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.