Tech Briefs

The Flexibility of ITO Films in Electronic Coating Applications

Indium-tin oxide (ITO) is used in nearly all flat panel displays, laptop screens, and mobile phones, in addition to solar panels and “smart” windows.

Indium Corporation, Utica, New York

ITO is a doped metal oxide semiconductor that combines two properties that usually are mutually exclusive in most materials: optical transparency and electrical conductivity. It is critical to understand the importance of this combination of optical transparency and electrical conductivity. A flat panel display cannot work without both properties. Yet the very nature of electrical conductivity normally excludes optical transparency. Doped metal oxide semiconductors conduct electricity in a different manner than metals, and hence, are not doomed to be opaque.

Of the transparent thin films, ITO is most commonly used because, at a given thickness, it has the highly desired combination of both better electrical conductivity and higher optical transparency compared to any other doped metal oxide. Other ITO advantages include resistance to moisture penetration and chemical stability.

Indium-tin oxide is a solid solution of indium(III) oxide (In2O3) and tin(IV) oxide (SnO2). Typically, ITO is 90% vIn2O3 and 10% SnO2 by weight. ITO is deposited on the substrate as a thin film, which is both highly electrically conductive and optically transparent, enabling the user to see easily through the film. The substrate onto which ITO is deposited is determined by the product’s end use, but is typically glass or some form of clear, flexible polymer. The most common deposition process for thin-film ITO is D.C. magnetron sputtering using plasma in a vacuum chamber. This method is the most widely used mass-production method for all major applications, including computer displays and television screens.

Thicknesses of ITO films are typically 40-280 nm, depending on the application and properties required. In general, the electrical carrier density increases and electrical resistivity decreases as the film thickness increases. A human hair is about 75,000 nm, or almost 2,000 times as thick as the thinnest ITO film. ITO film resistivity is typically less than 200 micro ohm·cm, and optical transparency is greater than 90% in the visible light range. The mechanical properties of ITO thinfilms produced by sputtering depend on the deposition parameters, most significantly the substrate temperature, oxygen partial pressure, and the distance between target and substrate. To evaluate these mechanical properties it is necessary to test them on the substrate onto which they will be deposited as the adhesion to the substrates and the substrates themselves dramatically affect ITO performance.

Since metal oxides, including ITO, are ceramic materials, a common criticism is their brittleness. However, the thinness of ITO allows for its flexibility. Glass, normally considered brittle, is quite flexible when produced in thin fibers. Typical glass fiber diameters are in the 10 micron or 10,000 nm range — much thicker than ITO. There is no flexibility concern for ITO deposited on the rigid display glass used in LCD and plasma TVs and computer displays, as the glass is very brittle. However, ITO brittleness is thought to be an issue for flexible plastic substrates used in future mobile display devices and in flexible solar panel applications.

It is doubtful that a flexible polymer flat panel display would often be bent to a radius of less than 5 mm. Once flexible displays arrive, they will still have the need for driver electronics and other supporting electronics parts that, for the foreseeable future, will have to be supplied by standard packaged chips, and therefore be inherently inflexible, progress in printed electronics notwithstanding.

The most convincing need for bendability in ITO does not derive from an application, but rather from the manufacturing process. Roll-to-roll processing is broadly discussed to make inroads into display manufacturing in the coming years because of its potential to reduce costs, and electrophoretic (eink) displays, as well as touch pad sensors for tablets, are already made this way. A simple way to manage ITO fatigue strain in the roll-to-roll process is to run the web over largediameter rolls with a minimal number of travels over the rollers. This reduces the number of bend cycles and helps to maintain the conductivity within specifications, all while streamlining the manufacturing process.

According to a Corning report, ITO-coated flexible glass can be wound around a 10"-wide spool containing a 6'' core (ID). In the few cases where ITO films may not be flexible enough, methods have been developed that can ameliorate this concern. If ITO mechanically fails on polymer films, it can manifest itself as cracking due to tensile stress or delaminating due to compressive stress. Several approaches have been developed to address these concerns.