Investigation on overplating high-aspect-ratio microstructure

摘要

Most previous research on electroplating in LIGA has focused on electrodeposition of metal into high aspect ratio resist molds. In overplating process how the metal grows up across the top of resist molds has been relatively neglected. Typical defects like holes formation at the top of cavities of electroplated metal mold usually occur due to improper process control especially when the space/linewidth ratio of microstructure increases. To help understand these problems, overplating process has been investigated. A model is developed to compute current density distribution based on LIGA mold feature using electroplating simulation tools. Results show that it is almost an isotropic growing model at the first stage of overplating. As the deposited metal grows bigger the space between electrodes is shortened and the current density distribution along electrode may be modulated by neighbor electrode. It doesn't show an isotropic growing model any more. The deposition rate in inward lateral direction is smaller than in vertical direction. The growing model based on calculation shows that the trench feature aspect ratio can reach a considerable magnitude especially when the LIGA mold feature space/linewidth ratio increases. In poor transport situation, ion depletion becomes significant and a stopped deposition may occur thus holes can be formed at the bottom of overlapped neighbor electrodes. An optimized experiment has been performed using low overpotentials at the stage before the overlapping of neighbor electrodes and a rigorous stirring of electrolyte. A nickel mold insert without holes-formation defects can be obtained.