ALD-Grown ZnO TFTs Patterned by High-Resolution Reverse-Offset Printing

Abstract

Zinc oxide (ZnO) is a benign and earth-abundant semiconductor material that has been applied in thin-film transistors (TFTs) for decades and can be used in biodegradable, transient, and biocompatible devices. Printing as an alternative fabrication method to conventional TFT manufacturing methods can deliver some benefits, such as simultaneous film deposition and patterning, good scalability, low cost, and material-saving features. However, the high annealing temperature needed for ink-to-metal oxide conversion and film densification, compounded by the poor patterning resolution of conventional printing methods, still limits the use of printing in the fabrication of flexible metal oxide TFTs. Atomic layer deposition (ALD) has recently emerged as a promising fabrication method for high-performance metal oxide TFTs that can offer more conformal film growth, precise film thickness, and higher film quality at low temperatures compared to sputtering, spin coating, or printing. Although ALD-based ZnO TFTs patterned with photolithography exhibit good electrical properties, they cannot be readily scaled to a high-throughput fabrication. Very little attention has been paid so far to the combination of low-temperature ALD growth with printing to obtain more scalable manufacturing of high-performance thin-film electronics. To overcome this challenge, we propose high-resolution reverse-offset printing (ROP) of a simple polymer resist to pattern an ALD-grown ZnO film at few μm resolution to fabricate TFTs. In this work, we report high-performance ZnO TFTs that are ALD-grown at a low temperature of 150 °C and ROP-patterned with promising stability and uniformity, a high field-effect mobility (μ_FE) of ∼16.6 cm² (Vs)⁻¹, an almost zero turn-on voltage (V_on) of ∼−0.49 V, a high current on–off ratio (I_on/I_off) of >10⁸, a low operation voltage (V_op) of ≤5 V, and a negligible hysteresis (V_hyst) of ∼0.13 V. The combination of ALD and the ROP-patterning process could be developed further to fabricate fully flexible high-resolution metal oxide TFT-based circuits in the future.