Failure of brittle functional layers in flexible electronic devices
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Failure of brittle functional layers in flexible electronic devices Judith de Goede1, Piet Bouten1, Léonard Médico 2, Yves Leterrier2, Jan-Anders Månson2, Giovanni Nisato1 1 Philips Research Laboratories, Eindhoven, The Netherlands, 2 Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
ABSTRACT In the present study, 90 nm thick conducting tin-doped indium oxide (ITO) layers on polymer substrates are used to study the failure behaviour of brittle layers. In a two-point bending test the resistance of uniform ITO-layers and narrow ITO-lines (10 to 300 µm width) are determined as a function of the applied tensile strain. At a certain strain, the thin layer will crack and the resistance will strongly increase. This characteristic strain is analysed using a large number of samples. Early stages of crack development are studied in the fragmentation test. This paper presents the results of different failure mechanisms of thin brittle layers on the electrical conductivity of ITO-layers. When the strain in the ITO-layer increases, stable cracks of a limited length are initiated at defects in the layer (crack initiation). At the critical strain εp the crack is no longer stable and it will propagate over the whole width of the layer (crack propagation). The experiments show a wide failure strain distribution for narrow ITO-lines. The wide distribution of defects, determining the crack initiation strain, controls the failure of narrow lines. The uniform layers show a narrow failure strain distribution. This is determined by the well-defined crack propagation strain εp. INTRODUCTION In recent years an increasing amount of attention is given to the development of flexible electronic devices, such as flexible displays and microelectronics on flexible carriers. Instead of the brittle carriers (glass and silicon) more flexible substrates, such as polymers and thin metal sheets are used. The functional performance of the device requires, however, still thin brittle layers. Examples of these layers are a transparent conductor like tin-doped indium oxide (ITO) in flexible displays, transparent inorganic hermetic coatings in Organic Light Emitting Devices (OLED) devices and silicon-based microelectronics in flexible, active matrix driven, Liquid Crystal Displays (LCD). Mechanical failure of the brittle functional layers seriously limits the allowed deformation of the flexible devices. ITO and polymers have different thermal expansion (CTE) and elastic properties. Therefore fabrication and personal use of a device can introduce strains into these material combinations. The application of an excessive strain leads to failure of a functional layer [1]. In previous papers [2, 3] the failure behaviour of tensile loaded ITO-layers and wide lines was studied. Only a small effect of the line width on the critical failure strain was observed. Model predictions [4] indicate a clear effect of the elastic mismatch on the crack evolution in the brittle
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