Directed Self Assembly Of Polydisperse Block Copolymers

Directed self-assembly of block copolymers(DSA of BCPs)on chemical patterns has attracted the interest of academia and industry because of its low cost and relatively easy manufacturing of sub-10nm nanopatterns.The International Roadmap for Devices and Systems(IRDS)predicted that DSA of BCPs would be applicable for contact-hole shrinkage and pattern multiplication in the fabrication of next-generation semiconductor devices.Due to the constant trade disputes between China and the United States,it is difficult to import advanced lithography equipment.However,DSA of BCPs is expected to be combined with the existing 193 nm immersion lithography equipment to prepare semiconductor devices with a sub-10 nm feature size.For the development of the process with smaller nodes,it is necessary to have a thorough understanding of the process of DSA.Currently,most DSA studies use BCPs that are synthesized by anionic polymerization and typically have relatively narrow molecular weight distributions(MWDs).The MWD contains the composition information of polymer chain lengths,which is the most important and fundamental property of polymers.In this dissertation,poly(styrene-b-methyl methacrylate,PS-b-PMMA)is used as a model BCP to systematically study the influence of MWDs of BCPs on DSA.A series of PS-b-PMMAs with various MWDs are prepared via atom transfer radical polymerization(ATRP).The MWDs of PMMA macroinitiators are modified by the type and content of initiators,copper salts,ligands and solvents.The most common parameter used to represent MWD is D which only describes the relative breadth of MWD but hardly the shape of MWD.As the TsCl/CuCl/bpy system initiates and catalyzes the ATRP of MMA,PMMA-Cls are synthesized with the relatively symmetric peak shape of GPC traces.The D of PMMA-Cl is modified by the solvent content in the reaction,while the skewness of the MWD is controlled by the feed rate of the initiator.The MWD of PMMA-Cl can be transferred to the MWD of PS-b-PMMA.In order to simplify the problem,PS-b-PMMAs that have similar domain periods(LO)and volume fractions of PS(φst)but different (?)s are selected for the study of self-assembly of thin films.The as-synthesized PS-b-PMMAs are purified by selectively removing PS and PMMA to eliminate the influence of homopolymers.The morphology of thin films on random copolymer mats and chemical patterns are used to investigate the influence of (?) in lamella-forming PS-b-PMMA.The thin films of MS-1～MS-4,which have similar LOS but different (?)s,are spin coated on the PS-r-PMMA(fst=55%)modified substrates,and the number of defects is counted from the SEM images.The defect density increases while the correlation length decreases as the (?) of MS-1～MS-4 increased.We also investigate the effect of the (?),the normalized chemical pattern period(LS/LO),the duty cycle(W/LS)of chemical patterns and the annealing time on the LER and LWR of domain boundaries of MS-1～MS-4 films on chemical patterns.The assembly kinetics was faster when the D of PS-b-PMMA is larger,while the (?) of PS-b-PMMA did not affect the LS/LO window for defect-free assembly when the annealing time was sufficient.D is the most critical parameter to control LER and LWR,and both increase with the increase of D on chemical patterns with same LS/LO.Besides (?),LS/LO also played an important role in the control of LER and LWR and the roughness increases with the increase of LS/LO.In addition,the annealing time and W/LS of chemical pattern had little effect on LER and LWR.This would provide insight to engineer LER and LWR of block copolymer domains in DSA.The influence of (?) of cylinder-forming PS-b-PMMAs(MS-5～MS-9)on the defect densities on the chemical pattern is investigated from the thin film morphologies on chemical patterns with LS ranging from 34 nm to 50 nm and annealed for 10～2400 min at different temperatures.When (?)≥1.27,defect-free assembly can be observed when the annealing temperature is 250℃;while when (?)<1.27,the LS/LO window of defect-free assembly is nearly unchanged(0.90～1.05)at various annealing temperatures.The morphologies of MS-5-MS-9 on the chemical patterns annealed at 250℃ for different times showing that for MS-5(D=1.10),the defect-free structures are observed on the chemical pattern with LS=LO after 10 s annealing,while for MS-9((?)=1.32),the defect-free assembly is observed on the chemical patterns with LS>LO after 720 min annealing.Cylinder-forming PS-b-PMMAs with broad (?) require higher annealing temperatures and more annealing time.In addition,the development of block copolymers with a high χ value is still a key point in the field of DSA.PS-b-PMMA is the most widely studied BCP in DSA because the surface energies(ys)of blocks are nearly equal at annealing temperatures.However,the χ of PS-b-PMMA is relatively low and the smallest LO is-22nm,which limits its application in the fabrication of next-generation semiconductor devices.Although many high-x BCPs have been developed for DSA,the majority of them have a significant difference in ys between two blocks.Here we report the synthesis and selfassembly of poly(styrene-b-vinyl acetate)(PS-b-PVAc).PS-b-PVAc has a higher χ than that of PS-b-PMMA and the smallest achievable period(LO)is 18.2 nm.PS and PVAc have nearly equal ys and a perpendicular orientation of domains is obtained on random copolymer brushes under thermal annealing.DSA of PS-b-PVAc is successfully demonstrated on chemical patterns,and the feature size down to 9.5 nm is obtained in DSA with 2× density multiplication,which demonstrates that PS-b-PVAc could be a candidate BCP for next-generation DSA lithography.