Design Rules for IGZO Logic Gates on Plastic Foil Enabling Operation at Bending Radii of 3.5 mm

Findings obtained from bending experiments with mechanically flexible InGaZnO-based thin-film transistors are used to derive design rules for flexible InGaZnO-based n-channel metal-oxide-semiconductor logic circuits. Based on the developed design rules, flexible NAND gates, inverters, and five-stage ring oscillators are fabricated directly on free-standing plastic foils at temperatures <= 150 degrees C. Geometrically well-designed circuits operated at a supply voltage of 5 V are exposed to tensile mechanical strains, induced by bending, up to 0.72% without performance degradation. This corresponds to a bending radius of 3.5 mm. At the same time, increases in the rise time by a factor of ca 2 and reductions in the high and low output voltage levels by ca 10% and 50% have been observed for circuits with disadvantageous geometrical design. Ring oscillators designed to be operated under strain show an increase in oscillation frequency from 22.9 kHz (flat substrate) to 23.32 kHz (bending radius: 3.5 mm). This demonstrates the field-effect mobility increase in a-IGZO-based circuits under tensile mechanical strain. Long-term reliability is evaluated with 20 000 cycles of repeated bending and reflattening without circuit failure.