Alexander Meinhardt

Doctoral project

Metrology of sub-10 nm block-copolymers to control the crystallization and microphase separation

Background and Scientific Challenge
Bottom-up molecular self-assembly of single-digit block copolymer (BCP) nanostructures with domain sizes <10 nm can play a crucial role in future nanotechnology and nanofabrication1 . Sub-10 nm nanofabrication requires novel metrology techniques capable to provide statistically relevant information on the mean size and the size uniformity of the nanostructures. Grazing Incidence Small Angle Xray Scattering (GISAXS) combined with Scanning-Electron and Scanning-Force Microscopy (SEM, AFM) can provide such information on the relevant length scales.2,3 Single-digit BCP candidates need to be tested with regard to their phase separation behaviour to form well-ordered sub-10 nm nanostructures, and also in view of their pattern fidelity throughout the subsequent nanofabrication processing steps. In this context, single-digit BCPs containing a crystallizable block are interesting macromolecular systems. The nanostructure formation in these systems is controlled not only by the microphase separation between the blocks, but the crystallization acts as a second driving force to form crystalline building blocks competing with the microphase separation.4 On one hand, microphase separation patterns can create a nanoconfinement for the crystallization or can be overwritten by a dominating crystallization front. On the other hand it was observed that crystallization can lead to varies morphological phases upon cooling, although from the volume fraction for the blocks an exclusive lamellar arrangement is expected over a wide temperature range.5,6 So far, it is however unknown, which transient surface morphologies form at higher temperatures in thin film geometry with affinities to substrate and free surface can modify the structure formation.7

Project Specific Aims
This project will elucidate the morphology in thin films of crystalline BCP films with sub10 nm domains and the interplay of the microphase separation and crystallization to form a surface morphology on the nanoscale. The first aim is to develop reliable metrology tools based on a combination of GISAXS and microscopy to determine statistically relevant structural information of sub-10 nm nanostructures in thin BCP films based on the existing technology and instrumentation. Using GISAXS, SEM and AFM, the morphology of thin BCP films with single digit domain sizes will then be characterized on chemically functionalized, planar silicon substrates. To induce a macroscopic orientation of the BCP domains, directed self-assembly (DSA) on topographic or chemical guiding patterns will be utilized. On the guiding pattern as well as on the free planar silicon surface the morphology in the BCP film shall be tracked by in situ GISAXS and AFM during heating and cooling in a controlled solvent environment. We hypothesize that the crystallization is size-selective in that chains of similar length will be more easily integrated into the growing crystal than shorter or longer chains, supporting an enhanced crystal size control and a high pattern fidelity. GISAXS will shed light on the role of the affinities to the substrate and the surrounding ambient air or solvent environment with impact on the structure formation and wetting. A combined GISAXS, SEM and AFM metrology will guide the path to create crystallization-assisted sub-10 nm heterogeneous surface nanostructures that could serve as templates for nanofabrication with technological relevance.

References
(1) Choi, J. W.; Li, Z.; Black, C. T.; Sweat, D. P.; Wang, X.; Gopalan, P. Patterning at the 10 Nanometer Length Scale Using a Strongly Segregating Block Copolymer Thin Film and Vapor Phase Infiltration of Inorganic Precursors. Nanoscale 2016, 8 (22), 11595–11601. https://doi.org/10.1039/c6nr01409g.
(2) Gottlieb, S.; Rösner, B.; Evangelio, L.; Fernández-Regúlez, M.; Nogales, A.; García-Gutiérrez, M. C. C.; Keller, T. F. F.; Fraxedas, J.; Ezquerra, T. A. A.; David, C.; Perez-Murano, F. Self-Assembly Morphology of Block Copolymers in Sub-10 Nm Topographical Guiding Patterns. Mol. Syst. Des. Eng. 2019, 4 (1), 175–185. https://doi.org/10.1039/c8me00046h.
(3) Müller-Buschbaum, P. GISAXS and GISANS as Metrology Technique for Understanding the 3D Morphology of Block Copolymer Thin Films. Eur. Polym. J. 2016, 81, 470–493. https://doi.org/10.1016/j.eurpolymj.2016.04.007.
(4) He, W. N.; Xu, J. T. Crystallization Assisted Self-Assembly of Semicrystalline Block Copolymers. Prog. Polym. Sci. 2012, 37 (10), 1350–1400. https://doi.org/10.1016/j.progpolymsci.2012.05.002.
(5) Weiyu, C.; Tashiro, K.; Hanesaka, M.; Takeda, S.; Masunaga, H.; Sasaki, S.; Takata, M. Relationship between Morphological Change and Crystalline Phase Transitions of Polyethylene-Poly(Ethylene Oxide) Diblock Copolymers, Revealed by the Temperature-Dependent Synchrotron WAXD/SAXS and Infrared/Raman Spectral Measurements. J. Phys. Chem. B 2009, 113 (8), 2338–2346. https://doi.org/10.1021/jp8092435.
(6) Cao, W.; Tashiro, K.; Masunaga, H.; Sasaki, S.; Takata, M. Relationship between Morphological Change and Crystalline Phase Transitions of Polyethylene-Poly(Ethylene Oxide) Diblock Copolymers. 3. Dependence of Morphological Transition Phenomena on the PE/PEO Segmental Lengths and Its Possible Origins. J. Phys. Chem. B 2009, 113 (25), 8495–8504. https://doi.org/10.1021/jp901442a.
(7) Fasolka, M. J.; Banerjee, P.; Mayes, A. M.; Pickett, G.; Balazs, A. C. Morphology of Ultrathin Supported Diblock Copolymer Films: Theory and Experiment. Macromolecules 2000, 33 (15), 5702–5712. https://doi.org/10.1021/ma990021h.