Experimental investigation of an embedded root method for stripping SU-8 photoresist in the UV-LIGA process

Abstract

In much previous research it has been popular to use SU-8 photoresist as a mold for electroplating, facilitating the production of low-cost microelectromechanical systems. However, the thickness of the electroplated structures standing on the substrate could only reach 50 mum or less due to the internal force and deformation of the photoresist in the 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, an experimental investigation of this method is conducted to characterize the relationship between the root depth, the linewidth and the achievable thickness of the electroplated structures. Some test patterns with embedded roots have been designed and fabricated to estimate the possible size of various structures associated with different depths of niches, which are completely defined through the SiO2 masking and KOH etching processes. Based on the established relationship between the root depth and the geometric sizes, a three-dimensional Ni coil, with a thickness of 200 mum, a width of 80 mum and a root depth of 4 mum, is successfully released by the SU-8 mold, which has a height of 400 mum. This cannot be achieved by the standard SU-8 molding process. The process parameters presented herein may be applied to the fabrication of other thick metal microstructures with similar needs.