guided BCP equal to 36.1 nm, since no change in the period is recorded after annealing at higher temperature, as indicated by the superimposition of the two sets of the 10 rods. In contrast, for the BCP self-assembled on a unpatterned area, a significant increase of the period from 35.8 to 36.9 nm is observed with increasing the annealing temperature. Thus, the self-assembly of BCP on a line grating commensurable with the polymer period does not only drive the orientation of the 2D hexagonal array leading to the formation of a single crystal, but also makes faster longrange ordering. This result clearly emphasizes the importance of the topography on the BCP ordering kinetics and has to be put in perspective with the recent results31 about the crucial role of the surface chemistry on the ordering kinetics. Indeed both the topography and the surface chemistry can enhance the self-assembly kinetics in BCP thin films.

the fact that the line grating is not completely perfect at a nanometric scale. During the lithographic process a lateral displacement of about 6 nm occurs every 33 μm. Such periodic displacement leads to a 2D lattice of rods in the (qx, qy) plane. The rods corresponding to negative values of qx also intercept the Ewald sphere leading to the additional semicircles. The intersections of these semicircles with the qz axis allow to determine a period along the beam direction, equal to 33 μm in agreement with the lithographic process. It is also worth noting that the misalignment of the incident beam with the line grating distroys totally the symmetry of the grating signal with respect to the qz direction, as demonstrated using a highly coherent Xrays beam30 and also attenuates the signal coming from the 2D hexagonal array of BCP. The intensity profiles along qy calculated around qz = 0.35 nm−1 (i.e., slightly above the Si Yoneda peak and the dark line corresponding to the junction between two modules of the XPAD detector) and integrated over 0.08 nm−1 are shown in Figure 8, parts e and f, for the BCP self-assembled on the patterned and unpatterned area, respectively. The 10 and 20 rods are more intense for the graphoepitaxy guided BCP due to the contribution of the rods of the commensurable line grating. Moreover, the ratio of the intensities I10/I20 is three times larger for the unguided BCP than for the guided BCP. This slower decrease for the graphoepitaxy guided BCP indicates an extent of hexagonal ordering over larger distances. For the guided BCP, a good fit of the 10 rod is obtained with two Gaussian components: a narrow one associated with a larger period (35.75 nm) likely coming from the polymer self-assembled in the trenches and another broader with a shorter period (32.4 nm) coming from the polymer self-assembled over the lines whose BCP thickness is only of 10 nm. This variation with the BCP thickness is in agreement with our previous results reported on unpatterned surfaces. As a matter of fact, this broad component was not observed for a film thickness of 30 nm, i.e., smaller than the line height (not shown). The larger value corresponds to a distance, a0, between two cylinder rows aligned along the [10] direction equal to 31 nm, which fulfills the commensurability conditions with the line and trench widths, namely 62 and 186 nm. The decrease of the polymer

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CONCLUSION The GISAXS measurements performed on PS-b-PMMA copolymers self-assembled jointly on an optimized line grating and unpatterned surface area have highlighted the advantages of the grating on long-range ordering: formation of a single hexagonal array domain with the [10] direction parallel to the lines, faster order kinetics, and higher stabilization of the 7227


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AUTHOR INFORMATION

Corresponding Authors

*(M.M.) E-mail: [email protected]. *(R.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Allocation of beamtime on the French CRG-BM02 beamline at the ESRF is gratefully acknowledged. The authors thank D. Babonneau for enlightening discussions about the FitGISAXS software, and J. F. Bérar, B. Caillot and S. Arnaud for their help at BM02.

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Figure 9. Horizontal profiles calculated around qz = 0.43 nm−1 and fits of the 10 peaks: for a copolymer film self-assembled jointly on the line grating (a) and a neighbor unpatterned area (b), after two different annealing treatments (220 °C/10 min and 240 °C/15 min).

polymer period with annealing temperature. The increase of the spacing between cylinders with the film thickness, observed for self-assembled BCP on unpatterned surfaces, is also found for the graphoepitaxy guided copolymer through its different periods adopted on the lines and trenches of the grating. A detailed analysis of the reflection rods of the hexagonal array has allowed us to resolve effects of the different PMMA removal treatments on the structure of the cylindrical holes, not attainable by standard laboratory techniques, which for future applications in nanolithography are of major importance. For it, PMMA was removed giving rise to a larger electronic density contrast between the cylinders and the PS matrix, the latter having a good resistance to beam exposure. Furthermore, the in situ GISAXS measurements performed on unpatterned surfaces have provided worthwhile information on the initial structure of the self-assembled copolymer films and the evolution of the polymer period and its gradient with annealing temperature. They have especially shown that the phase-separation started at both interfaces (air−polymer and polymer-Si) and also confirmed that annealing at 240 °C leads to fast and homogeneous self-assembly.

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ASSOCIATED CONTENT

S Supporting Information *

Figure showing the variation of the period of the BCP hexagonal array with annealing time at 240 °C measured on three different sample areas, where this variation was deduced from in situ GISAXS studies. This material is available free of charge via the Internet at http://pubs.acs.org. 7228


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Probing Self-Assembly of Cylindrical Morphology Block Copolymer

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