ITO Coating Thickness and Uniformity: How to Solve Process-Control Challenges

For transparent conductive coatings, average film thickness is only one part of the specification. A usable ITO layer must also meet the application’s requirements for spatial uniformity, electrical continuity, optical transmission, adhesion and surface quality. A coating can meet its average thickness target while still containing thin regions, edge build-up or local defects that affect device performance. This is why process engineers developing an Ultrasonic ITO Coating System need a measurement-led control strategy rather than relying on visual appearance or a single nominal machine setting.

Ultrasonic spray coating is a liquid deposition process. The atomizer converts an ITO-containing dispersion or precursor solution into droplets, which are directed onto a substrate. The deposited material then forms a wet layer and undergoes drying or further treatment. Thickness and uniformity are influenced by every stage: formulation preparation, liquid feed, droplet distribution, nozzle-to-substrate geometry, motion, pass overlap, substrate temperature and post-deposition processing. This article explains how to identify and control those variables without assuming one universal recipe.

Define what uniformity means for the application

Average thickness is not enough

Thickness specifications should define the measurement method, sampling locations and acceptable variation across the active area. For a small coupon, a few central measurements may appear consistent while missing edge effects. For a larger panel, a structured map is more informative. The sampling plan should include the centre, intermediate positions, corners and regions near the coating boundary. If the substrate has a functional pattern, measure both representative active regions and any areas known to be sensitive.

Connect thickness to electrical and optical performance

ITO is used because it combines electrical conductivity with optical transparency, but the balance depends on material composition, microstructure, thickness and post-treatment. Thickness alone does not predict sheet resistance or transmittance. Particle packing, interparticle contact, precursor conversion and defects can change the relationship. Establish correlated measurements for the actual material system rather than using a generic thickness-to-resistance assumption.

Separate coating variation from measurement variation

Before changing the spray process, confirm that the measurement method is repeatable. Probe pressure, contact condition, instrument calibration, sample handling and measurement location can all influence results. Use a documented measurement procedure and, where practical, repeat selected measurements to estimate measurement variation. Otherwise, the process may be adjusted in response to noise rather than a real coating defect.

Liquid formulation: the first source of thickness variation

Maintain dispersion stability

ITO nanoparticle dispersions can be sensitive to settling, agglomeration and changes in solvent composition. If the solids concentration delivered to the nozzle changes during a run, the deposited mass per unit area may also change. Define how the liquid is mixed before coating, whether gentle agitation is required during the run and how long a prepared batch can remain in use. Record the batch identity and preparation conditions.

Control viscosity and surface tension

Viscosity and surface tension influence atomization, droplet behaviour and wetting. A formulation change made to improve one property may affect another. For example, solvent composition can change evaporation behaviour and surface spreading at the same time. Establish a controlled formulation window with the materials team and avoid making multiple untracked adjustments during a coating trial.

Prevent changes in liquid concentration

Solvent evaporation from an open container or exposed feed line can gradually increase solids concentration. Temperature changes can also alter liquid properties. Use a consistent reservoir arrangement and limit unnecessary exposure to the environment. Where the formulation requires it, use a suitable cover or controlled supply arrangement. Confirm that filters, tubing and fittings are compatible with the dispersion and do not introduce contamination or remove functional particles.

Atomization and spray-footprint control

Ultrasonic atomization creates droplets through vibration at the atomizing surface. The resulting spray depends on the nozzle design, liquid properties and operating conditions. A stable spray is a prerequisite for repeatable deposition, but it does not automatically guarantee a uniform film. The spray footprint must be characterized at the intended stand-off distance and orientation, using the actual formulation where possible.

Check the spray footprint before coating a full substrate

Use witness substrates or a suitable collection method to examine the deposition footprint. Determine whether the centre receives more material than the edges and whether the pattern changes with stand-off distance or liquid feed. The footprint should be evaluated under the same conditions intended for the coating run. A pattern observed with a different test liquid may not represent the behaviour of the ITO formulation.

Manage nozzle-to-substrate geometry

Stand-off distance, nozzle angle and alignment affect where droplets land and how the plume spreads. If the substrate is not level or the nozzle is not perpendicular to the intended path, the deposited profile can become asymmetric. For large or irregular substrates, check the geometry at multiple positions across the motion area. Mechanical alignment should be verified after maintenance or fixture changes.

Motion planning, overlap and multi-pass deposition

Set consistent line spacing

When a spray path is rastered across a surface, adjacent passes need an appropriate overlap. Too little overlap can leave visible bands or thin regions; excessive overlap can create thicker tracks. Determine line spacing experimentally from the measured spray footprint and the required coating uniformity. Do not assume that the nominal spray width equals the effective uniform deposition width.

Control travel speed and acceleration

For a given liquid delivery condition, slower movement generally deposits more material per unit path length, while faster movement deposits less. However, the relationship is affected by spray distribution and wet-film behaviour. Acceleration and deceleration zones deserve particular attention: if the nozzle deposits material while slowing or turning, the ends of a path may receive more coating. Use motion profiles and path planning that minimize unintended dwell over the active area.

Build thickness in controlled passes

Multiple thinner passes may provide better process control than attempting to deposit the entire target amount in one wet layer, depending on formulation and substrate. Between passes, the coating may need time to level or partially dry. If the next pass arrives too early, it can disturb the previous layer; if the interval is too long, the surface condition may change and affect interlayer adhesion. Establish the pass sequence and interval through repeatable trials.

Substrate temperature and drying conditions

Substrate temperature influences solvent evaporation, droplet spreading and the time available for film formation. A warmer surface may accelerate drying, but overly rapid evaporation can prevent droplets from merging smoothly or can create surface texture. A cooler surface may allow more time for leveling, but can also increase the risk of liquid migration or pooling. The appropriate condition depends on solvent system, substrate and target layer.

Ambient temperature, humidity and airflow can also influence drying. For process development, record these conditions alongside the machine settings. If a coating result changes between shifts or seasons, environmental variation may be a contributing factor. Where necessary, use a controlled coating environment or define an acceptable operating window.

Use a measurement map to troubleshoot defects

A thickness map turns a visual impression into a diagnostic tool. Record thickness at fixed locations and compare the pattern with the spray path. A repeated band aligned with the scan direction may indicate line-spacing or overlap issues. A thicker perimeter may be associated with path-end behaviour or edge effects. A gradient across the whole substrate may indicate alignment, leveling or liquid-delivery variation. Correlate the map with electrical and optical measurements to determine whether the variation is functionally significant.

Observed pattern	Variables to investigate	Useful verification
Parallel bands	Line spacing, spray footprint, path overlap	Thickness map across adjacent passes
Thicker path ends	Acceleration, deceleration, dwell and turning sequence	Compare edge and centre measurements
One-sided gradient	Nozzle alignment, substrate level, stand-off distance	Geometry check across the motion area
Run-to-run drift	Dispersion stability, liquid concentration, environment	Batch records and repeated reference coupons
Local spots or pinholes	Contamination, agglomerates, wetting and drying	Surface inspection and formulation checks

Build process control around documented recipes

Once a stable process window is established, record the material batch, preparation method, liquid properties, feed conditions, nozzle configuration, spray geometry, motion path, pass count, substrate condition and drying or annealing sequence. A recipe should identify which parameters are fixed and which may be adjusted for a different substrate or target thickness. Change one variable at a time during early troubleshooting where practical, then confirm the result with repeated runs.

For production transfer, define incoming material checks, first-piece approval, in-process monitoring and final inspection. Include a response plan for out-of-window results. A process that produces one good sample is not yet a controlled process; repeatability across batches, operators and operating days is the real objective.

Frequently asked questions

What causes uneven ITO coating thickness?

Common contributors include unstable dispersion, changing liquid concentration, nonuniform spray footprint, poor pass overlap, inconsistent motion, substrate misalignment and variable drying conditions.

Can ultrasonic atomization guarantee uniform film thickness?

No single atomization method guarantees the final film. Uniformity depends on formulation, spray geometry, motion planning, substrate handling and post-deposition treatment working together.

How many thickness measurement points are needed?

The sampling plan should reflect substrate size, active-area geometry and product requirements. Include centre, intermediate and edge locations, and use a documented map that can be repeated between trials.

Should thickness be measured after drying or annealing?

Measure at the process stage that corresponds to the product specification. For process development, it can be useful to compare intermediate and final states to understand shrinkage or structural changes.

Turn thickness mapping into a process-control tool

Uniform ITO films are achieved by controlling the entire deposition sequence and verifying the result with meaningful measurements.

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.