Oxide TFTs with S/D-contacts patterned by high-resolution reverse-offset printed resist layers

Besides the metal oxide thin film transistors (TFTs) in flat-panel displays that are fabricated using vacuum-processes, there is a growing interest in the fabrication of metal oxide TFTs by means of scalable, low-cost solution and printing processes for applications such as flexible displays and biosensors. Although devices with printed semiconductor and gate insulator can exhibit good electrical performance, source/drain-contacts (S/D) printed from silver (Ag) nanoparticles (NPs) typically suffer from deteriorated electrical characteristics and stability problems. On the other hand, metals providing good contacts, such as aluminum (Al), titanium (Ti) and molybdenum (Mo), cannot be formed as air-stable NPs. To overcome these issues, we have developed a patterning method based on high-resolution reverse-offset printing (ROP) of a sacrificial polymer resist layer. ROP delivers patterns with micrometer-level resolution and steep sidewalls, which are ideal for patterning vacuum-deposited metal contacts at high resolution via lift-off process. Solution-processed indium oxide (In₂O₃) TFTs were successfully fabricated by using ROP lift-off process for patterning of gate and S/D-electrodes using Al. The fabricated In₂O₃-based TFTs with Al S/D-contacts exhibit good uniformity, constant mobility (μ _sat) ∼ 2 cm² (V s)⁻¹ over a wide range of width/length-ratios (W/L) and almost zero turn-on voltage (V _on) ∼ −0.2 V. TFTs down to 5 µm channel lengths were successfully patterned. Further development of the fabrication process could lead to flexible fully-print-patterned high-resolution TFT backplanes for flexible displays, biosensors, photosensors and x-ray detectors.