Thin film nanostructuring at oblique angles by substrate patterning

It is demonstrated that, besides classical nanocolumnar arrays, the oblique angle geometry induces the growth of singular structures in the nanoscale when using wisely designed patterned substrates. Well-ordered array of crosses, cylindrical nanorods or hole structures arranged in square or hexagonal regular geometries are reported as examples, among others. The fundamental framework connecting substrate topography and film growth at oblique angles is presented, allowing the use of substrate patterning as a feasible thin film nanostructuring technique. A systematic analysis of the growth of TiO2 thin films on 4 different lithographic patterned substrates in 4 different scale lengths is also presented. A first conclusion is the existence of a height-based selective growth in the initial stages of the deposition, by which the film preferentially develops on top of the tallest substrate features. This behavior is maintained until the film reaches a critical thickness, the so-called Oblivion Thickness, above which the film topography becomes gradually independent of the substrate features. A general formula relating the spatial features of the pattern, the coarsening exponent and the Oblivion Thickness has been deduced.
The authors thank the financial support from MCIN/AEI/10.13039/ 501100011033 projects PID2019-110430GB-C21, PID2020-112620GBI00, PID2020-114270RA-I00 and RTI2018-098117-B-C21 (also financed by FEDER Una manera de hacer europa), the Junta de Andalucía (PAIDI- 2020 through projects P18-RT-3480 and P18-RT-6079, and through its 2019 PhD Researcher Hiring Program), the CSIC (2019AEP161 and 201860E050), the Regional Government of Madrid (project IND2017/ IND-7668 and YEI contract PEJ-2019-AI/IND-14451 with support from FSE), the H2020-EU.1.2.1-FET OPEN program (grant 899352, project SOUNDofICE, and the EFRE Infra-Pro project ChAMP), and the University of Seville (VI PPIT-US). The work is supported by the Deutsche Forschungsgemeinschaft (DFG, grant Scha 632/24, “Tailored Disorder” and Scha 632/27, “DFGGer ¨atezentrum”). This work is also supported by the free state of Thuringia under grants 2015 FGI 0025 305 (FastμXRD) and B715-10009 (BioMacroNano2020), all co-financed by the European Union within the framework of the European Regional Development Fund (ERDF). The service from the MiNa Laboratory at IMN-CNM (CSIC), funded from CM (project S2018/NMT-4291 TEC2SPACE), MINECO (project CSIC13-4E- 1794) and EU (FEDER, FSE), is also acknowledged.