Plastic Deformation of Thin Foil Substrates with Amorphous Silicon Islands into Spherical Shapes
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Plastic Deformation of Thin Foil Substrates with Amorphous Silicon Islands into Spherical Shapes Pai-hui I. Hsu, Min Huang, Sigurd Wagner, Zhigang Suo, and J. C. Sturm Center for Photonics and Optoelectronic Materials (POEM) Princeton University Princeton, NJ 08544, U.S.A. ABSTRACT There is a growing interest in the application of large area electronics on curved surfaces. One approach towards realizing this goal is to fabricate circuits on planar substrates of thin plastic or metal foil, which are subsequently deformed into arbitrary shapes. The problem that we consider here is the deformation of substrates into a spherical shape, where the strain is determined by geometry and cannot be reduced by simply using a thinner substrate. The goal is to achieve permanent, plastic deformation in the substrates, without exceeding fracture or buckling limits in the device materials. Our experiments consist of the planar fabrication of amorphous silicon device structures onto stainless steel or Kapton polyimide substrates, followed by permanent deformation into a spherical shape. We will present empirical experiments showing the dependence of the results on the island/line size of the device materials and the deformation temperature. We have successfully deformed Kapton polyimide substrates with 100 µm wide amorphous silicon islands into a one steradian spherical cap, which subtends 66 degrees, without degradation of the silicon. This work demonstrates the feasibility of building semiconductor devices on plastically deformed substrates despite a 5% average biaxial strain in the substrate after deformation. INTRODUCTION The applications of traditional large-area electronics, such as displays, are limited by the fact that glass substrates are rigid and easily breakable. Previous work [1,2] has demonstrated that transistors on thin metal or plastic foil substrates can be rolled around a cylinder down to 0.5mm radius of curvature with no adverse effects. In these cases, the transistor is compressed when bending inwards and elongated when bending outwards. The strain due to such cylindrical deformation can be decreased by reducing the thickness of the substrates. In this work we permanently deform the substrates into the final shape of a spherical dome. To deform a flat sheet into a spherical structure, the initial substrate is stretched so that the surface area increases. The average strain in this case is determined by geometry and is independent of the substrate thickness. Another important difference is that while the device can be either compressed or stretched when rolling the substrate, spherically deforming the substrate leaves the devices in tension in all cases.
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EXPERIMENTS AND RESULTS The thin film structures are first processed on planar thin foil substrates of stainless steel or Kapton polyimide, with a typical thickness of 50 µm. The foil is placed over a circular hole and clamped by a circular ring (Figure 1). Pressurized gas is then used to deform the material inside the clamped ring into the shape of a
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