Lightweight touchscreen pressure sensor arrays
Touchscreens on mobile handheld devices can detect if and where a user is touching the screen, but standard technology cannot determine how much pressure is being exerted. Now, researchers at the University of California San Diego and the University of Texas at Austin have demonstrated a new technology for ‘force sensing’ that can be added to any type of display, including flexible devices, and potential other uses go far beyond touch screen displays on mobile devices.
Before he graduated from UC San Diego's Jacobs School of Engineering, Electrical and Computer Engineering (ECE) alumnus Siarhei Vishniakou (Ph.D. ’16) worked with colleagues including his advisor, ECE professor Shadi Dayeh, to spin off a startup company, Dimensional Touch. He was also accepted into the NSF I-Corps I and II programs that help academics commercialise new technology.
Since then, the team has demonstrated that zinc oxide (ZnO)-based thin-film transistor (TFT) sensors can be easily integrated with existing commercial integrated circuits (ICs) widely used to control touch screens (in which a variant of ZnO, Indium gallium zinc oxide [IGZO], is already used).
“It has been known for generations that ZnO has good piezoelectric properties and manufacturers already use IGZO in displays,” said Dayeh. “So it seemed that using ZnO in a thin-film transistor (TFT) would seamlessly integrate into the process flow already used by manufacturers of touch screens.”
Dayeh’s team developed and optimised the technology so that it simultaneously functions as a transistor and as a force sensor.
“We’ve determined that we could improve the transistor performance and the pressure-sensitivity by doing the ZnO deposition in an oxygen-rich environment,” said first-author Vishniakou. “The cost of the technology is also reduced because it can be integrated into a display at the backplane level.”
Dayeh is the senior author on a paper published in the journal Advanced Materials Technologies.
Force sensing permits users to draw on a transparent touch screen much as they might with a pencil or paint brush to draw a thinner or darker line by pressing lightly or with greater force on paper or a canvas.
While Apple’s force touch technology introduced in iPhone 7 required adding an extra layer under the display, the newest technology can add force sensing to any type of display, including flexible and lightweight displays.
Production could also scale up faster, because the ZnO TFT can be built on thin glass wafers that bend. According to Dayeh, the team worked with flexible glass from Corning Inc. that is 100 micrometers thick, and built an array of 16x16 elements that effectively can bend with the backplane.
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Image credit: University of California, San Diego.