PHYSICAL REVIEW LETTERS VOLUME 62, NUMBER 16 Neutron Reflectivity Studies of the Surface-Induced 17 APRIL 1989 Ordering of Diblock Copolymer Films S. H. Anastasiadis and T. P. Russell IBM Research DivisionA, lmaden Research Center, 650 Harry Road, San Jose, California 95120-6099 S. K. Satija and C. F. Majkrzak Reactor Radiation Division, Nationa/ Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 12 December 1988) Neutron refiectivity from annealed thin films of poly(styrene-b-deuterated-methylmethacrylate), P(S-b-D-MMA), reveals the formation of a multilayered morphology parallel to the film surface. This multilayer forms so that PS locates, preferentially, at the air/copolymer and D-PMMA at the interfaces with layer thicknesses at these interfaces one-half that found in the bulk. ordering of copolymers in the phase-mixed state characterized by an exponentially damped cosine function. substrate/copolymer P(D-S-b-MMA) of lower molecular weight shows the first evidence of surface-induced I'ACS numbers: 61. 12.Ex, 61.41.+e, 68.55.Jk, 68.90.+g efI'orts have focused Experimental' and theoretical on the static and dynamic properties of predominantly block copolymers in the bulk, i.e. , not near a free surface or interface with other materials. Block copolymers are increasingly being used as surfactants, compatibilizing agents, and adhesives in biomedical and microelectronics applications. Thus, a fundamental understanding of the behavior of copolymers at surfaces or interfaces is essential. Few studies have appeared in the literature on block copolymers near surfaces or interfaces. The morphology near surfaces can be dramatically affected by the difrerence in the surface free energies of the two blocks and their affinity for the substrate. The product of the number of segments in a symmetric diblock copolymer, N, and the interaction parameter between the two blocks, g, determines the mor10.5, the copolymer is phase mixed; phology. ' If gN i.e. , the copolymer morphology is homogeneous with no If gN 10.5, the symmetric microphase separation. copolymer forms lamellar microdomains. In the bulk the microdomains are, on average, randomly oriented, though locally a coherent packing of the microdomains is found. In thin films, the lamellar microdomains are found to orient parallel to the free surface. Such oriented morphologies are ideally suited to address details of the microphase-separated morphology. However, studies have been hindered by limitations in staining techniques or spatial resolution. In the bulk, small-angle scattering has been used to elucidate morphological details. However, empirical to eliminate procedures scattering from thermally induced density Auctuations limit the precision to which information on the interfacial region can be obtained. Recently, Fredrickson presented a mean-field theory to describe surface ordering phenomena in diblock copolymers near the microphase-separation transition temperature (MST). At temperatures above the MST an exponentially damped, oscillatory density profile is predicted normal to the film surface. The period of this oscillation depends on temperature and is predicted to be larger ( ~ 1852 than 2tr/qo, where qo is the wave vector that characterizes the peak in the equilibrium bulk scattering function. In this Letter a neutron reflectivity study on the behavior of thin films of symmetric poly(styrene-b-methylmethacrylate) diblock copolymers is presented. Neutron reAectivity provides a means by which the surface and interfacial behavior of polymers can be investigated with a spatial resolution of less than 10 A. ' ' Selective deuterium labeling of one block provides ample contrast and highlights specific blocks of the copolymer chain. P (S-b-D-M MA) [poly(styrene-b-deuterium-labeledmethylmethacrylate)], [molecular weight (MW), 29780; M„/M„=l. 1], and P (D-S-b-M MA) [poly(deuteriumlabeled-styrene-b-methylmethacrylate)], (MW, 27 600; M /M„=1.08), were purchased from Polymer Laboratories, Inc. , and were used as received. The total numbers of segments in the copolymers N are 281 and 263 with fractions of PS segments of 0.50 and 0.43, respectively. The copolymers were dissolved in toluene (ca. 3g per 100 cm3) and spin coated onto a highly polished Si disk (100 mm diam; 10 mm thick). The specimens were placed in a vacuum at 60 C to remove solvent and then annealed under vacuum at 170 C for ca. 24 h. Specular neutron reAectivity, where the grazing angle of incidence is equal to the angle of reflection, was measured as a function of neutron momentum. The experiments were performed at the BT-4 triple-axis spectrometer at the reactor experimental hall of the National Institute of Standards and Technology. A graphite monochromator was used to select neutrons of wavelength X=2.35 with a resolution dX/X=0. 02. Slit collimators reduced the angular divergence to ca. 0.02'. The detector aperture had an acceptance angle of ca. 0.4 . The principles of neutron and x-ray reAectivity have been extensively treated in the literature. ' The neutron reAectivity profile is solely a function of the component of the incident neutron momentum, ko = (2tr/X) sin 8, perpendicular to the film surface, where 9 is the angle of incidence of the neutron beam with the surface. In a medium i composed of nuclei with scattering amplitude b 1989 The American Physical Society f 4 in volume,V a — 4~(b/V);] L REVIEW LETTE PHYS&C VOLUME 62, NUMBER 16 that such k 0 is modified k;=[ko2 &/2 h re flectivity One metthod ec ivi y profile is the o too calculate the Th e seg rix method. egment density profi e perpendicular r to thee free surface can e a ith la ers of d i 8'eren t scattering length densities. The reflec tance r;, ;+i at t h e b oun"dar n b't (i+1)th layers is r;;+1= and the total reAectance o (k, +k, ed from a recursion re lation a ion combining the '3Th ~ t g the ratio of the re reAected beam at a mome incident beam, is g iven as the square of thee magnitude of Aectance r, . The sur facee rroughness is deaussian function witth'a me of (z ) This modifies Ro by t e in Debye-Wailer factor ' as anc;; „), At tR ""' u'""1 R(ko) =Ra(ko) exp( —4ko2 z 2 In the cases es p resented here, &zz &' was at most S A. instrumental resoluis then convoluted wit the t e ins a d Alternatively,1 iit has been shown 13(' 'hat interface can be aw of reAection att an in from the Fresnel law nitudee o of the Fourier s the s uare of the magnitu 1 "fil e transform of the derivative of t h'e d'n'" to the surface. Both approac es requ quire modeling to evaluate the densi it y p rofile norma 1 t o the surface with ic solutions are the exception o f a few cases where anaal y tic available. f tth e P (S-b-D-MMA) is rofile for i . 1. Three orders of Bragg re A hih 40K ' are ko 0 0195, 0 0375 and 0.0540 e due to a multilayered structure wit in D-PMMA layers. The refractionf alternating PS' and Dcorrected correc e expression for the Bra gg law is 15 R 2 (&/L ) kBragg 2 —k,c2 , = kobserved (2) ctor of o total reAection. the critical wave vector =17S A i of L = p riod ' m .' ' small-angle sca tterin e g data from bu t e scattering sc length Fi . 1 is the fit using the hf e hown in the inset.t Th'hi fro th' t't'1 th"k i e ness of the specimen, 1- an and 84-A layers of eriods each with 91'1 A res p ectively, wit h a i near gradient of microdomains. e o a S4 A between the micro was verified independent y y xthicknesses of t e diA'erences in thee molecular m weig ts annd mass densities d-orderer Bragg reAecof the blocks, giives rise to a second-or a ive domain sizes. r sensitive to t e re lative int"f'"'1 Ales are shown in Fig. 2 us 52 54, and 56 . W i th'nin th' th k ' th data and the mo el used, the in er a S4 nt with previous resu ts. e asize that such precision on portant to emp hasize as been, heretofore, una ttainable by other a her functional forms d fun't r radient, e.g. , a cosine-s u'ntl t e d't' a a. can yield an eequuall y good fit to th' ac' gradient canh inter t facial the exact functiona 1 form of the with these data. It s h ouuld also be noted P o rise near 1y that the interfaces compris specimen. a PS is preferentially louch tthat The multilayer forms such ~ """ ~ '"'"'" 'f C'"" ofh 1O-' 1O-' 10 17 A.PRIL 1989 2 10 10 R 10 1O-4 1O-4 10 10-6 I I 0.02 I 0.04 106 I 0.06 I I 0.02 0.08 ko(A 1) ko(A S-b-Dflectivit of neutrons for a diblock copo 1ymer mer film cast onto a si 1&con su b d C for 24 h. The soli1'd line ine is the calculated the scattering lengt }1 d ensii ofil ho the inset, w h ere re z =0 corresponds to t h e a' =0.0540 40 4 face. A, and 0.08 ) rison of the measured specu ar reflectivity or 24 70 oC for A) annealed at 170 e wee p o e where the interface M MA) i/oo 0.06 0.04 ' where w the interfacial wi t is (3) 56 A. 1853 VOLUME 62, NUMBER 16 PHYSICAL REVIEW LETTERS cated at the air/copolymer interface, due to the lower surface energy of PS, and D-PMMA is preferentially located at the Si surface due, most likely, to interactions between D-PMMA and the substrate. Electron microscopy studies have shown that the layer nearest the air interface is thinner than the layers in the bulk. From our analysis, the thicknesses of both the top and bottom layers were found to be one-half that of the respective layers within the multilayer. The scattering length densities of the two phases are found to be different from those predicted for the pure blocks [(2.65 ~0.05) X10 A compared with 1.43 X 10 comfor PS, and (6.25+ 0.05) &&10 A A A for D-PMMA]. This indipared with 6. 8X10 cates partial mixing of the components (24% and 10% for the PS and D-PMMA domains, respectively) which is consistent with the large interface thickness. In order to assess the degree of orientation of the multilayers parallel to the surface, the detector with a 0.04 acceptance slit was fixed at the angle corresponding to the Bragg refiection at 0.0195 A ', and the specimen was "rocked" around the angle of the peak. This moves the scattering vector to off-normal positions. The half width at half maximum of the rocking curve was 0.02 which includes contributions from the disorientation of the lamellae and instrumental broadening. Therefore, this result indicates a near-perfect orientation of the lamellar microdomains parallel to the surface over large x-ray lateral distances. Specular and off-specular on the same specimen, where reflectivity measurements' the beam collimation was tighter and the acceptance angle of the detector slit was smaller, support this finding. Thus, smearing of the density profile by misalignment of the lamellae is minimal. The specular refiectivity for the P(D-S-b-MMA) specimen is shown in Fig. 3. The most pronounced features of this profile are the high-frequency oscillations corresponding to a total specimen thickness of 1400 A and a broad shoulder occurring near ko=0. 025 A The absence of Bragg reflections clearly shows that there is no significant orientation of domains with respect to the surface. In fact, small-angle neutron scattering studies' on this copolymer in the bulk have shown that the MST for this copolymer occurs at 65'C since the interaction parameter g is given by @=0.0284+3.902/T, A maximum in the where T is the absolute temperature. correlation hole scattering is observed at qo=0. 043 A corresponding to a fiuctuation length scale of 146 A. Consequently, at 170 C, the annealing temperature, the copolymer is phase mixed. Calculations of the reflectivity profile assuming a film with a uniform scattering length density do not describe the observed data as seen in the uppermost curve in Fig. 3. Given the surface ener' and the affinity gy difference between PS and PMMA of PMMA to the silicon substrate, it is reasonable to expect excess concentrations of PS and PMMA at the air From the nature and substrate interfaces, respectively. 1854 17 APRIL 1989 100 10o 100 100 10 ' —2 10 10 10 4 10 106 I I I 0.02 0.04 0.06 ko(A 0.08 ) FIG. 3. Specular reffectivity of neutrons for a P(D-S-bMMA) diblock copolymer film annealed at 170'C for 24 h. The open circles are the experimental data. For clarity the comparisons with a film of (1) uniform scattering length density, (2) exponentially damped cosine variation from the air/ intercopolymer interface, (3) from the substrate/copolymer face, and (4) from both interfaces have been offset by a factor of 10. Inset: Scattering length density profiles used in the latter three calculations where, for (2) and (3), the oscillation opposite the interface of interest was omitted. of the block copolymer this would lead inevitably damped sinusoidal function from both interfaces. suming concentration profiles described by !t (z) =p, e '/~ sc(o2 z/trL)+p, to a As- (3) where g is the correlation length, L is the period, p, is the excess surface concentration, p is the average concentration, and z is the distance from the surface, calculated reflectivity profiles suitably described the experimental profiles. g and L should be independent of the surface and depend only on the copolymer and the distance from the MST. It was mandatory to assume such damped, oscillatory profiles from both surfaces. Neglect of either, as shown in Fig. 3, produced unacceptable fits to the data. From both interfaces L =150+ 10 A with 7 A. The only difference between the two oscilg =95 latory patterns was that the air/copolymer interface yielded a slight enrichment of PS at the surface, p+p, =0.65, whereas at the silicon interface the volume fraction of PMMA was essentially 1. Theoretical arguments of Fredrickson lead to a depth-dependent concentration profile similar in form to Eq. (3) where g and L are given by ~ PHYSICAL REVIEW LETTERS VOLUME 62, NUMBER 16 Germany, L =2tr[2I(1+ply, )1 '"Iqo, where =263 (gN), f)] g, =3'I /[f(l — I, (5) g=2gN —2(gN), +g„N is the total number of segments in the copolymer, is the value of gN at the spinodal, and qo has been previously defined. Substitution of these known quantities into Eqs. (4) and (5) yields (=104 A and L =150 A. These agree remarkably well with the experimental results. In summary, neutron reAectivity has been used to investigate thin films of symmetric diblock copolymers. For P(S-b-D-MMA) it has been shown that the lamellar microdomains orient parallel to the surface over very large distances. The interface between the P(S-b-DMMA) microdomains can be described with a linear profile having a width of 54~ 2 A. The first experimental evidence of surface-induced ordering at temperatures above the bulk MST for the P(D-S-b-MMA) copolymer is shown. An oscillatory segment density profile is observed that is damped exponentially in the bulk with a correlation length very close to the bulk correlation length. The measurements are in very good agreement with the current thermodynamic theory. This work was supported in part by Department of Energy, Office of Basic Energy Sciences Grant No. DE- FG03-88ER45 375. 'See, for example, M. Shibayama, T. Hashimoto, and H. Kawai, Macromolecules 16, 1434 (1983). 2See, for example, (a) E. Helfand and Z. R Wasserman, in Developments in Block Copolymers l, edited by I. Goodman (Applied Science, Essex, England, 1982); (b) D. J. 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