ITO Glass used in LCD modules

Many LCD technologies, such as monochrome character, dot matrix and segment displays, make use of ITO glass. Even though ITO glass has been in existence for some time, it is still an important aspect in LCD designs and will be covered in the article below.

The article about ITO glass was written by Barbara Dutra, an exchange engineering student from Brazil, who is currently an intern at Focus LCDs. Her current job responsibilities include ISO certification, Test and quality insurance of inbound LCD displays and writing technical articles.

What is ITO glass (Indium tin oxide)

The Indium tin oxide is a material used in modern devices that manipulates ambient light. ITO is a good material, because it has a good response time to conduct electricity and an appropriate transparency for the emission of light.

It has been used as a conductive coating in photovoltaic cells (solar cells) since the light absorption layer has an electrode at both the front and rear to form a circuit and convert the light into electricity.

It is also used in flat panel TVs where each pixel is turned ON or OFF by a pair of transparent electrodes ITO. Touch screen displays are the latest innovations using that material.

ITO is the main component in these devices because it is a conductive property and possesses an excellent image quality.

Application in displays

The first touchscreen devices were sold with a pen and were manufactured with two layers of ITO glass separated by a small gap. When the resistive screens were touched with a pen, the two layers came into contact, creating a short circuit and allowing a current to pass and be detected by the device.

New devices, implementing newer technology, use the finger to allow the short circuit that identifies the position without the need of a pen. The touch in the screen changes its capacitance at that location and this change is perceived by a single layer of ITO.

Characteristics of ITO glass

ITO glass is formed of tin oxide doped with indium. It contains approximately 10% of tin oxide (IV), SnO2, and 90% of indium oxide (III), In2O3.

The ITO glass is a thin transparent film similar to common glass, but unlike glass, it is a conductor of electricity because it is a kind of transparent conductive oxide (TCO). So it has the property of reflecting electromagnetic radiation in the infrared region (spectrum) and having a low electrical resistivity.

The ITO is the best TCO because it is a good combination of transparency and conductivity. Because of its transparency, ITO glass does not absorb light photons. Absorption occurs when the photon energy corresponds to what is necessary to let the electron in an excited state.

In a metallic conductive material, there is a good passing of electronics, but unfortunately, all the metals are highly opaque.

Manufacturing process of ITO Glass

To produce the thin films of ITO glass, it is common to use sputtering techniques with radio frequency assisted by constant magnetic field (RF magnetron sputtering). The sputtering technique involves the transport of molecules or atoms ejected from a source (also called the target) to a substrate.

The ejection takes place by means of ionic bombardment of the target surface. Before starting the deposition process, the chamber is evacuated and held at a low pressure (10-6 torr) for several hours to remove residual gases.

Once the chamber reaches solid state, the noble gas argon is injected into the chamber to 10-3 torr pressure (low pressure), forming an inert atmosphere. A high potential difference is then applied between the substrates and the target substrate containing the raw material to be spray, ionized gas in the area and forming plasma.

The plasma ions are accelerated by the potential difference (leaving the cathode to anode) acquiring enough energy to erode the target material ejecting the atoms or molecules that are accelerated by the electric field toward the substrate. The process can employ multiple targets, thus enabling to deposit a certain film material on a different successively forming a multilayer.

The sputtering current systems utilize permanent magnets to create a magnetic field that serves to confine the plasma in the target region, increasing process efficiency (magnetron sputtering). The high DC (Direct Current) voltage may be used for spraying metals or RF more suitable for insulating materials.

The conditions during the deposition process directly affect the electro-optical properties of the ITO (that is the quality of display).

The future of ITO Glass

The ITO glass is an expensive material because of the indium mineral, but this is not the biggest problem. The biggest challenge is the shortage expected to take place in a few years because indium is a rare mineral.

Much of the indium mineral reservations are located in China, which controls the exporting and marketing demand of this material.

The calculus performed by the University of Yale, by the working group of the United Nations program on international metal flow, predicts the end of the use of indium on a commercial scale will continue until 2020.

This is bad news because the market of touch screen devices is in an expansion mode and every day the demand increases since bigger and bigger screens are created using a larger amount of ITO.

Scientists are already looking for an ideal replacement for ITO glass.

Some hypotheses are the cadmium oxide, silver nanowires and graphene (carbon nanomaterial). The first is almost as transparent as ITO glass and has a greater capacity of conduct electricity, but it is very unstable and deteriorates quickly (half-life).

One solution to this problem is to apply 20% of ITO in the cadmium oxide just to create a film of protection on the material. Another problem, an environmental one, is that this oxide is more toxic and demands care in the manipulation and disposal of waste. This could be a future environmental problem.

The cadmium oxide is not still a viable solution. The second has an 85% transparency and conductivity only slightly smaller than the ITO.

Silver is very similar to high quality ITO, but it is quite flexible. Unfortunately silver nanowires are ten times more expensive to produce than the already expensive ITO and cheaper metals seem to not work.

Finally, graphene is manufactured in sheets and can be used to fabricate transistors of 1 micrometer, so it is slightly opaque because it is so much thinner. This material is very malleable, so this solves the problem of curved screens.

The graphene can be used for touch screen displays because of the characteristics but is an expensive technology today. In the future the price between ITO glass and carbon nanotubes will be equivalent because of the lack of indium and carbon segment growth cheapening their cost. So, the graphene looks a promising option.

There are indium free techniques in displays and they can be developed for utilization in commercial products. The first uses a mechanical switch behind each pixel, recording the force as the screen is touched. But the use of sensitive technology to pressure means removing the protective glass that is in front of the screen of the touchscreen devices, which leaves the display vulnerable to damage.

The second possibility is an optical technology that incorporates a light-sensing element at each pixel. These light sensors turn the screen in a kind of scanner that can detect and track finger touches on the screen. It also has problems: the “optical touch” demands processing power to keep under review the screen surface in search of rings and works with a quarter of the speed of a traditional laptop touch-pad.

But that does not rule out this option because the processing power is in expansion. Anyway these free indium techniques do not solve a fundamental problem: with or without touch, the electrodes that provide power to the pixels on the LCD screen depend on the ITO glass. This will be solved only with the development of new materials that emulate the highly desirable ability of the ITO glass to combine transparency and conductivity.

Cost factors for LCD Displays

So how is this affecting the cost of LCDs?

It is safe to say that the cost of LCD displays does not look to be decreasing any time soon, if at all. Part of the reason for higher cost displays is not only the potential cost increase due to an ITO shortage. But a labor shortage that is taking place in many LCD manufacturing locations.

Believe it or not, there is a labor shortage in China and this is causing two issues:

1. A higher cost
2. Quality issues

The quality issues arise since there is such a high turnover. Employees no longer stay at one manufacture for years, but instead, they only stay until a better deal comes along.

This means that many processes that require a high percentage of training and experience, such as soldering, have a high turnover rate.

Just as a manufacture completes training an employee, the employee leaves and the training process has to start over again.

What is the solution?

Look for US-based suppliers that offer an in-house Quality Assurance program. This reduces the number of failures that arrive on your dock.

Focus Display Solutions is located in Chandler, Arizona and has been a designer and supplier of LCD displays since 2001.