Gas flow in photoresist spray coating is numerically analyzed to reveal the flow in the vicinity of a substrate with trench structures. With increasing Reynold's number, gas flows enter the trenches to deeper positions, forming vortexes. Compared with experimental results, it is shown that bump formations, which are frequently observed around the convex corner of the trench sidewall, are related to the position of the boundary between the flow ejected from the spray nozzle and the vortexes confined inside the trenches. Moving laterally away from the center axis of the nozzle, the lateral flow along the substrate surface becomes dominant in the vicinity of the substrate and, consequently, photoresist particles tend to deposit on top of the trenches.

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The spray coating of a photoresist using a shield plate with an aperture is carried out for uniform deposition onto three-dimensional (3D) trench structures. The shield plate set over a sample blocks resist deposition onto the sample, except in the aperture area. Numerical analysis reveals that the vertical velocity component of gas flow is enhanced in the aperture area. In experiments on spray coating, the difference between the thicknesses of resist films deposited on top and bottom trench surfaces is decreased. On the trench sidewall, resist bump formation, which is frequently observed in spray coating, is suppressed. The profile of the resist film becomes conformal and uniform. Such resist deposition is necessary to realize 3D microdevices. In microfluidic devices using dielectrophoresis, aside from the top and bottom trench surfaces, the trench sidewall can be used for preparing device structures such as electrode. Electric interaction is enhanced for controlling the transport of micro-/nano-scale objects in micro-trench structures.

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