Enhancing ordering dynamics in solvent-annealed block-copolymer films by lithographic hard masks supports

We studied solvent-driven ordering dynamics of block copolymer films supported by a densely cross-linked polymer network designed as organic hard mask (HM) for lithographic fabrications. The ordering of microphase separated domains at low degrees of swelling corresponding to intermediate/strong segregation regimes was found to proceed significantly faster in films on a HM layer as compared to similar block copolymer films on silicon wafers. The ten-fold enhancement of the chain mobility was evident in the dynamics of morphological phase transitions and of related process of terrace-formation on a macroscale, as well as in the degree of long-range lateral order of nanostructures. The effect is independent of the chemical structure and on the volume composition (cylinder-/ lamella-forming) of the block copolymers. In-situ ellipsometric measurements of the swelling behavior revealed a cumulative increase in 1-3 vol. % in solvent up-take by HM-block copolymer bilayer films, so that we suggest other than dilution effect reasons for the observed significant enhancement of the chain mobility in concentrated block copolymer solutions. Another beneficial effect of the HM-support is the suppression of the film dewetting which holds true even for low molecular weight homopolymer polystyrene films at high degrees of swelling. Apart from immediate technological impact in block copolymer-assisted nanolithography, our findings convey novel insight into effects of molecular architecture on polymer-solvent interactions.Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Macromolecules, copyright \c{opyright} American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/ma500561

Controlled solvent vapor Annealing for Improved Processing of Thin Films of Block Copolymers - Revealing the Benefits of organic networks as supports for polymers in thin films

The aim of this work was to demonstrated the possibilities and advantages of the controlled solvent vapor annealing as an advanced technique for the processing of polymer films. Under investigation was the swelling behavior of polymer films and polymeric networks upon solvent vapor treatment under controlled and stable conditions as an advanced processing technique. The controlled parameters hereby are the temperature of the sample and the vapor as well as the vapor flow and partial vapor pressure dictating the solvent concentration inside the annealing chamber. Apart from the highly controlled and reproducible environment, this approach allows to utilize the advantages of polymeric networks as technology-compatible substrates. Investigated homopolymer films were used to access film stability. Thin films of block copolymers and the observation of their microphase separated structures allowed to draw conclusions about solvent uptake and ordering dynamics upon solvent vapor processing. The swelling behavior of a polymeric network (hardmask) and a bilayer system consisting of a network and a polymer layer were studied and the results were compared to the single layers arrangements. Additionally microwave annealing was employed as a novel and fast processing step. In chapter 4 the film preparation procedure and the film processing via controlled solvent vapor setup were established for employed systems and investigated in regards of reproducibility and stability throughout the process. The sensibility of the in-situ monitoring of the film thickness in the swollen state by spectroscopic ellipsometry was verified and the method was applied for the observation of a nanoscopic effect on the macroscopic swelling behavior. The film preparation conditions and preparation methods in combination with the setup for controlled solvent vapor annealing facilitate reproducible results regarding film thickness, film homogeneity, starting morphology, solvent uptake and target morphology. The solvent concentration was shown to be stable and the error of the adjustable partial vapor pressure was shown to be within 5 % leading to a reproducible polymer volume fraction in dependence of the partial vapor pressure of thin polymer films within an error of a few percent. The sensitivity of the ellipsometry measurements was found to be not sufficient to in-situ follow the reorientation of lamella structures. This is mainly due to the vast difference between the size of the area over which the data is averaged and the grain size for lamella reorientation. In chapter 5.1 the hardmask layer as model for a polymeric network was characterized by investigating the swelling behavior upon exposure to solvent vapor. The swelling experiments reveal a glassy behavior of the network and possible physical aging effects. Furthermore a dependency of the swelling behavior on the crosslinking density and different solvent (vapor) pretreatments were shown. The gradually altered crosslinking density has been achieved by varying the temperature during the curing process of the network. In chapter 5.2 the microphase separated and surface relief structures of block copolymer films were analyzed in order to draw conclusions regarding the ordering dynamics and polymer volume fraction upon exposure to solvent. The silicon supported films were compared to a multilayer system, composed of thin block copolymer films supported by network layers with varied crosslinking densities. Additionally, the swelling of block copolymer film, network layer and multilayer system was measured directly to verify a possible altered solvent uptake in the multilayer system. No altered wetting conditions or phase behavior could be shown for the network supported film. But a significant improvement of the ordering dynamics of block copolymer films supported by a network layer was found. Importantly this effect was shown to be especially pronounced for low solvent concentrations and short processing times. Furthermore the beneficial effects on the ordering dynamics are not altered by the crosslinking density and hence the swelling behavior of the network layer. A significant increase in solvent uptake of the network supported block copolymer films could not be revealed, excluding a higher degree of swelling as the reason for enhanced ordering dynamics. Chapter 5.3 presents a comparison of the stability of silicon or network supported homopolymer films with a supplemental variation of the chemical composition and molecular weight of employed polymers. The films were swollen in high solvent concentrations to induce dewetting and the degree of dewetting was compared. A retardation of dewetting effects by the network layer independent of its crosslinking density was revealed. The contradicting effects of enhanced ordering dynamics and retarded dewetting effects are advantageous for the implementation of the solvent vapor annealing process in industry, since they enable reduced processing times even at intermediate to strong segregation regimes while stabilizing films even at low molecular weights and high solvent concentrations. The presented results therefore are of great technological impact. In chapter 6 the effect of the initial morphology on the swelling behavior of block copolymer films was discussed. The experiments reveal different kinetics for the solvent uptake of films with varied initial morphologies. Lamellar films swell ~20 times faster than films with micellar morphology if the residual solvent and air voids were previously eliminated by drying. The experimental results were seconded by DPD simulation, and different diffusion pathways of the non selective solvent through the films were proposed to be the reason for the difference in swelling behavior. Chapter 7 gives an introduction on microwave annealing of block copolymers in thin films with and without the support of solvent vapor and discusses the benefits and drawbacks of the approach. Microwave annealing was shown to be a very fast and effective method to anneal block copolymer film employed throughout this work. The method enables the ordering of structures within a few minutes, whereby the addition of solvent merely causes an altered phase behavior and does not affect the ordering dynamics. Unfortunately, the benefit of significantly reduced processing times over controlled solvent vapor annealing as shown here is attenuated by the lack of control and understanding. However, since the work on microwave annealing is pushed forward the additional know-how should lead to a routine implementation of the microwave annealing process in the laboratory at the soonest