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Determination of the interfacial energies in chemical guiding patterns for directed self-assembly of block co-polymers

AutorEvangelio, Laura; Lorenzoni, Matteo; Fraxedas, J. ; Perez Murano, Francesc X.
Palabras claveDirected self-assembly of block co-polymers
Atomic force microscopy
Fecha de publicación2015
CitaciónMNE 2015
ResumenDirected self-assembly (DSA) of block co-polymers allows the generation of high-resolution patterns at wafer scale level. The characteristic feature size of the final pattern is dictated by the molecular weight of the block co-polymer, while its orientation is prompted by the pre-definition of guiding patterns on the surface. In chemical epitaxy DSA, the guiding patterns are defined as areas of the surface of varied chemical strength (affinity) with the blocks forming the co-polymer. In this communication, we present a method to experimentally determine the interface energies and a quantitative estimation between surface affinity strength and guidance capability of the chemical patterns. We use PS-b-PMMA as block copolymer and PS-OH as brush layer. Chemical guiding patterns are created by selective exposure of the brush layer to oxygen plasma]. The difference of surface free energy of each segment of the copolymer with the confining boundary (γSA - γSB), is the main driving force in chemical epitaxy DSA. It is experimentally estimated from de-wetting experiments using blends of homopolymers, which are performed under the same conditions than the ones used for DSA of as block copolymers. When thermally annealed, the two immiscible homopolymers exhibit phase separation and the contact angle ΦAB between both polymers (A and B) and the substrate (S) can be obtained (see figure 1). Then, the interfacial energy (Δγ) can be estimated by Youngís equation: Δγ = γSA - γSB = γAB.cos(ΦAB) where γSA and γSB are the interface tension between homopolymers A and B with the substrate, and γSA is the interface tension between both polymers, which depends on the annealing temperature. Figure 2 shows the typical morphology for a blend of PS/PMMA polymers on a pristine PSOH surface (a) and on a PS-OH surface chemically modified by oxygen plasma exposure (b). After selective removal of PMMA, the contact angle is estimated from cross-sectional SEM images. The properties of the formed droplet are further investigated by atomic force microscopy employing Peak ForceTM tapping mode. This technique allows the acquisition of multiple force distance curves with improved force resolution (0.1-10 nN), with real time calculation of mechanical properties at each point. Mechanical properties such as adhesion force and surface stiffness of polymer blends are mapped with nanometric resolution as shown in Figure 3. Two examples of the relevance of the interface energy in chemical epitaxy DSA are shown in figure 4 where the guiding stripes are defined by soft or strong oxygen plasma. A pristine PS-OH layer presents preferential affinity to the PS block. When it is exposed to oxygen plasma, it becomes more attractive to PMMA, being the affinity larger when the plasma is stronger (higher power and longer exposure time). As a result. two completely different morphologies of the DSA patterns are obtained: for the soft plasma case, we obtain vertical lamella oriented in the direction of the guiding stripes while for the strong oxygen plasma exposure, the PS-b-PMMA lamella are placed parallel to the substrate above the guiding stripes and turn vertical with perpendicular orientation above the non-guiding stripes. Quantitative results of the interface energy and a more complete investigation of their relation with the guiding pattern efficiency will be presented at the conference.
DescripciónTrabajo presentado a la 41st International Conference on Micro and Nano Engineering, celebrada en Hague (Netherlands) del 21 al 24 de septiembre de 2015.
Aparece en las colecciones: (CIN2) Comunicaciones congresos
(IMB-CNM) Comunicaciones congresos
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