ITO Target Material Spray Process: How to Achieve High-Quality Thin Films?
Indium tin oxide (ITO) is widely used where a transparent conductive layer is required, including display-related components, optical substrates, touch technologies and research samples. In spray-based coating development, the quality of the final ITO film depends on much more than the atomizer alone. Substrate condition, liquid formulation, droplet formation, spray pattern, coating path, wet-film management and post-deposition treatment all interact. An Ultrasonic ITO Coating System can be a useful process tool because ultrasonic atomization generates a controlled spray from a liquid feed without relying on high-pressure pneumatic atomization at the nozzle. The actual film properties, however, must be established through application-specific trials.
A terminology point is important for process engineers. A conventional ITO sputtering target is a solid ceramic target used in a physical-vapor-deposition process; it is not itself atomized into droplets by an ultrasonic spray nozzle. Ultrasonic spray coating normally deposits an ITO-containing liquid, such as a nanoparticle dispersion or a suitable precursor formulation. This article focuses on that liquid-based spray-coating route and on the process controls needed to produce a repeatable ITO-containing film.
Understanding the liquid-based ITO spray-coating route
ITO dispersion and precursor selection
The starting liquid may contain dispersed ITO particles or a chemistry designed to form an ITO-containing layer after deposition and thermal processing. These routes are not interchangeable. A nanoparticle ink brings considerations such as particle-size distribution, agglomeration, solids loading and sedimentation. A precursor solution has its own requirements for solvent compatibility, reaction behaviour and conversion during drying or annealing. The formulation must be selected according to the substrate, target electrical and optical properties, allowable thermal budget and downstream process.
Before atomization, characterize the liquid’s viscosity, surface tension, solids content, density and stability over the intended coating period. Confirm whether the formulation requires controlled mixing, filtration or gentle agitation. Excessive settling can cause the composition delivered at the start of a run to differ from that delivered later. Agglomerates may interrupt a stable spray or create visible defects. Filtration should only be used when the filter specification is compatible with the particle system and does not remove functional material.
Substrate preparation is part of coating quality
Surface contamination, moisture, particles and variations in surface energy can cause poor wetting, pinholes, dewetting or local thickness variation. Establish a repeatable cleaning and handling procedure for the selected glass, polymer, ceramic or other substrate. The cleaning sequence must be compatible with the substrate and the coating chemistry. Record the time between cleaning and coating, and avoid touching active surfaces after preparation.
For development work, use representative substrates with the same surface finish and dimensions as the intended component. A small laboratory coupon may not reproduce the wetting and edge behaviour of a larger panel. Where the production substrate has a patterned surface or a sensitive functional layer, include it in the qualification plan rather than relying only on a smooth reference coupon.
How ultrasonic atomization contributes to the process
In an ultrasonic spray nozzle, electrical energy drives a piezoelectric transducer and a vibrating atomizing surface. Liquid delivered to this surface is broken into fine droplets by the vibration. A carrier or shaping gas may be used to guide the spray toward the substrate, depending on the nozzle and system configuration. Compared with a conventional pressure nozzle, ultrasonic atomization can operate without requiring the liquid itself to be forced through a small high-pressure orifice. This can be beneficial for formulations where gentle, controlled atomization is preferred.
Droplet formation and spray stability
Droplet behaviour is influenced by the liquid properties and the atomizer design and operating conditions. Changes in viscosity, surface tension, solids loading or temperature can affect the spray. A formulation that atomizes well at the beginning of a test may become less stable if solvent evaporates from an open reservoir or if solids settle. Keep the liquid supply, feed path and nozzle operating conditions consistent, and monitor the spray during longer runs.
Spray pattern and deposition footprint
The atomized plume must be matched to the substrate size and the motion system. Spray width, stand-off distance, nozzle orientation and the path of the substrate or nozzle all influence how droplets overlap. A narrow footprint may require more passes; a wider footprint may reduce pass count but can make edge control more difficult. The optimum configuration is the one that provides repeatable coverage over the required active area while controlling overspray and material use.
Building a repeatable ITO coating process
Control liquid delivery
Liquid feed should remain stable throughout the coating cycle. Pulsation, intermittent flow or changes in reservoir level can create local variations in deposited mass. Select a feed arrangement compatible with the formulation and define how the liquid is mixed, replenished and protected from contamination. Record the formulation batch, preparation time and any conditioning steps so that coating results can be traced to the liquid used.
Coordinate nozzle motion and substrate movement
For a uniform coating, the spray path needs controlled overlap. Motion speed, line spacing, pass count, acceleration at the edges and the sequence of coating passes should be treated as process parameters. If the nozzle slows down at the end of a path while liquid delivery remains constant, more material may accumulate near the turning point. Motion profiles should therefore be designed to avoid unintended dwell or excessive deposition at the boundaries.
Manage wet-film formation and drying
After droplets reach the substrate, they spread, merge and evaporate. The final layer is influenced by droplet coalescence, solvent evaporation rate, substrate temperature, ambient conditions and the time between passes. If the surface dries too quickly, droplets may not merge into a continuous film. If the wet layer remains mobile for too long, material may migrate, form ridges or accumulate at edges. Multi-pass coating can help build a target layer progressively, but the interval between passes must be validated for the formulation.
Post-deposition treatment and film evaluation
Some ITO-containing formulations require drying, curing or annealing to remove solvent, form the intended oxide structure, improve particle contact or develop electrical properties. The appropriate thermal profile depends on the chemistry and substrate. Do not assume that a process suitable for glass can be transferred directly to a temperature-sensitive polymer. Confirm the thermal budget, atmosphere and dwell conditions with the material supplier and application requirements.
Film evaluation should combine physical, optical and electrical measurements. Depending on the project, relevant checks may include thickness mapping, sheet resistance, optical transmittance, haze, adhesion, surface morphology and visible defect inspection. A single measurement cannot establish overall quality: a film can have acceptable thickness but poor electrical continuity, or good conductivity with unacceptable optical loss. Define acceptance criteria before optimizing the spray parameters.
Common process defects and how to investigate them
- Pinholes or uncovered areas: Review substrate cleanliness, wetting behaviour, droplet overlap and pass spacing.
- Thickness bands: Check motion speed, liquid-feed stability, spray footprint and overlap between adjacent paths.
- Edge build-up: Review acceleration and deceleration zones, overspray management and the coating boundary strategy.
- Particles or roughness: Investigate dispersion stability, agglomeration, contamination and drying behaviour.
- Weak adhesion: Check surface preparation, formulation compatibility and drying or post-treatment conditions.
- Variable electrical response: Compare thickness and morphology maps with sheet-resistance data, and review the conversion or annealing process.
Frequently asked questions
Does an Ultrasonic ITO Coating System spray a solid ITO sputtering target?
No. Ultrasonic spray coating atomizes a liquid feed, such as an ITO nanoparticle dispersion or a suitable precursor formulation. A solid ITO sputtering target belongs to a different physical-vapor-deposition process.
Can ultrasonic spray coating be used on glass and polymer substrates?
It can be evaluated for different substrate types, but the liquid chemistry, wetting behaviour and drying or annealing temperature must be compatible with the substrate’s surface and thermal limits.
What determines the final ITO film quality?
Film quality depends on formulation stability, substrate preparation, atomization, liquid delivery, spray overlap, motion control, drying and post-deposition treatment. These factors must be optimized together.
How should a coating trial be evaluated?
Define application-specific criteria and assess thickness distribution, sheet resistance, optical properties, adhesion and visible defects across representative sample areas.
Develop the process around the material and substrate
A successful ITO spray process is a coordinated materials and equipment project. The atomizer must suit the liquid, the motion strategy must suit the substrate geometry, and the drying or annealing sequence must suit the formulation and thermal budget.
Explore Cheersonic Ultrasonic Coating Solutions
Learn more about Cheersonic’s ultrasonic coating equipment solutions and ultrasonic atomizing coating technology. Qualified manufacturers, R&D teams and production engineers are invited to contact Cheersonic to discuss their coating project and arrange a sample coating test. Email: market2@cheersonic.com.
