(524e) Effect of Asymmetric Homopolymer Addition on Structural Characteristic of Lamellae Forming Block Copolymers Aligned Via Directed Self-Assembly

Block copolymers (BCPs) offer an attractive pathway to the formation of nanoscale patterned features in semiconductor integrated circuit manufacturing and other applications due to their inherent propensity to microphase separate into morphologies (lamellae, cylinders, spheres, etc.) with feature pitches in the range of approximately 5 nm to 100 nm. While the native microstructures formed in such block copolymer microphase-separated assemblies are generally highly defective due to entropic effects, it has been shown that directed self-assembly (DSA) techniques (e.g. graphoepitaxy and chemoepitaxy) which rely on the formation of larger “guide patterns” using often traditional lithographic techniques at larger length scales can provide an energetic landscape on surfaces in which the location and orientation of the microphase separated regions formed from the block copolymer can be controlled. These controlled, essentially templated, micro-phase separation processes can produce structures with extremely low numbers of defects in nearly perfectly ordered morphologies over large area. Furthermore, the sub-division of the larger guiding structure pitch into much smaller pitch structures through the block copolymer micro-phase separation and self-assembly process is the critical advantage of this process which results from combining a top-down lithographic patterning method with a self-assembly process. It is exactly this ability to form virtually perfect nanometer scale assemblies over large areas that presents the real opportunity for DSA of block copolymers to extend and enhance the current top-down lithographic capabilities engineered by the semiconductor community over the last six decades.

In engineering the materials used for such DSA patterning methods, the pitch of the features formed is controlled not by the lithographic techniques used to form the guide patterns, but instead are determined by the molecular weights of the blocks formed in the copolymers. This allows synthetic control over feature sizes which can be tightly determined using for example modern living polymerization methods, and this ultimately is the critical advantage that allows DSA methods to surpass the diffraction limits of state-of-the-art lithographic processes. However, this also means that any small desired variation in feature size using DSA of pure block copolymers must be accomplished by synthesizing new block copolymers targeted for that new feature size. It also means that exquisite molecular weight control is required to yield useful batches of copolymer targeted at specific pattern feature sizes as any variation in molecular weight of the block copolymer directly translates into feature size changes in the resulting assembled block copolymer film pattern. It is therefore of interest to explore methods whereby adjustments or variations in the pattern (i.e. morphology) pitch that can be achieved with a given block copolymer can be made. Two such methods that are straightforward to imagine are: (1) addition of homopolymer of one or more blocks in the copolymer and (2) blending of block copolymers of different molecular weight. The first methods offers the advantage that in principle it should be possible to vary somewhat independently the size of the different domains in the resulting morphology (e.g. the size of the two different lamellae domains forms in a lamellae forming diblock copolymer could be independently adjusted while the overall pitch of the pattern is also modified through addition of different amounts of homopolymer of the two blocks constituting the block copolymer). However, the effect of the addition of such homopolymer additives is not obvious given the different types of interactions and constraints in the material system for homopolymer as compared to the block copolymer. The goal of the work reported here is to elucidate the effect of such homopolymer addition on the lithographically relevant characteristics of block copolymer DSA processes.

Addition of homopolymer is already known to increase the achievable pitch of block copolymer materials, but the details of this process have not been deeply investigated. Furthermore, patterned features in integrated circuits and other possible applications are not all spaced by distances equal to the feature size of interest (i.e. not all patterns are so-called 1:1 patterns), and thus may exist at more complex geometries and pattern size ratios than allowed by standard di-block copolymer self-assembly may allow. The addition of homopolymer to the block copolymer not only allows for an increase in the overall pitch of the repeating morphology, but the addition of different amounts of the homopolymer constituents of the block copolymer can allow for variation in the domain sizes of a morphology (e.g. it is possible to achieve lamellae patterns where the lamellae widths are not equal even though a 50:50 volume fraction diblock copolymer is used for the assembly). This allows for formation of non-equal block copolymer pattern domain feature sizes in morphologies not achievable by the block copolymer alone (e.g. it may be possible to go to different domain size ratios in a lamellae forming diblock copolymer) that if attempted from a single diblock copolymer would result in phase transition to different morphologies such as transitioning from lamellae to cylinder phases when varying to far from 50:50 volume fraction blocks. In this work, coarse grained molecular dynamics simulations have been used to probe the effect that asymmetric homopolymer addition has important DSA patterning behaviors and pattern characteristics. For example, the effect of homopolymer addition on the line edge- and line width roughness (LER/LWR) of lamellae forming BCPs has been probed as a function of both homopolymer to block copolymer loading and as a function of varying the ratio of the two homopolymers that are constituents of the block copolymer. The limits of such processes, in the form of effective process windows, will be presented and the general trade-offs presented by such processes will be discussed.