Effect of potassium persulfate on chemical mechanical planarization of Cu/Ni microstructures for MEMS

Abstract

Electrochemical FABrication (EFAB) can fabricate complex metal microstructures. Planarization is crucial in EFAB in terms of flatness and surface quality. Accordingly, chemical mechanical polishing (CMP) was employed to planarize Cu/Ni microstructures. The oxidant K2S2O8 is critical in regulating the material removal rate (MRR) selectivity between copper and nickel. In the presence of the complexing agent ethylenediamine sulfate and corrosion inhibitors benzotriazole and sodium dodecyl benzene sulfonate, the MRR selectivity of 1 (∼47 nm/min) and satisfactory surface quality can be achieved by tuning the K2S2O8 concentration to 20 mM. A two-step planarization method was proposed. After lapping for 7 min and CMP for 21 min, the step height of the Cu/Ni microstructures rapidly decreases from 251 μm to −46 nm on average, and the surface roughness Sa of copper and nickel are significantly improved to 8.7 nm and 2.9 nm, respectively. For the CMP mechanism, as the K2S2O8 concentration increases, the copper corrosion is enhanced, and thus the copper MRR quickly increases; The nickel surface film gradually transforms from Ni(OH)2 to NiOOH, and the intermediate state can be removed relatively easily, and thereby the nickel MRR increases and decreases. This study may provide a feasible high-precision planarization method for EFAB.