Elsevier

Thin Solid Films

Volume 501, Issues 1–2, 20 April 2006, Pages 346-349
Thin Solid Films

Improvement of indium–tin oxide films on polyethylene terephthalate substrates using hot-wire surface treatment

https://doi.org/10.1016/j.tsf.2005.07.147Get rights and content

Abstract

Indium–tin oxide (ITO) thin films have been studied extensively in flat panel displays because they combine unique transparent and conducting properties. A continuous roll-to-roll sputter system was used to deposit ITO/SiO2 thin films on polyethylene terephthalate (PET) substrates with no intentional heating. Typical transmittance and resistivity of the ITO/SiO2/PET sample were 78% (400∼700 nm) and 7.5 × 10− 3 Ω cm, respectively. Changes in the characteristics of ITO/SiO2/PET after hot-wire surface treatment in oxygen were examined in terms of resistivity, transmittance, surface roughness and bond configuration. The effects of process parameters like iridium wire temperature, chamber pressure and process duration on the properties of the ITO films are studied in details and compared with the works by plasma or furnace annealing. Under optimum conditions, the resistivity of the ITO/SiO2/PET sample can achieve a minimum value of 1 × 10− 3 Ω cm with a transmittance of 82%. These results indicate that the hot-wire system can be utilized to realize better quality film and to overcome the technological limits imposed by the role-to-role sputtering conditions.

Introduction

Indium–tin oxides (ITO) are the most widely used transparent conducting oxides [1], [2] for flat panel displays, primarily because they have high optical transmittance in the visible region, high electrical conductivity, surface uniformity and process compatibility. ITO films can be prepared by a variety of techniques: dc-sputtering [3], r.f.-sputtering [4], [5], electron beam evaporation [6], chemical vapor deposition [7] and spray pyrolysis [8]. The nature of the substrate and experimental conditions such as oxygen partial pressure, substrate temperature and post deposition annealing were confirmed to have large effects on the microstructure, electrical and optical properties of the ITO films [3], [4], [5], [6], [7], [8]. For example, an oxygen vacancy can donate free electron for conduction, but the oxygen-deficient film shows poor optical transmittance and poor ‘Sn’ doping efficiency. Whereas the Sn ion doping can provide one free electron for electrical conduction without significant reduction of visible transmittance, it may also create a locally disordered structure upon the doping concentration.

Recently, plastic materials are desirable in the flat panel display industry, where the demand for remote information access is driving the development of rugged, lightweight, power efficient displays. There are many reports on the properties of the ITO films prepared on rigid plastic substrates [9], [10]. Various chemical modification protocols have been used to improve the physical and electronic properties of ITO. In this paper, the properties of the ITO films after the hot-wire surface treatment is reported. Polyethylene terephthalate (PET) was used as the substrate material. The effects of treatment parameters like iridium (Ir) wire temperature, O2 pressure, and process duration on the electrical and optical properties of the flexible ITO/SiO2/PET thin films are studied in details and compared with the works by plasma or furnace annealing. It will be shown that the hot-wire system can be utilized to realize better quality film and to overcome technological limits imposed by the compatibility of deposition conditions (e.g. role-to-role sputtering).

Section snippets

Experimental details

A continuous roll-to-roll sputter system was used to deposit ITO/SiO2 thin films on PET substrates with no intentional heating. Before the sputter deposition, the PET substrate is cleaned and textured by argon ion plasma. Hot-wire surface treatment of the ITO/SiO2/PET samples was performed in O2 at a chamber pressure between 2.5 and 40 Pa. It is very important to select a catalyst material that endures the oxidative atmosphere at high temperature. Tungsten cannot be used in oxygen atmosphere

Results and discussion

The as-deposited ITO (50 nm)/SiO2 (30 nm)/PET sample has a resistivity of 7.5 × 10− 3 Ω cm. Fig. 1 shows the effect of the Ir wire temperature on the resistivity of the ITO films under vacuum or oxygen ambience. There is no change in the ITO thickness after the hot-wire treatment. It was found that the resistivity of the vacuum-treated ITO film increased when the Ir wire temperature increased. This could be due to the increase of oxygen vacancies in the ITO films caused by the higher wire

Conclusion

We have reported the characterization of hot-wire-treated ITO/SiO2/PET samples in O2 at low chamber pressures. The optimum chamber pressure and Ir wire temperature were found to be approximately 20 Pa and 1300 °C, respectively. Under the optimum conditions, there is a decrease in resistivity of an ITO film from 7.5 × 10− 3 to 1 × 10− 3 Ω cm and the transmittance from 78.3% to 82.5%. It was found that the control of oxygen vacancies is very important to improve the resistivity and transmittance of ITO

Acknowledgement

This work was supported by the National Science Council of Republic of China under contract No. NSC 93-ET-7-005-001-ET.

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