Direct Spray Photoresist on MEMS Wafers

Direct Spray Photoresist on MEMS Wafers: Managing 3D Coverage Challenges

MEMS wafers rarely behave like simple, flat substrates. Trenches, steps, cavities, suspended features and tall sidewalls can make photoresist application more difficult than coating an uninterrupted surface. Direct spray deposition is worth evaluating when conventional spin coating leaves shadowed areas or struggles with non-planar geometry. The process still needs to be designed around the wafer, resist chemistry and downstream lithography rather than assumed to provide conformal coverage automatically.

Why three-dimensional MEMS features complicate coating

On a flat wafer, coating uniformity is often discussed as a center-to-edge thickness map. A structured MEMS surface adds several dimensions: top surfaces, sidewalls, recessed floors, corners and regions hidden from the spray trajectory. A film that looks continuous from above may be thin at a sidewall or absent in a narrow recess. Feature aspect ratio, pitch, orientation and surface roughness all influence where droplets land and how the liquid redistributes after impact.

Geometry should therefore be documented before choosing process settings. Representative coupons or test wafers should include the deepest recesses and tallest structures that matter to the product. Inspection should distinguish visible coverage from functional resist coverage: a thin trace on a sidewall may not survive exposure, development or etching.

How ultrasonic spray deposition can help

Ultrasonic atomization uses vibration at a nozzle to break liquid into droplets. In a suitable process window, this can produce a controlled spray without relying on the high liquid pressure used by some conventional atomizers. A motion stage or scanning system then distributes the spray across the target. For photoresist, the result depends on droplet characteristics, flow, nozzle-to-wafer distance, traverse speed, overlap, resist formulation and the way solvent evaporates between passes.

The method can be attractive for non-planar surfaces because the coating path can be adapted and deposited material can be directed toward selected regions. However, line-of-sight limitations remain. A spray does not inherently reach every hidden surface, and small droplets do not guarantee equal thickness on horizontal and vertical faces.

Build a coverage-focused process window

Start with representative structures

Choose coupons that reproduce actual feature dimensions, materials and surface preparation. Record the wafer orientation and identify the regions most likely to be shadowed. Include a flat reference area so the team can compare nominal film thickness with local coverage.

Separate the key variables

Change one group of parameters at a time: liquid delivery, atomization setting, nozzle distance, scan speed or pass overlap. Keep resist conditioning and substrate cleaning consistent. If several settings change together, a better-looking sample may not reveal which change helped. Inspect both as-coated and developed patterns where appropriate.

Verify after the complete process

Assess coverage after soft bake and after exposure and development, not only immediately after spraying. Solvent loss, film flow, bake conditions and developer access can change the final result. Cross-sections or suitable microscopy may be needed to examine sidewalls and recessed regions that a top-down image cannot resolve.

Common failure modes and how to investigate them

Thin sidewall films may point to spray shadowing, insufficient local deposition or poor wetting. Pooling at trench corners may indicate excess liquid, slow solvent loss or a geometry-driven capillary effect. Discontinuous coverage can also arise from particles, contamination or incompatibility between the resist and surface treatment. Use defect maps and repeatable inspection locations to distinguish a systematic geometry effect from random contamination.

There is no single universal recipe for every MEMS wafer. The appropriate endpoint is a validated combination of coverage, pattern fidelity, process stability and compatibility with the device materials. If the target structure is especially deep or enclosed, confirm whether direct spray is physically suitable before investing in extensive parameter tuning.

Trial notes for complex structures

For high-relief features, make sure the sample represents both the nominal design and the worst-case geometry. A test coupon with shallower features may be useful for screening settings, but it cannot establish performance in a deeper cavity. Capture the orientation of the wafer relative to the spray path and keep it unchanged during comparisons. If a feature is inaccessible to direct spray, document that limitation rather than interpreting an absence of visible coating as a minor uniformity issue. This distinction helps the engineering team decide whether a path change is sufficient or whether the application requires a different coating strategy.

Frequently asked questions

Does ultrasonic spray guarantee conformal coating?

No. It can support controlled deposition, but actual sidewall and recess coverage must be demonstrated on representative structures.

What should be inspected first?

Prioritize the deepest recesses, tall sidewalls, corners and any regions that are difficult to reach with a direct spray path.

Can one recipe coat every MEMS design?

Usually not. Geometry, resist formulation, substrate material and downstream process requirements differ between designs.

Discuss a MEMS coating trial with Cheersonic

MEMS manufacturers and research teams can define a practical test plan around representative wafers, target features and post-lithography inspection. Contact Cheersonic to discuss sample coating evaluation rather than relying on an assumed result.

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