Solvent-Vapor-Induced Tunability of Self-Assembled Block Copolymer Patterns

2009 WILEY-VCH Verlag Gmb Well-controlled monolayer patterns of microdomains of block copolymers (BCPs) have been widely pursued for applications in sub-30-nm nanolithography. BCP film processing is scalable and low cost, and is compatible with existing semiconductor fabrication techniques. Di-BCP with a molecular weight of a few tens to a few hundreds of kgmol 1 have been known to spontaneously form periodic arrays of well-defined nanoscale features, such as dots, holes and lines, which have been used as masks in the fabrication of arrays of nanoscale functional features after the selective elimination of one block. Long-range ordering and positional registration of the features has been imposed by using chemical or topographical templates. The morphology of the patterns has been further diversified by using multi-BCPs or employing various confinement geometries. The morphology and length scale of the microdomain arrays of BCPs are governed by the degree of polymerization of each block, and thus to obtain different geometries and feature sizes, polymers with different chain lengths or BCP–homopolymer blends have been employed. However, in terms of device fabrication, it would be advantageous to be able to manipulate the shape and dimensions of features composed of a single BCP by simply altering the processing conditions. Polymeric materials often provide an exceptional degree of controllability in their molecular configurations due to their weak intermolecular forces, which are largely based on Van der Waals interactions. Solvent vapor annealing, which has been employed to increase chain flexibility in BCPs and to promote their self-assembly into themicrophase-separated state, may be used as an effective lever to engineer the resulting structures. It has been shown that treatments with selective or nonselective vapor result in different morphologies or orientations, demonstrating solvent-induced controllability. However, precise adjustment of pattern size and morphology using controlled mixed solvent vapors has not been studied to the best of our knowledge. In this Communication, we report on the systematic tunability of the dimensions and morphology of patterns of microphaseseparated BCP; this was achieved by controlling the solventannealing conditions, where the key parameters are solvent vapor pressure and the mixing ratio of selective and partially selective solvent vapors. Vapor pressure can control the degree of solvent uptake by the film, which changes both the chainmobility and the interfacial interaction between the blocks of the BCP. We will also propose the theoretical model explaining the increasing size of the pattern period with decreasing vapor pressure. It has been recently reported that a change in the effective volume fraction of the blocks can be accomplished using selective and nonselective vapors. We will show independent control of pattern size and periodicity by using a mixed vapor containing selective (heptane) and partially selective (toluene) solvents. We will also demonstrate that a cylinder-forming BCP can be transformed into a perforated lamellar structure by increasing the portion of selective solvent in the mixed vapor. This study suggests a way to relieve the constraints, imposed by the molecular structure of BCPs, on the achievable microdomain geometries. We have previously presented high quality nanoscale lines, dots and rings composed of a poly(styrene-b-dimethylsiloxane) (PS–PDMS) di-BCP. This polymer is useful for nanolithography in that its self-assembled microdomain pattern has a large correlation length that can exceed several micrometers and a low defect density, even in the absence of templating. High etch selectivity between its two blocks is also possible due to the existence of Si in the PDMS backbone. As described in Figure 1a, a PS-PDMS BCP with a molecular weight of 45.5 kgmol 1 and 33.5 vol% PDMS was spin-coated onto a Si substrate and solvent-annealed forming a PS matrix containing a monolayer of PDMS cylinders that were parallel to the substrate. There was also a PDMS layer at the interfaces of the BCP with the substrate and air due to its lower surface energy compared to PS. Etching away the surface layer of PDMS and some of the PS matrix revealed the arrangement of the now-oxidized PDMS cylinders. We consider first the effect of solvent vapor pressure on the morphology of the cylinder patterns. The vapor pressure is characterized by the ratio between S, the surface area of the solvent while in a beaker placed within the solvent-annealing chamber, and V, the volume of air that will mix with the solvent vapor, which is equivalent to the amount of air in the chamber at atmospheric pressure. The chamber had a small leak path through which solvent vapor could escape. The amount of solvent vapor in the chamber was determined by three flux components: evaporation from the solvent surface (F1), condensation onto the solvent surface (F2), and leak flow (F3). At steady state, the concentration (C) of the solvent vapor in the chamber is given by