On the design and fabrication of metal molds in LIGA

Most previous research on electroplating in LIGA has been focused on electrodeposition of metal into high aspect ratio resist cavities. The process in which the metal grows up crossing the top of resist cavities has been largely neglected. In this process, various defects may occur. A typical defect is the hole- formation at the top of electroplated metal mold, especially when the space/linewidth ratio of a microstructure is large. In order to understand and hence, to eliminate these problems, the electroplating process is investigated in details in this paper. First, the current density distribution is investigated using an electroplating simulation tool. The simulation results show that it is almost an isotropic growing model at the first stage of the process. As the deposited metal grows the space between electrodes is shortened and the current density distribution between the electrodes will be modulated and hence, will not be isotropic any more. The simulation also shows that a trench will form and its aspect ratio can reach a considerable magnitude especially when the space/linewidth ratio of the mold is large. As a result, ion depletion may become significant causing the deposition being stopped and thus forming holes at the bottom of overlapped neighbor electrodes. In order to solve this problem, an auxiliary electrode is introduced. The experiment results show the holes are therefore effectively eliminated in a nickel mold. This work also provides some design suggestions for the LIGA parts with various space / linewidth ratios.