Investigation on removal of SU-8 electroplating mould using fuming sulphuric acid

In some microelectromechanical system device fabrication, using fuming sulphuric acid to remove crosslinked SU-8 mould from integrated thick metal structures is an inexpensive and effective wet etching technique. The fabrication of stator for a suspended rotational microgyroscope, which has surface-micromachined structures integrated, was investigated to study the removal process of 250-µm thick SU-8 plating mould. Immersion time in the acid was a critical factor for complete removal of SU-8 mould without detectable damage to the nickel structures. Variation of etch rate was investigated every 0.5 h in the range from 0.5 to 6 h; 5 h was found to be a suitable time setting to remove 250-µm thick SU-8 mould. During the process, etch rate was measured slowing down from 110 µm/h in the first 0.5 h to 18 µm/h in the last 0.5 h. It was better to finish the whole removal process in a consistent manner because although taking sample out of the acid, cleaning, and immersing it back into the acid again was able to accelerate subsequent etch rate, metal structures were likely to be etched black. The etch rate which slowed down with the increase of etch time based on continuous etching provided useful reference for time control during SU-8 application for ultra-thick integrated microstructure fabrication.