ZnO TFTs revealed as next-gen in-screen force sensors

January 31, 2018 // By Julien Happich
Researchers at the University of California San Diego and the University of Texas at Austin have demonstrated a novel transparent force sensing technology that can be readily integrated into any type of display, as a one-micron thin layer deposited on top of the backplane, using commonly used materials and processes.

Adding force sensing to capacitive touch screens gives many more features to a display, notably for stylus-based drawing applications or in gaming. Until now, adding 3D touch (touch location and force) meant additional processing electronics and sensing layers. But the researchers who leveraged the known piezoelectric properties of ZnO to create force sensing thin film transistors have now demonstrated that they could seamlessly integrate their force sensors onto the Indium gallium zinc oxide [IGZO] active matrix of most displays (the materials and deposition processes being similar). The force sensors can replace the capacitive sensing layer as a whole since each pressure sensor (transistor) also acts as a switch.

In a paper titled "Improved Performance of Zinc Oxide Thin Film Transistor Pressure Sensors and a Demonstration of a Commercial Chip Compatibility with the New Force Sensing Technology" published in the Journal of Advanced Materials Technologies, first author Siarhei Vishniakou describes how the ZnO TFTs' piezoelectric characteristics and transistor performance had been optimized through oxygen-rich deposition conditions. For a 16x16 transparent pressure sensor array, the paper reports a sensitivity of 4nA kPa−1 and a latency below 1ms (an order of magnitude faster than today's touchscreens response).