Block copolymers (BCPs) have the ability to microphase separate into morphologies with feature-to-feature spacings (pitch) ranging from 5 -100nm. This quality makes them of interest to the microelectronics industry in their attempt to continue Moore’s Law by extension or replacement of optical lithography. Particularly, BCPs may also offer a cheaper route to the continuation of Moore’s Law compared to Extreme Ultra-Violet Light (EUVL) lithography. BCPs microphase separate when the enthalpic penalty between the two blocks of the BCP interacting is greater than the entropic penalty for them not mixing. The enthalpic penalty for mixing is the product of the degree of polymerization (
N) and the Flory-Huggins interaction parameter (χ). These two parameters also dictate what the pitch of the BCP will be, though the pitch is more dependent on
N than χ. This means that to reach a low pitch, a high χ, low
N material is needed. Thin films of the material are then brought above their glass transition temperature and allowed to phase separate to defect free states. Guidance of the morphology is controlled either by chemically preferential or topographic features in the underlayer, a process known as directed self-assembly (DSA). One example of this is graphoepitaxy where by a series of trenches are patterned in the underlayer with chemically preferential side walls. These preferential side walls not only dictate the orientation of the features but also guide the features along their direction. By removal of one of the BCP’s blocks, an etch mask of line-space or contact holes can be produced over a substrate. The number of full pitch features that can form within a trench is known as the density multiplication. The fact that the density multiplication is can be higher than 1x (the width of the trench being equal to the pitch of the BCP) is one of the most attractive qualities of DSA of BCPs. This means that pre-patterns can be made at a lithographically cheaper cost than the patterns that are ultimately formed by the BCP.
Defects in phase separated BCPs come in the form of common line defects such as jogs, dislocations, and disclinations. To remove a dislocation defect, for example, first the disconnected domains in the defect must penetrate through the opposite block. This increase in interactions leads to an enthalpic penalty and an energetic barrier to the defect’s annihilation. While high-χ materials are needed to reach small feature sizes, they are also suspected to have a high energetic barrier to defect annihilation during the annealing process. Low-χ materials may require higher molecular weights to phase separate, but their barrier to defect annihilation may be reduced.
Here we report on the anionic polymerization and characterization of a low-χ BCP, poly(4-tertbutylstyene)-b-poly(propyl methacrylate) (PtBS-b-PPMA). Small angle X-ray scattering (SAXS) was performed on bulk samples to discern the morphology, pitch, and χ of the material. Thin film samples were thermally annealed over a neutral underlayer synthesized from the random copolymerization of the blocks of the BCP. Thin film imaging was done using top down SEM. The etch contrast between PtBS and PPMA was measured as well using an O2/Ar etch chemistry. Thin films of the BCP were then cast on graphoepitaxy underlayers with trenches of varying widths and thermally annealed.
