We investigated the effect of plating parameters on the coating morphology and grain size of copper using a novel citrate bath. The structure and deposit were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The copper plating bath was applied to a micro-electro-mechanical system (MEMS). Results show that an even and compact copper coating was obtained for the fine grains using plating parameters of 6 g . L-1 Cu2+, pH=7.0-8.5, 1-2 A . dm(-2), 45 C degrees and bath agitation. The deposit consists of pure copper in a fcc polycrystalline structure. A planar inductor was successfully fabricated by MEMS using this bath and its maximum quality factor was 12.75, which satisfies the design requirements. ; National Natural Science Foundation of China [20873114, 20833005]; National Key Basic Research Program of China [2009CB930703]