Selective Growth of ZnO nanowires for Electronic Skin

Abstract

Touch sensors have studied intensively to mimic the sense of human touch by using resistive, or ca-pacitive sensors and so on for mobile devices and android robot. However, most of touch sensors simply detect pressure or pressure distribution without psychological feelings such as soft, roughness or pain. To produce artificial psychological feeling, we tried to develop touch sensor arrays based on piezoelectric. Touch sensors based on piezoelectric materials have several advantages such as self-powered, high resolu-tion, multi touch, and simple design. Various piezoelectric characteristics of ZnO nanowire have been stud-ied for a novel tactile sensor concept. To make device structure with high performance efficiency, the crystal orientation of seed layer, the kind of metals, the length of wire, the change of pressure, and the cell size ef-fect have been studied in the point of piezoelectric effect. The combination of Au electrode and thin ZnO seed layer formed by sputtering at room temperature shows a good piezoelectric power generation from the grown ZnO nanowire, since ZnO nanowires on the gold (Au) layer makes a Schottky barrier with good crys-tallinity. The length of ZnO nanowires are proportional to the growth time. With increasing length of wire, the electrical signal was increased from about 80 mV to 150 mV. The current was also increased from 250 nA to 400 nA. Smaller cell structure produces higher piezoelectric power density due to the increase of ef-fective edge area. Because the resolution of human fingers is very small, about 1mm2, the small size effect of ZnO wire array cell gives other design merit for touch sensor application. To produce touch feeling using ZnO wire sensor concept, 3x3 array patterns were fabricated, and then, the power signals were measured when giving external pressure by using several material such as sharp tip or blunt tip. The generated signals are different with materials, shapes and degree of pressure. Therefore, we can produce some psychological feelings using the ZnO wire array structure and signal processing. Since the maximum of process tempera-ture growing ZnO is 90°C, the structure design and fabrication process is quite proper to apply to a flexible substrate and tactile sensor concept. This result can be applied to other fields such as self-power generation or piezo-mechanical device. ⓒ 2014 DGIST