Photoresist Wettability and Viscosity for Spray Coating

Wettability and Viscosity: Tuning Photoresist for Ultrasonic Spray Deposition

A stable ultrasonic spray process depends on more than the atomizer. The photoresist must atomize under suitable conditions, wet the target surface, spread enough to form a continuous film and then dry in a way compatible with lithography. Viscosity and wettability are linked but distinct: viscosity describes resistance to flow, while wettability reflects how the liquid interacts with a particular surface. Changing one does not automatically correct the other.

Why viscosity matters during atomization and film formation

Photoresist viscosity affects liquid delivery, droplet formation and how the deposited material spreads. A change in temperature, solvent loss, storage history or formulation can shift viscosity. If the liquid becomes difficult to deliver consistently, the resulting film may show changes in local build or visible texture. At the same time, a very fluid material can spread or drain differently on a structured wafer, especially around corners and recessed features.

There is no universal ideal viscosity for every ultrasonic spray head and resist. The usable range depends on the equipment design, atomization conditions, delivery method and material supplier’s limits. Keep resist conditioning consistent and document any permitted temperature control or preparation step.

Understand wetting on MEMS materials

Silicon, silicon oxide, metals, polymers and surface-treated regions can present different surface energies. A resist that wets a clean reference wafer may dewet on a patterned wafer containing several materials or residues. Roughness, contamination, adsorbed moisture and previous process steps can also affect spreading. This matters especially at transitions between materials and around small structures.

Contact-angle measurements can help compare surface conditions on suitable witness samples, but a single number cannot describe every feature inside a complex MEMS structure. Combine such measurements with visual inspection and film evaluation on representative geometries. Avoid selecting a surface treatment solely to lower contact angle; the treatment must remain compatible with adhesion, exposure, development and device materials.

Separate material issues from equipment settings

When coverage changes, first verify the resist lot, age, storage, mixing instructions and delivery path. Check for evidence of solvent loss or contamination and consult the resist supplier before altering the formulation. Then confirm that the atomizer and liquid delivery are stable. Only after the material condition is understood should the team tune scan speed, pass overlap, nozzle distance or other machine variables.

Change one factor at a time and record the resulting film appearance and measured thickness. If the same material behaves differently on two substrates, the cause may be surface condition rather than the atomizer. If multiple materials all show the same banding, motion or path overlap may deserve earlier attention.

Use a small, structured design of experiments

Begin with a baseline recipe and a small matrix that covers a limited range of supplier-approved material conditions and machine settings. Do not vary viscosity, flow, atomization and scan speed simultaneously. Select observable outputs in advance: delivery stability, continuity, thickness map, defect count, coverage of representative structures and post-development pattern quality. Use repeated runs to determine whether an apparent improvement is reproducible.

Include the time between preparation and coating as a controlled factor when it may affect solvent loss or material state. Record room conditions when they plausibly affect evaporation. These records help distinguish a true process effect from an uncontrolled change in the material.

Compatibility matters after deposition

A film that wets well is not necessarily a good photoresist film. Solvent behavior, soft-bake response, adhesion and pattern transfer still need validation. Any proposed adjustment should be checked against the resist datasheet and the complete lithography sequence. Never assume that adding solvent or changing a surface treatment is harmless.

Keep formulation changes controlled

Resist formulation is part of the process specification, not simply another machine setting. Record any approved material conditioning and the elapsed time between preparation and coating. If a supplier recommends limits for temperature, storage or working life, include those limits in the trial plan. Do not compensate for a delivery issue with an undocumented solvent addition or other improvised adjustment. A formulation change can affect drying, adhesion, exposure and development, so any permitted change should be evaluated through the complete intended process.

Frequently asked questions

Can viscosity alone predict spray quality?

No. Atomization, surface tension, substrate condition, solvent evaporation and equipment settings also influence the result.

Why does the same resist behave differently across materials?

Surface energy, contamination, roughness and previous processing can change wetting and adhesion.

Should solvent be added to improve spraying?

Only if the resist supplier explicitly permits it and the change is validated. Unapproved dilution can alter film and lithography behavior.

Discuss material compatibility with Cheersonic

MEMS process engineers can bring their resist type, substrate stack and target geometry to a Cheersonic sample-coating discussion. The goal is to evaluate a documented process window, not to promise a universal setting.

Learn more about Cheersonic’s ultrasonic coating equipment solutions and ultrasonic atomizing coating.