In‐plane and out‐of‐plane defectivity in thin films of lamellar block copolymers

Abstract

We investigate the ordering of poly(styrene-bmethyl methacrylate) (PS-PMMA) lamellar copolymers (periodicity L 0 5 46 nm) confined between a free surface and brushed poly(styrene-r-methyl methacrylate) silicon substrate. The processing temperature was selected to eliminate wetting layers at the top and bottom interfaces, producing approximately neutral boundaries that stabilize perpendicular domain orientations. The PS-PMMA film thickness (t 5 0.5L 0 2 2.5L 0 ) and brush grafting density (R 5 0.2-0.6 nm 22) were systematically varied to examine their impacts on in-plane and out-of-plane ordering. Samples were characterized with a combination of highresolution microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering. In-plane order at the top of the film (quantified through calculation of orientational correlation lengths) improved with t n , where the exponent n increased from 0.75 to 1 as R decreased from 0.6 to 0.2 nm 22 . Out-ofplane defects such as tilted domains were detected in all films, and the distribution of domain tilt angles was nearly independent of t and R. These studies demonstrate that defectivity in perpendicular lamellar phases is three-dimensional, comprised of in-plane topological defects and out-of-plane domain tilt, with little or no correlation between these two types of disorder. Strong interactions between the block copolymer and underlying substrate may trap both kinds of thermally generated defects. KEYWORDS: block copolymers; defectivity; directed self-assembly; grazing-incidence small-angle X-ray scattering; thin films INTRODUCTION Thin films of symmetric diblock copolymers can spontaneously self-assemble into nanoscale lamellar domains (i.e., nanolines). 1,2 These materials could improve the resolution of projection lithography by "shrinking" the sizes of patterned features, 3-6 so the leading semiconductor manufacturers are considering their use in next-generation integrated circuit manufacturing. Lithographic processes require precise control over the placement and orientation of domains with respect to the underlying substrate. Although many methods have been developed to direct the placement of domains, the in-plane and out-of-plane defectivity remains too high for production. The objective of our current work is to examine in-plane and out-of-plane defect structures in thin films of perpendicular poly(styrene-bmethyl methacrylate) (PS-PMMA) lamellae. In-plane defects are well documented in these systems, and they include dislocations and disclinations that disrupt lateral order. Out-ofplane defects are not well studied, but could include tilted, bent, or discontinuous domains.

Ordering of lamellar block copolymers on oxidized silane coatings

“…These works demonstrate that ordering improves as t n , where n ranges from 0.4-1. 14,[35][36][37] The data for low energy ODS surfaces (θ w ≈ 90 • ) are consistent with this power-law behavior, and the dashed blue line in Fig. 4 marks the trend for n = 0.9.…”

“…42 We note that the energetic penalty for bending a soft lamellar domain is very low, so suppressing these defects would require an external guide such as an epitaxial template. 14,43 Coexistence between perpendicular and parallel domain orientations is clearly detected in GISAXS patterns from some of the samples, and this occurrence is designated by the "x" and "+" at φ = 90 • for θ w ≈ 90 • and 87 • , respectively. There are only a few apparent discrepancies between GISAXS and microscopy analysis of domain orientations: First, when t/L 0 ≈ 0.5, there is no sign of parallel lamellae in GISAXS patterns for either surface, although featureless patches were detected in microscopy ( f perp ≈ 0.9).…”

“…13,14,41 Disorder is quantified by the maximum mis-orientation angle φ , where φ = 0 • indicates a perfect perpendicular orientation, and φ = p • includes all domain orientations in the range of 0 to p • . 13,14 Tilt could not be identified in samples with t/L 0 ≤ 1, but these samples have broad line shapes due to poor lateral order, so the partial Debye-Scherrer rings might be obscured (resolution limit). 14 In thicker films, the perpendicular phase is characterized by large misorientation angles up to approximately 40 • , so the idealized cartoons inferred from surface microscopy in Fig.…”

“…5,[8][9][10][11][12][13][14][15] In these systems, the optimal proportion of styrene to methyl methacrylate is determined by the composition of the block copolymer, 5,[7][8][9][10] as well as the coating thickness and density. [12][13][14][15] While there are a few types of commercially-available random copolymer coating materials, most are synthesized in a university laboratory to target a specific composition and molecular weight. Furthermore, parameters like coating thickness and density are partially controlled by the deposition process and can be difficult to accurately characterize.…”

Improved block copolymer domain dispersity on chemical patterns via homopolymer-blending and molecular transfer printing

“…Furthermore, the increased polymer brush density also contributed to the minimization of defective structures. [52] As a result, the hydroxyl-terminated homopolymers healed the boundary conditions for density multiplication and further narrowed the domain width distribution (FWHM = 4.8 nm, Figure 4C). After directed assembly on one-to-one chemical patterns from MTP, SMMA2 formed nanopatterns with highly controlled domain width distribution (FWHM = 4.1 nm, Figure 4D).…”