Tatsuhiro Iwama

Field-theoretic simulations of directed self-assembly in cylindrical confinement: placement and rectification aspects

We have investigated the directed self-assembly (DSA) of cylinder-forming block copolymers inside cylindrical guiding templates. To complement and corroborate our experimental study, we use field-theoretic simulations to examine the fluctuations-induced variations in the size and position of the cylindrical microdomain that forms in the middle of the guiding hole. Our study goes beyond the usual mean-field approximation and self-consistent field theory simulations (SCFT) and incorporates the effects of thermal fluctuations in the description of the self-assembly process using complex Langevin (CL) dynamics. In both our experimental and modeling efforts, we focus on minor-block-attractive sidewalls and bottom substrates and neutral top surfaces and explore the properties of the formed cylinders, including fluctuations in the center position and the size of the domain, for various prepattern conditions. Our results indicate robust critical dimensions (CD) of the DSA cylinders relative to the incoming CD, with a sigma CD < 0.9nm. Likewise, we find that the DSA cylinders are accurately registered in the center of the guiding hole, with deviations in the hole-inhole distance on the order of ≈ 0.7-1nm, translating to errors in the hole-to-hole distance of ≈ 1-1.5nm.

Computational studies of shape rectification in directed self-assembly

We use self-consistent field theory (SCFT) to study shape rectification in overlapped cylindrical and non-cylindrical prepatterns. Specifically, we examine the potential of directed self-assembly (DSA) of block copolymers to not only reduce critical dimensions relative to the template, but also repair defects in the guiding prepatterns and produce defectfree contact holes. In our study over a wide range of prepattern dimensions, we found that defects in the central minorblock domain arise with decreasing center-to-center distance of the prepattern. Increasing the minor-block fraction in the block copolymer was observed to remove some of the defects. We also studied the effect of adding homopolymer to the block copolymer melt and show how blends can successfully eliminate defects and increase the range of the process window relative to the neat diblock case without influencing domain properties such as the critical dimension and the hole-to-hole distance.

Computational study of directed self-assembly for contact-hole shrink and multiplication

Abstract. We use three-dimensional self-consistent field theory (SCFT) to study the directed self-assembly (DSA) of cylinder-forming block copolymers in a peanut-shaped (also called egg-box) prepattern. The design of the prepattern shape will target the pitch reduction of the contact holes. The idea is that the DSA of block copolymers will not only lead to reduced critical dimensions relative to the template but will also repair defects in the guiding prepatterns and produce defect-free contact holes. We also study blends of block copolymers and homopolymers with various lengths and volume fractions. Using SCFT simulations, we establish the effects of the added homopolymer on defectivity, the process window, and the properties of the formed cylinders. In an attempt to quantify the effect of thermal fluctuations on the placement of the cylinders, we resort to complex Langevin simulations and perform a stochastic sampling of the assembled morphologies.

Effects of thermal fluctuations on directed self-assembly in cylindrical confinement

Abstract. We investigate the directed self-assembly (DSA) of cylinder-forming block copolymers inside cylindrical guiding templates. To complement and corroborate our experimental investigations, we use field-theoretic simulations to examine the fluctuation-induced variations in the size and position of the cylindrical microdomain that forms in the middle of the guiding hole. Our study goes beyond the usual mean-field approximation and self-consistent field theory simulations (SCFT) and incorporates the effects of thermal fluctuations in the description of the self-assembly process using complex Langevin (CL) dynamics. In addition to CL simulations, we present an efficient SCFT-based approach that can inform about the positional error of the formed cylinders. In this new scheme, an external chemical-potential field is applied to displace the inner cylinder away from its centered, lowest energy configuration. In both our experimental and modeling efforts, we focus on two wall-wetting conditions: (1) minor-block-attractive sidewalls and bottom substrates and neutral top surfaces and (2) neutral sidewalls, substrates, and top surfaces. For both cases, we explore the properties of the formed cylinders, including fluctuations in the center position and the size of the domain, for various prepattern conditions. Our results indicate robust critical dimensions (CDs) of the DSA cylinders relative to the prepattern CD, with a standard deviation <0.9  nm. Likewise, we find that the DSA cylinders are accurately registered in the center of the guiding hole, with deviations in the hole-in-hole distance on the order of ∼0.7 to 1.4 nm, translating to errors in the hole-to-hole distance of ∼1 to 2 nm.