Durability under mechanical bending of the indium tin oxide films deposited on polymer substrate by thermionically enhanced sputtering

Abstract

The emerging demand for flexible display panels has created a demand for coating techniques capable of depositing high-quality transparent conductive coatings (TCOs) on polymeric materials. This study demonstrates indium tin oxide (ITO) films deposited on polyethylene terephthalate (PET) substrates using a thermionic emission (TE) enhanced DC magnetron sputtering system to improve electrical and optical properties under static and dynamic mechanical bending.

Experimental results show that the ITO-coated PET specimen prepared with TE and without TE presents an average transmittance in the visible region of the spectrum of 83% and 70%, and an electrical resistivity of 4.5 × 10⁻⁴ and 1.7 × 10⁻³ Ω cm, respectively. By contrast, the static bending test revealed that the critical radius of curvature (R_c) is 13.9 and 17.0 mm for the ITO-coated PET with and without TE. The dynamic bending test showed that the ITO-coated PET with TE is durable for more than 1200 cycles, while the corresponding number for the ITO-coated PET without TE is smaller than 50 cycles for the ITO-coated PET without TE. These results evidence that the ITO film deposited using the TE approach exhibits a higher degree of crystallinity and film adhesion than that deposited without TE. TE improves optical and electrical properties of the ITO coatings and their longer durability under mechanical bending.

Introduction

Indium tin oxide (ITO) films have been extensively applied in flat panel displays [1], [2] and photovoltaic devices [3], [4] as transparent conductive oxide (TCO) electrodes because of their low electrical resistivity and high optical transmittance. The emerging demand for flexible display panels has created a demand for coating techniques capable of depositing high-quality TCOs on polymeric materials to particularly achieve light-weight, compactness, impact tolerance [5], [6] and mechanical bending durability [7], [8]. Among commonly used polymeric materials, PET (polyethylene terephthalate) and PC (polycarbonate) have been considered as substrate materials for flexible displays because of their satisfactory optical transmittance and low material cost compared to other materials [9].

Modified sputter deposition techniques have directly obtained promising results to deposit ITO films on flexible substrates at a low deposition temperature. Yang [10] found a resistivity of 6.3 × 10⁻⁴ Ω cm and an average optical transmittance of 80% by utilizing an RF magnetron sputtering system with supplemental substrate biasing to deposit ITO film on PPA (polypropylene adipate) substrates. Kim [11] added negative cesium ions into a glow discharge sputtering system to impart additional energy into the growing ITO film, to achieve an electrical resistivity of 6.2 × 10⁻⁴ Ω cm and an average optical transmittance of 87%. Hoshi and Kiyomura [12] accomplished an electrical resistivity of 3.5 × 10⁻⁴ Ω cm and an average optical transmittance over 80% by using an RF applied grid in the DC sputtering system to increase ion density. Our previous study also demonstrated a thermionic electron emission (TE) approach as a supplement to the DC sputtering system to obtain an ITO film with an electrical resistivity of 4.5 × 10⁻⁴ Ω cm and an average optical transmittance of 80% [13]. The TE approach can be utilized to deposit high-quality ITO deposition at relatively low substrate temperature with low-cost equipment. The quality of the film thus formed, in terms of electrical resistivity and optical transmittance, is improved by the increased crystallinity of the film under intense ion bombardment. However, thermal expansion and elastic properties for both TCO films and polymer substrates are very different, resulting in large mechanical stresses. A TCO film on a PET polymer substrate subjected to such stress cracks and eventually fractures, markedly reduces its electrical conductivity. Cairns et al. [14] investigated the mechanical properties of ITO films on a PET substrate, focusing on the cracking mechanism and its relationship to electrical properties. In addition, Fortunato et al. [15] also observed that ZnO:Al cracking under increasing strain increases its resistance. This result is consistent with similar studies of silicon oxide deposited on the PET substrate [16].

Most studies on the above subject have been performed using thin films on polymer substrates and have demonstrated initial cracking and the mechanism failure of electrical conductivity of thin films. The current authors’ earlier study sought to obtain high-quality ITO films deposited by TE, and motivates this study. The goal of this study is to improve electrical response the ITO-coated PET under static and dynamic mechanical bending to improved crystallinity and the film adhesion through the TE technique which contributes to resist damage under mechanical action. This paper investigates the effect of mechanical bending of an ITO film deposited by DC magnetron sputtering on a PET substrate with and without TE. A discussion follows about the change in electrical resistance of ITO-coated PET specimens under a single static bending event and their durability under dynamic repeated bending.