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 conductive indium tin oxide screens widely used today.
Early resistive touchscreens were pressure activated double-screen systems. Today, capacitive touchscreens use conductive materials like indium tin oxide on a single layer to create a current. We disrupt this current when we touch it; detectors can pick up on the exact location of the touch.
Touchscreens in the Early Days
Using Light to Capture Touch
The first consumer PC to use touchscreen technology was the HP-150. First hitting shelves in 1983, the public was ecstatic about being able to use their fingers to control what was happening on the monitor.
The HP-150 used a system of crisscrossed infrared beams that covered the entire display. When the system was interrupted by a finger or stylus, infrared detectors would register the location of the obstructions and translate it into an input on the PC.
However, the main issue was that dust and dirt would enter the infrared system and cause it to malfunction. This meant that users had to constantly clean and vacuum the computer, which, unsurprisingly, led to poor sales of the HP-150.
Resistant to Touch!
To get around the issue of cleaning between the layers, we needed to switch to a fully enclosed touchscreen system. Luckily, such a system had already been invented in the 70s by American inventor George Samuel Hurst, known as a resistive touchscreen.
Resistive touchscreens quickly made their way into consumer electronics in the 1990s. Remember when PDAs and PalmPilots were all the rage? Resistive touchscreens were once so popular, in fact, that 90% of touchscreen devices employed this technology in 2007.
Although mostly replaced by newer capacitive touch technologies, they haven’t been made entirely obsolete. think about those terrible entertainment systems on planes and those 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.
How Resistive Touch Works
Resistive touch systems consist 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.
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.
Since the system is purely pressure-based, you can use anything that’s pointy to elicit a response from a resistive touchscreen. They are also durable, since the resistive top layer can be made to be rather thick to prevent damage.
There are certain 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 the resistive and conductive surfaces are made of coated plastic, it results in a blurry image you receive from the screen. Remember that there are TWO plastic surfaces your image must transmit through before reaching your eyes.
Touchscreens Today: Capacitive Touch
How Capacitive Touchscreens Work
Alas, the era of touchscreen frustration is behind us. Today, we have touchscreens that are thin, responsive and can capture multiple POCs, swipes and even fingerprints! All thanks to capacitive touchscreen technology.
If you own a smartphone, it almost certainly employs capacitive touchscreen technology. It gets its name from capacitors, devices that can store electric charge. Capacitors are widely used in virtually all electronic devices.
In this case, the screens themselves are can act as capacitors, temporarily storing an electric charge in a conductive layer. This electric field runs between a protective layer and the glass display, removing the air gap that we see in resistive touchscreens. This makes the screen thinner and lighter.
When a conductor (such as your fingers, or conductive rubber pens) makes contact with the surface, the electric field within the conductive film is distorted. This changes the current that runs through the capacitor, which is detected by multiple sensors that surround the screen.
The location of the change is current is translated into coordinates. Modern detectors like the ones in our smartphones can also register movement from a gesture such as a swipe, as the current changes from one point to another. Many screens come with multiple detectors that can detect multiple POCs.
The critical component in capacitive touchscreens is the conductive layer that is also transparent. While there are multiple ways we can do this by far the most common is to coat a glass panel with indium tin oxide.
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 conductivity. This 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 many aspects, ITO-coated glass is much more expensive to produce than resistive touchscreen systems.
Perhaps the next generation of touch screens will incorporate novel semiconductors such as graphene?
Reference
- 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.
About the Author
Sean is a consultant for clients in the pharmaceutical industry and is an associate lecturer at La Trobe University, where unfortunate undergrads are subject to his ramblings on chemistry and pharmacology.
