Introduction: MEMS and the coating challenge

MEMS fabrication combines mechanical structures with electrical, optical or fluidic functions. Process flows can include sacrificial layers, structural films, deep etch masks, electroplating moulds and high-aspect-ratio resist features. Substrates may be silicon, glass, metals or multilayer stacks, and surface topography can vary significantly across a wafer. Ultrasonic spray deposition is worth evaluating where non-contact coating or coverage over non-planar features may offer process flexibility, but it cannot be assumed to reproduce the thickness control or defect performance of a qualified spin process.

For MEMS, the critical question is often how the resist behaves over steps, trenches, cavities and wafer edges. A spray plume may reach regions that are difficult to coat with a planar liquid puddle, yet geometric shadowing, drainage and local accumulation remain possible. Design test coupons with representative step heights and trench widths. Include structures near the centre and edge of the wafer, and examine both the top surface and sidewall coverage where the application requires it. For thick resist moulds, assess stress, cracking, adhesion and development through the full film thickness. If the resist is sacrificial, verify complete removal without residue or damage to released structures.

Process definition and application boundary

The first task is to translate the application into a measurable coating specification. Define the substrate material, dimensions, thickness, flatness, surface finish, active area, edge exclusion and handling method. Identify whether the resist is used for imaging, etch masking, lift-off, mould definition, temporary protection or another function. Those uses impose different requirements for film thickness, adhesion, chemical resistance and pattern fidelity. A coating method should not be selected until the process owner has defined the downstream steps and the failure modes that matter most.

Resist chemistry and fluid preparation

Photoresist is a formulated material whose response depends on resin chemistry, photoactive components, solvent blend, solids content, viscosity, surface tension and age. Positive-tone, negative-tone, chemically amplified, thick-film and specialty resists are not interchangeable. Use the current supplier technical data sheet and safety data sheet for the exact grade and lot. Record storage, conditioning, filtration and dispense practices. Temperature control matters because viscosity and solvent evaporation can change with temperature. Any dilution, filtration, degassing or solvent substitution must be permitted by the resist supplier and validated rather than assumed harmless.

Surface preparation and wetting

Substrate cleanliness and surface energy are often decisive. Develop a cleaning sequence for the actual substrate and prior process history; a recipe suitable for polished glass may be unsuitable for a metal stack, polymer, coated wafer or delicate optical surface. Compatible detergent cleaning, high-purity rinsing, controlled drying, dehydration and qualified plasma or UV-ozone activation may be considered where allowed. Record the interval between cleaning and coating. Contact-angle measurements at several positions can help identify surface variation, but contact angle alone does not prove film adhesion or lithographic quality. Excessive spreading can be as problematic as poor wetting.

Ultrasonic atomization and spray-path design

An ultrasonic atomizing nozzle uses high-frequency mechanical vibration at a wetted surface to generate droplets from a supplied liquid. The droplets are transported toward the substrate, where a controlled motion system distributes the material. Candidate variables include liquid flow, drive condition, carrier-gas flow where applicable, nozzle-to-substrate distance, traverse speed, raster pitch, overlap, substrate temperature and number of passes. These variables interact. A higher flow may increase deposited mass but create pooling; a longer stand-off may broaden the footprint but increase sensitivity to airflow; excessive overlap can form bands. Map the spray footprint on sacrificial coupons before selecting the raster strategy.

Film formation, solvent loss and bake

A useful film requires droplets to arrive, spread, merge and lose solvent in a controlled sequence. If droplets dry before coalescing, roughness, islands or pinholes may result. If too much liquid accumulates, ridges, runs, edge build-up or drying marks may appear. Evaporation depends on solvent volatility, ambient airflow, substrate temperature, droplet distribution and the time between passes. A smooth wet appearance does not guarantee a uniform dry film. Begin with the resist supplier’s bake recommendations and measure hotplate uniformity, substrate contact and transfer delays. Do not change bake conditions without checking residual solvent, stress, adhesion and photosensitivity.

Metrology and defect classification

No single measurement establishes coating suitability. Select thickness metrology appropriate to the film and substrate, such as profilometry, ellipsometry, reflectometry or a validated optical method. Map centre, intermediate positions, edges and corners. Optical microscopy can reveal particles, streaks, pinholes, dewetting and edge defects; surface profilometry or interferometry may be useful for roughness and topography. Define the calculation for within-substrate non-uniformity before comparing methods. Keep raw maps, not only averages. Link each defect to its position relative to the spray path, substrate geometry and handling sequence.

Lithography and downstream acceptance

The coating must be judged by the patterned result and its downstream function. Evaluate adhesion, exposure latitude, critical-dimension variation, sidewall profile, residual scum, defectivity and compatibility with development, etch, lift-off or bonding. Select test structures that represent real feature sizes, pattern density and topography. Keep exposure, focus, bake and development controlled while the deposition process is being screened; changing every variable at once makes root-cause analysis difficult. A film that appears uniform can still fail in exposure or development, so functional test patterns are essential.

Experimental design and repeatability

A structured design of experiments is more informative than adjusting many settings by intuition. Begin with a small screening matrix for the most influential factors, then refine the promising region. Use replicate substrates, a qualified baseline process and agreed acceptance criteria. Record material lot, fluid temperature, substrate preparation, nozzle settings, motion path, environment, bake, metrology and operator. Repeat promising recipes across days and material lots. A single good coupon demonstrates only that a condition may be feasible; it does not establish process capability, production yield or long-term stability.

Chemical safety and equipment compatibility

Many photoresist solvent systems are volatile, flammable or harmful. Use the controls specified by the chemical supplier and facility EHS team, including appropriate ventilation, approved enclosures, waste collection, spill response and personal protective equipment. Confirm compatibility of every wetted component—reservoir, tubing, seals, fittings and cleaning agents—with the complete formulation, concentration, temperature and exposure duration. Do not infer compatibility from a polymer family name or from water-based testing. Prevent uncontrolled aerosol escape and assess exhaust loading before operating a spray process.

Scale-up, maintenance and manufacturing readiness

Moving from coupons to production substrates changes the problem. Larger areas introduce stage straightness, acceleration, support, flatness, airflow and thermal-gradient effects. Establish loading and unloading controls, nozzle inspection and cleaning intervals, fluid changeover procedures, recipe traceability and preventive maintenance. Measure cycle time, consumables, material use, cleaning downtime and rework alongside film metrics. A technically successful laboratory trial is not evidence of manufacturing readiness until the process is repeatable across operators, days, lots and realistic equipment start-up conditions.

Application-specific engineering considerations

For MEMS, the critical question is often how the resist behaves over steps, trenches, cavities and wafer edges. A spray plume may reach regions that are difficult to coat with a planar liquid puddle, yet geometric shadowing, drainage and local accumulation remain possible. Design test coupons with representative step heights and trench widths. Include structures near the centre and edge of the wafer, and examine both the top surface and sidewall coverage where the application requires it. For thick resist moulds, assess stress, cracking, adhesion and development through the full film thickness. If the resist is sacrificial, verify complete removal without residue or damage to released structures.

Map thickness over planar and stepped regions; quantify edge bead or edge build-up; inspect voids, pinholes, cracks and delamination; measure patterned dimensions and sidewall shape; and test release or sacrificial-layer removal where relevant.

For this application, the coating specification should be written before machine optimization begins. It should state the acceptable film thickness range, spatial uniformity definition, active area, edge exclusion, defect classes, adhesion requirement and downstream lithography response. If the final product is optical, electrical, mechanical or fluidic, include a functional measurement that reflects its intended use. This avoids optimizing a convenient surrogate such as wet appearance while overlooking the performance that actually determines acceptance.

Recommended qualification sequence

Qualify the deposition route on coupons that reproduce the real MEMS topography, then pattern and process the coupons through etch, plating, release or bonding steps. Acceptance must be based on the final structure and device function, not merely on the as-coated film.

In the first phase, verify material compatibility and stable liquid delivery using non-production coupons. In the second phase, map the spray footprint and establish a repeatable motion strategy. In the third phase, optimize the dry film using thickness maps and defect inspection. In the fourth phase, expose and develop representative patterns, then complete the relevant etch, plating, bonding, stripping or optical test. Keep a baseline sample at each stage so that changes can be attributed to a defined process variable.

Interpreting results without overclaiming

Report the number of substrates tested, the number of measurement locations, the resist grade and lot, and the conditions under which the data were collected. Separate measured values from proposed targets. A result obtained on one substrate size, one chemistry or one surface finish should not be generalized to every application. Where the process has not yet been tested, describe the expected mechanism as a hypothesis and identify the experiment needed to confirm it. This discipline makes the technical evaluation more useful to process engineers and researchers.

When comparing coating methods, use equivalent acceptance criteria and include the full process cost. A low material feed rate does not automatically mean lower material consumption if overspray, purge losses or rework are high. A visually smooth film does not establish lithographic yield. A short coating cycle may be offset by longer cleaning or inspection. Record both coating metrics and the downstream results that determine whether the substrate can be accepted.

Conclusion and sample coating evaluation

Qualify the deposition route on coupons that reproduce the real MEMS topography, then pattern and process the coupons through etch, plating, release or bonding steps. Acceptance must be based on the final structure and device function, not merely on the as-coated film.

Cheersonic welcomes qualified manufacturers, process engineers and research teams evaluating ultrasonic atomizing coating for precision photoresist applications. To discuss a meaningful sample test, share the substrate material and dimensions, resist family and solvent system, target dry-film thickness, coating area, surface topography, downstream process and key acceptance criteria. A representative sample-coating evaluation can help determine whether the process is suitable for the specific application; performance should be established by measurement rather than assumed in advance.

Explore ultrasonic coating solutions

Learn about ultrasonic coating equipment solutions and ultrasonic atomizing coating technology.