Depth-Dependent Structural Changes in PS-b-P2VP Thin Films

Article pubs.acs.org/Macromolecules

Depth-Dependent Structural Changes in PS‑b‑P2VP Thin Films Induced by Annealing Jan Wernecke,*,† Hiroshi Okuda,‡ Hiroki Ogawa,⊥ Frank Siewert,§ and Michael Krumrey† †

Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyoku, Kyoto, 606-8501, Japan ⊥ SPring-8, Japan Synchrotron Radiation Research Institute , 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan § Institute Nanometre Optics and Technology, Helmholtz Zentrum Berlin (HZB), Albert-Einstein-Strasse 15, 12489 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: GISAXS measurements with scattering contrast matching at the silicon K-edge were performed to obtain depthresolved information on structural changes in as-spun and annealed PS-b-P2VP thin films on silicon substrate. Depthsensitive GISAXS measurements of the as-spun film revealed a vertically oriented fingerprint-like lamellar structure with a microphase separation distance of 59 nm throughout the entire film. The annealed film showed a significantly reduced ordering at the surface to a depth of about 30 nm, while the order is preserved toward the substrate interface. At the same time, no significant transition to horizontal ordering was observed after 2 h of annealing at 105 °C. We conclude that the transition process from vertical to horizontal ordering is incomplete after the annealing time and remains in a frozen state at room temperature. Moreover, the transition starts as a disorder increase at the top of the film, which indicates a higher mobility of the coalescing microdomains at the surface.

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INTRODUCTION Block copolymer (BCP) thin films forming self-organized nanostructures1−5 have been very attractive materials in both science and cutting-edge applications for decades. Such structures offer a wide range of attractive applications, for example, in organic photovoltaics,6 for medical applications,7 as nanostructuring templates of other materials,8−12 or even as sensors. 13 Polystyrene-block-poly(2-vinylpyridine) (PS-bP2VP) is a commonly used material14−16 that is a typical example of the class of block copolymers with a large surface energy difference between the components.17 It has been reported in many studies that it forms well-developed selforganized nanostructures11,18,19 which can be controlled and changed by varying the environment (addition of solvent, concentration change, annealing, electric fields, vapor annealing, etc.).17,20−24 However, despite the large number of reports on the variation of chemical and physical process parameters, detailed studies of the structure along the thin film depth with nondestructive techniques are scarce so far, due to the very limited number of suitable measurement methods. Understanding the characteristics of the self-organization, for example after annealing, is the key to control the shape of the nanostructures and tailor it to specific needs. This requires tools to assess the film morphology in both lateral and vertical direction in a nondestructive way. Grazing incidence smallangle X-ray scattering (GISAXS)25,26 is a widely used and © 2014 American Chemical Society

sometimes the only applicable technique for such studies in polymer films.27−31 Depth resolution can be achieved by variation of the X-ray incidence angle around the critical angle of total reflection.32 Yet major difficulties arise from the strong scattering contribution of the film interfaces.33 Consequently, scattering induced by roughness, reflection from the film substrate as well as multiple scattering occur and overlay the weak film scattering on the GISAXS image.29 The X-ray scattering contrast can be adjusted by varying the photon energy around an absorption edge of a specific element.34−36 In this way, it is possible to minimize the interface scattering effects by matching the scattering contrast between the polymer film and the silicon substrate.33,37,38 A major challenge of this approach is that it requires GISAXS measurements to be performed around the absorption K-edge of silicon at 1839 eV, which is impossible to reach for most digital large-area X-ray detectors. Nondestructive X-ray investigations on the depth structure of BCP thin films are, therefore, so far restricted to the use of image plates,33,38 which have excellent imaging properties, but are time-consuming in handling and, thus, limited in the amount of data that can be acquired. In principle, the same could be achieved with hard X-rays and, for example, a Received: March 28, 2014 Revised: July 8, 2014 Published: August 13, 2014 5719

dx.doi.org/10.1021/ma500642d | Macromolecules 2014, 47, 5719−5727

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Instrumentation. The X-ray reflectometry and GISAXS measurements shown in this work were performed at the four-crystal monochromator (FCM) beamline49 in the PTB laboratory50 at the synchrotron radiation facility BESSY II of the Helmholtz-Zentrum Berlin (HZB). The FCM beamline offers an energy range from 1.75 to 10 keV with a beam spot size of (0.3 × 0.3) mm2. Samples are mounted in a load-lock UHV sample chamber, which has six axes for sample movement as well as several detectors mounted on a detector arm, including highly linear photodiodes for XRR measurements.51 Attached to the sample chamber is the SAXS instrument of HZB, by which the sample−detector distance can be varied continuously between 2.1 m to about 4.5 m by movable bellows without breaking the vacuum.52 A typical sample−detector distance of the presented GISAXS measurements is 3200 mm. The 2D detector that is used for GISAXS, in particular in the X-ray energy regime below 3 keV, is a custom-made in-vacuum version of a PILATUS 1 M detector48,53 that is now routinely operated at the FCM beamline. The instrument has been extensively characterized by radiometric and geometric means and it is able to cover the full energy range of the FCM beamline down to 1.75 keV with sufficient quantum efficiency.47 Typical total exposure times of the GISAXS data presented here are in the range of 30 to 1200 s (stated along with the data), depending on the photon energy and the incidence angle. XRR. X-ray reflectometry is a technique used to characterize thin films, monolayers, and multilayers in terms of vertical correlation lengths, i.e. thickness and interface roughness, as well as the electron density depth profile.54,55 A monochromatic X-ray beam with a photon energy Eph (or, equivalently, wavelength λ) is specularly reflected at grazing incidence. The reflected intensity is measured as a function of the incidence angle αi with a photodiode and normalized by the intensity of the incident beam, which yields the reflectance curve. GISAXS. GISAXS combines the techniques of small-angle X-ray scattering (SAXS) with the grazing incidence reflection geometry of XRR. Hence, GISAXS can be used to probe vertical and lateral structures in the nanometer to submicrometer range or, with an extended sample-to-detector distance, even well into the micrometer range.56 In GISAXS geometry, a two-dimensional scattering pattern is recorded with a large area detector. Each position of the detector image can be referred to by the azimuthal scattering angle θf and the vertical scattering angle αf (Figure 1). The monochromatic X-ray beam

gold substrate. But the critical angles at high photon energy are much smaller, which results in less precise depth control, a much larger beam footprint, and possibly shadowing by surface undulations or artifacts due to a curved surface within the large illuminated area. However, combining GISAXS with advanced micro- and nanobeam instrumentation39−42 greatly reduces the problems of overillumination and surface curvature due to a large beam footprint. Invasive techniques (e.g., cross-section electron microscopy, plasma etching) or techniques only sensitive to the surface (e.g., atomic force microscopy) carry the risk of inducing morphological changes during preparation or of missing relevant depth information. An alternative for noninvasive polymer thin film investigations is grazing incidence small angle neutron scattering (GISANS). A number of depth resolved GISANS studies on triblock43−45 and diblock46 copolymer thin films profit from the widely tunable scattering contrast by deuteration of hydrogen bonds, but this requires a considerable alteration of the sample preparation process. As drawbacks, the limited availability of GISANS, long exposure times, low counting statistics and low q-resolution have to be mentioned. In this work, we present the contrastmatching X-ray pendant to the depth-resolved GISANS studies, which forego the specific advantages of neutrons (deuteration; probing from the substrate side), but can, in principle, be more widely performed and circumvent most of the mentioned drawbacks. We have studied structural changes in as-spun and annealed PS-b-P2VP thin films on silicon substrates by depth-resolved GISAXS with contrast matching around the silicon absorption K-edge at 1839 eV. The scattering images were recorded with the in-vacuum PILATUS 1 M detector that was developed in a cooperation between Dectris Ltd. and the PhysikalischTechnische Bundesanstalt. With this detector it is possible to record scattering images down to 1750 eV with all the benefits of a digital hybrid pixel detector such as ease of use, fast image acquisition, high dynamic range, and a very low dark count rate.47,48 Prior to GISAXS, X-ray reflectometry (XRR) was performed to determine the thickness and density of the film as well as to observe the contrast matching. Atomic force microscopy (AFM) on both samples was applied to obtain complementary information on the surface structure before and after annealing.

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EXPERIMENTAL SECTION

Sample Material and Preparation. The asymmetric block copolymer PS-b-P2VP purchased from Polymer Source has a polydispersity of MW/Mn = 1.07, a PS volume fraction of 0.5 and molar masses of MW = 40 500 g/mol (PS block) and 41 000 g/mol (P2VP block), respectively. The PS-b-P2VP was dissolved in a 2% w/w toluene solution and spin-coated at 2000 rpm for 30 s on cleaned commercial Si(100) wafers. The films were then annealed at 60 °C for 24 h under vacuum to remove the residual solvent. The glass transition temperature of the PS-b-P2VP is at 102.7 °C, the annealed thin film sample was kept at a temperature of 105.0 °C for 2 h. The as-cleaned silicon substrate has a native SiOx oxide (thickness around 1 nm) and has a surface energy of 65.7 mJ/m2. It has been reported44 that the substrate surface treatment and, thus, surface energy influences the morphology of block copolymer thin films. No chemical modification of the substrate other than cleaning in an ultrasonic bath, followed by a hot acid bath and thorough rinsing was applied. Thus, the Si/SiOx interface represents a (hydrophilic) neutral wall44 for the PS-b-P2VP film, which means that the interface interactions of both blocks with the substrate are similar and result in an expected perpendicular orientation of the microphase separation structure.

Figure 1. GISAXS scattering geometry. impinges on the sample at a fixed vertical grazing incidence angle αi. It is common practice to address each point of the scattering pattern in reciprocal space coordinates q⃗ = (qx, qy, qz), which are defined as qx = k(cos θf cos αf − cos αi) qy = k(sin θf cos αf ) qz = k(sin αi + sin αf ),

(1)

with the wavevector k = 2π/λ. The detector plane can be usually seen as being congruent with the (qy, qz)-plane due to the distance of several meters between the sample and the detector. The complex refractive index is defined as n = (1 − δ) + iβ. The real part (1 − δ) is slightly smaller than unity in the X-ray regime (δ ≈ 10−5···10−4). This implies that there is a critical incidence angle 5720


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αc ≈ (2δ)1/2 below which total external reflection at the vacuum interface occurs. The penetration depth Λ, i.e., the depth where the penetrating evanescent beam is attenuated by 1/e, is the reciprocal of the linear absorption coefficient μ and can be derived54 from the complex refractive index n

Λ = [2k 2( (αi 2 − αc 2)2 + 4β 2 − (αi 2 − αc 2))]−1/2

(2)

Thus, it is possible to tune the penetration depth Λ of the X-ray beam by varying αi around αc. Consequently, the averaged scattering from the top (“top” refers here to the side where the X-ray or neutron beam impinges on the sample) of the film down to the penetration depth is measured (which is why eq 2 is sometimes43−45 also referred to as the scattering depth). In other words, the detected signal contains the average structure of the film weighted by an exponential damping factor of this scattering or penetration depth. AFM. Atomic force microscopy is a scanning probe microscopy technique providing height resolution on an atomic scale.57,58 It is, thus, a surface sensitive technique that probes real-space lengths, which is complementary to the X-ray scattering methods. The used instrument is a Bruker SIS-ULTRAObjective AFM with a 40 μm × 40 μm scanner. The instrument is based on a PICOStation system with active vibration damping. The tip applied for the measurements is a silicon SPM sensor for the noncontact mode with a resonance frequency of 190 kHz and a force constant of 48 J/m2. The tip is shaped like a polygon-based pyramid with a height of 10 to 15 μm. The tip radius is less than 8 nm. Thus, the achievable lateral resolution is in the range of about 20 nm. After every 4 scans, the tip was exchanged to avoid having the measurement be influenced by tip wear. In addition to tapping-mode amplitude (topography) images, phase images of the tip frequency phase shift were recorded. Phase images yield information on the local stiffness and friction and, thus, on the spatial distribution of different materials.

Figure 2. (a) XRR data measured at different energies close to the contrast matching energy (1820 to 1845 eV) and far away from it (1770 and 3500 eV). The reflectance curves were simultaneously fitted with a single layer model to obtain the common parameters of film thickness, density, surface roughness and substrate interface roughness (solid lines; profiles vertically shifted for clarity). (b) Magnification of the XRR data in the low q range (vertically shifted for clarity). The black line indicates the strongest damping of wiggles due to contrast matching at Ematch = 1827 eV.

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RESULTS XRR Measurements. In order to perform depth-resolved GISAXS under the scattering contrast-matching condition, it is helpful to determine the optical constants, the film thickness, the surface roughness and the substrate interface roughness of the film. These values are then used to calculate the intersection of the δ-curves of polymer thin film and Si substrate, which is the contrast matching energy Ematch. The optical constants are also needed for GISAXS intensity modeling, for example within the framework of DWBA,59 to obtain the amplitude reconstruction of reflected X-rays. XRR measurements were carried out on the as-spun sample at different photon energies from 1770 to 3500 eV, that is, close to the silicon K-edge and far away from it (see Figure 2a). Since the X-ray scattering contrast between both blocks of (δ(PS))/(δ(P2VP)) = 1.1 is low, the inner structure of the film is not visible. Consequently, the film is regarded as a homogeneous film with an average density, which is also supported by the observation of only one critical angle. A possible intermediate layer of SiOx between the polymer film and the substrate is neglected in the model because the index of refraction is very similar to that of silicon.31 All reflectance curves were simultaneously fitted (least-squares minimization of modified Fresnel functions (Parratt formalism) with Névot−Croce roughness60−62) with common parameters for film thickness, surface roughness, and substrate interface roughness. The best fit values of the parameters resulted in a film thickness of (85 ± 3) nm and a film density of (1.16 ± 0.03) g/cm3. The determined surface roughness of around 3 nm is about 1 order of magnitude higher than that of the substrate interface. For more details on the uncertainty determination, the reader is referred to the Supporting Information (Figure S1 and the text therein). The

noticeable deviations in the oscillation amplitude close to the contrast matching energy (reflectance curves at 1820 to 1835 eV in Figure 2a) are due to the pronounced change of δ and β of the substrate close to the silicon K-absorption edge and the limited accuracy of the tabulated values of the optical constants. No Bragg peaks of any possible horizontal layering become visible in any of the reflectance curves (as-spun and annealed samples, the latter is not shown). Displayed in Figure 2b is a magnification of the low q-region of XRR measurements around the silicon K-edge. A strong damping of the oscillation amplitude is observed around a photon energy of Ematch = 1827 eV (Figure 2b). This is due to the minimized scattering contrast at the film−substrate interface. The film density can be used to calculate the real part δ and the imaginary part β of the refractive index of the film around the energy of contrast matching Ematch. Figure 3 shows the energy dependence of δ and β of the PS-b-P2VP thin film and bulk Si substrate as calculated with reference data for Si63 and the Henke data64 for P2VP (the applied sum formula is C21H21N3). An intersection of the δ curves of P2VP and Si is found in the region around 1815 eV. The deviation from the experimentally observed Ematch is due to the steep slope around the absorption edge and the uncertainty of the film density. The uncertainty of the density, or critical angle, contributes to the uncertainty of δ multiplied by a factor of 2 (due to αc = (2δ)1/2), which explains the discrepancy between theoretical and experimental values. 5721


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which is far away from the silicon K-edge (shown is the as-spun sample, the annealed sample shows the same pattern). Two symmetric diffuse scattering rods are visible at around qy = ± 0.11 nm−1. These are more intense in the so-called Yoneda region at qz = 0.21 nm−1, which appears at αf = αc due to the constructive interference of the dynamic scattering contributions.59 The presence of scattering rods (first order Bragg peaks) extended along qy in GISAXS is indicative of an in-plane ordering of structures with a characteristic separation distance P in the PS-b-P2VP film. It is well-known from the literature5,24,65 that PS-b-P2VP forms mircodomains of vertically extended lamellae under the present preparation conditions, which possess a characteristic microphase separation distance due to self-ordering of both blocks. Thus, the Bragg peak qy-position corresponds to a period length of P = 2 π/0.11 nm−1 = 58 nm, which is in full agreement with values from the literature.20 Also visible in Figure 4a are Kiessig-like fringes along qz that are superimposed with the scattering rods of the film nanostructures. The pattern in Figure 4b was measured at the contrast matching energy Ematch = 1827 eV (αi = 0.65°) and shows no indication of any fringes. Consequently and as expected, the oscillations originate from the interference of the incident beam and reflections from the film−substrate interface and reflect the film thickness and resonant diffuse scattering (RDS) due to correlated roughness.66 Thus, RDS is strongly attenuated at 1827 eV because of the minimized scattering contrast at the interface, while the scattering rods of the film are preserved in the GISAXS pattern. RDS can be used to study the degree of correlation of film surface and substrate interface.27,67,68 The observed period length in qz corresponds to a correlation length of dcorr = 85 nm, thus, there is full correlation of the surface and the substrate interface. This is no contradiction to the different rms roughness values that were determined by XRR fitting because the correlation does not extend over the full in-plane length scale spectrum.67 The damping of the oscillations with increasing qy indicates a loss of correlation and a cutoff at a minimal in-plane length Rc, thus, the polymer film acts as a band-pass filter for the interface height fluctuations.68 Consequently, the short-wavelength roughness spectrum is more than an order of magnitude below the cutoff length and is, thus, statistically independent from the surface-interface correlation.27 Depth-Resolved GISAXS with Contrast Matching. Depth-resolved GISAXS measurements at 1827 eV were performed on the as-spun and annealed thin films by changing the incidence angle and, thus, the scattering depth Λ, see Figure 5 and eq 2. Parts a and b of Figure 6 show line cuts along qy through the Yoneda region at αf = αc (dashed line in Figure 4b) for all incident angles of the as-spun and the annealed films, shifted vertically for clarity. In order to quantitatively characterize the depth-dependent film structure of both films, a similar analysis as presented in a depth-resolved GISANS study by Metwalli et al.46 and in an in situ GISAXS study of BCP solvent vapor annealing shown by Gu et al.69 is carried out. The qy-profiles were fitted with the function69

Figure 3. Component of the real part δ (solid lines) and the imaginary part β (dashed lines) of the complex refractive indices of Si and P2VP. The calculations were made with reference data for Si63 (ρ = 2.3 g cm−3) and the Henke data64 for P2VP (C21 H21 N3) with ρ = 1.16 g cm−3, as obtained by XRR fitting (Figure 2a).

Figure 3 also shows that there is no intersection of the βcurves of both materials, which is why the full matching of the complex refractive indices is not possible. A consequence of the unmatched β (in XRR as well as in GISAXS) is that the oscillations are attenuated, but still present at low q and less suppressed at higher q values. For a more detailed discussion of the topic, see the work of Ishiji et al.37 (especially Figure 6) and Okuda et al.33 GISAXS Contrast Matching. On the basis of the information on the contrast matching energy obtained by XRR, GISAXS images were recorded with the in-vacuum PILATUS 1 M detector. Displayed in Figure 4a is a GISAXS pattern measured at αi = 0.65° and a photon energy of 2500 eV,

Figure 4. GISAXS pattern of the as-spun sample measured at (a) αi = 0.65°, Eph = 2500 eV (300 s exposure time) and (b) 0.65°, 1827 eV (1200 s exposure time). The dashed black line indicates the Yoneda line, i.e., the condition of αf = αc. 5722


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functions and with a perturbation potential δV for perpendicular lamellae31 did not result in meaningful fits, which is probably due to the absence of well-defined structure shapes. AFM data (see next section) also confirmed that the sample surface just consists of fingerprint-like lamellae, which can be characterized by the size and shape of the Bragg peak that corresponds to the average separation distance. Thus, the profiles are adequately characterized by the Bragg peak position qy0, the full width at half-maximum (fwhm) and the peak height (Figure 6c−e). The Bragg peak position qy0 of the as-spun film (blue triangles) remains constant in the qyrange from 0.105 to 0.107 nm−1 (59 nm) throughout the entire film thickness. The fwhm decreases slightly with increasing scattering depth, which is due to the fact that it resembles the degree of ordering.44 With increasing scattering depth, the scattering signal is averaged over a larger volume (from the surface down to the scattering depth), which implies that the ordering slightly decreases with increasing film depth. The peak height, which is the difference of the minimum of the approximated peak function and the peak maximum (black squares in Figure 6a) increases slightly up to the critical angle and decreases with increasing scattering depth. The former is the same dynamic effect that causes the intensity increase at αf = αc, the Yoneda peak. The latter can be mainly attributed to

Figure 5. Scattering depth Λ in PS-b-P2VP at 1827 eV as calculated with eq 2

ln 2 G1 π ⎡ ⎛ qy − qy 0 ⎞2 ⎤ ⎢ exp − 4 ln 2⎜ ⎟⎥ ⎝ FWHM ⎠ ⎥⎦ ⎢⎣

I(qy ) = B1 + B2 qy + qy−B3 + 2

(3)

that accounts for the background (fit parameters B1,2), the exponential decay along qy−B3, and a Gaussian-shaped function for the Bragg peak at around 0.11 nm−1 (G1, qy0, and fwhm). It should be noted that DWBA modeling70 with various form factors (cylinders, boxes, anisotropic pyramids), interference

Figure 6. GISAXS qy profiles through the Yoneda region of the as-spun (a) and annealed (b) samples for different incidence angles around the critical angle αc = 0.70° (300 s exposure time). The profiles are fitted with a function (eq 3) to characterize the Bragg peak shape (solid lines in parts a and b), black triangles indicate the local minimum and maximum positions of the peak. Extracted (c) Bragg peak positions, (d) peak fwhm, and (e) peak heights of the as-spun and annealed films as a function of incidence angle and with an indication of the scattering depth. 5723


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Figure 7. Representative AFM amplitude images (a, b) and phase images (c, d) of the as-spun film (a, c) and the annealed film (b, d) on measurement fields of 10 μm × 10 μm and 1 μm × 1 μm (insets in parts a−d). (e) Radial power spectral densities as a function of spacial frequency k of the as-spun film and the annealed film of the 10 μm × 10 μm amplitude images (a and b).

height is zero in the surface-sensitive region of αf < αc, increases in the first 30 nm of the film and follows the same trend as that of the as-spun film at greater depth. For better comparison, the Bragg peak heights of both samples were normalized to the corresponding peak height at full film penetration (largest incidence angle). In absolute numbers, the peak height in the annealed film is lower by a factor of about 5. AFM. AFM measurements were carried out to get complementary information on the surface structure of both films. Different fields with sizes of 1 μm × 1 μm, 10 μm × 10 μm (Figure 7 and insets) and 35 μm × 35 μm (Supporting Information, Figure S2) were scanned on both samples and

the reduction of reflectance (and increase of absorption) at higher incidence angles above the critical angle. On the annealed sample (Green circles in Figure 6c−e), a significantly different depth-dependent behavior is observed. Parts b and e of Figure 6 clearly show that there is no Bragg peak at incidence angles below around 0.65°, only a flat plateau is present, while the peak reappears at around the critical angle and above. The data may also suggest that there is a slight increase of the separation distance toward a higher scattering depth. The fwhm of the annealed film generally follows the slope of the curve of the as-spun sample, which indicates that the subsurface order has not changed after annealing. The peak 5724


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of the Bragg peak heights and the fwhm in the annealed film, compared to the as-spun film. The Bragg peak center position seems to slightly decrease with increasing scattering depth. This would suggest that the lamellar separation distance increases, which might be due to strain induced by the transition close to the film surface. A very similar observation was made21 after exposing PS-b-P2VP thin films to a temperature leap of 200 °C and subsequent cooling. Because of the complete suppression of RDS in GISAXS under the contrast-matching condition, the absence of Bragg peaks along the qz-axis in GISAXS and XRR as well as due to the presence of only one critical angle in the XRR measurements, there is no indication of the formation of parallel lamellae toward the substrate interface as, for example, during solvent vapor annealing.24,69 However, this is difficult to conclude from X-ray scattering measurements because of the low scattering contrast of both monomer units and due to the restriction to probing the film only from the free surface. Depth-resolved GISANS investigation with the neutrons impinging from the substrate side44,46 would add valuable insights in the film structures close to the substrate interface. The discussed results are summed up in a sketch of the depth structure of the annealed PS-b-P2VP thin film in Figure 8. In terms of reorientation processes of the monomer units after thermal treatment, it seems that the coalescence of microdomains to form horizontal lamellar structures has not been taken place after 2 h of annealing and mainly resembles a disordered state close to the film surface. We suggest that the annealed sample is being frozen within the transition process from vertical to horizontal lamellar ordering, which takes much longer than the applied annealing time. The glass transition temperature is above 100 °C, which would mean that the transition process practically comes to rest at room temperature. The depth profiles imply that the transition starts at the surface within the topmost layer down to about half of the interlamellar separation distance. This could be explained by a higher mobility of the PS-b-P2VP microdomains at the surface in comparison to a greater film depth. It can also be noted that AFM measurements alone would not have been sufficient to observe the full range of structural changes. Depth-resolved GISAXS is needed to observe the preservation of vertical order in greater depths of the film.

recorded as amplitude images (Figure 7a,b) and phase images (Figure 7c,d). Especially the phase images show the fingerprintlike surface distribution of the PS and P2VP blocks as vertical lamellae on the as-spun and annealed films. The structures appear less pronounced on the annealed sample (Figure 7b,d), which is also confirmed by a comparison of the average height roughness σrms. Representative values of σrms are 2.1 nm for the as-spun film surface and 1.45 nm for the annealed film. The radial power spectral densities (PSDs) of the 10 μm × 10 μm amplitude images, Figure 7e, exhibit a maximum at a spacial frequency of around 0.107 nm−1 (59 nm correlation length) which is again due to the spacing of the lamellae. No formation of islands was found on any of the AFM images up to the maximal length scale of 35 μm.

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DISCUSSION Depth-resolved GISAXS and AFM measurements of the PS-bP2VP thin films under the condition of matched contrast reveal structural changes after annealing on the surface and along the film thickness. From the combined results of XRR, AFM, and GISAXS data, we propose a depth structure of the annealed film as sketched in Figure 8.

Figure 8. Schematic representation of the depth structure of the asspun and annealed PS-b-P2VP thin films (see text).

At the surface, the same fingerprint-like lamellar structure is observed by AFM on the as-spun and annealed films surface. The maxima of the radial PSD of the AFM amplitude images fully agree with the qy-positions of the GISAXS Bragg peaks and correspond to the lamellar spacing of 59 nm. However, the inplane lamellar ordering at the surface of the annealed sample is significantly reduced. This can be concluded from the absence of a Bragg peak in depth-resolved GISAXS measurements at incidence angles below αc, from the reduced rms roughness (lower by a factor of 1.4 compared to the as-spun sample) and from the lower AFM PSD intensity at higher spacial frequencies (shorter correlation lengths). The presence of RDS in GISAXS measurements without contrast matching on both films reveals a pronounced morphology correlation of film surface and interface at higher in-plane lengths. Consequently, also in GISAXS there is no indication of any additional surface structuring after annealing. With increasing X-ray scattering depth, αi > αc, the GISAXS Bragg peak height significantly increases in the qy-profiles of the annealed sample and follows the same trend as that of the asspun film when probing further down through the film. Hence, the in-plane ordering of the vertical lamellae is preserved after about 30 nm of film depth after annealing. The Bragg peak fwhm and the absolute peak heights of the annealed film follow the slope of the as-spun sample peak shapes, but are systematically lower. A reduced in-plane order at the surface and a preserved in-depth order causes a reduced total degree of order in the probed scattering volume from the film surface to the scattering depth, which results in a constant negative offset

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CONCLUSIONS In this study, we have shown that GISAXS measurements with scattering contrast matching at the silicon K-edge can now be routinely performed with the in-vacuum PILATUS 1 M detector at the PTB FCM beamline. The contrast matching technique is not restricted to PS-b-P2VP, but it can also be generally applied to studies of nanostructured layers of light materials, such as polymers, on silicon, the most commonly used substrate material. A benefit of contrast matching is the minimization of interface scattering effects, which overlay the weak scattering features of the film structure. GISAXS contrast matching can be seen, to some extend, as the X-ray pendant of depth-sensitive GISANS studies with contrast variation by deuteration. Depth-sensitive GISAXS measurements at the contrast matching energy of 1827 eV on PS-b-P2VP thin films showed structural changes along the film depth after the annealing process. The as-spun film exhibited fingerprint-like vertical lamellae with a separation distance of 59 nm throughout the entire film thickness of 85 nm. The annealed film showed a significantly reduced ordering toward the surface, within a 5725


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depth of about 30 nm, while the order is preserved toward the bottom of the film. Contrast-matched GISAXS and AFM measurements were used to clarify the state of in-plane ordering in the annealed film. Neither technique showed an indication of a significant formation of horizontal lamellae throughout the film thickness. We suggest that the transition from vertical to horizontal ordering is not completed after 2 h of annealing and the transition process is frozen at room temperature. It was found that the transition starts in the topmost film layer, which suggests a higher mobility of the coalescing microdomains at the film surface.

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ASSOCIATED CONTENT

* Supporting Information S 2

χ -values of XRR fitting and additional AFM data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Levent Cibik and Stefanie Langner (both from PTB) for their valuable assistance during the experiments, Armin Hoell (HZB) for the continuous cooperating research with the HZB SAXS instrument, as well as Christian Gollwitzer and Victor Soltwisch for constructive and valuable discussions.

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Depth-Dependent Structural Changes in PS-b-P2VP Thin Films

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