The Evolution of Touch Screen Technology
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.
Table of Contents
Resistive Touch – The Early Days
Once upon a time, resistive touchscreens were commonplace (and in the present – on those terrible entertainment systems on planes and ‘interactive’ directories in shopping malls, urgh). They required significant pressure before touch was registered, leading to some 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 resistive. In its resting state, an electric current is allowed to run between the conductive bottom layer.
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 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’s one upside though, since the system is effectively pressure-based, you can use almost anything that’s pointy to elicit a response from a resistive touchscreen.
The Next Step – Capacitive Touch
Nowadays in the age of smartphones, capacitive touchscreens are more widespread. They are effectively capacitors – they are able to temporarily store an electric charge when touched by conductive material – and are named as such.
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 (which we will go into more detail about later).
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.
The advantages of using capacitive touchscreens are the ability of the system to detect multiple POCs as well as generating movement (such as swiping). Also, due to having an outer glass layer and the nature of indium tin oxide as a conductor, their system provides a much clearer image of the screen.
However, this comes at a 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 IR region as well as having almost metallic conductivity1 – which makes it perfect for use in touch screens! You can read more about n-type and p-type semiconductors here.
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 the first two, 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?
- 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.
- Mozdzyn, L. (2008). U.S. Patent Application No. 12/210,140.