Characterization of embedded root method in UV-LIGA process

SU-8 photoresist has been popularly used as a mold for electroplating and facilitated low-cost MEMS production in many previous researches. However, the reachable thickness of electroplated structures standing on the substrate were limited to 50 μm below due to the internal stress and deformation of the SU-8 resist in final stripping process. In order to fabricate thicker structures, an embedded root method has been proposed to consolidate the adhesion of the metal structures to the substrate during the SU-8 removal process. In this paper, detailed investigation on this method is conducted to characterize the relationship among the root depth, the line-width and the allowable thickness of the electroplated structures. Some test patterns with embedded roots are designed and tested to generalize the possible extent of various structural scales associated with different niches in depth, which are completely defined through SiO₂ masking and KOH etching processes. Based on establishment of the relationship between the root depth and the geometric sizes, a 3-D integrated coil with 200 μm in thickness, 80 μ in width and 4 μm in root depth is successfully released by the SU-8 mold with 400 μm in height, which can not be achieved by the standard SU-8 molding process. The UV-LIGA process presented herein may be applied to the fabrication of other microstructures and microactuators.