FULL PAPER 1700107 (1 of 8) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmattechnol.de Simple and Versatile High Aspect Ratio Nanostructuring via Zinc Oxide Masking Agnieszka Gwiazda,* Anna Rumyantseva, Anisha Gokarna, Komla Nomenyo, Clotaire Chevalier-César, and Gilles Lérondel* DOI: 10.1002/admt.201700107 form of a thin film can be easily struc- tured by wet etching wherein a photoresist pattern is used or by direct photodissolu- tion [15] as has also been recently demon- strated by Chevalier-César et al. [16] and also by photo-crosslinking. [17] At the same time ZnO has a low pulverization rate. Therefore, structuration using dry etching technique becomes extremely difficult. A few articles report application of ZnO as a sacrificial layer in dry etching, namely, in chemically roughened ZnO thin films for glass texturing [18] or ZnO hydrothermal nanorods which have been used in the fabrication of GaN micropillars and nano- pillars on LED wafers. [19] With a low pul- verization rate, ZnO is expected to be a good hard mask material for dry etching, however, its application as a mask has not been reported so far. Nowadays, material science presents a growing need of material structuring at micro and nanoscale. Controlled and precise structuring becomes crucial for numerous applications, such as micro- and nanoelectronics, [20] photonics, [21] and MicroElectroMechanical Systems (MEMS). [22] These requirements have led to the devel- opment of various methods of dry plasma-assisted etching, such as conventional reactive-ion etching (RIE) or inductively coupled plasma (ICP-RIE) as well as deep reactive-ion etching (DRIE) techniques such as the cryogenic or Bosch process. The DRIE processes allow one to obtain a higher aspect ratio structures but require specialized equipment. Most of these techniques are well developed by now but are often limited to silicon [23,24] only, while there is a great interest to structure any kind of material. Micro and nanostructuring provides new properties to the material allowing creation of multifunctional surfaces that can have potential applications in many areas. Structuring or roughening of the surfaces improves the wetting properties of the materials. [25,26] This proves to be an advantage for biomedical devices. [27,28] For some applications, well-defined and ordered structures are required, as it allows creation of super-repellent materials for self-cleaning [29] and antireflective surfaces for solar cells. [30] For structuration of any kind of materials, the choice of the mask plays a crucial role as far as plasma-etching process is concerned. The mask should offer a high selectivity and at the same time its deposition and structuring should be simple to implement. Soft masks such as the photoresists require very This article reports a new universal masking technique based on ZnO. This technique combines two main properties of the material. ZnO has an extremely low pulverization rate making the material well adapted for dry etching, and second, it can be easily chemically dissolved in solution making the material well suited for masking. Using ZnO as a mask on silicon, one can achieve selectivity as high as 60. This allows one to etch holes through a 30 μm thick wafer. Sub-100 nm features are obtained using interferential lithography as a masking technique. There is no intrinsic limitation in this technique excepting the texture of the initial ZnO thin film. To demonstrate the versatility of this technique, it has been successively applied to other materials such as Si 3 N 4 , TiO 2 , and an organic resist (SU-8) with a selectivity of up to 15:1, 5:1, and 35:1, respectively. The technique reported in this article opens the way to universal masking which is extremely important for the development of multifunctional nanostructured surfaces in any kind of mate- rials with direct applications in antireflection coating, hydrophobicity, and hydrophilicity to only name a few. A. Gwiazda, Dr. A. Rumyantseva, Dr. A. Gokarna, Dr. K. Nomenyo, Dr. C. Chevalier-César, Prof. G. Lérondel Laboratoire de Nanotechnologie et d’Instrumentation Optique Institut Charles Delaunay CNRS UMR 6281 Université de Technologie de Troyes 12 Rue Marie Curie, CS 42060, 10004 Troyes, France E-mail: agnieszka.gwiazda@utt.fr; gilles.lerondel@utt.fr Laser Interference Lithography 1. Introduction Zinc oxide (ZnO) is a multifunctional material because of its unique physical and chemical properties. As a semiconductor with a wide electronic bandgap (3.36–3.4 eV) and large exciton binding energy (60 meV), it finds various applications in opto- electronic devices such as UV lasers, [1] UV photodetectors, [2] and light-emitting diodes (LEDs). [3] The piezo and pyroelectric properties of ZnO allow one to use it in converters, [4] energy generators, [5] and sensor [6,7] applications while its photocatalytic properties serve largely in production of hydrogen. [8] ZnO pre- sents a very rich variety of structures which can be obtained by several methods including chemical bath deposition (CBD), [9,10] hydrothermal synthesis, [11] sol-gel process, [12] as well as chem- ical vapor deposition (CVD), [13] pulsed laser deposition (PLD), and physical vapor deposition (PVD) technique. [14] ZnO in the Adv. Mater. Technol. 2017, 1700107