The Evolution of Touch Screen Technology

woman pointing at smartphone ftloscience post

Nowadays, you would be hard-pressed to find someone who didn’t own a phone or a piece of technology that uses touch screen technology. From early resistive touch screens to the indium tin oxide coated glass we use today, the development of touch screen capability over the years is fascinating. We look at the evolution of the touch screen from early resistive touch to the powerful indium tin oxide screens widely used today.

Resistive Touch in the Early Days

Once upon a time, resistive touchscreens were commonplace. They haven’t been made completely obsolete: think about those terrible entertainment systems on planes and ‘interactive’ directories in shopping malls, urgh. Resistive touchscreens, as their name suggests, require significant pressure before a touch is registered, leading to much frustration on the part of the users.

Invented in the 70s, it consists of a glass screen base with two metallic layers mounted, one on top of the other but separated ever so slightly by spacers. The bottom layer is conductive, while the top is resistive. In its resting state, an electric current is allowed to run between the conductive bottom layer.

airbus 380 touchscreens
Both screens exist on the same A380 plane (Flickr)

Once pressure is applied, however, there is contact between these layers in a specific spot. With the top layer being resistive, the material interrupts this current on the bottom layer. The exact coordinates, known as the point of contact (POC), can then be identified and processed.

Although cheap, there are certain big downsides to using this system. Firstly, resistive touch technology is somewhat primitive as only one POC can be registered at a time. This makes it impossible to ‘swipe’ or incorporate multi-touch gestures into resistive touchscreens.

Also, since resistive touch screens are made of plastic, it results in a blurry image you receive from the screen. Remember that there are TWO plastic screens your image has to transmit through. There is some upside to using resistive touchscreens, though. Since the system is effectively pressure-based, you can use almost anything that’s pointy to elicit a response. And since all that is required is a resistive top layer, they can be made to be rather thick to prevent damage.



The Next Step: Capacitive Touch

Nowadays in the age of smartphones, capacitive touchscreens are more widespread. They get their name from being capacitors; the screens are able to temporarily store an electric charge when touched by conductive material. Capacitors are widely used in virtually all electronic devices.

There are multiple ways that this technology can be implemented, but the most common is to coat a glass layer on top of the screen with a transparent conductor such as indium tin oxide.

This creates an electric field between the inner screen and layer, kind of like in a resistive touchscreen. The difference is that when a conductor (such as your fingers, or conductive rubber pens) makes contact with the surface, the electric field is distorted. The change in capacitance that occurs is then picked up by multiple detectors surrounding the screen, which is translated into coordinates.

capacitive touch on phone

The advantages of using capacitive touchscreens are the ability of the system to detect multiple POCs as well as to generate movement (such as swiping). Also, due to having an outer glass layer and the transparency of indium tin oxide as a conductor, their system provides a much clearer image of the screen.

However, this also comes at a greater financial cost. And I guess, also the cost of not being able to properly work your phones when your fingernails get too long.

Indium Tin Oxide (ITO)

Tin-doped indium oxide is perfect for use in this case as it is optically transparent while being a good conductor. ITO is an example of a heavily ‘doped’ n-type semiconductor, which works by having excess electrons in the lattice structure. Therefore the movement of ‘free’ electrons is what gives it conductivity.

Due to this bandgap, ITO is highly transparent in the visible region, reflective in the infrared region as well as having almost metallic conductivity1. This makes it perfect for use in touch screens! You can read more about n-type and p-type semiconductors here.

piece of transparent indium tin oxide
ITO coated glass – what, were you expecting something magical? Yeah, so was I. (Flickr)

When it comes to designing a semiconductor for use in touch screens, the main challenges to address are its speed (electron transfer), durability and manufacturing costs. While ITO does well in many aspects, it is still rather expensive to produce and process it onto the glass. Perhaps the next generation of touch screens will incorporate novel semiconductors such as graphene?



Reference

  1. Bel Hadj Tahar, R., Ban, T., Ohya, Y., & Takahashi, Y. (1998). Tin doped indium oxide thin films: Electrical properties. Journal of Applied Physics, 83(5), 2631-2645.
  2. Mozdzyn, L. (2008). U.S. Patent Application No. 12/210,140.

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