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deposit materials one atomic layer at a time. ALD is particularly advantageous for creating conformal
coatings with excellent uniformity and control over film thickness. It has been used to deposit materials
such as Al O and HfO 2 [92,111,113,123,135] . Recently, ALD and PEALD have also been used for the deposition of
3
2
semiconductors such as IGZO, ZnO and InO x [134-136] . These materials are gaining attention for their excellent
electrical properties and suitability for use in advanced electronic devices, including TFTs and
optoelectronic components. PEALD is a variant that uses plasma to enhance the reactivity of the precursors,
allowing for lower-temperature processing. ALD and PEALD are well-suited for depositing channel layers,
gate dielectrics, and passivation due to their ability to produce films with high resistance, low pinhole
density, and good sidewall coverage .
[183]

Spin-coating is a widely used technique for fabricating oxide thin films on flexible substrates due to its
simplicity, uniformity, reproducibility, and compatibility with various substrates [8,15] . The process involves
depositing a liquid precursor on a substrate, which is then rapidly spun to evenly distribute the solution,
balancing centrifugal and viscous forces. This results in a thin, uniform film treated by drying and annealing
after spin coating. The film thickness is influenced by factors such as rotational speed, duration, and
solution viscosity. Spin-coating can deposit films with a minimum thickness of around 5 nm and is suitable
for solutions with viscosity between 1 and 5,200 cP [8,184,185] . Despite its advantages, spin-coating has
limitations, including material wastage, difficulty with large or curved substrates, and low throughput. In
contrast, methods such as slit-die coating can improve uniformity. Spray-coating is a cost-effective,
noncontact method for depositing thin films over large areas using small amounts of MO solution
deposition. This technique allows for material doping and multicomponent film through multiple coating
processes. It uses an atomizer or nebulizer to create fine droplets from low-viscosity solutions, which are
directed onto a substrate by carrier gases and gravity, with an electrostatic field aiding in the formation of
thin-film coatings. The thickness, roughness, and quality of the films are influenced by droplet size, coating
cycles, substrate temperature, and solution viscosity during thin-film deposition. While capable of achieving
film thicknesses of 10-600 nm and handling viscosities of 0.1-1 cP [8,186] , spray-coating faces challenges such
as high film roughness, limited low-viscosity solutions, and lengthy processing times. Despite these issues, it
remains valuable for large-area thin-film deposition due to its simplicity and adaptability. Inkjet printing, a
noncontact thin-film deposition technique, is valued for its precision and efficiency. It uses micrometer-
scale nozzles in printer heads, with thermal inkjet expelling ink via solvent evaporation and piezoelectric
inkjet using voltage-induced nozzle shape changes. Factors including nozzle diameter, temperature, and
type affect the morphology of printed patterns. Ink viscosities range from 1 to 50 cP, and printed lines are
20 to 50 μm wide . While inkjet printing offers advantages involving photolithography-free deposition,
[8]
selective small-area coating, and efficient material use, it faces challenges such as lower throughput
compared to roller-based methods, limited ink choices, and potential nozzle clogging. Moreover, 3D
printing is emerging as a promising fabrication method for flexible MO TFTs due to its ability to create
complex, custom geometries with precise control. This technique enables the layer-by-layer deposition of
functional materials allowing for the formation of TFTs on flexible substrates. For MO TFTs, 3D printing
offers several benefits, including the precise deposition of MOs and integration with flexible substrates
without the need for high-temperature processing, which is often incompatible with flexible materials.
Additionally, 3D printing reduces waste and provides greater design flexibility compared to conventional
methods. Recent studies have demonstrated successful printing of oxide semiconductors using inkjet or
aerosol jet printing techniques, showcasing the potential of 3D printing in this area. However, challenges
remain, particularly in ensuring the uniformity and electrical performance of printed MOs; nevertheless,
continued advancements in 3D printing materials and techniques are making flexible MO TFT fabrication
increasingly feasible [187-190] .