Highly Robust Flexible Vertical-Channel Thin-Film Transistors Using Atomic-Layer-Deposited Oxide Channels and Zeocoat Spacers on Ultrathin Polyimide Substrates

Mechanically flexible vertical-channel-structured thin-film transistors (VTFTs) with a channel length of 200 nm were fabricated on 1.2 μm thick colorless polyimide (CPI) substrates. All layers composing the gate stacks were prepared by atomic-layer deposition (ALD) with a good step coverage, and the process thermal budget was designed below 180 °C. Zeocoat was introduced as a spacer material to improve the device characteristics by properly determining the process conditions for clearly forming the vertical sidewalls. The transfer characteristics of the fabricated flexible ZnO and IGZO VTFTs showed the field-effect mobility of 3.31 and 6.57 cm2 V–1 s–1, the threshold voltages of 2.34 and 1.52 V, the subthreshold swings of 1.42 and 0.34 V/dec, and ION/IOFF of 1.13 × 104 and 5.16 × 105, respectively, after the postannealing at 150 °C. The threshold voltage shifts (ΔVTH) under positive and negative bias stresses (PBS and NBS) were estimated to be +1.0, +1.8 V and −1.8, −3.8 V for 104 s for ZnO and IGZO VTFTs, respectively. The flexible IGZO VTFTs delaminated by means of a laser lift-off process did not show any marked degradation in device characteristics under mechanically strained conditions at various radii of curvature (Rc). The ΔVTH values were also estimated to be +1.2 and −2.1 V under PBS and NBS at Rc of 1 mm, respectively, demonstrating stable bending reliability. Appropriate choices of spacer materials, optimization of patterning process for spacer patterns, and ALD process of IGZO active channel can be proposed as key process parameters for guaranteeing excellent device performance of the oxide-channel VTFTs in this work.