Stabilization of Thin Polymeric Bilayer Films on ... - ACS Publications

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Langmuir 2003, 19, 8511-8520

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Stabilization of Thin Polymeric Bilayer Films on Top of Semiconductor Surfaces O. Wunnicke,† P. Mu¨ller-Buschbaum,*,‡ M. Wolkenhauer,§ C. Lorenz-Haas,§ R. Cubitt,| V. Leiner,| and M. Stamm⊥ Infineon Technologies, Koenigsbruecker Strasse 180, 01099 Dresden, Germany, TU Mu¨ nchen, Physik-Department, LS E13, James-Franck-Strasse 1, 85747 Garching, Germany, MPI fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, France, and Institut fu¨ r Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany Received March 20, 2003. In Final Form: July 18, 2003 The enhanced thermal stability of thin polymer bilayer films against dewetting is investigated. Stabilization results from the addition of a random functional copolymer. The model system consists of a silicon substrate covered with a bilayer of amorphous polyamide (PA), followed by a fully deuterated polystyrene (PSd). While the PA sublayer is stable against dewetting, the addition of a PSd homopolymer film allows dewetting to occur during annealing. By blending the PSd top layer with a fully protonated copolymer poly(styrene-co-maleic anhydride) (SMA2), containing 2% maleic anhydride groups in the chain, its dewetting process is retarded. This increase in stability is investigated as a function of copolymer and annealing time. Scanning force microscopy (SFM) is applied to determine the surface root-mean-square roughness and to check the stabilization effect. Information about the density profile is aquired from specular neutron reflectivity measurements. In addition, grazing incidence small angle neutron scattering (GISANS) is applied. GISANS utilizes the averaging capabilities of scattering methods, which is compared to local information obtained by SFM. Furthermore, GISANS enables the detection of buried structures in contrast to the SFM. An amount of 5% by volume SMA2 is sufficient to stabilize the bilayer film due to the creation of an enrichment layer of SMA2 at the PA:PSd interface. With creation of a brushlike interface, the mobility of the PSd molecules is decreased, which suppresses dewetting.

Introduction The thermal stability of polymeric films is of fundamental importance for numerous industrial thin film applications. While macroscopic films are stable, the wetting behavior of mesoscopic thin films is controlled by an interplay between short- and long-range forces.1,2 As a result of miniaturization, spatial dimensions are reduced (“shrink technology”) which gives rise to instability of polymer layers. For industrial applications as well as for scientific research, the spin-coating technique is applied for the preparation of thin films on a large variety of substrates including nonwettable surfaces such as polymer sublayers.3 In the case of nonwettable surfaces, the prepared films are metastable.4-7 The thermal load in processing can act as an annealing step above the glass transition temperature of the polymer. Annealing above the glass transition temperature results in a relaxation toward the thermodynamical equilibrium. As a consequence, these films rupture and holes are created in the films. The former homogeneous surface is destroyed. The * To whom correspondence may be addressed. † Infineon Technologies. ‡ TU Mu ¨ nchen. § MPI fu ¨ r Polymerforschung. | Institut Laue-Langevin. ⊥ Institut fu ¨ r Polymerforschung Dresden e.V. (1) Dietrich, S. In Phase Transitions and Critical Phenomena; Domb, C., Lebowitz, J. L., Eds.; Academic: New York, 1988; Vol. 12. (2) Israelachvili, J. N. In Intermolecular and surface forces, 2nd ed.; Academic Press: London, 1991. (3) Schubert, D. W. Polym. Bull. 1997 38, 177. (4) Vrij, A.; Overbeek, J. T. G. J. Am. Chem. Soc. 1968, 90, 3074. (5) Brochard, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (6) Vrij, A. Farady Discuss. 1966, 42, 22. (7) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682.

destabilization of metastable thin films is called dewetting and cannot be utilized further. For example, the rupture of an insulting polymer layer will destroy any electronic device. Dewetting has been studied in detail for many different systems;8-24 however the inverse process of thin film stabilization is much less investigated,25-27 even though it is a fundamental requirement for industrial applications.28 In the semiconductor industry the adhesion (8) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (9) Reiter, G. Langmuir 1993, 9, 1344. (10) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. Rev. Lett. 1996, 76, 1110. (11) Bishof, J.; Scherer, D.; Herminghaus, S.; Leiderer, P. Phys. Rev. Lett. 1996, 77, 1536. (12) Mu¨ller-Buschbaum, P.; Vanhoorne, P.; Scheumann, V.; Stamm, M. Europhys. Lett. 1997, 40, 655. (13) Jacobs, K.; Herminghaus, S.; Mecke, K. R. Langmuir 1998, 14, 965. (14) Jacobs, K.; Seemann, R.; Schatz, G.; Herminghaus, S. Langmuir 1998, 14, 4961. (15) Sferrazza, M.; Heppenstall-Butler, M.; Cubitt, R.; Bucknall, D.; Webster, J.; Jones, R. A. L. Phys. Rev. Lett. 1998, 81, 5173. (16) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251. (17) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602. (18) Segalman, R. A.; Green, P. F. Macromolecules 1999, 32, 801. (19) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Macromolecules 2000, 33, 4886. (20) Reiter, G. Phys. Rev. Lett. 2001, 87, 186110-1. (21) Suh, K. Y.; Lee, H. H. Phys. Rev. Lett. 2001, 87, 135502-1. (22) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Lorenz-Haas, C.; Wunnicke, O.; Stamm, M.; Petry, W. Macromolecules 2002, 35, 2017. (23) Ashley, K. M.; Meredith, J. C.; Amis, E.; Raghavan, D.; Karim, A. Polymer 2003, 44, 769. (24) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261. (25) Yerushalmi-Rozen, R.; Klein, J.; Fetters, L. J. Science 1994, 263, 793. (26) Renger, C.; Mu¨ller-Buschbaum, P.; Stamm, M.; Hinrichsen, G. Macromolecules 2000, 33, 8388. (27) Wang, C.; Krausch, G.; Geoghegan, M. Langmuir 2001, 17, 6269.

10.1021/la0344837 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/26/2003

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of thin films such as photoresist onto silicon (Si) or silicon oxide (SiO) is essential for optical lithography applications. A primer, hexamethyldisilozane (HMDS), is applied onto the Si or SiO before spin-coating the resist. This addition preparation step is necessary to promote adhesion.28 In the presented investigation, a technique is used which can be applied successfully for bulk systems. This technique increases the compatibility between two immiscible polymers, thereby achieving a retardation of dewetting in the thin film geometry. In the case of bulk applications, a blend of two polymers, A and B, can become compatible by the addition of a third component which is a functional copolymer.29-35 For this technique, referred to as “reactive compatibilization”, the functional copolymer must be miscible with one component, A, and capable of forming a blend (reaction) to the second polymer, B. In the polymer blend system of polyamide (PA) and polystyrene (PS), the reactive random copolymer poly(styrene-co-maleic anhydride) (SMA) was applied with various amounts of maleic anhydride (MA) groups in the chain to increase adhesion.30-35 The focus of past investigations toward this system is primarily due to the unique properties of the individual polymers, PA and PS. The enhanced compatibility between the two-phase PA and PS system results from an enrichment of the functional copolymer at the PA:PS interface during melt extrusion. SMA containing a small percentage of MA lower than 3% is reported to be miscible with PS. Furthermore SMA of low molecular weight increases the effectiveness of the compatibilization.34-35 Analogous to the bulk experiments, the samples in this investigation are bilayer films consisting of an amorphous polyamide (PA) sublayer and a fully deuterated polystyrene top-layer (PSd) blended with a copolymer. The reactive random copolymer poly(styrene-co-maleic anhydride) containing 2% MA groups in the chain (SMA2) is utilized. For neutron scattering experiments deuteration was performed to increase the contrast of the system. The large number of interfaces between PA and PS in the bulk is replaced by one interface in the thin film geometry. The reported improvement of the adhesion between both components, PA and PS, is expected to be transformed into a retardation of the dewetting or stabilization in the thin film geometry. In earlier experiments, dewetting of thin polystyrene (PS) homopolymer films upon a PA sublayer was reported.26 As a consequence, the pure bilayer system PA: PSd upon a silicon surface is unstable. However, PA itself remains stable on a silicon surface26 giving rise to welldefined starting conditions for the present investigation of routes toward stability. To prove the feasibility of the stabilization concept, the enhanced thermal stability as well as the stabilization mechanism (the enrichment of SMA2 at the PA:PSd interface) is investigated. SMA2 is expected to be miscible with PS due to the reactive random copolymer poly(styrene-co-maleic anhydride) which was reported to be miscible up to a MA content of 3%.31 Deuteration of PS is not expected to result in a nonmiscibility. In addition, (28) Microlithography; Sheats, J. R., Smith, B. W., Eds.; Marcel Dekker: New York, 1998. (29) Park, I.; Barlow, J. W.; Paul, D. R. J. Polym. Sci. Polym. Phys. 1992, 30, 1021. (30) Dedecker, K.; Groeninckx, G. Pure Appl. Chem. 1998, 70, 1289. (31) Dedecker, K.; Groeninckx, G. Polymer 1998, 39, 4985. (32) Dedecker, K.; Groeninckx, G. Polymer 1998, 39, 4993. (33) Dedecker, K.; Groeninckx, G.; Inoue, T. Polymer 1998, 39, 5001. (34) Dedecker, K.; Groeninckx, G. Macromolecules 1999, 32, 2472. (35) Dedecker, K.; Groeninckx, G. J. Appl. Polym. Sci 1999, 73, 889.

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SMA2 was shown to form hydrogen bonds to the amino groups of PA.32,33 With respect to the experimental techniques applied, neutron scattering has the advantage of displaying a large contrast between protonated and deuterated material. To reduce the background from protonated materials in the neutron scattering experiments, a protonated SMA2 (minority component) and a fully deuterated PSd (majority component) were chosen. From specular neutron reflectivity experiments, the scattering length density (sld) profiles perpendicular to the silicon surface result. The sld profile enables the investigation of a possible structuring perpendicular to the surface, e.g., due to enrichment of one component. SFM (scanning force microscopy) complements the analysis and yields the surface topography. Additionally, by applying GISANS (grazing incidence small angle neutron scattering), the chemical composition parallel to the silicon surface is obtained. In the early stages of dewetting, the surface roughness increases. Thus a detailed analysis of the surface topography is of fundamental importance to evaluate the stabilization effect. However, a change of a surface roughness might also result from the blend nature of the top layer. Thus GISANS is applied to distinguish both lateral structuring processes. The combination of techniques operating in real and reciprocal space was successfully applied previously to study dewetting processes.36-38 With the present investigation, it is demonstrated that these techniques are well suited for the investigation of stabilization effects as well. In this article the introduction is followed by an Experimental Section describing the sample preparation and the techniques used. The section Results and Discussion is followed by a Summary and Outlook. Experimental Section Sample Preparation. Silicon (100) wafers (Si) covered with approximately 1.6 nm of native silicon oxide were used as substrates (provided by Wacker Siltronic, Burghausen, Germany). Prior to spin-coating, the substrates were cleaned:26 The Si substrates were placed in dichloromethane in an ultrasonic bath for 5 min and rinsed with Millipore water shortly after. To clean the surface of organic traces, the Si substrates were stored for 2 h in an oxidation bath at 75 °C consisting of 1400 mL of Millipore water, 120 mL of H2O2, and 120 mL of NH3. Thereafter the samples were stored in Millipore water. The Si substrates were rinsed with Millipore water at last five times to remove possible traces of the oxidation bath. The samples were dried using compressed pure nitrogen directly before spin-coating the Si surface. The bilayer films on the Si surfaces were prepared by two successive spin-coating steps. The sublayer of the bilayer consists of amorphous polyamide 6,I (PA) from Bayer AG (Leverkusen, Germany; trademark: Durethan T40) with a molecular weight Mw ) 28900 g/mol (Mw/ Mn ) 3.28, Tg ) 130 °C). A homogeneous PA layer was obtained by first applying spin-coating of a 1,2-chlorphenol solution for 120 s at 1950 rpm.26 Immediately after the first spin-coating step, the top layer was spin-coated from a tetrahydrofuran solution. Fully deuterated polystyrene with Mw ) 2210 g/mol (Mw/Mn ) 1.05, Tg ) 65 °C) synthesized by Polymer Standard Service (Mainz, Germany) was used and blended with the reactive random copolymer poly(styrene-co-maleic anhydride) (SMA2) from Bayer AG (Leverkusen, Germany) containing 2% maleic anhydride (MA) (36) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Phys. Chem. Chem. Phys. 1999, 1, 3857. (37) Mu¨ller, M.; MacDowell, L. G.; Mu¨ller-Buschbaum, P.; Wunnicke, O.; Stamm, M. J. Chem. Phys. 2001, 115, 9960. (38) Mu¨ller-Buschbaum, P.; Hermsdorf, N.; Gutmann, J. S.; Stamm, M.; Cunis, S.; Gehrke, R.; Petry, W. Submitted for publication in J. Macrmol. Sci. Phys.

Polymer Bilayer Films groups in the chain (Mw ) 170000 g/mol, Mw/Mn ) 1.7, Tg ) 109 °C). Several identical samples of each blend composition (0, 1, 5, 10, 30 vol % SMA2 in the blend film) were prepared by varying the volume content of the copolymer in solution. After preparation, the samples were annealed under vacuum conditions at 119 °C. After the desired annealing time, the samples were quenched to room temperature and examined. No dewetting of the PA sublayers was observed during annealing at temperatures below T ) 195 °C for 1000 h.26 Optical Microscopy. The sample surfaces were observed with optical microscopy using a Zeiss Axiotech 25H optical microscope with magnifications between 4 times and 50 times. A Hitachi KP-D50 CCD camera recorded the micrographs. Scanning Force Microscopy. The sample topography was investigated by a Digital Instruments DI3100 scanning force microscope (SFM). The SFM was used in tapping mode to reduce tip-induced surface degradation and sample damage. The image acquisition was performed in air. The cantilevers were operated slightly below their resonance frequency of around 275 kHz. Different scan ranges were applied ranging from 5 µm × 5 µm (to achieve a high lateral resolution) up to 40 µm × 40 µm (to detect possible dominant length scales). The background due to the sample stage movement was fully subtracted from the data to determine the surface root mean square (rms) roughness over the complete scan area. For presentation, the z-ranges of all images were scaled to maximize the contrast. To enable a comparison with the GISANS experiments, a statistical treatment of the SFM images was necessary. From the surface topography images, the two-dimensional power spectral density (PSD) was calculated using a two-dimensional fast Fourier algorithm (FFT). In every case the FFT displays an isotropic intensity distribution resulting from an isotropic surface topography. Therefore, a radial averaging of the FFT was applied. PSD from various scan sizes of one sample were combined into one PSD master curve39 to enlarge the q-range in comparison to the individual PSD of one limited scan image. Nevertheless, SFM images contain only local information about the surface topography in contrast to scattering experiments under grazing incidence, where a large surface area is illuminated. To underline the statistical relevance of the SFM data and to enable an investigation of the compositional origin of the structures measured, all PSD master curves were compared with neutron scattering data. Specular Neutron Reflectivity. The specular neutron reflectivity (NR) experiments were carried out at the ADAM beamline of the Institute Laue-Langevin, Grenoble (France).40 In the probed region near the total external reflection, a dynamic range of 6 orders of magnitude or more was achieved. A graphite monochromator PG (001) was used to select a wavelength of λ ) 0.441 nm with a resolution of ∆λ/λ ) 0.006. By use of a zero dimensional 3He detector, the specular intensity was recorded with a low background noise of 2 counts/min to enable a q-range of qz,max ) 3 nm-1. The fitting of the NR data was performed using a nonlinear least-squares routine based on a matrix formalism.41 The interfaces were modeled by a tanh-shaped scattering length density (sld) profile with an interfacial roughness σ. This type of profile is commonly used for substrates42 as well as for the interface between polymers.43 Although in the case of real copolymers, the profile will differ from a tanh shape.43 The parametrization with tanh profiles was successfully used in the past. Deviations between different profiles are mainly visible at large wave vectors qz42 and will not yield significantly different fits in the case of these data. However, in the case of an interface roughness from the range of the layer thickness, the values of thickness and interface roughness are ambiguous. Thus the corresponding parameters may be misleading and were restricted to the resulting sld profile for further analysis. In contrast to (39) Gutmann, J. S.; Mu¨ller-Buschbaum, P.; Stamm, M. Farady Discuss. 1999, 112, 285. (40) Schreyer, A.; Siebrecht, R.; Englisch, U.; Zabel, H. Physica B 1998, 248, 349. (41) Parratt, L. G. Phys. Rev. 1954, 95, 359. (42) Bahr, D.; Press: W.; Jebasinski, R.; Mantl, S. Phys. Rev. B 1993, 47, 4385. (43) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. Stamm, M.; Schubert, D. W. Annu. Rev. Mater. Sci. 1995, 25, 325.

Langmuir, Vol. 19, No. 20, 2003 8513 homogeneous layers, the sld profile of a dewetted layer differs strongly from the one of a homogeneous layer. The basic origin of this difference is the absence of a continuous layer in the case of dewetting.9 The fit to the reflectivity data was always performed with the smallest number of layers enabling a reasonable description of the data. Different sets of thickness, interface roughness, and refractive indices may lead to similar sld profiles.44 However, taking the known stack sequence into account, the calculated profile of the sld itself is unambiguous. The surface excess was therefore determined from the sld profiles for further analysis. To prove the significance of the sld profiles, the surface roughnesses determined from specular neutron reflectivity and SFM were compared. Grazing Incidence Small-Angle Neutron Scattering. The grazing incidence small-angle neutron scattering (GISANS) experiments were performed at the D22 beamline at Institute Laue Langevin ILL/Grenoble (France). With a wavelength selector (∆λ/λ ) 10%) a mean wavelength of λ ) 0.6 nm was installed. Extremely narrow cross-slits, a large collimation distance (d ) 17.67 m), and evacuated beam paths were used to obtain a resolution of ∆q ) 4.45 × 10-3 nm-1. A detailed description of the D22 beamline, which is usually operated in transmission geometry, is given in ref 45. The GISANS technique is based on the same principal ideas as grazing incidence smallangle X-ray scattering (GISAXS), but using neutrons instead of X-rays as a probe.46 Both techniques GISAXS and GISANS are operated in a reflection geometry. GISAXS was previously described in detail.47,48 In this section only a short description of GISANS is presented. While in specular reflectivity, the incident and exit angle are varied obeying Ri ) Rf, GISANS is performed at one fixed angle of incidence Ri. The intensity scattered from the sample is detected by a two-dimensional detector (128 × 128 pixels). GISANS does not consider the specular reflection but deals with the off-specular scattering collected with the two-dimensional detector. Perpendicular to the sample surface, the scattered intensity distribution is characterized by the exit angle Rf and parallel to the sample surface by the out-of-plane angle ψ. Consequently different information is obtained from vertical and horizontal (with respect to the sample surface) slices of the two-dimensional intensity distribution. So-called out-of-plane scans result from a slice performed at a constant exit angle Rf along the horizontal direction. From the out-of-plane angle ψ, the reciprocal space vector component qy is calculated using qy ) 2π sin(ψ) cos(Rf)/λ. The component qy lies in the sample surface perpendicular to the incident neutron beam. Thus, these horizontal slices exhibit intensity which was scattered from in-plane structures of the sample surface. Statistics of these slices were improved by integrating the intensity (∆qz ) (1.91 × 10-2 nm-1). As a consequence, horizontal slices are well suited for the detection of lateral structures. Since the rms surface roughness is small, satisfying σrmsqy < 1, the diffuse, scattered intensity is calculated in the framework of the distorted-wave Born approximation (DWBA). For a given illuminated surface area A, wavelength λ, and Fresnel transmission functions Ti,f, the differential cross section is given by49,50

dσ Aπ2 ∼ 4 | Ti|2 |Tf|2 F(q b) dΩ λ

(1)

Because Ri and Rf are fixed in GISANS, the Fresnel transmission (44) Tolan, M. X-ray Scattering from Soft-Matter Thin Films; Springer-Verlag: Berlin, 1999. (45) Guide to Neutron Research Facilities at the ILL; Bu¨ttner, H. G.,Lelievre-Berna, E., Pinet, F., Eds.; 1997; p 32. (46) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M.; Cubitt, R.; Cunis, S.; von Krosigk, G.; Gehrke, R.; Petry, W. Physica B 2000, 283, 53. (47) Levine, J. R.; Cohen, J. B.; Chung, Y. W.; Georgopoulos, P. J. Appl. Crystallogr. 1989, 22, 528. (48) Naudon, A.; Thiaudiere, D. J. Appl. Crystallogr. 1997, 30, 822. (49) Salditt, T.; Metzger, T. H.; Peisl, J.; Reinecker, B.; Moske, M.; Samer, K. Europhys. Lett. 1995, 32, 331. (50) Naudon, A.; Babonneau, D.; Thiaudiere, D.; Lequien, S. Physica B 2000, 283, 69.

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Figure 1. Optical micrographs of the surface of a bilayer film consisting of a PSd film with an initial film thickness of 64 nm on top of a 41 ( 4 nm PA sublayer before (left) and after annealing for 100 h (middle) and 300 h (right) at 119 °C. Before annealing the initial sample surface is homogeneous and uniform. After annealing for 100 h at 119 °C, the top layer is completely dewetted: large drops of PSd are created on the PA sublayer. functions Ti,f act only as overall scaling factors and the diffuse scattering factor F(q b) is directly detected. For N identical and centrosymmetrical objects with a random orientation, the diffuse scattering factor can be approximated to depend on the form factor of the individual objects P(q b) and on the structure factor S(q b)51

F(q b) ∼ N S(q b) P(q b)

(2)

A mathematical description of the form factor depends on the type of object, while the structure factor directly yields the most prominent in-plane length ξ.43-45,49 Usually a distribution of the size of the scattering objects and of the distance between them is taken into account. In the case of non-monodisperse objects, the form factor contribution gives no longer rise to wellpronounced scattering features and consequently cannot be accurately determined from the scattering data. Polymer blends are one example of a system in which it is quite likely that a size distribution is rather wide and the form factor degenerates into an intensity decay P(q b) ∼ q-β. A statistical distribution of lateral sizes without any dominant in-plane length or length scales beyond the resolution limit results in a monotonic decrease of the scattered intensity toward larger out-of-plane angles ψ as well.36 Thus in the model fit, a power law decay of the intensity was taken into account. In addition, the experimental resolution was accounted for. Due to a selective deuteration of the PSd matrix, a compositional contrast between the fully protonated SMA2 molecules and the PSd matrix is achieved. While SFM provides only topographic surface information, GISANS enables information about the chemical nature of the dominant in-plane length investigated. From a combination of SFM and GISANS, a detailed analysis of the surface topography of thin polymer films is possible.

Results and Discussion In the bulk, the stabilization effect is based on the enrichment of SMA2 at the PA:PS interface. Consequently, the investigation is focused not only on the stabilization itself but also on the creation of an enrichment layer at the interface. Samples were annealed at 119 °C, which is well above the glass transition temperature of both components of the top layer, PSd and SMA2. In contrast, the PA sublayer remains highly viscous as discussed in detail earlier.26 Thus, only the polymers in the top layer are mobile. A. Enhanced Thermal Stabilization of Blended Bilayer Films. The dewetting kinetics of PA:PS bilayer films as a function of the molecular weight and of the thickness of the PS films were recently investigated.26 A homopolymer PS layer upon a PA layer is unstable and, although prepared initially homogeneously, decays into an arrangement of drops. In contrast, the PA sublayer remains stable and resists dewetting upon the silicon surface. Because deuterated PS was used in the present (51) Williams, C. E.; May, R.-P.; Guinier A. In Materials Science and Technology; VCH: Weinheim, 1993; Vol. 2B, Chapter 21, p 1.

investigation, the reference sample for verifying the retardation effect of added copolymer is a PA:PSd bilayer. Figure 1 shows the surface topography as visible with optical microscopy. Following preparation the surfaces are smooth; however after annealing for 100 h, drops are visible. These drops remain stable during further annealing on the order of 300 h. No significant rounding of the contact line between the PSd drops and the PA/air interface is visible. Therefore the driving force of the destabilization, resulting from a minimization of the surface free energy density, is quite small. Regardless, a pure PSd layer is unstable, and the deuteration has no stabilizing effect, as expected. To address the stabilization of the top layer, the homopolymer PSd is blended with a statistical copolymer. The added amount of reactive random copolymer poly(styrene-co-maleic anhydride) containing 2% MA groups in the chain (SMA2) was varied in this investigation. For simplicity, the sample names are abbreviated by the added volume percentage, e.g., SMA2-1% for 1 vol % of SMA2 added to PSd, SMA2-5% for 5 vol %, and so on. Because optical microscopy is restricted toward small in-plane lengths, the characterization in real space was performed by SFM. SFM is established to study the onset of the dewetting process.13,16,22 At the beginning of the dewetting process, the surface roughness increases due to a nucleation and growth process of a binodal process13,14 or an amplification of capillary forces of a spinodal process.16,22 Irrespective of the dominating process, the first changes in the surface roughness are in the regime of a few nanometers only. Figure 2a-d shows SFM images before (left-hand side) and after annealing (right-hand side) of the bilayers containing SMA2. Data from samples without SMA2 are not shown for comparison because the surface roughness introduced by the drops is too large to be visible by SFM on a 10 µm scan range. The prepared samples (a) SMA2-1%, (b) SMA2-5%, (c) SMA2-10%, and (d) SMA230% exhibit a grainy surface roughness. The lateral extension of the grains and the height of the surface amplitude change with increasing SMA2 content. After being annealed, the surface is modified irrespective of the amount of SMA2 added. In the case of 1% SMA2 added, well-developed deep holes are visible (Figure 2a, right side). The holes extend to the PA sublayer; therefore these holes are a fingerprint of a starting dewetting process. As compared to the pure homopolymer film, a significant retardation of the dewetting process is present. It requires 300 h of annealing to install the first small holes with a diameter on the order of micrometers, while 100 h is sufficient to destroy the nonstabilized film completely. As shown by SFM, samples with a higher SMA2 content exhibit flat and uniform surfaces. In contrast to a rupture, the surface has moreover smoothed as compared to the measurement right after preparation. To qualify this

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Figure 2. SFM images (scan range 10 × 10 µm2) of the topography signal as observed before (left) and after annealing (right) for 300 h (1, 5, 10 vol % SMA2) and 500 h (30 vol % SMA2) at T ) 119 °C. The height scaling is different for each pattern to depict the in-plane structure more clearly. (a) SMA2-1%, (b) SMA2-5%, (c) SMA2-10%, and (d) SMA2-30%.

trend, the rms surface roughness σ is calculated from the SFM data of different scan ranges. Figure 3 shows the averaged values (squares). The error bars account for deviations between different sample positions as well as for the error resulting from averaging. Before the sample was annealed, a mean surface roughness of 0.85 nm was obtained while a significantly higher one of 1.5 nm resulted

in the case of 1 SMA2 added. In addition, the surface roughness developed differently after annealing. In the case of the SMA2-1% sample, the roughness increases strongly due to the holes created in contrast to the samples consisting of 5, 10, and 30% vol. With SMA2 in the blend film, the surface roughness decreases to 0.3 nm after annealing. This behavior of thin film smoothing is

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Figure 3. Surface roughness σ of the as-prepared samples (open symbols) and after annealing (filled symbols) for 300 h (1, 5, and 10 vol % SMA2) and 500 h (30 vol % SMA2) at T ) 119 °C determined by SFM (squares) and specular neutron reflectivity (circles). The straight and the dashed lines indicate the average root mean square surface roughness of the as prepared and of the annealed samples (5, 10, and 30 vol % SMA2), respectively.

Figure 4. Specular neutron reflectivity curves of the (a) asprepared and (b) annealed (for 300 and 500 h, respectively) samples. The filled symbols correspond to the PA sublayer. The open symbols represent the bilayer films. For the bilayer films, from top to bottom the SMA2 content decreases (a, 30, 10, 5, and 1 vol %; b, 30, 10, and 5 vol %). The solid lines are calculated from the best-fit scattering length density profiles shown in Figure 5. For clarity, the curves are shifted along the y-axis.

completely different from a starting dewetting process.26 Thus 5% SMA2 is sufficient to stabilize the bilayer system. A further increase in the amount of added SMA2 has no influence in the opposite direction toward a reentering destabilization. One main reason is the miscibility between PSd and SMA2, which prevents the creation of nucleations sites and a phase separation. B. Enrichment of SMA2 at the PA:PSd Interface. To gain information about the internal composition of the bilayer system, specular neutron reflectivity experiments were performed. In Figure 4 the reflectivity data of the prepared samples (Figure 4a) and of the annealed samples (Figure 4b) are shown. In each graph from the top to the

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bottom, the SMA2 content decreases (30, 10, 5, 1 vol %). For comparison, data of the pure as-prepared PA sublayer are added to Figure 4a. The reflectivity data of the single PA film exhibit well-pronounced fringes due to a uniform, smooth, and homogeneous sample surface. The surface roughness of the PA layer was determined by the abovedescribed model fit. The thickness of all prepared PA layers is 41 ( 4 nm. The error accounts for the deviation between different samples, whereas the error of the individual fits is smaller. In the case of all bilayer samples, the PA interface roughness of the prepared samples σn ) 0.25 ( 0.1 nm agrees well with the values of the surface roughness of the prepared PA sublayers (σn ) 0.2 ( 0.05 nm). Thus, a swelling or roughening of the PA sublayer during the preparation of the top layer by spin coating is negligible. This results mainly from the use of a solvent for preparing the top layer, which does not solve the sublayer (PA). As shown earlier, the PA layer is stable up to temperatures above 195 °C.26 As a consequence, it was assumed that modifications in the neutron reflectivity curve after annealing at 119 °C only result from changes in the top layer. The once determined interface roughness of the PA sublayer was kept constant during the fitting procedure of the bilayer reflectivity data. Only the parameters of the top layer were modified during fitting. A total thickness of all prepared top layers, containing the PSd/SMA2 blend, of 63 ( 4 nm, was determined. The given error bar includes deviations between different samples, whereas the errors of the fits are on the order of (0.1 nm only. In Figure 3, the rms-roughness data obtained from specular neutron reflectivity σn (circles) are compared with the values calculated from the SFM data σsfm (squares). The determined surface roughness σn agrees well with σsfm. Deviations can be attributed to the specifics of both experimental techniques: SFM probes only a local surface area whereas neutron reflectivity addresses several square centimeters. SFM only detects the topography whereas with neutrons the density profile is investigated. In general, directly after preparation, the sample surfaces are rather smooth with the exception of sample SMA2-1%. Figure 4b shows the specular neutron reflectivity curves after annealing at 119 °C for 300 h (SMA2-5%, SMA210%) and for 500 h (SMA2-30%). The specular reflectivity data of the sample SMA2-1% was not recorded after annealing due to the creation of deep holes in the top layer as detected with SFM. The lateral structures are studied in detail in the next section. In specular reflectivity experiments holes influence the refractive index of a layer due to the detection of a mean refractive index averaged over holes (n ) 1) and polymer material (n < 1).44 To avoid misinterpretation of the sld profiles, the reflectivity curve of this sample was not taken into account. After annealing, the fringes in the reflectivity data are damped out. This may be due to an increased surface roughness which is in contradiction to the SFM observations. From the model fits yielding the sld profiles, a smearing out of the internal PA:PSd/SMA2 interface was detected. Figure 5 depicts the sld profiles giving rise to the model fits shown in Figure 4. The protonated PA has a lower sld as compared to the Si substrate or the PSd/SMA2 top layer. With increasing amounts of added protonated copolymer SMA2, the mean sld of the top layer decreases. From the sld profile, information about the internal sample structure perpendicular to the Si surface is available: with increasing distance from the PA interface, the sld increases slowly for each sample. Since the PA surface roughness was kept fixed, this results from changes in the PSd/SMA2 layer. The local gradient at the

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Figure 5. Scattering length density profiles yielding the fits shown in Figure 4 of the (a) as-prepared and (b) annealed (for 300 and 500 h at 119 °C) samples. The dash-dot, the solid, the dashed, and the dashed-dot-dot lines correspond to 30, 10, 5, and 1 vol % SMA2, respectively. The curve of the sample consisting of 1 vol % SMA2 is shifted to lower z′ values according to the thicker PA sublayer (see text) in such a way that the PA-blend interface is at the same z′ position for all profiles.

PA:PSd/SMA2 interface is a figure of merit for the enrichment of protonated polymers at the PA interface due to the lower sld of protonated material (FSMA2 ) 1.41 × 10-4 nm-2) in comparison to deuterated material (FPSd ) 6.51 × 10-4 nm-2). After annealing, the gradient seems to be smoothed out, which results from an increased amount of protonated SMA2 at the PA interface. Following this SMA2 enrichment layer, a constant sld is observed according to a composition resulting from blend film of protonated SMA2 and deuterated PSd. After annealing, an additional increase of the sld toward the film surface is visible. It corresponds to an enrichment of deuterated polystyrene or a depletion of protonated SMA2 at the free surface. Nevertheless, the sld value of the pure deuterated material is not reached, which directly shows that SMA2 is present at the surface as well. Before annealing, no additional enrichment layer at the free surface is detected. To describe the amount of SMA2 molecules at the internal interfaces, the interface excess is calculated from the concentration profile. The concentration of SMA2 is calculated from the scattering length molecules cSMA2 z density assuming a linear superposition of the scattering length densities of SMA2 and PSd

Fz ) xFSMA2 + (1 - x)FPSd z z

(3)

to the where x denotes the contribution of SMA2 FSMA2 z overall scattering length density Fz based on the model fit. From the scattering length density profile of SMA2, FSMA2 , the concentration of SMA2 as a function of the z depth z is determined52

cSMA2 z

)

FSMA2 MM z NA

∑j bj

(4)

MM is the molecular weight of the monomer, NA is the Avogardo number, and ∑jbj is the summation over the coherent scattering length of the monomer. Figure 6 (top)

Figure 6. (top) Concentration profile of the top layer containing 5 vol % SMA2 as-prepared (solid line) and after annealing (dashed line) for 300 h at 119 °C. The vertical lines indicate the position of interface between the SMA2 enrichment layer and the PSd:SMA2 blend film. (bottom) Interface excess of SMA2 molecules at the interface between the PA sublayer and the blend film as-prepared (open circles) and after annealing (filled square) for 300 h (10 and 5 vol % SMA2) and 500 h (30 vol % SMA2) at 119 °C.

depicts, as an example, the concentration profile of the SMA2-5% blend film as-prepared and after annealing for 300 h at 119 °C in the vicinity of the PA-blend interface. The vertical lines illustrate the width of the SMA2 enrichment layer. Within the blend film, the concentration SMA2 of SMA2 molecules cblend is constant. After annealing, the blend film exhibits a lower concentration of SMA2 molecules. On the other hand, the thickness of the SMA2 enrichment layer is increased. To verify the increasing amount of SMA2 molecules at the PA interface, the surface excess is calculated using53

Γ(n) SMA2 )

SMA2 - cblend ) dz ∫z zPA. then both values, cSMA2 z Within the blend film the integration is also zero (cSMA2 z SMA2 ) cblend ). Thus, the calculated amount of SMA2 molecules at the PA:PSd/SMA2 interface is independent of the chosen position of the interface z*.53 This analysis of the scattering length density profile enables the comparison of the SMA2 enrichment layer before and after annealing without defining the interface z*. For an investigation of the diffusion process of SMA2 molecules within the PSd

(52) Pethrick, R. A.; Dawkins, J. V. Modern Techniques for Polymer Characterisation; Wiley: Chichester, 1999. (53) Schwuger, M. J. Lehrbuch der Grenzfla¨ chenchemie; Georg Thieme Verlag: Stuttgart, 1996.

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Figure 7. GISANS data recorded at the D22 beamline. The left and the right image depict the two-dimensional intensity distribution of the as-prepared and of the annealed (300 h at T ) 119 °C) sample containing 1 vol % SMA2.

matrix, additional experiments will be necessary, which is beyond the scope of this investigation. Figure 6 (bottom) exhibits the surface excess of SMA2 molecules at the PA interface as a function of the volume content of SMA2 as-prepared and after annealing of the samples. With increase in the amounts of added copolymers, the surface excess of SMA2 at the interface increases before as well as after annealing. It must be mentioned that no direct information about the thickness of the enrichment layer is gained from the surface excess. From the scattering length density profiles as well as from the concentration profiles, the thickness of the enrichment layer is estimated to be in the order of the radius of gyration of the SMA2 molecules (Rg(SMA2) ≈ 11 nm) after annealing. A surface excess of 7 mg/m2 is sufficient to stabilize the top layer. A similar increase of the surface excess of PSd at the free surface was observed in a miscible blend of protonated and deuterated polystyrene.54-56 For the blend system PS/ PSd, a good agreement between the surface excess calculated with theoretical predictions was reported.56 C. Lateral Structures. Specular reflectivity experiments only address the density profile perpendicular to the sample surface and thus give no information about the lateral distribution. A diffuse scattering experiment is required to become sensitive to lateral structures. GISANS has been proven to detect possible dominant inplane length resulting from a chemical contrast between the protonated and the deuterated polymers.22,46 In the present case, fully protonated SMA2 within a fully deuterated PSd matrix is studied. The inverse chemical system would yield a higher background due to the large incoherent scattering from hydrogen, which is extremely disadvantageous for the sophisticated scattering technique GISANS. In the reversed case, the mean sld is significantly smaller than that compared to the investigated sample type. As a consequence, the related critical angles would have been smaller as well because Rc ∼ (sld)1/2. As an example, the two-dimensional intensity distribution recorded at the D22 beamline is shown in Figure 7 for the sample SMA2-1 before as well as after annealing. The out-of-plane angle axis was transformed into a qyaxis, and the exit angle Rf was kept unchanged to avoid (54) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989, 62, 280. (55) Leonhardt, D. C.; Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 685. (56) Genzer, J.; Faldi, A.; Composto, R. J. Phys. Rev. E 1994, 50, 2373.

Figure 8. (a) PSD master curves (open symbols) and GISANS (filled symbols) data of the samples directly after preparation (squares) and after annealing (triangles) for 300 h at T ) 119 °C of the samples containing 1 vol % SMA2. The solid lines are fits to the data as explained in the text. The position of structure factor peaks is marked with qA,B,C. The dashed lines indicate the resolution limit. (b) PSD master curves (open symbols) and GISANS (filled symbols) data of the samples directly after preparation (squares) and after annealing (triangles) of 500 h at T ) 119 °C of the samples containing 30 vol % SMA2. The solid lines are fits to the data as explained in the text. The position of structure factor peaks is marked with qA,B. The dashed lines indicate the resolution limit. For clarity, the curves are shifted along the y-axis.

the usual deformation in the q-space and to overcome a detector pixel based presentation. The specular peak (visible as the peak of maximum intensity) is located in both images at equal position (qy ) 0; Rf ) 0.57°) due to the fixed angle of incidence. From the two-dimensional data, a strong change due to annealing is visible. Mainly the shape of the Yoneda peak (intensity located at Rf < 0.57°) is changed indicating changes of the lateral density distributions. The two-dimensional scattering pattern can be understood to be built-up from several horizontal lines at different exit angles Rf or from several vertical lines at different out-of-plane angles ψ. The denotation horizontal and vertical is referred to the sample surface. The grazing incidence small-angle scattering information is obtained from horizontal cuts at a fixed angle Rf and thus depends on ψ only. For the horizontal cut, an exit angle Rf,0 ) 0.486° equal to the critical angle of PSd was chosen. Figure 8 compares these GISANS data with the PSD master curves calculated from SFM images of the samples

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SMA2-1% and SMA2-30%. The GISANS data are shown with filled symbols and the PSD master curves with open symbols in the common double logarithmic presentation. The solid lines are fits based on a model (see eqs 1 and 2) which takes into account a structure factor, the experimental resolution as well as scattering from surface roughness and the form factor contribution. The structure factor is assumed to have a Gaussian distribution and yields the most prominent in-plane length ξ. The resolution was determined experimentally. The surface roughness gives rise to a power law decay of the intensity for large qy-values. The wide distribution of the object size degenerates the contribution of the form factor to an intensity decay as well. The comparison between GISANS and PSD master curves helps to distinguish between surface topography and chemical structures. For presentation, we have chosen the examples with 1 and 30 vol % SMA2, due to the fact that 1 vol % SMA2 represents an insufficient stabilization and 30 vol % SMA2 a sufficient one. The data of the samples SMA2-5% and SMA2-10% are comparable to Figure 8b (sample SMA2-30%). The resolution limit is shown by the dashed lines, and prominent features in the fits are marked with arrows and labeled qA,B,C. Directly after preparation, a broad peak at the position qA is present in the PSD master curves (Figure 8, open squares). It corresponds to the structure factor of the grainy surface topography visible in parts a and d of Figure 2. The amount of SMA2 only effects the actual position of this peak, or in other terms the corresponding real space length ξ ) 2π/qA. Thus on comparison of parts a and b of Figure 8, the position of qA depends on the amount of SMA2 added. For a SMA2 concentration higher than 1 vol %, qA shifts toward larger q values or smaller ξ values, respectively. Figure 2 illustrates this change in the lateral size of the surface structure in real space. In contrast to the surface topography sensitive data, the GISANS data (Figure 8, filled squares) depend on the lateral scattering length density distribution and thus on the amount of SMA2 added. In the case of 1 vol % SMA2 (Figure 8a), a peak at a position qB > qA is visible and at the position qA the scattered intensity remains at a comparable high level. An additional peak at a position qB > qA can be explained only by the presence of an additional internal length scale inside the PSd/SMA2 layer, which is unaccessible to the SFM.57 The contribution resulting from the surface is due to these additional dominant length scales, and the marked differences in the scattering length densities are not that strong. It gives rise to an intensity plateau for q < qB. If 30 vol % SMA2 were added (Figure 8b), the peak at qA is present as well, but significantly weaker as compared to the PSD master curve. Thus no additional internal length was detected with GISANS. In other terms for SMA2 concentration above 1 vol %, no buried structures are present directly after preparation. After the samples were annealed, the surfaces changed significantly as visible by the SFM pictures (Figure 2). As a consequence, the intensity distributions of the PSD master curves (Figure 8, open triangles) are also changed. Qualitatively the PSD master curves of the sample SMA21% and SMA2-30% differ. In the case of 1 vol % SMA2 added (Figure 8a) two strong peaks are present. They result from the deep holes which are formed by the onset of dewetting. If a larger amount of SMA2 was added, dewetting is prevented. The PSD master curve of the sample SMA2-30% exhibits a smeared out peak at a (57) Mu¨ller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Cubitt, R.; Stamm, M.; Petry, W. Langmuir 2001, 17, 5567.

Langmuir, Vol. 19, No. 20, 2003 8519

Figure 9. Dominant in-plane length ξ determined from SFM data (open symbols) and from a fit to the GISANS (filled symbols) data directly after preparation (circles) and after annealing (triangles).

position qB. Again, the GISANS signals (Figure 8, filled triangles) differ from the PSD master curves. In the sample SMA2-1% (Figure 8a), only one peak at a position qC remains from the two peaks as detected with SFM. One possible reason for the deviation between PSD master curve and GISANS results from the difference in the probed surface area. SFM probes a rather local surface area and GISANS averages over a large area. It is quite probable that on a small area the surface structure appears to be more uniform as compared to a larger area. In the sample SMA2-30 (Figure 8b), GISANS detects a better pronounced peak as compared to SFM; therefore the structural information corresponds to a chemical one. The surface of the blend top layers consists of PSd and SMA2 and is not randomly rough. The presence of both components PSd and SMA2 at the surface is in good agreement with the sld profile as determined from specular neutron scattering (see Figure 5). In Figure 9, the dominant length scales ξ of the bilayer samples are shown as a function of the SMA2 content. Values resulting from the PSD master curves are plotted with open symbols and the ones resulting from GISANS with filled symbols. As may be expected in the case of blend systems, the morphology is not given only by the surface topography of the sample. The existence of buried structures in thin polymer blend films was reported for many systems. Usually, immiscible blends yield surface as well as internal structures.58-67 However, the chosen copolymer poly(styrene-co-maleic anhydride) was reported to be miscible with PS up to a MA content of 3%,31-33 which means that SMA2 should be miscible and no buried (58) Strawhecker, K. E.; Kumar, S. K.; Douglas, J. F.; Karim, A. Macromolecules 2001, 34, 4669. (59) Jung, W. G.; Fischer, E. W. Makromol. Chem., Macromol. Symp. 1998, 16, 281. (60) Jones, R. A. L.; Norton, L. J.; Kramer, E. J.; Bates, F. S.; Wiltzius, P. Phys. Rev. Lett. 1991, 66, 1326. (61) Krausch, G. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 446. (62) Krausch, G. Mater. Sci. Eng. 1995, R14, 1. (63) Affrossman, S.; Henn, G.; O’Neill, S. A.; Pethrick, R. A.; Stamm, M. Macromolecules 1996, 29, 5010. (64) Schnell, R.; Stamm, M. Physica B 1997, 234-236, 247. (65) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995. (66) Karim, A.; Slawecki, T. M.; Kumar, S. K.; Douglas, J. F.; Satija, S. K.; Han, C. C.; Russell, T. P.; Liu, Y.; Overney, R.; Sokolov, J.; Rafailovich, M. H. Macromolecules 1998, 31, 857. (67) Mu¨ller-Buschbaum, P.; O’Neill, S. A.; Affrossman, S.; Stamm, M. Macromolecules 1998, 31, 5003.

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Figure 10. Model to explain the enhanced thermal stability of the polymer bilayer films due to an enrichment, bonding, and entanglement of the SMA2 molecules at the PA interface. The gray scale in the top layer indicates the concentration of SMA2 polymers.

structures should result. Addressing this point in more detail, structures directly after preparation via spin coating and structures created during long annealing have to be distinguished. The spin-coating process gives rise to nonequilibrium structures. The influence of the solvent evaporation on the surface roughness and structure creation has to be taken into account.58,67 The spin-coating process is modeled as a competition between the flow of material to the wafer edge and the evaporation of the solvent. THF is a more polar solvent than toluene. Thus THF fits well to the polar SMA2 molecules in contrast to the apolar PSd polymers. Different solubilities of two polymers for a particular solvent lead to different viscosity of the polymer species. On the other hand PSd and SMA2 are miscible. Thus the increasing amount of SMA2 leads to an increasing compatibilization of PSd and THF. This might be the origin of both, the decreasing surface roughness and the increasing in-plane length, with increasing amount of SMA2 right after preparation. On the other hand sufficiently long annealing yields a relaxation of the structures toward the equilibrium structure, which is determined by the minimization of the surface free energy density. As a consequence of bulk miscibility all buried structures vanish and only surface near structures remain. In the stabilized bilayer samples, although the surface has smoothed it is chemically heterogeneous. As detected from the neutron reflectivity, PSd has enriched at the free surface, but SMA2 is still present at the free surface as well. This explains why in the sld profile (Figure 5) the value of pure PSd is not reached at the surface. D. Model. Dominant lateral structures are observed only at the bilayer surface in the case of stabilized films and are therefore not of interest for the stabilization mechanisms itself. This gives confidence to restrict on the projected information as gained from the reflectivity. Figure 10 pictures a side view sketch to model the enhanced thermal stability of the bilayer films. SMA2 molecules are considered to be able to connect to PA via hydrogen bonds. It has to be noted, that a direct proof of the presence of hydrogen bonds was not possible in the thin film geometry. During further annealing, SMA2 copolymers segregate to the PA:PSd interface building up and increasing the SMA2 enrichment layer.68 Only copolymers connected via hydrogen bonds stick at this interface while others diffuse back into the PSd/SMA2 (68) Kuhlmann, T.; Kraus, J.; Mu¨ller-Buschbaum, P.; Schubert, D. W.; Stamm, M. Noncryst. Solids 1998, 235-237, 457.

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film. As a consequence the enrichment layer remains on the order of the radius of gyration of SMA2. Because SMA2 is a random functional copolymer, the hydrogen bond is created at a random position in the SMA2 chain. The parts of the SMA2 chain which are not directly connected to PA create a brushlike interface as pictured in Figure 10. The length of the brush is random again. This brush reduces the possible movement of PSd polymer chains in the top layer, and dewetting is retarded. Increasing the amount of SMA2 increases the amount of SMA2 molecules connected to the PA surface until saturation is reached. After the buildup of a complete brush layer, no free positions for additional hydrogen bonds are present. At a critical SMA2 concentration (on the order of 5 vol %) stabilization occurs. Thus, this effect is comparable to the earlier observed stabilization effect of thin polymer films on top of a brush consisting of the same polymer (wet brush),25,69-71 where the film sinks into the brush. Interpenetration between the brush and the polymer film stabilizes the film. Summary and Outlook The concept of compatibilization was successfully transferred from a bulk system to the thin film geometry. In the bulk the random functional copolymer SMA2 was proved to strengthen the PS-PA interface in fibers. In a bilayer system the addition of SMA2 stabilizes the PSd layer against dewetting on top of the PA sublayer. Already the addition of 1 vol % SMA2 retards the dewetting significantly and might be sufficient for all applications which are focused on a moderate annealing time. Above a critical concentration, e.g., at 5 vol % SMA2, no sign of dewetting was detectable, at least within the experimentally investigated time window. During annealing the blended films tend to smooth and stabilize themselves. A combination of scattering methods, neutron reflectivity, and GISANS, with real space techniques, microscopy, and SFM, was necessary to enable a modeling of the stabilization mechanism. Although the experimental conditions between melt extrusion (bulk) and spin-coating (thin film) are extremely different, our data suggest that a similar basic mechanism is present: the aggregation of SMA2 molecules at the PA:PSd interface. The addition of a small amount of a random copolymer can replace the two-step procedure with primers which commonly is necessary to stabilize polymeric bilayers on top of semiconductor surfaces. The use of a random copolymer instead of a diblock copolymer, which has the same effect, is extremely advantageous, because random copolymers are cheaper in their chemical synthesis. Thus, although the presented investigation is based on the model system PA:PS, it might have future implifications. Perhaps the principle of reactive compatibilization of bulky blends can be transferred to the enhanced thermal stability of thin polymer bilayer films in general. Acknowledgment. The used silicon substrates were kindly provided by Wacker Siltronic, Burghausen (Germany). S. Iacono helped with the manuscript. This work was supported by DFG-Schwerpunktsprogramm “Benetzung und Strukturbildung an Grenzfla¨chen” (STA324/8). In addition the support by the Max-Buchner-Stiftung and fruitful discussions with M. Mu¨ller and K. Binder are acknowledged. LA0344837 (69) Yerushalmi-Rozen, R.; Klein, J. Langmuir 1995, 11, 2806. (70) Reiter, G.; Sharma, A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551. (71) Edgecombe, S. R.; Gardiner, J. M.; Matsen, M. W. Macromolecules 2002, 35, 6475.