Reverse‐Offset Printing of Metal‐Nitrate‐Based Metal Oxide Semiconductor Ink for Flexible TFTs

Abstract

Reverse‐offset printing (ROP) is a novel printing technique capable of forming electronics‐industry‐relevant linewidths (≈1 µm) with good thickness control and sharp edge definition. It is demonstrated that through a controlled oxygen‐plasma treatment, the energy of the surfaces related to the process steps of ROP can be optimized to allow the patterning of metal‐oxide semiconductor layers using a simple printing ink based on metal nitrates dissolved in an organic solvent. The steps of the ROP process are analyzed using surface‐energy measurements and Fourier transform infrared spectra of the ink during drying. Thin‐film transistors (TFTs) fabricated using a roll‐to‐plate ROP of In2O3 semiconductor and evaporated Al source/drain (S/D) contacts show, on average, mobilities of 3.1 and 3.5 cm2 V−1 s−1, and ON/OFF‐ratios up to 108 and 107 on a Si/SiO2 substrate and on a flexible polyimide‐type substrate, respectively. TFTs on the flexible substrate with also the S/D‐contacts printed with ROP using Ag nanoparticle ink exhibit a 1.4 cm2 V−1 s−1 mobility. To demonstrate the scalability of the process, continuous lines of In2O3 are printed using a roll‐to‐roll‐compatible (R2R) ROP with linewidths down to ≈2 µm. This process is expected to lead to miniaturized metal‐oxide circuits as required by flexible high‐resolution sensor arrays and displays.

Printed, Highly Stable Metal Oxide Thin‐Film Transistors with Ultra‐Thin High‐κ Oxide Dielectric

“…Under electrical and mechanical stress, the devices showed great stability. Although the devices have large size, the nitrate‐based inks used in the flexography‐ and inkjet‐printing could be tuned to novel high‐resolution printing methods such as reverse‐offset printing that could lead to printed oxide TFT miniaturization 27. The electrodes were not printed in this work, since it is not possible yet to print a smooth conductive layer with less than 40 nm thickness, which is crucial for the bottom gate electrode and low‐temperature solutions for printable ohmic S/D contact materials are not readily available.…”

“…Although the devices have large size, the nitratebased inks used in the flexography-and inkjet-printing could be tuned to novel high-resolution printing methods such as reverse-offset printing that could lead to printed oxide TFT miniaturization. [27] The electrodes were not printed in this work, since it is not possible yet to print a smooth conductive layer with less than 40 nm thickness, which is crucial for the bottom gate electrode and low-temperature solutions for printable ohmic S/D contact materials are not readily available. However, the steps taken in this work will allow, in the future, the production of fully-printed oxide TFTs with low operation voltage and, thus, help to reshape the application window for printed flexible electronics like wearable devices or flexible displays.…”

“…New demands and challenges are emerging with the Internet of Things (IoT), which can also be considered as part of the next-generation of electronics. [1] The quick growth of ubiquitous electronics is expected to help revolutionize modern life through applications such as flexible displays, wearables, www.advelectronicmat.de (e.g., flexographic printing), [20] high printing resolution (e.g., reverse-offset printing), [27] and low substrate surface roughness (e.g., smooth polyimide) [20] would need to be achieved simultaneously. Furthermore, to boost reproducibility and stability in printed TFTs, a number of printability requirements (smooth substrates, surface energy, ink formulation, printing resolution) and annealing conditions (using low-temperature for oxides, e.g., UV-assisted curing) must be well controlled.…”

“…Furthermore, to boost reproducibility and stability in printed TFTs, a number of printability requirements (smooth substrates, surface energy, ink formulation, printing resolution) and annealing conditions (using low-temperature for oxides, e.g., UV-assisted curing) must be well controlled. [8,27,[31][32][33][34] It has been shown that high quality oxide films can be obtained at low process temperature by using UV irradiation. When the films are exposed to high-energy photons, the cleavage of alkoxy groups into active metals and oxygen atoms occurs to simplify the metal-oxygen-metal (M-O-M) network formation through radical-mediated chemical reactions.…”

“…The high cost associated with processing metal oxide materials using capital‐cost‐intensive vacuum deposition methods over large areas on flexible substrates could be reduced by investments in readily‐scalable printing methods (PM) 4,25. Generally, printing is a reproductive process and various printable functional materials have been developed and optimized for different printing approaches, including screen, inkjet, gravure, reverse‐offset, and flexographic printing 26,27. However, for enabling the flexible printed TFTs to meet the demands of electronics manufacturing industry, the challenges that are reported to be solved separately such as low annealing temperature (e.g., UV‐assisted annealing),28–30 high in‐line speed (e.g., flexographic printing),20 high printing resolution (e.g., reverse‐offset printing),27 and low substrate surface roughness (e.g., smooth polyimide)20 would need to be achieved simultaneously.…”