Mechanical Durability of Flexible/Stretchable a-IGZO TFTs on PI Island for Wearable Electronic Application

In this work, we examined the mechanical durability of island-type a-IGZO thin-film transistors (TFTs). Island TFTs were fabricated on polyimide (PI) islands and were transferred to a thermoplastic polyurethane (TPU) film. In repeated bending tests with a 1.5 mm bending radius, island TFTs showed less electrical property deterioration than TFTs on a PI film. We confirmed that the TPU, which has a lower elastic modulus compared to PI, effectively reduced the curvature of PI island even under the same bending test conditions. Furthermore, an organic passivation layer was applied on the upper part of the PI island. The 3 μm thick organic passivation layer made the TFT layer more stable against bending and elongation stress. Island TFTs with an organic passivation layer showed a change in the saturation mobility of only −2.3% and a change in the threshold voltage of −0.22 V even after 250 000 repetitive bending tests. Additionally, no change in electrical properties was observed even after 10 000 repeated stretching test cycles under 30% uniaxial elongation. Finally, we fabricated island-type logic circuits based on a-IGZO TFTs for wearable electronic applications. Using the organic passivation layer, we showed that the NMOS pseudoinverter and NAND gate also operated without significant deterioration in 100 000 repeated bending cycles and 5000 repeated stretching cycles. After applying repeated mechanical stresses, the high output voltage (VOH) and low output voltage (VOL) of the inverter only changed from 8.85 to 8.93 V and from 0.44 to 0.50 V, respectively. In NAND gates, VOH and VOL only changed slightly from 8.46 to 8.56 V and from 0.45 to 0.55 V, respectively.