COMMUNICATION © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 7) 1400038 wileyonlinelibrary.com Stretchable and Conformable Oxide Thin-Film Electronics Niko Münzenrieder,* Giuseppe Cantarella, Christian Vogt, Luisa Petti, Lars Büthe, Giovanni A. Salvatore, Yang Fang, Renzo Andri, Yawhuei Lam, Rafael Libanori, Daniel Widner, André R. Studart, and Gerhard Tröster Dr. N. Münzenrieder Sensor Technology Research Center School of Engineering and Informatics University of Sussex BN1 9QT Falmer, Brighton, UK E-mail: n.s.munzenrieder@sussex.ac.uk Dr. N. Münzenrieder, G. Cantarella, C. Vogt, L. Petti, L. Büthe, Dr. G. A. Salvatore, Y. Fang, R. Andri, Y. Lam, Prof. G. Tröster Electronics Laboratory Department of Information Technology and Electrical Engineering ETH Zurich, Gloriastrasse 35, 8092 Zürich, Switzerland Dr. R. Libanori, D. Widner, Prof. A. R. Studart Complex Materials Department of Materials ETH Zurich, Wolfgang-Pauli-Str. 10, 8093 Zürich, Switzerland DOI: 10.1002/aelm.201400038 gains its elasticity from its accordion-like structure. Such struc- tures are generally realized using prestrained substrates, where the relaxation of the substrate after the fabrication leads to the formation of wrinkles on the surface. Wavy transistors made of organic materials or silicon nanomembranes demonstrated stretchability >200%. [6,21,24] This method also enabled stretch- able magnetic field sensors, [25] and organic light-emitting diodes. [26] Even more challenging than the fabrication of stretchable electronics devices is the realization of conformal electronics which is stretchable in multiple dimensions. [12] 3D deformed a-Si TFTs on a spherical dome can survive strains of 6%, [27] and wavy organic electronic components on a biaxial prestretched elastomer are able to withstand a 35% area decrease. [6] The challenges concerning the fabrication of stretchable active electronic devices can be summarized as follows: Stretch- able substrates need to provide thermal, mechanical, and chemical stability, as well as a surface roughness compatible with the device fabrication process. [28] Additionally, wavy elec- tronics need to survive extremely small bending radii, in the micrometer range, caused by the wrinkles, elastomers with stiff islands at the same time have to overcome the delamina- tion problem caused by stress localization at the interfaces. Here, two approaches based on wavy electronics, as well as on locally reinforced composite substrates are investigated. Both techniques result in inorganic electronic devices reversibly stretchable to strain values >200%. Furthermore, the developed technology is used to demonstrate the basic components of a stretchable electronic system, including a sensor and integrated circuits for signal processing and power transmission wrapped around 3D surfaces. This proves the potential of the presented technology for electronic skins and smart implants. The fabrication of electronic devices in general requires temperatures above 100 °C, the use of different etchants and solvents, as well as a substrate surface roughness in the nano- meter range. Since these requirements are hardly compatible with elastic substrates, [28] the proposed stretchable electronics is manufactured in a two-step process: The devices are fabri- cated on a rigid substrate and afterwards transferred to an elas- tomeric polymer. A silicon wafer covered with polyvinyl alcohol (PVA) and parylene is used as substrate. After the device fab- rication is finished, the PVA is dissolved in water and the 1 μm-thin parylene membrane carrying the electronic devices is released (Figure S1, Supporting Information), [8] and transferred to any arbitrary new substrate. Since transistors are the most important building blocks for all electronic systems, Figure 1a shows a schematic of the fabricated thin-film transistors based on amorphous indium-gallium-zinc-oxide (IGZO) as semicon- ductor, high-k Aluminum oxide ( ε r ≈ 9.5) as gate insulator and Nowadays, electronics is diverging from being bulky and rigid and is becoming lightweight and flexible. This development not only leads to new applications covering all aspects of wear- able electronics, [1] ranging from smart textiles [2] to skin mount devices, [3] but enables also new cost efficient fabrication tech- niques. [4] Extremely bendable electronics based on amorphous silicon, [5] organic, [6,7] and oxide [8] semiconductors have been realized by the use of micrometer thin substrates. However, epidermal electronics, [9,10] smart implants, [11] or artificial elec- tronic skins for robots [12] require stretchable electronic devices. Since the stretchability of human skin varies between 20% and 70%, [9,13] elastic electronics have to survive similar elongations. State-of-the-art elastic electronics are classified into three main groups: first, conductive interconnection lines can be realized by using intrinsically elastic conductors, [14] air-bridge structures, [15] and metal lines on prestretched substrates [16] or patterned into meander-like geometries. [17] Examples are carbon nanotubes embedded into a rubber matrix (stretchable up to 100%), [14] or metal films on porous polydimethylsiloxane (PDMS) (stretchable by 80%). [18] Furthermore, oxide transistors roll-transferred to elastic PDMS substrates can be stretched by 5%. [19] Significantly more stretchable active devices made from brittle materials are realized by two other approaches, namely by the fabrication on elastomeric substrates with stiff islands and by the use of “wavy” layouts using prestretched substrates. [20,21] The use of stiff islands allows minimizing the strain experienced by the devices. Here 20% stretchability was achieved for amorphous silicon and oxide thin-film transistors (TFTs) fabricated on PDMS patterned with stiff polyimide or epoxy-based photoresist islands. [22] At the same time off-the- shelf LEDs on an elastic composite substrate stayed functional while strained by 150%. [23] On the other hand, wavy electronics www.MaterialsViews.com www.advelectronicmat.de Adv. Electron. Mater. 2015, 1, 1400038