Reliability of thin films: Experimental study on mechanical and thermal behavior of indium tin oxide and poly(3,4-ethylenedioxythiophene) Atif Alkhazaili a,b , Mohammad M. Hamasha c,⇑ , Gihoon Choi a , Susan Lu a , Charles R. Westgate a a State University of New York at Binghamton, Binghamton, NY, USA b Department of Industrial Engineering, Hashemite University, Zarqa, Jordan c Department of Industrial Engineering, University of Business and Technology, Jeddah, Saudi Arabia article info Article history: Received 5 August 2014 Received in revised form 11 December 2014 Accepted 26 January 2015 Available online 7 February 2015 Keywords: Bending fatigue Damp heat PCER ITO PEDOT Reliability abstract In order to improve the performance of various flexible electronic devices, the research on the transpar- ent conductive thin films becomes very intensive in the recent years. In this work, we studied mechanical and thermal behaviors of two types of transparent conductive thin films, Poly(3,4-ethylenedioxythio- phene) (PEDOT) as an example on polymer conductive thin film, and indium tin oxide (ITO) as an exam- ple on transparent conductive oxide thin film. Both films are deposited on polyethyleneterephthalate (PET) substrate. Two sheet resistances for PEDOT (i.e., 150 X/sq and 225 X/sq) and one sheet resistance for ITO (i.e., 60 X/sq) were involved in the study. PEDOT showed good mechanical properties with a small electrical resistance change and no clear cracks or deformation on the film surface under the condition of cyclic bending. However, the resistance of ITO significantly increased with the cyclic bending and cracks were seen initiated in the center of sample and propagated toward the edges. Further, design of experi- ment approach was used to study the effect of different cyclic bending parameters, such as bending diam- eter and frequency. Additionally, damp heat experiment on similar samples was conducted by applying 85 °C temperatures and 85% relative humidity on them for 1000 h. The electrical resistance was dramat- ically increased for both films. Scanning electronic microscopy (SIM), Energy Dispersive Spectrometry (EDX) and transmission tests were also used to determine the change of films’ compositions and transparencies. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Flexible electronic displays have unique features, such as light weight, low cost, and flexibility. They are used in many applica- tions such as, solar cell, organic light emitting diode (OLED), and electronic sensors [1]. There were many efforts on improving con- ductive films to increase the solar cell efficiency. Two of the most common conductive transparent thin films are polyethylene dioxy- thiophene (PEDOT) and tin indium oxide (ITO). They are very com- mon in liquid crystal displays (LCDs), sensors, and emissions devices applications [2,3]. ITO is a brittle transparent conductive oxide, while PEDOT is a ductile polymer. Many researchers paid attention to show the response of the ITO films to mechanical and thermal stresses. For example, Leterrier et al. [4] noticed that process-induced internal stresses were systematically compressive, and the tensile cracks in the ITO layer always start from surface defects. Further, they noticed that transition from stable to unstable crack growth starts at crack length of several hundred times of the coating thickness. Furthermore, they found that polarization of hard coats provides a good quality surface for polymer substrate and enhances cohe- sive specifications of ITO thin film. Many researchers studied mechanical properties of different thin film materials that were exposed to cycling bending and ther- mal stresses [5]. Alzoubi et al. [6] investigated the effect of high and low cycle bending on electrical properties of copper thin film coated flexible substrate. In their study, they observed that the fati- gue failure was developed faster at strain of 2.6% or higher. Also, Hamasha et al. [7] carried out an experiment to study the effect of cyclic bending fatigue on the surface integrity of ITO under harsh environmental condition. They conducted, however, the same experiment on similar samples, but without involving any bending to make a comparison and figure out the effect of bending fatigue itself. They found that the combination of high temperature and humidity under bending fatigue condition is critical on the conduc- tivity and the surface integrity of the thin film. Park et al. [8] conducted a bending test on the single crystalline silicon coated plastic substrate. They observed three types of http://dx.doi.org/10.1016/j.microrel.2015.01.013 0026-2714/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Microelectronics Reliability 55 (2015) 538–546 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel