Effect of twisting fatigue on the electrical reliability of a metal interconnect on a flexible substrate
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Seol-Min Yi Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Korea
Byoung-Joon Kima) School of Materials Science and Engineering, Andong National University, Andong, Gyeongbuk-do 36729, Korea
Young-Chang Joob) Department of Materials Science & Engineering, Seoul National University, Seoul 08826, Korea (Received 17 July 2017; accepted 28 September 2017)
To secure the reliability of flexible electronics, the effect of multicomponent stress on the device properties during complex mechanical deformation needs to be thoroughly understood. The electrical resistances of metal interconnects are investigated by in situ monitoring at different twisting angles and with different pattern positions. As the twisting angle increased, the electrical resistance increased earlier. Furthermore, in the line pattern located far from the central axis, severe electrical degradation and fatigue damage formation were observed. Multicomponent stress evolution during twisting was analyzed by the finite-element simulation method. For easy practical application for estimating the representative twisting strain, an analytic solution of twisting deformation was formulated and compared with the simulation. Using the equivalent strain, the fatigue lifetime was fitted, and the exponents were obtained for lifetime expectation. This systematic study provides the guidelines for highly reliable flexible devices and the tools for determining the expected fatigue lifetime.
I. INTRODUCTION
Currently, flexible electronic devices such as sensors, batteries,2 displays,3 and solar cells4 are attracting great interest as next-generation devices due to their high portability and light weight. These flexible devices are supposed to be operated under repeated mechanical deformation unlike conventional rigid electronics, which is very susceptible to mechanical impact. Repeated bending or stretching generally results in the gradual degradation of device properties. Specifically, severe and complex deformation such as twisting deformation is more dangerous to the reliability of the device. Therefore, an understanding of the effect of complex mechanical deformation on the electrical reliability of flexible devices is essential to guarantee their long-term reliability. A metal layer, used as an interconnect or an electrode, is a basic component in flexible electronics and covers a large area in a flexible device. The conductivity of flexible metal under mechanical deformation is directly related to the electrical performance and reliability of an entire device. Therefore, many studies have been 1
Contributing Editor: C. Robert Kao Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.422
conducted to investigate the effect of mechanical deformation on the deformability of a metal layer. The mechanism of damage formation5,6 and associated parameters such as the thickness of the metal film,7 the width of the metal line,8 different kinds of metal,9 and the thickness of over-layer
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