Dual Block Copolymer Morphologies in Ultrathin Films on Topographic Substrates: The Effect of Film Curvature

Abstract

The ability to create mixed morphologies using easily controlled parameters is crucial for the integration of block copolymers in advanced technologies. We have previously shown that casting an ultrathin block copolymer film on a topographically patterned substrate results in different deposited thicknesses on the plateaus and in the trenches, which leads to the co-existence of two patterns. In this work, we highlight the dependence of the dual patterns on the film profile. We suggest that the steepness of the film profile formed across the plateau edge affects the nucleation of microphase-separated domains near the plateau edges, which influences the morphology that develops on the plateau regions. An analysis of the local film thicknesses in multiple samples exhibiting various combinations of plateau and trench widths for different trench depths enabled the construction of phase diagrams, which unraveled the intricate dependence of the formed patterns not only on the curvature of the film profile but also on the fraction of the film that resides in the trenches. Our analysis facilitates the prediction of the patterns that would develop in the trenches and on the plateaus for a given block copolymer film of known thickness from the dimensions of the topographic features.

Non-Bulk Morphologies of Extremely Thin Block Copolymer Films Cast on Topographically Defined Substrates Featuring Deep Trenches: The Importance of Lateral Confinement

“…Lamellar polystyrene- block -poly(methyl methacrylate) (PS- b -PMMA) diblock copolymer ( M n 312 kDa, PDI 1.27, f PS = 0.48, L 0 = 84 nm) was spin coated from toluene solutions over topographically patterned substrates yielding films with nominal film thicknesses, h , in the range of 8–24 nm. The high limit of this thickness range overlaps with the nominal film thicknesses studied in our previous works (22–23 nm) [ 71 , 74 ], which are already considered ultra-confined (i.e., h < 0.5 L 0 ), whereas the other film thicknesses extend the investigations to even stronger confinement. The topographically patterned substrates were made of silicon wafers that were etched with an array of 38 nm-deep, parallel trenches (the trench depth was kept constant throughout this study).…”

“…Although the films are continuous and do not dewet from the sidewalls of the trenches (possibly also because of the slightly increased roughness of the sidewalls compared with the horizontal surfaces [ 78 , 79 ]), the overlays shown in Figure 3 d indicate that the surface of the film, regardless of trench width, is extremely close to the edge of the plateau, which suggests pinning of the film to the plateau edges. The formation of stripes at the edges of all of the plateaus shown in Figure 1 and Figure 2 (which were not observed with the thicker films in our previous studies [ 74 ]) may be templated by such pinning.…”

“…For plateau widths of 240 nm or less, a stripe oriented parallel to the topographic features is also seen at the center of the plateau ( Figure 1 d). It is interesting to note that while the surface patterns exhibited by thicker films (22–23 nm nominal thickness) cast over shallower trenches, which were described in our previous study, showed a trivial dependence on the plateau width (i.e., the width merely dictated the number of dots across the plateau) [ 74 ], the 18 nm-thick films investigated in this study exhibited a different morphology on the narrowest plateaus. This observation shows that increasing the ratio between trench depth and film thickness and utilizing narrow plateaus provides access to non-bulk patterns of co-existing domain types that have not been observed before.…”

“…These thicknesses were determined by referencing the measured heights to the SFM height values of the corresponding, completely exposed features (see Figure S2 ) [ 71 ]. Film height contrast was measured using step analysis from the highest area of the plateau to the lowest area of the trench [ 74 ]. For films where the substrate became exposed by dewetting (i.e., BCP droplets), cross-section analysis was used to measure the film thickness at the desired location.…”

“…This behavior was rationalized by the topography-driven variation in film thickness between the film coating the plateaus and the film deposited in the trenches, which is coupled to the extreme sensitivity of the morphology to the local film thickness for films confined to thicknesses lower than half the lamellar period [ 72 , 73 ]. In a follow-up study, we have unraveled the intricate relationship between the trench depth, lateral topographical dimensions, and film surface profile, allowing us to predict the morphology of the film under various experimental conditions while the nominal film thickness remained constant [ 74 ].…”