Inorganic islands on a highly stretchable polyimide substrate
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Nanshu Lu School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138
Juil Yoon Department of Mechanical Systems Engineering, Hansung University, Seoul 136-792, South Korea
Kyu-Hwan Oh Department of Material Science and Engineering, Seoul National University, Seoul 151-742, South Korea
Zhigang Suo and Joost J.Vlassaka) School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received 23 June 2009; accepted 2 September 2009)
For a flexible electronic device integrating inorganic materials on a polymer substrate, the polymer can deform substantially, but the inorganic materials usually fracture at small strains. This paper describes an approach to make such a device highly stretchable. A polyimide substrate is first coated with a thin layer of an elastomer, on top of which SiNx islands are fabricated. When the substrate is stretched to a large strain, the SiNx islands remain intact. Calculations confirm that the elastomer reduces the strain in the SiNx islands by orders of magnitude. I. INTRODUCTION
Flexible electronic devices may experience one-time or repeated large deformation during manufacture or in service.1–7 Such a device often integrates a polymer substrate with diverse inorganic materials, such as semiconductors, metals, and ceramics. While the polymer can deform substantially, the inorganic materials usually fracture at strains below 1%.8 Consequently, the device is stretchable only when the polymer and the inorganic materials are suitably integrated. Examples include inorganic islands5,9–11 and buckled inorganic films12,13 on polymer substrates. This paper focuses on inorganic islands on a polymer substrate. For example, islands of diamond-like carbon on an elastomeric (PDMS) substrate remain intact when the substrate is stretched beyond 25%.9 Some applications may require substrates much stiffer than PDMS, but SiNx islands on much stiffer polyimide (PI) substrates crack and debond when the substrates are stretched by only a few percent.14,15 These two examples illustrate a trade-off between the stiffness of the substrate and the stretchability of the structure.
a)
Address all correspondence to this author. email: [email protected] DOI: 10.1557/JMR.2009.0417
3338
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 11, Nov 2009 Downloaded: 13 Apr 2015
Here we demonstrate that, when a thin layer of PDMS is sandwiched between the polyimide substrate and the SiNx islands, the substrate can be stretched beyond 20% without causing the islands to crack or debond. The PDMS interlayer behaves as a buffer, isolating the islands from most of the strain of the substrate. The resulting SiNx/PDMS/PI structure is both stretchable and relatively stiff. II. EXPERIMENTAL
We fabricated two sets of specimens, SiNx/PI and SiNx/PDMS/PI, using the lift-off technique illustrated in Fig. 1(a). The substrates for the SiNx/PI specimens were 25 mm thick Upilex-S (UBE Industries, Tokyo, Japan), while the substrates for the SiNx/P
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